Polymer composite femoral stems do not have a standardized in vitro mechanical testing method. The testing device for metallic stems, which simulates proximal resorption of the femur, is inappropriate for composite stems, which they are expected to reduce or prevent proximal resorption. The objective of this study was to determine a mechanical testing method for pressfit composite stems, using finite element analysis (FEA). The goal was to reproduce the strains of the stem implanted in a femur.

Three simplified models used for preliminary studies were beam-on-elastic-foundation, two-dimensional plus sideplates FEA, and axisymmetric FEA. These models showed that stem stresses were affected by design changes to the testing device, and gave some results equivalent to those found with the more complex models.

The effects on the composite stem of varying the testing device were studied using full 3-D FEA models. The stem strains were affected by design changes to the testing device. The maximum normal and interlaminar shear strains of the composite stem in the testing device were either lower than or about equal to the maximum strains of the composite stem in the femur.

A single-material testing device made of birchwood was chosen as the best testing device from the parametric study. Birchwood is an orthotropic material with a longitudinal stiffness in the range of bone. This testing device improved the stem strains which lead to composite interlaminar failure, relative to the other testing devices studied.

The viability of the testing device, as measured by stresses on the bone cement or testing device and by interface motion, was also affected by changes to the testing device and stem material. Changing from a metallic stem to a composite stem increased the bone cement stresses by about twofold. Some testing device configurations had cement stresses that were close to or above static cement strengths. The birchwood testing device had high interface motion.

Other stem materials were compared to the composite stem. The strains on a metallic stem and the composite stem were not always affected in the same way by changes to the testing device. An isotropic stem was a better simplification of the composite stem than was a transversely isotropic stem.