Biomechanical evaluation of the Less Invasive Stabilization System, angled blade plate, and retrograde intramedullary nail for the internal fixation of distal femur fractures

Objective: To evaluate the stability of the retrograde intramedullary nail (IMN), angled blade plate (ABP), and a locked internal fixator (Less Invasive Stabilization System [LISS], Synthes, Paoli, PA) for internal fixation of distal femur fractures. Design: Destructive biornechanical testing of matched pairs of fresh-frozen human cadaveric bone-implant constructs. Setting: Biomechanical laboratory. Methods: A fracture model was created to simulate an AO/OTA33A3 fracture. Forty-eight matched pairs of specimens were used. Six groups of 8 pairs each were tested to failure: LISS versus ABP and LISS versus IMN (axial, torsional, and cyclical axial). Main Outcome Measurement: Load to failure, mode of failure, energy to failure, displacement at the load to failure, and stiffness. Results: Fixation strength (load/moment to failure) of the LISS constructs was 34% greater in axial loading (P = 0.01) and 32% less in torsional loading (P = 0.05) compared with ABP constructs and 13% greater in axial loading (P = 0.35) and 45% less in torsional loading (P < 0.01) compared with IMN constructs. Loss of distal fixation in axial loading occurred in 1 of 16 cases with the LISS, in 3 of 8 cases with the ABP, and in 8 of 8 cases with the IMN. Cyclical axial loading demonstrated significantly less plastic deformation for the LISS construct compared with ABP constructs (P < 0.01) and similar plastic deformation compared with IMN constructs (P = 0.98). Conclusions: All 3 fixation devices (LISS, ABP, and IMN) offer sufficient torsional stability and sufficient proximal fixation that withstands axial loading without failing. The LISS provides improved distal fixation, especially in osteoporotic bone, at the expense of more displacement at the fracture site.