title: Femoral cam deformity due to anterior capsular force: A theoretical model with MRI and cadaveric correlation.
abstract: BACKGROUND
  Cam deformity is 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 pressure concentrated where the zona orbicularis (ZO) joins the iliofemoral ligament (IFL). An animal model has shown that growth plates deflect laterally and distally when exposed to forces perpendicular to growth. These observations raise the question of whether capsular pressure against the epiphysis can stimulate cam formation.
  
  
  OBJECTIVE
  The purposes are to measure: (1) the distance from the ZO/IFL confluence to the maximal epiphyseal extension (MEE) and cam apex; and (2) acetabular depth at this location, since less coverage increases capsular contact on the physis.
  
  
  METHODS
  MRI scans of 39 subjects (47 hips) were measured. Acetabular depth was compared between those with and without a cam deformity. Secondarily, anatomic findings were correlated on a cadaveric specimen.
  
  
  RESULTS
  The cam apex and MEE were adjacent to the ZO/IFL confluence in all subjects (mean, 6.3 mm). Controlling for sex, acetabular depth was less (12.5%, p = 0.012) in the group with cam deformity. Contact points were confirmed in the specimen.
  
  
  CONCLUSIONS
  The cam apex and MEE occur at the ZO/IFL confluence in the thickest region of the anterosuperior capsule and vary with acetabular depth. This supports a theoretical model postulating that capsular forces against an immature epiphysis may induce cam formation, particularly in individuals who repetitively tension the anterior capsule.
authors.affiliation: Center for Hip Preservation, Virginia Mason Medical Center, 925 Seneca Street, Lindeman Pavilion, Level 6, Seattle, WA 98111, USA.
authors.forename: Cara Beth
authors.initials: CB
authors.lastname: Lee
authors.affiliation: Southern California Permanente Medical Group, Orthopedics Department, 3460 E, La Palma Avenue, Anaheim, CA 92806, USA.
authors.forename: Hillard T
authors.initials: HT
authors.lastname: Spencer
authors.affiliation: Center for Hip Preservation, Virginia Mason Medical Center, 925 Seneca Street, Lindeman Pavilion, Level 6, Seattle, WA 98111, USA.
authors.forename: Kirsten F
authors.initials: KF
authors.lastname: Nygaard
keywords: ARA, acetabular roof angle; Cam deformity; EER, epiphyseal extension ration; FAI; FAI, femoroacetabular impingement; Femoroacetabular impingement; Hip capsule; Hip impingement; IFL, iliofemoral ligament; MEE, maximal epiphyseal extension; ZO, zona orbicularis