TRICALCIUM PHOSPHATE AND OSTEOGENIN - A BIOACTIVE ONLAY BONE-GRAFT SUBSTITUTE

The disadvantages of autogenous bone grafts has prompted a search for a dependable onlay bone graft substitute. A combination of tricalcium phosphate, a resorbable ceramic, and osteogenin, an osteoinductive pro tein, was evaluated as an onlay bone graft substitute in a rabbit calv arial model. Twenty-eight tricalcium phosphate implants (15 mm diamete r X 5 mm; pore size, 100-200 mu m) were divided into experimental and control groups and placed on the frontal bone of 14 adult New Zealand White rabbits. In the experimental animals, 185 mu g of osteogenin was added to each implant. In the control animals, the implants were plac ed untreated. Implants were harvested at intervals of 1, 3, and 6 mont hs, and evaluated using hematoxylin and eosin histology, microradiogra phy, and histomorphometric scanning electron microscope backscatter im age analysis. At 1 month there was minimal bone ingrowth and little tr icalcium phosphate resorption in both the osteogenin-treated and contr ol implants, At 3 months, both the osteogenin-treated and control impl ants showed a modest increase in bone ingrowth (8.85 percent versus 5. 87 percent) and decrease in tricalcium phosphate (32.86 percent versus 37.08 percent). At 6 months, however, the osteogenin-treated implants showed a statistically significant increase in bone ingrowth (22.33 p ercent versus 6.96 percent; p = 0.000) and decrease in tricalcium phos phate (27.25 percent versus 37.80 percent; p = 0.004) compared with th e control implants. The bone within the control implants was mostly wo ven at 6 months, whereas the osteogenin-treated implants contained pre dominantly mature lamellar bone with well-differentiated marrow. All i mplants maintained their original volume at each time interval studied . The tricalcium phosphate/osteogenin composite, having the advantage of maintaining its volume and being replaced by new bone as the trical cium phosphate resorbs, may be applicable clinically as an onlay bone graft substitute.