슬개 건의 기능과 생체역학 - 반드시 정리해야...

[문형철]

슬개골과 경골조면을 열결하는 구조는 patella tendon이다.

이는 인대가 아니라 tendon이다. 

뼈와 뼈를 연결하는 구조는 ligament인데.. 

panic bird....

Mechanical and structural properties of collagen fascicles from the human patellar tendon

Mechanical and structural properties of collagen.pdf

SUMMARY

Tendon tissue behavior plays an important role in the overall function of the muscle-tendon complex. The structure and function of skeletal muscle has received considerable attention in past decades, while much less is known about tendon. It is, however, becoming increasingly appreciated that the tendon properties significantly influence the behavior of the entire muscle-tendon complex and that afflictions related to physical activity, involve tendon and its associated connective tissue.

The available information on tendon structural and biomechanical properties is largely based on animal models, which may be of some but limited value. Further, only few attempts have been made to couple tendon structure to mechanical function. However, with newly developed techniques it is possible to determine the stress-strain characteristics of individual human tendon fascicles, in vitro. Combined with other techniques such as transmission electron microscopy and analysis of extracellular components it is now possible to significantly better our understanding of human tendon functional and structural characteristics.

Tendon overuse injuries are common in both recreational and elite sports and corticosteroid administration is a common treatment option, although the direct effects on the material properties of the tendon are unknown.

Study I

To examine the biomechanical properties of isolated collagen fascicles from the anterior and posterior aspect of the human patellar tendon, tissue specimens were obtained from young men during elective anterior cruciate ligament reconstruction. Isolated collagen fascicles were dissected out of the specimen and tested in a mechanical rig. The main findings of the study were that fascicles from the anterior aspect of the tendon had greater yield and peak stress levels than fascicles from the posterior aspect (56.6 􀁲 10.4 MPa vs. 31.6 􀁲 2.9 MPa and 76.0 􀁲 9.5 MPa vs. 38.5 􀁲 3.9 MPa, respectively)(P<0.05). Tangent modulus was also greater for fascicles from the

anterior aspect (1232 􀁲 198 MPa) than for the posterior fascicles (584 􀁲 123 MPa, P<0.05) while the strain properties did not differ. The results indicate a region specific difference in the biomechanical properties of the human patellar tendon.

Study II

The aim of this study was to reproduce the biomechanical data from study I and further to analyze the collagen fascicles from the anterior and posterior aspects for structural parameters. This was done in order to address what may be the contributing factors to the region specific differences in biomechanical properties within the human patellar tendon. Transmission electron microscopy was used to analyze cross-sections of single fascicles for cross-sectional area density and diameter distribution of collagen fibrils. The main results where that the biomechanical results from the previous study were confirmed. The fascicles from the anterior aspect displayed greater peak tensile

stress levels and greater tendon modulus than those from the posterior aspect (57.3 ± 4.5 MPa vs. 39.6 ± 7.1 MPa (P<0.05) and (606 ± 41 MPa vs. 308 ± 50 MPa (P<0.001), respectively). The fascicles from the posterior portion of the tendon displayed higher strain levels at fascicle peak stress (18.4 ± 1.2 % vs. 14.0 ± 1.2 %, respectively)(P<0.01).

The fibril diameter profile also differed between the two regions of the tendon. Fascicles from the posterior aspect of the tendon had greater fibril density than those from the anterior aspect (96.6 􀁲 6.7 #/μm2 vs. 69.0 􀁲 8.7 #/μm2, respectively)(P<0.05) There was a tendency towards greater fibril mean area in the anterior fascicles (7819 􀁲 1104 nm2 vs. 4897 􀁲 631 nm2, P=0.1), while the volume fraction was similar to that of the posterior fascicles.

These results support the notion that tissues containing fibrils with small diameter have larger contact surface between the fibrils and the extracellular matrix and thereby greater compliance. Also, tissues containing larger fibril diameter will have a greater capacity for inter

molecular cross-links as larger fibrils have a greater ratio of internal volume to fibril periphery and are therefore capable of withstanding greater tensile forces. Further, this indicates that the loading of the human patellar tendon is non-homogenous with the load being greatest through the anterior portion of the tendon.

Study III

The purpose of this study was to estimate the magnitude of lateral force transmission between two adjacent collagen fascicles from two different types of tendons from the human body, the patellar and the Achilles tendon. The patellar tendon specimens were obtained from 7 healthy men during elective anterior cruciate ligament reconstruction. A thin bundle of collagen (~35 mm in length and

~3.5 mm in diameter) was obtained from the anterior portion of the harvested tendon. During open surgical repair of Achilles tendon rupture, collagen fascicles were obtained from 6 healthy men. Similarly, a thin bundle of collagen (~40 mm in length and ~2 mm in diameter) was isolated proximal to the rupture site, i.e. from the healthy part of the tendon. 

