Effect of controlled release of platelet-derived growth factor from a porous hydroxyapatite implant on bone ingrowth

Platelet-derived growth factor (PDGF) is one of several osteogenic factors which affect bone growth and fracture healing. This study examined the potential of hydroxyapatite (HA) rods with interconnected pores of mean diameter 200 microns to be used as a matrix for the release of PDGF to enhance bone ingrowth into the implant. In the initial phase of the study the sustained release of PDGF from the HA rods was characterized in vitro for two different PDGF loadings, 10 and 100 micrograms per implant. The second phase of the study examined bone ingrowth in HA implants placed into the medullary canals of rabbit femora. The specimens were dumb-bell shaped, with a reduced central diameter so that bone growth across a gap could also be determined. Bone ingrowth into HA implants was compared with growth into HA implants loaded with 100 micrograms of PDGF. Pushout measurements were made of average shear strength across the bone-implant interface and backscatter scanning electron microscopy of thick sections was used to quantify the amount of bone ingrowth into the implant. Although greater interfacial shear strength and area of ingrowth were observed, especially across gap sites, in specimens loaded with PDGF, no difference was statistically significant.