Contributions of the Capsule and Labrum to Hip Mechanics in the Context of Hip Microinstability.

Background: Hip microinstability and labral pathology are commonly treated conditions with increasing research emphasis. To date, there is limited understanding of the biomechanical effects of the hip capsule and labrum on controlling femoral head motion.

Purpose/hypothesis: The purpose of this study was to determine the relative role of anterior capsular laxity and labral insufficiency in atraumatic hip microinstability. Our hypotheses were that (1) labral tears in a capsular intact state will have a minimal effect on femoral head motion and (2) the capsule and labrum work synergistically in controlling hip stability.

Study Design: Controlled laboratory study.

Methods: Twelve paired hip specimens from 6 cadaveric pelvises (age, 18-41 years) met the inclusion criteria. Specimens were stripped of all soft tissue except the hip capsule and labrum, then aligned, cut, and potted using a custom jig. A materials testing system was used to cyclically stretch the anterior hip capsule in extension and external rotation, while rotating about the mechanical axis of the hip. Labral insufficiency was created with a combined radial and chondrolabral tear under direct visualization. A motion tracking system was used to record hip internal-external rotation and displacement of the femoral head relative to the acetabulum in the anterior-posterior, medial-lateral, and superior-inferior directions. Testing variables included baseline, postventing, postcapsular stretching, and postlabral insufficiency.

Results: When comparing the vented state with each experimental pathologic state, increases in femoral head motion were noted in both the capsular laxity state and the labral insufficiency state. The combined labral insufficiency and capsular laxity state produced statistically significant increases ( < .001) in femoral head translation compared with the vented state in all planes of motion.

Conclusion: Both the anterior capsule and labrum play a role in hip stability. In this study, the anterior hip capsule was the primary stabilizer to femoral head translation, but labral tears in the setting of capsular laxity produced the most significant increases in femoral head translation.

Clinical Relevance: This study provides a physiologic biomechanical assessment of the hip constraints in the setting of hip microinstability. It also sheds light on the importance of the hip capsule in the management of labral tears. Our study demonstrates that labral tears in isolation provide minimal changes in femoral head translation, but in the setting of a deficient capsule, significant increases in femoral head translation are seen, which may result in joint-related symptoms.