Cyclic loading of periprosthetic fracture fixation constructs

Femur fractures are a common complication of hip arthroplasty. When the stem is well fixed, fracture fixation is the preferred treatment option. Numerous fixation methods have been advocated, using plates or allograft struts.

Vancouver type B1 periprosthetic femur fractures were created distal to a cemented hip stem in 15 third-generation composite femurs. The fractures were fixed with (1) a nonlocking plate and allograft strut, (2) a locking plate and allograft strut, or (3) a locking plate alone. The struts were fixed with cables. After fixation, the constructs underwent cyclic loading for 100,000 cycles. Stiffness of the constructs was determined during bending, torsion, and axial compression before and after cyclic loading. Load to failure was also determined.

Overall, cyclic loading had little effect on the mechanical properties of these constructs. The two constructs with allografts were significantly stiffer in bending than the construct consisting of only a locking plate. There were no significant differences in axial or torsional stiffness between the constructs. Load to failure of the two constructs with allografts was significantly greater than the locking plate alone.

Allograft strut-plate constructs are stiffer in bending and have a higher load to failure than a stand-alone locking plate. When an allograft plate construct is chosen, locking screw seemed to provide no mechanical advantage. All three constructs tested retained their mechanical characteristics after 100,000 cycles of loading. Our initial concern that the cables fixing the allograft strut would loosen appears unfounded.