Microporosity enhances bioactivity of synthetic bone graft substitutes

This paper describes an investigation into the influence of microporosity on early osseointegration and final bone volume within porous hydroxyapatite (HA) bone graft substitutes (BGS). Four paired grades of BGS were studied, two (HA70-1 and HA70-2) with a nominal total porosity of 70% and two (HA80-1 and HA80-2) with a total-porosity of 80%. Within each of the total-porosity paired grades the nominal volume fraction of microporosity within the HA struts was varied such that the strut porosity of HA70-1 and HA80-1 was 10% while the strut-porosity of HA70-2 and HA80-2 was 20%. Cylindrical specimens, 4.5 mm diameter x 6.5 mm length, were implanted in the femoral condyle of 6 month New Zealand White rabbits and retrieved for histological, histomorphometric, and mechanical analysis at 1, 3, 12 and 24 weeks. Histological observations demonstrated variation in the degree of capillary penetration at 1 week and bone morphology within scaffolds 3-24 weeks. Moreover, histomorphometry demonstrated a significant increase in bone volume within 20% strut-porosity scaffolds at 3 weeks and that the mineral apposition rate within these scaffolds over the 1-2 week period was significantly higher. However, an elevated level of bone volume was only maintained at 24 weeks in HA80-2 and there was no significant difference in bone volume at either 12 or 24 weeks for 70% total-porosity scaffolds. The results of mechanical testing suggested that this disparity in behaviour between 70 and 80% total-porosity scaffolds may have reflected variations in scaffold mechanics and the degree of reinforcement conferred to the bone-BGS composite once fully integrated. Together these results indicate that manipulation of the levels of microporosity within a BGS can be used to accelerate osseointegration and elevate the equilibrium volume of bone.