Biomechanics of knee ligaments: injury, healing, and repair

Knee ligament injuries are common, particularly in sports and sports related activities. Rupture of these ligaments upsets the balance between knee mobility and stability, resulting in abnormal knee kinematics and damage to other tissues in and around the joint that lead to morbidity and pain. During the past three decades, significant advances have been made in characterizing the biomechanical and biochemical properties of knee ligaments as an individual component as well as their contribution to joint function. Further, significant knowledge on the healing process and replacement of ligaments after rupture have helped to evaluate the effectiveness of various treatment procedures. This review paper provides an overview of the current biological and biomechanical knowledge on normal knee ligaments, as well as ligament healing and reconstruction following injury. Further, it deals with new and exciting functional tissue engineering approaches (ex. growth factors, gene transfer and gene therapy, cell therapy, mechanical factors, and the use of scaffolding materials) aimed at improving the healing of ligaments as well as the interface between a replacement graft and bone. In addition, it explores the anatomical, biological and functional perspectives of current reconstruction procedures. Through the utilization of robotics technology and computational modeling, there is a better understanding of the kinematics of the knee and the in situ forces in knee ligaments and replacement grafts. The research summarized here is multidisciplinary and cutting edge that will ultimately help improve the treatment of ligament injuries. The material presented should serve as an inspiration to future investigators.     

New parameters describing how knee ligaments carry force in situ predict interspecimen variations in laxity during simulated clinical exams

Knee laxity, defined as the net translation or rotation of the tibia relative to the femur in a given direction in response to an applied load, is highly variable from person to person. High levels of knee laxity as assessed during routine clinical exams are associated with first-time ligament injury and graft reinjury following reconstruction. During laxity exams, ligaments carry force to resist the applied load; however, relationships between intersubject variations in knee laxity and variations in how ligaments carry force as the knee moves through its passive envelope of motion, which we refer to as ligament engagement, are not well established. Thus, the objectives of this study were, first, to define parameters describing ligament engagement and, then, to link variations in ligament engagement and variations in laxity across a group of knees. We used a robotic manipulator in a cadaveric knee model (n = 20) to quantify how important knee stabilizers, namely the anterior and posterior cruciate ligaments (ACL and PCL, respectively), as well as the medial collateral ligament (MCL) engage during respective tests of anterior, posterior, and valgus laxity. Ligament engagement was quantified using three parameters: (1) in situ slack, defined as the relative tibiofemoral motion from the neutral position of the joint to the position where the ligament began to carry force; (2) in situ stiffness, defined as the slope of the linear portion of the ligament force–tibial motion response; and (3) ligament force at the peak applied load. Knee laxity was related to parameters of ligament engagement using univariate and multivariate regression models. Variations in the in situ slack of the ACL and PCL predicted anterior and posterior laxity, while variations in both in situ slack and in situ stiffness of the MCL predicted valgus laxity. Parameters of ligament engagement may be useful to further characterize the in situ biomechanical function of ligaments and ligament grafts.     

A Three-Dimensional Musculoskeletal Model of the Human Knee Joint. Part 2: Analysis of Ligament Function

A three-dimensional model of the knee is used to study ligament function during anterior-posterior (a-p) draw, axial rotation, and isometric contractions of the extensor and flexor muscles. The geometry of the model bones is based on cadaver data. The contacting surfaces of the femur and tibia are modeled as deformable; those of the femur and patella are assumed to be rigid. Twelve elastic elements are used to describe the geometry and mechanical properties of the cruciate ligaments, the collateral ligaments, and the posterior capsule. The model is actuated by thirteen musculotendinous units, each unit represented as a three-element muscle in series with tendon. The calculations show that the forces applied during a-p draw are substantially different from those applied by the muscles during activity. Principles of knee-ligament function based on the results of in vitro experiments may therefore be overstated. Knee-ligament forces during straight a-p draw are determined solely by the changing geometry of the ligaments relative to the bones: ACL force decreases with increasing flexion during anterior draw because the angle between the ACL and the tibial plateau decreases as knee flexion increases; PCL force increases with increasing flexion during posterior draw because the angle between the PCL and the tibial plateau increases. The pattern of ligament loading during activity is governed by the geometry of the muscles spanning the knee: the resultant force in the ACL during isometric knee extension is determined mainly by the changing orientation of the patellar tendon relative to the tibia in the sagittal plane; the resultant force in the PCL during isometric knee flexion is dominated by the angle at which the hamstrings meet the tibia in the sagittal plane.     

