Background: The Bone Material Strength index (BMSi), measured by Impact Microindentation, is a parameter to assess bone material properties at the tissue-level in vivo. BMSi is lower in patients with fragility fractures compared with those without fractures independently of Bone Mineral Density (BMD) values but there is no information about its long-term changes with treatments for osteoporosis.

Objective: To evaluate changes in BMSi in patients with low bone mass treated with antiresorptive agents.

Methods: Therapy-naïve patients who started bisphosphonates or denosumab were assessed with a 24-month interval. Subjects without treatment were used as controls.

Results: 61 patients (38 women) with a mean age of 55.8 years (range 27-79) were included. Baseline characteristics were similar between treatment (n=37) and control groups (n=24) but previous fragility fractures were more frequent and BMSi was lower [78.9(0.8) vs. 81.8(0.8), (p=0.02)] in the treatment group. Femoral neck BMD did not change with treatment while it decreased in the control group (p=0.02). In the treatment group BMSi significantly increased to 82.8(0.8),(p<0.001), with highest increases observed in patients on denosumab [78.3(1.2) to 85.4(1.7), (p=0.02)]. In contrast, BMSi values did not change in the control group. Changes in BMSi with treatment were significantly correlated with baseline values (r=-0.46; p=0.02) but were not related to changes in BMD.

Conclusion: BMSi values increase with antiresorptive treatment in patients with low bone mass and the highest increases were observed in patients with low baseline values. Whether changes in BMSi with treatment will predict fracture outcome remains to be established.