Contact area, pressure distribution and mechanical stability in external arthrodesis of the ankle joint

Abstract

The ankle joint is often affected by arthritis, giving a joint that is painful, stiff, and restricts movement. This can result in a huge loss of mobility for the sufferer. Unlike replacement of the hip, the replacement of a diseased ankle joint is not as straightforward and the outcomes do not reach the same success levels. The preferred surgical choice is arthrodesis, a procedure whereby the two bones forming the joint are fused together to eliminate the joint and hence pain. The success of the procedure is dependent upon several factors, two of the most significant being the levels of contact area and pressure achieved during the compression period, during which bone growth occurs across the two bones being compressed together. These factors influence joint stability and micromotion at the bone to bone interface during this growth phase. This study investigates the levels of contact areas and pressures that can be achieved for different arthodesis variables. These variables include the joint shape, which can be curved or flat, and the position of the compression pin within the talus, namely anteriorly or centrally positioned with reference to the talar dome. Influence of the Achilles tendon in joint stability is also investigated. A test rig was developed allowing load/deflection curves to be determined for various configurations of these variables. Models representing the bones under consideration, together with pressure sensitive film, allowed measurement of contact areas and pressures within the joint under compression, achieved using pins and instrumented compression rods. Results indicate there is little significant variation in contact area and pressure for the different shaped joint cuts, however, if the talar pin is placed in a more anterior position then the contact area can be improved over a centrally positioned pin. Anterior pin placement also gives increased resistance to motion and mechanical stability. It has been established that the athrodesis construct is especially weak in terms of rotation about the tibial axis, and the results from this study indicate that through the use of a curved joint shape the resistance to this motion can be improved greatly