The clinical importance of ultrasound detectable forefoot bursae in rheumatoid arthritis

Sir, RA commonly affects the feet-reducing mobility. This is generally thought to be due to synovitis or joint damage in the MTP joints [1]. However, the role of soft tissues, such as bursae, has been less well explored. Musculoskeletal ultrasound (MSUS) is a valuable clinical tool that is comparable with and more readily accessible than MRI in assessing soft tissues in RA [2]. Using MSUS, a higher prevalence of bursae within the forefoot has been reported than in control participants [3]. Others have suggested that bursae within the foot in RA may cause clinical symptoms when they enlarge or become inflamed [4]. The relevance of bursae within the forefoot in RA patients detected by MSUS that are not clinically apparent, however, remains speculative. The aim of our study was to investigate the prevalence of forefoot bursae in patients with RA using MSUS and to assess their contribution to patient-related foot disability. We investigated patients with RA who were diagnosed according to the ACR criteria [5] and 50 gender-matched healthy participants. Ethics committee approval (Southampton and SW Hampshire Ethics Committee) was obtained, and all participants gave informed written consent. The feet of all participants were examined for the presence of bursae by an experienced podiatrist. The impact of foot disease was measured using both subscales of the Leeds Foot Impact Scale (LFIS), impairment/footwear (LFISIF) and activity limitation/participation restriction (LFISAP) [6]. Before starting the study, the reliability of scanning technique for the detection of forefoot bursae had been demonstrated against an expert radiologist [7]. MSUS foot scans were performed using a Diasus ultrasound system with a broadband linear 5–12 MHz probe (Dynamic Imaging, Scotland, UK) and recorded according to the EULAR (European League Against Rheumatism) guidance of longitudinal and transverse scan technique [8]. A total of 149 RA patients [119 females and 30 males; 33 seronegative and 114 seropositive; mean age 59.3 (range 25–87) years] completed the study. The mean disease activity was moderate (DAS28 µ 3.9; range 1.1–6.9) with mean disease duration of 12.3 (range 0.5–43) years. The use of MSUS to detect pathology in RA that is not clinically apparent and to guide treatment decisions is gaining recognition [9, 10]. Our results confirmed a significantly higher (P < 0.001) prevalence of bursae in patients with RA detectable by MSUS (92.6%; n = 138) than detectable by clinical examination (23.5%; n = 35). No bursae were observed clinically in the control participants, although on MSUS, 38% (n = 19) had detectable bursae. The mean number of MSUS detectable bursae per individual were, however, significantly smaller (P < 0.001) for controls (µ 0.68, range 0–3) than for RA participants (µ 3.54, range 0–9). The most common location for MSUS-detectable bursae in both groups was the 4/5 intermetatarsal space for both feet (seen in Fig. 1, which represents a cross-section of a left forefoot at the level of the metatarsal heads). Differences between MSUS-detectable bursae of the control participants and study RA participants were statistically significant (P < 0.001), even after adjusting for age. A cross-section of a left forefoot at the level of the metatarsal heads. M1: first metatarsal head; M2: second metatarsal head; M3: third metatarsal head; M4: fourth metatarsal head; M5: fifth metatarsal head; c: control participants; MS: medial sesamoid; LS: lateral sesamoid. A key and unique finding within our study was the significant association between the number of MSUS-detectable bursae and impact of RA disease on the foot for both LFISIF [Pearson's correlation coefficient (PCC) = 0.671; P = 0.026] and LFISAP (PCC = 0.216; P = 0.009), which was independent of age, weight and disease duration. When the association of MSUS-detectable bursae and LFIS was adjusted for disease activity ESR, CRP, DAS28 and global well-being (visual analogue scale) and forefoot deformity (MTP joint subluxation), there was still a significant association, suggesting that bursae do contribute to patient-related foot disability independent of overall disease activity and forefoot deformity. This study has several strengths and a number of potential limitations. It was a large, pragmatic clinical study representative of secondary care in the UK using patient-reported clinical outcomes including disease activity and foot-specific measures. Unfortunately, we did not have access to MRI to validate the use of MSUS in identification of the presence of bursae within the forefoot. Data from dynamic plantar pressure measurements and gait assessments to better define foot status and determine foot loading would have also been useful and will form the subject of future study. In summary, our results further demonstrate the use of MSUS in evaluation of forefoot bursae that are often missed by clinical examination. They also provide new knowledge that MSUS-detectable bursae in the RA forefoot are common and appear to be a significant contributory factor to patient-related foot disability independent of disease activity. These findings suggest that bursae within the forefoot deserve increased clinical attention, and that further research on the potential benefits of the use of MSUS to facilitate diagnosis and timely interventions for foot symptoms in RA is needed. Disclosure statement: The authors have declared no conflicts of interest.