The aims of Total Knee Arthroplasty (TKA) are to relieve pain and to recreate joint function and stability. Knee joint stability is intimately associated with the concept of joint motion. A stable knee joint is one that maintains an appropriate minimum contact force between the articulating surfaces throughout the functional range of motion of the joint. Thus a TKA is stable when moving through its range of motion it can carry the required functional loads without pain, maintaining contact on non-peripheral located regions and produce joint contact force of normal intensity on the polyethylene insert. Any factors causing an abnormal joint contact force and/or abnormal eccentric position of joint contact force might lead to polyethylene and component loosening. The TKA stability and function are strictly related to the interplay among the implant component alignment, articular surface geometry (flat or congruent polyethylene insert), cruciate-retaining or cruciate-substituting prosthesis design, soft tissue balancing and muscle action. Tibial component loosening continues to be a common mode of failure following TKA. Tibial component fixation is critically dependent on an equilibrium between the mechanical loads and bone resistance to them. Even if it is difficult to find a strict correlation between locomotor lower limb function and knee kinematics, TKR kinematics and position of the point of contact between femur and tibial insert are fundamental biomechanical parameters to understand the reason of extensor mechanism deficit often found in TKA patients and the risk of polyethylene wear. In the present study we will present TKA kinematics and position of the point of contact between femur and tibial insert in fixed and mobile insert focusing in TKA design features. Different knee joint kinematic patterns has been found between fixed and mobile TKA design particularly when congruent artificial joint surface is coupled with mechanical constraint such as the spine-cam mechanism.
Only recently has the mobility of the ankle joint been elucidated. Sliding/rolling of the articular surfaces and slackening/tightening of the ligaments have been explained in terms of a mechanism guided by the isometric rotation of fibres within the calcaneofibular and tibiocalcaneal ligaments. The purpose of this investigation was to design a novel ankle prosthesis able to reproduce this physiological mobility. A four-bar linkage computer-based model was used to calculate the shapes of talar components compatible with concave, flat and convex tibial components and appropriate fully congruous meniscal bearings. Three-component designs were analysed, and full congruence of the articular surfaces, appropriate entrapment of the meniscal bearing and isometry of the two ligaments were required. A convex tibial component with 5 cm arc radius gave a 2 mm entrapment together with a 9.8 mm amount of tibial bone cut, while maintaining ligament elongation within 0.03 % of the original length. The physiological patterns of joint motion and ligament tensioning were replicated. The talar component slid backwards while rolling forwards during dorsiflexion. These movements were accommodated by the forward displacement of the meniscal bearing on the tibial surface under the control of the ligaments. The complementary surfaces provide complete congruence over the entire range of flexion, such that a large contact area is achieved in all positions. To restore the physiological mobility at the ankle joint, not only should the components be designed to be compatible with original ligament pattern of tensioning, but also these should be mounted in the appropriate position. A suitable surgical technique was devised and relevant instrumentation was manufactured. Five below-knee amputated specimens replaced with corresponding prototype components showed good agreement with the model predictions. Current three-component designs using a flat tibial component and physiological talar shapes cannot be compatible with physiological ligament function.