Intrinsic innervation of the rat knee joint articular capsule and ligaments

In spite of the practical importance of having a detailed knowledge of knee joint innervation to understand the pathophysiologic aspects, little information is now available concerning the density and pattern of the nerve fibres which are distributed to it. The present study has been designed to investigate the density and distribution of nerve fibres and receptor corpuscles in the knee joint articular capsule, cruciate and collateral ligaments in the rat, using the acetylcholinesterase (AChE) histochemical in toto staining technique. The investigation was performed on male Wistar rats of 3 months of age, some of which had been treated with capsaicin to deplete their afferent 'C' fibres of their content of neuropeptides. AChE-positive nerve fibres and different types of receptor corpuscle endings were found within articular capsule and ligaments. The highest density of AChE-positive nerve fibres was noticeable in the fibular collateral ligament followed by the tibial collateral ligament, the posterior cruciate ligament, the anterior cruciate ligament and the articular capsule. In the articular capsule the number of type I endings was higher than in the ligaments. The opposite is true for the other type of receptor corpuscles found as well as for nerve endings. Capsaicin treatment significantly reduced the density of AChE-positive nerve fibres in knee joint ligaments but did not affect nerve fibres in the articular capsule. Moreover, it caused the disappearance of some kind of receptor corpuscles within the collateral and cruciate ligaments. The above data collectively suggest that the AChE in toto staining technique may represent a good method for investigating joint innervation and that a significant percentage of nerve fibres supplying knee joint ligaments is represented by C fibre afferents.     

A Three-Dimensional Musculoskeletal Model of the Human Knee Joint. Part 2: Analysis of Ligament Function

A three-dimensional model of the knee is used to study ligament function during anterior-posterior (a-p) draw, axial rotation, and isometric contractions of the extensor and flexor muscles. The geometry of the model bones is based on cadaver data. The contacting surfaces of the femur and tibia are modeled as deformable; those of the femur and patella are assumed to be rigid. Twelve elastic elements are used to describe the geometry and mechanical properties of the cruciate ligaments, the collateral ligaments, and the posterior capsule. The model is actuated by thirteen musculotendinous units, each unit represented as a three-element muscle in series with tendon. The calculations show that the forces applied during a-p draw are substantially different from those applied by the muscles during activity. Principles of knee-ligament function based on the results of in vitro experiments may therefore be overstated. Knee-ligament forces during straight a-p draw are determined solely by the changing geometry of the ligaments relative to the bones: ACL force decreases with increasing flexion during anterior draw because the angle between the ACL and the tibial plateau decreases as knee flexion increases; PCL force increases with increasing flexion during posterior draw because the angle between the PCL and the tibial plateau increases. The pattern of ligament loading during activity is governed by the geometry of the muscles spanning the knee: the resultant force in the ACL during isometric knee extension is determined mainly by the changing orientation of the patellar tendon relative to the tibia in the sagittal plane; the resultant force in the PCL during isometric knee flexion is dominated by the angle at which the hamstrings meet the tibia in the sagittal plane.     

