Comparison of in vivo bioactivity and compressive strength of a novel superporous hydroxyapatite with beta-tricalcium phosphates

Superporous hydroxyapatite (HAp-S) is a novel bone substitute that contains three-dimensionally interconnected macropores with micropores, which stimulate bone ingrowth into the material. We investigated the in vivo behaviour of HAp-S by comparing its bioactivity and biomechanical properties with beta-tricalcium phosphates (beta-TCP). HAp-S or beta-TCP was implanted in the lateral femoral condyle of rabbits. In vivo bioactivity of each material, including bone ingrowth and material resorption, was quantitatively evaluated by micro-CT and the ultimate compressive strength of the bone-material composite was also measured. Micro-CT showed that bone ingrowth in the HAp-S group significantly increased over time, while no significant increase was observed after 8 weeks in the beta-TCP group. Although both materials showed gradual material resorption, beta-TCP resorption was significantly greater than HAp-S. The ultimate compressive strength in the HAp-S group significantly increased over time up to six times its original value, while there was no significant increase in the beta-TCP group. These results show that HAp-S resorption is concurrent with bone ingrowth, resulting in increasing compressive strength over 12 weeks. On the other hand, beta-TCP resorption is fast but unaccompanied by bone ingrowth; consequently, it remains relatively fragile at least in the early period after implantation. Although these highly porous materials themselves are structurally and mechanically similar, there are significant differences in in vivo behaviour depending on the material composition. These findings should be kept in mind when choosing the highly porous ceramics.