Strontium substituted hydroxyapatite promotes direct primary human osteoblast maturation

Strontium-substituted hydroxyapatite (SrHAp) materials are known to actively promote bone formation. However, the optimum level of Sr inclusion needed to elicit a physiologically relevant response from bone cells is unclear and can vary dependent on the fabrication process employed. In this work hydroxyapatite (HAp), SrHAp powders (2, 5 and 10 wt% (i.e., 1, 2 and 5 at%) with respect to [Sr/(Sr + Ca) ∙100]), were synthesized with the purity and Sr-substitution was confined in the range of 1–8 wt% (1–4 at%). All SrHAp samples contained rod-like crystals (<106 nm in length), which decreased in length with increasing Sr content, and exhibited larger flatter crystals (>300 nm in length). TEM-EDX confirmed the presence of Sr and maintenance of the HAp lattice structure for both types of crystals. Qualitative in vitro evaluation using primary human ostesoblast cells (HOBC) cultured in contact with the SrHAp over 28 days showed that the presence of Sr (in particular with the highest Sr content) directly promotes the maturation of osteoblasts into osteocytes as compared to the response observed for HAp. As these materials contain no additives other than Sr, the effects observed here can only be attributed to the physiologically important levels of Sr in the samples.