FEMORAL PROSTHESIS IMPLANTATION INDUCES CHANGES IN BONE STRESS THAT DEPEND ON THE EXTENT OF POROUS COATING

The objective of this study was to evaluate the effect of implantation of porous-coated anatomic medullary fitting prostheses on stress in t he proximal femur. Three-dimensional finite element models of a cadave ric femur before and after implantation were used to evaluate the resu lting changes in stress in the bone. Models of the femur were generate d automatically from computed tomographic scan data with use of an inn ovative mesh-generation technique. The models were analyzed for three levels of porous coating (proximal, 5/8, and full), with the assumptio n of ideal ingrowth (perfect bonding) over porous areas and a friction less, tension-free surface on smooth areas. All models were loaded and restrained to represent conditions of normal gait. The stresses predi cted in the implanted femur are consistent with clinical observations of proximal cortical atrophy (normal stress reduced to 6-9% of normal at the calcar and 50-55% at mid-prosthesis) and of hypertrophy at the porous coating junctions (normal stress at the 5/8-coating junction, 1 23% of stress proximal to the junction) and hypertrophy near the dista l tip of the prosthesis (anterior and posterior normal stresses 200-80 0% of normal). The fully coated prosthesis induced stresses in the bon e near the tip of the prosthesis that were most like stresses in the n ormal femur (medial and lateral normal stress 105 and 102% of the stre ss in the normal femur). Below the collar, the normal stress associate d with the proximally coated prosthesis was 6% greater than that produ ced with the other two levels of coating but still was only 2% of norm al. The 5/8-coated prosthesis appeared to combine the worst features o f the fully coated and proximally coated prostheses-greater stress-shi elding at the calcar and higher stress near the tip of the prosthesis.