Changes of elastic constants and anisotropy patterns in trabecular bone during disuse-induced bone loss assessed by poroelastic ultrasound

Abstract

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 lCT and ultrasound wave propagation. Microstructure values determined from lCT 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 invBMD in both immobilization and risedronate treated groups. Interestingly, changes inapparent 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.

Fig. 1

3D images from μCT scanning. (a) shows the CN + VEH; (b) displays IM + VEH, in (c) an example from the CN + RIS group and a sample form IM + RIS is shown in (d). The different thickness in trabeculae can be observed in each group.

Fig. 2

Global changes in microarchitecture, TMD and vBMD as a consequence of immobilization and antiresorptive treatment. (a)–(c) show the Tb.Th, Tb.Sp, and Tb.N, respectively, and (d)–(f) correspond to BV/TV, TMD, and vBMD. There is a similar pattern for Tb.Th, Tb.N, BV/TV, TMD, and vBMD, and Tb.Sp exhibits the opposite trend. However, significant differences were only found in Tb.Th and BV/TV when comparing CN + VEH versus IM + VEH, indicating a significant effect of immobilization in Tb.Th and BV/TV, and when comparing CN + RIS versus IM + RIS, indicating that immobilization has an effect in Tb.Th and BV/TV, but not in Tb.N, Tb. Sp, TMD, or BMD. It was also found a difference in Tb.Th between IM + VEH and IM + RIS groups, but there is not a difference in Tb.Th or BV/TV when comparing CN + VEH versus IM + RIS, indicating that RIS treatment was effective in slowing the bone loss produced by immobilization.

Fig. 3

Magnitude of Fabric components F₁, F₂, and F₃ (eigenvalues) from trabecular bone samples in the CN + VEH, IM + VEH, CN + RIS, and IM + RIS groups. No significant differences were observed due to immobilization or risedronate treatment; however, the comparison of F₁, F₂, and F₃ within each treated group indicates that bone is orthotropic and remained orthotropic after immobilization and risedronate treatment.

Fig. 4

Fabric anisotropy in all three anatomical planes are shown as ellipsoidal plots for the CN + VEH, IM + VEH, CN + RIS, and IM + RIS groups. Each group shows mean-SD values (red solid inner ellipsoids), the intermediate ellipsoids (blue) represent the mean values and the open mesh ellipsoids show mean + SD. There is a clear fabric anisotropy (p < 0.05) given by directional dependent microarchitecture within each group. Such DA was found similar in all groups (p > 0.05), indicating that immobilization and risedronate treatment had no significant effect on anisotropy of microarchitecture.

Fig. 5

Ultrasonic wave velocities and apparent EC as a function of vBMD (a), theoretical PEUS wave velocities (b), and theoretical EC (c). Coefficient of correlation between wave velocities and vBMD was R ²= 0.58. Theoretical poroelastic wave propagation theory was able to predict 81% of experimental wave velocity variability (R ²= 0.81), and the theoretical poroelastic constants were higher correlated to experimental constants (R ²= 0.91) that vBMD or wave velocity.

Fig. 6

The shift from an ellipsoid (CN + VEH) to a more spherical shape (IM + VEH) shows that bone loss is not uniform in all directions. The antiresorptive therapy during immobilization (IM + RIS) exhibit an ellipsoid smaller than CN + RIS, consistent with bone loss, but it maintains the same anisotropy ratio as the control group, indicating a conservation of mechanical function.