뼈, 인대, 건, 피부 등의 stress strain curve와 손상 메카니즘

[문형철]

장력-응력 커브와 손상메커니즘에 대한 탐구가 좀더 필요하다

인대는 건보다 콜라겐이 적고, 탄력섬유가 많음. 

그래서 건보다 더 늘어남. 

panic bird....

과학적 실험과 실제 인체에서 일어나는 기전의 차이를 상상해야

1. tendon은 근육과 직렬로 연결되어 muscle to bone linkage

2. ligament는 bone to bone linkage

3. 근육의 기능부전 상황, tendinopathy상황,  관절의 틀어짐 상황 등을 고려해야

General mechanical characteristic of soft tissues

Before describing a model for soft tissues it is beneficial and instructive to give some insight in their general mechanical characteristic. Soft tissues behave anisotropically because of their fibers which tend to have preferred directions. In a microscopic sense they are non-homogeneous materials because of their composition. 

- 연부조직은 선택된 방향 배열에 따라서 기능함.

The tensile response of soft tissue is nonlinear stiffening and tensile strength depends on the strain rate. In contrast to hard tissues, soft tissues may undergo large deformations. Some soft tissues show viscoelastic behavior (relaxation and/or creep), which has been associated with the shear interaction of collagen with the matrix of proteoglycans [16] (the matrix provides a viscous lubrication between collagen fibrils).

- 연부조직의 늘어남 반응은 비선형 stiffening이고, 장력은 응력비율에 의존함. 

- 단단한 조직에 반하여 연부조직은 많은 변형을 가짐. 연부조직은 점탄성 특성을 가지고 있어서 프로테오글리칸 기질내 콜라겐의 전단상호작용과 연관되는 역할을 함. 

In a simplified way we explain here the tensile stress-strain behavior for skin, an organ consisting mainly of connective tissues, which is representative of the mechanical behavior of many (collagenous) soft connective tissues. For the connective tissue parts of the skin the three-dimensional network of fibers appears to have preferred directions parallel to the surface. However, in order to prevent out-of-plane shearing, some fiber orientations also have components out-of-plane. 

- 피부의 장력-응력 커브로 설명할 수 있음. 

- 이는 연부조직의 기계적 특성을 대표함. 

Figure 1 shows a schematic diagram of a typical J-shaped (tensile) stress-strain curve for skin.

- 피부의 장력-응력커브는 J 형태의 장력-응력커브를 가짐. 

This form, representative for many soft tissues, differs significantly from stress-strain curves of hard tissues or from other types of (engineering) materials. In addition, Figure 1 shows how the collagen fibers straighten with increasing stress. The deformation behavior for skin may be studied in three phases I, II and III:

- 피부가 장력에 반응하여 어떻게 늘어나는 지를 보여주는 그래프. 3단계로 나누어 설명가능함. 

Phase I. In the absence of load the collagen fibers, which are woven into rhombic-shaped pattern, are in relaxed conditions and appear wavy and crimped. Unstretched skin behaves approximately isotropically. Initially low stress is required to achieve large deformations of the individual collagen fibers without requiring stretch of the fibers. In phase I the tissue behaves like a very soft (isotropic) rubber sheet, and the elastin fibers (which keep the skin smooth) are mainly responsible for the stretching mechanism. The stress-strain relation is approximately linear, the elastic modulus of skin in phase I is low (0.1-2 MPa).

- 1단계는 콜라겐 늘어남이 없고, 엘라스틴 섬유가 주로 늘어남 기전에 관여함. 

- 위 그림과 같이 늘어나는 정도가 직선그래프를 보임. 

Phase II. In phase II, as the load is increased, the collagen fibers tend to line up with the load direction and bear loads. The crimped collagen fibers gradually elongate and they interact with the hydrated matrix. With deformation the crimp angle in collagen fibrils leads to a sequential uncrimping of fibrils. Note, that the skin is normally under tension in vivo.

- 2단계에는 부하가 증가하여 콜라겐이 늘어나는 단계

- 곱슬머리같은 콜라겐이 점차로 늘어나는 단계

Phase III. In phase III, at high tensile stresses, the crimp patterns disappear and the collagen fibers become straighter. They are primarily aligned with one another in the direction in which the load is applied. The straightened collagen fibers resist the load strongly and the tissue becomes stiff at higher stresses. The stress-strain relation becomes linear again. Beyond the third phase the ultimate tensile strength is reached and fibers begin to break.

- 3단계는 높은 장력에 곱슬머리 패턴이 사라지고 콜라겐이 반듯한 배열로 늘어남. 

- 늘어난 콜라겐 섬유는 부하에 강하게 저항하고 점점 단단해짐. 

- 늘어나는 정도가 다시 직선 그래프를 보임. 

The mechanical properties of soft tissues depend strongly on the topography, risk factors, age, species, physical and chemical environmental factors such as temperature, osmotic pressure, pH, and on the strain rate. The material properties are strongly related to the quality and completeness of experimental data, which come from in vivo or in vitro tests having the aim of mimicking real loading conditions. Therefore, to present specific values for the ultimate tensile strength and strain of a specific tissue is a difficult task. Nevertheless, Table 1 attempts to present ranges of values of mechanical properties and collagen/elastin contents (% dry weight) in some representative organs mainly consisting of soft connective tissues.

- 연부조직의 기계적 특성은 형태, 위험인자, 나이, 종, 물리 화학적 환경요소(온도, 삼투압, PH, 응력비율 등)에 의존함.