An experimental study of a high-strength hydroxyapatite (ceratite®). The second report: Osteoconduction in hydroxyapatite tricalciumphosphate composites with altered porosities

Ceratite®(NGK Spark Plug Co, Ltd, Komaki, Japan), a hydroxyapatite(HAP) tricalciumphosphate(TCP) composite, has superior mechanical strength coupled with the capability of being easily shaped intraoperatively in contrast to other porous hydroxyapatites. In our first report, complete new bone coverage of the porous ceratite® was noted at 12 weeks following implantation, but no osteoconduction into the interior was seen. Based on this finding, we thus prepared two bone substitutes with larger pore sizes than conventional ceratite® and conducted animal in situ implantations for evaluation. The new bone substitutes, one with 20 μm pores and the other with 50 μm pores, were sintered at 1, 100°C and 1,300°C, respectively, and had two differing 3- point bending strengths: one with one-eighth the bending strength of conventional porous ceratite® and the other with the same bending strength of conventional porous ceratite®. When under-decalcified specimens were investigated at 4 weeks after implantation, new bone tissue formation was seen around both tested bone substitutes; similarly, at 12 weeks after implantation, the new bone tissue also covered the surrounding regions and exhibited direct bonding without interstitial tissue in both bone substitutes. However, on histological inspection of the boundary of this new bone tissue, the margin of the bone substitute sintered at 1,100°C had a saw-shaped appearance, whereas the bone substitute sintered at 1,300°C had a smooth margin. Further, osteoconduction into the interior of the bone, which had been anticipated in these larger-pore bone substitutes, was not seen. Inspection of the pore-size distribution showed that the minimum diameter of the paths between the smaller pores ranged from 1 to 2 μm, whereas the diameter of paths between the maximum-size pores ranged from 20 to 50 μm. It thus was concluded that a new HAP·TCP bone substitute must be developed with continuous pores that are much larger, and that these pores be connected by much larger paths, so as to achieve osteoconduction into the interior of the bone substitute.