Geometrical changes of knee ligaments and patellar tendon during passive flexion

Patterns of fibre elongation and orientation for the cruciate and collateral ligaments of the human knee joint and for the patellar tendon have not yet been established in three-dimensions. These patterns are essential for understanding thoroughly the contribution of these soft tissues to joint function and of value in surgical treatments for a more conscious assessment of the knee status. Measurements from 10 normal cadaver knees are here reported using an accurate surgical navigation system and consistent anatomical references, over a large flexion arc, and according to current recommended conventions. The contours of relevant sub-bundles were digitised over the corresponding origins and insertions on the bones. Representative fibres were calculated as the straight line segments joining the centroids of these attachment areas. The most isometric fibre was also taken as that whose attachment points were at the minimum change in length over the flexion arc. Changes in length and orientation of these fibres were reported versus the flexion angle. A good general repeatability of intra- and inter-specimens was found. Isometric fibres were found in the locations reported in the literature. During knee flexion, ligament sub-bundles slacken in the anterior cruciate ligament, and in the medial and lateral collateral ligaments, whereas they tighten in the posterior cruciate ligament. In each cruciate ligament the two compounding sub-bundles have different extents for the change in fibre length, and also bend differently from each other on both tibial planes. In the collateral ligaments and patellar tendon all fibres bend posteriorly. Patellar tendon underwent complex changes in length and orientation, on both the tibial sagittal and frontal planes. For the first time thorough and consistent patterns of geometrical changes are provided for the main knee ligaments and tendons after careful fibre mapping.     

An improved OpenSim gait model with multiple degrees of freedom knee joint and knee ligaments

Musculoskeletal models are widely used to investigate joint kinematics and predict muscle force during gait. However, the knee is usually simplified as a one degree of freedom joint and knee ligaments are neglected. The aim of this study was to develop an OpenSim gait model with enhanced knee structures. The knee joint in this study included three rotations and three translations. The three knee rotations and mediolateral translation were independent, with proximodistal and anteroposterior translations occurring as a function of knee flexion/extension. Ten elastic elements described the geometrical and mechanical properties of the anterior and posterior cruciate ligaments (ACL and PCL), and the medial and lateral collateral ligaments (MCL and LCL). The three independent knee rotations were evaluated using OpenSim to observe ligament function. The results showed that the anterior and posterior bundles of ACL and PCL (aACL, pACL and aPCL, pPCL) intersected during knee flexion. The aACL and pACL mainly provided force during knee flexion and adduction, respectively. The aPCL was slack throughout the range of three knee rotations; however, the pPCL was utilised for knee abduction and internal rotation. The LCL was employed for knee adduction and rotation, but was slack beyond 20° of knee flexion. The MCL bundles were mainly used during knee adduction and external rotation. All these results suggest that the functions of knee ligaments in this model approximated the behaviour of the physical knee and the enhanced knee structures can improve the ability to investigate knee joint biomechanics during various gait activities.     

Role of knee ligaments in proprioception and regulation of muscle stiffness

Physiologic evidence for the sensory role of the knee joint ligaments are reviewed. The cruciate and collateral ligaments accomodate morphologically different sensory nerve endings with different capabilities of providing the central nervous system (CNS) with information not only about noxious and chemical stimuli but also about mechanical events, e.g., movement- and position-related stretches of the ligaments. Available data show that low-threshold joint/ligament receptor (i.e., mechanoreceptor) afferents evoke only weak and rare effects in skeletomotor neurons (α-motoneurons), whereas they frequently and powerfully influence fusimotor neurons (γ-motoneurons). The effects on the γ-muscle-spindle system in the muscles around the knee are so potent that even stretches of the cruciate ligaments at relatively moderate loads (not noxious) may induce major changes in responses of the muscle spindle afferents. As the activity in the primary muscle spindle afferents modifies stiffness in the muscles, the cruciate ligament receptors may, through the γ-muscle-spindle system, participate in regulation and preparatory adjustment of the stiffness of the muscles around the knee joint and thereby of knee joint stiffness. Thus, the sensory system of the cruciate ligaments is able to contribute significantly to the functional stability of the knee joint. The possible role of (ligamentous) joint receptors in genesis and spread of muscular tension in occupational muscle pain and in chronic musculoskeletal pain syndromes is also discussed.     

