Variation of the bone forming ability with the physicochemical properties of calcium phosphate bone substitutes

Because of their bioactive properties and chemical similarity to the inorganic component of bone, calcium phosphate (CaP) materials are widely used for bone regeneration. Six commercially available CaP bone substitutes (Bio-Oss, Actifuse, Bi-Ostetic, MBCP, Vitoss and chronOs) as well as two tricalcium phosphate (TCP) ceramics with either a micron-scale (TCP-B) or submicron-scale (TCP-S) surface structure are characterized and their bone forming potential is evaluated in a canine ectopic implantation model. After 12 weeks of implantation in the paraspinal muscle of four beagles, sporadic bone (0.1 +/- 0.1%) is observed in two Actifuse implants (2/4), limited bone (2.1 +/- 1.4%) in four MBCP implants (4/ 4) and abundant bone (21.6 +/- 4.5%) is formed in all TCP-S implants (4/ 4). Bone is not observed in any of the BioOss, Bi-Ostetic, Vitoss, chronOs and TCP-B implants (0/4). When correlating the bone forming potential with the physicochemical properties of each material, we observe that the physical characteristics (e.g. grain size and micropore size at the submicron scale) might be the dominant trigger of material directed bone formation via specific mechanotransduction, instead of protein adsorption, surface mineralization and calcium ion release.