Influence of First Ray Positioning on Ankle Contact Stresses in the Setting of a Subtalar Arthrodesis

Category: Ankle; Hindfoot Introduction/Purpose: Subtalar arthrodesis is a commonly performed procedure for primary and post-traumatic subtalar arthritis and in the setting of deformity correction. Following arthrodesis, loss of motion at the subtalar joint alters loads experienced at the ankle joint. The influence of the first ray, an essential aspect of the foot tripod, in protecting or overloading the ankle joint in the setting of a fused subtalar joint is unknown. The purpose of this study was to assess the influence of first ray positioning on cartilage contact mechanics in the ankle joint in a native subtalar joint and following subtalar arthrodesis. Methods: Twelve below-knee cadaveric specimens (6 L/6 R) were mechanically loaded in neutral ankle flexion with an MTS-applied axial load of 600N and a pneumatically applied 45N of Achilles tendon tension to simulate 2-legged standing. A calibrated piezoresistive pressure sensor placed within the ankle joint measured cartilage contact pressure on the talar dome under load for 10 frames at 2Hz. In each specimen, contact pressure was measured pre- and post-subtalar fusion and after the application of two progressively larger dorsal opening wedges (4mm and 8mm Cotton osteotomy) in both the fused/unfused conditions. Computed tomography scans of specimens were segmented to generate 3D talus surfaces on which to overlay measured contact pressures. Peak and mean contact pressure, contact area, and center of pressure were compared between fused and unfused conditions with each first-ray correction. Results: Fusion of the subtalar joint increased contact pressures and shifted the contact patch laterally on the talar dome. First ray plantarflexion increased contact pressure and medialized the contact patch in both the fused and unfused conditions. (Figure 1). Joint contact area significantly decreased (by an average 18±9.3%) in all fused conditions compared to the unfused baseline (p < 0.001). Peak pressure in the unfused conditions increased over baseline by an average of 4±11.5% with the 4mm wedge and by 11±17.9% with the 8mm wedge, however neither increase was significant. Compared to the unfused baseline, subtalar fusion significantly increased peak contact pressures (32±21.7%, 38±23.4% and 49±30.5%) and mean contact pressures (23±22.2%, 23±19.8%, 21±19.1%) for the fused baseline, fused 4mm, and fused 8mm, respectively. Conclusion: This cadaveric biomechanical study found that subtalar fusion has a much stronger effect on ankle cartilage contact stress than changes in first-ray position. Alterations of the first ray position in specimens with subtalar fusion resulted in an increase in peak and mean contact pressure relative to the native ankle joint. Plantarflexion of the first ray modestly shifted the location of peak cartilage pressure in both the subtalar-fused and unfused foot but did not significantly increase contact pressure over the non-dorsiflexed condition.