Specializations within the lumbosacral spinal cord of the pigeon

The prominent accessory lobes of Lachi in birds are considered to be marginal nuclei; similar nuclei have been implicated in mechanoreceptive functions in snakes and lampreys. Reptile studies emphasized the involvement of the denticulate ligament with this mechanoreceptive function. This investigation examines the fine structure of the accessory lobes of Lachi in pigeons and their interaction with ligaments for features which might support such a mechanoreceptive function. In the lumbosacral area of the spinal cord, the lateral longitudinal ligaments and the ventral longitudinal ligament are hypertrophied. The ventral transverse ligaments are present only within the lumbosacral segments of the spinal cord and they interconnect with the lateral and ventral longitudinal ligaments. The lateral longitudinal ligament makes intimate contact with the spinal cord, and many glial processes from the spinal cord mingle with and are firmly attached to collagenous fibers of the ligament. The lobes lie dorsal to the lateral longitudinal ligament in the exact area where it interconnects with the transverse ligament. The lobe's multipolar neurons have a number of synaptic contacts but no unusual specializations were noted. Most of each lobe is composed of interdigitating saccular structures filled to varying degrees with flocculent material. The sacs are extensions of the cytoplasm of neuroglial cells, which also give rise to membranes surrounding neuronal processes and the sacs themselves. A possible functional relationship of the lobes and the ligaments of the lumbosacral spinal segments within the vertebral column is described.     

Évaluation clinique et biologique d’un ligament synthétique bioactif chez la brebis

The anterior cruciate ligament, which plays a key role in the knee stabilization, is commonly injured mainly during sport practicing such as soccer or skiing. Although it seems that ligament replacement by a tendon autograft is a better solution, the reconstruction with an artificial ligament provides a shorter recovery time. Polyethylene terephthalate (PET) is the best polymer to fabricate ligament prosthesis but its biocompatibility still needs to be improved. Radical graft polymerization of sodium salt of styrene sulfonate (NaSS) on PET surface was performed using the “grafting from” technique. The grafting ratio is about 5 μmol/g and found to be perfectly reproducible. Polymer grafted ligaments and non-grafted ligaments were implanted in sheep for a 3-month observation. The clinical and biological evaluation of the knee synovial liquid of implanted sheep evidenced an early functional recuperation and an excellent tolerance of pNaSS reflecting a significant absence of articular inflammation.     

Comparison of material properties in fascicle-bone units from human patellar tendon and knee ligaments

The fascicle material properties in bone-fascicle-bone units were determined for the anterior and posterior cruciate ligaments (ACL, PCL), the lateral collateral ligament (LCL) and the patellar tendon (PT) from three young human donor knees. Groups of fascicles from each tissue were isolated with intact bone ends and failed at a high strain rate in a saline bath at 37 degrees C. In each knee tested the load related material properties (linear modulus, maximum stress and energy density to maximum stress) for the patellar tendon were significantly larger than corresponding values for the cruciate and collateral ligaments. Bundles from different ligaments in the same knee were similar to each other in their mechanical behavior. In addition, no significant differences were present in the maximum strains recorded for any of the four tissue types examined. The results presented have implications in studies of ligament injury. They are also important in the design and use of synthetic and biological ligament replacements and in tissue and whole knee modeling.     

The influence of ligament modelling strategies on the predictive capability of finite element models of the human knee joint

In finite element (FE) models knee ligaments can represented either by a group of one-dimensional springs, or by three-dimensional continuum elements based on segmentations. Continuum models closer approximate the anatomy, and facilitate ligament wrapping, while spring models are computationally less expensive. The mechanical properties of ligaments can be based on literature, or adjusted specifically for the subject. In the current study we investigated the effect of ligament modelling strategy on the predictive capability of FE models of the human knee joint. The effect of literature-based versus specimen-specific optimized material parameters was evaluated. Experiments were performed on three human cadaver knees, which were modelled in FE models with ligaments represented either using springs, or using continuum representations. In spring representation collateral ligaments were each modelled with three and cruciate ligaments with two single-element bundles. Stiffness parameters and pre-strains were optimized based on laxity tests for both approaches. Validation experiments were conducted to evaluate the outcomes of the FE models.Models (both spring and continuum) with subject-specific properties improved the predicted kinematics and contact outcome parameters. Models incorporating literature-based parameters, and particularly the spring models (with the representations implemented in this study), led to relatively high errors in kinematics and contact pressures. Using a continuum modelling approach resulted in more accurate contact outcome variables than the spring representation with two (cruciate ligaments) and three (collateral ligaments) single-element-bundle representations. However, when the prediction of joint kinematics is of main interest, spring ligament models provide a faster option with acceptable outcome.     

