Ankylosing spondylitis (AS) is a chronic inflammatory arthritis that primarily affects the spine and sacroiliac joints, leading to pain, stiffness, and potential fusion of vertebrae over time. While the condition is most commonly associated with spinal symptoms, its impact extends far beyond the back, significantly affecting peripheral joints and tissues throughout the body. Among these peripheral manifestations, foot involvement represents one of the most common and debilitating aspects of AS, occurring in approximately 50-90% of patients during the course of their disease. Understanding how ankylosing spondylitis affects the feet is crucial for both patients and healthcare providers, as early recognition and appropriate management of foot symptoms can significantly improve quality of life and prevent long-term complications.
The Inflammatory Process in AS and Foot Involvement
Ankylosing spondylitis belongs to a group of conditions called spondyloarthropathies, characterized by inflammation at sites where tendons, ligaments, and joint capsules attach to bone—areas known as entheses. This process, called enthesitis, is central to understanding how AS affects the feet. The feet contain numerous entheses, making them particularly vulnerable to the inflammatory processes characteristic of AS. The chronic inflammation associated with AS is driven by an overactive immune system, often triggered by genetic factors, particularly the HLA-B27 gene, which is present in approximately 90% of AS patients.
When inflammation occurs in the feet, it typically begins at the entheses and can spread to surrounding tissues, including joints, tendons, and even bone. This inflammatory cascade leads to pain, swelling, stiffness, and eventually structural changes that can permanently alter foot function and biomechanics. The inflammatory process in AS is systemic, meaning that foot symptoms often coincide with or precede spinal symptoms, making foot involvement an important early indicator of the disease.
Specific Foot Manifestations of Ankylosing Spondylitis
Plantar Fasciitis and Heel Pain
One of the most common foot manifestations of AS is plantar fasciitis, which affects approximately 40-60% of patients with the condition. In AS-related plantar fasciitis, inflammation occurs at the insertion point of the plantar fascia into the heel bone (calcaneus). Unlike mechanical plantar fasciitis seen in the general population, AS-related plantar fasciitis tends to be more persistent, bilateral, and resistant to conventional treatments. Patients typically experience sharp, stabbing pain in the heel, particularly with the first steps in the morning or after periods of rest.
The inflammatory nature of AS-related plantar fasciitis means that pain may not follow the typical pattern of mechanical plantar fasciitis. Instead of improving with activity, AS patients may experience persistent pain throughout the day, accompanied by significant stiffness and swelling. This can lead to altered gait patterns and compensatory mechanisms that further stress other parts of the foot and lower extremity.
Achilles Tendonitis and Retrocalcaneal Bursitis
The Achilles tendon insertion at the posterior heel is another common site of inflammation in AS patients. Achilles tendonitis in AS typically presents as pain, swelling, and stiffness at the back of the heel, often accompanied by inflammation of the retrocalcaneal bursa—a small fluid-filled sac that cushions the area between the Achilles tendon and heel bone. This condition, known as retrocalcaneal bursitis, can cause significant disability and may lead to chronic changes in the tendon structure.
AS-related Achilles problems are often more severe and persistent than those seen in athletic injuries or overuse syndromes. The inflammation may cause thickening of the tendon, formation of bone spurs, and in severe cases, partial tears or ruptures. The chronic nature of the inflammation can also lead to calcification within the tendon, further compromising its function and flexibility.
Dactylitis (Sausage Digits)
Dactylitis (sausage toe) , characterized by diffuse swelling of entire digits that gives them a “sausage-like” appearance, is another important manifestation of AS in the feet. This condition typically affects the toes and involves inflammation of multiple structures within the digit, including joints, tendons, and surrounding soft tissues. Dactylitis can affect one or multiple toes simultaneously and may be asymmetric, affecting different toes on each foot.
The swelling associated with dactylitis is usually painful and can significantly impact foot function, making it difficult to wear normal footwear and affecting balance and mobility. In some cases, the inflammation may be so severe that it causes the affected toe to appear red and warm to the touch. Chronic dactylitis can lead to permanent structural changes, including joint damage and deformity.
Peripheral Joint Involvement
While AS primarily affects axial joints (spine and pelvis), peripheral joint involvement in the feet is common and can significantly impact daily activities. The small joints of the feet, including the metatarsophalangeal joints (where the toes meet the foot) and interphalangeal joints (within the toes), may become inflamed, leading to pain, swelling, and stiffness.
Joint involvement in AS feet often follows an asymmetric pattern, affecting different joints on each foot. This can lead to biomechanical imbalances and altered weight distribution during walking and standing. Over time, chronic joint inflammation may result in erosive changes, joint space narrowing, and eventual fusion of affected joints, leading to permanent deformity and functional limitations.
