Elsevier

Journal of Biomechanics

Volume 31, Issue 12, December 1998, Pages 1127-1136
Journal of Biomechanics

Ligaments and articular contact guide passive knee flexion

https://doi.org/10.1016/S0021-9290(98)00119-5Get rights and content

Abstract

The aim of this study was to test the hypothesis that the coupled features of passive knee flexion are guided by articular contact and by the isometric fascicles of the ACL, PCL and MCL. A three-dimensional mathematical model of the knee was developed, in which the articular surfaces in the lateral and medial compartments and the isometric fascicles in the ACL, PCL and MCL were represented as five constraints in a one degree-of-freedom parallel spatial mechanism. Mechanism analysis techniques were used to predict the path of motion of the tibia relative to the femur. Using a set of anatomical parameters obtained from a cadaver specimen, the model predicts coupled internal rotation and ab/adduction with flexion. These predictions correspond well to measurements of the cadaver specimen’s motion. The model also predicts posterior translation of contact on the tibia with flexion. Although this is a well-known feature of passive knee flexion, the model predicts more translation than has been reported from experiments in the literature. Modelling of uncertainty in the anatomical parameters demonstrated that the discrepancy between theoretical predictions and experimental measurement can be attributed to parameter sensitivity of the model. This study shows that the ligaments and articular surfaces work together to guide passive knee motion. A principal implication of the work is that both articular surface geometry and ligament geometry must be preserved or replicated by surgical reconstruction and replacement procedures to ensure normal knee kinematics and by extension, mechanics.

Introduction

The functional anatomy of the knee must be understood when designing or assessing surgical procedures. The mechanical functions of the structures at the tibiofemoral joint include guiding the relative motion of the tibia and femur and transmitting load between these two bones. Surgeons seek to preserve or restore both of these mechanical functions during joint reconstruction or replacement. During surgery, both the geometry of joint structures and their mechanical properties are often changed. In anterior cruciate ligament (ACL) reconstruction, geometric parameters include the sites of origin and insertion of the graft, while in total knee arthroplasty, geometric parameters include the shapes and sizes of the articular surfaces and their locations relative to the ligament and muscle tendon insertions. Intra-operatively, the surgeon flexes the knee passively and observes knee motion to assess component or graft placement. If flexion is resisted or if the knee appears unstable, component alignment or graft length may be adjusted. How geometric changes to the joint structures affect the three-dimensional movement of the joint has not been explained completely.

It is not clear from the literature which anatomical structures guide the knee in passive flexion and how their geometric arrangement produces the unique path of passive knee motion. The knee can be moved passively through a full range of flexion/extension (0–130°) with minimal resistance. Both internal rotation (Meyer, 1853) and posterior translation of the femur on the tibia (Weber and Weber, 1836) are coupled to flexion. This movement path must be guided by the anatomical structures of the tibiofemoral joint. Several mathematical models have been developed that predict three-dimensional motion of the knee by solving the equations of mechanical equilibrium (and, sometimes, simultaneously solving equations of geometric constraint) (Andriacchi et al., 1983; Blankevoort et al., 1991; Essinger et al., 1989; Wismans et al., 1980). However, these models have been developed to study the response of the joint to moderate loads and they have not been used to predict the unique three-dimensional path of motion of the unloaded joint. In a contrasting approach, the knee has been studied as a mechanical linkage or mechanism (Goodfellow and O'Connor, 1978; Huson 1974; Menschik, 1974; O'Connor et al., 1989; Strasser, 1917). These models predict knee motion from the geometry of the anatomical structures. By comparing model predictions to experimental measurements of knee motion, hypotheses concerning which structures guide knee motion and the influence of changes to the geometry of the structures on knee motion can be tested. Although two-dimensional linkage models have successfully explained posterior translation of the femur on the tibia, no linkage model has yet predicted the simultaneous coupled internal tibial rotation and posterior femoral translation that characterizes passive knee flexion. To explain the relationship between knee anatomy and knee flexion completely, a linkage model must be developed that predicts these simultaneous coupled features of passive knee flexion from the geometry of the anatomical structures.

The aim of this study is to test the hypothesis that passive knee motion is guided by the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL) and medial collateral ligament (MCL) and articular contact in the medial and lateral compartments. This article describes how this hypothesis was tested by formulating and analyzing a three-dimensional mechanism model of the knee and by comparing its predictions to measurements of knee motion.

Section snippets

Method

A three-dimensional mechanism model of the knee was formulated by making assumptions about joint anatomy based on evidence in the literature. Experimental studies suggest that within the ACL and PCL (Fuss, 1989; Rovick et al., 1991; Sidles et al., 1988) and the MCL (Rovick et al., 1991) there is a fascicle that remains nearly isometric through the range of flexion of the knee. The posterior femoral condyles are nearly spherical (Kurosawa et al., 1985) and the tibial condyles are curved slightly

Results

The model predicts coupled internal rotation of the knee in passive flexion. Motion is predicted from 6° of hyperextension to 106° of flexion. As the knee is flexed, the tibia rotates internally and ab/adducts slightly (Fig. 5). Experimental measurements of internal/external rotation and ab/adduction on the knee from which the model parameters were taken match the model predictions closely (Fig. 5).

The model also predicts posterior translation of the femur on the tibia. In both compartments,

Discussion

To test the hypothesis that passive knee motion is guided by the ACL, PCL and MCL and articular contact in the medial and lateral compartments, the knee was modelled as a three-dimensional parallel spatial mechanism. In a novel approach, a systematic method was used to model joint structures as links and pairs in a spatial mechanism. This approach can easily be applied to any joint in which the appropriate assumptions of rigidity and isometricity can be made.

The development of the

Acknowledgments

The authors thank Prof. K. H. Hunt, Department of Mechanical and Manufacturing Engineering, University of Melbourne and Dr Ross McAree, Department of Engineering Science, University of Oxford, for their advice on theoretical kinematics, and CAMARC-II, the Arthritis and Rheumatism Council (UK), the Fonds FCAR (Québec, Canada), and the Overseas Research Studentship Scheme (UK) for support.

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