Osteoclastic responses to various calcium phosphates in cell cultures

Disks made of hydroxyapatite, beta-tricalcium phosphate, carbonate apatite, tetracalcium phosphate, alpha-tricalcium phosphate, dicalcium phosphate dihydrate, and octacalcium phosphate were incubated in osteoclastic cell cultures for 2 days. The first five salts were sintered and the last two were compressed before incubation. Osteoclasts resorbed only the sintered carbonate apatite disks. However, osteoclasts were able to resorb octacalcium phosphate disks that were preincubated for 1 day in medium without cells, indicating that surface conditioning was important for osteoclastic resorption of this calcium phosphate. Although resorption did not occur, medium calcium and phosphorus changed to an appreciable extent after a 2-day incubation of beta-tricalcium phosphate, tetracalcium phosphate, alpha-tricalcium phosphate, and dicalcium phosphate dihydrate. These changes in the medium calcium and phosphate concentrations could explain why osteoclasts appeared to have lost their activity on these calcium phosphate disks and were not capable of resorbing them. With hydroxyapatite disks no changes were observed in the medium calcium and phosphorus before and after incubation. Moreover, the osteoclasts appeared to be essentially the same as with the sintered carbonate apatite disks and with bone slices used as a control. Nevertheless, no pits or lacunae were observed on the hydroxyapatite disks, indicating that sintered carbonate apatite should be superior to sintered hydroxyapatite as a bioresorbable bone substitute.