Posterior sloping angle as a predictor of contralateral slip in slipped capital femoral epiphysis.

Background: Slipped capital femoral epiphysis is a condition with potentially severe complications. Controversy remains as to when to undertake prophylactic pinning. We aimed to assess the utility of the posterior sloping angle as a predictor for contralateral slip in a large, multi-ethnic cohort including Polynesian children with a high incidence of slipped capital femoral epiphysis.

Methods: All patients presenting to our hospital between 2000 and 2009 were identified and records were reviewed to determine demographic data and determine whether they subsequently developed a contralateral slip. The presenting radiographs were reviewed and the posterior sloping angle was measured. Patients with bilateral slips at presentation and those without initial radiographs were excluded.

Results: Records and radiographs of 132 patients were analyzed for the posterior sloping angle in the unaffected hip. Forty-two patients who had subsequently developed a contralateral slip had a mean posterior sloping angle (and standard deviation) of 17.2° ± 5.6°, which was significantly higher (p < 0.001) than that of 10.8° ± 4.2° for the ninety patients who had had a unilateral slip. Children who had developed a subsequent contralateral slip were significantly younger (11.1 years) than those who had developed a unilateral slip (12.2 years) (p < 0.001). If a posterior sloping angle of 14° were used as an indication for prophylactic fixation in this population, thirty-five (83.3%) of forty-two contralateral slips would have been prevented, and nineteen (21.1%) of ninety hips would have been pinned unnecessarily. The number needed to treat to prevent one subsequent contralateral slip is 1.79.

Conclusions: To our knowledge, this is the largest study to date that confirms that the posterior sloping angle is a reliable predictor of contralateral slip and can be used to guide prophylactic pinning. The posterior sloping angle is applicable in the high-risk Polynesian population and could be useful in preventing future slips in populations that are difficult to follow up.