Arthritic diseases, such as rheumatoid arthritis (RA), are commonly characterized by inflammation-associated deterioration of joint surfaces which can lead to structural joint changes and joint degeneration. This can result in the loss of gross (e.g., walking, sitting, standing, etc.) and fine (e.g., writing, grabbing objects, etc.) motor skills, leading to an overall decrease in quality-of-life (QOL)1–3. This gradual loss of joint function is in part due to the loss of cartilage, leading to reduced joint space and eventual bone-on-bone contact. Clinically, the diagnosis of RA-associated joint degeneration can be determined by the two-dimensional (2D) radiographic evidence of joint space narrowing4. While clinically, joint space changes are assessed and graded as an estimated proportion of lost space via 2D X-ray radiographs, these assessments are not sensitive to smaller joint space changes and overlapping anatomy (i.e. (sub)luxation), such as those found when imaging the metacarpophalangeal joints of the hand. Thus, high resolution 3D imaging techniques may improve the assessment, detection, and quantification of changes in joint space metrics.
High-resolution peripheral quantitative computed tomography (HR-pQCT) is an X-ray based imaging technique that provides unparalleled 3D in vivo images of extremity joints, such as the MCP joints of the hand commonly affected by RA5–8. The three dimensionality of HR-pQCT, in contrast to 2D radiographs, allows for accurate and precise, quantitative 3D derived joint space metrics including joint space width, maximum, and minimum9. These metrics have been applied to HR-pQCT to assess disease course and treatment effects in RA10,11. However, while HR-pQCT-derived 3D joint space metrics differed between RA and healthy controls in a proof-of-concept study, the small sample size limited evaluation of the individual variability that can impact 3D joint space metrics12.
Age, sex, and obesity have been shown to negatively impact joint space narrowing or disease progression in hand osteoarthritis (OA). The continual wear and tear of cartilage with increasing age can contribute to a cartilage thinning, resulting in joint space narrowing13–15. In addition to known sex-based joint size differences 16–18, the inverse relationship between obesity and immune response is known to affect OA disease progression19,20. However, while OA and RA can affect the hand, different joints are typically involved (distal interphalangeal and proximal interphalangeal joints vs metacarpophalangeal joints, respectively). Thus, for RA, an immune-mediated musculoskeletal diseases, understanding the effects of these non-disease related factors (i.e. age, sex and obesity) on 3D joint space width (JSW) metrics from HR-pQCT images is necessary to interpret results JSW results.
Therefore, in this study, we aimed to evaluate whether HR-pQCT-derived 3D joint space metrics could differentiate between RA and age- and sex-matched controls. Additionally, we assessed the effects of age, sex, and obesity (assessed in terms of body mass index, BMI) on HR-pQCT-derived 3D joint space metrics. To accomplish these objectives, we compared 3D joint space metrics from HR-pQCT images of 2nd MCP (MCP2) and 3rd MCP (MCP3) joints between RA patients and their age- and sex-matched controls. By computing various 3D joint space metrics (i.e., volume, width, standard deviation of width, maximum width, minimum width, and asymmetry) from these groups, we determined whether variation in age, sex, and BMI plays a significant role in these acquired quantitative joint metrics. Additionally, we explored how these 3D joint space metrics are related to functional outcomes.