This study is being undertaken in response to the current wave of worldwide enthusiasm for using porous coated implants. This enthusiasm is growing despite the fact that very little fundamental information is available on the actual process of biological ingrowth. This study will use two configurations of porous coating on cylindrical femoral total hip replacements in a canine model. The overall goals of this project are to: a) determine the percent, extent, and viability of bone ingrowth into the porous regions of the implant, b) determine the mechanical properties of the bone-prosthesis interface and c) to relate the density changes in the bone to the changes in stress computed from finite element models of the canine femur. Specially bred dogs (black labrador) will receive bilateral porous acetabular and femoral components. These animals will be followed for 6, 12 and 18 months. Four animals will be involved in each of the followup periods. Each animal will have one fully coated femoral prosthesis (100%) in one hip and a proximally coated device (50%) in the contralateral hip an conjunction with a chamfered cylinder acetabular component. After sacrifice push out tests will be performed on the femoral specimens at predetermined levels to determine the shear stiffness and strength at the interface. Image processing techniques will be used to quantify the density changes, cortical dimension alternations, and the amount of bone present within the porous layer at the time of sacrifice. Thin sections above and below the mechanical test section will provide the best representation of the material which is present in these interfaces. Finite Element Models will be constructed to determine the state of stress in the bone with and without implants. Microradiographs and calcified tissue histology will be used to quantify the bone ingrowth response. The information obtained from this study will provide fundamental insight into the factors affecting the remodeling response to porous implants in a full weightbearing situation. The information obtained will be quantitative rather than the qualitative assessments that have been reported previously.
