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.
Orthopaedic biomechanics laboratory.
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
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.
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.
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.