Idiopathic Cam Morphology Is Not Caused by Subclinical Slipped Capital Femoral Epiphysis: An MRI and CT Study

BACKGROUND: Cam impingement as a known sequelae of slipped capital femoral epiphysis (SCFE) has led to speculation that subclinical SCFEs may be the causative factor in idiopathic cam morphology; alternatively, others have implicated an abnormal extension of the growth plate as a causative factor. HYPOTHESIS/PURPOSE: To investigate the growth plate tilt angle in 4 patient cohorts: normal patients, patients with idiopathic cam morphology (Cam(IP)), patients with cam morphology secondary to known SCFE (Cam(SCFE)), and patients with incidental findings of an asymptomatic cam (Cam(asymp)). STUDY DESIGN: Case-control study; Level of evidence, 3. METHODS: A database of 192 computed tomography scans of abdomens/pelvises of patients (ages, 5-19 years) with no known orthopaedic issues, reformatted to neutral tilt, inclination, and rotation, were utilized for the normal cohort, the Cam(asymp) cohort, and to create an age- and sex-matched control cohort. In addition, a retrospective review of all patients treated for femoroacetabular impingement (FAI) with preoperative advance imaging was conducted, and patients were separated to Cam(IP) and Cam(SCFE) cohorts. The alpha angle and tilt angle were measured on each hip. Statistical analysis was performed. RESULTS: The mean tilt angle among the normal patients was 12.1°, with 1.9% of the variation in tilt angle being explained by age; each additional year of age decreased the tilt angle by 0.27° (P = .008). The tilt angle for the Cam(SCFE) cohort (mean, 44.5°) was found to be significantly greater than both the Cam(IP) cohort (mean, 5.9°; P < .001) and the control cohort (mean, 12.8°; P < .001). The tilt angle for the Cam(IP) cohort was found to be significantly less than the control cohort (P = .003). The alpha angle and tilt angle were positively correlated in the Cam(IP) cohort, but no correlation was found in the other cohorts. The mean tilt angle of the 18 hips in the Cam(asymp) cohort was 13.9° ± 11.5° (range, –12° to 37°), with 12 hips (67%) in the tilt angle range of Cam(IP) cohort and 6 in the tilt angle range of Cam(SCFE). CONCLUSION: The proximal femoral growth plate normally has a posterior tilt that becomes more anterior through maturation. Idiopathic cam morphology has a drastically different growth plate tilt angle than cam morphology secondary to SCFE, suggesting that a majority of idiopathic cam morphology is not the result of subclinical SCFEs.