BMP-2 liberated from biomimetic implant coatings induces and sustains intramembranous bone growth in an ectopic rat model

Introduction: Using a rat model, we evaluated the kinetics and histomorphometry of ectopic bone formation in association with biomimetic implant coatings containing BMP-2.

Materials and methods: One experimental and three control groups were set up: titanium-alloy discs coated with a biomimetically co-precipitated layer of calcium phosphate and BMP-2 [1.7 μg per disc (incorporated-BMP group)]; uncoated discs (control); discs biomimetically coated with a layer of calcium phosphate alone (control); and discs biomimetically coated with a layer of calcium phosphate bearing superficially adsorbed BMP-2 [0.98 μg per disc (control)]. Discs (n = 6 per group) were implanted subcutaneously in rats and retrieved at 7-day intervals over a period of 5 weeks for kinetic, histomorphometrical, morphological and histochemical analyses.

Results: In the incorporated-BMP-2 group, osteogenic activity was first observed 2 weeks after implantation and thereafter continued unabated until the end of the monitoring period. The net weekly rates of bone formation per disc were 5.8 mm3 at 2 weeks and 3.64 mm3 at 5 weeks. The total volumes of bone formed per disc at these junctures were 5.8 mm3 and 10.3 mm3, respectively. Bone tissue, which was formed by a direct ossification mechanism, was deposited at distances of up to 340 μm from the implant surfaces. The biomimetic coatings were degraded gradually, initially by foreign body giant cells alone and then also by osteoclasts. Forty percent of the coating material (and thus presumably of the incorporated BMP-2) remained at the end of the monitoring period. Hence, 60% of the incorporated BMP-2 had been released. At this 5-week juncture, no bone tissue was associated with any of the control implants.

Conclusion: BMP-2 incorporated into biomimetic calcium phosphate coatings is capable not only of inducing bone formation at an ectopic site in vivo but also of doing so with a very high potency at a low pharmacological level, and of sustaining this activity for a considerable period of time. The sustainment of osteogenic activity is of great clinical importance for the osseointegration of dental and orthopedic implants.