Nanocrystalline Hydroxyapatite-Poly(thioketal urethane) Nanocomposites Stimulate a Combined Intramembranous and Endochondral Ossification Response in Rabbits

Resorbable bone cements are replaced by bone through osteoclastic resorption and osteoblastic new bone formation near the periphery. However, the ideal bone cement would be replaced by new bone through processes similar to fracture repair, which occurs by a variable combination of endochondral and intramembranous ossification. In this study, nanocrystalline hydroxyapatite (nHA)-poly(thioketal urethane) (PTKUR) cements were implanted in femoral defects in New Zealand White rabbits to evaluate ossification at 4, 12, and 18 months. Four formulations were tested: An injectable, flowable cement and three moldable putties with varying ratios of calcium phosphate to sucrose granules. New bone formation and resorption of the cement by osteoclasts occurred near the periphery. Stevenel's Blue and Safranin O staining revealed infiltration of chondrocytes into the cements and ossification of the cartilaginous intermediate. These findings suggest that nHA-PTKUR cements support combined intramembranous and endochondral ossification, resulting in enhanced osseointegration of the cement that could potentially improve patient outcomes.