Primary cancellous bone formation with BMP and micro-chambered beads: experimental study on sheep

The physiological reconstruction of cancellous bone defects in surgery of the locomotor system is an unsatisfactorily solved problem.

The aims of this study are to examine whether micro-chambered ß-tricalcium-phosphate (ß-TCP) beads provide a certain capillary force suctioning in blood and bone marrow thus forming a stable "negative"-replica of the bone marrow spaces. If so, a new approach for osteoconduction would yield primarily a scaffold of lamellar cancellous bone under load without a long-lasting remodeling process. Recombinant human bone morphogenetic protein (rhBMP) might even enhance all processes of defect healing, remodeling and ß-TCP resorption; gentamicin-loaded ε-caprolactone might protect the implant.

Ten sheep were operated on; the patella-groove model and the tibial head were used. A defect of 9.4 × 20 mm was created using wet-grinding-diamond instruments. Micro-chambered ß-TCP-beads of 4-6 mm with 0.35 mg rhBMP-7 + 0.1 g collagen per animal, or 1.5 g demineralized bone matrix (DBM) paste on the contra-lateral side were implanted. Both osteoinduction groups were compared with the defect in the tibial heads where plain micro-chambered ceramic beads were inserted. Added to the beads was 12.5 mg gentamicin sulphate in 12.5 mg ε-caprolactone-carrier. Outward diffusion was prevented using a 1-mm-thick press-fit inserted ceramic lid. The bone healing, remodeling and resorption of the ceramic in a right-left comparison of the patella groove and the tibial head was examined at 6 weeks, 2 and 3 months; one animal in reserve was followed for 14 months. The animals were perfusion-fixed, the vasculature micro-casted with an acrylate and nondemineralized processed, and with μ-CT and microscopically documented.

A primary load-bearing spongiosa had developed around the beads, which shortened the remodeling process. The strong micro-chambered, resorbable ß-TCP-beads demonstrate high capillary strength, resorb blood and bone marrow, and represent a stable formative material which, as a carrier for the controlled local release of BMP, that accelerates bone healing, shortens resorption and remodeling compared with plain and DBM loaded implants.

Micro-chambered beads represent the bone-forming element, BMP yields a fast defect healing and enhanced remodeling of bone and resorption of ß-TCP compared to delayed and incomplete reconstruction and resorption of ß-TCP on the DBM-side, the plain implants reached nearly the same reconstruction, but far later compared with the BMP loaded implants.