Background: Parathyroid hormone (PTH) is important for bone mass maintenance and integrity. It’s continuous excess in primary hyperparathyroidism (PHPT), however, increases bone turnover in favor of bone resorption leading to loss particularly of cortical bone with relative preservation of trabecular bone. Accordingly, Bone Mineral Density (BMD) is decreased typically at cortical sites with relative preservation at trabecular sites. Despite these findings, fracture risk is increased at both non-vertebral and vertebral sites, and fractures occur at higher BMD values than expected. We therefore hypothesized that components of bone quality other than BMD such as tissue-level properties also are affected.

Aim: To evaluate tissue-level properties of bone using Impact Microindentation (IMI) in vivo in patients with PHPT.

Methods: We measured Bone Material Strength index (BMSi) by IMI at the midshaft of the tibia in 25 patients with PHPT (21 women), 10 of whom had prevalent fragility fractures (FF), and in 25 euparathyroid controls matched for age, sex and FF status (21 women).

Results: Mean age of PHPT patients and controls was 63.7±13.7 and 62.6±12.5 years, respectively. Calcium and PTH levels were significantly higher in PHPT patients as expected. Lumbar spine and femoral neck BMD were comparable between groups. However, BMSi was significantly lower in PHPT patients compared to controls (76.7±5.5 vs. 82.2±4.6, p<0.001). In addition, BMSi was significantly lower in the 10 PHPT patients with FF compared to 15 PHPT patients without FF (74.0±5.5 vs. 78.4±5.2, p=0.03).

Conclusion: BMSi was significantly lower in patients with PHPT compared to euparathyroid matched controls, independently of BMD values, and PHPT patients with prevalent fragility fractures had the lowest BMSi. Our data indicate that tissue-level properties of bone are impaired in patients with PHPT, which might help explain their increased global fracture risk. Our findings also suggest that Impact Microindentation might be a valuable additional tool in the evaluation of bone fragility in patients with PHPT.