Effects of graded talar external rotation on distal tibial articular surface stress and contact pressure: a finite element study.

Background: Abnormal talar biomechanics alter tibiotalar local mechanics and are implicated in several ankle-foot disorders. However, the relationship between graded talar external-rotation moments and local articular changes under controlled loading remains unclear. This study investigated the variation pattern and spatial migration of local mechanical responses on the distal tibial articular surface using a patient-specific three-dimensional finite element ankle model under closed-chain single-leg stance. Methods: CT data from the right ankle and foot were used to construct a three-dimensional finite element model including cortical bone, cancellous bone, articular cartilage, and major peri-ankle ligaments using Mimics, Geomagic, and SolidWorks. Mild plantarflexion was simulated by a 6° rotation about the line connecting the malleoli. Cartilage contact was defined as low-friction frictional contact, and ligaments were represented by equivalent spring elements. After mesh sensitivity analysis, a 2.0 mm mesh was selected. Fixed support was applied to the plantar surface of the calcaneus, and axial load was distributed between the tibia and fibula at a 5:1 ratio (500 N and 100 N). Graded talar external-rotation moments of 0-5.0 N·m were applied about the tibial long axis. Peak surface stress, peak contact pressure, and relative talar rotation were extracted. Results: Peak surface stress first decreased and then increased, from 7.56 MPa at 0 N·m to 6.96 MPa at 2.2 N·m, before rising to 8.45 MPa at 5.0 N·m, indicating a relatively low-stress interval within 2.0-2.4 N·m. Peak contact pressure increased continuously from 2.55 MPa to 3.14 MPa, with a slower rise within 0-2.4 N·m and a more pronounced increase beyond 2.6 N·m. Relative talar rotation increased progressively from 0° to 1.61°. Both peak surface stress and peak contact pressure migrated overall from the anterolateral to the anteromedial tibial plafond. Conclusion: Under closed-chain single-leg stance loading, talar external rotation redistributes the local mechanical environment of the distal tibial articular surface. Peak surface stress shows a relatively low-stress interval, whereas peak contact pressure increases overall. These findings suggest that talar external rotation alters both local stress levels and the location of the primary tibiotalar contact zone, and should therefore be interpreted using multiple mechanical indicators.