Role of the Acetabular Labrum and the Iliofemoral Ligament in Hip Stability An In Vitro Biplane Fluoroscopy Study

Background: Recent biomechanical reports have described the function of the acetabular labrum and iliofemoral ligament in providing hip stability, but the relative stability provided by each structure has not been well described. Hypothesis: Both the iliofemoral ligament and acetabular labrum are important for hip stability by limiting external rotation and anterior translation, with increased stability provided by the iliofemoral ligament compared with the acetabular labrum. Study Design: Controlled laboratory study. Methods: Fifteen fresh-frozen male cadaveric hips were utilized for this study. Each specimen was selectively skeletonized down to the hip capsule. Four tantalum beads were embedded into each femur and pelvis to accurately measure hip translations and rotations using biplane fluoroscopy while either a standardized 5 N.m external or internal rotation torque was applied. The hips were tested in 4 hip flexion angles (10 degrees of extension, neutral, and 10 degrees and 40 degrees of flexion) in the intact state and then by sectioning and later repairing the acetabular labrum and iliofemoral ligament in a randomized order. Results: External rotation significantly increased from the intact condition (41.5 degrees +/- 7.4 degrees) to the sectioned iliofemoral ligament condition (54.4 degrees +/- 6.6 degrees) and both-sectioned condition (61.5 degrees +/- 5.7 degrees; P < .01), but there was no significant increase in external rotation when the labrum alone was sectioned (45.6 degrees +/- 5.9 degrees). The intact and fully repaired conditions were not significantly different. External rotation and internal rotation significantly decreased when the hip flexion angle decreased from 40 degrees of flexion to 10 degrees of extension (P < .01) regardless of sectioned condition. Anterior translation varied significantly across sectioned conditions but not across flexion angles (P < .001). The ligament-sectioned (1.4 +/- 0.5 mm), both-sectioned (2.2 +/- 0.2 mm), and labrum-repaired (1.1 +/- 0.2 mm) conditions all resulted in significantly greater anterior translation than the intact condition (-0.4 +/- 0.1 mm) (P < .001). Conclusion: The iliofemoral ligament had a significant role in limiting external rotation and anterior translation of the femur, while the acetabular labrum provided a secondary stabilizing role for these motions. Clinical Relevance: These results suggest that, if injured, both the acetabular labrum and iliofemoral ligament should be surgically repaired to restore native hip rotation and translation. In addition, a careful repair of an arthroscopic capsulotomy should be performed to avoid increased external hip rotation and anterior translation after arthroscopy.