Original ArticleIdentification of Rheumatoid Arthritis Patients With Vertebral Fractures Using Bone Mineral Density and Trabecular Bone Score
Introduction
Bone involvement is the main extra articular complication of rheumatoid arthritis (RA). Patients with RA have a greater risk of osteoporosis and fracture than the general population (1). The most important factor involved in the pathogenesis of osteoporosis is the inflammation because of disease activity through the effect of inflammatory cytokines. Other well-known risk factors are glucocorticoids (GCs) use, menopausal status, low body mass index (BMI, kg/m2), and reduced physical activity (1). Population-based controlled studies have shown that the relative risk of having at least 1 vertebral fracture (VF) is 1.7–2.3 2, 3, 4 and up to 6.2 (5) in RA patients. Consequences of VFs such as chronic back pain, thoracic kyphosis, functional impairment, and back-related disability, are added to the disability of the RA itself (1).
A low bone mineral density (BMD) is also a determinant of VF risk. Prevalence of osteoporosis in RA is 20–30% at the spine and 7–26% at the hip 6, 7, 8, 9, 10. However, there is a discrepancy between low BMD and fracture risk, and a number of fractures are observed in patients with T-scores, which are not in the osteoporotic range. T-scores in RA patients with VFs are between −1.2 and −2.7 at the spine 2, 11, and between −1.4 and −1.7 at total hip 2, 11, but the risk of VFs has been described to be higher in RA compared with patients with postmenopausal osteoporosis or controls 2, 4, 5, 11. This discrepancy may be related to alterations of bone, which are not captured by BMD measurements, that is, changes in bone quality. Such decreases in bone quality (including parameters of mineralization, bone matrix, microarchitecture) have been described to be related to both inflammation (1) and long-term GCs treatment 12, 13. A challenge in clinical practice is thus to have a tool able to detect patients with a risk of having fractures although their BMD is not in the osteoporotic range.
The trabecular bone score (TBS) is a texture parameter assessing the pixel gray-level variations in dual-energy X-ray absorptiometry (DXA) images. The method was initially based on micro-computed tomography (μCT, 3 dimensional [3D]) images, then adapted for 2-dimensional projections obtained by DXA. The method builds an experimental variogram from the gray-scale variations in pixels in multiple random directions, and TBS is the slope at the origin of this variogram (on a log-log representation). There is no direct relation of TBS with microarchitectural parameters, nor trabecular network. On trabecular bone specimens, TBS measured with an experimental tool is correlated with the main 3D microarchitectural parameters, measured by μCT 14, 15.
The software for TBS computation can be installed on DXA machines, and TBS is automatically calculated consecutively on BMD measurement. A low TBS value indicates few gray-level variations of large amplitude and is intuitively interpreted as a low quality of bone texture.
In postmenopausal women, TBS is lower in patients with osteoporotic fractures compared with BMD-matched women without fracture (16). In a retrospective analysis of the Manitoba Study, TBS predicts osteoporotic fractures independent of bone density (17).
The aim of this study was to test TBS, BMD, and their combination in the detection of RA patients with VF.
Section snippets
Study Subjects Selection
Participants were 185 women with RA who fulfilled the American College of Rheumatology criteria (18). They were consulting in the tertiary Department of Rheumatology of Cochin Hospital, Paris, France between February 2009 and July 2010 for a BMD measurement as part of the routine procedure in RA.
Clinical assessment included demographic data: age, height, weight, and BMI (kg/m2). Disease duration was defined as the time elapsed between the onset of first disease-related symptoms and enrollment.
Patients’ Characteristics
Between February 2009 and July 2010, 185 women with RA (mean age of 56.0 ± 13.5 yr) were included in the study. Their main characteristics are summarized in Table 1. Among them, 133 (71.9%) women were rheumatoid factor positive. The mean disease duration of RA was 15.5 ± 9.9 yr. In our population, 162 women (88.1%) were treated with DMARDs, 130 (70.3%) with biological agents, and 112 (60.5%) were currently treated with GCs at a mean daily dose of 6.4 ± 4.3 mg per day equivalent prednisone.
Thirty-three
Discussion
This study is the first assessing the value of TBS in patients with RA. A high proportion of these patients have VFs although their bone density is above the osteoporotic threshold, and a low TBS is associated with a higher risk of having such fractures.
Our results confirm that a generalized osteoporosis is observed in RA: 30% of our patients with a mean age of 56 yr had a T-score below −2.5, and 18% had at least 1 VF. Both hip BMD and lumbar spine TBS were correlated to HAQ, which assesses the
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