Biomechanical study of 16-mm threaded, 32-mm threaded, and fully threaded SCFE screw fixation

The initial goals of the treatment of slipped capital femoral epiphysis (SCFE) are to stabilize the epiphysis, prevent slip progression, and avoid complications. In situ fixation with a single screw is the most accepted procedure to provide an optimal fixation, but fixation failure and slip progression suggest that the procedure might be improved. The aim of the present study was to biomechanically compare partially threaded screws (16 mm and 32 mm) and fully threaded screws in an in vitro porcine model.

An unstable/acute SCFE was created in 18 skeletally immature porcine femurs through a type 1 Salter-Harris fracture. Each femur was fixed using a 6.5-mm stainless-steel cannulated screw inserted through the physis under fluoroscopic guidance. The screw was either 16 mm threaded, 32 mm threaded, or fully threaded (n=6 each group). Each specimen was cyclically tested to failure fixation by anterior-to-posterior loading through the femoral head.

No significant difference was detected between the Newton cycles to failure of the 3 screw-thread groups. The 16-mm threaded group had the highest frequency of femoral neck failure.

The 16-mm threaded screws had the highest rate of neck failure and did not demonstrate additional fixation stability over the 32-mm threaded and fully threaded screws. As femoral neck fracture is a rare but devastating complication in pediatric patients, clinical use of the 32-mm threaded and fully threaded screws may be indicated in SCFE fixation.

The use of 32-mm or fully threaded screws is a valid SCFE treatment option. The increased number of threads in the metaphysis with these screws may confer additional biomechanical strength to the femoral neck.