The Role of Anterior Capsular Laxity in Hip Microinstability: A Novel Biomechanical Model

Background: Hip microinstability is an increasingly recognized source of hip pain and disability. Although the clinical entity has been well described, the pathomechanics of this disease remain poorly understood. Purpose/Hypothesis: The purpose of this study was to determine the role of capsular laxity in atraumatic hip microinstability. Our hypothesis was that cyclic stretching of the anterior hip capsule would result in increased hip range of motion and femoral head displacement. Study Design: Controlled laboratory study. Methods: In this study, 7 hip specimens met inclusion criteria (age, 18-46 years). Specimens were stripped of all soft tissue, aligned, cut, and potted by use of 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. A motion tracking system was used to record hip rotation and displacement of the femoral head relative to the acetabulum in the anterior-posterior, medial-lateral, and superior-inferior directions. Testing was conducted at baseline, after venting, and after capsular stretching. Results: With the hip in anatomic neutral alignment, cyclic stretching of the anterior hip capsule resulted in increased hip rotation (P < .001). Femoral head displacement significantly increased relative to the vented state in the medial-lateral (P < .001), anterior-posterior (P = .013), and superior-inferior (P = .036) planes after cyclic stretching of the anterior hip capsule. Conclusion: The anterior hip capsule plays an important role in controlling hip rotation and femoral head displacement. This study is the first to display significant increases in femoral head displacement through a controlled cyclic stretching protocol of the anterior hip capsule. Clinical Relevance: This study is directly applicable to the treatment of atraumatic hip microinstability. The results quantitatively define the relative importance of the hip capsule in controlling femoral head motion. This allows for a better understanding of the pathophysiological process of hip microinstability and serves as a platform to develop effective surgical techniques for treatment of this disease.