관절 압력센서(기계적 수용체), 인대, 근방추, 골지건기관, 신경전달경로 탐구를 통한 문치연 통증치료법!!

[문형철]

https://cafe.daum.net/panicbird/QrbQ/66

전신 조절자로서 근막 fascis 문제 해결, 그 효과의 모든 것!! 최신 논문의 통합

https://cafe.daum.net/panicbird/QrbQ/72

1990년대 이후 TrP(통증유발점) 기념비적 논문 탐구

인류의 지식탐구를 통해 우리가 얻고 싶은 것은? phronesis 실천적 지혜

1. 통증치료를 넘어선 올바른 자세와 움직임(CMP therapy)

2. 자신의 몸 적용

인체 움직임 기능 - muscle(extrafusal fiber), muscle spindle(intrafusal fiber), GTO

관절의 기능 - Deep mechanoreceptor

Type I (Ruffini corpuscles / Ruffini endings) 나뭇가지처럼 여러 방향으로 갈라진 클러스터(cluster) 형태.

1) 특징: 천천히 적응(slowly adapting), 저역치(low-threshold).

2) 기능: 관절의 정적 위치 감각(static joint position),  지속적인 압력·인장(stretch)을 감지.

            자세 유지와 근육 긴장 조절에 중요.

3) 위치: 관절낭의 바깥층 (fibrous layer).

4) 구심신경 : 얇은 수초(thinly myelinated) 섬유, 직경 6–9 μm.

                    주로 Aδ 또는 Group II에 해당 (중간 크기 myelinated fiber).

Type II (Pacinian corpuscles / Vater-Pacini corpuscles) 긴 타원형 동심원 층(concentric lamellae)이 감싸고 있는 형태.

1) 특징: 빠르게 적응(fast-adapting).

2) 기능: 관절의 동적 움직임(dynamic movement, acceleration/deceleration) 감지

3) 위치: 관절낭의 더 깊은 층

4) 구심신경  : 두꺼운 수초(thickly myelinated) 섬유, 직경 9–12 μm 정도

                     Aβ 또는 Group II에 해당

Type III (Golgi tendon organ-like receptors) 가지가 많고 복잡한 수상돌기(arborization) 형태.

1) 특징: 고역치(high-threshold), 천천히 적응.

2) 기능: 인대의 강한 긴장(tension)이나 과신전을 감지하여 보호 반사(inhibitory reflex)를 유발.

            극단적 운동 범위에서 활성화.

3) 위치: 인대(ligaments, e.g., ACL/PCL)와 관절 근처 힘줄 끝부분

4) 구심신경: : 큰 수초(large myelinated) 섬유, 직경 13–17 μm.

                      Aα 또는 Group I (Ib)에 해당

Type IV (Free nerve endings) 가지가 불규칙하게 뻗은 나뭇가지 모양.

1) 특징: 캡슐이 없음(unencapsulated), 다중양상(polymodal).

2) : 통증(nociception), 염증, 손상 신호 전달.

      고유수용감각보다는 보호·경고 역할.

3) 위치 : 관절낭 (Joint capsule), 활액막 (Synovium), 지방패드 (Fat pads), 골막 (Periosteum)까지

4) 구심섬유 :  통증 전달( A-δ , C fiber)

                     대부분이 C-fiber (Group IV): 관절 통증의 60~80%를 차지.

                    일부 A-δ (Group III): 빠른 보호 반응 담당.

Type IV 수용기는 주로 얇은 유수 또는 무수 섬유로 구성.

구분              섬유 종류               직경 & 수초         전도 속도          주요 전달                                      신호특징

Group III	A-δ (A-delta)	1~5 μm, 얇은 유수	5~30 m/s	날카로운 통증 (Sharp pain), 빠른 경고	초기 급성 통증, 기계·열 자극
Group IV	C fiber	<1 μm, 무수	0.5~2 m/s	둔한 통증 (Dull pain), 염증·화학 자극	지연성 통증, polymodal, 만성염증

근방추와 골지건기관 

항목                             근방추 (Muscle Spindle)                             골지건 기관 (GTO)

위치	근육 내부 (intrafusal fiber)	근육-힘줄 접합부
감지하는 것	근육 길이와 길이변화 정도	근육 장력 (force), 장력변화 정도
주요 섬유	Ia (primary), II (secondary)	Ib
반사	신전반사 (Stretch reflex)	역신전반사 (Inverse stretch reflex) — 보호
역할	자세 유지, 운동 조절	과도한 힘으로부터 근육 보호

근방추는 물(Fluid)안에 들어 있는 구조

근육 및 골격 기계수용기 Muscle and skeletal mechanoreceptors			사지 고유수용감각 Limb proprioception
근방추 1차 말단 (Muscle spindle primary)	Aα	Ia	근육 길이와 속도 (Muscle length and speed)
근방추 2차 말단 (Muscle spindle secondary)	Aβ	II	근육 신전 (Muscle stretch)
골지건 기관 (Golgi tendon organ)	Aα	Ib	근육 수축에 의한 장력, 장력변화
관절낭 수용기 (Joint capsule receptors)	Aβ	II	관절 각도 (Joint angle)
신전 민감 자유 말단 (Stretch-sensitive free endings)	Aδ	III	과도한 신전 또는 힘 (Excess stretch or force)

기계적 수용기

근방추, 골지건기관의 정보를 신경, 척수, 대뇌로 전달하는 과정의 이해

Type I~III(기계수용기, mechanoreceptors)는 주로 고유수용감각(proprioception)과 촉각 정보를,

Type IV(통각수용기)는 통증·염증 정보를 전달.

공통 전달 구조 (3차 뉴런)

• 1차 뉴런 (1st order neuron): 주변 수용기 → 척수 후각(dorsal horn) 또는 후주(dorsal column). 세포체는 후근신경절(Dorsal Root Ganglion).
• 2차 뉴런 (2nd order neuron): 척수 또는 뇌간에서 교차(decussation) 후 시상(thalamus)으로.
• 3차 뉴런 (3rd order neuron): 시상 VPL (Ventral Posterolateral nucleus) → 대뇌피질 일차체성감각피질(Primary Somatosensory Cortex, S1) (postcentral gyrus, Brodmann 영역 3a, 3b, 1, 2).

전침의 통증 치료 기전(descending pain control)

TENS의 기전(통증의 Gate control theory)

통증을 완치를 못하면 일어나는 일은? 

AMI(Arthrogenic muscle inhibition) - 관절기원성 근억제

관절, 움직임 기능회복의 주요 핵심 

추나??

관절가동 테크닉(joint mobilization)

비유

마른 논에 물대기

관절 가동술(Joint Mobilization)의 이론적 배경을 정리하고,
다양한 기법(Kaltenborn, Maitland, McKenzie, Mulligan 등)들이
근골격계 장애(musculoskeletal disorders) 치료에 미치는 효과를 문헌고찰을 통해 비교·분석

주요 내용

1. 역사적 배경

• 고대(히포크라테스)부터 시작된 수기요법 → Still(골다공증), Palmer(카이로프랙틱), Cyriax(정형의학) 등을 거쳐 발전.
• 물리치료 분야에서 Kaltenborn(1940년대, grade 3단계, arthrokinematics/osteokinematics) → Maitland(grade 4+manipulation) → Mulligan(Mobilization with Movement) 등으로 발전.

2. 핵심 이론

• Osteokinematics (골 운동) vs Arthrokinematics (관절면 운동)
• Convex-Concave Rule (칼텐본): convex-concave 이론에 따른 glide 방향 결정
• Joint Position: Open Pack Position(OPP, distraction에 유리), Closed Pack Position(CPP)
• Mobilization Grade: Kaltenborn(3단계), Maitland(4단계 + manipulation)

3. 증거 요약

• 최근 연구(2015~2020) 체계적 고찰을 바탕으로, Kaltenborn, Maitland, McKenzie, Mulligan 등 모든 관절 가동 기법 간에 근골격계 장애 치료 효과에 큰 차이가 없다.
• 어떤 기법을 사용하든 통증 감소, 관절가동범위 증가, 기능 개선에 긍정적인 효과를 보임.
• 수기요법의 진통 효과(analgesic effect)와 근육 기능 개선 효과도 이론적으로 여전히 타당함.

치료사들은 특정 기법 하나만 고집할 필요가 없으며,
기본 이론(arthrokinematics, grade, convex-concave rule)을 정확히 이해하고
환자 상태에 맞게 적용하면 된다.

모든 관절 가동술 기법은
근골격계 문제에 비슷한 수준의 효과를 보인다

그럼 치료란 무엇인가? 

아래는

articular mechanoreceptors (joint mechanoreceptors) 분야의

기념비적·고인용지수 논문을 년도순으로 정리한 것입니다.

1.  1950년대~1960년대 (기초 해부·생리학 확립)

1956 — Skoglund S. "Anatomical and physiological studies of knee joint innervation in the cat." Acta Physiol Scand (Suppl. 124).

이 논문은

고양이(cat) 무릎관절의 신경 지배(innervation)에 대한

해부학적(anatomical) 및 생리학적(physiological) 연구입니다.

무릎관절(특히 관절낭, 인대, 근육 등)의 감각 신경 분포를 자세히 조사

관절의 기계수용체(mechanoreceptors)와

신경 섬유의 위치, 형태, 기능을 해부학적으로 기술

의의: 1950년대 초반에 발표된 이 연구는 관절 고유수용감각(proprioception) 연구의 초기 기초 자료 중 하나

Freeman MAR, Wyke B. "The innervation of the knee joint. An anatomical and histological study in the cat." J Anat. 인용 ≈779회 (가장 높은 인용 중 하나).

이 논문은

고양이(cat)의 무릎관절 신경 지배(innervation)에 대한

해부학적(anatomical) 및 조직학적(histological) 연구.

→ 가장 기념비적인 연구.

    관절 capsule·ligament의 신경 분포를 상세히 밝히고,

    Type I~IV mechanoreceptor를 체계적으로 분류.

      현대 proprioception 연구의 기준 논문.

2. 1980년대 (인간 적용 + 기능 증명)

Schultz RA, Miller DC, Kerr CS, Micheli LJ. "Mechanoreceptors in human cruciate ligaments. A histological study." J Bone Joint Surg Am. 인용 ≈881회. 

 ACL 재건술 후 proprioception 중요성의 임상적 토대.

이 논문은

인간의 십자인대(전방십자인대 ACL + 후방십자인대 PCL)에서

기계수용체(mechanoreceptors)의 존재를 최초로 조직학적으로 증명한 연구.

연구 방법:

• 전반관절치환술(total knee replacement) 시 제거된 십자인대, 부검 및 절단标本에서 채취
• 조직 염색: Bodian, Bielschowsky, Ranvier gold-chloride stain (축삭과 신경 종말을 염색하는 고전적 방법)
• 현미경 관찰로 신경 종말의 형태와 위치를 분석

주요 발견:

• 각 십자인대(ACL, PCL) 당 1~3개의 기계수용체가 관찰됨
• 위치: 인대 표면(superficial layer), 특히 활액막(synovial membrane) 바로 아래에 집중
• 형태: Golgi tendon organ과 유사한 구조 (고역치, 천천히 적응하는 수용체)
• 관절낭(joint capsule)이나 다른 부위에서는 발견되지 않음
• 이 수용체들은 고유수용감각(proprioceptive information)을 제공하고, 반사(reflexes)에 기여할 가능성이 높다고 제안

의의:

• 이전까지 동물 모델(고양이 등) 연구(Freeman & Wyke 1967 등)는 있었지만, 인간 십자인대에서 기계수용체를 직접 확인한 최초의 연구입니다.
• ACL 손상 후 고유수용감각(proprioception) 저하와 재활 전략의 이론적 근거를 제공한 획기적인 논문
• 이후 거의 모든 ACL 기계수용체 관련 연구에서 기초 문헌으로 필수 인용

Wood L, Ferrell WR.  "Response of slowly adapting articular mechanoreceptors in the cat knee joint to alterations in intra-articular volume." Ann Rheum Dis. 인용 ≈46회. 

이 논문은

고양이(cat) 무릎관절 후방 관절낭(posterior aspect of the knee joint capsule)에 분포하는

천천히 적응하는 기계수용체(slowly adapting articular mechanoreceptors)의

반응을 연구한 생리학적 실험.

연구 방법:

• 고양이 무릎관절에 미세전극을 이용해 단일 신경 섬유(single afferent fiber)에서 신경 방전(neural discharge)을 직접 기록
• 관절 내 용적(intra-articular volume)을 인위적으로 증가시키면서 (관절 내 압력과 함께) 수용체의 방전 패턴 관찰

주요 발견:

• 관절 내 용적이 증가함에 따라 초기에는 관절 내 압력(intra-articular pressure)과 신경 방전율(neural discharge)이 함께 증가
• 그러나 용적이 계속 증가해도 신경 방전은 일정 수준에서 평평해짐(levelled out) → 더 이상 증가하지 않음
• 이 현상은 유체 주입 속도(rate of fluid accumulation)에 따라 달라짐
• 결론: 관절 후방 캡슐에는 팽창 한계(capsular distension limit)가 존재하며, 그 이상의 유체는 관절 다른 부위로 이동(diverted)된다는 것

의의:

• 관절 내 삼출액(effusion)이나 부종(swelling) 시 slowly adapting mechanoreceptors (주로 Ruffini-like 수용체)가 어떻게 압력/팽창 신호를 전달하는지를 보여줌
• 관절 부종이 고유수용감각(proprioception)과 근육 반사(예: quadriceps inhibition)에 미치는 기전을 설명하는 중요한 기초 연구
• 이전 Freeman & Wyke (1967), Schultz (1984) 등과 함께 관절 기계수용체의 기능적 역할을 밝히는 고전적 생리학 논문

Zimny ML. "Mechanoreceptors in articular tissues." Am J Anat. 인용 ≈395회. 

동물·인간 관절 capsule, ligament, meniscus, TMJ 등에서

Ruffini, Pacinian, Golgi, free nerve ending의 형태·분포·기능을 종합 정리.

후속 연구에서 가장 많이 인용되는 고전.

이 논문은

관절 조직(articular tissues) 내 기계수용체(mechanoreceptors)의

형태(morphology), 분포(distribution), 기능(function)을

동물(주로 고양이 등)과 인간을 포함해 종합적으로 검토한 리뷰 논문.

