Biomechanical Properties of Bone Ingrowth Cement

　　　　　We investigated the effect of hydroxyapatite (HA) and human cancellous bone particles on the bone ingrowth of polymethylmethacrylate (PMMA) bone cement in rabbits. Twenty-four rabbits were divided into four groups of six animals each. The first group went through the implantation of pure cement, and the 2nd, 3rd, and the 4 ?? groups went through the implantation of fine cancellous bone (particle size < 400 μ m), coarse cancellous bone (particles size between 400 μ m and 500 μ m), and HA impregnated cement respectively on both femoral condyles of each animal. Two animals of each group were killed three, six and twelve weeks postoperatively for biomechanical push-out testing. The results analyzed by ANOVA test revealed that the shear strength for each group was insignificant at three weeks postoperatively (p > 0.05). The shear strength for the other periods, six weeks (p < 0.001, HA = pure > fine = coarse) and twelve weeks (p < 0.001, HA > pure > fine = coarse), however was significantly different. The shear strength of pure and HA impregnated cement increased with increasing implantation periods, but decreased for fine and coarse bone impregnated cement. Histologic study of the specimen reveals more reactive fibrous tissue around the PMMA rod, which contains fine or coarse bone granules. There is less amount of fibrous tissue and more bone tissue around the pure PMMA rod and hydroxyapatite impregnated PMMA rod. These findings correlated with biomechanical push out test study that higher force needed in the groups made of pure PMMA or hydroxyapatite-PMMA.