Long Segment Blocking Screws Increase the Stability of Retrograde Nail Fixation in Geriatric Supracondylar Femur Fractures: Eliminating the "Bell-Clapper Effect"

Objectives: To determine the change in stiffness and horizontal translations of a geriatric supracondylar femur fracture model with the addition of distal segment blocking screws versus proximal (long) segment blocking screws to the standard retrograde intramedullary nail construct. Methods: Unstable supracondylar femur fractures (OTA/AO 33-A3) were created; all specimens were instrumented with a retrograde intramedullary nail. Specimens were divided into 2 groups (6 matched pairs per group). Group 1 compared the standard construct (1 proximal screw and 3 distal screws) to a distally augmented construct, with blocking screws placed in the distal metaphyseal segment. Group 2 compared the distally augmented construct to one in which blocking screws were placed just proximal to the fracture (long segment blocking screws). Specimens were then axially loaded and cycled to failure or run-out. Results: There was no significant difference in baseline stiffness, survival through cyclic loading, stiffness after cyclic loading, or cycles to failure between femurs treated with distal blocking screws and femurs treated without blocking screws (group 1). Femurs with blocking screws in the long proximal segment had significantly greater baseline stiffness, stiffness after cyclic loading, and less horizontal translation at the fracture site (group 2). There was no difference in survival through cyclic loading or cycles to failure. Conclusion: Long segment blocking screws are biomechanically superior to blocking screws in the distal segment or no blocking screws initially and after cyclic loading in an unstable geriatric supracondylar femur fracture model treated with intramedullary nail.