Background: There is limited literature available to assess the impact of talar component rotation on total ankle contact biomechanics.
Materials and methods: Six male cadaveric below-knee specimens were implanted with Agility(R) total ankles. The sequence of talar rotation for each specimen was randomized between: Neutral, 7.5 degrees internal and 7.5 degrees external rotation. Contact pressure was measured using Tekscan ankle sensors during sequential static axial loadings and 10 simulated dynamic strides under 650 N axial load.
Results: The peak pressure (PP) increased for the internally (PP(static)=7.0 +/- 0.27 MPa (mean +/- SD), p < 0.001; PP(dynamic)=7.8 +/- 0.22 MPa, p = 0.001) and externally rotated talar component positions (PP(static)=6.2 +/- 0.22 MPa, p = 0.011; PP(dynamic)=7.6 +/- 0.29 MPa, p = 0.004) as compared to neutral (PP(static) =5.5 +/- 0.13 MPa; PP(dynamic) = 6.3 +/- 0.11 MPa). The contact area under 650 approximately N load was reduced for both talar component internal (97.38 +/- 17.7 mm(2), p = 0.001) and external rotation (152.66 +/- 16.8 mm(2), p = 0.022) as compared to neutral (190.02 +/- 13.8 mm(2)). There was a significant rotational torque for the malrotated talar components as compared to neutral, that increased with axial loading (p = 0.044).
Conclusion: Near the extremes of talar malrotation, there was a consistent change from a continuous tibiotalar contact area to a pattern of two-point contact; the orientation of which opposed the direction of talar component malrotation. Talar component malrotation resulted in: increased peak pressure, decreased contact area and increased rotational torque that resisted the malrotation.
Clinical relevance: Talar component malrotation may contribute to premature polyethylene wear as well as potential talar loosening secondary to the rotational torque generated as the geometry of the prosthesis attempts to seek congruency.