Porous hydroxyapatite as a bone-graft substitute in metaphyseal defects. A histometric study

Porous hydroxyapatite (Interpore 500) formed by conversion of the Porites goniopora coral exoskeleton has pores averaging 600 micrometers and pore interconnections averaging 260 micrometers in diameter. In the proximal tibial metaphysis of 8 dogs, a defect one cubic centimeter in size was created unilaterally and was fitted with a block of Interpore 500. Both proximal tibial metaphyses were retrieved at 2, 4, 6, and 12 months. Stained undecalcified sections were examined by light microscopy and quantitated by histometric methods. The implant-side specimens contained compact bone along the external surface and trabecular bone interiorly. The interior of these specimens was composed of 51.9 ± 1.3% soft tissue, 13.0 ± 1.2% bone, and 35.1 ± 1.2% Interpore 500 (mean and standard error). The interior of the normal specimens was composed of 79.7 ± 1.4% soft tissue and 20.2 ± 1.4% bone. The allocation of implant pore space between bone and soft tissue was proportional to that of bone and soft tissue in the normal tibiae. The stereological distribution of regenerated bone in the porous hydroxyapatite was also the same as in the normal tibiae. The appositional process of incorporation of the implant was confirmed by the finding that 66.5% of the surface of the Interpore 500 was covered with bone ingrowth at 12 months. Clinical relevance: This study supports the concept that a morphologically compatible and biocompatible implant framework may duplicate, in part, the success of cancellous grafts. The potential for similar findings in clinical practice is suggested by morphological comparisons between this model and clinical applications and by morphological comparison between Interpore 500 and clinical autografts.