Xenograft bone inclusion improves incorporation of hydroxyapatite cement into cancellous defects

Hydroxyapatite cement (HAC) is biocompatible and osteoconductive, but its slow resorption limits new bone formation. The purpose of this investigation was to determine the effects of adding partially demineralized xenograft cortical bone in an established animal model.

Eight 6-month-old female New Zealand white rabbits were used. Drill-hole defects (8.0 mm long, 5.0 mm diameter) were prepared and filled with either HAC alone or HAC mixed with processed xenograft bone particles from young pigs (XBC) at a volumetric ratio of approximately 25%. The particles were approximately 5 mm long and 1 mm diameter and were extensively washed, demineralized in dilute hydrochloric acid, and rewashed. Microcomputed tomography scanning and histology were performed after 10 weeks. New bone and inflammatory/immune response were graded on a 0 to 3 scale and calcein labeling was quantified as percent area new bone. Statistical analyses were by Student's t tests.

XBC showed significantly more new bone formation than HAC throughout the defect (P < 0.05). XBC also showed significantly more inflammatory/immune response than HAC (P < 0.05). The three-dimensional microcomputed tomography reconstructions showed that the HAC was basically inert, whereas the XBC took on an appearance suggestive of more extensive incorporation.

Adding xenograft to HAC creates a bioactive composite that is more rapidly incorporated, resorbed, and replaced by new bone. The presence of processed xenograft particles creates a vigorous inflammatory response, but there may be some benefit to the resorption rate of the HAC as a result of the infiltration of cells. Future research should focus on the longer-term incorporation and remodeling of XBC.