Morphologic Features of the Contralateral Femur in Patients With Unilateral Slipped Capital Femoral Epiphysis Resembles Mild Slip Deformity: A Matched Cohort Study.

Background: Hip osteoarthritis has been reported in the contralateral hip in patients who had been treated for unilateral slipped capital femoral epiphysis (SCFE) during adolescence. Although this might be related to the presence of a mild deformity, the morphologic features of the contralateral hip in unilateral SCFE remains poorly characterized.

Questions/purposes: Do measurements of (1) femoral head-neck concavity (α angle and femoral head-neck offset), (2) epiphyseal extension into the metaphysis (epiphyseal extension ratio and epiphyseal angle), and (3) posterior tilt of the epiphysis (epiphyseal tilt angle) differ between the contralateral asymptomatic hips of patients treated for unilateral SCFE and hips of an age- and sex-matched control population without a history of hip disease?

Methods: From January 2005 to May 2015, 442 patients underwent surgical treatment for SCFE at our institution. Patients were included in this study if they had a pelvic CT scan and unilateral SCFE defined by pain or a limp in one hip without symptoms or obligatory external rotation with flexion in the contralateral hip and no evidence of SCFE findings on available radiographs. Seventy-two (16%) patients had a pelvic CT scan; however, 32 patients with bilateral involvement and one patient with CT imaging of inadequate quality for multiplanar reformatting were excluded. Thirty-nine control subjects were identified from a preexisting database of patients who underwent pelvic CT between January 2008 and January 2014 for assessment of abdominal pain in the setting of suspected appendicitis. Patients in the contralateral asymptomatic hip group then were matched to control subjects using a modified nearest-neighbor approach based on sex and age. Patients in the contralateral asymptomatic hip group were separated in males and females and control subjects were assigned to an appropriate sex category. Then subjects closest in age were matched with each patient. If more than one subject was available as a match for a given patient, the control subject with the closest BMI was selected. The contralateral asymptomatic hip and matched groups had 19 (49%) male patients and 20 (51%) female patients, with mean ages (± SD) of 16 (± 3) years and 16 (± 3) years, respectively (p = 0.16). Matched subjects had a mean BMI of 25 ± 4 kg/m and the mean BMI difference among groups was 5 ± 5 kg/m (p < 0.001). According to the Southwick radiographic criteria nine patients (23%) had a mild slip, 10 (26%) had a moderate slip, and 19 (49%) had severe SCFE. The α angle and femoral head-neck offset, epiphyseal extension ratio and epiphyseal angle, and epiphyseal tilt were assessed in the anterior, anterosuperior, and superior femoral planes on radially reformatted CT by one observer not involved in clinical care of the patients. Inter- and intrarater reliability were determined on 10 randomly selected hips assessed by the same observer and another observer and it was found to be excellent for all femoral measurements (intraclass correlation coefficients > 0.85). Paired t-tests were used to compare the contralateral asymptomatic hip of patients with SCFE and control hips.

Results: The head-neck junction showed decreased concavity in the contralateral femur of patients with unilateral SCFE compared with control subjects as assessed by slightly higher mean α angle in the anterosuperior plane (51° ± 6° versus 48° ± 7°; mean difference, 2°, 95% CI, 0°-5°; p = 0.04) and slightly higher median α angle in the superior plane (45° [range 37°-72°] versus 42° [range, 36°-50°], median shift, 4° [range, 2°-5°], p < 0.001), and slightly lower head-neck offset (anterosuperior: 5 mm ± 2 mm versus 6 mm ± 2 mm, mean difference, -1mm [range, -1 mm to 0 mm], p = 0.009; superior: median, 6 mm [range, 1 mm-8 mm] versus 7 mm [range, 5 mm-9 mm]; median shift, -1 mm [range, -1 mm to 0 mm], p < 0.001). There was less epiphyseal extension in the anterosuperior plane as evidenced by lower epiphyseal extension ratio (72% ± 6% versus 75% ± 6%; p = 0.005) and higher epiphyseal angle (64° ± 7° versus 60° ± 7°; p = 0.003). The epiphysis was slightly more posteriorly tilted (anterior plane tilt: 8° ± 6° versus 5° ± 4°; p = 0.03) and more vertically oriented (superior plane tilt 11° ± 5° versus 14° ± 4°; p = 0.006) in the contralateral asymptomatic hip of patients with SCFE.

Conclusions: The contralateral femur in patients treated for unilateral SCFE shows decreased concavity of the head-neck junction assessed by a higher α angle and reduced head-neck offset compared with age- and sex-matched control subjects. Because we noted lower epiphyseal extension but a more posteriorly tilted epiphysis, the reduced concavity resembles a mild slip deformity rather than an idiopathic cam morphologic feature.

Clinical Relevance: Although we noted a difference in the morphologic features of the head-neck junction between the two groups, the clinical significance is unclear because most differences were rather small. However, our findings suggest that the uninvolved hip in patients with unilateral SCFE may have a subtle asymptomatic cam morphologic feature that may be identified only with advanced imaging (CT or MRI). Future studies should investigate whether these morphologic changes influence development of contralateral SCFE or symptomatic femoroacetabular impingement in the contralateral hip of patients with unilateral SCFE and establish thresholds for indication of prophylactic fixation to avoid further slip and worsening of the morphologic features of the cam-femoroacetabular impingement.