The collateral ligaments of the knee joint in the cat and man. Morphological and functional study of the internal arrangement of fibers

The present investigation of fiber arrangement in the collateral ligaments of the knee was carried out in cats and man in various positions of flexion and extension, without compression load. In all knee joint positions, the fibers of the collateral ligaments are twisted except for the fibers in the meniscal part of the medial collateral ligament which have a parallel arrangement. Furthermore, most of the fibers in the collateral ligaments are taut in all positions of the knee joint in both cat and man. By means of planar models representing different fiber arrangements, the kinematic behavior of the collateral ligaments was analyzed. It appears that a crossed (twisted) arrangement of the fibers is most effective in rotatory movements, whereas a parallel orientation is most effective in translation. Our data further indicate that, in measuring the changes in lengths of ligaments during joint motion, one cannot neglect the internal arrangement of fibers and the geometry of the articular surfaces and menisci.     

A dynamic multibody model of the physiological knee to predict internal loads during movement in gravitational field

Obtaining tibio-femoral (TF) contact forces, ligament deformations and loads during daily life motor tasks would be useful to better understand the aetiopathogenesis of knee joint diseases or the effects of ligament reconstruction and knee arthroplasty. However, methods to obtain this information are either too simplified or too computationally demanding to be used for clinical application. A multibody dynamic model of the lower limb reproducing knee joint contact surfaces and ligaments was developed on the basis of magnetic resonance imaging. Several clinically relevant conditions were simulated, including resistance to hyperextension, varus–valgus stability, anterior–posterior drawer, loaded squat movement. Quadriceps force, ligament deformations and loads, and TF contact forces were computed. During anterior drawer test the anterior cruciate ligament (ACL) was maximally loaded when the knee was extended (392 N) while the posterior cruciate ligament (PCL) was much more stressed during posterior drawer when the knee was flexed (319 N). The simulated loaded squat revealed that the anterior fibres of ACL become inactive after 60° of flexion in conjunction with PCL anterior bundle activation, while most components of the collateral ligaments exhibit limited length changes. Maximum quadriceps and TF forces achieved 3.2 and 4.2 body weight, respectively. The possibility to easily manage model parameters and the low computational cost of each simulation represent key points of the present project. The obtained results are consistent with in vivo measurements, suggesting that the model can be used to simulate complex and clinically relevant exercises.     

Comparison of elastic,viscoelastic and failure tensile material properties of knee ligaments and patellar tendon

The knee ligaments and patellar tendon function in concert with each other and other joint tissues, and are adapted to their specific physiological function via geometry and material properties. However, it is not well known how the viscoelastic and quasi-static material properties compare between the ligaments. The purpose of this study was to characterize and compare these material properties between the knee ligaments and patellar tendon.Dumbbell-shaped tensile test samples were cut from bovine knee ligaments (ACL, LCL, MCL, PCL) and patellar tendon (PT) and subjected to tensile testing (n = 10 per ligament type). A sinusoidal loading test was performed at 8% strain with 0.5% strain amplitude using 0.1, 0.5 and 1 Hz frequencies. Subsequently, an ultimate tensile test was performed to investigate the stress-strain characteristics.At 0.1 Hz, the phase difference between stress and strain was higher in LCL compared with ACL, PCL and PT (p < 0.05), and at 0.5 Hz that was higher in LCL compared with all other ligaments and PT (p < 0.05). PT had the longest toe-region strain (p < 0.05 compared with PCL and MCL) and MCL had the highest linear and strain-dependent modulus, and toughness (p < 0.05 compared with ACL, LCL and PT).The results indicate that LCL is more viscous than other ligaments at low-frequency loads. MCL was the stiffest and toughest, and its modulus increased most steeply at the toe-region, possibly implying a greater amount of collagen. This study improves the knowledge about elastic, viscoelastic and failure properties of the knee ligaments and PT.     

