Antiresorptive properties of strontium substituted and alendronate functionalized hydroxyapatite nanocrystals in an ovariectomized rat spinal arthrodesis model

Purpose: The purpose of this study was to comparatively investigate the posterolateral fusion rate in ovariectomized (OVX) rats using two new bone graft materials: strontium (Sr) substituted hydroxyapatite (HA) nanocrystals and alendronate (AL) functionalized HA nanocrystals. SrHA was synthesized in presence of different Sr concentrations (SrHA5; SrHA10) and HA-AL nanocrystals at increasing bisphosphonate (BP) content (HA-AL7; HA-AL28). Methods: A posterolateral spinal fusion model in twenty-five Sham operated and in twenty-five OVX female rats was used and materials were bilaterally implanted between transverse processes of lumbar vertebrae. Sham and OVX animals were divided in five groups depending on the material: HA, SrHA5, SrHA10, HA-AL7 and HA-AL28. The assessment of bone fusion was carried out by μCT, histology and histomorphometry. Results: Some gaps between the transverse processes were observed by μCT in OVX–HA group, while they were not present in all the other groups. These results were consistent with the histomorphometrical analyses showing that in OVX animals SrHA and HA-AL materials displayed significantly higher BV/TV and Tb.Th and significantly lower Tb.N and Tb.Sp in comparison with HA alone. Conclusions: Results of this study suggest that in spinal fusion the incorporation of bioactive ions or drugs as Sr and AL improves the biological performance of HA representing a promising strategy especially in osteoporosis patients with high risks of spinal fusion failure. Results also suggest the existence of a Sr and AL dose response effect and that HA containing the highest AL dose could be the candidate biomaterial for spinal fusion in osteoporotic subjects.