Paper #15 - ISHA Annual Scientific Meeting 2016

Femoral CAM Deformity Due to Anterior Capsular Force: A Theoretical Model with MRI and Cadaveric Correlation

Cara Beth Lee, MD, Seattle, WA UNITED STATES
Hillard T. Spencer, MD, Anaheim, CA UNITED STATES
Kirsten Nygaard, PA-C, Seattle, WA UNITED STATES

Virginia Mason Medical Center, Seattle, Washington, UNITED STATES

FDA Status Not Applicable

Summary: This MRI study localizes the maximal epiphyseal extension and apex of cam deformity adjacent to the thickest region of the anterior capsule where the superior band of the iliofemoral ligament coalesces with the zona orbicularis.

Femoral cam deformity is prevalent in athletes and associated with epiphyseal extension onto the anterosuperior femoral head-neck before physeal closure. A century ago, anatomists speculated that this femoral prominence acts as a pulley bar to withstand capsular compression in hip extension with maximal pressure occurring where the zona orbicularis (ZO) joins the superior band of the iliofemoral ligament (IFL). An animal model has shown that epiphyseal growth plates deflect laterally and distally when exposed to forces perpendicular to the direction of growth. Integrating these observations raises the question of whether capsular pressure against the immature epiphysis can induce cam formation. As a first step to explore this question, the objectives of this study are to quantify the distance from the ZO/IFL confluence to: (1) the maximal epiphyseal extension in the proximal femur in subjects with and without a femoral cam lesion (alpha angle less than 55 degrees), and (2) the apex of the cam deformity in those with the lesion. We then compared acetabular measures between the two groups, since acetabular depth influences the trajectory and degree of contact the capsule has against the proximal femur. Less acetabular coverage exposes the physis to increased capsular contact and pressure, which may stimulate physeal overgrowth. Methods: We measured the distance from the ZO/IFL confluence to the point of maximal epiphyseal extension and cam apex, if present, on magnetic resonance imaging scans of 39 subjects (47 hips, ages 15-25) with and without a cam lesion and compared acetabular depth between groups. Secondarily, we correlated imaging and anatomic findings on a cadaveric specimen. Results: In all subjects, the ZO/IFL confluence was in close proximity to the maximal epiphyseal extension (5.7 mm in the cam group and 8.1 mm in the normal offset group; mean 6.3 mm; p=0.27). Similarly, in subjects with an alpha angle greater than 55 degrees, the capsular confluence closely corresponded to the location of the apex of the cam deformity (mean, 4.98 mm; 95%CI 4.00-5.96). At the location of maximal epiphyseal extension, the acetabular roof angle was larger in the group with normal offset compared to the cam group (70.9 degrees vs. 64.1 degrees, mean difference 6.8 degrees, p=0.04). In multivariable analysis, controlling for sex, the acetabular depth was significantly less (12.5%, p=0.012) in the cam group compared to the normal offset group. Contact points were confirmed with dynamic testing and histologic evaluation in the specimen. Conclusions: The apex of the cam lesion and the point of maximal epiphyseal extension lie deep to the ZO/IFL confluence in the thickest region of the anterosuperior capsule. Acetabular depth is lower in subjects with cam deformity, which increases capsular contact with the femoral head-neck junction in this location. This lends support to a theoretical model postulating that capsular forces against an immature epiphysis may contribute to cam formation, particularly in individuals who participate in activities that repetitively tension the anterior capsule in extension and external rotation.