DESIGNING A NOVEL ANKLE PROSTHESIS

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.