From each sample two adjacent strands of fascicles (ø 300 - 530 μm) enclosed in a fascicular membrane were dissected. The specimen was deformed to ~3 % strain in three independent load-displacement cycles in a small scale mechanical rig. Cycle 1: the fascicles and the fascicular membrane were intact. Cycle 2: one fascicle was transversally cut while the other fascicle and the fascicular membrane were kept intact.

Cycle 3: both fascicles were cut in opposite ends while the fascicular membrane was left intact. A decline in peak force of 45 % and 55 % from cycle 1 to cycle 2, and 93 % and 92 % from cycle 2 to cycle 3 was observed in the patellar and Achilles tendon fascicles, respectively. A decline in stiffness of 39 % and 60 % from cycle 1 to cycle 2, and of 93 % and 100 % from cycle 2 to cycle 3 was observed in the patellar and Achilles tendon fascicles, respectively.

The main finding of the study was that lateral force transmission between adjacent collagen fascicles in human tendons is small or negligible, suggesting that tendon fascicles largely act as independent structures and that force transmission principally takes place within the

individual fascicles. 

Study IV

The purpose of the study was to examine the effect of a corticosteroid environment on the tensile strength of isolated collagen fascicles and to examine the effects of corticosteroids on cross-links (hydroxylysyl pyridinoline and lysyl pyridinoline) density in isolated collagen fascicles from rat tail-tendon. A ~60 mm section was cut from the proximal end of the skinned rat-tail, subsequently single fascicles were teased out and divided into a proximal and distal section. The strands were incubated in either high (1mL of 40 mgmL-1 mixed with 0.5 mL saline 9% - HC) or low concentration (1mL of 40 mgmL-1 mixed with 2 mL saline 9% - LC) of methylprednisolone acetate

(Depo-medrol?) for 3 or 7 days while the control segment from the same fascicle was kept in saline. There was an equal number of fascicles strands from the proximal and the distal part of the specimen in each of the intervention groups and the corresponding control group. The main findings in this study were that in the HC groups the strength was reduced after 3 (16.6±4.6 MPa) and 7 (8.6±1.7 MPa) days compared to the controls (30.2±5.0 MPa, 25.6±4.6 Mpa respectively), P<0.05. In the LC groups the strength was also reduced after 3 (12.0±3.1 MPa) and 7 days (10.9±2.5 MPa) compared to the controls (31.5±5.0 MPa, 32.4±5.6 MPa respectively), P<0.05. The amount of

cross-linking was unaffected by the intervention.

Study V

The aim of the study was to examine if the biomechanical effects observed in the in vitro study (study IV) could be extended to an in vivo model of intratendinous corticosteroid injections on healthy rat-tail tendon collagen fascicles. A total of 24 animals were divided into two groups. In the corticosteroid treated group (n=12) the animals were injected in the tail tendon with methylprednisolone acetate (Depo-Medrol?), 1.0 mL of 40 mg/mL mixed with 1.0 mL saline 9 % (1:1). The animals in the control group (n=12) were injected with saline 9 %. Each rat-tail was injected at four sites with 100 μl per injection for a total of 400 μl / animal. Three days after the injections the animals were sacrificed and the tails were collected. Subsequently, single isolated collagen fascicles were teased out and subsequently tested in a mechanical rig. 

The main finding of the study was that corticosteroid administration significantly reduced fascicle tensile yield strength by 16 % compared to sham treatment (10.5±0.2 MPa vs. 12.4±0.1 MPa, P􀂔0.05). Peak stress was also reduced for the treated fascicles compared to the controls, although it did not reach significant level (15 􀁲 1.0 MPa vs. 13 􀁲 1.0 MPa)(P=0.1). Further, Young’s modulus was reduced by 14 % in the corticosteroid treated group (537±155 MPa vs. 641±185 MPa, P<0.05) while the strain properties were unaffected.

Corticosteroid administration may weaken specific regions in the treated tendon and leave it further risk of injury. This weakening effect is manifested in the individual collagen fascicles that constitute the tendon.

