Effects of bone ingrowth on the strength and non-invasive assessment of a coralline hydroxyapatite material

The dependence of strength on the amount of bone growth into a hydroxyapatite material made from coral was investigated. Block and granular forms of the material were implanted into cortical and trabecular regions of the skeletons of 16 dogs. The results were examined after 4, 8, 12 and 16 wk, with four dogs in each experimental group. When implanted into cortical bone, the bending strength of the implant material was found to be highly correlated with the amount of pore space which had become occupied by bone (r = 0.92, P < 0.005 for the block form; r = 0.84, P < 0.005 for the granular form). Multiple regression analysis showed that six histomorphometric measures of ingrowth accounted for 96% of the variability in bending strength of the block material, and there were no significant differences between block and granular forms of the material. On the other hand, when implanted into trabecular bone, the block form of the material achieved greater compressive strength than the granular form. While both strength and ingrowth increased with time, there were poor correlations between these two variables. Finally, when the material is implanted into trabecular bone, it becomes stronger in compression than the surrounding bone; when implanted in cortical bone, linear modelling suggests that resorption and replacement of the implant would be required to approximate the bending strength of the surrounding bone.