In vitro interface and cement mantle analysis of different femur stem designs

The aim of this study is to define stem design related factors causing both gaps in the metal–bone cement interface and cracks within the cement mantle. Six different stem designs (Exeter; Lubinus SP II; Ceraver Osteal; Mueller-straight stem; Centega; Spectron EF) (n=15 of each design) were cemented into artificial femur bones. Ten stems of each design were loaded, while five stems served as an unloaded control. Physiologically adapted cyclical loading (DIN ISO 7206-4) was performed with a hip simulator. After loading both interfaces and the bone cement itself were analysed regarding gaps and cracks in the cement mantle. Significant differences between the stem designs concerning gaps in the metal–bone cement interface and cracks in the cement mantle became apparent. Additionally, a high correlation between gaps in the metal–bone cement interface and cracks within the cement mantle could be proven. Gaps in the metal–bone cement interface but no cracks within the cement mantle were seen in the unloaded specimens. Differences between the unloaded control groups and the cyclical loaded stems regarding the longitudinal extension and width of gaps in the metal–bone cement interface were obvious. The designs of cemented femoral stems have an influence on both the quality of the metal–bone cement contact and the failure rate of the cement mantle. Less interface gaps and less cement defects were found with anatomically formed, collared, well-rounded stem designs without undercuttings.