The Stability of Biological Apatite in the Presence of Strontium

INTRODUCTION: Strontium plays important role in stimulating the growth of osteoblasts and enhancing the formation of new bone. Therefore, a strontium related compound strontium ranelate (SR) has recently been suggested for use as a daily supplement to treat osteoporosis. However, the true effect of strontium to the stability of hydroxyapatite (HAp) has not been well understood yet, such as why higher concentration of strontium was normally detected in new bone, but seldom in old bone. In particular, strontium-substituted apatite was widely reported much higher soluble than pure HAp, although some report postulated that Sr40-HAp may be more stable than pure HAp, thus HAp should be the stable phase, not a strontium-substituted apatite. The most explanation was only attributed to calcium ion easily substituted by strontium ion on bone surface, but lacking of thermodynamic principle to support it. Therefore a recent reported solid titration method has re-checked the strontium system and found solubility of strontium-substituted apatite indeed increased with increase of strontium content (Fig. 1), the formation of such phase therefore seems not to be thermodynamically favored, which may be a kinetic issue. Thus, the solid titration method by seeding biological apatite in SBF solution may simulate the effect of strontium in human body and help to understand why strontiumsubstituted apatite was formed as new bone.