Foot involvement is not uncommon and occurs early in the disease course of rheumatoid arthritis (RA). Inflammation and ongoing synovitis of foot joints lead to joint destruction and instability, tendon dysfunction, and eventually collapse of the medial longitudinal arch and pes planovalgus that contributes to difficulty in walking and gait abnormalities. This article reviews foot-related problems in patients with RA, focusing on the prevalence, natural history and role of imaging in both diagnosis and management of midfoot and subtalar joint disease in RA.
Rheumatoid arthritis (RA) is a multisystemic, chronic progressive inflammatory disease affecting all ethnic groups with overall prevalence of 1–2% of the population. Joint pain, stiffness and swelling are the most notable presenting complaints among patients with RA. The severity of joint disease may fluctuate over time and its clinical course is often unpredictable. Ongoing synovitis with joint inflammation leads to joint destruction, deformity, chronic pain and disability. Early diagnosis of RA followed by the early use of synthetic and biologic disease-modifying anti-rheumatic drugs (DMARDs) may further modify the disease course. In early disease, the wrists, metacarpophalangeal joints, proximal interphalangeal joints of fingers and metatarsophalangeal joints are most commonly affected. As the disease progresses, the shoulders, elbows, knees, feet and ankles may also be involved if diagnosis is delayed and treatment is not initiated early.[4, 5]
The rheumatoid foot
Foot problems are not uncommon in RA and approximately 90% of patients report foot-related complains within 10 years of RA onset.[6-8] Minaker et al. who studied the prevalence of foot problems in 55 RA patients reported foot pain at some stage during the course of disease in up to 90% of their patients. Of these, 86% had clinical involvement and 92% had radiological changes in their feet. Overall, 16–19% of patients being treated for RA presented with signs and symptoms of foot and ankle involvement.[9, 10] Hallus valgus, splaying of forefoot, pes planus and valgus hindfoot are the most typical foot deformities in RA. In a recent study conducted in a cohort of 40 RA patients with disease duration of more than 10 years, frequency of foot deformities was determined as 78%, in which 62% of them had metatarsus primus varus and 41% had splaying of the forefoot. Besides articular pathologies of the feet and ankles, patients with RA may have associated tendinopathies, although the incidence has only been reported to be approximately 7%. Overall, the involvement of the peroneal tendons is more common than the posterior tibial tendon and other extensor tendons of the foot.
Foot problems in early RA
Clinical signs of foot disease in RA are often subtle. Discrepancies between clinical examination and true synovitis or tendon abnormalities have been observed and clinical examination alone is unable to diagnose the precise extent of joint, tendon and soft tissue involvement in RA patients.[7, 13-15] In fact, patients may complain of ill-defined “ankle pain”, swelling behind the malleoli, or dorsum of the feet, and localization of signs may be difficult to pinpoint to specific structures/joints in the ankles/feet. A recent study in a cohort of RA patients with early disease of < 2 years' duration noted that 90% of the patients experienced foot pain at some point of their illness. Among patients with disease duration < 1 year, individual joints of the foot, especially the fifth metatarsophalangeal joint (MTPJ), have been shown to erode more frequently than the individual joints of the hands over a year. In another study, the first MTPJ was shown to be affected in 15% within 1 year, and 28% within 3 years in early RA patients who were on DMARDs. Using magnetic resonance imaging (MRI) as an assessment tool, Calisir et al. observed that there was no significant difference between the MCPJ and MTPJ with respect to RA-based changes obtained in the MRI, with synovitis being evident in 81% of hands and 71% of feet.
Subtalar and midfoot joint involvement in RA
The subtalar joint, or the talocalcaneal joint, is one of the three hindfoot joints. It controls eversion and inversion of the foot on the talus. The midfoot is the link-bridge between the hindfoot and forefoot. It includes the midtarsal (talonavicular and calcaneocuboid), naviculocuneiform (medial, intermediate and lateral), cuboidocuneiform and Lisfranc joints. The prevalence of subtalar and midfoot joint involvement in RA has been reported by Vainio et al. as early as 1956, in which subtalar, talonavicular and calcaneocuboid joint pathologies occurred in 70% of RA patients compared with the ankle, which occurred in 9%. Vidigal et al. who examined the feet of 200 consecutive admissions with chronic RA found that 104 of these patients had foot pain or deformity. Radiologically, midtarsal joint involvement was seen in 62% (124 feet) and subtalar joint disease was noted in 32% (64 feet). In order of decreasing frequency, arthritis in the foot affects the forefoot, midtarsal, subtalar and ankle.
