Validity and Clinical Consequences of a Rotational Mechanism for Slipped Capital Femoral Epiphysis

Background: Recent anatomic data supports a mechanism for slipped capital femoral epiphysis (SCFE) where the metaphysis rotates on the epiphysis through the eccentrically located epiphyseal tubercle as a pivot. The validity of such a mechanism and its clinical consequences have not been well investigated. Methods: This cadaveric study defined the normal location of the calcar ridge line in 100 paired femora and compared them with 11 SCFE specimens, and with 25 immature femora where the metaphysis was rotated 30, 60, and 90 degrees on the epiphysis to model progressive SCFE. Coronal, sagittal, and axial plane deformity were measured on the rotational model to define the characteristic deformity caused by rotation, and lateral epiphyseal foramina stretch was measured to quantify the protective effect of an eccentric pivot. Results: The posterior displacement of the fovea with respect to the calcar ridge line was 1 +/- 5 mm for the controls, versus 23 +/- 10 mm for the SCFE specimens (P < 0.0005), and posterior displacement increased with incremental rotation in the rotational model (P < 0.0005). The rotational model found minimal varus deformity, but substantial extension and retroversion, with deformities of 0 +/- 5, -16 +/- 12, and -38 +/- 9 degrees, respectively, at 90 degrees of rotation. The eccentric pivot mitigated lateral epiphyseal vessel stretch by 55% to 70%. Conclusions: This study provides strong anatomic support for a rotational mechanism for stable SCFE. When the metaphysis is rotated on the epiphysis acutely, minimal varus deformity is created, while substantial retroversion occurs. As this rotation occurs the eccentric pivot protects the lateral epiphyseal vessels, and likely accounts for the low rate of avascular necrosis observed in stable SCFE. Clinical Relevance: If SCFE is treated in an open manner then the presence of a chronic rotational deformity should be considered. When planning osteotomies for deformity after a stable SCFE, any suspected varus component should be carefully investigated as it may represent retroversion deformity brought into view by external rotation posturing of the leg.