A laboratory model to evaluate cutout resistance of implants for pertrochanteric fracture fixation

Objectives: To establish a laboratory model of implant cutout, which can evaluate the effect of implant design on cutout resistance in a clinically realistic worst case scenario. Setting: Orthopaedic biomechanics laboratory. Design: Implant cutout was simulated in an unstable pertrochanteric fracture model, which accounted for dynamic loading, osteoporotic bone, and a defined implant offset. For model characterization, lag screw cutout was simulated in human cadaveric specimens and in polyurethane foam surrogates. Subsequently, foam surrogates were used to determine differences in cutout resistance between 2 common lag screws (dynamic hip screw, Gamma) and 2 novel blade-type implant designs (dynamic helical hip system, trochanteric fixation nail). Main Outcome Measures: Implant migration was continuously recorded with a spatial motion tracking system as a function of the applied loading cycles. In addition, the total number of loading cycles to cutout failure was determined for specific load amplitudes. Results: Implant migration in polyurethane surrogates closely correlated with that in cadaveric specimens, but yielded higher reproducibility and consistent cutout failure. The cutout model was able to delineate significant differences in cutout resistance between specific implant designs. At any of 4 load amplitudes (0.8 kN, 1.0 kN, 1.2 kN, 1.4 kN) dynamic hip screw lag screws failed earliest. The gamma nail lag screw could sustain significantly more loading cycles than the dynamic hip screw. Of all implants, trochanteric fixation nail implants demonstrated the highest cutout resistance. Conclusions: Implant design can significantly affect the fixation strength and cutout resistance of implants for pertrochanteric fracture fixation. The novel cutout model can predict differences in cutout resistance between distinct implant designs.