The Effectiveness of External Rotators and Ischiofemoral Ligament for the Prevention of Hip Dislocation in Hip Flexion Position: A Cadaveric Study

Abstract BackgroundThe purpose of this study was to examine the magnitude of the impact of each external rotator muscle and ischiofemoral ligament on prevention of joint dislocation, depending on the hip flexion angle.MethodNine normal hips were studied; the pelvis was fixed in the lateral decubitus position. An intramedullary nail was inserted into the femur and fixed using screws. A circular plate with a protractor was fixed on the nail. The circular plate was pulled by 3.15 Nm force in the direction of internal rotation, and this angle was defined as the baseline angle. External rotators of the right hip were resected stepwise using the posterior approach; in contrast, external rotators of the left hip were resected stepwise in the reverse order, and finally the ischiofemoral ligament was cut. After each muscle and ischiofemoral ligament resection, the increase in the angle by the traction of was measured at the neutral and 30° and 60° hip flexion positions.ResultsIn posterior approach, there was a significant difference between measurements after piriformis and ischiofemoral ligament resection at 0° of flexion (p=0.02). At 30° of flexion, there were significant differences between measurements after each external rotator and ischiofemoral ligament resection (p<0.01). At 60° of flexion, there were significant differences between ischiofemoral ligament resection and piriformis or inferior gemellus resection (p=0.04, p=0.02, respectively). In the reverse order of the posterior approach, there were significant differences between ischiofemoral ligament resection and the obturator externus, inferior gemellus, or obturator internus at 0° of flexion (p<0.01, p<0.01, p=0.01, respectively). At 30° of flexion, there was a significant difference between the ischiofemoral ligament and each external rotator (p<0.01). ConclusionThe ischiofemoral ligament is the main restrictor of posterior dislocation. The combination of the piriformis and obturator internus may restrict internal rotation at 0°and 60° of flexion.