Biomechanics of Slipped Capital Femoral Epiphysis: Evaluation of the Posterior Sloping Angle

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. Clinical Relevance: The number needed to treat with (minimally invasive) prophylactic fixation to prevent contralateral SCFE can be minimized if the above-mentioned threshold is used.