DO BONE INGROWTH PROCESSES PRODUCE A GLOBALLY OPTIMIZED STRUCTURE

A topology optimization program was applied to test the hypothesis tha t bone adaptation to porous coated implants produces a structure which minimizes the global strain energy density. The program was used to p redict the optimal material layout around a porous coated tibial compo nent with multiple cones [Goldstein et al., Trans. 37th ORS. p. 92 (19 91)]. The sensitivity of the predicted adaptation to analysis assumpti ons was assessed and the predicted bone ingrowth and apposition were c ompared with the experimental findings of Goldstein et al. The results showed that apposition occurred consistently at the cone tips regardl ess of analysis assumptions. The specific topology of apposition at th e cone tips was most sensitive to the assumed loading conditions. A co mparison with the experimental results for 11 subdivisions showed that the general predicted location of material agreed with the experiment al results (R2 greater-than-or-equal-to 0.59). However, the program pr edicted a consolidated bone greater than 1000 mum in thickness at the cone tips, which differed from the porous bone structure found experim entally. This discrepancy was reflected in a refined comparison over 3 1 subdivisions which did not produce a significant correlation (R2 les s-than-or-equal-to 0.3). The program also predicted little ingrowth (< 6-7%), indicating that ingrowth past the first bead layer contributed little to the overall bone-implant interface layer stiffness. Based o n these results, we conclude, within limitations of a two-dimensional analysis, that bone adaptation to porous coated implants does not prod uce a structure solely optimized to minimize the global strain energy density. We hypothesize that the final bone structure reflects the nee d to meet both mechanical and nutritional demands.