Defining the three-dimensional deformity in slipped capital femoral epiphysis
The purpose of this study was to define a novel angle measure (theta) characterizing true slipped capital femoral epiphysis (SCFE) deformity; use theta to differentiate between SCFE hips, contralateral unaffected hips, and normal hips; and to compare theta to the Southwick slip angle (SSA). 3DCT reconstructions of the pelvis and femurs were obtained and pelvic position was standardized. The center point and direction vector of the femoral epiphysis was determined. The femoral neck axis was defined. The angle between the femoral neck axis and epiphysis vector defined the 3D angle of deformity (theta). The 3D translation of the femoral epiphysis, measured as a percentage of femoral neck diameter, was measured in three planes. The average theta angle was significantly greater in SCFE hips (44.9±22.5°) compared to control (14.5±8.8°) or normal (14.0±6.5°) hips (p<0.001). There was no significant difference in theta angle between control and normal hips (p=0.83). Theta angle correlated strongly with SSA (rs=0.774, p<0.001). Its high but imperfect correlation with SSA may indicate theta as a better measure, implicating SSA as underestimating the true deformity in nearly 25% of cases.