Multi-axial loading micromechanics of the cement-bone interface in postmortem retrievals and lab-prepared specimens

Abstract

Maintaining adequate fixation between cement and bone is important for successful long term survival of cemented total joint replacements. Mixed-mode loading conditions (combination of tension/compression and shear) are present during in vivo loading, but the micromotion response of the interface to these conditions is not fully understood. Non-destructive, multi-axial loading experiments were conducted on laboratory prepared (n=6) and postmortem (n=6) human cement-bone interfaces. Specimens were mounted in custom loading discs and loaded at 0 degrees , 30 degrees , 60 degrees , and 90 degrees relative to the interface plane where 0 degrees represents normal loading to the interface, and 90 degrees represents shear loading along the longitudinal axis of the femur. Axial compliance did not depend on loading angle for laboratory prepared (p=0.96) or postmortem specimens (p=0.62). The cement-bone interface was more compliant under tensile than compressive loading at the 0 degrees loading angle only (p=0.024). The coupled transverse to axial compliance ratio, which is a measure of the coupled motion, was small for laboratory prepared (0.115 +/- 0.115) and postmortem specimens (0.142 +/- 0.101). There was a moderately strong inverse relationship between interface compliance and contact index (r(2)=0.65). From a computational modeling perspective, the results of the current study support the concept that the cement-bone interface could be numerically implemented as a compliant layer with the same initial stiffness in tension and shear directions. The magnitude of the compliance could be modified to simulate immediate post-operative conditions (using laboratory prepared data set) or long-term remodeling (using postmortem data set).