Joint Fusion: Understanding Ankylosis and Arthrodesis

What is fusion of a joint called?

Joint fusion, whether natural or surgically induced, is primarily known as ankylosis when it occurs pathologically, and arthrodesis when it is performed as a surgical procedure to stabilize a joint.

Understanding Joint Fusion: Ankylosis and Arthrodesis

Joints are critical structures enabling movement and flexibility within the skeletal system. When a joint "fuses," it means the bones that once articulated freely become rigidly connected, eliminating or severely restricting movement at that specific articulation. This process can occur due to various physiological or pathological reasons, or it can be intentionally induced through surgical intervention. Understanding the distinction between these two primary terms – ankylosis and arthrodesis – is crucial for anyone involved in exercise science, rehabilitation, or musculoskeletal health.

Ankylosis: Natural or Pathological Joint Fusion

Ankylosis refers to the natural stiffening or fusion of a joint, leading to a loss of mobility. This process typically results from disease, injury, or chronic inflammation, where the body's repair mechanisms lead to the formation of bone or fibrous tissue across the joint space, effectively "welding" the bones together.

• Types of Ankylosis:
  • Bony Ankylosis: The most severe form, where the bones of the joint become completely fused by new bone growth, eliminating the joint space entirely. This results in permanent immobility.
  • Fibrous Ankylosis: Involves the formation of dense fibrous connective tissue within the joint capsule, which severely restricts movement but may not completely eliminate it.
• Common Causes of Ankylosis:
  • Arthritis: Conditions like rheumatoid arthritis, osteoarthritis, and especially ankylosing spondylitis (a chronic inflammatory disease primarily affecting the spine) are frequent causes. In ankylosing spondylitis, inflammation leads to new bone formation that can fuse vertebrae, leading to a "bamboo spine."
  • Infection: Septic arthritis, if left untreated, can destroy joint cartilage and lead to bony fusion.
  • Trauma: Severe fractures or dislocations that disrupt the joint surface can lead to abnormal bone healing and subsequent fusion.
  • Autoimmune Conditions: Beyond specific arthritides, other autoimmune disorders can sometimes lead to joint destruction and fusion.
• Impact of Ankylosis: The primary consequence is a significant loss of range of motion and often chronic pain. This can profoundly impact an individual's functional abilities and quality of life, depending on the affected joint.

Arthrodesis: Surgical Joint Fusion

Arthrodesis is the surgical procedure deliberately performed to induce fusion of a joint. Unlike pathological ankylosis, arthrodesis is a controlled medical intervention aimed at achieving specific therapeutic goals.

• Purpose of Arthrodesis:
  • Pain Relief: Often the primary reason, especially in cases of severe arthritis or joint degeneration that has not responded to other treatments. By eliminating movement, the source of pain is removed.
  • Joint Stability: Used to stabilize hypermobile or unstable joints, such as those affected by severe ligamentous injury or neurological conditions.
  • Deformity Correction: Can correct severe joint deformities that impair function or cause significant discomfort.
• Commonly Fused Joints: While any joint can theoretically be fused, arthrodesis is most commonly performed on:
  • Spine (Spinal Fusion): To stabilize vertebrae in cases of severe disc degeneration, scoliosis, or spinal instability.
  • Ankle: For severe ankle arthritis or instability.
  • Wrist: For severe wrist arthritis or instability, often following trauma.
  • Foot (e.g., Subtalar, Triple Arthrodesis): To address deformities, instability, or arthritis in the hindfoot.
  • Fingers and Toes: For severe arthritis or deformities in smaller joints.
• The Arthrodesis Procedure: The surgery typically involves removing the remaining cartilage from the joint surfaces, roughing up the bone ends, and then joining them together with internal fixation devices (e.g., plates, screws, rods) to hold them stable while they heal and fuse. Bone grafts (autograft from the patient or allograft from a donor) are often used to promote fusion. The goal is for the bones to grow together over several months, forming a single, solid bone.
• Outcomes and Considerations: While effective for pain relief and stability, arthrodesis permanently eliminates movement at the fused joint. This often places increased stress on adjacent joints, which may, over time, lead to degenerative changes in those areas.

Biomechanical Implications of Joint Fusion

Whether natural or surgical, joint fusion fundamentally alters the biomechanics of the affected limb or spinal segment.

• Loss of Movement: The most direct impact is the complete cessation of movement at the fused joint. This means that activities requiring that specific range of motion must be adapted or become impossible.
• Compensation by Adjacent Joints: The body is remarkably adaptive. When one joint loses its mobility, the burden of movement is often shifted to neighboring joints. For example, a fused ankle may lead to increased motion at the knee or hip during gait, or a fused spinal segment may increase compensatory movement at segments above or below.
• Altered Movement Patterns: Daily activities like walking, lifting, or reaching will require modified movement strategies. This can affect balance, efficiency, and potentially lead to overuse injuries in other areas.
• Muscle Adaptations: Muscles that once moved the fused joint may atrophy or change their function. Other muscles may become overactive to compensate for the lost motion.

Living with a Fused Joint

Individuals with fused joints, whether from ankylosis or arthrodesis, often require significant adjustments and rehabilitation.

• Rehabilitation: Post-surgical arthrodesis requires a period of immobilization followed by physical therapy to strengthen surrounding muscles, improve balance, and teach compensatory movement patterns. For pathological ankylosis, physical therapy aims to maintain function in non-fused joints and manage pain.
• Activity Modifications: Certain activities may need to be avoided or modified. For example, a fused ankle may preclude high-impact sports, while a fused spine may limit heavy lifting or extreme twisting motions.
• Long-Term Outlook: While a fused joint provides stability and often pain relief, it is a permanent change. Long-term management often involves monitoring adjacent joints for signs of compensatory stress and adapting lifestyle to optimize function within the new biomechanical realities.

Conclusion

The terms ankylosis and arthrodesis both describe the fusion of a joint, but they denote distinct processes: ankylosis being a pathological or natural occurrence, and arthrodesis being a deliberate surgical intervention. Both processes result in a loss of joint mobility, fundamentally altering the body's biomechanics. Understanding these concepts is essential for appreciating the complexities of musculoskeletal health, rehabilitation, and the profound impact joint fusion can have on an individual's movement and quality of life.