Radiopaque Strontium Fluoroapatite Glass-Ceramics

Abstract

The controlled precipitation of strontium fluoroapatite crystals was studied in four base glass compositions derived from the SiO2–Al2O3–Y2O3–SrO–Na2O–K2O/Rb2O/Cs2O–P2O5–F system. The crystal phase formation of these glasses and the main properties of the glass-ceramics, such as thermal and optical properties and radiopacity were compared with a fifth, a reference glass-ceramic. The reference glass-ceramic was characterized as Ca-fluoroapatite glass-ceramic. The four strontium fluoroapatite glass-ceramics showed the following crystal phases: (a) Sr5(PO4)3F – leucite, KAlSi2O6, (b) Sr5(PO4)3F – leucite, KAlSi2O6, and nano-sized NaSrPO4, (c) Sr5(PO4)3F – pollucite, CsAlSi2O6, and nano-sized NaSrPO4, and (d) Sr5(PO4)3F – Rb-leucite, RbAlSi2O6, and nano-sized NaSrPO4. The proof of crystal phase formation was possible by X-ray diffraction. The microstructures, which were studied using scanning electron microscopy, demonstrated a uniform distribution of the crystals in the glass matrix. The Sr-fluoroapatites were precipitated based on an internal crystallization process, and the crystals demonstrated a needle-like morphology. The study of the crystal growth of needle-like Sr-fluoroapatites gave a clear evidence of an Ostwald ripening mechanism. The formation of leucite, pollucite, and Rb-leucite was based on a surface crystallization mechanism. Therefore, a twofold crystallization mechanism was successfully applied to develop these types of glass-ceramics. The main focus of this study was the controlled development of glass-ceramics exhibiting high radiopacity in comparison to the reference glass-ceramic. This goal could be achieved with all four glass-ceramics with the preferred development of the Sr-fluoroapatite – pollucite-type glass-ceramic. In addition to this main development, it was possible to control the thermal properties. Especially the Rb-leucite containing glass-ceramic showed the highest coefficient of thermal expansion (CTE). These glass-ceramics allow optical properties, especially the translucency and color, to be tailored to the needs of biomaterials for dental applications. The authors conclude that it is possible to use twofold crystallization processes to develop glass-ceramic biomaterials featuring different properties, such as specific radiopacity values, CTEs, and optical characteristics.

Apatite Glass-Ceramics: A Review

“…Fluorapatite is part of the hexagonal group of crystals; therefore, it exhibits a kinetically favored growth in the crystallographic c-direction, which is seen in Höland et al (2015)'s findings, whereby crystal length as a function of time due to OR process was more pronounced as opposed to crystal expansion at crystallographic a-direction. OR process can be used to produce a highly homogenous and mechanically superior microstructure of a GC.…”

“…Thermodynamically, OR is a favorable process because larger particles exhibit lower surface energy in contrast to smaller particles. Apatite crystal growth through OR process is observed in apatite glass-ceramics, such as reported by Höland et al (2015). One of the key characteristics of an OR process is the reduction in the number of droplets/ crystals and an increase in volume as a function of time, pressure or temperature.…”

“…Höland et al (2015) were also able to obtain secondary and tertiary crystal phases, in addition to Sr-FAp, including leucite (KAlSi2O6), rubidium leucite (RbSi2O6), cesium pollucite (CsAl2S2O6), and sodium strontium orthophpshate (NaSrPO4). However, both leucite phases and pollucite phases showed surface crystallization.…”

“…However, both leucite phases and pollucite phases showed surface crystallization. Höland et al (2015) found that some of the base glasses that were rapidly quenched into water (to prevent crystallization) did not avoid crystallization completely. Based on XRD analyses of all parent glasses, Höland et al (2015) found that all glasses, except reference glass No.…”

“…Strontium-Substituted A-L Glass-Ceramics Höland et al (2015) developed radiopaque strontium FAp (Sr-FAp) containing glass-ceramics (Table 6) for dental applications where Sr-FAp phase crystallized through homogenous mechanisms. Höland et al (2015) were also able to obtain secondary and tertiary crystal phases, in addition to Sr-FAp, including leucite (KAlSi2O6), rubidium leucite (RbSi2O6), cesium pollucite (CsAl2S2O6), and sodium strontium orthophpshate (NaSrPO4).…”