Changes of Elastic Constants and Anisotropy Patterns in Trabecular Bone during Disuse-Induced Bone Loss Assessed by Poroelastic Ultrasound

Currently, the approach most widely used to examine bone loss is the measurement of bone mineral density (BMD) using dual X-ray absorptiometry (DXA). However, bone loss due to immobilization creates changes in bone microarchitecture, which in turn are related to changes in bone mechanical function and competence to resist fracture. Unfortunately, the relationship between microarchitecture and mechanical function within the framework of immobilization and antiresorptive therapy has not being fully investigated. The goal of the present study was to investigate the structure-function relationship in trabecular bone in the real-world situations of a rapidly evolving osteoporosis (disuse), both with and without antiresorptive treatment. We evaluated the structure-function relationship in trabecular bone after bone loss (disuse-induced osteoporosis) and bisphosphonate treatment (antiresorptive therapy using risedronate) in canine trabecular bone using μCT and ultrasound wave propagation. Microstructure values determined from μCT images were used into the anisotropic poroelastic model of wave propagation in order to compute the apparent elastic constants (EC) and elastic anisotropy pattern of bone. Immobilization resulted in a significant reduction in trabecular thickness (Tb.Th) and bone volume fraction (BV/TV), while risedronate treatment combined with immobilization exhibited a lesser reduction in Tb.Th and BV/TV, suggesting that risedronate treatment decelerates bone loss, but it was unable to fully stop it. Risedronate treatment also increased the tissue mineral density (TMD), which when combined with the decrease in Tb.Th and BV/TV may explain the lack of significant differences in vBMD in both immobilization and risedronate treated groups. Interestingly, changes in apparent EC were much stronger in the superior-inferior (SI) direction than in the medial-lateral (ML) and anterior-posterior (AP) anatomical directions, producing changes in elastic anisotropy patterns. When data were pooled together, vBMD was able to explain 58% of ultrasound measurements variability, a poroelastic wave propagation analytical model (i.e., BMD modulated by fabric directionality) was able to predict 81% of experimental wave velocity variability, and also explained 91% of apparent EC and changes in elastic anisotropy patterns. Overall, measurements of vBMD were unable to distinguish changes in apparent EC due to immobilization or risedronate treatment. However, anisotropic poroelastic ultrasound (PEUS) wave propagation was able to distinguish functional changes in apparent EC and elastic anisotropy patterns due to immobilization and antiresorptive therapy, providing an enhanced discrimination of anisotropic bone loss and the structure-function relationship in immobilized and risedronate-treated bone, beyond vBMD.