The results of simple biomechanical unloading in models of acute-disuse osteoporosis are influenced by systemic and regional effects of the method used to generate the bone loss. A model in which strain-gauge measurements confirmed that the os calcis was unloaded in healthy ewes during ambulation was assessed by histomorphometry. Twelve nonovariectomized adult female Welsh mountain sheep were submitted to hock joint immobilization by an external fixation procedure from the tibia to the metatarsus for a period of 12 wk. Histomorphometric analysis showed that this model was able to produce pure local bone loss, as transiliac bone biopsies failed to reveal any difference between the initial and final results. Immobilized and nonimmobilized calcanei were both removed postmortem. After the 12 wk of the study, osteoclastic activity was increased in accordance with the usual disuse process. An unexpected increase of osteoblastic activity was also observed, possibly related to recovery after the initial dramatic bone loss, but an artifact of the surgical procedure such as a regional acceleration phenomenon cannot be definitively excluded. However, the increased osteoblastic activity was not sufficient to prevent accentuation of the negative bone balance, resulting in a 29% decrease of trabecular bone volume in immobilized calcanei compared with nonimmobilized calcanei. This reduction was due to thinning of trabeculae (72.4 +/- 12.1 vs. 98.9 +/- 15.9 microns; P < 0.05) without any change in trabecular number (2.74 +/- 0.72 vs. 2.79 +/- 0.40/mm2; not significant). In conclusion, this model only locally increased both osteoclastic and osteoblastic activities leading to bone loss and architectural modifications. The decreased bone formation usually observed in other models of disuse osteoporosis may therefore not constitute a local phenomenon generated by unloading.

BackgroundMany patients affected by spondyloarthritis are at risk of fracture due to the bone's fragility although their bone mineral density (BMD) is not significantly decreased. Other tools are needed to assess changes in the bone of these patients. The Trabecular Bone Score (TBS) is a bone quality assessment tool that analyses variations of the grey level of a densitometric image. It was evaluated among post-menopausal women and in secondary osteoporosis. It may help to discriminate patients with osteoporosis fractures.ObjectivesTo evaluate the TBS in a population of spondyloarthritis as a bone quality indicator and look for the clinical and biological factors correlating to the values of the TBS.MethodsPatients who met the criteria of a spondyloarthritis diagnosis and who had a measure of BMD were selected. All these patients came for the setting up or the changing of a biotherapy. Clinical and laboratory data were collected at the same time as the BMD measurement.The TBS measurements were obtained immediately from dual-energy X-ray absorptiometry spinal images. A TBS was considered normal above 1.34, distorted between 1.34 and 1.2 and very distorted below 1.2.ResultsNinety-eight patients participated in the study. The average age was 40.2 years and the average duration of the disease's evolution was of 8.2 years. A spinal BMD lower than -1 and -2.5 T-score was respectively observed in 17% and 3% of patients. The average TBS was lowered to 1.34. The spinal BMD correlated positively to the values of TBS (r=0.61), whereas the duration of the disease's evolution (r = -0.24), its activity level (r= -0.33) and the serum PTH levels (r= -0.27) correlated negatively to the TBS. This association persisted in multivariate analysis. More than half of patients with a BMD above the threshold of a -2.5 T-score had an altered TBS. The relashionship observed between the rates of the PTH and the TBS, which we do not find with the BMD, suggests that a part of the changes of the bone structure in inflammatory rheumatism such as the spondyloarthritis, would be under the dependence of the PTH regardless of the BMD.ConclusionsThis study shows that TBS provides additional and independent information from the simple BMD measure on bone status of patients with inflammatory rheumatism. It appears necessary to carry out studies on a larger scale to analyse the discriminating power of the TBS in the prediction of fragility fracture to a greater extent.Disclosure of InterestNone declared

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