HIGH ASSEMBLY STRAINS AND FEMORAL FRACTURES PRODUCED DURING INSERTIONOF UNCEMENTED FEMORAL COMPONENTS - A CADAVER STUDY

The assembly strains produced in cadaver femurs during uncemented femo ral arthroplasty were measured using strain gages and photoelastic coa tings. Resecting the femoral neck, reaming the canal with power drills , and rasping with an optimal size rasp, as determined by preoperative radiographic templating, produced small assembly strains, up to 300 m icrostrain. Insertion of an optimal-size prosthesis after preparing th e femoral canal with instruments the same size as the prosthesis produ ced moderate assembly strains, up to 1,000 microstrain. Half a millime ter press-fit of optimal prostheses produced larger assembly strains, up to 2,000 microstrain. Half a millimeter press-fit of a prosthesis t hat was also one size (1.0 mm) larger than that determined to be optim um produced even larger assembly strains (2,000-6,000 microstrain) and longitudinal linear fractures in the femoral cortex. Insertion of pro stheses that were smaller than the rasps produced minimal strains in t he femoral cortex. The magnitude of peak strains produced by press-fit ting the femoral components and the small amounts of disparity between the size of the recess and the prosthesis necessary to produce these strains show the narrow range of tolerances available to the surgeon. Cementless femoral arthroplasty requires great care in preparing the f emoral canal to the appropriate size as determined from preoperative t emplating, using accurate and precisely toleranced instrumentation and prosthetic components in order to avoid femoral fractures, yet obtain a stable fit.