[Experimental studies of healing process on compound blocks of hydroxyapatite (HAP) particles and tricalcium phosphate (TCP) powder implantation in rabbit mandible--comparison of HAP/TCP ratios and plastic methods]

Abstract

Bone substitutes consisting in 2 mold-poured compound blocks of HAP:TCP (mixing ratio 1:1) and HAP:TCP (mixing ratio 3:1) and one compound block (mixing ratio 1:1) prepared in a rubber press were implanted in defective areas in rabbit mandibles. The conditions under which the implants were made were identical in all cases. Histological studies and contact microradiography performed on ossification of the surfaces and surroundings of each block, ossification maturation, and the absorptive process of TCP and the extent to which it was replaced by new bone produced the following results. 1. At one week after surgery, new bone had grown along the surfaces of the blocks. It remained in tight contact with the block surfaces as it matured. These findings make it clear that the blocks are superior in terms of biocompatibility and osteoconduction. 2. In the course of time, in the 2 mold-poured compound blocks, the hydrated parts of alpha-TCP, which was a binder, were absorbed and replaced by new bone. At forty eight weeks after surgery, new bone had grown into the pores of the HAP particles to form a bone-HAP composite. 3. Even as late as forty eight weeks after surgery, some of the hydrated parts of alpha-TCP remained unabsorbed; and mold-poured compound blocks (mixing ratio 1:1) retained their shapes. On the other hand, in the mold-poured compound blocks (mixing ratio 3:1), the hydrated parts of alpha-TCP were almost absorbed and replaced by new bone in forty eight weeks after surgery. Amounts of the binder alpha-TCP present appeared to cause differences in biological reactions. 4. In the case of compound blocks of HAP:TCP (mixing ratio 1:1) formed in a rubber press, the binder was beta-TCP. Because of its high sintering properties and great density, absorption and ossification were gradual; and the blocks retained their complete original forms even in forty eight weeks after surgery. 5. Past reports have claimed that beta-TCP is absorbed in the initial stage. But, as is clear from experiments with compound blocks formed by means of a rubber press, even when the binder is beta-TCP in all cases, absorption speed differs depending on the mold used. 6. These results indicate that compound blocks of HAP and TCP used as artificial substitute bone materials have excellent properties of biocompatibility, osteoconduction, and plasticity. In addition, however, they make it clear that, owing to differences in absorption of the binder, ossification speed, and dynamic properties from block to block, it is essential to use the mold that properly suits conditions prevailing in the defective area.