Background Rheumatoid arthritis (RA) is characterized by synovitis leading to bone erosions and joint destruction, but little is known about bone structure in the joints of patients before they are diagnosed with RA.

High Resolution peripheral Quantitative Computed Tomography (HR-pQCT) is a new imaging technique allowing detailed automatic evaluation of bone structure.

Objectives The aim was to quantify the cortical and trabecular bone structure in the metacarpophalangeal (MCP) joints of anticitrullinated peptide antibody (ACPA) positive patients with arthralgia and compare with healthy controls.

Methods Using a cross-sectional study design patients were recruited from local rheumatologists and controls from a website for test subjects. All individuals underwent medical history interview, clinical examination, and biochemical screening including ACPA. Patient inclusion criteria were positive ACPA and arthralgia. Exclusion criteria were fulfillment of the 2010 ACR/EULAR criteria for RA and other rheumatologic or metabolic bone diseases. Exclusion criteria for controls were known rheumatologic or metabolic bone disease, current or prior arthralgia, arthritis, and positive ACPA.

The right hand MCP joints were imaged with HR-pQCT using an isotropic voxel size of 82 μm.

A region of 1.2 cm located proximal to the articular surface of the MCP head was evaluated in the 2nd and 3rd finger using a semiautomatic program. Volumetric bone mineral density (vBMD), bone volume fraction (BV/TV), trabecular number (Tb.N), trabecular thickness (Tb.Th), trabecular separation (Tb.Sp), average cortical thickness (Ct.Th), and cortical area (Ct.Ar) were evaluated. Nine patients were rescanned after repositioning and the coefficient of variation (CV) was calculated. Values are mean (SD).

Results Twenty-nine ACPA positive patients without clinical arthritis (age 51 (13)) and 29 healthy controls (age 50 (14)) were included. CV of the HR-pQCT parameters was 0.64–3.53%.

The overall bone density was similar in the groups since vBMD and BV/TV did not differ. In addition, the cortical bone was not affected in patients, as we found no difference in Ct.Th and Ct.Ar between the groups. In contrast, the trabeculae were significantly (p <0.05) thinner in both 2nd (87.8 (11.1) μm vs 93.9 (9.8) μm) and 3rd (84.3 (13.5) μm vs 89.8 (9.8) μm) MCP head compared with controls, whereas Tb.N and Tb.Sp did not differ.

Conclusions It has earlier been demonstrated that inflammation of the synovium and bone marrow is crucial for development of erosions in RA. In the present study we demonstrate that the trabecular thickness is affected very early, i.e. before the onset of clinical arthritis. Therefore, up-regulation of bone resorption in the bone marrow may be an early feature of RA. Bone loss before clinical RA warrants earlier diagnosis and treatment.

Acknowledgement This work was supported by grants from the Danish Rheumatism Association and the Scandinavian Journal of Rheumatology.

Disclosure of Interest None declared