INTRODUCTION

The basic function of tendon tissue is force transmission from muscle to bone to hereby induce joint movement. Therefore are the material properties and the mechanical behavior of tendon tissues of great importance for the overall and optimal function of the muscle-tendon unit. Depending on the anatomical location and function of the tendon the structural composition of tendons vary: For example, long and thin tendons facilitate storage and recovery of elastic energy during human movement, while thick and short tendons deliver time efficient force transmission. (1). 

Thus proper tendon structure is essential for optimal function during locomotion/movement, and it is well accepted that tendon tissue has the capacity to adapt to loading by altering its structural components and mechanical properties according to the functional demands of the entire muscle-tendon complex. However, if mechanical loading exceeds the capability of the structure tissue injury will occur, and tendon overload injuries seem to be one of the most frequent injury types among elite and recreational athletes (2;3), where patellar tendinopathy has been reported to be as high as 55% among elite jumping athletes (3-6). In order to develop optimal prevention and rehabilitation strategies against tendon overload injuries we need to understand the basic behavior and biomechanics of the tendon structure.

- 엘리트 점프 운동선수의 55%에서 발생하는 슬개건병증..

Tendon structure

Tendon is to a large extent made up of fibrillar collagen type I, which is the most abundant collagen type in the human body. Type II collagen is also present where tendons wrap around bony or fibrous ‘pulleys’, and at the tendon entheses (the bony attachments) (7). In addition, collagen type III can be found in the periphery of the fibril and may contribute to fibrillogenesis (8). The collagen molecule is a trimeric molecule, i.e. it consists of three polypeptide chains in a left-handed helix, which can intertwine with two other helices to form a unique triple-stranded, right-handed super-helical structure (9;10).

Each collagen fibril consists of millions of short collagen molecules which are approximately 300 nm in length and 1,5 nm in diameter, and to secure optimal force transduction the molecules are connected and stabilized by covalent cross-linking (11-14). Cross-linking involves two different mechanisms, one precisely controlled enzymatic modification by the enzyme lysyloxidase during development and maturation and one non-enzymatic mechanism that seems to increase with age and involves reactions with glucose and is generally referred to as glycations (11- 15). These intermolecular cross-links are fundamental to the proper behavior and function of the tendon tissue, and lack of cross links likely causes the molecules to slide relative to each other thus leaving the tissue extensible and weakened (11;13;16;17).

The tendon extracellular matrix (ECM) consists largely of collagen fibers, proteoglycans, glycoproteins and glycosaminoglycans, where the first two are preval‎ent. The various structures or components of the ECM here included collagen are produced by tenocytes (18-20). The ECM contributes in important ways to the mechanical integrity of the tendon, and its tensile strength is established via intra- and intermolecular cross links, the orientation, density, and the length of the collagen fibrils and fibers (21). In addition, the proteoglycans along with water seem to have a spacing and lubricating role for the tendon, an important feature for sliding between fibrils and fascicle within the tendon (21;21;22).

Tendon tenocytes lie parallel with and in between the collagen fibrils. They sense and respond to mechanical loading of the tissue by altering their function and composition. This mechanotransduction is an important mechanism in the initiation of tissue adaptation to mechanical loading (21). These adaptations include increased production of collagen and other components of the EMC (23-25). It appears that tension is required to maintain normal function of the tenocytes (26;27).

The internal architecture of tendons is organized in a complex hierarchical structure primarily composed of collagen molecules (Figure 1). Collagen fibrils are generally considered the most fundamental molecular structure and the basic force-transmitting unit of the tendon.

The Collagen fiber: A bundle of fibrils in a parallel alignment is termed a collagen fiber, which with a diameter ranging from 1 to 12 μm, may be straight or wavy or branch at acute angles. In the resting state, the collagen fibers have a crimped waveform appearance, which disappears if the tendon is stretched slightly corresponding to a straightening of the collagen fibers (28). 

The fascicle is a collection of fibers enclosed in fibrous connective tissue, the endotenon which is a thin reticular network of connective tissue that has a well-developed crisscross pattern of collagen fibers (18;29). The fascicles, which have a crimped appearance due to the axial alignment of the fibrils, vary in diameter with the size of the tissue. The endotenon encloses the fascicles and binds them together in bundles, which again are surrounded by another sheath the epitenon, which also envelops the whole tendon. The epitenon is connected with paratenon on its outer surface and with endotenon on its inner surface (30). Tendons are formed from groups of collagen fascicles

enclosed within the paratenon sheaths. The paratenon functions as an elastic ‘sleeve’ permitting free movement of the tendon against surrounding tissues (28;30). In tendons the collagen fiber bundles are aligned parallel to each other and are essentially co-linear with the line of pull of the muscle. It is the difference in the arrangement or alignment of these fascicles that is thought to affect the mechanical responses of various tendons and ligaments (31;32). 

