Rheumatoid arthritis (RA) is a systemic inflammatory polyarthropathy characterized by progressive joint damage and nonarticular complications such as osteoporosis (1), accelerated atherosclerosis, and increased risk of malignancy (2). The earliest appropriate initiation of treatment after disease onset offers the best chance of permanent remission and a normal lifespan. In this regard, early diagnosis is essential but is frequently hampered by the lack of sufficient clinical evidence early in the disease course (3).
Joint erosions identified by conventional radiography are late findings indicating a poor prognosis (4). More sensitive diagnostic tools for the early detection of joint damage include magnetic resonance imaging (MRI) and ultrasound (US) (5, 6). MRI provides excellent detail for articular defects but is expensive and less accessible to rheumatologists compared with US, which can be used at the bedside or in the clinic (7).
The results of this study indicate that US examination of the fifth MTP joint in patients who present with early inflammatory arthritis aids in the identification of those with a poor prognosis and has considerable advantages over conventional radiography in the detection of early erosive disease.
We selected the fifth MTP joint for study since it has previously been identified as an area of early joint damage in patients with RA (6, 8, 12). Although the reasons why this joint appears to be involved at an early stage of the erosive process remain speculative, it has been suggested that this area may be prone to early subluxation because of its location (8), therefore subjecting it to early joint damage. In terms of scrutiny, however, the fifth MTP joint provides particular advantages, as it is superficial and readily accessible to imaging. Therefore, it is ideally suited to US examination at the patient's bedside or in the clinic.
Several studies have indicated that US is more sensitive than conventional radiography in the detection of joint damage (5, 6). Although US provides less detail compared with MRI, it nevertheless offers valuable insights into articular and periarticular pathology, thereby helping to guide treatment (13). In recent years, musculoskeletal US has become increasingly accessible to rheumatologists as a bedside diagnostic tool that can be performed as part of an outpatient assessment (14). Although US examination of all inflamed joints would be time-consuming in the clinic setting (7), selection of the joints most likely to yield pathology, such as the fifth MTP joint, could have significant diagnostic benefit and help early therapeutic decision making.
The development of joint damage is thought to result from the influence of local inflammatory cells on the cytokine and enzymatic milieu, causing cartilage destruction and bony erosion (15). Therefore, it is assumed that ongoing synovitis leads to joint erosions. Although this study noted a correlation between the presence of US-detected synovitis and radiographic erosions, US detected more cases of erosions than synovitis. It is possible that US was not sufficiently sensitive to identify synovial inflammation in every case, particularly in a small joint where erosions are more easily identifiable as breaks in the cortical surface of bone rather than soft tissue abnormalities. Alternatively, the synovitis may have preceded the development of bone damage in a condition whose time course is unknown, particularly in early disease. Treatment may also have influenced the US findings, since 11 of the 20 patients without evidence of synovitis had received corticosteroids within 4 weeks of assessment. In this regard, although 53% of patients with US-detected erosions of the fifth MPT joint had a PD signal, this finding did not reach statistical significance (P = 0.12), perhaps because erosions are a retrospective reflection of an inflammatory process, while a PD signal suggests current inflammation.
In this study, the presence of fifth MTP joint erosions did not differentiate RA from UA, but did highlight patients with otherwise undetected joint damage and future erosive potential. A larger cohort and a more prolonged study period may demonstrate such a distinction. A comparison of the total number of erosions between radiography and US was not performed since this study was undertaken primarily to demonstrate the ease with which targeted US can be utilized in a clinical setting and to identify a marker of poor prognosis at an earlier stage than can be demonstrated by radiography.
The current ACR criteria for the diagnosis of RA do not include inflammatory markers, disease activity scores such as the DAS28, anti-CCP antibody positivity, or US-detected erosions (9). However, recent evidence supports inclusion of these variables in any future diagnostic criteria (3, 16–18). We believe that this study contributes to the current knowledge in the field of both early inflammatory arthritis and the application of US in rheumatology clinics.
In patients who present with undifferentiated inflammatory arthritis, US of the fifth MTP joint identifies patients with early joint damage and has the potential to help in the diagnosis of RA where the presence of joint erosions is part of the current diagnostic criteria. This study shows that targeted US of this joint is a quick screening tool for the detection of bony damage and is more sensitive than conventional radiography in this regard. Longitudinal studies with larger patient cohorts would be useful. However, as US becomes more widely available in the rheumatology clinic setting, examination of the fifth MTP joint may yield rapid and valuable information to guide early therapeutic decisions.