An extended OpenSim knee model for analysis of strains of connective tissues.

BACKGROUND: OpenSim musculoskeletal models provide an accurate simulation environment that eases limitations of in vivo and in vitro studies. In this work, a biomechanical knee model was formulated with femoral articular cartilages and menisci along with 25 connective tissue bundles representing ligaments and capsules. The strain patterns of the connective tissues in the presence of femoral articular cartilage and menisci in the OpenSim knee model was probed in a first of its kind study. METHODS: The effect of knee flexion (0°-120°), knee rotation (- 40° to 30°) and knee adduction (- 15° to 15°) on the anterior cruciate, posterior cruciate, medial collateral, lateral collateral ligaments and other connective tissues were studied by passive simulation. Further, a new parameter for assessment of strain namely, the differential inter-bundle strain of the connective tissues were analyzed to provide new insights for injury kinematics. RESULTS: ACL, PCL, LCL and PL was observed to follow a parabolic strain pattern during flexion while MCL represented linear strain patterns. All connective tissues showed non-symmetric parabolic strain variation during rotation. During adduction, the strain variation was linear for the knee bundles except for FL, PFL and TL. CONCLUSIONS: Strains higher than 0.1 were observed in most of the bundles during lateral rotation followed by abduction, medial rotation and adduction. In the case of flexion, highest strains were observed in aACL and aPCL. A combination of strains at a flexion of 0° with medial rotation of 30° or a flexion of 80° with rotation of 30° are evaluated as rupture-prone kinematics.

Estimation of forces on anterior cruciate ligament in dynamic activities.

In this work, a nonlinear strain rate dependent plugin developed for the OpenSim® platform was used to estimate the instantaneous strain rate (ISR) and the forces on the ACL's anteromedial (aACL) and posterolateral (pACL) bundles during walking and sudden change of direction of running termed as 'plant-and-cut' (PC). The authors obtained the kinematics data for walking via optical motion capture. PC movements, along with running kinematics, were obtained from the literature. A nonlinear plugin developed for ligaments was interfaced with OpenSim® platform to simulate walking and PC motions with a flexed knee and an extended knee. PC phase is sandwiched between an approach phase and take-off phase and was studied at various event velocities (1.8, 3, and 4.2 m s-1), and angles of PC (23°, 34°, and 45°) as encountered in adult ball games. In both cases of PC-with-extended knee and PC-with-flexed-knee, the maximum forces on both the ACL bundles were observed after the take-off phase. A maximum force of ~ 35 N kg-1 of body weight (BW) was observed on aACL after the take-off phase for an event velocity of 4.2 m s-1. In the posterolateral bundle (pACL), the maximum forces (~ 40 N kg-1 of BW) were observed towards the end of the mid-swing phase (after the take-off phase) for the various combinations of the parameters studied. The forces observed in the simulation of PC-with-flexed-knee and PC-with-extended-knee has resulted in magnitude higher than sustainable by the adults. This study is novel in attempting to incorporate differing rates-of-strain that have been shown to alter soft tissue properties into the OpenSim® musculoskeletal model. The proposed model can be used by researchers to predict the forces during various kinematic activities for other soft tissues.

A cadaveric study on the rate of strain-dependent behaviour of human anterior cruciate ligament.

PURPOSE: Failure of anterior cruciate ligament often occurs in young sports personnel hampering their career. Such ACL ruptures are quite prevalent in sports such as soccer during dynamic loading which occurs at more than one rate of loading. In this work, a structural constitutive equation has been used to predict the forces acting on ACL for different rates of loading. METHODS: Ligaments with distal femur and proximal tibia were subjected to tensile loading to avoid crushing of tissue ends and slipping at higher rates of strain. Custom designed cylindrical grippers were fabricated to clamp the distal femur and proximal tibial bony sections. To estimate parameters for the model, eighteen fresh cadaveric femur-ACL-tibia complex (FATC) samples were experimented on by pure tensile loading at three orders of rates of strain viz., 0.003, 0.03, and 0.3 s^-1. The experimental force-elongation data was used to obtain parameters for De-Vita and Slaughter's equation. The model was validated with additional tensile experiments. RESULTS: Statistical analysis demonstrated failure stress, Young's modulus and volumetric strain energy to vary significantly as a function of rate of strain. Midsection failure was observed only in samples tested at 0.03 s^-1. Femoral or tibial insertion failure were observed in all other experiments irrespective of rate of strain. CONCLUSION: Human FATC samples were tensile tested to failure at three rates of strain using custom-designed cylindrical grippers. A structural model was used to model the data for the ACL behaviour in the linear region of loading to predict ligament behaviour during dynamic activities in live subjects.

