Ankylosing spondylitis (AS) is a chronic, inflammatory rheumatic disease that primarily affects the axial skeleton, including the spine and sacroiliac joints. However, AS is a systemic condition that can also involve peripheral joints, entheses (sites where tendons or ligaments attach to bone), and even extra-articular structures such as the eyes, heart, and lungs. While much attention is given to spinal manifestations, the foot is frequently affected in AS, leading to significant pain, stiffness, and functional impairment. This essay explores how ankylosing spondylitis affects the foot, including its pathological mechanisms, clinical manifestations, diagnostic challenges, and management strategies.

Pathophysiology of Ankylosing Spondylitis in the Foot

Ankylosing spondylitis is classified as a seronegative spondyloarthropathy, characterized by inflammation, fibrosis, and eventual ossification of affected structures. The disease primarily targets entheses, leading to a condition known as enthesitis. In the foot, common entheseal sites include:

• Achilles tendon insertion (calcaneus)
• Plantar fascia attachment (calcaneal tuberosity)
• Metatarsophalangeal (MTP) joints
• Tarsal joints (subtalar, talonavicular, and calcaneocuboid joints)

The inflammatory process begins with immune-mediated infiltration of lymphocytes and macrophages, leading to localized swelling, pain, and eventual structural damage. Over time, chronic inflammation can result in bone erosion, followed by abnormal bone formation (enthesophytes), contributing to stiffness and reduced joint mobility.

Clinical Manifestations in the Foot

1. Enthesitis

Enthesitis is one of the most common foot-related symptoms in Ankylosing spondylitis. The most frequently affected sites are:

• Achilles Tendon Enthesitis: Inflammation at the insertion of the Achilles tendon into the calcaneus causes posterior heel pain, swelling, and difficulty walking, especially during the first steps in the morning or after prolonged rest.
• Plantar Fasciitis: Inflammation of the plantar fascia near its calcaneal attachment leads to sharp, stabbing heel pain, often worse upon weight-bearing after rest.

2. Arthritis of the Foot and Ankle

Peripheral arthritis occurs in up to 30% of Ankylosing spondylitis patients, with the foot and ankle commonly involved. Key presentations include:

• Tarsitis: Inflammation of the tarsal joints (subtalar, talonavicular) causes diffuse midfoot pain, swelling, and stiffness, often mimicking other conditions like rheumatoid arthritis.
• Metatarsophalangeal (MTP) Joint Involvement: The first MTP joint is frequently affected, leading to pain and limited toe movement, sometimes resembling gout or osteoarthritis.

3. Dactylitis (“Sausage Toe”)

Dactylitis is a hallmark feature of spondyloarthropathies, characterized by diffuse swelling of an entire toe due to synovitis and tenosynovitis. It most commonly affects the second, third, or fourth toes and can be extremely painful.

4. Structural Deformities

Chronic inflammation and abnormal bone remodeling can lead to structural changes, including:

• Pes Planus (Flatfoot): Due to ligamentous laxity and tarsal joint damage.
• Hallux Valgus (Bunion): Progressive deformity of the first MTP joint.
• Ankylosis (Fusion): In severe cases, chronic inflammation leads to bony fusion of affected joints, significantly impairing mobility.

5. Extra-Articular Foot Manifestations

• Nail Changes: Psoriasis-like nail dystrophy (pitting, onycholysis) may occur, especially in patients with overlapping psoriatic arthritis features.
• Skin Lesions: Rarely, AS patients may develop plantar pustulosis or keratoderma blennorrhagicum (associated with reactive arthritis).

Diagnostic Challenges

Diagnosing foot involvement in Ankylosing spondylitis can be challenging due to overlapping symptoms with other conditions, such as:

• Plantar fasciopathy (mechanical vs. inflammatory)
• Gout or pseudogout (acute MTP/tarsal pain)
• Rheumatoid arthritis (symmetrical small joint involvement)
• Osteoarthritis (degenerative joint changes)

Diagnostic Tools

1. Clinical Examination: Tenderness at entheseal sites, joint swelling, and restricted motion.
2. Imaging:

• X-rays: May show erosions, enthesophytes, or joint fusion.
• MRI: Detects early enthesitis, synovitis, and bone marrow edema.
• Ultrasound: Useful for assessing tendon and ligament inflammation.

1. Laboratory Tests:

• Elevated CRP/ESR (indicating inflammation)
• HLA-B27 positivity (present in ~90% of AS patients, though not diagnostic alone)

Management Strategies

1. Pharmacological Treatment

• NSAIDs: First-line therapy for pain and inflammation (e.g., ibuprofen, naproxen).
• DMARDs: Sulfasalazine or methotrexate may help peripheral arthritis but are less effective for axial disease.
• Biologics: TNF inhibitors (e.g., adalimumab, etanercept) and IL-17 inhibitors (e.g., secukinumab) are highly effective for enthesitis and arthritis.
• Corticosteroid Injections: Local injections into affected joints or entheses can provide short-term relief.

2. Non-Pharmacological Approaches

• Footwear Modifications: Supportive shoes with cushioned insoles, arch support, and heel pads reduce pressure on inflamed entheses.
• Orthotics: Custom orthoses can correct biomechanical abnormalities (e.g., pes planus).
• Physical Therapy: Stretching (Achilles, plantar fascia) and strengthening exercises improve mobility.
• Weight Management: Reducing excess body weight decreases mechanical stress on the feet.

3. Surgical Interventions

Surgery is rarely needed but may be considered for severe cases:

• Enthesophyte Removal: For refractory Achilles or plantar enthesophytes.
• Joint Fusion (Arthrodesis): For painful, unstable tarsal joints.
• Bunion Correction: If hallux valgus causes significant disability.

Ankylosing spondylitis significantly impacts the foot through enthesitis, arthritis, dactylitis, and structural deformities. These manifestations contribute to pain, stiffness, and functional limitations, often requiring a multidisciplinary approach for effective management. Early diagnosis and targeted treatment—including NSAIDs, biologics, orthotics, and physical therapy—are crucial in preventing long-term disability. Increased awareness of foot involvement in AS among clinicians can lead to better patient outcomes and improved quality of life for affected individuals. Future research should focus on optimizing therapeutic strategies to mitigate foot-related complications in AS.