주요 검토 내용:

• 관절낭(joint capsule), 인대(ligaments), 무릎 반월상 연골(knee-joint menisci), 측두하악관절(temporomandibular joint)의 관절 디스크(articular disks) 등에 분포하는 기계수용체를 체계적으로 정리
• 자유 신경 종말(free nerve endings) 외에 대부분의 관절에서 공통적으로 발견되는 세 가지 주요 유형:
  1. Ruffini-like receptor — 주로 관절낭(capsule)에 위치, 천천히 적응(slowly adapting), 정적 위치 감각(static position sense)과 관절 압력에 관여
  2. Golgi tendon organ-like receptor — 인대(ligaments)와 반월상 연골(menisci)에 주로 분포, 고역치(high-threshold), 인대 긴장(tension) 감지, 극단적 운동 범위에서 활성화
  3. Pacinian-like corpuscle (encapsulated) — 동적 운동(dynamic movement), 가속도·진동 감지, 빠르게 적응(fast-adapting)
• 각 수용체의 미세구조, 위치(표재 vs 심부), 자극 특성, 그리고 고유수용감각(proprioception)과 반사(reflex)에서의 역할을 설명
• 무릎 반월상 연골(meniscus)에서도 기계수용체가 존재한다는 점을 강조 (이전에는 인대 중심으로만 알려짐)

의의:

• 1950~80년대 관절 신경 연구(Freeman & Wyke 1967, Schultz 1984, Wood & Ferrell 1984 등)를 종합하여 관절 기계수용체의 통합적 이해를 제공한 중요한 리뷰 논문
• ACL(십자인대)을 포함한 관절 조직이 단순 기계적 구조물이 아니라 감각 기관(sensory organ)이라는 개념을 강화
• 이후 ACL 손상 후 proprioception 저하, neuroplasticity, 재활 전략 관련 연구(예: 2026년 리뷰 논문)의 핵심 기초 문헌으로 자주 인용됨

3. 1990년대 (인간 PCL·임상 확장)

Katonis PG et al. "Mechanoreceptors in the posterior cruciate ligament. Histologic study on cadaver knees." Acta Orthop Scand. 인용 ≈162회. 

→ 인간 PCL에서 mechanoreceptor 확인, PCL 보존술의 proprioception 근거 제공.

이 논문은

건강한 인간 후방십자인대(PCL)에서

기계수용체(mechanoreceptors)의 존재를 조직학적으로 확인한 연구.

(Schultz 1984가 ACL에 초점을 맞춘 데 이어 PCL을 대상으로 한 중요한 후속 연구)

연구 방법:

• 10명의 신선한 사체(fresh cadavers)에서 총 10개의 PCL을 채취
• 광학현미경(light microscopy)으로 조직학적 검사 (염색 기법 사용)
• 주로 PCL의 대퇴골 부착부(femoral attachment)와 경골 부착부(tibial attachment)를 중점 관찰

주요 발견:

• 자유 신경 종말(free nerve endings, afferent) 외에 두 가지 유형의 캡슐화된 기계수용체(encapsulated mechanoreceptors)가 확인됨
• 이 수용체들은 주로 인대 부착부 표면(near the surface of the ligament)에 위치
• Ruffini corpuscles와 Pacinian corpuscles (또는 유사 형태)로 추정되는 수용체가 관찰됨
• PCL도 ACL과 마찬가지로 단순한 기계적 구조물이 아니라 감각 기관(sensory organ)으로서의 역할을 가진다는 점을 뒷받침

의의:

• 인간 PCL에서 기계수용체의 존재를 직접 증명한 초기 연구 중 하나
• ACL과 PCL 모두에 기계수용체가 존재한다는 사실을 강화 → 무릎관절 전체의 고유수용감각(proprioception) 이해에 기여
• 이후 PCL 손상, 재건술 후 감각 회복, 재활 전략 연구의 기초 자료로 인용됨

이 논문은

인간의

추간판(intervertebral disc) 및 전종인대(anterior longitudinal ligament)에서

기계수용체(mechanoreceptors)의 존재, 형태(morphology), 분포(distribution),

그리고 관련 신경펩티드(neuropeptides)를 조사한 조직학적 연구.

연구 방법:

• 인간: 요통(low back pain) 환자와 척추측만증(scoliosis) 환자의 추간판 조직
• 소: 미추(coccygeal) 디스크
• 면역조직화학(immunolocalization) 기법으로 신경과 신경펩티드를 연속 절편에서 관찰

주요 발견:

• 기계수용체 위치: 인간 추간판의 섬유륜(anulus fibrosus) 바깥쪽 2~3개 lamellae(외측 2-3층)에 주로 존재. 이는 deep mechanoreceptor에 해당하며, 표재층이 아닌 비교적 깊은 섬유층입니다. 전종인대에서도 관찰됨.
• 형태 및 유형:
  • Pacinian corpuscles (빠르게 적응, 동적 운동 감지)
  • Ruffini endings (천천히 적응, 정적 위치·압력 감지)
  • Golgi tendon organ-like (가장 빈번하게 관찰, 긴장·고역치 감지)
• 발견 빈도: 요통 환자 디스크에서는 50%에서 기계수용체 발견, 척추측만증 환자에서는 15%로 유의하게 낮음.
• 기능: 자세(posture)와 운동(movement) 감각 제공 (고유수용감각 proprioception), 근육 긴장(muscle tone) 유지, 반사(reflexes) 조절. Golgi-type 수용체는 nociception(통증)에도 관여할 가능성 제시.

의의:

• 추간판(허리 디스크)이 단순한 쿠션 구조가 아니라 감각 기관(sensory organ)으로서 proprioception과 통증 전달에 기여한다는 점을 명확히 보여준 초기 중요 연구입니다.
• 정상 디스크에서는 기계수용체가 외층에 제한적으로 존재하나, 병적 상태(요통)에서 더 잘 관찰됨 → 디스크 퇴행·손상 시 deep mechanoreceptor의 역할 변화와 discogenic pain(디스크성 요통)의 기전을 이해하는 데 핵심 기초 논문.
• 이전 ACL 기계수용체 연구(예: Schultz 1984, Zimny 1988)와 유사하게, 디스크도 mechanoreceptor를 통해 sensorimotor control에 관여한다는 통합적 관점을 제공

4. 2000년대 이후 (OA·ACL·현대 리뷰)

이 논문은

요추 추간판(lumbar intervertebral disc)의 신경 공급(nerve supply)에 대한 문헌 고찰(review article)로,

특히

정상 vs 퇴행성 디스크에서의 감각 신경 종말(sensory nerve endings) 분포 차이를

중점적으로 다룹니다.

주요 발견:

• 정상 디스크 (normal lumbar disc): 신경 종말(nerve endings)은 주로 annulus fibrosus(섬유륜)의 바깥층(outer lamellae)에만 존재하며, nucleus pulposus(수핵) 깊숙이 침투하지 않습니다. 따라서 정상 상태에서는 디스크 내부가 통증에 둔감(insensitive)합니다.
• 퇴행성·손상 디스크 (degenerative lumbar disc): 감각 신경 종말(sensory nerve endings)이 깊숙이 침투(deep penetration)하여 파괴된 수핵(disrupted nucleus pulposus)까지 도달합니다. 이 신경들은 기계수용체(mechanoreceptors)와 통증 수용체(nociceptors)를 포함하며, discogenic low back pain(디스크성 요통)의 주요 원인이 됩니다.
• 신경 공급 경로: sinu-vertebral nerve (재발신경)를 통해 들어오는 dual innervation 패턴을 강조. 이전 연구(Roberts 1995 등)에서 확인된 mechanoreceptors (Ruffini, Pacinian, Golgi-like)가 annulus 외층에 있음을 바탕으로, 퇴행 시 nerve ingrowth(신경 침투)가 어떻게 통증과 sensorimotor 이상을 유발하는지 설명.

의의:

• 허리 디스크가 정상 상태에서는 “무감각”하지만, 퇴행·탈출 시 deep nerve ingrowth로 인해 만성 통증과 고유수용감각(proprioception) 장애가 발생한다는 기전을 명확히 정리한 고전적 리뷰 논문입니다.
• deep mechanoreceptor와 허리 디스크의 관계를 이해하는 데 핵심: 정상에서는 deep layer에 거의 없으나, 병적 상태에서 mechanoreceptor/nociceptor가 깊이 침투 → 악순환(통증 → 근육 경련 → 불안정 → 더 많은 퇴행).
• 이전 논문(Roberts 1995: 정상 annulus 외층 mechanoreceptor)과 직접 연결되어, “정상 vs 병적 변화”를 보여주는 중요한 발전 단계입니다.

이 논문은

정상 성인 사체(normal adult cadaveric donors)의

하위 요추 추간판(lower lumbar intervertebral discs,

주로 L4-5와 L5-S1)에서 기계수용체(mechanoreceptors)의 존재와 유형을

면역조직화학(immunohistochemical) 방법으로 조사한 연구.

연구 방법:

• 25개의 정상 요추 추간판(L4-5, L5-S1) 조직을 사체에서 채취
• S-100 protein 면역염색을 이용해 신경 및 기계수용체를 선택적으로 염색하여 광학현미경으로 관찰
• 두 개의 최하위 디스크를 중점 분석

의의:

• 정상 인간 요추 디스크에도 기계수용체가 풍부하게 존재한다는 점을 면역염색으로 확인한 연구입니다.
• 이전 Roberts (1995) 연구(annulus 외층 2-3 lamellae)와 Edgar (2007) 리뷰를 보완하며, 정상 상태에서의 deep mechanoreceptor 역할(고유수용감각, 자세 제어, 근육 반사)을 강조
• 디스크 퇴행 시 nerve ingrowth와 mechanoreceptor 변화(discogenic pain 기전)를 이해하는 중요한 기초 자료

https://pmc.ncbi.nlm.nih.gov/articles/PMC8185559/

Innervation of the Human Intervertebral Disc: A Scoping Review - PMC

이 논문은 인간 추간판(intervertebral disc, IVD)의 신경 분포(topography), 형태(morphology), 면역반응성(immunoreactivity)을 종합적으로 매핑하는 스코핑 리뷰(scoping review)입니다. PRISMA-ScR 가이드라인을 따랐으며, 6개 데이터베이스(PubMed, Scopus, EMBASE 등)에서 1940~2020년까지 검색 후 33개 연구를 최종 포함했습니다. 주로 조직학(histology)과 면역조직화학(immunohistochemistry, IHC: PGP9.5, CGRP, SP 등 마커 사용) 연구를 중심으로 분석했습니다.

주요 발견 (Key Findings)

1. 정상(비퇴행성) 디스크에서의 신경 분포

• 신경 요소는 주로 annulus fibrosus(AF, 섬유륜)의 바깥 1/3 (outer third)에 집중됩니다.
• 수핵(nucleus pulposus, NP)에는 정상적으로 신경이 거의 없음 (controls에서 0%).
• 기계수용체(mechanoreceptors): Ruffini-type (가장 흔함), Pacinian-type, Golgi-like가 outer AF에 존재. Free nerve endings도 관찰됨.
• 신경은 perivascular (혈관 주위)와 independent sensory nerve (혈관과 무관)로 나뉨.
• 이는 이전 고전 연구(Roberts 1995, Dimitroulias 2010)와 일치: 정상 디스크에서는 deep penetration이 거의 없음 → 디스크 내부가 통증에 둔감.

2. 퇴행성 또는 통증 디스크에서의 변화

• Hyperinnervation (신경 과증식)과 nerve ingrowth가 특징적입니다.
• 신경이 inner two-thirds of AF와 NP(수핵)까지 깊게 침투(deep penetration)합니다. 특히 annular fissure(섬유륜 균열), granulation tissue 부위에서 두드러짐.
• Type IV free nerve endings와 nociceptive fibers (SP, CGRP 양성)가 증가하여 discogenic low back pain의 핵심 기전.
• Encapsulated mechanoreceptors (Type I~III, Ruffini 등)는 outer AF에 주로 남아 있지만, 일부 연구에서 inner AF나 NP까지 관찰됨. 기능 저하나 위치 변화 가능성 제시.
• 염증 사이토카인(IL-1, TNF-α), neurotrophins(NGF 등)가 nerve ingrowth를 촉진.

정량적 데이터 예시:

• 정상 AF: outer layer 중심, NP 신경 거의 없음.
• 통증 디스크: inner AF 신경 침투 57% 이상, NP 침투 10/33 연구에서 확인 (Freemont 1997 등).
• Ruffini-type이 mechanoreceptor 중 가장 빈번 (Dimitroulias 2010 직접 인용).

임상적 함의 (Clinical Implications)

• 정상 디스크는 outer AF에만 제한적 innervation으로 안정적.
• 퇴행 시 deep nerve ingrowth + nociceptive sensitization → 만성 discogenic 요통 발생.
• 고유수용감각(proprioception) 측면: mechanoreceptor (Ruffini, Golgi-like)의 변화가 자세 제어와 sensorimotor control 저하를 유발할 수 있음 → 척추 불안정성 악순환.

피부 및 피하 기계수용기 Cutaneous and subcutaneous mechanoreceptors			촉각 (Touch)
마이스너 소체 (Meissner corpuscle)	Aβ, Aα	RA1	가벼운 쓰다듬기, 진동 (Stroking, flutter)
메르켈 원반 수용기 (Merkel disk receptor)	Aβ, Aα	SA1	압력, 질감 (Pressure, texture)
파치니 소체 (Pacinian corpuscle)	Aβ, Aα	RA2	진동 (Vibration)
루피니 말단 (Ruffini ending)	Aβ, Aα	SA2	피부 신전 (Skin stretch)
털-틸로트리히, 털-가드 (Hair-tylotrich, hair-guard)	Aβ, Aα	G1, G2	쓰다듬기, 떨림 (Stroking, fluttering)
다운 털 (Hair-down)	Aδ	D	가벼운 쓰다듬기 (Light stroking)
장(field) 기계수용기 (Field mechanoreceptor)	Aβ, Aα	F	피부 신전 (Skin stretch)
C 섬유 기계수용기	C	-	쓰다듬기, 성감적 촉각 (Stroking, erotic touch)
온도 수용기 Thermal receptors			온도 (Temperature)
냉각 수용기 (Cool receptors)	Aδ	III	피부 냉각 (<25°C)
온난 수용기 (Warm receptors)	C	IV	피부 온난 (>35°C)
열 통각수용기 (Heat nociceptors)	Aδ	III	뜨거운 온도 (>45°C)
냉 통각수용기 (Cold nociceptors)	C	IV	매우 추운 온도 (<5°C)

Ruffini-type mechanoreceptor는
Freeman & Wyke (1967) 분류에서 Type I에 해당하는
대표적인 천천히 적응(slowly adapting, SA) 기계수용체입니다.

관절, 인대, 관절낭, 피부 깊은 층(dermis/hypodermis), 건(tendon),
그리고 추간판(허리 디스크)의 annulus fibrosus 외층 등 결합조직(connective tissue)에 광범위하게 분포.

ACL(무릎) vs 척추(허리 디스크)에서의 비교

• ACL/관절: 관절낭과 인대 표재층에 많음. 정적 위치 감각과 근육 억제(arthrogenic muscle inhibition) 방지에 중요. 손상 시 proprioception 저하의 주요 원인.
• 허리 디스크: annulus fibrosus 외층에 가장 흔한 유형 (Dimitroulias 2010: 88%가 Ruffini-type). L5-S1 전방부에서 더 높은 빈도 (높은 shear force 때문). 정상: outer AF 집중 → proprioceptive feedback. 퇴행 시: Type I~III (Ruffini 포함) 기능 저하나 deep ingrowth와 함께 discogenic pain 악순환에 관여.

임상적·병리적 의미

• 손상/퇴행 후: Ruffini-type 수 감소 또는 기능 저하 → 고유수용감각 저하, 자세 불안정, 근육 약화, maladaptive neuroplasticity 유발 (ACL 손상이나 요통에서 공통).
• 퇴행성 변화: 일부 연구에서 병적 디스크(경추·요추)에서 Ruffini corpuscle 밀도가 증가 (염증·신경 재생 관련). 그러나 전체적으로 proprioceptive deficit가 dominant.
• 재활 전략: Ruffini 자극을 유도하는 지속적 stretch, balance training, sensorimotor exercise가 proprioception 회복에 효과적 (neuroplasticity-oriented 재활).

