Mechanical and bone ingrowth properties of a polymer-coated, porous, synthetic, coralline hydroxyapatite bone-graft material.

CHAG, that is, porous hydroxyapatite hydrothermally converted from the calcium carbonate exoskeleton of a coral (genus Goniopora), has been shown to be effective as a scaffold for bone ingrowth. The large pores in the material, however, resulted in low compressive strengths. Compressive testing was performed to assess the changes in mechanical properties by coating the internal surfaces of CHAG with DL-PLA. Plugs of CHAG with thick (3:1 chloroform to DL-PLA by weight), medium (10:1), and thin (30:1) coatings as well as uncoated CHAG were then implanted transcortically in the proximal third of the diaphysis of rabbit tibiae to assess the in vivo response. The mechanical tests demonstrated significantly improved compressive strength, stiffness, and energy absorption for coated specimens compared with uncoated specimens. Coated specimens were not significantly different from canine tibial cancellous bone in strength and stiffness although they achieved only 36% of the energy absorption capacity. Specimens from rabbit tibiae were harvested at 3, 12, and 24 weeks for interface shear strength determination and contralaterally for histological and histomorphometric assessment. At 12 weeks, uncoated CHAG plugs developed an average ultimate interface shear stress of 26.7 MPa compared with 17 MPa for specimens with 30:1 coatings and 8 MPa for specimens with 10:1 and 3:1 coatings. At 24 weeks, there were no significant differences in shear stress between any of the specimens. Histomorphometric assessments showed that the ratio of area fraction of new bone to area fraction of new bone and void space increased from 68-70% for specimens with 3:1 and 10:1 coatings at 3 weeks to 85.5-89.5% at 24 weeks. In comparison, uncoated and 30:1 specimens had area fraction ratios of about 82% at 3 weeks and 93% at 24 weeks. Histologic sections demonstrated direct apposition of new bone to both the coating and the hydroxyapatite as well as degradation of the coating.