Finite element models of the articulating surfaces of diarthrodial joints have typically depicted the joints as having a "Hertzian" type of contact, having a round convex surface sitting on a flat or concave surface with a higher radius of curvature. However, careful investigation of the major diarthrodial joints in the human body reveals that the mechanism of load bearing in these joints is different. Contact starts at the periphery of the joint at the site of an accessory peripheral circumferential structure and the contact area extends inwards as more load is applied. Finite element modelling of typical diarthrodial joints was performed and the new mechanism of load bearing was shown to be more efficient in reducing the matrix stresses and the fluid flow inside the articulating surfaces. The accessory peripheral circumferential structures act to confine the joint and to reduce the matrix fluid-flow-induced stresses.
The memscus of the knee joint is the largest of the peripheral circumfen::ntial structures. The structure of the meniscus is unique as it varies from the inner part to the outer peripheral part. The finite element models presented here are the first to show that this variation in the physical structure matches the variation in the stress states inside the different zones of the meniscus. The cartilage-like inner zone of the meniscus is under a stress state that is similar to that in the cartilage layers, high compressive stress and high fluid pressures. The ligament-like outer zone of the meniscus is subjected to high circumferential stresses aligned with the circumferential fibres. The very low shear properties of the meniscus allow it to deform and take the shape of the space between the two articulating surfaces without overstressing the articular cartilage layers. Due to the high flexibility of the meniscus and under certain loading configurations, however, a tear can develop inside the meniscus. Surgeons now tend to preserve as much as they can of these important tissues. The criteria of a successful repair mechanism are discussed based on the structure and the mechanical behaviour of the meniscus. The available repair techniques are compared and new repair techniques are presented.
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