Pacinian 기계수용체 (Pacinian corpuscles / Type II) 

Pacinian corpuscle은
Freeman & Wyke (1967) 분류에서 Type II에 해당하는 빠르게 적응(fast-adapting, rapidly adapting) 기계수용체.


한 줄 요약 비교:

• Ruffini = “지속적인 자세 정보 제공자” (천천히 적응, 정적)
• Pacinian = “빠른 움직임·진동 감지기” (빠르게 적응, 동적)

ACL(무릎) vs 척추(허리 디스크)에서의 분포와 역할 비교

• ACL (무릎 인대):
  • Pacinian corpuscles가 명확히 관찰됨 (Zimny 1988, Schultz 1984 등).
  • 동적 운동 피드백에 중요: 방향 전환, 착지, 스포츠 동작 시 관절 위치 변화 감지.
  • 손상 후: 동적 proprioception 저하 → 재손상 위험 증가.
• 허리 디스크 (lumbar intervertebral disc):
  
  • 전체적으로 척추 디스크에서는 Ruffini-type이 주를 이루고, Pacinian은 동적 측면(갑작스러운 움직임이나 진동)에서 보조 역할. 퇴행 시 deep ingrowth는 주로 Type IV (free nerve endings)와 nociceptive fiber 중심.

임상적·병리적 의미

• 손상/퇴행 후: Pacinian 기능 저하 → 동적 운동 감지 능력 감소 → 불안정성, 특히 스포츠나 일상 동작에서 문제.
• 재활 전략: Pacinian을 자극하려면 빠른 진동, sudden movement, balance perturbation 훈련
  (예: wobble board, plyometric, vibration platform)이 효과적. (Ruffini는 지속적 stretch나 static balance 훈련)
• ACL 재활과 디스크성 요통 재활 모두에서 Type I (Ruffini) + Type II (Pacinian) 균형 잡힌
  sensorimotor training이 중요.

골지 기계 유사수용체 (Golgi Tendon Organ-like mechanoreceptor / Type III) 

골지 기계 유사 수용체는 Freeman & Wyke (1967) 분류에서 Type III에 해당하며,
고역치(high-threshold), 천천히 적응(slowly adapting)하는 기계수용체.

원래 건(tendon)에 있는 Golgi tendon organ (GTO)과 매우 유사한 구조와 기능을 가지기 때문에
Golgi tendon organ-like receptor 또는 Golgi-like ending이라고 불립니다.



한 줄 요약 비교:

• Ruffini → “지속적인 자세 감시자”
• Pacinian → “빠른 움직임 감지기”
• Golgi (Type III) → “과부하 경보 시스템 + 보호 브레이크”

임상적 의미와 재활

• 손상 후: Golgi-like receptor 손상 → 과신전이나 과부하에 대한 보호 반사가 약해져 불안정성과 재손상 위험이 증가.
• 재활 전략:
  • End-range training: 관절의 극단적 범위에서 천천히 제어하는 운동 (controlled end-range loading).
  • Eccentric exercise: 근육이 길어지면서 강한 긴장을 받는 운동.
  • Proprioceptive training: balance board, perturbation training 등으로 Golgi 자극 유도.
  • ACL 재활 후기나 만성 요통 재활에서 특히 중요합니다.

골지 기계수용체 (Golgi-like / Type III) vs 근육 방추 (Muscle Spindle) 비교

골지 기계수용체(Golgi tendon organ-like, GTO-like)와
근육 방추(muscle spindle)는
모두 고유수용감각(proprioception)에 핵심적인 역할을 하는 수용체이지만,
위치, 감지하는 정보, 반사 유형, 기능이 완전히 반대입니다.

이 둘은
서로 상호보완적으로 작용하며,
근육-건-관절의 안전과 정밀한 운동 제어를 담당합니다.


임상적·재활적 의미 (ACL 손상 및 허리 디스크 연계)

• 정상: Muscle spindle (길이 감지) + Golgi (긴장 감지)가 함께 작용 → 정밀한 sensorimotor control.
• 손상 후:
  • Muscle spindle 손상/억제 → stretch reflex 약화 → 근육 약화 (arthrogenic muscle inhibition).
  • Golgi-like 손상 → 보호반사 약화 → 과신전·과부하에 취약 → 재손상 위험 증가.
• 재활 전략:
  • Muscle spindle 자극: 빠른 stretch, vibration, perturbation training.
  • Golgi-like 자극: eccentric exercise, end-range controlled loading, heavy slow resistance training (강한 장력 유도).
  • ACL 재활 후기나 만성 요통 재활에서 두 수용체의 균형을 맞추는 sensorimotor training이 핵심.

한 줄 요약:

• Muscle Spindle = “근육이 얼마나 늘었나?” → 수축 유도 (보호 + 자세 유지).
• Golgi-like = “근육이 얼마나 세게 당기고 있나?” → 이완 유도 (과부하 보호).

관절염(Osteoarthritis, OA)에서의 기계수용체 변화 

관절염, 특히 무릎관절(knee OA)에서
기계수용체(mechanoreceptors)의 변화는
고유수용감각(proprioception) 저하의 주요 원인 중 하나입니다.

이는
연골 파괴, 인대·관절낭 퇴행, 염증과 함께 악순환(cycle)을 형성하며,
관절 불안정성,
근력 약화(arthrogenic muscle inhibition),
자세 흔들림 증가,
낙상 위험 상승으로 이어집니다.


1. 주요 변화 요약

• 기계수용체 수의 감소 (Quantitative decline):
  • Ruffini corpuscles (Type I), Pacinian corpuscles (Type II), Golgi-like corpuscles (Type III), free nerve endings (Type IV) 모두 감소.
  • 특히 PCL에서 Golgi, Ruffini, free nerve endings, total nerve endings 수가 유의하게 낮음.
  • 작은 혈관(small vessels) 수도 함께 감소 → 혈관 재생과 신경 재분포(reinnervation) 장애.
• 심각도와의 상관관계:
  • OA 중증도(WOMAC score)가 높을수록 PCL 내 mechanoreceptor 수가 유의하게 감소 (Chen et al. 2023).
    • WOMAC 낮은 군: 평균 43.5개
    • 중간 군: 25.0개
    • 높은 군: 15.2개 (p < 0.001)
  • 연령(age)은 mechanoreceptor 수에 거의 영향 없음 (주요 요인은 OA severity).
• 조직학적·기능적 변화:
  • 형태학적 퇴행(degeneration): 수용체의 캡슐 손상, 신경 섬유 탈수초(demyelination), 밀도 저하.
  • Proprioceptive deficit: Joint position sense (JPS), kinesthesia, postural sway 검사에서 악화. 특히 severe OA에서 두드러짐.
  • 연골 병변(cartilage lesions)만 있어도 proprioception 저하 관찰 → mechanoreceptor 변화가 연골 손상과 독립적으로 또는 함께 작용.

2. Type별 변화 특징 (Freeman & Wyke 분류 기준)

• Type I (Ruffini): 정적 위치·지속적 stretch 감지 → 수 감소로 자세 유지 능력 저하.
• Type II (Pacinian): 동적 운동·진동 감지 → 급격한 움직임 피드백 약화 (일부 연구에서는 상대적으로 덜 감소).
• Type III (Golgi-like): 강한 긴장·과신전 보호반사 → 보호 기능 약화로 과부하 취약.
• Type IV (Free nerve endings): 통증·염증 신호 → OA에서는 전체적으로 감소하나, 일부 deep ingrowth나 과민화로 만성 통증 기여 (discogenic pain과 유사한 기전).

https://pmc.ncbi.nlm.nih.gov/articles/PMC7769215/

Regulating muscle spindle and Golgi tendon organ proprioceptor phenotypes - PMC

이 리뷰 논문은

근방추(Muscle Spindle, MS)와

골지건 기관(Golgi Tendon Organ, GTO)의 고유수용감각(proprioceptor) 뉴런이

어떻게 다양한 아형(subtype)으로 분화하고,

그 분자적·발생학적 기전을 최근 연구(특히 single-cell RNA sequencing) 결과를 바탕으로 정리한 것.

1. 고유수용감각 뉴런의 주요 아형

• Muscle Spindle (MS) afferents:
  • Group Ia: Primary ending — 근육 길이 변화 속도와 위치를 민감하게 감지 (dynamic sensitivity 높음).
  • Group II: Secondary ending — 주로 근육 길이 감지.
• Golgi Tendon Organ (GTO) afferents:
  • Group Ib: 근육 긴장력(force)을 감지 → 과도한 수축 시 보호 반사(역신전반사).

2. 발달 과정 (3단계)

1. Generic Proprioceptor Fate (초기 공통 운명 결정)
   • Runx3 전사인자와 NT3/TrkC 신호가 핵심.
   • Retinoic Acid (RA)가 Runx3 발현을 조절해 DRG 내 proprioceptor 수를 조절.
2. MS vs GTO 아형 분화
   • Heg1: MS afferents (Ia/II) 마커
   • Pou4f3: GTO afferents (Ib) 마커
   • 분화 시점: 말초 근육 표적 침범 이후 (e14.5 ~ 출생 후) → 표적(수용기)과의 상호작용이 중요.
   • MS 아형은 postnatal 기간까지 더 세분화 (Calretinin 등).
3. Muscle-type Identity (근육 특이성)
   • Hox 유전자: 축성(axial) vs 사지(limb) 수준의 광범위한 정체성 결정 (intrinsic).
   • Limb mesenchyme나 근육으로부터의 외부 신호(extrinsic): dorsal vs ventral, proximal vs distal 근육 특이성 결정.
   • 예: distal hindlimb에서 dorsal/ventral Ia afferents에 특이적 분자 마커 존재.

3. 핵심 메시지

• Proprioceptor의 다양성은 늦은 발달 단계 (표적 침범 후)에서 주로 결정되며, intrinsic genetic program (Hox, Runx3)와 extrinsic target-derived signals의 상호작용으로 형성된다.
• MS afferents가 GTO보다 더 다양한 molecular subtype을 가짐 (정밀한 길이·속도 정보 필요).
• 새로운 genetic tool (Cre/Flp 등)로 특정 아형 연구가 가능해지면서, 운동 제어에서의 세부 기능 규명이 기대됨.

꼭 알아야 할 큰 개념

extrafusal fiber(우리가 아는 근육)

intrafusal fiber(muscle spindle, 근방추, 근육의 길이와 길이변화 감지 섬유)

https://pmc.ncbi.nlm.nih.gov/articles/PMC7788844/

이 논문은

근육 스핀들(muscle spindle)의 발달, 정상 기능,

그리고 다양한 신경근육 질환(특히 근이영양증)에서 나타나는 변화에 대한 리뷰.

근육 스핀들은

근육의 길이 변화와 신장 속도를 감지하는 중요한 고유수용기(proprioceptor)로,

운동 제어, 자세 유지, 안정적인 보행에 필수적.

주요 내용

1. 정상 기능 (Healthy Muscle)
   • 거의 모든 골격근에 존재하며, CNS(중추신경계)에 근육 길이와 신장 속도를 전달.
   • 구조: 캡슐로 둘러싸인 특수 근섬유(intrafusal fibers: nuclear bag1, bag2, chain fibers)로 구성.
     • Ia afferent (1차 감각신경): 동적(dynamic) 및 정적(static) 감각.
     • II afferent (2차): 주로 정적 감각 (인간 기준).
     • γ-motoneuron: intrafusal fiber를 수축시켜 감도를 조절.
   • 근육이 늘어나면 spindle도 함께 늘어나 action potential을 발생시켜 뇌가 사지 위치·운동을 인지하게 함.
   • Proprioception(고유감각)의 핵심으로, 운동 조절, 균형, 골절 치유, 척수 손상 회복 등에 중요.
   •  
2. 질환에서의 변화 (Diseased Muscle)
   • 많은 신경근육 질환에서 muscle spindle 기능이 손상되어 불안정한 보행, 잦은 낙상, 운동실조(ataxia) 등을 유발.
   • 특히 근이영양증(muscular dystrophies)에서 spindle 구조·기능 변화가 두드러짐 (DGC: dystrophin-glycoprotein complex 관련).
   • spindle은 근육 기능 분석이나 치료 전략에서 자주 간과되지만, proprioceptive deficit이 운동 장애의 주요 원인 중 하나임.

결론 및 시사점

근육 스핀들은 단순한 stretch receptor가 아니라,

운동 제어와 비의식적 골격 기능 유지에 핵심적입니다.

질환 시 spindle 기능 저하를 고려한 치료가 필요하며,

proprioception 회복이 운동 기능 개선에 중요

Salamanna F et al. (2023) "Proprioception and Mechanoreceptors in Osteoarthritis: A Systematic Literature Review." J Clin Med. 인용 ≈40회.

→ OA 환자에서 mechanoreceptor 감소와 proprioception 저하를 체계적으로 분석한 최근 고인용 리뷰.

이 논문은

골관절염(Osteoarthritis, OA) 환자에서

고유수용감각(proprioception) 저하와

기계수용체(mechanoreceptors) 변화에 대한

체계적 문헌 고찰(Systematic Literature Review).

목적:

OA에서 관절의 감각 기능이 어떻게 손상되는지,

특히 기계수용체의 형태학적·기능적 변화와

proprioceptive deficit의 관계를

체계적으로 분석하고, 임상적 함의를 도출하는 것.

방법:

• 2013년 9월부터 최근까지 PubMed, Scopus 등 데이터베이스에서 체계적 검색
• OA 환자(주로 무릎관절 중심)를 대상으로 한 임상·조직학·신경생리학 연구 선별
• PRISMA 가이드라인 준수

주요 발견:

• OA 환자에서 고유수용감각이 유의하게 저하됨 (joint position sense, kinesthesia, postural control 모두 악화)
• 관절낭, 인대(ACL, PCL 포함), 반월상 연골 등 관절 조직 내 기계수용체의 수와 기능이 감소 또는 퇴행성 변화(degeneration) 관찰
  • Ruffini, Pacinian, Golgi-like mechanoreceptors의 밀도 감소, 형태 이상, 신경 섬유 탈수초(demyelination) 등
• 이러한 변화는 연골 파괴, 염증, 관절 불안정성과 밀접한 관련이 있으며, 악순환(cycle)을 형성
• OA 진행 정도와 proprioceptive deficit 사이에 상관관계가 존재 (중증 OA에서 더 심함)
• 양측성 변화(bilateral effects)도 일부 관찰됨

https://www.mdpi.com/2411-5142/10/4/454

Knee Health Is a Major Determinant of Mobility Across the Healthspan

무릎 건강(knee health)은

healthspan(건강수명) 동안

이동성(mobility)과 보행(locomotion)의 핵심 결정 요인이다.

무릎 기능 저하는

생역학적·생리적 악순환을 일으켜 공중보건 및 경제적 부담을 초래한다.

이 리뷰는

무릎의 구조적·신경근육적·생역학적 요소가 서로 연결되어

mobility를 유지하는 메커니즘을 종합적으로 분석하며,

Integrated Knee Health Mobility Model (IKHMM)을 제안.

