Biomechanical evaluation of external ankle arthrodesis contact area and pressure distribution

The ankle is a joint that is often adversely affected by arthritis. This results in a painful and stiff joint. Ankle replacement is only suitable in certain situations, and so the surgical technique of choice is often arthrodesis, where the two bones are fused together to remove the joint. Key factors that affect the successful outcome of such procedures are the levels of contact and pressure across the joint during the arthrodesis compression period. This study evaluates the difference in contact area and pressures that can be expected during an external arthrodesis procedure. Comparisons are obtained for different shaped joint surfaces, namely flat or curved, and compression pin placement, namely central to the talar dome or anterior. The effect of the Achilles tendon upon this structure is also considered. The mechanical stability of these constructs was then evaluated resulting in load/deflection curves for the different situations. Contact areas and pressures were measured by installing models representing the ankle joint into a test rig specifically designed for the purpose. Pressure sensitive film was used to measure the pressure across the joint, whilst measured loads were applied using compression pins through the talar model to achieve compression. Loads were then applied to these models to simulate motion forces at the ankle, and the resulting deflections recorded. Results indicate that there is little difference in terms of joint contact area and pressure between curved and flat cut scenarios. An anterior talar pin position does, however give the best contact area for both flat and curved shapes. Mechanical stability was also influenced by pin position, with anterior pins giving the optimum resistance to motion when combined with an Achilles tendon load. Curved joint shapes gave a marked improvement in stability when tested for rotation, previously identified as the weakest and most important micromotion at the arthrodesis site. (C) 2004 Elsevier B.V. All rights reserved.