Osteogenic response of rabbit tibia to hydroxyapatite particle–Plaster of Paris mixture

A mixture of hydroxyapatite (HA) particles and plaster of Paris (POP) with saline solution was injected into the bone marrow of 30 unilateral tibiae of 30 rabbits. As a control, POP with saline solution was similarly injected into the bone marrow of 30 unilateral tibiae of 30 additional rabbits. Five rabbits in each group were killed 2, 4, 8, 12, 24 and 40 weeks after injection. Histological examination was performed on three tibiae of five killed animals, and X-ray diffraction analysis of the bone marrow was performed on the remaining two tibiae. In two of three tibiae, bridges of new bone were formed between the hydroxyapatite particles and the cortical bone, 4 weeks after the injection of the HA particle–POP mixture. New bone formation was seen in all three tibiae at 8 weeks and thereafter. No new bone formation was observed in the tibiae of the control group. The affinity index, a parameter of osteogenesis, was 0% at 2 weeks, 52.2±14.0% at 4 weeks, 74.8±5.3% at 8 weeks, 86.1±6.1% at 12 weeks, 90.1±8.3% at 24 weeks and 92.8±4.1% at 40 weeks after injection. Statistically significant increases were found between the affinity indexes of 2 weeks and 4 weeks and between those of 4 weeks and 8 weeks, p<0.03 and p<0.05, respectively. Between 12, 24 and 40 weeks, the indexes showed tendencies toward increases. X-ray diffraction of bone marrow of two tibiae into which HA particle–POP mixture or POP alone was injected showed that POP had disappeared 8 weeks after injection.