title: Is head-shaft angle a valuable continuous risk factor for hip migration in cerebral palsy?
abstract: BACKGROUND
  Reimer's migration percentage (MP) is the most established radiographic risk factor for hip migration in cerebral palsy (CP), and it assists surgical decision-making. The head-shaft angle (HSA) measures the valgus of the head and neck in relation to the shaft and may also be a useful predictor of hip migration at a young age. This study first defined normal values and investigated whether the head-shaft angle (HSA) is a continuous risk factor for hip migration in CP.
  
  
  METHODS
  Three hundred and fifty AP pelvic radiographs of 100 consecutive children comprising the hip surveillance programme in our region were analysed for MP and HSA. Inclusion criteria were children with spastic CP and Gross Motor Function Classification System (GMFCS) levels of III-V, along with a minimum follow-up of 5 years. The mean age was 8.8 (range 3-18) years and the mean follow-up time was 7.5 (range 5-10) years. Radiographs of 103 typically developing children (TDC) were selected for the control group. The reliability of the measurements was determined. A random effects analysis was used to assess the relationship between MP and HSA for all data and for MP > 40 %.
  
  
  RESULTS
  The TDC cohort had a mean HSA of 157.7° whilst that for the CP cohort was 161.7°. The value declined with age in both groups but remained consistently higher in the CP group. A random effects analysis considering the longitudinal data showed that there was no significant effect of HSA on MP. Similarly, when excluding CP patients with MP < 40 %, there was no significant effect of HSA on MP.
  
  
  CONCLUSIONS
  This study found no correlation between HSA and hip migration in children with CP in this age group. Using the HSA as a routine radiographic measure in the management pathway across childhood does not offer any added value. Early enrolment onto the hip surveillance programme could offer a better prediction of hip migration using the HSA at a very young age.
  
  
  METHODS
  II retrospective prognostic study.
authors.affiliation: East Surrey Hospital NHS Trust, Redhill, RH1 5RH, UK.
authors.forename: Sanjay
authors.initials: S
authors.lastname: Chougule
authors.affiliation: St Georges Hospital NHS Foundation Trust, London, SW17 OQT, UK.
authors.forename: John
authors.initials: J
authors.lastname: Dabis
authors.affiliation: University College London NHS Foundation Trust, London, WC1X 8LD, UK.
authors.forename: Aviva
authors.initials: A
authors.lastname: Petrie
authors.affiliation: St Georges Hospital NHS Foundation Trust, London, SW17 OQT, UK.
authors.forename: Karen
authors.initials: K
authors.lastname: Daly
authors.affiliation: St Georges Hospital NHS Foundation Trust, London, SW17 OQT, UK. yaelgelfer@gmail.com.|, 21 Arcadian place, London, SW18 5JF, UK. yaelgelfer@gmail.com.
authors.forename: Yael
authors.identifier: http://orcid.org/0000-0001-8212-9999
authors.initials: Y
authors.lastname: Gelfer
keywords: Cerebral palsy; Head shaft angle; Hip migration