Biomechanical analysis of single screw fixation for slipped capital femoral epiphysis: are more threads across the physis necessary for stability?

PURPOSE To evaluate single screw fixation stability, in the treatment of slipped capital femoral epiphysis, as a function of screw thread distribution across the physis. STUDY DESIGN In vitro biomechanical study. METHODS Thirty porcine proximal femurs were sectioned through the physeal line and stabilized with a cannulated 7.3-mm stainless steel AO screw. The distal 16 mm of each screw was threaded (5 threads). The femurs were randomized into 5 groups (1, 2, 3, 4, or 5 threads across the physis) and biomechanically tested to determine failure load (N) and stiffness (N/mm). RESULTS Femurs with 2 or 3 threads across the physis had a significantly greater load to failure and stiffness compared with femurs with 1, 4, or 5 threads across the physis (P < 0.05). CONCLUSIONS Thread distribution across the physis seems to be important. When using screws with a 16-mm thread, greatest strength and stiffness are achieved when 40%-60% of threads engage the epiphysis, with a significant decrease when greater than 80% of threads cross the physis. Too few threads in the epiphysis as well as too few in the metaphysis both lead to decreased stability. CLINICAL RELEVANCE This study challenges the belief that compression across the physis maximizes slipped capital femoral epiphysis fixation stability. We recommend equal distribution of threads across the physis when using 16-mm thread screws, and we postulate that screws with a greater thread length (32 mm or fully threaded) would increase fixation stability even further. Optimizing purchase may decrease the incidence of slip progression, especially as the prevalence of obesity increases in the adolescent population.