Can we still use X-rays to predict contralateral slip in primary unilateral slipped capital femoral epiphysis?

The objective of this study was to assess subsequent contralateral slip (SCS) in the unaffected hip in patients with primary unilateral slipped capital femoral epiphysis (SCFE) using three radiographic parameters: posterior sloping angle (PSA), center-edge angle (CEA) and triradiate cartilage (TC) appearance. A total of 152 patients admitted to two pediatric units between 2001 and 2015 were divided into three groups: A - underwent prophylactic fixation of the unaffected hip at the time of index surgery- high clinical risk of SCS; B - no clinical risk factors but SCS occurred; C - no issues regarding the contralateral hip during follow-up. The mean PSA for groups A, B and C were 22°(6-49), 17°(9-24) and 13°(0-27), respectively. PSA was significantly higher in Group A than in Group C ( P < 0.0001). The differences in PSA between groups A and B, but also B and C were insignificant ( P = 0.12 and p=0.21, respectively). The mean CEA in groups A, B and C was 33(25-43), 35(26-42) and 37(17-53), respectively. CEA did not differ significantly between groups A, B and C ( P = 0.25). Assessment of TC did not differ significantly between the groups ( P = 0.66). Observation of TC in groups B and C combined revealed that the cartilage was open in 65% of 77 patients and 14% of them developed SCS; whereas among the 35% of patients with ossified TC only 7% developed SCS (OR=2.0). PSA and CEA alone have no predictive value in determining the risk of contralateral slip. The absence of TC results in a two-fold decrease in the likelihood of developing an SCS. The decision of prophylactic surgical treatment of the contralateral hip in primary unilateral SCFE should not be based solely on radiographic findings.