The Rheumatoid Foot and Ankle

The foot and ankle are involved in over 90 percent of patients with rheumatoid arthritis at some point in the disease. The forefoot is the most common site of pain and deformity and the single most frequent reason an RA patient sees a foot and ankle surgeon. The hindfoot is the second most common region, and its involvement correlates with disease duration and seropositivity.

Who. Women are affected 2-3 times more often than men, and seropositive disease carries the highest foot burden. The foot is the initial presentation of RA in 15-20 percent of patients.

Where. Prevalence by region:

• Forefoot - symptoms in 70-80 percent of RA patients at 10-year follow-up
• Midfoot - 20-30 percent
• Hindfoot - valgus in 30-50 percent of established RA
• Ankle - 10-30 percent; less common than forefoot or hindfoot disease, but produces most disability

The forefoot dominates by prevalence, but the hindfoot dominates by functional impact.

What it costs the patient. Forefoot pain is the leading cause of walking disability in RA, and in long-standing disease plantar callosities and ulceration form over the prominent metatarsal heads. Footwear modification fails once fixed MTP dislocation is established. Fall risk rises with hindfoot valgus and ankle arthritis, and quality-of-life scores (HAQ, AOFAS) drop sharply with bilateral hindfoot disease.

View larger

Synovitis drives every deformity. Chronic synovial inflammation is the primary driver of every deformity in the RA foot. Pannus, cytokines (TNF-alpha, IL-6, IL-17) and erosive enzymes destroy cartilage and ligament: the pannus erodes articular cartilage from the joint margins inward, weakens the collateral ligaments and plantar plate, and invades adjacent tendons, especially tibialis posterior, peroneus longus and the FDL and FHL to the toes. Once the static stabilisers fail, muscle imbalance, gravity and ground reaction force drive the foot into the classic RA deformities.

The forefoot. The MTP joint is the first casualty. Synovitis erodes its cartilage and capsule and attenuates the plantar plate and collateral ligaments, and plantar plate attenuation is the single most important step in forefoot collapse: the plate no longer restrains dorsiflexion, so the proximal phalanx drifts dorsally over the metatarsal head and the head is left plantar.

The fat pad and the claw. The plantar fat pad does not atrophy. It is dragged distally with the subluxing phalanx, so it still exists but no longer sits under the metatarsal heads, and painful callosities form over the prominent heads and often ulcerate. The long flexors (FDL, FHL) now pull obliquely: FDL slips dorsally, becoming a dorsiflexor of the PIP and a plantarflexor of the DIP, and the toe claws.

The first ray and the lesser toes. In the arthritic hallux valgus the first ray drifts medially and the first MTP subluxes laterally. The lesser toes crowd laterally and dislocate dorsolaterally.

The hindfoot. Chronic tenosynovitis and intrasubstance degeneration of tibialis posterior drive hindfoot valgus, and the tenosynovitis is present on ultrasound in 30-50 percent of RA feet. With the medial arch unsupported and the spring ligament elongated by chronic synovitis, the talar head uncovers medially, the navicular abducts, the calcaneus drifts into valgus and forefoot abduction follows. The foot flattens, yet this is a different disease entity from adult-acquired flatfoot deformity.

Around the hindfoot. Subtalar and talonavicular arthritis often coexist. Lateral impingement (calcaneofibular abutment) gives sinus tarsi pain, and gastrocnemius tightness contributes equinus.

The ankle. Marginal erosions and cartilage loss at the tibiotalar joint lead to varus or valgus collapse and an end-stage, painful, stiff ankle.

The rheumatoid foot is described by region, by the severity of the forefoot deformity, and by disease activity at the time of surgery. Severity guides the choice between joint-preserving (early) and joint-sacrificing (late) surgery.

History. Establish where the pain is (forefoot most commonly, then hindfoot or ankle) and how long the disease has run, since longer disease brings more deformity and disability. Record the current regimen (methotrexate, anti-TNF, rituximab, JAK inhibitors), previous forefoot or hindfoot surgery, and any hip or knee arthroplasty. A patient who can no longer tolerate off-the-shelf shoes has severe deformity; ask about current or recurrent ulceration of plantar callosities.

Examination. Always examine the contralateral foot and the footwear as well.

• Forefoot - MTP synovitis, dorsal subluxation, fixed or flexible claw, hallux valgus angle
• Callosities - plantar under the lesser metatarsal heads, and medial over the first metatarsal in hallux valgus
• Hindfoot - heel valgus, sinus tarsi tenderness, and tibialis posterior function (below)
• Ankle - the tibiotalar line, range of motion, and whether the pain is tibiotalar or subtalar
• Skin and nails - capillary refill, prior ulceration, onychomycosis
• Gait - antalgic, externally rotated, or vaulting over a stiff ankle

Radiographs first. RA is largely a clinical and X-ray diagnosis, and standing films are the cornerstone. Take them weight-bearing:

• Foot - AP, lateral and oblique
• Ankle - AP, mortise and lateral

Templating for a first MTP arthrodesis or TAA is done on standing films, not supine.

What the films show. The hallmarks are marginal erosions, not the central erosions of OA, juxta-articular osteopenia rather than subchondral sclerosis, and symmetric joint space loss. Erosions appear first at the metatarsal heads and PIP joints, and the 5th metatarsal head is often the first site to show erosive change, which makes it a useful early sign. In established disease the subchondral bone is moth-eaten rather than sclerotic as in OA, and advanced disease forms cysts, occasionally leading to pathological fracture of a metatarsal head. Look also for hindfoot valgus and tibiotalar arthritis.

Ultrasound. It detects active synovitis when the radiographs are normal and confirms tibialis posterior tenosynovitis. Power Doppler grades synovial activity, guides injection and monitors the response to treatment, and because it is bedside, cheap and dynamic it can assess tibialis posterior dysfunction in real time.

MRI. It shows early RA change (bone marrow oedema, early erosions), tendon pathology including tibialis posterior tenosynovitis, spring ligament injury and sinus tarsi synovitis, and it is indicated for atypical pain. Before surgery it defines cartilage status before TAA, the extent of talar dome AVN and midfoot involvement.

CT. For planning an arthrodesis, for complex hindfoot deformity and after prior failed surgery.

Bloods and aspiration. CRP, ESR, full blood count, renal and liver function, and DMARD levels if available. Aspirate the joint if there is any suspicion of septic arthritis, especially in a patient on biologics.

Before any operation. Before the patient is listed, agree the medical regimen with rheumatology and assess the cervical spine.