Calcium phosphate implants coatings as carriers for BMP-2

The osteoconductivity of dental implants can be improved by coating them with a layer of
calcium phosphate (CaP), which can be rendered osteoinductive by functionalizing it with an osteogenic agent, such as bone morphogenetic protein 2 (BMP-2). In the present study, we wished to compare the osteoindcutive efficacies of three different types of BMP-2-bearing calcium-phosphate coating: one of hydroxyapatite (C-HA), which was prepared
conventionally, viz., under extremely unphysiological conditions of temperature (> 1000°C).
by plasma spraying, and two that were prepared biomimetically, viz., under physiological
conditions of temperature (37°C). One of the latter two was an amorphous layer of calcium
phosphate (B-A), which serves as a seeding substratum during the two-stage biomimetic
coating procedure; the second was a composite of this amorphous seeding layer and a
subsequently deposited crystalline one of octacalcium phosphate (B-A/OCP). Titanium-alloy discs (lOmm in diameter) were coated with a layer of either C-HA, B-A or B-A/OCP, which was then functionalized by the direct adsorption of BMP-2 (2.5, 5.0 or 7.5 flg per disc). The discs (n =: 6 per group) were inserted at an ectopic (subcutaneous) site in rats, and retrieved 1, 2 or 3 weeks later for a histomorphometric analysis of the volumes of bone and foreign-body giant cells (a gauge of coating biocompatibility) and of the rate of coating degradation. Using the lowest quantity of BMP-2 (2.5 pg per disc), ectopic bone formation at the I-week juncture was associated only with the C-HA coatings; the bone volume in this group peaked between the first and the second weeks. In the groups with B-A or B-A/OCP coatings that likewise bore the lowest quantity of BMP-2, bone tissue fist became apparent after 2 weeks, and its volume had increased by the third. Using the intermediate (5.0 flg per disc) and highest (7.5 pg per disc) quantities of BMP-2, bone was associated with the B-A and BA/OCP coatings also at the I-week juncture, and its volume increased progressively thereafter.
The temporal course of bone fOl'mation around the C-HA coatings was similar to that
observed using the lowest quantity of BMP-2. Our data indicate that an adsorbed depot of
BMP-2 is released more gradually from biol11imetically-prepared than from conventionallyprepared coatings of calcium phosphate, with the consequence that the osteoinductive efficacy of the former is raised above that of the latter.