Formation of stable strontium-rich amorphous calcium phosphate: Possible effects on bone mineral

Abstract

Bone, tooth enamel, and dentin accumulate Sr2+, a natural trace element in the human body. Sr2+ comes from dietary and environmental sources and is thought to play a key role in osteoporosis treatments. However, the underlying impacts of Sr2+on bone mineralization remain unclear and the use of synthetic apatites (which are structurally different from bone mineral) and non-physiological conditions have led to contradictory results. Here, we report on the formation of a new Sr2+-rich and stable amorphous calcium phosphate phase, Sr(ACP). Relying on a bioinspired pathway, a series of Sr2+ substituted hydroxyapatite (HA) that combines the major bone mineral features is depicted as model to investigate how this phase forms and Sr2+ affects bone. In addition, by means of a comprehensive investigation the biomineralization pathway of Sr2+ bearing HA is described showing that not more than 10 at% of Sr2+, i.e. a physiological limit incorporated in bone, can be incorporated into HA without phase segregation. A combination of 31P and 1H solid state NMR, energy electron loss spectromicroscopy, transmission electron microscopy, electron diffraction, and Raman spectroscopy shows that Sr2+ introduces disorder in the HA culminating with the unexpected Sr(ACP), which co-exists with the HA under physiological conditions. These results suggest that heterogeneous Sr2+ distribution in bone is associated with regions of low structural organization. Going further, such observations give clues from the physicochemical standpoint to understand the defects in bone formation induced by high Sr2+ doses. Statement of SignificanceUnderstanding the role played by Sr2+ has a relevant impact in physiological biomineralization and provides insights for its use as osteoporosis treatments. Previous studies inspired by the bone remodelling pathway led to the formation of biomimetic HA in terms of composition, structures and properties in water. Herein, by investigating different atomic percentage of Sr2+ related to Ca2+ in the synthesis, we demonstrate that 10% of Sr2+ is the critical loads into the biomimetic HA phase; similarly to bone. Unexpectedly, using higher amount leads to the formation of a stable Sr2+-rich amorphous calcium phosphate phase that may high-dose related pathologies. Our results provide further understanding of the different ways Sr2+ impacts bone.