Tendon biomechanics

Tendon tissues possess a very high tensile strength compared to other soft tissue in the body due to the collagen structure and the parallel arrangement of the collagen fibrils to the direction of force. The mechanical properties of the collagen describe the reaction of the tissue to tensile loading, and depend on the architecture of the collagen fibers and the interaction of the collagen with the extracellular matrix (31). These properties are commonly determined by tensile tests during which an external force (N) is applied causing the structure to elongate (m) (1;32).

The initial concave portion of a normal stress-strain curve is termed the toe region and is followed by a linear loading region that extends up to the yield point (figure 2). It is suggested that the toe region is related to straightening out of the collagen molecules and the crimped fibrils while further stretching of the molecules and gliding of the molecules relative to each other within the fibrils takes place in the linear region (33-35). Tissue stiffness (N/m) is calculated in the linear region of the force-deformation curve and corresponds to the slope of the curve (1;32). As long as strain does not exceed ~4-8%, the tissue is almost perfectly elastic in the sense that it returns to its original shape when stress is removed. Beyond this strain micro-ruptures begin to occur in the structure, and the tissue does not return to its original length when stress is removed (1;36).  

Obviously the dimensions of the tendon (cross-sectional area and length) affect the mechanical properties. If tissue is added in parallel to increase the cross-sectional area both strength and stiffness will increase but the deformation at failure will not change. Also, if tissue is added in series to increase tendon length the stiffness will increase, the elongation to failure will increase while the strength will remain the unchanged (1;32;34). Thus, for a better comparison between tendons the force-elongation curve can be transformed into a stress-strain curve by dividing the force (N) with the original CSA (m2) (stress = N/m2) and by dividing the elongation (m) of the tendon by the initial tendon length (strain = 􀇻L/LO). Consequently, stress is a measure of the relative load of the structure while strain is a relative measure for tendon elongation.

The relationship between stress and strain is independent of structural dimensions and gives the modulus of the tissue (􀇻 stress/􀇻 strain, Pa), which can be regarded as a measure of tissue quality (32;37). During elongation elastic energy is stored within the tendon (the area under the force elongation curve) and subsequently released upon unloading. Hysteresis is a measure of the energy lost during the loading and the unloading of the tissue (the area between the loading and the unloading force-displacement curves) (1;38). The elongation of tendons does not only depend on the amount of the applied force but also on the time and history of the force application, or the viscoelastic properties (31). Creep describes the time-dependent increase in tissue elongation of the tissue under constant load, while stress-relaxation is defined as the time-dependent decrease in load when tissue is subjected to a constant elongation. The history-dependent behavior of tendons means that the shape of the load-elongation curve will wary depending on the previous loading. 

Preconditioning is another viscoelastic feature in tendons, and is somewhat similar to what is

termed ‘warm-up’. The first few cycles of displacement after inactivity reveal larger areas of

hysteresis. Following this conditioning the behavior of the tissues (force-displacement curves)

becomes more similar between repeated loading cycles. The preconditioning step therefore is

important for biomechanical testing to avoid experimental error. After preconditioning the

recovered elastic strain energy of a tendon is 90-96% when loaded in the physiologic range (31).

Tendon function

The basic function of tendon is to transmit the muscle generated force to the bone, thus making limb

and joint movement possible. Therefore tendons must be capable of resisting high tensile forces

with limited elongation (36). The system with the most effective force transfer would be an

inextensible load transmitting structure but tendons nonetheless transmit loads with minimal energy

loss and deformation and concurrently exhibit spring-like characteristics that serve additional

important functions. Dynamic interaction between muscle and tendon influence not only force

transmission but also energy storage and return for locomotion, joint position control, and injury

protection (1;39;40).

It is generally accepted that strain energy is stored in elastic elements upon loading

and subsequently released upon unloading (38;41). The tendon is capable of returning over 90% of

the stored energy, but some of the stored energy is not recovered due to energy dissipation (42;43).

During movement a part of the kinetic energy created by the muscle is transiently stored as strain

energy in the tendon (36). Further, it appears that the strain energy recovery is larger during running

than normal walking demonstrating the importance of elastic savings of energy for economy of

motion (41;44).

A thinner tendon will experience greater stress for a given force because of a reduced

cross-sectional area and also greater strain that will favor increased storage of elastic strain energy.

With respect to the tendon acting as a spring that can store and recover elastic energy, a thin and

long tendon would be advantageous. But in terms of force transfer for joint movement a thick

tendon may be advantageous, as it would decrease the average force per area and therefore the

potential for injury (1).

