Ankylosis: Causes, Types, Pathophysiology, and Management

What is ankylosis caused by?

Ankylosis is the pathological stiffening and immobility of a joint due to fusion of the bones across the joint space, most commonly caused by chronic inflammation, severe trauma, infection, or degenerative processes that lead to abnormal bone or fibrous tissue formation.

Understanding Ankylosis: A Joint's Loss of Motion

Ankylosis refers to a condition where a joint, which is designed for movement, becomes stiff and loses its normal range of motion, often progressing to complete immobility. This occurs when the joint space is obliterated by the abnormal growth of bone or fibrous tissue, effectively "fusing" the bones together. From a biomechanical perspective, this loss of articular cartilage and the subsequent bridging of the joint represents a severe disruption of normal joint kinematics and kinetics, leading to significant functional impairment.

There are two primary forms of ankylosis:

• Fibrous Ankylosis: Characterized by the formation of dense, inflexible fibrous tissue within the joint capsule, which severely restricts movement but does not involve complete bony fusion. This is often an intermediate stage or a less severe form.
• Bony Ankylosis (True Ankylosis): The most severe form, where the bones on either side of the joint fuse completely, forming a single, solid bone structure. This results in the complete and irreversible loss of joint motion.

Understanding the underlying causes is crucial for diagnosis, prognosis, and potential management strategies, as the etiology dictates the course of the disease and available interventions.

Primary Causes of Ankylosis

Ankylosis is not a disease in itself but rather a consequence of various underlying conditions that lead to joint damage and abnormal healing or bone formation.

Inflammatory Joint Diseases

Chronic inflammatory conditions are a leading cause of ankylosis, particularly bony fusion, due to persistent inflammation that triggers pathological bone remodeling.

• Ankylosing Spondylitis (AS): This is the quintessential cause of bony ankylosis, predominantly affecting the spine and sacroiliac joints. AS is a chronic autoimmune inflammatory arthritis where inflammation at entheses (points where tendons/ligaments attach to bone) leads to new bone formation (syndesmophytes). Over time, these bony bridges can fuse vertebrae, leading to the characteristic "bamboo spine" and severe spinal rigidity.
• Rheumatoid Arthritis (RA): A chronic autoimmune disease causing systemic inflammation, primarily of the synovial lining of joints. While typically leading to joint destruction, deformity, and instability, severe, long-standing RA can, in some cases, result in fibrous or bony ankylosis, especially in smaller joints (e.g., wrist, hand, foot). This occurs due to extensive cartilage and bone erosion followed by reparative processes that bridge the joint.
• Psoriatic Arthritis (PsA): Similar to AS, PsA can cause inflammation of the spine and sacroiliac joints, leading to syndesmophyte formation and spinal fusion. It can also affect peripheral joints, potentially leading to ankylosis.
• Juvenile Idiopathic Arthritis (JIA): Various forms of chronic arthritis affecting children can lead to significant joint damage, growth disturbances, and the potential for ankylosis if not well-controlled.

Degenerative Joint Disease (Osteoarthritis - OA)

While primarily characterized by cartilage degradation and osteophyte formation, severe and long-standing osteoarthritis can contribute to ankylosis, particularly in smaller joints. The extensive loss of articular cartilage leads to bone-on-bone friction, and the body's attempt to repair this damage can result in bony bridging of the joint space, especially in the interphalangeal joints of the fingers and toes, or in severe spinal OA.

Trauma and Injury

Significant joint trauma can directly or indirectly lead to ankylosis by disrupting joint integrity and triggering abnormal healing responses.

• Intra-articular Fractures: Fractures that extend into the joint surface can damage articular cartilage, disrupt joint congruity, and lead to post-traumatic arthritis. The body's healing response, including excessive callus formation or heterotopic ossification, can bridge the joint space.
• Severe Dislocations: Traumatic dislocations, especially if recurrent or poorly reduced, can cause extensive soft tissue and cartilage damage, leading to inflammation, fibrosis, and potentially bony fusion.
• Burns and Severe Soft Tissue Injury: Extensive burns, particularly those crossing joint lines, can lead to contractures from scar tissue and, in severe cases, heterotopic ossification around the joint, restricting or eliminating motion.

Infection (Septic Arthritis)

Bacterial or fungal infection within a joint (septic arthritis) is a medical emergency. The infectious process rapidly destroys articular cartilage and subchondral bone. If the infection is severe or treatment is delayed, the extensive damage and subsequent inflammatory and reparative processes can lead to fibrous or bony ankylosis.

