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Calcium phosphates (CaPs) have a widespread use in bone repair and regeneration due to their osteoconductive and bioactive (bone bonding) nature. In the past decade, it has become apparent that a sub-group of CaPs with specific physico-chemical properties have the ability to induce bone formation in ectopic sites (ie, intramuscular or sub-cutaneously). In this chapter, we summarize the scientific evidence of CaP induced bone formation, describe material properties required for bone induction and discuss possible underlying mechanisms. In addition, we also provide data on the clinical efficacy of this unique group of CaPs.

Utilizing several in vitro and in vivo studies, a variety of calcium phosphates were examined for their bone-bonding and biodegradation properties. For the in vitro studies, use was made of osteoblast and/or osteoclast enriched rat bone marrow cultures that were grown on fluorapatite, tricalcium phosphate and various types of hydroxyapatite that differed in degree of crystallinity. For the in vivo studies, the same range of hydroxyapatite was used and implanted either in femora of young adult male Wistar rats, or in the acetabulum of goats.The results of these studies show that more extensive dissolution/reprecipitation occurs on degrading and low crystalline calcium phosphate surfaces. As a consequence, bone tissue is formed more rapidly on these dynamic material surfaces. Irrespective of the type of material, osteoblasts first form afibrillar mineralized globular accretions, that subsequently fuse to form a cementing substance in which collagen fibres become incorporated. Osteoclasts only appear to have the ability to resorb low crystalline hydroxyapatite and tricalcium phosphate. In culture, osteoclastic resorption of these calcium phosphates occurred only after osteoblasts had been grown on the various materials. This suggests that osteoblast-derived substances initiate osteoclastic resorption activity.