Mechanical and morphological characterization of PMMA/bone composites in human femoral heads

PMMA bone cement has gained an important place in a variety of orthopaedic applications in the femur. However, appropriate data on the mechanical properties of bone-cement composites from the human femur are lacking. Therefore, the goal of this study was to determine the morphological and quasi-static compressive properties of proximal femoral bone-cement composites. Thirty trabecular bone specimens were extracted from fifteen pairs of human femoral heads using specimen-specific cutting guides to ensure an accurate alignment with the main trabecular direction (MTD). One specimen from each pair was augmented with PMMA bone cement, while the other one was left untreated. Specimens were scanned with μCT to determine morphological parameters and tested in quasi-static compression until failure. We found that the long axis of the specimens was highly aligned with the MTD (mean error < 5°). A higher compressive modulus and ultimate strength were observed for the bone-cement composite specimens (E = 5.7 ± 0.4 GPa; σ_u = 77.9 ± 5.1 MPa) compared to the bone only specimens (E = 2.9 ± 0.7 GPa; σ_u = 19.0 ± 5.8 MPa). Furthermore, the composites had a higher modulus, but lower strength than cement itself (E = 5.0 ± 0.3 GPa; σ_u = 85.9 ± 2.7 MPa) and the composite modulus was significantly correlated with the bone volume fraction (BV/TV). These results are in contrast to previous findings on human vertebral bone, where the composite was more compliant than cement and no correlation was found between BV/TV and the composite modulus. Thus, properties of bone-cement composites cannot simply be applied across different anatomical sites; the site-specific differences in bone density and trabecular alignment should be taken into account. Collectively, the present results suggest that at low BV/TV, cement dominates the composite properties, while at high BV/TV, the contribution of bone becomes apparent, revealing a positive relationship between BV/TV and the on-axis modulus.