MATTERS ARISING Sonographic erosions of the rheumatoid little toe

We read with interest the pictorial essay on ultrasonography of bone erosions by Grassi and colleagues. The presented site-specific comparison of radiographic and sonographic imaging of metacarpophalangeal (MCP) and metatarsophalangeal (MTP) joint sites in rheumatoid subjects suggests strongly a homology of the erosive lesions, as visualised by these diVerent imaging modalities. A recently published study by an independent group, comparing radiographic and sonographic imaging of MCP joints in patients with rheumatoid arthritis (RA) supports this impression. The same study observed magnetic resonance imaging (MRI) changes corresponding to specific sites at selected radiograph-negative joints that had sonographic erosions, and found (depending on disease duration) 3.4to 6.5-fold more erosions with ultrasound than with radiography. In our experience, based on pilot data on rheumatoid patients with a disease duration of up to six years, sonographic erosions could be shown in half of all 5th MTP joints examined. Fifteen patients with RA, according to standard criteria, (median age (range) 48 (23–78) years; eight female; median disease duration (range) 13 (1.5–72) months; 12 rheumatoid factor positive; 12 receiving disease modifying treatment), were examined for erosions by a rheumatologist, trained in musculoskeletal ultrasound (RK). An L12–5, 38 mm linear array, high frequency transducer with an ATL HDI 5000 ultrasound system (Advanced Technologies Laboratories, Bothell, WA, US) was used. The following seven sites were chosen for ease of transducer access, as well as early, characteristic, and/or representative involvement by RA erosions: ulnar head/styloid; radial head/styloid; 2nd MCP joint (ulnar aspect); 3rd proximal interphalangeal joint (ulnar and radial aspect); 1st MTP joint (medial aspect); and the 5th MTP joint (lateral aspect). All four limbs were examined and to ensure comparability of sites, only bone lesions in the coronal plane were considered. All sites were examined in longitudinal and transverse planes in joint extension, and were classified as erosive, if they had at least one “break” in the cortical contour, visible in both planes and associated with an irregular floor. The latest available posteroanterior x ray of hands and feet (median time interval preceding ultrasound (range) 3 (0–18) months) was assessed for the presence of erosions at corresponding sites by a radiologist with a special interest in musculoskeletal imaging (DG), who was unaware of the sonographic findings. The 1st MTP joint was excluded from the analysis wherever the radiograph showed osteoarthritic change, because sonographic assessment for erosions was felt to be unreliable. A total of 13 sites (in seven subjects) had radiographic erosions; all except for one ulnar site were identified by ultrasound. Sonography detected a total of 56 erosive sites (in 11 subjects)—that is, four times as many as radiography. Two patients without radiographic erosions at the study sites had erosions elsewhere in the radiographs of their hands and feet, but both had erosive sites on ultrasound. Table 1 shows the frequency of radiographic and sonographic sites with erosions. Figure 1 shows an example of a sonographic erosion at the 5th MTP joint that was not seen on radiography. Recently a Dutch study of patients with early RA, followed up radiographically for six years, found the 5th MTP joint to be the most common hand or foot joint aVected by erosions at baseline, as well as by new and progression of erosions in the first and fifth year of follow up. Although our study is limited by lack of data on sonographic reliability or corroborative MRI imaging, its findings add support to the notion that the rheumatoid 5th MTP joint is probably the most common site of sonographic as well as radiographic erosions. This oVers yet further potential for earlier diagnosis and treatment of erosive arthritis, justifying more studies into the diagnostic specificity of sonographic erosions of this and other MTP joints.