Surgical Procedures (Arthrodesis)

Arthrodesis is a surgical procedure specifically designed to induce bony ankylosis. It is an intentional fusion of a joint performed to eliminate pain, stabilize a severely damaged or unstable joint, or correct a deformity. Common sites for arthrodesis include the spine, ankle, wrist, and small joints of the hand and foot.

Congenital Conditions

In rare instances, individuals may be born with joints that are abnormally fused or prone to fusion. Examples include some forms of synostosis, where bones are fused from birth.

Other Less Common Causes

• Heterotopic Ossification (HO): This is the abnormal formation of mature lamellar bone in non-osseous tissues, often occurring after severe trauma (e.g., spinal cord injury, traumatic brain injury, severe burns, major joint surgery like total hip arthroplasty). If HO forms across a joint, it can lead to severe ankylosis.
• Certain Metabolic Disorders: Chronic and severe cases of conditions like gout or pseudogout can lead to extensive joint damage and secondary ankylosis due to crystal deposition and inflammatory responses.
• Tumors: Rarely, tumors originating near or within a joint can cause significant bone destruction and subsequent ankylosis.

The Pathophysiology of Joint Fusion

Regardless of the specific cause, the underlying mechanism of ankylosis involves a complex interplay of inflammation, tissue damage, and abnormal healing. In a healthy joint, articular cartilage provides a smooth, low-friction surface. When this cartilage is damaged or destroyed, and the subchondral bone is exposed, the body's repair mechanisms can become dysregulated.

Persistent inflammation, as seen in autoimmune conditions, stimulates osteoblasts and other cells to lay down new bone where it shouldn't be. In traumatic or infectious scenarios, the extensive tissue destruction and subsequent scarring can lead to dense fibrous tissue formation that stiffens the joint. If this process progresses, the fibrous tissue can undergo metaplasia and ossify, or the exposed bone surfaces can directly bridge, leading to complete bony fusion and the permanent loss of the joint's function.

Risk Factors for Developing Ankylosis

Several factors can increase an individual's risk of developing ankylosis:

• Genetic Predisposition: Strong genetic links exist for inflammatory conditions like Ankylosing Spondylitis (e.g., HLA-B27 gene).
• Chronic Inflammatory Conditions: Pre-existing autoimmune diseases significantly increase risk.
• History of Severe Joint Trauma or Infection: Previous damage to a joint makes it susceptible.
• Age: While not a direct cause, older age is associated with an increased prevalence of osteoarthritis, which can lead to ankylotic changes.
• Specific Surgical Procedures: Arthrodesis is an intentional cause, but complications from other joint surgeries can sometimes lead to unwanted ankylosis.

Clinical Implications and Management Considerations

Understanding the specific cause of ankylosis is paramount for clinical management. Diagnosis typically involves a combination of medical history, physical examination (assessing range of motion), and imaging studies such as X-rays, CT scans, and MRI, which can visualize the extent of joint space narrowing, bone erosion, and fusion.

Management strategies vary widely based on the cause and severity:

• For inflammatory causes: Disease-modifying anti-rheumatic drugs (DMARDs) and biologics aim to control inflammation and prevent further joint damage and fusion. Physical therapy is crucial to maintain existing mobility.
• For post-traumatic or infectious causes: Acute treatment focuses on managing the primary injury or infection. Rehabilitation aims to restore function and prevent stiffness.
• For established ankylosis: Once bony fusion occurs, the change is typically irreversible. Management shifts to pain control, maximizing function in surrounding joints, and adapting to the functional limitations. In some cases, surgical procedures like osteotomy (cutting and realigning bone) or arthroplasty (joint replacement) may be considered to restore some motion or improve alignment, though these are complex and not always feasible.

Conclusion

Ankylosis represents a profound loss of joint function, stemming from a diverse range of underlying pathologies. From autoimmune diseases like Ankylosing Spondylitis that systematically fuse the spine, to acute infections that rapidly destroy joint surfaces, the common thread is a pathological process that obliterates the joint space. For fitness enthusiasts, personal trainers, and kinesiologists, a clear understanding of these causes is vital for recognizing risk factors, interpreting client limitations, and guiding appropriate exercise interventions or referrals to medical professionals. While often a debilitating condition, ongoing research into inflammatory pathways and regenerative medicine offers hope for future preventative and therapeutic strategies.