The Effect of Bone/Implant Deformation on In-Vitro Femoral Micromotion Measurements

In order to promote bony ingrowth into an uncemented, porous coated femoral component, a stable interface between the bone and implant is required. Poor initial stability can inhibit bony ingrowth, leading to possible early failure of the femoral component. To determine initial implant stability many researchers have measured relative motion between the bone and implant (micromotion) using a variety of techniques. Typically this involves measuring motion of a target fixed to the implant with sensors such as linear variable differential transducers (LVDTs) fixed to the cortical bone. These techniques require the assumption of bone and implant rigidity to calculate micromotion using a linear transformation. Recently, some researchers have expressed concern about this assumption of bone and implant rigidity. Callaghan et al., proposed a way of testing the rigidity assumption by measuring pseudo-micromotion of targets mounted directly onto the bone (prior to mounting on the implant). They found that bone deformation had little effect on micromotion, however their technique could not determine the effect of cancellous bone deformation.