연구 배경 및 목적

• 무릎은 단순한 경첩 관절이 아니라 복잡한 생역학 시스템 (cartilage, ligaments, muscles, sensorimotor system의 상호작용).
• ACL 손상, meniscus tear, knee OA 등으로 인한 무릎 문제는 청소년기부터 노년기까지 mobility 저하의 주요 원인.
• 목적: knee health와 mobility의 다층적 관계를 설명하고, 악순환(cascade of deterioration)을 막는 중재 전략의 근거를 제시.

주요 내용 구조

1. 사회적·경제적 맥락
   • 전 세계 40세 이상 6.54억 명이 knee OA를 앓음. 1990년 이후 disability years 132% 증가.
   • ACL/PTOA(외상 후 OA)로 젊은 층부터 시작 → 중장년·노년 악화.
   • 경제적 비용: OA 관련 연간 1,850억 달러 (직접+간접 비용), 낙상·심혈관 질환·당뇨 등 2차 비용 추가.
2. 정상 보행의 생역학 및 에너지학
   • 무릎은 kinetic chain의 핵심 연결고리 (지지·충격 흡수·power generation).
   • Vasti 근육(대퇴사두근)이 보행 속도 증가 시 주요 활성화.
3. Sensorimotor System과 Mechanoreceptors (이전 대화와 직접 관련)
   • Figure 3에서 Ruffini endings (slow-adapting, stretch), Pacinian corpuscles (fast-adapting, vibration), Golgi tendon organs, free nerve endings 등의 위치와 역할을 상세히 설명.
   • 관절낭·인대·건·근육에서 proprioception과 nociception을 담당.
4. 악순환 메커니즘
   • Quadriceps weakness → arthrogenic muscle inhibition (AMI) → cartilage degradation → compensatory gait (hip/ankle 과부하) → metabolic cost 증가.
   • Knee power (K1~K4 phase) 감소, joint moment redistribution.
   • Muscle co-contraction 증가 → 관절 압축력 ↑ → OA 진행 가속.
     • 근육별 기여도 (gluteus maximus, gluteus medius, hamstrings, vasti)
     • 속도가 빨라질수록 (특히 fast) vasti (대퇴사두근)의 수직 가속 기여도가 크게 증가 (**).
     • Gluteus maximus와 medius도 속도 증가 시 기여 ↑ (early stance에서 몸을 지지하고 전진을 제어).
     • 의미: 빠른 보행일수록 대퇴사두근(vasti)과 둔근이 초기 충격 흡수와 몸 지지에 더 중요해짐.
     • Early stance: gluteus maximus + vasti → backward acceleration (전진을 제동, braking).
     • Late stance: gastrocnemius + soleus (비복근 + 가자미근) → forward acceleration (추진력).
     • Gluteus maximus는 early stance에서, soleus/gastrocnemius는 late stance에서 주요 역할.
     • 속도가 빨라질수록 전체적인 근육 기여도가 증가.
     • 상단: Knee flexion-extension 곡선 (4가지 속도)
       • 속도가 빨라질수록 무릎 굴곡(flexion) 범위와 peak가 증가.
     • 하단: Rectus femoris (실선) vs Vastus medialis (점선) EMG (대퇴사두근 활성도)
       • Fast walking에서 rectus femoris 활성도가 더 뚜렷하게 증가 (swing phase 준비).
       • Vastus medialis는 stance phase에서 안정적으로 활성.
     • 보행 속도가 증가하면:
       1. Early stance: Gluteus maximus + Vasti가 몸을 더 강하게 지지하면서 전진을 제동.
       2. Late stance: Plantarflexors (soleus, gastrocnemius)가 추진력을 크게 증가.
       3. 무릎: 더 큰 flexion-extension 운동과 quadriceps 활성화.
     • 임상적 의미: ACL 손상·재건술 후 재활 시, 보행 속도별 근육 훈련(특히 vasti, gluteus, plantarflexors)이 고유수용감각 회복과 안정성 향상에 중요

무릎 치료 통합 모델 4가지 주요 영역

1. Structural Integrity (구조적 완전성) → 연골, 뼈, 반월판, 인대, 활액막
2. Neuromuscular Function (신경근육 기능) → 근육 품질, 신경근육 조절 (Ruffini, Pacinian 등 mechanoreceptor 포함)
3. Biomechanical Adaptations (생역학적 적응) → 보행 패턴, 하중 분배, 보상 동작
4. Functional Mobility (기능적 이동성) → 실제 걸음, 일상생활 능력, 삶의 질

작동 원리

• 이 4가지가 피드백 루프(Positive: 악화 / Negative: 안정화)로 서로 영향을 주고받음.
• Modifying Factors: 나이, 비만, 이전 부상(악화) vs 운동, 유전(보호)

결론: 무릎은 한 부분만 망가져도 전체가 점점 악화되는 연쇄적 시스템이므로,

        조기 예방과 다각적 관리가 중요하다는 모델

 Lee Z et al. "Disrupted sensorimotor control after ACL injury..." Annals of Medicine. → ACL 손상 후 mechanoreceptor degeneration과 neuroplasticity를 최신으로 종합 (현재 가장 최근 landmark 리뷰).

이 논문은

전방십자인대(ACL) 손상 후 발생하는

감각운동 제어(sensorimotor control) 장애를 다룹니다.

주로 기계수용체(mechanoreceptor) 퇴행성 변화에서 시작해

중추신경계(CNS) 신경가소성(neuroplasticity) 변화까지 연결하고,

이를 바탕으로 한 재활 전략을 제안하는 리뷰 논문.

1. 배경과 목적 (Introduction)

ACL 손상은 젊은 활동 인구에서 흔하며,

단순한 기계적 불안정성(mechanical instability)뿐 아니라

고유수용감각(proprioception) 장애를 유발합니다.

고유수용감각이란

관절 위치 감각(joint position sense),

운동 감각(kinesthesia),

반사적 근육 조절 등을 포함하는 감각으로,

ACL 내 기계수용체가 이를 담당합니다.

손상 후 기계수용체가 손상되면

말초(afferent) 신호가 약해지고,

중추신경계에서 부적응성 가소성(maladaptive neuroplasticity)이 발생해

기능 회복이 지연되고 재손상 위험이 증가합니다.

논문은

말초-중추 통합 관점에서 평가 방법과 재활 전략을 체계적으로 정리합니다.

2. ACL 내 기계수용체의 해부학과 기능 (Mechanoreceptors in the ACL)

ACL은 단순한 안정 구조물이 아니라

감각 기관(sensory organ)으로,

전체 부피의 1~2.5%를 기계수용체가 차지합니다.

주로 경골(tibial)과 대퇴(femoral) 부착부 근처, 활액막 아래에 분포합니다.

네 가지 주요 유형 (Figure 1 참조):

• Ruffini corpuscles (Type I): 방추형, 저역치(low-threshold), 천천히 적응(slowly adapting). 정적 위치 감각(static position sense)과 압력 감지를 담당. 표재성(superficial)에 많아 자세 제어에 중요.
• Pacinian corpuscles (Type II): 난형(ovoid), 빠르게 적응(fast-adapting). 가속도( acceleration)와 진동 감지. 동적 운동 피드백(dynamic feedback)에 핵심.
• Golgi tendon organs (Type III): 고역치(high-threshold), 천천히 적응. 과신전(hyperextension) 방지를 위한 긴장(tension) 감지. 부착부 근처에 위치.
• Free nerve endings (Type IV): 불규칙형, 다중양상(polymodal) 통증/염증 수용체(nociceptors).

Figure 1 설명 (논문의 주요 모식도): ACL 기계수용체의 4가지 유형을 도식화한 그림.

각 수용체의 형태(Ruffini: fusiform, Pacinian: ovoid 등), 위치(표재/심부), 자극 유형(정적 위치, 가속도, 긴장, 통증)을 화살표와 라벨로 표시. ACL 전체를 선으로 나타내고, 수용체가 관절 운동 시 어떻게 신호를 생성하는지 개념적으로 보여줍니다. 이 그림은 기계수용체가 관절 각도·긴장·가속도를 CNS로 전달해 neuromuscular control을 조절한다는 점을 강조합니다.

함의: 손상 시 이 신호망이 붕괴되어 sensorimotor loop가 깨짐.

Table 1은 위 유형을 기능별로 정리한 표입니다. (정적 vs 동적, 방어 vs 통증 역할)

이 수용체들은

시각·전정계와 통합되어 무릎 안정성과 운동 조절을 유지합니다.

3. ACL 손상 후 고유수용감각 변화 (Changes in Proprioception Following ACL Injury)

손상 직후 기계수용체 수가 감소하고,

시간이 지날수록 퇴행성 변화(degeneration)가 진행됩니다.

원숭이 모델 연구에서

관절 위치 감각이 점진적으로 악화되고,

양측성 변화(bilateral effects)가 관찰됩니다

(손상된 쪽뿐 아니라 반대쪽도 영향).

결과적으로:

• Joint position sense (JPS)와 kinesthesia 정확도 저하
• Quadriceps activation inhibition (arthrogenic muscle inhibition, AMI)
• 자세 흔들림(postural sway) 증가 → 기능적 불안정성
• 재손상 위험 상승

중추신경계에서는

피질 재조직화(cortical reorganization)가 일어나

maladaptive plasticity가 발생합니다.

예를 들어,

somatosensory cortex와 motor cortex의 mapping이 변화하고,

bilateral hemisphere involvement가 증가합니다.

Table 2는 이러한 변화(감소된 정확도, bilateral 영향, 시간 의존적 악화)를 요약합니다.

Figure 2 (추정: neurophysiological mechanisms linking proprioceptive impairment and AMI): 손상 후 말초 신호 감소 → 척수·뇌간·피질 수준에서의 억제 메커니즘을 연결한 다이어그램.

AMI와 proprioceptive deficit의 신경생리학적 경로를 보여줍니다. (화살표로 afferent 감소 → gamma motor neuron 억제 → muscle spindle sensitivity 저하 등 표시)

4. 고유수용감각 평가 방법 (Measurement Methods)

직접 측정이 어렵기 때문에 간접 방법 4가지를 분류:

• Joint Position Reproduction (JPR): 능동/수동으로 특정 각도를 재현. Absolute error, constant error, variable error로 정량화. 신뢰도 ICC 0.60–0.95.
• Threshold to Detect Passive Motion (TDPM): 아주 느린 수동 운동을 감지하는 최소 각도 차이.
• Postural Sway Test: 눈 감은 상태에서 force plate로 중심압(COP) 변동성 측정. 가장 기능적(functional) 평가.
• Somatosensory Evoked Potentials (SEPs): 관절 자극 후 EMG 또는 피질 전위(latency/amplitude) 측정.

다중 모달(multimodal) 평가를 권장하며,

contralateral 비교는 bilateral effect 때문에 부정확할 수 있습니다.

5. 재건술 및 일차 봉합의 영향 (Reconstruction and Primary Repair)

• ACL Reconstruction: 기계적 안정성은 회복되지만, 고유수용감각 회복은 불완전. 조기 수술(<3개월)이 receptor 보존에 유리. Remnant-preserving technique(잔존 조직 보존)은 혈관 재생과 재신경지배(reinnervation)를 촉진해 JPS와 strength 회복을 향상시킴. Graft 종류(자가/동종) 간 proprioception 차이는 크지 않음.
• Primary Repair (with augmentation): native 조직과 receptor를 보존해 flexion angle sense에서 더 나은 결과를 보임.

6. 신경가소성 지향 재활 (Neuroplasticity-Oriented Rehabilitation)

전통 재활(근력 강화, neuromuscular training)은 제한적입니다.

논문은 중추신경계 재조직화를 목표로 한 접근을 강조:

• Remnant-preserving reconstruction + neuromuscular training
• Neurofeedback (EEG나 fMRI 기반 실시간 피드백)
• Virtual reality (VR) 또는 sensor-based technology (착용형 센서로 운동 중 피질 활성화 유도)
• 장기 neurophysiological monitoring (SEPs 등)

목표는 maladaptive plasticity를 adaptive plasticity로 전환하는 것. 예: proprioceptive training은 position sense, hopping, balance를 개선하지만 strength에는 영향이 적음.

결론 및 임상적 함의 (Conclusion)

ACL 손상 후 sensorimotor disruption은 말초 기계수용체 퇴행 → 중추 maladaptive neuroplasticity의 연속 과정입니다. 효과적인 회복을 위해서는:

1. 조기 remnant-preserving 수술로 말초 receptor 최대한 보존
2. 표준화된 proprioception 평가 (postural sway 중심)
3. neuroplasticity-driven 재활 (neurofeedback, VR, individualized sensor tech)
4. 장기 추적 모니터링

이 프레임워크는 기존 strength 중심 재활을 넘어 functional recovery와 reinjury prevention을 강화할 수 있습니다. 미래 연구로는 대규모 RCT와 advanced neuroimaging (fMRI, TMS)이 필요합니다.

이 논문은 스포츠의학·재활 분야에서 통합적 관점(integrative perspective)을 제공하며, 임상에서 “기계적 안정성만 복구하면 된다”는 단순 사고를 넘어 신경-근육-인지 통합 재활의 필요성을 강조합니다

https://www.nature.com/articles/s41413-025-00472-7

만성 요통(Low Back Pain, LBP)의 주요 원인 중 하나인

discogenic low back pain(디스크성 요통)의 병태생리, 진단 어려움, 실험 모델, 현재 치료의 한계,

그리고 미래 치료 방향을 체계적으로 정리.

특히

추간판 퇴행(IVD degeneration, IVDD)이

항상 통증을 유발하지 않는 이유(30~95%는 무증상)를 강조하며,

hyperinnervation(신경 과증식), 염증, 신경감작(sensitization)을 중심으로

discogenic pain을 별개의 병적 실체로 규정합니다.

주요 발견 및 병태생리 (Key Findings)

1. 정상 vs 퇴행성 추간판 신경 분포
   • 건강한 추간판: inner annulus fibrosus (AF)와 nucleus pulposus (NP)는 거의 무신경(aneural). 신경은 outer AF에만 제한적으로 존재.
   • 퇴행성 추간판: aberrant nerve and vascular ingrowth (비정상적 신경·혈관 침투)가 발생. 신경이 inner AF와 NP까지 깊게 침투(deep penetration) → hyperinnervation.
   • 이는 이전 고전 연구(Roberts 1995, Edgar 2007, Groh 2021)와 일치하며, Type IV free nerve endings와 nociceptors의 증가가 discogenic pain의 핵심 기전.
2. 분자·세포 기전
   • ECM degradation: MMP3, MMP13, ADAMTS 등 protease 과발현 → 디스크 탈수, 높이 감소.
   • 염증: DAMPs → PRR 활성화 → NF-κB/MAPK 경로 → TNF-α, IL-1β, IL-6 등 사이토카인 폭풍. 산성 pH가 이를 악화.
   • 신경성장 및 감작: NGF (Nerve Growth Factor)와 BDNF가 TrkA/TrkB 수용체를 통해 axonal sprouting과 sensitization 유발 (MAPK/ERK, PI3K/Akt, PLC-γ 경로).
   • 이온 채널: TRPV1, TRPV4가 기계적·염증성 자극 감지 → 통증 과민화.
   • 면역 세포 침윤: macrophage, neutrophil 등이 inflammation과 nerve ingrowth를 촉진.
3. 기계수용체(mechanoreceptors)와 proprioception 관련
   • 직접적으로는 적게 다루지만, low-threshold mechanoreceptors가 sensitization 과정에서 pain pathway로 재편입될 수 있음.
   • TRPV4가 osmotic/mechanical 변화 감지 → IL-6 upregulation.
   • IVDD로 인한 biomechanical 변화(경직성 증가, 탈수)는 proprioceptive feedback을 간접적으로 저하시킬 가능성 제시 (이전 ACL/OA 논문과 유사한 sensorimotor disruption 기전).

