Porous hydroxyapatite is resorbable and osteoconductive, so it is slowly replaced by ingrowing new bone. Optimal pore geometry for osteoconduction, however, has not been determined yet. The objective of this investigation was to assess both the histological response and the reinforcing effects of bone ingrowth within the porous hydroxyapatite implants depending on pore size.

Four kinds of cylindrical types (50, 100, 300, 500 micrometer) of porous hydrox-yapatite were prepared. Fifty-six white rabbits were divided into 4 groups and porous hydroxyapatite block was inserted through the medial cortical window of a proximal tibia. Histomorphological changes were examined using light microscopy and scanning electron microscopy. Biomechanical compression test was performed using a material test machine.

Hydroxyapatite implants appeared to have no early adverse effects, such as inflammation and foreign body reaction. Osteoconduction through the pores was found in all four implants and new bone was found on the surface of pores with no histologically demonstrable intervening nonosseous tissue. At four weeks after implantation, new bone was arranged in a concentric pattern around the vessel, similar to osteon. New bone formation through pores was most evident at 300 micrometer-sized type. At 8 weeks, active osteoconduction was also found at 50 micrometer-sized type. Evidence of bone marrow formation within porous hydroxyapatite was found. In a biomechanical study, ultimate compression strength significantly increased in the 300 micrometer-sized type, after 8 weeks implantation compared to preimplantation.

Porous hydroxyapatite implanted into rabbit tibia showed biological fixation and osteointegration. A pore size of 300 micrometer was most effective for bone ingrowth. Osteoconduction also took place in 50 micrometer-sized cylindrical pores.