title: Biomechanics of Slipped Capital Femoral Epiphysis: Evaluation of the Posterior Sloping Angle.
abstract: BACKGROUND
  The posterior sloping angle (PSA) has been shown to be an objective and reproducible predictor of the risk of patients developing contralateral slipped capital femoral epiphysis (SCFE); however, prophylactic fixation remains controversial. This in vitro study investigates the biomechanical basis of using a 15-degree PSA as a threshold for prophylactic fixation.
  
  
  METHODS
  Synthetic bone in vitro models of the proximal femur were constructed with a PSA of 10 degrees as a control (normal) group (n=6) by performing an osteotomy at the physis and gluing the head back onto the neck. SCFE groups were created with a PSA of 15, 20, 25, 30, 50, or 60 degrees, by excising a wedge from the posterior neck and gluing them back at the new angle with corresponding posterior translation proportional to the slip angle, and loaded superoinferiorly in compression, to failure. Ultimate strength, energy to failure, and stiffness were recorded.
  
  
  RESULTS
  Increasing the PSA from 10 to 15 degrees only reduced ultimate strength by 13% (P>0.05; CI, -0.21 to -0.06), though a significantly lesser energy to failure was required (-58%, P<0.05; CI, -0.68 to -0.48). Increasing the angle to 20 degrees resulted in a further significant decrease in strength (-19%, P<0.05; CI, -0.28 to -0.10) and energy to failure (-45%, P<0.05; CI, -0.53 to -0.84). The severe SCFE (60-degree PSA) was significantly weaker and less rigid that the control, and the mild and moderate SCFE models (P<0.01).
  
  
  CONCLUSIONS
  These biomechanical data support the threshold of 15-degree PSA as an objective measure for prophylactic fixation of the contralateral hip in SCFE.
  
  
  CONCLUSIONS
  The number needed to treat with (minimally invasive) prophylactic fixation to prevent contralateral SCFE can be minimized if the above-mentioned threshold is used.
authors.affiliation: Department of Orthopaedic Research & Biotechnology, Kids Research Institute, The Children's Hospital at Westmead, Westmead, NSW, Australia.
authors.forename: Jeremy M
authors.initials: JM
authors.lastname: Bellemore
authors.forename: Eleanor C
authors.initials: EC
authors.lastname: Carpenter
authors.forename: Nicole Y C
authors.initials: NY
authors.lastname: Yu
authors.forename: Oliver
authors.initials: O
authors.lastname: Birke
authors.forename: David G
authors.initials: DG
authors.lastname: Little
mesh_terms: D053587:Arthrometry, Articular / Q000379:methods*; D001696:Biomechanical Phenomena; D005269:Femur / Q000000981:diagnostic imaging; D005592:Fracture Fixation / Q000379:methods*; D006132:Growth Plate; D006621:Hip Joint / Q000000981:diagnostic imaging; D006801:Humans; D008953:Models, Anatomic; D011859:Radiography / Q000379:methods; D060048:Slipped Capital Femoral Epiphyses / Q000175:diagnosis / Q000503:physiopathology / Q000601:surgery