Discogenic LBP는 IVDD 자체가 아니라

염증 + hyperinnervation + sensitization의 복합 결과물입니다.

MCL (내측 측부인대)의 epiligament (EL, 인대외막)이

인대 치유에 중요하다는 기존 연구를 바탕으로,

EL 내 신경섬유와 mechanoreceptor의 분포·밀도·형태를

처음으로 상세히 조사했습니다.

특히

Meissner’s corpuscles가

EL에 존재한다는 것을 최초로 보고한 점이 핵심.

이 연구는

Freeman & Wyke의 전통적 Type I~IV 분류를 언급하면서,

MCL EL의 proprioception, nociception, 혈류 조절 역할을 강조합니다.

연구 방법

• 표본: 12명 신선 사체 (평균 연령 55세, 남 5·여 7)의 MCL proximal, mid, distal 부위 EL.
• 염색: H&E, Luxol fast blue/Cresyl violet (수초 확인), S100B·MBP 면역조직화학 (신경·Schwann cell 표시).
• 정량: 50 mm² 영역에서 100× 배율로 Meissner’s corpuscles와 nerve endings 계수.
• 통계: ANOVA + Tukey post-hoc test.

주요 결과

• Meissner’s corpuscles (빠르게 적응하는 mechanoreceptor, discriminative touch/vibration 감지): EL 전반에 분포. Proximal > Distal > Mid 순으로 밀도 높음. 크기(길이·너비·면적)는 3부위 간 유의한 차이 없음.
• 신경섬유 (myelinated + unmyelinated): Proximal에서 가장 밀도 높음. Mid-portion에서는 큰 구경(large-caliber) 신경이 많고, proximal/distal로 갈수록 작은 가지로 분지됨 → “primary nerve branches가 mid에서 들어와 branching” 패턴.
• 모든 신경 조직 주변에 모세혈관이 풍부하게 관찰됨 (혈류 조절 역할).
• Free nerve endings, myelinated fibers 등도 확인.

임상적 의의 (Discussion & Conclusion)

• EL은 MCL의 주요 신경 저장소로, proprioception과 ligament healing에 핵심.
• 수술 시 EL 보존이 중요 (paratenon 보존처럼) → 수술 후 안정성·재생·기능 회복 향상.
• Meissner’s corpuscles의 EL 존재는 ligament mechanoreception 이해를 확장.

Article Info

Publication History:

Received March 27, 2025; Revised July 7, 2025; Accepted July 29, 2025; Published online August 20, 2025

DOI: 10.1016/j.knee.2025.07.019 External LinkAlso available on ScienceDirect External Link

Copyright: © 2025 The Author(s). Published by Elsevier B.V.

User License: Creative Commons Attribution (CC BY 4.0) | Elsevier's open access license policy

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Graphical abstract

AbstractBackground

This cadaveric descriptive study was aimed at eval‎uation of the nerve fibers and mechanoreceptors of the epiligament of the medial collateral ligament (MCL) of the human knee – their distribution, density and potential clinical implications.

Methods

Tissue samples were obtained from 12 fresh cadavers, and 5-μm sections corresponding to the proximal, mid, and distal portions of the epiligament were stained with hematoxylin and eosin, Luxol fast blue/Cresyl violet, and antibodies against S100B and myelin basic protein. ANOVA and post hoc Tukey tests were used to assess the differences in density distributions of the neural elements in the different portions of the epiligament.

Results

The proximal region of the epiligament exhibited the highest density of both myelinated and unmyelinated nerve fibers and Meissner’s corpuscles, followed by the distal region, while the mid-portion, despite a lower density, contained larger-caliber nerve fibers –suggesting that primary nerve branches enter through the mid-region and subsequently branch into smaller fibers towards the proximal and distal areas.

Conclusions

This investigation is the first to document the presence of Meissner’s corpuscles within the epiligament of the MCL, thereby expanding our understanding of ligament mechanoreception. These findings underscore the pivotal role of the epiligament in mediating mechanoreception, nociception, proprioception, and blood flow regulation, and highlight its potential impact on ligament regeneration and post-surgical outcomes. Overall, our study advances the current knowledge of MCL innervation and lays the groundwork for future research into its therapeutic implications in knee joint biomechanics and ligament healing.

Keywords

1. Epiligament
2. Medial collateral ligament
3. Innervation
4. Knee
5. Human

1 Introduction

Ligaments contribute to joint stability by resisting forces during movement, ensuring static stability, and providing dynamic stability through proprioceptive control of muscular forces acting on the joint [1]. Proprioceptive signals from ligaments, muscles, and tendons, originating from their mechanoreceptors, play a key role in movement coordination and are involved in the pathophysiology of arthritis [2]. The knee joint, which allows for a wide range of motion, relies on the support of surrounding ligaments, tendons, and muscles. Proprioceptive feedback from these structures is essential for maintaining knee stability [3].

Ligaments and tendons are not solely passive structures; they actively contribute to proprioception and dynamic neuromuscular stability [4]. Impairments in the neuromuscular system can result in muscle-related instability of the knee [5]. A thorough understanding of the neuroanatomy of mechanoreceptors around the knee is crucial for recognizing proprioceptive impairments caused by ligament injuries and making informed decisions about autograft selection and post-surgical rehabilitation. Additionally, this knowledge is essential for understanding the precise control of movements such as walking and maintaining posture [3].

The medial region of the knee has garnered considerable attention in orthopedics due to the frequent occurrence of medial collateral ligament (MCL) injuries [6–8]. The MCL is composed of two parts: the superficial MCL (sMCL) and the deep MCL (dMCL) [6]. This ligament complex is a key stabilizer, providing both static and dynamic resistance, aided by its muscular connections, against valgus stress. It also plays a critical role in limiting rotational motion and controlling anterior–posterior translation [9].

Despite the well-established role of mechanoreceptors in joint proprioception and neuromuscular control, research on their presence and classification within the MCL, particularly its epiligament (EL), remains limited [3,10]. Freeman and Wyke [11] classified articular neuroreceptors into four types based on light microscopy and physiological studies in cats. The first three – Ruffini endings (type I), Pacinian corpuscles (type II), and Golgi tendon organs (type III) – are encapsulated. In contrast, Type IV free nerve endings lack a connective tissue capsule [12]. While these mechanoreceptor classifications have been well described in various articular structures, their distribution and function within the MCL and its EL remain largely unexplored. Given the critical role of the MCL in knee stability, a detailed neuroanatomical understanding of its mechanoreceptors is essential for improving surgical repair strategies and rehabilitation protocols.

Previous studies by Georgiev et al. [13,14] described that the EL of the MCL is essential for the ligament's ability to heal. This layer is the main reservoir of progenitor cells and contains a rich supply of blood vessels required to deliver vital oxygen and nutrients to the ligament proper. When the MCL is injured, the EL provides a steady stream of these healing components, allowing the ligament to mend itself effectively. Essentially, this vital layer is the primary reason the MCL has a strong natural capacity for repair compared to the anterior cruciate ligament (ACL) [14].

The present study aims to expand our understanding of MCL innervation by (1) characterizing the types, densities, and distribution patterns of mechanoreceptors and (2) developing a comprehensive neuroanatomical map of the MCL and its enveloping EL. Our findings are expected to underscore the critical importance of primary EL repair during ligament suture procedures by elucidating the localization of key receptors responsible for knee stability.

2 Materials & methods

The study was conducted in accordance with the requirements of the Declaration of Helsinki (64th WMA General Assembly, Fortaleza, Brazil, October 2013). The present study protocol did not require approval by our institutional ethics committees.

2.1 Tissue preparation

This study conducted histological and immunohistochemical (IHC) analyses on tissue samples from the proximal and distal regions of the MCL of 12 fresh European cadavers of Caucasian race. The cadavers, aged between 49 and 62 years at death with a mean age of 55 years, included seven females and five males. None of the individuals had any history of knee osteoarthritis or visible scars from prior knee surgeries. A skin incision was made to collect the samples, and the subcutaneous tissue was carefully dissected to reveal the MCL and the outer layer of the EL. This approach ensured precise sampling from the desired areas. Samples were taken from the MCL and then processed using standard fixation methods [15].

2.2 Light microscopy

The specimens were carefully prepared for light microscopy using a Leica microtome (Wetzlar, Germany). Thin sections, 5 μm thick, were sliced from the samples and mounted on to microscope slides. The slides were stained with hematoxylin and eosin according to standard protocols [15].

2.3 Luxol fast blue /Cresyl violet

The tissue sections were placed on gelatin-coated slides, followed by deparaffinization and rehydration to 95 % alcohol. They were then incubated in a 0.01 % Luxol fast blue (LFB) solution for 6 h at 58 °C. Following incubation, the sections were differentiated in a 0.05 % lithium carbonate solution and counterstained with Cresyl violet. This staining process resulted in myelinated fibers appearing blue, the neuropil turning pink, and nerve cells showing up as purple. After drying for 24 h, the sections were mounted with Entellan for preservation.

2.4 Immunohistochemistry

S100B and myelin basic protein (MBP) antibodies were used following standard protocol. Following deparaffinization – three successive 10‐min immersions in xylene (Merck Catalogue No. 1082984000) – the sections were rehydrated using a descending series of alcohols (100 %, 95 %, 90 %, 80 %, and 70 % ethanol; Merck Catalogue No. 1009835000) followed by distilled water, each for 5 min. Next, antigen retrieval‎ was carried out by heating the slides at 95 °C for 20 min in Citrate Plus (10×) HIER Solution (pH 6.0; Cat. No. CPL500, ScyTek Laboratories Inc., Logan, UT, USA). After cooling, the slides underwent three 5‐min washes in TTBS (Tris-Buffered Saline with 0.05 % Tween 20; E-BC-R335, Wuhan Elabscience Biotechnology Co., Ltd., Hubei Sheng, China). Endogenous peroxidase activity was then inhibited by incubating the sections in 3 % hydrogen peroxide in distilled water for 10 min, followed by another series of three 5‐min TTBS washes. The sections were subsequently treated with Super Block (ScyTek Laboratories Inc., Logan, UT, USA) for 5 min at room temperature. To block endogenous biotin, a specific kit (Cat. No. BBK120, ScyTek Laboratories Inc.) was applied as directed – 15 min with component A, a washing step, and then 15 min with component B. Additionally, a mouse-to-mouse blocking reagent (Cat. No. MTM015, ScyTek Laboratories Inc.) was used for 1 h at room temperature to inhibit endogenous mouse immunoglobulins. After a final TTBS wash, the sections were incubated overnight at 4 °C with primary antibodies – FLEX Polyclonal Rabbit Anti-S100, Ready-to-Use (code number GA504, Dako Omnis) and mouse monoclonal anti-MBP (sc-271524, Santa Cruz Biotechnology Inc.), at a 1:300 dilution – to label oligodendrocyte progenitors. The immunoreactivity for S100B and MBP was then visualized using the UltraTek HRP Anti-Polyvalent Staining System (Cat. No. AFN600, ScyTek Laboratories Inc.) according to the manufacturer’s protocol. Finally, the sections were counterstained with hematoxylin, dehydrated through an ascending ethanol series, cleared in xylene, and coverslipped. For control experiments, the primary antibody was replaced with an antibody diluent.

For each specimen, three slides were examined: one from the proximal bony insertion site, another from the distal bony insertion, and a third from the central region. Meissner’s corpuscles and nerve endings were quantified within standardized areas (50 mm2) and observed under 100× magnification.

2.5 Statistical analysis

In the present study, statistical analysis was performed using one-way analysis of variance (ANOVA) to eval‎uate differences in the density and morphology of nerve fibers and Meissner’s corpuscles across three anatomical regions of the EL of the MCL: proximal, mid-substance, and distal. ANOVA was employed to determine whether significant variations existed among these regions. Following a significant ANOVA result, post hoc Tukey’s Honestly Significant Difference (HSD) test was applied to identify specific pairwise differences. A standard significance level of p < 0.05 was used for all statistical tests.

3 Results

The MCL of the human knee displayed a complex neural network. We report Meissner’s corpuscles and nerve endings in the EL of the MCL. Additionally, capillaries were observed surrounding all types of neural tissue.

Meissner’s corpuscles were visualized as enclosed by a connective tissue capsule and contained a central core made up of layers of modified, flattened Schwann cells. Each corpuscle essentially consists of three main elements: a group of elongated Schwann cells, an encasing connective tissue capsule, and a central axon. The flattened Schwann cells are arranged in distinct layers within an interlamellar matrix primarily composed of collagen and microfilaments consistent with previous descriptions [16] (Figures 1–3).

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Figure 1 Meissner corpuscles visualized in the epiligament of the proximal part of the medial collateral ligament using (a) hematoxylin and eosin, (b) Luxol fast blue/Cresyl violet, (c) S100 immunohistochemistry, and (d) myelin basic protein immunohistochemistry. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Figure 2 Meissner corpuscles visualized in the epiligament of the middle portion of the medial collateral ligament using (a) hematoxylin and eosin, (b) Luxol fast blue/Cresyl violet, (c) S100 immunohistochemistry, and (d) myelin basic protein immunohistochemistry. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Figure 3 Meissner corpuscles visualized in the epiligament of the distal part of the medial collateral ligament using (a) hematoxylin and eosin, (b) Luxol fast blue/Cresyl violet, (c) S100 immunohistochemistry, and (d) myelin basic protein immunohistochemistry. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Nerve fibers were most commonly found in the EL layer, often closely associated with the vasculature. Nerve fibers were visualized as distinct structures within the EL, which displays a complex histological organization composed of a functional parenchyma and a supportive stroma. Each nerve fiber consisted of an axon encased by myelin-forming glial cells. The surrounding stroma was organized into three connective tissue layers: the innermost endoneurium, a delicate reticular network that individually wraps each fiber and supplies capillaries; the perineurium, a dense layer that bundles fibers into fascicles and maintains the nerve’s internal environment; and the outermost epineurium, composed of dense irregular connective tissue that encases multiple fascicles. Our observations align with previous histological descriptions [17]. These epiligamentous nerve fibers ran parallel to the collagen strands before entering the main body of the MCL, either as free nerve endings or alongside blood vessels, branching into deeper layers via the endoligament. While most axons were myelinated, some were not (Figures 4–6). These findings underscore the MCL's complex proprioceptive and mechanoreceptive functions within the knee joint. Interestingly, Meissner’s corpuscles, known for their rapid adaptation and activation during movement, were frequently found near the ligament's entheses, suggesting that dynamic proprioception plays a key role at these insertion points.