Subtalar joint pain is felt mainly in the lateral hindfoot on activity due to chronic inflammation and destruction. If left untreated, progressive eversion at the subtalar joint, together with dysfunction of peritalar ligaments and the tibialis posterior tendon, subsequently lead to instability of the subtalar and midtarsal joints.[20, 21] Lateral subluxation beginning in the midfoot, causes the collapse of the medial longitudinal arch, pes planovalgus or valgus deformity that contributes to difficulty in walking.[21, 22] The gait abnormalities detected in early RA patients are similar to those reported in established disease. Turner et al. who examined foot function in a small cohort of 12 early RA patients with disease duration < 2 years, found small but clinically important changes and disability in these patients when compared to controls. These included slower walking speeds, a longer double-support phase, reduced heel rise angle in terminal stance, lower medial arch height and greater peak eversion in stance. Pressure analysis indicated lesser toe contact area, elevated peak forefoot pressure and a larger midfoot contact area in these patients.
Imaging of subtalar and midfoot joint disease in RA
Imaging plays a crucial role in the assessment of RA. Among all the imaging techniques, plain radiographs remain the initial screening test for RA patients. In the midfoot, characteristic radiographic features include diffuse joint space loss, bony sclerosis and osteophytosis, with osseous erosion being uncommon. The differentiation of RA involvement from degenerative, post-traumatic or neuropathic disorders may be difficult in these regions. For radiological progression of RA, either the modified Sharp method or the Larsen method is used, but both methods do not specifically address midfoot or subtalar joint involvement. As an assessment and monitoring tool, conventional radiography, despite its low cost, high availability and reproducibility, provides very little information because changes usually take months or even years to develop, by which time the “window of opportunity” for early escalation of treatment may have passed. Moreover, X-ray of the foot is limited by multiple factors, including projectional superimposition caused by the 2-dimensional representation of a 3-dimensional pathology, use of ionizing radiation, relative insensitivity to early bone damage and total insufficiency for assessment of soft tissue changes, including synvoitis (Fig. 1).
It is well known that synovitis, bone marrow edema and bone erosion are important pathologies associated with RA. Imaging modalities should be able to address such changes in the joint, especially in the early stage of disease. MRI and computed tomography (CT) provide useful information about both the features and the extent of anatomic damage in selected RA patients. MRI is very sensitive in detecting bone marrow edema, while CT is good at detection of bone erosion (Fig. 2). However, the high cost, availability of the machines and high radiation exposure hinder their use in clinical practice.
Ultrasonography is one of the techniques that has gained wide acceptance for studying joint, tendon, bursal and bone involvement in RA (Figs 3, 4). It has been increasingly used in rheumatology clinics for assessment and follow-up of these patients as it provides real-time visualization as well as direct identification of bone lesions and extent of synvoitis (Fig. 5). Wakefield et al. reported that ultrasound (US) detected 3.5 times more erosions than radiography in RA. This difference was even greater with early disease. Ultrasound has other benefits, including guidance of steroid injections, thus ensuring accurate treatment applications.[28-31] In recent years, standardized US definitions for different pathologies and scanning guidelines were published by the Outcome Measures in Rheumatology Clinical Trials (OMERACT) US group, although further validation is still pending.[32-34]
Advances in imaging have led to the ability to distinguish between active synovitis and joint destruction. The fifth MTPJ has been reported to be the most common sonographic site of erosion in the foot in patients with RA, suggesting US assessment should be included in the baseline approach to patients with arthritis.[13, 35, 36] MRI and US have also been shown to be more sensitive than clinical examination for detecting synovitis in the forefoot in RA. Further, low-field MRI and US were superior to clinical examination for detection of joint inflammation in RA feet.[13, 37] Using MRI as the gold standard, Wakefield et al. reported that US was more specific in identifying hindfoot and midtarsal joint synovitis and tenosynovitis compared with clinical examination in patients with established RA. Woodburn et al. who utilized MRI techniques to compare geometric architecture of subtalar and midtarsal joints in normal and symptomatic feet of 23 RA patients, concluded that only subtalar joint synovitis was predictive of abnormal subtalar and midtarsal architecture.
To date, most published reports on foot and ankle involvement in RA have focused predominantly on forefoot and hindfoot pathologies. More studies are needed for better understanding of the impact of the RA foot, especially on the prevalence, pattern of involvement and imaging of subtalar and midfoot joint disease in RA. With the help of different imaging techniques in rheumatology practice, such as ultrasonography, MRI and CT, detection of early or subclinical foot problems is facilitated, which allows prompt pharmacological and non-pharmacological treatment, ultimately improving foot function and quality of life for RA patients.