gms | German Medical Science

Questionnaire: Periimplant bone formation in vivo by recombinant human bone morphogenetic protein 2 (rhBMP-2) applied together with diverse carriers has only been variably successful or negative. These unsatisfactory results indicate that the mechanisms underlying bioactive BMP-2 coatings are yet only poorly understood. The present work attempts to shed some light on the mechanisms involved.

Methods: Ti-alloy-miniplates (Ti-6Al-4V, 10 x 5 x 1.6 mm) plated with cp titanium by plasma vapor deposition (PVD) and cylindrical PVD-plated Ti-alloy dumbbell shaped implants (length 10 mm) with a bar (length 7.5 mm, diam. = 5.0 mm) and a disk for fixation (diam. = 7 mm) at each end were used. rhBMP-2 was expressed in and prepared from E.coli. The surfaces were functionalized either with aminopropyltriethoxysilane (APS) alone or with APS followed by 1,1´-carbonyl diimidazole (CDI). The functionalized surfaces were then incubated for 12-14 h either in 2 ml ¹²⁵I-rhBMP-2 (ca. 10 µCi/mg) or unlabelled rhBMP-2 (0.25-0.30 mg/ml). Three sheep (§8 of the Animal Protection law observed) received a total of 24 implants into the left and right distal femur condylus leaving a circular gap of ~1000 µm circumferentially around the bar. They were sacrificed after 4 and 9 weeks.

Results and Conclusions: rhBMP-2 (5-8 µg/cm²) was immobilized by two methods on porous PVD titanium surfaces (miniplates, dumbbell implants). Release of the immobilized ¹²⁵I-rhBMP-2 from the surface occurred as an exponential decay in two-phases: a first rapid phase (ca. 15% of immobilized BMP-2) with half-lives of 1-2 days and a second slow sustained release phase (ca. 85% of immobilized BMP-2) with half-lives of 40-60 days. In spite of the very high binding constant (K_A) of adsorbed rhBMP-2 estimated to be in the range of 10¹² M^-1 , the released rhBMP-2 was biologically active in vitro and in vivo. Osteoinductivity of rhBMP-2 coated implants was tested in vivo in a gap-healing model in the trabecular bone of the distal femur condylus of sheep. Neither inflammation nor capsule formation occurred during healing. The concentration of released rhBMP-2 in the 1 mm gap was calculated to 20-98 mM, well above the half-maximal response concentration (K_0.5) of rhBMP-2 for inducing alkaline phosphatase in vitro in MC3T3-E1 cells. The bone density (BD) and bone-to-implant contact (BIC) of the implants explanted after 4 and 9 weeks were assessed histomorphometrically. Control implants without rhBMP-2 were not osseointegrated. Implants with immobilized rhBMP-2 displayed a significant 2-4 fold increase in BD and BIC values versus negative controls after 4-9 weeks. Integration of implants by trabecular bone was achieved after 4 weeks, indicating a mean "gap-filling rate" of ~250 µm/week. Integration of implants by cortical bone was observed after 9 weeks. We conclude that the enhancement of periimplant osseointegration and gap bridging by immobilized rhBMP-2 on implant surfaces is feasible and may serve as a model for future clinical applications.