Impact on Foot Biomechanics and Function
The various manifestations of ankylosing spondylitis in the feet collectively contribute to significant alterations in foot biomechanics and function. Inflammation and structural changes affect the foot’s ability to act as a shock absorber, provide stability during stance, and facilitate efficient propulsion during walking. These biomechanical changes can create a cascade of problems that extend beyond the feet to affect the entire lower extremity and spine.
When heel pain limits normal heel strike during walking, patients often develop compensatory gait patterns, such as toe walking or shortened stride length. These adaptations can place additional stress on other parts of the foot, ankle, and leg, potentially leading to secondary problems such as calf tightness, knee pain, or hip dysfunction. The altered biomechanics may also contribute to increased spinal stress, potentially exacerbating the primary spinal symptoms of ankylosing spondylitis.
Chronic foot inflammation and structural changes can also affect proprioception—the body’s ability to sense joint position and movement. This can lead to balance problems and increased fall risk, particularly in older AS patients or those with more advanced disease. The combination of pain, stiffness, and altered proprioception can significantly impact a patient’s confidence in mobility and may lead to reduced activity levels, further contributing to deconditioning and disease progression.
Diagnostic Considerations
Diagnosing ankylosing spondylitis-related foot problems requires a comprehensive approach that considers both clinical presentation and imaging findings. Healthcare providers must differentiate between AS-related foot symptoms and other common foot conditions, as treatment approaches may differ significantly. The inflammatory nature of AS-related foot problems is often evident through the presence of morning stiffness, bilateral involvement, and systemic inflammatory markers.
Imaging studies play a crucial role in diagnosing and monitoring AS-related foot involvement. X-rays may show characteristic changes such as enthesophytes (bone spurs at tendon insertions), erosive changes at joint margins, and in advanced cases, ankylosis (joint fusion). Magnetic resonance imaging (MRI) is particularly valuable for detecting early inflammatory changes and can identify bone marrow edema, soft tissue inflammation, and enthesitis before structural changes become apparent on X-rays.
Ultrasound imaging has emerged as a valuable tool for assessing AS-related foot involvement, particularly for evaluating enthesitis and monitoring treatment response. Power Doppler ultrasound can detect increased blood flow associated with active inflammation, helping to distinguish between active inflammatory processes and chronic structural changes.
Treatment Approaches and Management Strategies
Managing ankylosing spondylitis-related foot problems requires a multidisciplinary approach that addresses both the local foot symptoms and the underlying systemic inflammatory process. Treatment strategies typically combine pharmacological interventions, physical therapy, biomechanical support, and in some cases, surgical intervention.
Pharmacological management often involves systemic treatments that address the underlying ankylosing spondylitis, including nonsteroidal anti-inflammatory drugs (NSAIDs), disease-modifying antirheumatic drugs (DMARDs), and biologic therapies. Local treatments such as corticosteroid injections may provide temporary relief for specific areas of inflammation, though their use should be judicious due to potential complications.
Physical therapy plays a crucial role in maintaining foot and ankle mobility, strengthening supporting muscles, and teaching patients appropriate exercises and stretching techniques. Biomechanical support through orthotic devices can help redistribute pressure, support inflamed structures, and improve overall foot function. Custom-made orthotics are often necessary to address the specific needs of AS patients, as over-the-counter devices may not provide adequate support or accommodation for inflamed areas.
Long-term Implications and Prognosis
The long-term implications of ankylosing spondylitis-related foot involvement vary significantly among patients and depend on factors such as disease severity, treatment response, and adherence to management strategies. Early recognition and appropriate treatment can significantly improve outcomes and prevent or delay structural changes that may lead to permanent disability.
Without proper management, chronic foot involvement in AS can lead to progressive structural changes, including joint deformities, tendon ruptures, and functional limitations that significantly impact quality of life. However, with modern treatment approaches, including biologic therapies that can effectively control systemic inflammation, many patients can maintain good foot function and prevent serious complications.
Regular monitoring by healthcare providers familiar with AS and its peripheral manifestations is essential for optimizing outcomes. This includes periodic assessment of foot symptoms, functional status, and response to treatment, with adjustments made as necessary to maintain optimal foot health and overall disease control.
The impact of ankylosing spondylitis on the feet represents a significant aspect of this complex autoimmune condition that requires specialized attention and management. Understanding these manifestations enables healthcare providers and patients to work together effectively in developing comprehensive treatment plans that address both local foot symptoms and the broader implications of this systemic inflammatory disease.