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Figure 4 Peripheral nerves visualized in the epiligament of the proximal part of the medial collateral ligament using (a) hematoxylin and eosin, (b) Luxol fast blue/Cresyl violet, (c) S100 immunohistochemistry, and (d) myelin basic protein immunohistochemistry. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Figure 5 Peripheral nerves visualized in the epiligament of the middle portion of the medial collateral ligament using (a) hematoxylin and eosin, (b) Luxol fast blue/Cresyl violet, (c) S100 immunohistochemistry, and (d) myelin basic protein immunohistochemistry. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Figure viewer

Figure 6 Peripheral nerves visualized in the epiligament of the distal part of the medial collateral ligament using (a) hematoxylin and eosin, (b) Luxol fast blue/Cresyl violet, (c) S100 immunohistochemistry, and (d) myelin basic protein immunohistochemistry. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The quantitative analysis revealed that receptor density in the EL of the MCL was highest in the proximal region, lowest in the midsubstance, and intermediate in the distal region. ANOVA demonstrated statistically significant differences among these regions, and post hoc Tukey tests confirmed significant pairwise differences between the proximal and midsubstance, midsubstance and distal, and proximal and distal regions (Figure 7(a)). In addition, receptor dimensions (length, width, and area) were measured across these regions. Meissner receptors exhibited no significant differences in these dimensions across the three regions (Figure 8). Furthermore, nerve quantification indicated that nerve density was highest in the proximal EL, followed by the distal region, with the midsubstance showing the lowest density; both ANOVA and post hoc tests confirmed significant differences among these regions (Figure 7(b)). Measurements of nerve diameter and area revealed that the midsubstance contained nerves with significantly larger diameters and areas than those in the proximal and distal regions, which were similar and approximately half the size of the midsubstance nerves. Post hoc analyses confirmed significant differences in nerve diameter between the proximal and midsubstance regions and between the midsubstance and distal regions, while no significant difference was found between the proximal and distal regions; for nerve area, the pattern was similar, though the difference between the proximal and midsubstance regions did not reach statistical significance (Figure 9).

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Figure 7 Graphical representation of the number of receptors (a) and nerves (b) in the proximal, middle and distal part of the epiligament of the medial collateral ligament presented with box and whisker plot showing the mean (x), surrounded by a ‘box’, the vertical edge of which is the interval between the lower and upper quartile [25–75 %]. ‘Whiskers’ originating from this ‘box’ represent the non-outlier range. * P < 0.001.

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Figure 8 Graphical representation of the length (in µm) (a) width (in µm) (b) and area (in µ2) (c) of Meissner’s receptors in the proximal, middle and distal part of the epiligament of the medial collateral ligament presented with box and whisker plot showing the mean (x), surrounded by a ‘box’, the vertical edge of which is the interval between the lower and upper quartile [25–75 %]. ‘Whiskers’ originating from this ‘box’ represent the non-outlier range. * P < 0.001.

Figure viewer

Figure 9 Graphical representation of the diameter (in µm) (a) and area (in µm2) (b) of Meissner’s receptors in the proximal, middle and distal part of the epiligament of the medial collateral ligament presented with box and whisker plot showing the mean (x), surrounded by a ‘box’, the vertical edge of which is the interval between the lower and upper quartile [25–75 %]. ‘Whiskers’ originating from this ‘box’ represent the non-outlier range. * P < 0.001.

4 Discussion

Myelinated and unmyelinated nerve fibers were predominantly found in the EL layer and frequently associated with blood vessels in this area. These epiligamentous nerve fibers followed the direction of collagen strands before penetrating the main body of the MCL, often accompanied by blood vessels, and branching into deeper layers via the endoligament. Those findings support the existing literature data where similar results are described in ankle, elbow, and knee capsules, and those nerves are associated with proper blood flow [5,10,18].

This puts an even higher emphasis on EL innervation as the intact nerve fibers are crucial for optimal blood flow and, therefore, regeneration of the whole ligament. From the morphometric and statistical analysis that we performed regarding the nerve fibers, we found a higher density of nerves compared with capsulated receptors in all parts of the EL of MCL, which aligns with similar findings in the synovial coverings of the cruciate ligaments, the posterior oblique tendon, and the medial and lateral retinaculum [3,19]. It is important to note that Grigg et al. [20] reported that those free nerve endings form the nociceptive system in the ligaments, and they remain inactive in normal conditions but get activated by mechanical and chemical stimuli. Splitting the EL of the MCL into three thirds, we also detected significant differences in the nerve density between them, with the highest density in the proximal part followed by the distal part and, lastly, the middle part. Interestingly, we also found that the largest nerves are located in the middle part of the EL of MCL. In contrast, those in the proximal and the distal part were significantly smaller and similar in between. Our morphometric analysis reveals, for the first time, morphological values characterizing the innervation of the EL within the MCL. Our data indicate that primary nerve branches enter the mid-substance of the EL and subsequently subdivide into smaller-caliber fibers in both the proximal and distal regions, underscoring the complex neural architecture of this structure and suggesting significant implications for its functional role. However, correctly interpreting these findings will require further experiments and comprehensive analysis.

While most myelinated fibers terminated in simple nerve endings, some concluded as specialized structures such as Meissner receptors. These observations highlight the intricate proprioceptive and mechanoreceptive functions of the MCL in knee joints. Another important finding of the present study is that, for the first time, we described the presence of Meisner corpuscles in the EL of MCL. Most commonly, Meisner receptors are described as tactile corpuscles found in the superficial layers of the dermal papillae, responsible for detecting discriminative touch and vibration [16,21]. The receptors we observed in the EL match the description of Meisner receptors as ellipsoid mechanoreceptors and the average length of the ones described in the skin [21]. The diameter of the Meisner receptors found in the EL was around 40–70 μm, which is higher than those described in the skin, where the diameter varies between 20 and 40 μm [21]. Notably, when we compared the morphological data for the Meisner corpuscles from the EL, there were no significant differences between the proximal, middle, and distal parts, unlike the data we gathered for the nerves and the Pacinian corpuscles. Future experiments and analysis will be required to unveil the role of Meisner receptors in the EL of MCL and potentially localize them in other ligaments as well.

Our results demonstrate that the neural supply to the MCL resides primarily within the EL tissue. It can therefore be inferred that the EL plays a crucial role in knee proprioception and is integral to normal ligament function. Drawing a parallel with Achilles tendon surgery, where preserving the paratenon is vital for recovery, we propose that meticulous preservation of the EL during MCL repair is essential. This biological approach may lead to improved functional recovery, dynamic coordination, and joint stability. While the present study establishes the morphological foundation for the rich innervation of the MCL EL, prospective clinical investigations are essential to validate our central hypothesis: that preserving and restoring this neural-rich tissue is crucial for optimal proprioceptive feedback and functional recovery after injury.

Despite providing novel insights, our study has several limitations. First, the relatively small sample size (n = 12) and older age (49–62 years) may affect the generalizability of our findings and limit the statistical power necessary to detect subtle variations in neural architecture. Additionally, the exclusive use of cadaveric specimens constrains our ability to assess mechanoreceptor activation under physiological conditions, particularly during dynamic knee motion. Consequently, the functional implications of these receptors remain only partially elucidated. Future research should include larger and more diverse sample populations and in vivo assessments to better understand the mechanistic role and activation of these neural structures.

5 Conclusion

The present study elucidates the EL of the human MCL as a densely innervated structure whose mechanoreceptors and nerve fibers are pivotal for proprioceptive acuity and ligament homeostasis. Preservation or reconstruction of EL integrity during surgical intervention may potentiate ligamentous regeneration, enhance functional restoration, and mitigate postoperative joint instability. Our results not only build on the existing EL theory by underscoring the importance of an intact EL in ensuring proper innervation and facilitating ligament regeneration, but also imply that optimal recovery of the EL after trauma or rupture is essential for improved post-surgical outcomes. Overall, our study underscores the complexity of the innervation within the EL of the human MCL and paves the way for future research into the morphology and therapeutic implications of the EL in ligament function and healing.

CRediT authorship contribution statement

Nikola Stamenov: Writing – review & editing, Writing – original draft, Software, Methodology, Formal analysis, Conceptualization. Lyubomir Gaydarski: Writing – review & editing, Validation, Supervision, Project administration, Investigation, Formal analysis, Conceptualization. R. Shane Tubbs: Writing – review & editing, Visualization, Validation, Supervision, Project administration, Formal analysis. Joe Iwanaga: Writing – review & editing, Visualization, Validation, Supervision, Formal analysis, Conceptualization. Maria Piagkou: Writing – review & editing, Visualization, Validation, Supervision, Methodology, Conceptualization. Svetoslav A. Slavchev: Writing – review & editing, Methodology, Investigation, Funding acquisition, Conceptualization. Pavel Rashev: Writing – review & editing, Supervision, Resources, Methodology, Investigation, Data curation. Julian Ananiev: Writing – review & editing, Visualization, Supervision, Formal analysis, Data curation, Conceptualization. Boycho Landzhov: Writing – review & editing, Supervision, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Georgi P. Georgiev: Writing – original draft, Visualization, Project administration, Investigation, Formal analysis, Data curation, Conceptualization.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Georgi P. Georgiev reports a relationship with Medical Iniversity of Sofia that includes: employment.

Acknowledgements

The authors sincerely thank those who donated their bodies to science such that anatomical research could be performed. The results from such research can potentially increase mankind’s overall knowledge that can then improve patient care. Therefore, these donors and their families deserve our highest gratitude [22].

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Ann Med

. 2025 Dec 24;58(1):2604403. doi: 10.1080/07853890.2025.2604403

Disrupted sensorimotor control after ACL injury: from mechanoreceptor degeneration to neuroplasticity-oriented rehabilitation

Zelong Lee a,b,#, Ying Zhang a,c,#, Peng Wang a,b,#, Zhenming Kan a,b, Peng Wu a,b, Yirui Han a,b, Weiliang Zhong a,b,✉

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PMCID: PMC12777884  PMID: 41439739

AbstractBackground

Anterior cruciate ligament (ACL) injury is one of the most common sports-related injuries, often resulting in not only mechanical instability but also long-term proprioceptive dysfunction. The ACL is richly innervated with mechanoreceptors that contribute to sensorimotor control. Injury-induced degeneration of these receptors leads to disrupted afferent signaling and maladaptive central nervous system (CNS) responses, which can compromise functional recovery and increase the risk of reinjury.

Objective

This review aims to summarize current knowledge on ACL-associated proprioception, including mechanoreceptor anatomy, injury-induced changes in neural signaling, and advances in eval‎uation and rehabilitation techniques. Emphasis is placed on the integration of peripheral and central mechanisms and their implications for clinical interventions.

Content

We detail the types and distribution of mechanoreceptors within the ACL and describe how their disruption alters joint position sense and kinesthesia. We further explore CNS neuroplasticity, such as cortical reorganization and bilateral sensorimotor changes. Traditional and emerging methods for proprioceptive assessment are critically eval‎uated. Finally, we discuss surgical and rehabilitative strategies—including remnant-preserving reconstruction, neuromuscular training, and neurofeedback—that target proprioceptive recovery through neuroplastic adaptation.

Conclusion

Restoration of proprioceptive integrity following ACL injury requires a multifaceted approach that addresses both peripheral mechanoreceptor preservation and central sensorimotor reorganization. Future research should focus on standardized assessments, long-term neurophysiological monitoring, and the integration of sensor-based technologies to support individualized, neuroplasticity-driven rehabilitation strategies.

Keywords: Anterior cruciate ligament, central nervous system adaptation, mechanoreceptors, neuroplasticity, proprioception, rehabilitation, sensorimotor control

Introduction

Anterior cruciate ligament (ACL) injuries are among the most preval‎ent and debilitating musculoskeletal injuries, particularly affecting young, active populations [1,2]. While the biomechanical consequences of ACL rupture—such as joint instability and altered kinematics—are well recognized [3], increasing evidence has underscored the pivotal role of proprioceptive impairment in functional deterioration and recurrent injury [4]. Proprioception, a composite sensory modality involving joint position sense, kinesthesia, and neuromuscular reflexes, is largely mediated by mechanoreceptors embedded within the ACL [5]. These receptors form a critical feedback loop with the central nervous system (CNS), regulating joint stability, movement precision, and motor control [6].

Recent advancements in neurophysiology, histology, and imaging have expanded our understanding of proprioceptive degradation following ACL trauma, highlighting both peripheral receptor damage and central neural reorganization [6–9]. Moreover, the clinical relevance of proprioceptive deficits has prompted a paradigm shift in ACL treatment [4,5,10]—from purely structural repair to functional restoration—including novel strategies in graft selection, remnant preservation, early intervention, and sensorimotor rehabilitation. Several recent reviews have refined the current understanding of proprioceptive rehabilitation after ACL reconstruction. Most have confirmed that proprioception-oriented training improves passive joint position sense, single-leg hop performance, and postural balance, but its impact on muscle strength and range of motion remains limited [11,12]. Other analyses have compared emerging approaches such as virtual-reality-based training, which may enhance balance control through greater central engagement [13]. From a surgical perspective, remnant-preserving reconstruction has been linked to improved proprioceptive and stability outcomes, though long-term evidence remains inconclusive [14]. Building on these findings, the present review differs from previous work by integrating peripheral mechanoreceptor biology, central neuroplastic adaptations, and clinical rehabilitation practice.

This review comprehensively synthesizes current evidence on ACL proprioception, covering anatomical substrates, post-injury alterations, eval‎uation techniques, and therapeutic interventions. By critically examining recent literature and identifying key knowledge gaps, we propose an integrative framework for optimizing proprioceptive recovery and improving long-term functional outcomes after ACL injury. Although extensive research has addressed biomechanical instability following ACL rupture, the contribution of proprioceptive deterioration and its neurophysiological consequences has not been systematically consolidated. Here, we integrate emerging evidence on peripheral mechanoreceptor degeneration and central nervous system neuroplasticity to explain persistent sensorimotor dysfunction, and we highlight how bidirectional interactions between neural adaptations and rehabilitation interventions can inform neuroplasticity-oriented, precision rehabilitation that goes beyond conventional strength-based approaches.

Mechanoreceptors in the ACL

Emerging research highlights the ACL as a sensory hub, endowed with mechanoreceptors intricately woven into its structure, primarily concentrated at its tibial and femoral junctures beneath the synovial membrane [7,10]. These mechanoreceptors, constituting about 1% to 2.5% of the ligament’s total volume, are pivotal in the proprioceptive matrix of the knee joint [6,8,9,15,16]. Recent histological studies have identified four principal types of mechanoreceptors within the ACL—Ruffini corpuscles (Type I), Pacinian corpuscles (Type II), Golgi tendon organs (Type III), and free nerve endings (Type IV) [6,7]—each specialized in relaying nuanced information regarding joint dynamics, such as static position, motion direction, and mechanical stress (Figure 1). This rich sensory input is essential for the fine-tuning of neuromuscular responses, underpinning joint stability and movement precision.

Figure 1.

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Classification and morphology of mechanoreceptors in the anterior cruciate ligament (ACL). This schematic diagram illustrates the four major types of mechanoreceptors identified within and around the ACL: Ruffini endings (type I), Pacinian corpuscles (type II), Golgi tendon organs (type III), and free nerve endings (type IV). Each receptor type plays a distinct role in proprioception, responding to different mechanical stimuli such as tension, pressure, vibration, and nociception. These mechanoreceptors are distributed variably along the ligament, contributing to afferent signaling necessary for joint position sense and reflexive motor control.