A global verification study of a quasi-static knee model with multi-bundle ligaments

The ligaments of the knee consist of fiber bundles with variable orientations, lengths and mechanical properties. In concept, however, these structures were too often seen as homogeneous structures, which are either stretched or slack during knee motions. In previous studies, we proposed a new structural concept of the ligaments of the knee. In this concept, the ligaments were considered as multi-bundle structures, with nonuniform mechanical properties and zero force lengths. The purpose of the present study was to verify this new concept.

For this purpose, laxity characteristics of a human knee joint were compared as measured in an experiment and predicted in a model simulation study. In the experiment, the varus-valgus and anterior-posterior laxities of a knee-joint specimen containing the ligaments and the articular surfaces only, were determined. From this knee-joint, geometric and mechanical parameters were derived to supply the parameters for a three-dimensional quasi-static knee-joint model. These parameters included (i) the three-dimensional insertion points of bundles, defined in the four major knee ligaments, (ii) the mechanical properties of these ligament, as functions of their relative insertion orientations and (iii) three-dimensional representations of the articular surfaces. With this model the experiments were simulated. If knee-model predictions and experimental results agree, then the multi-bundle ligament models are validated, at least with respect to their functional role in anterior-posterior and varus-valgus loading of the joint.

The model described the laxity characteristics in AP-translation and VV-rotation of the cadaveric knee-joint specimen reasonably well. Both display the same patterns of laxity changes during knee flexion. Only if a varus moment of 8 N m was applied and if the tibia was posteriorly loaded, did the model predict a slightly higher laxity than that measured experimentally.

From the model-experiment comparisons it was concluded that the proposed structural representations of the ligaments and their mechanical property distributions seem to be valid for studying the anterior-posterior and varus-valgus laxity characteristics of the human knee-joint.     

Increasing quadriceps loads affect the lengths of the ligaments and the kinematics of the knee.

The relationships between the lengths of the ligaments and kinematics of the knee and quadriceps load, for low to physiologic levels of quadriceps loads, have not previously been studied. We investigated the effects of increasing levels of quadriceps force, necessary to balance increasing levels of externally applied flexion moments, on the kinematics of the tibiofemoral joint and on the separation distances between insertions of selected fibers of the major ligaments of the knee in twelve cadavera. Static measurements were made using a six-degree-of-freedom digitizer for flexion angles ranging from 0 to 120 deg in 15 deg increments. Quadriceps generated extension of the knee was performed by applying loads to the quadriceps tendon to equilibrate each of four magnitudes of external flexion moments equivalent to 8.33, 16.67, 25.00, and 33.33 percent of values previously reported for maximum isometric extension moments. The magnitude of quadriceps force increased linearly (p < 0.0001) as external flexion moment increased throughout the entire range of flexion. Anterior translation, internal rotation, and abduction of the tibia increased linearly (p < 0.0001, p < 0.001, p < 0.001) as external flexion moment and, hence, quadriceps load increased. For the fibers studied, the anterior cruciate ligament (p < 0.0076), posterior cruciate ligament (p < 0.0001), and medial collateral ligament (p < 0.0383) lengthened linearly while the lateral collateral ligament (p < 0.0124) shortened linearly as quadriceps load increased. Based on these results for low to physiologic levels of quadriceps loads, it is reasonable to assume that the ligament lengths or knee kinematics expected with higher quadriceps loads can be extrapolated.     