The mechanical properties of the tendon may also serve to protect the muscle fiber

from damage. If the tendon were inextensible any elongation of the muscle-tendon unit during joint motion would be taken up by the muscle fiber. Which means that the muscle fiber would likely

have to operate on its descending limb of the length-tension curve (45), where it may be more

susceptible to injury (40). Thus, the tendon has the capacity to act as a mechanical buffer and

protect the muscle fiber from stretch and possible damage (39). The mechanical properties of the

tendon may also affect the muscle fiber contraction properties. During an isometric muscle

contraction, a stiffer tendon would tend to reduce sarcomere velocity and shortening rate, and

thereby augment the contractile force (40). Theoretically, a stiffer tendon may put the muscle at an

increased risk of damage since less strain would be taken up by the tendon relative to the muscle

fiber (40).

The anatomy and biomechanics of the patellar tendon

Basso et al. (2000) reported after examining twenty-two human cadaver knees that the majority of

the parallel oriented fascicles in the patellar tendon attached to the distal two thirds of the anterior

surface of the patella. The distance between patellar and tibial attachments of such bundles varied,

depending on the layer and across the width of the patellar tendon. The anterior fascicles were

longer than the corresponding posterior fascicles, since their attachment was more proximal on the

patella and more distal on the tibia than that of the corresponding posterior fascicles. The tendon is

thin and broad proximally, becoming thick and narrow distally, since the fiber bundles converge as

they run towards the tibial tuberosity (46).

The existing data on the biomechanical properties of the patellar tendon are scarce. It

is e.g. unknown how the force is distributed through the tendon during movement and muscular

contraction, i.e. if the force is distributed equally or how the different parts of the structure (the

anterior-, posterior-, proximal-, distal-, medial-, central- and lateral part) are loaded during

contraction. Furthermore, knee joint positions could also influence the loading pattern. The

available data is mostly based on cadaver or animal studies. However, the two studies that have

examined regional strain differences in the human patellar tendon show very different and at times

contradicting results (47;48). 

Patellar tendinopathy

The term patellar tendinopathy, also known as jumpers knee (49), describes the overuse injury that

is characterized by pain during activity, localized tenderness under palpation, swelling and impaired

performance (50-52). The proximal posterior region of the patellar tendon is more frequently

affected (65-79 %) than the distal posterior region of the tendon (~15 %)(5;53;54). The reason for

this region specific injury remains elusive (53), but a plausible explanation could be that the

posterior region has the smallest CSA and therefore undergoes greater stress during loading (55).

However, this does not explain why injury occurs mainly on the posterior aspect of the tendon.

It has been suggested that pain in this region is due to tensile overload or impingement

of the inferior patellar pole against the patellar tendon during knee flexion (53). However, Schmid

et al. (2002) did not find any support for patellar tendon impingement since no difference was

observed between symptomatic and asymptomatic knees when analyzed for patella shape, tendon

insertion location and patellar tendon angle on dynamic open configuration MRI (56). Therefore,

the magnitude and the rate of loading of the patellar tendon is generally believed to play a major

causative role in patellar tendinopathy (4;5;55-58). Ground reaction forces during jump landing

may reach up to 11 times body weight (58), and the patellar tendon may be subjected to forces

exceeding 4000 N during countermovement jumps (59). When the patellar tendon is subjected to

extreme forces micro-ruptures in the tendon structural units can occur (4;5;49;56;57;60;61).

Within recent years another tendon overload injury model has been presented. This

theory suggest that damage to isolated fascicles and the subsequent stress deprivation to the

tenocytes will elicit a catabolic response that results in degradation of the ECM and consequent loss

of tendon material properties. This mechanical under stimulation of the tenocytes may well be

initiated by overuse or overloading of the tissue (62).

Corticosteroid administration

The use of corticosteroid injection is a common treatment option for tendinopathies (63-65) although its direct effect on the material properties of the tendon are unknown. There have been several case reports of tendon rupture after intratendinous corticosteroid administration in both the upper and the lower extremities (66-70). However, to what extent such ruptures can be attributed the steroid treatment itself and/or a progression of the tendinopathy remains unknown.

Several animal studies have shown that local injection of corticosteroids leads to a reduction in tensile strength of the tendons (71-76), but in contrast at least two animal studies fail to demonstrate such a corticosteroid associated reduction in mechanical strength (77;78). Thus, the use of corticosteroid injections may well be the most debated and controversial issue in the treatment of tendinopathies.