Ruffini corpuscles are fusiform, low-threshold, and slowly adapting receptors situated superficially within the ligament. They detect static joint position and intra-articular pressure changes, thereby contributing to sustained postural control and the perception of joint motion direction [6,8]. Pacinian corpuscles are ovoid, fast-adapting receptors embedded in the deeper layers of the ligament; while inactive in static conditions, they rapidly respond to sudden joint movements and acceleration, providing feedback about dynamic changes in joint position [6,10]. Golgi tendon organs are high-threshold, slowly adapting receptors localized near ligament insertions. They are activated primarily at the extremes of joint motion or under high tensile stress, functioning to prevent excessive joint rotation or hyperextension [9,17]. Free nerve endings, which are irregularly distributed throughout the ligament, act as polymodal nociceptors responsible for pain perception, inflammatory responses, and local vasoregulation [5,15].

Table 1 summarizes the main types and functions of mechanoreceptors found in the ACL. These mechanoreceptors constitute an intricate afferent network that continuously transmits information about joint angle, position, mechanical tension, and acceleration to the central nervous system. Following integration with visual and vestibular inputs, this proprioceptive feedback enables the fine-tuning of neuromuscular responses necessary to maintain dynamic knee stability and coordinated movement [6,18,19].

Table 1.

Types and functions of mechanoreceptors in the ACL.

TypeMorphologyFunctional role

Ruffini corpuscles (Type I)	Fusiform, located superficially	Low-threshold, slow-adapting; detect static position and intra-articular pressure [6,8]
Pacinian corpuscles (Type II)	Ovoid, found deeper in the ligament	Low-threshold, fast-adapting; respond to sudden movements and acceleration [6,10]
Golgi tendon organs (Type III)	Fusiform near ligament insertions	High-threshold; respond to extreme joint movements and tension [9,17]
Free nerve endings (Type IV)	Irregular, widely distributed	Nociceptors; detect pain and participate in inflammation [5,15]

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Evolution and function of ACL proprioception

The conceptualization of proprioception has evolved since its inception by Sir Charles Sherrington, who envisaged it as the body’s intrinsic awareness of limb positioning and movement through space [20]. This concept has evolved, now broadly recognized not just as joint position sense but also incorporating kinesthetic awareness—subconscious perceptions of movement and spatial orientation. This expanded understanding aligns with recent neurophysiological research suggesting proprioception as a composite sensory modality, integrating signals from mechanoreceptors within the ACL to facilitate joint stability and dynamic neuromuscular adjustments [21]. This complex interplay is critical for executing precise movements, safeguarding the knee from injury, and orchestrating rehabilitation strategies post-ACL reconstruction. Some scholars view proprioception as merely joint position sense [22,23], others argue it encompasses both joint position and movement senses, with movement sense as a subset of proprioception [24–26]. This broader interpretation aligns with Bastian’s description, emphasizing the intertwined nature of position and movement senses in daily activities. Thus, proprioception can be considered synonymous with ‘movement sense’ [27].

Proprioception refers to the sensation generated by muscles, tendons, joints, and other motion-related organs in different states (movement or stillness), also known as deep sensation [28,29]. Proprioception mainly has three functions: 1) static perception of joint position; 2) perception of joint movement (perception of joint movement or acceleration); 3) reflex response and regulation of muscle activity. The first two reflect the afferent capabilities of proprioception, while the latter reflects the efferent capabilities of proprioception [28,30].

The proprioception of the knee joint protects the joint from excessive movement, maintains joint stability, and coordinates joint movement [31–33]. Within the proprioceptive structure of the knee joint, the ACL plays a crucial role. ACL injury leads to changes in knee joint stability not only due to biomechanical changes caused by ligament deficiency but also due to changes in proprioception after ligament injury [34]. Therefore, the proprioceptive function of the anterior cruciate ligament is essential for maintaining the stability of the knee joint.

Changes in proprioception following ACL injury

The precise mechanisms through which ACL injuries precipitate alterations in proprioception are yet to be fully elucidated. The ACL injury is known to induce significant alterations in knee proprioception, primarily due to the damage to the mechanoreceptors embedded within the ligament and subsequent impairment in the proprioceptive signal transmission pathways [35,36]. These alterations reduce the accuracy of proprioceptive feedback and neuromuscular control, diminishing quadriceps activity, compromising knee stability and strength, and increasing the risk of reinjury [37]. Early-stage experimental research conducted by Zhang et al. in a cynomolgus monkey model demonstrated a significant decline in joint position sense following unilateral ACL injury, which progressively worsened over time, suggesting bilateral proprioceptive adaptation and long-term sensory impairment [38]. Complementary findings by Gao et al. reveal a reduction in the number and volume of mechanoreceptors in the ACL remnant as time post-injury progresses [17], highlighting the time-sensitive nature of proprioceptive deterioration.

Moreover, the reduction in proprioceptive input is thought to trigger neuroplastic changes within the CNS, as a compensatory mechanism to the loss of incoming signals [39]. This adaptability of the CNS, while beneficial in some respects, may also lead to a diminished capacity for processing and integrating complex proprioceptive information. Research has shown that individuals with ACL injuries exhibit changes in cortical activity related to movement and sensation, suggesting that the brain undergoes a form of ‘reprogramming’ in response to altered proprioceptive input [40]. This CNS adaptation potentially recalibrates the brain’s perception of knee joint proprioception, further influencing joint stability and function.

Additionally, it has been confirmed that unilateral ACL injury not only affects the injured side but may also lead to proprioceptive changes in the contralateral, uninjured limb [5,41,42]. This bilateral impact suggests that proprioceptive deficits post-ACL injury are not confined to the local site of injury but have a more systemic influence, affecting posture balance, joint stability, and muscle strength of the non-injured limb as well [43,44]. Table 2 outlines the characteristic proprioceptive changes observed after ACL injury.

Table 2.

Proprioceptive changes following ACL injury.

Injury stageNeural changesImpact on proprioception

Acute phase	Receptor disruption, loss of afferent signals	Decreased joint position sense and neuromuscular control [17,38]
Chronic phase	Progressive receptor degeneration	Impaired postural balance and contralateral deficits [41,42]
Postoperative phase	Partial reinnervation possible with intervention	Dependent on graft type, surgical timing, and rehabilitation [45–47]

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Measurement methods of proprioception following ACL injury

Due to the diversity in the content and form of proprioception, it cannot be directly assessed. This has led to a lack of standardized, universally accepted clinical methods for measuring proprioception following ACL injuries. Recent studies have identified several commonly used methods for assessing proprioception, which can be categorized into four main types.

Active and passive joint position reproduction techniques

The active and passive joint position reproduction techniques (JPR) are the most commonly used methods to measure joint position sense (JPS) [48,49]. These techniques assess the individual’s ability to perceive joint position by examining the active or passive replication of predetermined flexion and extension angles of the knee [50,51]. The difference between the replicated angle and the preset angle is calculated to gauge the knee joint’s positional awareness [18,27,52]. Several variables can be manipulated during this test to create various JPS testing protocols [30,53], such as the method of angle reproduction (active or passive), direction of movement (extension or flexion), limb demonstrated (ipsilateral or contralateral), body posture (lying, sitting, or standing), measurement range (starting and target angle), load bearing status (weighted or unweighted), testing instruments (such as goniometers or dynamometers), and the method of result measurement (absolute error, constant error, variable error). Recent investigations have demonstrated moderate-to-excellent test–retest reliability of knee JPR, with intraclass correlation coefficients (ICCs) ranging approximately from 0.60 to 0.95, depending on task posture, active or passive mode, and device standardization [51,54].

Passive motion perception threshold measurement

The threshold to detection of passive motion (TDPM) is another common method used to measure the capacity of proprioception to perceive joint dynamics without other sensory mechanisms. This technique involves slowly and continuously moving the knee joint passively, typically starting from a stationary position, with visual, auditory, and tactile senses masked. The test progresses with the knee joint flexing and extending at a low angular velocity until the participant perceives the start of the movement. The angles at the onset of movement and when the movement is detected by the participant are measured to quantify and compare the differences, thereby assessing the accuracy of knee joint proprioception [27,28,55]. Reported test–retest reliability for knee TDPM varies across studies, with ICCs reported approximately between 0.52 and 0.70 in healthy cohorts, and evidence in ACL-injured populations remains inconsistent; reliability depends on the test protocol, starting angle, and measurement setup [56].

Postural sway test

Some researchers suggest that instability following ACL rupture typically occurs under load-bearing conditions [57,58] The postural sway test provides a representative assessment of proprioceptive capability in such conditions. In this method, participants stand on a force plate with eyes closed (to control for visual influence) and arms crossed over the chest. Changes in the center of pressure (COP) and its distribution displayed on the screen reflect anterior–posterior and medial–lateral sway. The variability index of COP displacement quantifies the magnitude of sway, thereby reflecting postural stability under load-bearing conditions [58,59]. This method has demonstrated moderate-to-high test–retest reliability in ACL-deficient or reconstructed cohorts, with ICCs approximately spanning 0.60–0.95, contingent on stance condition and the specific COP parameter assessed [57,60–62].

Somatosensory evoked potentials (SEPs) measurement

Following mechanical or electrical stimulation to the knee joint, changes in surface electromyography and/or cortical potentials are measured, providing a comprehensive eval‎uation of the neuromuscular circuitry. Although a few studies use implanted electrodes for selective stimulation of the ACL, most results are obtained through external perturbations. The sensory signals acquired depend on the integrated input from skin, joint capsules, ligaments, tendons, muscle receptors, and even visual-vestibular systems. The latency and amplitude of the evoked potentials provide indirect insights into proprioception. This method is advantageous when comparing the neuromuscular excitation patterns of patients and healthy individuals during functional exercises [28,53]. However, sensory thresholds can vary individually, often influenced by factors such as gender, age, psychological states, and external environment. While SEPs are valuable for assessing central sensory processing, published data on test–retest reliability in ACL-injured or reconstructed populations are extremely limited; thus, specific ICC values for this context cannot currently be confidently stated.

When conducting these proprioceptive tests, reliance is on the sum of information transmitted to the CNS. Therefore, interference from additional proprioceptive information should be minimized. Furthermore, many studies use the uninjured contralateral limb as a control, but proprioceptive decline in the injured limb may also affect the uninjured side. Thus, comparisons with control groups (uninjured groups) may not always be accurate.

Among the commonly used proprioceptive assessments following ACL injury, each test provides distinct insights into sensorimotor function. Joint position reproduction and passive motion threshold tests quantify the static and dynamic components of joint position sense, postural sway testing eval‎uates functional balance under load-bearing conditions, and somatosensory evoked potentials characterize central sensory processing mechanisms. Based on current evidence, postural sway testing demonstrates the greatest functional relevance for eval‎uating proprioception during daily and sport-specific activities. Nevertheless, a multimodal approach that integrates several assessment methods is recommended to achieve a comprehensive eval‎uation of proprioceptive function.

The impact of ACL reconstruction on proprioception

Following ACL injuries, particularly complete ruptures or severe damage, arthroscopic reconstruction has become the mainstream treatment approach. Research has demonstrated that, compared to repair surgeries, reconstruction offers higher joint stability, lower joint laxity, and reduced failure rates [63]. Ligament reconstruction can restore the mechanical functions of the damaged ligament, and extensive studies have confirmed that it can also improve the proprioceptive function of the knee joint to a certain extent [53,64].

Effects of graft selection on proprioceptive recovery

Currently, three primary types of grafts are utilized in ACL reconstruction surgery: autografts, allografts, and synthetic ligaments. Ozenci et al. conducted a comparative study including 80 participants divided into four groups (autograft, allograft, ACL-deficient, and healthy controls; mean age ≈ 29 years), and found no significant difference in proprioceptive recovery between autograft (bone–patellar tendon–bone) and allograft groups following reconstruction [45]. Young et al. analyzed 10 patients (7 men, 3 women; mean 33 years) undergoing revision or second-look arthroscopy after ACL reconstruction (5 autograft and 5 allograft recipients). Histological examination showed that the concentration of neurofilament protein–positive mechanoreceptors was markedly reduced in both graft types compared to native ACL tissue, with no significant difference between autografts and allografts [65]. Angoules et al. performed a prospective trial involving 40 patients (34 men, 6 women; mean 31 years) who underwent ACL reconstruction using either hamstring tendon or bone–patellar tendon–bone autografts. Proprioception, assessed by joint position sense (JPS) and threshold to detection of passive motion (TTDPM) at 3, 6, and 12 months postoperatively, demonstrated that knee proprioception returned to normal by 6 months, with no significant difference between the two graft types at any time point [66]. Xu et al. conducted a randomized controlled trial of 40 patients (31 men, 9 women; aged 17–51 years) comparing hamstring autografts and synthetic LARS ligaments. Using passive–passive JPS testing at 45° and 75° of knee flexion, they found no significant difference in proprioceptive recovery between the two groups at 3 or 12 months after surgery [46]. These findings suggest that the type of graft may have a minimal impact on the recovery of proprioception post-ACL reconstruction. However, harvesting the semitendinosus and gracilis tendons for autografts could potentially damage the tendons’ proprioceptors, thereby affecting knee joint proprioception.

Timing of surgery

The optimal timing for ACL reconstruction surgery remains controversial. Sha et al. studied quantitative changes in mechanoreceptors in the tibial remnant and found that the number of mechanoreceptors did not significantly decline post-ACL rupture, and residual mechanoreceptors could be preserved for up to one year [67]. Adachi et al. also found no relationship between the time from injury to surgery and the number of residual mechanoreceptors [68]. However, Dhillon and Denti suggested that the longer the duration from ACL injury to surgery, the fewer the number of residual mechanoreceptors [69,70]. Zhang et al. found that ACL remnants collected within three months post-injury had higher healing potential, and early surgical intervention (within 90 days post-injury) could lead to better clinical outcomes [71].

Recent literature increasingly supports early ACL reconstruction surgery for optimal outcomes. Lee et al. quantitatively assessed proprioception and postural stability in patients with acute ACL tears (within 3 months post-injury) versus chronic tears (over 3 months post-injury). The study revealed that longer delays from injury to surgery resulted in significantly poorer proprioception and stability in the affected knees [47]. Furthermore, immunohistochemical analyses have shown that the timing of surgery critically affects the quantity of mechanoreceptors in the ACL stump, with delays potentially leading to a reduction in their number [72]. This evidence underscores the importance of timely surgical intervention to preserve proprioceptive function and enhance postoperative knee stability.

