The role of the ischiofemoral ligament in hip dislocation resistance: a cadaveric study

The ischiofemoral ligament (ISFL) contributes to posterior hip stability. However, its independent role and mechanical limitations remain unclear. This study aimed to quantify the stabilizing effect of the ISFL on posterior dislocation under progressive hip flexion. Nine fresh-frozen cadaveric hips were tested using a custom jig under three conditions: (1) capsular incision distal to the ISFL with the ISFL preserved, (2) capsular repair with the ISFL preserved, and (3) capsular incision with the ISFL transected. Condition (1) represented a posteroinferior capsular incision performed distal to the ISFL, thereby preserving the ligament. Dislocation occurrence and internal rotation angle under 2 N·m torque were measured to evaluate passive capsuloligamentous restraint at flexion angles from 0°─105° in 15° increments. In condition (1), dislocation occurred in four hips at 60°, eight at 75°, and all nine at 90°. Under condition (2), no dislocations occurred at any flexion angle. Under condition (3), dislocation began at 30° and became universal at 60°. Following capsular repair under condition (2), the internal rotation angle under constant torque increased by 10.8% relative to the pre-capsular incision state, indicating reduced resistance to internal rotation. At flexion angles ≥ 60°, the increase was 15.2%, which was statistically significant. The ISFL provides measurable resistance to posterior hip dislocation in shallow flexion; however, its stabilizing capacity diminishes in mid-to-deep flexion. Repair of the inferior capsule located distal to the ISFL sufficiently re-establishes mechanical restraint to prevent dislocation. These findings clarify the biomechanical limitations of the ISFL and emphasize the importance of inferior structures in maintaining posterior hip stability, with implications for capsular management strategies in posterior approach THA.