Biomechanical and histological studies of particulate hydroxylapatite implanted in femur bone defects of adult dogs

The purpose of this study was to investigate the biomechanical behavior and the histology of particulate hydroxylapatite (HA) implanted in bone defects created in femurs of 48 adult mongrel dogs. The bone defects, measuring 10x10x25 mm with or without HA implantation, were allowed to heal until the end of the 1(st), 2(nd), 3(rd), 6(th), 9(th) and 12(th) months after surgery. The harvested tissue specimens from the implant sites were processed into long cubic or rectangular prismatic forms. Their mechanical strengths were assessed using compression and shearing tests to measure the maximal compressive and sheer stress by force loading on the middle portions of the specimens. Histological sections of each stage were processed with both decalcified and nondecalcified methods. The results showed that the mechanical strength of the tested specimens was closely related to the tissue regeneration within the bone defect. Progressive regeneration of new bone was observed at each stage, with complete bone formation at the 9-month period. A consistent increase in both maximal compressive stress and maximal shear stress was noted at each stage, responsive to the ossification and maturation of the regenerating tissue within the bone defect. This animal model provides a novel approach to directly estimate the tissue strength of the HA-implanted bone defect.