Remnant-preserving ACL reconstruction

There is some debate about whether to preserve the ruptured ACL remnant during reconstruction surgery. Traditional methods involve removing the tibial remnant to facilitate arthroscopic visibility and surgical manipulation. In recent years, preserving the remnant has shown potential benefits in enhancing vascular reconstruction, cellular proliferation, and promoting vascular ingrowth of the graft. The ACL remnant contains various types of mechanoreceptors that can better preserve and restore proprioceptive functions. Increasingly, scholars are focusing on the benefits of preserving the intraoperative remnant [67,68]. Takahashi et al. conducted an animal study comparing remnant-preserved and remnant-removed ACL reconstruction in goats, finding that remnant preservation promoted cell proliferation, vascular reconstruction, reduced anterior translation, and enhanced proprioceptive function regeneration [73]. Lee et al. confirmed in a retrospective human clinical study that remnant preservation during ACL reconstruction surgery better restored proprioceptive functions than remnant removal, suggesting enhanced reinnervation and mechanoreceptor maintenance [74]. Similarly, conducted a prospective clinical study in 46 patients and reported that preserving at least one-third of the native ligament length during surgery led to significantly improved proprioceptive recovery compared with complete remnant resection [75]. Mahmoud Ahmed et al. included 109 patients with intact ACL remnants, where 56 underwent remnant-preserving surgery and 53 had remnant removal; the results showed that remnant-preserving reconstruction produced functional outcomes similar to remnant removal and better restored proprioception [76]. Igdir et al. included 44 patients with ACL tears, half of whom underwent remnant-preserving surgery, finding better proprioceptive recovery and muscle strength in the remnant-preserved group [77]. Although remnant preservation during ACL reconstruction may increase the risk of Cyclops syndrome, recent studies by Webster et al. and Bierke et al. have shown that remnant preservation does not increase the incidence of Cyclops syndrome compared to traditional ACL reconstruction methods [78,79]. These findings align with the latest systematic review by Chen et al. which demonstrated that remnant-preserving ACL reconstruction offers measurable improvements in knee stability and proprioceptive recovery without elevating complication rates, although the overall quality and consistency of evidence remain limited across studies [14].

The impact of ACL primary repair on proprioception

Primary repair of the ACL appears to significantly influence proprioception recovery compared to traditional reconstruction methods. Studies have shown that primary ACL repair can preserve proprioceptive function due to the retention of the natural ACL tissue, which contains mechanoreceptors crucial for joint position sense [80,81]. A cohort study indicated that patients who underwent primary ACL repair exhibited significantly better proprioception at various angles of knee flexion compared to those who had ACL reconstruction [80,82]. Another study assessing proprioceptive function post-repair found that patients who underwent ACL repair with additional augmentation had proprioceptive function almost identical to their healthy contralateral knees and significantly better than those who had ACL reconstruction using semitendinosus muscle tendons [83]. Supporting these findings, Müller et al. conducted a study comparing primary ACL repair with augmentation to ACL reconstruction [84]. Their study suggested that techniques preserving the native ACL offer potential benefits in maintaining proprioceptive function, muscle strength, and overall functional outcomes. Histological and clinical evidence indicates that primary ACL repair can preserve vascularity and proprioceptors within the native ligament, facilitating better proprioceptive recovery compared with reconstruction [47,72,85]. Furthermore, early surgical intervention—preferably within three months of injury—has been associated with greater mechanoreceptor preservation and improved proprioceptive and functional outcomes [5,86].

The evidence collectively underscores the importance of timely surgical intervention and the potential benefits of primary ACL repair in selected patient groups. Primary repair not only preserves native ACL tissue but also provides better short-term proprioceptive outcomes, which are essential for knee stability and function [47,80]. These findings suggest a paradigm shift towards considering primary repair as a viable option for specific ACL injuries, particularly those with proximal femoral avulsion tears and suitable stump quality. Further research is warranted to explore the long-term proprioceptive and functional outcomes of primary ACL repair compared to reconstruction.

Proprioception recovery training after ACL injury

ACL injuries significantly impact knee joint function, and the long-term success of ACL reconstruction surgery largely depends on consistent post-operative rehabilitation. Research indicates that systematic, regular rehabilitation exercises can effectively alleviate pain, nourish joint cartilage, reduce knee contracture and scar formation, decrease the incidence of post-operative patellofemoral pain, lessen muscle atrophy, enhance functional mobility, and maintain knee stability, which is as crucial as the surgery itself [87]. As understanding of ACL proprioception deepens, its critical role in knee joint function is increasingly recognized. However, ACL injuries result in decreased proprioception, leading to reduced joint stability. Traditional rehabilitation has primarily focused on restoring joint mobility and muscle strength, with a lack of targeted training for proprioception and neuromuscular control [88]. Currently, there is no clear, standardized protocol for restoring proprioceptive function after an ACL injury [89]. Table 3 presents key intervention strategies aimed at enhancing ACL proprioception along with their proposed neurophysiological mechanisms.

Table 3.

Intervention strategies for enhancing ACL proprioception and underlying mechanisms.

InterventionMechanismObserved outcome

Neuromuscular training	Stimulates afferent pathways and CNS plasticity	Improves proprioception, reduces risk of reinjury [91,93]
Proprioceptive training	Enhances joint awareness via sensory feedback integration	Enhances postural control and dynamic stability [90,96]
Remnant-preserving surgery	Retains native mechanoreceptors	Better proprioceptive recovery compared to full removal [73,74]
Primary ACL repair	Maintains original ligament and neural structures	Superior proprioception in short-term compared to reconstruction [80,85]

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Proprioception training

Proprioception training is designed to enhance balance, flexibility, and agility. During training, the CNS regulates proprioceptive signals from the limbs, trunk, and neck, as well as sensory signals from the vestibular and visual systems, to maintain joint stability and improve motor functions [90]. The primary training methods include closed kinetic chain exercises, balance and control training on balance boards, joint stability training, perturbation training, visual feedback motion programs, curvature envelopment exercises for the feet and knees, lateral bounding drills, and running drills in various directions and speeds, including reverse lay-ups and ‘S’ runs. Recent systematic reviews have further clarified the clinical value of proprioceptive-focused and virtual-reality–assisted rehabilitation after ACL reconstruction. Huang et al. reported that proprioception-oriented training significantly improves passive joint position sense, single-leg hop performance, and postural balance, whereas its effect on muscle strength and range of motion remains limited [11]. Likewise, Shamsi Majelan et al. found that virtual-reality-based balance training yields comparable or even superior gains in postural stability compared with conventional proprioceptive exercises, highlighting the potential role of immersive feedback in central sensorimotor engagement [13]. These findings emphasize that proprioceptive and VR-assisted interventions complement rather than replace traditional strength and mobility programs, and they reinforce the need for individualized, multi-modal rehabilitation strategies.

Neuromuscular proprioceptive enhancement training

Neuromuscular training addresses some shortcomings of traditional rehabilitation. Studies by Ghaderi et al. [91]. have demonstrated that neuromuscular training can enhance proprioception in the knee after ACL reconstruction. Defined as training that enhances unconscious motor responses by stimulating sensory input and central mechanisms responsible for dynamic joint control, it aims to improve lower limb biomechanics, neuromuscular control, and dynamic stability in the affected area, reducing the risk of athletic injuries and enhancing overall athletic performance [92,93]. Neuromuscular training includes strength training, balance exercises, rapid expansion and contraction exercises, proximal control training, and agility training. The training progresses from static to dynamic control and is generally divided into six phases: early post-operative recovery (1–2 weeks), walking phase (2–4 weeks), balance and dynamic knee stability training phase (5–10 weeks), strength training phase (11–18 weeks), running and jumping training phase (19–24 weeks), and progressive training and agility phase (25–48 weeks). Although neuromuscular training has advantages in promoting isokinetic strength and recovery of movement, starting it early can increase the risk of joint swelling. Therefore, the optimal intensity and timing for early neuromuscular training still require further research [94].

Vibration training

Vibration training is an innovative method of somatosensory stimulation that can partly restore proprioception. During vibration training, the patient stands on a continuously vibrating platform that transmits vibration waves from the soles of the feet upwards, activating mechanoreceptors in and around the ACL, thereby enhancing the knee’s perception of position and movement. This method also strengthens lower limb muscle power and enhances the neuromuscular feedback mechanism, thereby improving the dynamic stability of the knee joint. Kruse et al. reviewed 29 Level I–II clinical trials on postoperative ACL rehabilitation and found that vibration-based neuromuscular training may confer modest improvements in proprioception, although the supporting evidence remains limited [95]. They emphasized that vibration training should be regarded as an adjunct rather than a substitute for standard strengthening and functional rehabilitation [95]. Maghbouli et al. in a meta-analysis, demonstrated the effectiveness of vibration training in rehabilitating patients with ACL reconstructions, showing significant benefits in strengthening the hamstring and quadriceps muscles, especially when the local muscle vibration frequency exceeds 100 Hz [96].

Mechanisms linking AMI and proprioceptive impairment

Arthrogenic muscle inhibition (AMI) is a condition characterized by the reflexive inhibition of muscles surrounding a joint following injury or surgery, preventing full voluntary muscle activation [97]. This phenomenon is commonly observed after ACL injuries and significantly impacts rehabilitation and functional recovery [98–100]. The relationship between AMI and proprioceptive impairment is characterized by a complex feedback loop where disrupted sensory input from joint mechanoreceptors leads to reflexive muscle inhibition [101] (Figure 2). This inhibition, in turn, exacerbates proprioceptive deficits, creating a challenging cycle that impacts joint stability and functional recovery (Figure 2). Effective rehabilitation strategies must address both aspects to ensure comprehensive recovery post-ACL injury or surgery [102].

Figure 2.

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Neurophysiological mechanisms linking proprioceptive impairment and AMI following ACL injury. This diagram illustrates the central mechanisms by which proprioceptive deficits following ACL injury contribute to AMI. Damage to mechanoreceptors reduces afferent input, leading to maladaptive cortical reorganization—including altered somatosensory and motor cortex activation—and reduced descending motor output. These neuroplastic changes impair quadriceps activation and functional motor control, thereby reinforcing the cycle of sensorimotor dysfunction.

Disruption of sensory feedback loops

AMI primarily arises from altered sensory feedback from the injured joint [101,103]. Proprioceptive impairment involves a diminished ability to sense joint position and movement, disrupting the afferent signals from mechanoreceptors in the joint. These mechanoreceptors are crucial for providing the CNS with information about joint status, which is necessary for appropriate muscle activation [46].

Reflexive inhibition

Proprioceptive deficits lead to changes in the reflex pathways that control muscle activity [104]. Specifically, the altered input from the joint’s mechanoreceptors affects spinal reflexes, leading to the inhibition of alpha motor neurons that innervate the muscles around the joint [97]. This reflexive inhibition is a protective mechanism to prevent further damage but unfortunately results in muscle weakness and atrophy [46].

Joint stability and neuromuscular control

Proper proprioceptive function is essential for joint stability and dynamic neuromuscular control. When proprioception is impaired, the body’s ability to stabilize the joint through coordinated muscle contractions is compromised [105]. This lack of stability can exacerbate AMI, as the muscles are less able to respond effectively to joint movements, leading to further inhibition and weakness [5].

Impact on rehabilitation

The interplay between AMI and proprioceptive impairment complicates rehabilitation efforts. Effective rehabilitation requires re-establishing both muscle strength and proprioceptive function. Techniques such as neuromuscular training, proprioceptive exercises, and modalities that enhance sensory feedback (e.g. vibration therapy) are critical in addressing both AMI and proprioceptive deficits to restore normal joint function [5]. Recent studies have demonstrated the impact of proprioceptive training on mitigating AMI [106]. For instance, proprioception-oriented exercises such as closed-chain weight-shift tasks, perturbation training, and balance-board exercises have been shown to enhance joint position sense and neuromuscular activation by improving afferent feedback and postural control [107–109]. In addition, joint-repositioning or angle-reproduction drills directly target position-sense accuracy and contribute to improved sensorimotor function following ACL injury [110]. Furthermore, interventions that preserve or restore mechanoreceptor function, such as remnant-preserving ACL reconstruction, have been associated with better proprioceptive outcomes and reduced AMI [5].

Neural measurements of neuromuscular function after ACL injury

Accumulating electromyography (EMG) evidence indicates altered quadriceps–hamstrings activation patterns after ACL injury and reconstruction, characterized by diminished quadriceps drive, compensatory hamstrings co-activation, and task-specific asymmetries that may persist even beyond return to sport [111,112]. Recent syntheses have broadly confirmed these activation patterns across gait, landing, and hopping paradigms, though differences in experimental protocols and EMG normalization methods remain substantial [113,114].

Reflex-based electrophysiological measures offer additional insight into both spinal and supraspinal contributions to arthrogenic muscle inhibition (AMI) [115]. Suppression of the quadriceps Hoffmann reflex (H-reflex) and reductions in the H:M ratio are frequently reported after ACL injury/ACL reconstruction, consistent with heightened presynaptic inhibition at the spinal level; related indices such as the V-wave further suggest impaired efferent drive [115]. Longitudinal and cross-sectional studies also describe altered corticospinal excitability and intracortical inhibition using transcranial magnetic stimulation (TMS) in ACL reconstruction cohorts, aligning with the persistence of activation failure despite strength rehabilitation [116].

Collectively, EMG and reflex-based assessments help quantify multi-level neurophysiological adaptations that extend beyond mechanical instability alone. Integrating these neural measures with proprioceptive and functional tests may refine longitudinal monitoring of recovery trajectories and enable more personalized interventions aimed at mitigating AMI and improving neuromuscular control [117]. Future work combining EMG, reflex indices, and imaging could help delineate the relative contribution of spinal and cortical mechanisms in long-term motor adaptation.

Limitations

Current evidence on ACL-related proprioception remains constrained by several methodological issues. Proprioceptive assessments vary widely in testing protocols and outcome measures, which limits comparability across studies. Many mechanoreceptor and neuroplasticity findings are based on small observational cohorts or animal work, reducing their direct clinical applicability. In addition, few longitudinal studies have examined how peripheral receptor changes relate to central nervous system adaptations over time. Rehabilitation approaches designed to target proprioception also differ substantially in timing, intensity, and exercise content, making it difficult to determine the most effective strategy. Finally, objective tools capable of reliably capturing proprioceptive function in clinical settings are still lacking. These factors should be considered when interpreting the current literature and designing future research.

Conclusion

Proprioception plays an essential role in maintaining dynamic stability and functional integrity of the knee joint, and its disruption following ACL injury represents a critical challenge in modern orthopaedics and sports medicine. As this review highlights, ACL injuries lead to both peripheral mechanoreceptor degeneration and central neuromuscular adaptations, collectively impairing motor coordination and increasing the risk of reinjury. Despite advances in surgical reconstruction and rehabilitative protocols, the restoration of proprioceptive function remains suboptimal in many patients. Emerging strategies—such as remnant-preserving techniques, early surgical intervention, neuromuscular training, and proprioceptive biofeedback—have shown promise but lack standardization and robust comparative data. Furthermore, the development of objective, reliable proprioception assessment tools is urgently needed to eval‎uate treatment efficacy and personalize interventions. Looking forward, future research should focus on longitudinal studies integrating histological, neurophysiological, and clinical endpoints. The incorporation of wearable sensors, machine learning, and neurorehabilitation technologies may further refine proprioceptive diagnostics and therapeutics. Ultimately, restoring ACL proprioception requires a multidisciplinary, patient-specific approach that bridges molecular science, clinical practice, and rehabilitative innovation.

Acknowledgments

Z.L., Y.Z., and P.W. contributed equally to the conception and drafting of the manuscript. Z.L. and P.W. conducted the literature review and figure design. Y.Z. contributed to manuscript organization and critical content integration. Z.K. and P. Wu assisted with literature analysis and revisions. Y.H. provided input on neurophysiological mechanisms. W.Z. supervised the project and revised the manuscript critically for important intellectual content. All authors read and approved the final manuscript.

Funding Statement

This work was supported by The Medical Science Research Project of Dalian (Grant No. DF2023006) and Joint Fund of Liaoning Provincial Natural Science Foundation (Grant No. 2024-MSLH-107).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

No data were generated in this work. All data discussed in the manuscript are derived from previously published studies, which are cited in the reference list.

References

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