Abstract
The effect of cross-linking of a hydroxyapatite/collagen (HA/Col) nanocomposite, in which HA nanocrystals and collagen fibers are aligned like natural bone by a self-organization mechanism between HA and collagen in vitro, on mechanical properties was examined. The influence of degree of cross-linking, as well as rhBMP-2 preadsorption to the composite on the substitution pattern and rate with bone, was examined. In Experiment 1, anterior fusion was carried out at the C3–C4 vertebrae on 10 dogs and they were implanted as follows: without cross-linking and without adsorbed rhBMP-2 (three dogs), with cross-linking and without adsorbed rhBMP-2 (three dogs), without cross-linking and with adsorbed rhBMP-2 (two dogs), and with cross-linking and adsorbed rhBMP-2 (two dogs). Implants were removed from each dog for histology determinations after 12, 16, and 24 weeks in the non-rhBMP-treated groups, and after 16 and 24 weeks in the rhBMP-treated groups. In Experiment 2, the HA/Col composites with cross-linking and both with and without rhBMP-2 pretreatment were implanted into a bone defect of 20 mm made in the central part of tibiae in dogs (N = 3 in each group). As a control, bone defects of 20 mm remained without implantation (N = 3). The dogs were allowed to walk using an Ilizarov extra skeletal fixator. The implants were removed after 12, 16, and 24 weeks from one dog in each group. The cross-linking of the HA/Col composite was effective in controlling both the mechanical strength and bioresorbability. A “self-organization process” on the HA/Col implant surface resulted in the formation of bone remodeling units in and around the implant. Radiographic and histological findings suggest that a combined treatment of cross-linking of the HA/Col composite with preadsorption of rhBMP-2 molecules may be a very suitable replacement of existing ceramic systems in the anterior fusion of the cervical spine, as well as inlay grafting of bone defects in weight-bearing sites.