Interfacial conditions between a press-fit acetabular cup and bone during daily activities: implications for achieving bone in-growth

Interfacial gaps and relative micromotions during activities are widely believed to restrict the boney in-growth process of non-cemented acetabular cups. Using finite element modeling of the cup-bone system, relative micromotions and interfacial gaps are calculated for walking slow, normal and fast and for climbing upstairs, downstairs and standing up from a chair. A 2mm press-fit is simulated and interfacial conditions in the immediate postoperative period (i.e. prior to boney in-growth) calculated between paired nodes covering the whole of the interface. In regions of 'safe' micromotions and 'allowable' gaps, boney in-growth is simulated by specifying zero relative displacement between nodal pairs. The modified model is then resubjected to the loads associated with climbing upstairs, which was shown to be the worst activity. Interfacial conditions are recalculated for subsequent iterations. The procedure is repeated until no further in-growth is predicted. The final pattern of in-growth calculated with the model compares reasonably well with histological evidence from explanted canine cups (Cha et al., 1998. Transactions of the Orthopaedic Research Society, 23, p. 373). Bridging between adjacent regions of in-growth is observed. Notably, in-growth occurs at most of the periphery but not in the polar region. The lack of polar in-growth is caused by the interfacial gap assumed to exist after cup implantation. It is suggested that increasing/decreasing hip-joint loads would have little effect on this lack of polar in-growth. However, excessive micromotions as a result of high hip-joint loads cause a lack of in-growth in the anterior region of the periphery in the model. Although such results were not found in the canine study, if relevant to the general human population, the avoidance of harsh weight-bearing activities may encourage complete peripheral in-growth but is speculated to do little for polar in-growth.