title: The alpha angle as a predictor of contralateral slipped capital femoral epiphysis.
abstract: OBJECTIVE
  Contralateral hip involvement in slipped capital femoral epiphysis (SCFE) is common. Femoral head-neck asphericity, as measured by an elevated alpha angle, has not previously been assessed with respect to SCFE risk. Our aim was to assess the utility of the alpha angle in predicting contralateral SCFE.
  
  
  METHODS
  We retrospectively reviewed 168 patients (94 males) managed surgically for unilateral SCFE between 2001 and 2013 who had a minimum of 18 months follow-up. The alpha angle, the posterior sloping angle (PSA), and the modified Oxford score were recorded for every patient at the time of initial SCFE presentation. Follow-up clinical records and radiographs were assessed to determine the presence of absence of contralateral SCFE.
  
  
  RESULTS
  Forty-five patients (27 %) developed a contralateral SCFE. Patients who developed a contralateral SCFE had a significantly higher alpha angle (51° vs 45°, p < 0.001) than patients who did not develop a contralateral SCFE. There was no significant difference in PSA or modified Oxford score (both p > 0.10) between patients who developed a contralateral SCFE and those who did not. Using a proposed alpha angle of 50.5° as a threshold for prophylactic fixation, 26 (58 %) of the 45 cases of contralateral SCFE in our study would have been prevented and 18 (15 %) of 123 patients would have undergone fixation unnecessarily.
  
  
  CONCLUSIONS
  We found the alpha angle to positively correlate with contralateral SCFE risk. Patients with significantly elevated alpha angles may be at greater risk of contralateral SCFE and benefit from further investigation or prophylactic hip fixation.
authors.affiliation: Department of Orthopaedic Surgery, Harvard Medical School, Children's Hospital Boston, Boston, USA. mboyle@adhb.govt.nz.|Department of Pediatric Orthopaedics, Starship Children's Hospital, Auckland Central, Private Bag 92 024, Auckland, 1142, New Zealand. mboyle@adhb.govt.nz.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Boyle
authors.affiliation: Department of Orthopaedic Surgery, Harvard Medical School, Children's Hospital Boston, Boston, USA.|Department of Pediatric Orthopaedics, Hospital Universitario Virgen del Rocío, Seville, Spain.
authors.forename: Jose F
authors.initials: JF
authors.lastname: Lirola
authors.affiliation: Department of Orthopaedic Surgery, Harvard Medical School, Children's Hospital Boston, Boston, USA.|Department of Pediatric Orthopaedics, University of Texas Health Science Center at San Antonio, San Antonio, TX, USA.
authors.forename: Grant D
authors.initials: GD
authors.lastname: Hogue
authors.affiliation: Department of Orthopaedic Surgery, Harvard Medical School, Children's Hospital Boston, Boston, USA.
authors.forename: Yi-Meng
authors.initials: YM
authors.lastname: Yen
authors.affiliation: Department of Orthopaedic Surgery, Harvard Medical School, Children's Hospital Boston, Boston, USA.
authors.forename: Michael B
authors.initials: MB
authors.lastname: Millis
authors.affiliation: Department of Orthopaedic Surgery, Harvard Medical School, Children's Hospital Boston, Boston, USA.
authors.forename: Young-Jo
authors.initials: YJ
authors.lastname: Kim
keywords: Alpha angle; Fixation; Hip; Slipped capital femoral epiphysis