BONE INGROWTH AND MECHANICAL-PROPERTIES OF CORALLINE HYDROXYAPATITE 1YR AFTER IMPLANTATION

A previous study of coralline hydroxyapatite as a bone-graft Substitut e was extended from 4 to 12 months to determine better the relationshi ps between implantation time, bone ingrowth and mechanical properties. The model consisted of a 10 x 30 mm window defect in the shaft of the canine radius (a cortical site), and a 10 mm diameter cylindrical def ect in the head of the humerus (a cancellous site). In the new study, these two defects were made bilaterally in eight dogs, and filled with block-form coralline hydroxyapatite. The radius defects were supporte d by a metal fixation plate which was removed after 9 months. After 12 months, the dogs were killed and the left-side implants were analysed histomorphometrically and mechanically. The right-side radius and hum erus were reserved for structural analysis. The results were combined with those previously measured after 4, 8, 12 and 16 wk of implantatio n. In the cortical site, bone ingrowth increased from 52% at 16 wk to 74% at 1 yr. In the cancellous site, bone ingrowth was 38% after 4 wk, then fell monotonically, reaching 17% at 1 yr. Bending and compressiv e strength and stiffness of the radius implants increased throughout t he post-implantation year, but compressive strength and stiffness of t he humerus implants did not change after the first 2-4 months. Mechani cal properties were strongly correlated to bone ingrowth in the cortic al, but not the cancellous, site. The volume fraction of the coralline hydroxyapatite material diminished significantly with time in the cor tical, but not the cancellous, site.