Effects of porous coating, with and without collar support, on early relative motion for a cementless hip prosthesis

In theory, porous or rough coatings could be used to reduce early post-operative relative motion about cementless hip prostheses. To investigate this theory, we used detailed, non-linear finite element analysis to compare early relative motion about a well-fit Anatomical Medullary Locking (AML) prosthesis for different amounts of porous coating (full, proximal 2/3, and no coating), both with and without collar support. Details of the model included quantitative computed tomography-derived (QCT-derived) geometric and material properties for the bone, and a no-tension interface condition at all bone-prosthesis interfaces, with Coulomb friction (mu = 1.73) over coated surfaces and zero friction elsewhere. Predicted values of relative motion for this well-fit device were in the range of approximately 1-550 microns. The distribution of relative motion was relatively insensitive to the amount of porous coating but was sensitive to collar support, while the magnitude of relative motion was sensitive to the porous coating and collar support. In addition, a reduction in the porous coating caused larger increases in relative motion when there was no collar support, indicating an interaction between the effects of porous coating and collar support. For example, distal twist increased (full vs 2/3 partial coating) by 38% with collar support and by 58% without collar support. These data suggest that porous coating, or other surface treatments which result in a high coefficient of friction at the bone-prosthesis interface, may well be used to control the magnitude of early relative motion, particularly when there is no collar support.