Bone ingrowth into polymer coated porous synthetic coralline hydroxyapatite

Porous hydroxyapatite, converted hydrothermally from the calcium carbon exoskeleton of the coral genus Goniopora (CHAG), has been shown to be effective as a scaffold for bone ingrowth (2,3,5-7,9). However, the large pores in the material resulted in low compressive strengths. In a previous study, we found that microcoating the internal surfaces of CHAG with dilactic-polyactic acid (DL-PLA) improved significantly its compressive properties. The objective of this study was to determine the effect of PLA microcoating on bone ingrowth into CHAG plugs. Plugs of thick- (3:1 chloroform to DL-PLA by weight), medium- (10:1), and thin- (30:1) coated as well as uncoated CHAG were implanted transcortically in the proximal third of the diaphysis of the rabbit tibia. Specimens were harvested at 3, 12, and 24 weeks for mechanical testing 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 30:1 and 8 MPa for 10:1 and 3:1 coated specimens. At 24 weeks, there were no significant differences in shear stress among any of the specimens. Histomorphometric assessments showed that the ratio of area fractions of new bone to area fractions of new bone and void space increased from 68-70% for 3:1 and 10:1 coated specimens at 3 weeks, and 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.