Strontium-coating of orthodontic miniscrews provides higher osseointegration – A rat model studied by histology, synchrotron X-ray fluorescence and diffraction imaging

The objective of this animal study was to evaluate the osseointegration of loaded strontium-functionalized orthodontic miniscrews (Ti-Sr-O) and the effect of the Sr coating on the elemental composition of the bone as well as the coating's effects on bone mineral size and orientation. Strontium-containing coated (Ti-Sr-O) miniscrews and grade 4 titanium miniscrews (Ti) were inserted into either tibia of thirty male Wistar rats and loaded through a coil spring. After a two-, four- and six-week healing period, specimens were analyzed via histomorphometry to measure bone-to-implant contact (BIC%) and peri-implant bone formation (BF%) in defined regions of interest (ROI-1; ROI-2). Furthermore, samples were investigated at the P06 synchrotron beamline to simultaneous measure the atomic composition with X-ray fluorescence (XRF) and the crystallite size/orientation with X-ray diffraction (XRD). After two weeks Ti-Sr-O functionalized miniscrews showed significantly more bone formation in ROI-1 than in the control group with Ti miniscrews at the tension (p**=0.005) and compression side (p*=0.018). The investigation by 2D XRF and XRD Mapping with a 0.4 µm beam resulted in multiple significant changes in comparison to the controls. The XRF showed a significant increase in the Sr/Ca level in the peri implant bone, and XRD showed significant changes in degree of orientation of the crystallographic c-axis, and for the apparent crystallite size perpendicular to the crystallographic c direction. These changes depend on the treatment and Sr/Ca level. The results suggest advantageous osseointegration of Sr-coated miniscrews under immediate loading. This resulted in changes in the microscopic organization of the bone biomineral.