Biomechanical comparison of fully and partially threaded screws for fixation of slipped capital femoral epiphysis

Background: Previous data have shown that an equal number of threads on each side of the physis maximizes stability for slipped capital femoral epiphysis (SCFE) fixation. The purpose of the current study was to determine if a fully threaded cancellous screw provides greater stability compared with a partially threaded screw in a porcine model. Methods: Twenty skeletally immature porcine femurs were sectioned, and a 30-degree angular wedge was resected from the femoral neck to simulate SCFE. Femora were randomly assigned to partially threaded (16 mm) or fully threaded screw groups (n = 10/group). Kirschner wires were inserted in a retrograde fashion to stabilize the proximal fragment. Each 7.3-mm-diameter screw was placed using fluoroscopic guidance to obtain 3 threads crossing the physis. Specimens were fixed in custom fixation rigs, and the epiphyseal fragment was loaded at 0.5 mm/s in a posterior-inferior direction to simulate slip progression. Data for displacement (in millimeters) and force (in newtons) were collected for the entire test. Forces at 2, 4, 6, and 8 turn of fragment displacement were compared between groups using a 1-way analysis of variance (p < 0.05). Results: Increasing epiphyseal displacements were associated with incremental increases in loading. There were no significant differences between fully threaded or partially threaded screws for loads at each displacement. Each stepwise increase in displacement was associated with approximately 325 N of force. Conclusion: There was no biomechanical benefit when using a fully threaded screw for stabilization of an in vitro SUE model. Clinical Relevance: Although there were no differences between screw types in an in vitro model, bone healing around the fully threaded screw may eventually provide greater stability. The use of fully threaded screw remains a reasonable option in the treatment of SCFE, and implant removal may be easier with such a system. Further studies are warranted to verify these 2 points.