Knee ligament mechanical properties are not influenced by estrogen or its receptors

Women are at greater risk of tearing their knee anterior cruciate ligament (ACL) than men participating in similar athletic activities. There is currently no conclusive explanation for this disparity; however, as ACL injuries in women have been linked with estrogen fluctuations during the menstrual cycle, one hypothesis is that estrogen has a direct detrimental effect on knee ligament mechanical properties. This study investigated the influence of estrogen and its receptors (ER alpha and ER beta) on knee ligament mechanical properties. This was achieved by testing the viscoelastic and tensile mechanical properties of knee medial collateral ligaments (MCL) and ACLs from: 1) male Sprague-Dawley rats treated with either estrogen (17alpha-ethynylestradiol; 0.03 mg/kg) or an ER alpha-specific agonist (propyl pyrazole triol; 2 mg/kg), and 2) female mice with a null mutation of the gene encoding for ER beta. Estrogen treatment had no significant effects on the viscoelastic or tensile mechanical properties of the rat MCL or ACL. Similarly, pharmacological stimulation of ER alpha using a selective agonist in rats and genetic modulation of ER beta by null mutation of its gene in mice did not influence MCL or ACL properties. These data indicate that estrogen does not have a major direct effect on ligament mechanical properties. Energies for the prevention of the disproportionately high rate of knee ligament injuries in women may be better spent focusing on more established and modifiable risk factors, such as abnormalities in neuromuscular control about the knee.     

Water content alters viscoelastic behaviour of the normal adolescent rabbit medial collateral ligament.

Testing environment is an important factor in the outcome of mechanical tests on connective tissue. The purpose of this investigation was to determine the effect of ligament water content on ligament mechanical behaviour by altering the test environment. Water content of medial collateral ligament (MCLs) from 19 three-month-old New Zealand White rabbits was varied in subsets of ligaments pairs by means of immersion in 2, 10 or 25% sucrose or 0.9% phosphate-buffered saline (PBS) solutions for 1 h. One knee joint was cycled 50 times in the designated solution (experimental), while the contralateral knee (uncycled control) was simultaneously soaked in the same tank. Following cycling, the water contents of both test and control ligaments were determined. Water contents of 22 normal MCLs were determined immediately post-sacrifice and served as 'normal water content' controls. Normalized peak cyclic load changes were used as a measure of the viscoelastic behaviour of each MCL. Results demonstrated that only ligaments soaked (but not cycled) in a 10% sucrose solution had water contents (60.5 +/- 2.5%) which were statistically similar to the 22 fresh normal MCLs (63.9 +/- 6.0%). Ligaments soaked in PBS (74.0 +/- 1.3%) or 2% sucrose (69.2 +/- 2.3%) had significantly higher water contents compared to fresh normal MCLs. Ligaments with higher water contents (e.g. soaked in PBS or 2% sucrose) demonstrated greater cyclic load relaxation compared to ligaments with lower contents (e.g. soaked in 25 or 10% sucrose). Different fluid test environments can significantly alter ligament water content and, in turn, significantly affect ligament viscoelastic behaviour.     

Biomechanics of changes in ACL and PCL material properties or prestrains in flexion under muscle force-implications in ligament reconstruction

The effects of changes in cruciate ligament material and prestrain on knee joint biomechanics following ligament reconstruction surgery by a tendon are not adequately known. A 3D nonlinear finite element model of the entire knee joint was used to investigate the joint response at different flexion angles under a quadriceps force while varying ACL and PCL initial strains or material properties. The ACL and PCL forces as well as tibiofemoral contact forces/areas substantially increased with greater ACL or PCL initial strains or stiffness. The patellofemoral contact force slightly increased whereas the tibial extensor moment slightly decreased with tenser or stiffer ACL. Reverse trends were predicted with slacker ACL. Results confirm the hypotheses that changes in the prestrain of one cruciate ligament substantially influence the force in the other cruciate ligament and the entire joint and that the use of the patellar tendon (PT) as a replacement for cruciate ligaments markedly alters the joint biomechanics with trends similar to those predicted when increasing prestrains. Forces in both ACL and PCL ligaments increased as one of them became tenser or stiffer and diminished as it became slacker. These results have important consequences in joint biomechanics following ligament injuries or replacement and tend to recommend the use of grafts with smaller prestrains (i.e. slacker than intact) when using the PT as the replacement material with stiffness greater than that of replaced ligament itself.