Volumetric locking free 3D finite element for modelling of anisotropic visco-hyperelastic behaviour of anterior cruciate ligament.

Solids such as polymers, soft biological tissues display visco-hyperelastic, isochoric and finite deformation behaviour. The incompressibility constraint imposed severe restriction on the displacement field results in volumetric locking. Many techniques have been developed to address the issue such as reduced integration, mixed formulation, B-Bar and F-Bar methods, each of them with their own merits and demerits. In this work, we have developed a 3D finite element (hereby referred as J-Bar method) to counter volumetric locking in visco-hyperelastic solids. To validate the proposed J-Bar method, rheological characteristics of the human anterior cruciate ligament (ACL) were predicted and compared with the experimental results.

A Review on Biomechanics of Anterior Cruciate Ligament and Materials for Reconstruction.

The anterior cruciate ligament is one of the six ligaments in the human knee joint that provides stability during articulations. It is relatively prone to acute and chronic injuries as compared to other ligaments. Repair and self-healing of an injured anterior cruciate ligament are time-consuming processes. For personnel resuming an active sports life, surgical repair or replacement is essential. Untreated anterior cruciate ligament tear results frequently in osteoarthritis. Therefore, understanding of the biomechanics of injury and properties of the native ligament is crucial. An abridged summary of the prominent literature with a focus on key topics on kinematics and kinetics of the knee joint and various loads acting on the anterior cruciate ligament as a function of flexion angle is presented here with an emphasis on the gaps. Briefly, we also review mechanical characterization composition and anatomy of the anterior cruciate ligament as well as graft materials used for replacement/reconstruction surgeries. The key conclusions of this review are as follows: (a) the highest shear forces on the anterior cruciate ligament occur during hyperextension/low flexion angles of the knee joint; (b) the characterization of the anterior cruciate ligament at variable strain rates is critical to model a viscoelastic behavior; however, studies on human anterior cruciate ligament on variable strain rates are yet to be reported; (c) a significant disparity on maximum stress/strain pattern of the anterior cruciate ligament was observed in the earlier works; (d) nearly all synthetic grafts have been recalled from the market; and (e) bridge-enhanced repair developed by Murray is a promising technique for anterior cruciate ligament reconstruction, currently in clinical trials. It is important to note that full extension of the knee is not feasible in the case of most animals and hence the loading pattern of human ACL is different from animal models. Many of the published reviews on the ACL focus largely on animal ACL than human ACL. Further, this review article summarizes the issues with autografts and synthetic grafts used so far. Autografts (patellar tendon and hamstring tendon) remains the gold standard as nearly all synthetic grafts introduced for clinical use have been withdrawn from the market. The mechanical strength during the ligamentization of autografts is also highlighted in this work.

Effect of preservation methods on tensile properties of human femur-ACL-tibial complex (FATC) - a cadaveric study on male subjects.

PURPOSE: Deep freezing and storing in formalin are some of the common techniques of human tissue preservation. However, the preservation modes affect the biomechanical properties of the tissues. In this work, the effects of the above-stated preservation tech- niques are compared with that of fresh cadaveric samples. METHODS: FATC samples from male cadavers of age between 60 and 70 years were tested under tensile loading at a strain rate of 0.8 s-1. Fourteen FATC samples from soft embalmed cadavers were preserved for 3 weeks by two methods: (a) 10% formalin and (b) deep freezing at -20 ° C followed by thawing. Seven FATC samples from fresh ca- davers were experimented as control samples. The results were evaluated by a two-stage statistical process of Kruskal-Wallis H test and Mann-Whitney U-test. RESULTS: It was observed that the failure force of fresh cadavers was the highest while that of preserved samples were approximately half the value. Failure elongation of frozen samples exceeded fresh samples while formalin samples failed at lesser elongations. Higher incidence of tibial insertion point or mid-section failures were observed in fresh samples while the higher incidence of ruptures at femoral insertion point was observed in the two preservation methods. CONCLUSION: Tensile properties of fresh tissues vary significantly from that of formalin preserved or frozen preserved samples.