Introduction: Uncemented proximally filling porous-coated femoral components must be designed with an optimal level of press-fit. Excessive press-fit yields higher femoral stress which can result in periprosthetic femoral fracture (PPFx), whereas insufficient femoral stress can lead to a lack of initial mechanical stability, which “is necessary to achieve bone ingrowth into the porous surface” (Manley P.A. et. al., J Arthroplasty10:63–73, 1995) of the implant. An optimal press-fit design should also provide an accurate and repeatable femoral stem seating height in all patients.

A battery of cadaveric tests, physical “bench-top” tests, and finite element analyses (FEA) should be used in order to both quantitatively and qualitatively optimize a femoral press-fit design. In this study, a method is proposed to quantitatively rank candidate press-fit stem designs relative to successful predicates based on stem seating height and PPFx risk by recreating impact loading applied during surgery through a controlled “bench-top” model.

Methods: Three press fit candidate designs A, B & C and two clinically successful predicate proximal fit and fill stems (Secur-Fit™ Max (Fit & Fill 1) and Meridian^® TMZF^® (Fit & Fill 2), Stryker, Mahwah NJ) were evaluated. Five foam cortical shell Sawbones^® femur samples (Item# 1130, Pacific Research Laboratories, Inc., Vashon, WA) were prepared for each press-fit design. A stem impactor was attached to the stem and then the stem was hand inserted in the femur. Then the construct was mounted in the drop tower using a vice and initial drop height was set to generate approximately 5500 N of impaction force when fully seated. Each stem was serially impacted until stable then step loaded until PPFx occurred. The height above/below the medial resection plane was measured after each impaction.

Results: All press-fit designs had an initial stable seating height within the desired range without causing PPFx, using an average impaction load of 5341 N. All of the press-fit designs required, on average, roughly a 200% increase in impact load (10925 N) to cause PPFx. The press-fit deign which ranked first based on seating height accuracy, defined as the design closest to zero at stable, was Design C at −0.02 mm countersunk. Design A with a standard deviation of 0.09 mm ranked first for repeatability, defined as the design with the smallest standard deviation at stable. Finally the press-fit design which ranked first for lowest PPFx risk, defined as the design that is most countersunk prior to PPFx, was Fit & Fill 1 at 6.30 mm countersunk.

Discussion: This controlled “bench-top” impact loading model successfully showed that it can quantitatively evaluate stem seating height and PPFx risk for several different femoral press-fit designs. In order to determine the optimal design, each press-fit design was ranked with equal weight given to seating height and fracture risk. Using this test method one design alternative, press-fit Design C, ranked first as the optimal combination of seating height accuracy and consistency with a low risk of PPFx. A limitation of this impaction model is that it does not directly predict PPFx rate, it only quantifies risk of fracture. Another limitation is that this model does not simulate all of the variably that is inherent to actual patient bone types. This test is one step in a battery of tests, including cadaveric evaluation and FEA, which should be used in order to optimize a femoral press-fit design.