Influence of First Ray Positioning on Ankle Contact Stresses in the Setting of a Subtalar Arthrodesis: A Cadaveric...

Loss of subtalar (ST) motion after arthrodesis for cavovarus can alter loads through the ankle. In this context, first ray position has the potential to protect or further overload the ankle joint. This study's purpose was to assess the influence of first ray plantarflexion on tibiotalar cartilage contact mechanics in a native ST joint and following ST arthrodesis.

Twelve below-knee cadaveric specimens were mechanically loaded to simulate 2-legged standing (neutral ankle flexion, 600 N axial load, 45 N of Achilles tendon tension). A piezoresistive pressure sensor measured cartilage contact pressure in the loaded ankle joint both before and after ST fusion, and before and after application of a 4-mm or 8-mm dorsal opening wedge (Cotton osteotomy) in each fusion condition. Peak and mean contact pressure, contact area, and center of pressure were compared between ST-fused and unfused conditions with each first-ray correction.

Peak pressure in the unfused ST condition moved anteromedially and increased slightly over baseline by an average of 4% ± 11.5% and 11% ± 17.9% with the 4-mm and 8-mm wedges, respectively. Relative to the unfused baseline conditions, ST fusion lateralized and decreased joint contact area by an average of 18% ± 9.3% (p < 0.001). This resulted in significantly increased peak (32% ± 21.7%, 38% ± 23.4%, and 49% ± 30.5%, P < .05) and mean contact pressures (23% ± 22.2%, 23% ± 19.8%, 21% ± 19.1%, P < .05) for the fused baseline, fused 4 mm, and fused 8 mm, respectively.

Overall, ST fusion had a greater effect on ankle contact pressures than changes in first-ray position. ST fusion combined with increasing plantarflexion of the first ray shifted peak contact stress anteromedially. These findings, although speculative because of the utilization of non-deformed specimens, and the use of a static loading model that does not account for dynamic muscle forces during gait, are important in the setting of a ST arthrodesis for forefoot driven cavus. As the forefoot assumes a more cavus position in the absence of ST motion, the increased contact stress may put the ankle joint at higher risk of arthritic progression.

These findings suggest that subtalar arthrodesis inherently increases tibiotalar contact stresses, potentially predisposing the ankle to degenerative changes regardless of the degree of first ray correction.