Mass spectrometric monitoring of Sr-enriched bone cements-from in vitro to in vivo.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a well-established technique in materials science, but is now increasingly applied also in the life sciences. Here, we demonstrate the potential of this analytical technique for use in the development of new bone implant materials. We tracked strontium-enriched calcium phosphate cements, which were developed for the treatment of osteoporotic bone, from in vitro to in vivo. Essentially, the spatial distribution of strontium in two different types of strontium-modified calcium phosphate cements is analysed by SIMS depth profiling. To gain information about the strontium release kinetics, the cements were immersed for 3, 7, 14 and 21 days in alpha-MEM and tris(hydroxymethyl)-aminomethane solution and analysed afterwards by ToF-SIMS depth profiling. For cements stored in alpha-MEM solution an inhibited strontium release was observed. By using principal component analysis to evaluate TOF-SIMS surface spectra, we are able to prove the adsorption of proteins on the cement surface, which inhibit the release kinetics. Cell experiments with human osteoblast-like cells cultured on the strontium-modified cements and subsequent mass spectrometric analysis of the mineralised extracellular matrix (mECM) prove clearly that strontium is incorporated into the mECM of the osteoblast-like cells. Finally, in an animal experiment, the strontium-doped cements are implanted into the femur of osteoporotic rats. After 6 weeks, only a slight release of strontium was found in the vicinity of the implant material. By using ToF-SIMS, it is proven that strontium is localised in regions of newly formed bone but also within the pre-existing tissue

ToF-SIMS analysis of osteoblast-like cells and their mineralized extracellular matrix on strontium enriched bone cements

Commonly used implants for therapeutic approaches of non-systemically impaired bone do not sufficiently support the healing process of osteoporotic bone. Since strontium (II) has been proven as an effective anti-osteoporotic drug new types of strontium enriched calcium phosphate bone cements were developed. As osteoporosis is characterized by an imbalance of osteoblast and osteoclast activity the influence of this newly generated strontium enriched biomaterials on the cellular behavior of osteoblast-like cells was investigated by time of flight secondary ion mass spectrometry (ToF-SIMS). ToF-SIMS is used to analyze whether strontium is incorporated in the mineralized extracellular matrix (mECM) and whether there is strontium uptake by osteogenically differentiated human mesenchymal stem cells (hMSCs). Therefore hMSCs were cultured in osteogenic differentiation medium for 21days on two different strontium enriched bone cements (S100 and A10) and for reference also on the pure calcium phosphate cement (CPC) and on a silicon wafer. The distribution of strontium in the osteoblast-like cells and within their mineralized extracellular matrix was analyzed. A higher intensity of the strontium signal could be detected in the region of the mECM, synthesized by cells cultivated on the Sr- substituted bone cement (S100) in comparison to the reference groups. The osteoblast-like cells used the released strontium from the biomaterial to synthesize their mECM. Apart from that a uniform strontium distribution was measured within all investigated cells. However, different amounts of strontium were found in cells cultured on different biomaterials and substrates. Compared to the negative controls the strontium content in the cells on the strontium enriched biomaterials was much higher. A higher concentration of strontium inside the cells means that more strontium can take part in signaling pathways. As strontium is known for its beneficial effects on osteoblasts by promoting osteoblastic cell replication and differentiation, and reducing apoptosis, the newly developed strontium enriched calcium phosphate cements are promising implant materials for osteoporotic bone