Joint Fixation: Understanding Immobility, Ankylosis, and Arthrodesis

What is the fixation of a joint so it cannot move?

The fixation of a joint so it cannot move primarily refers to a state of immobility, which can be a natural anatomical classification, a pathological condition, or the result of a surgical procedure designed to eliminate movement.

Understanding Joint Mobility: A Spectrum

Joints, or articulations, are the points where two or more bones meet. Their primary function is to allow movement, provide stability, or a combination of both. The human body features a remarkable spectrum of joint mobility, ranging from highly mobile (e.g., the shoulder) to completely immobile. Understanding this spectrum is crucial to comprehending what "fixation" truly means in an anatomical or clinical context. Joints are broadly classified by their structure and the degree of movement they permit.

Synarthroses: The Naturally Immovable Joints

In the context of normal human anatomy, joints that are naturally designed to permit little to no movement are classified as synarthroses. Their primary role is to provide strong, stable connections between bones, protecting underlying structures or facilitating force transmission.

• Fibrous Joints: These joints are connected by dense connective tissue, offering very little to no movement.
  • Sutures: Immovable joints found only in the skull, uniting the cranial bones. Their interlocking edges provide significant strength and protection to the brain.
  • Gomphoses: A specialized fibrous joint that anchors a tooth to its bony socket in the jaw (alveolus). The periodontal ligament provides slight give, but the joint is essentially immobile.
  • Syndesmoses: Joints where bones are connected by a band of fibrous tissue, such as a ligament or interosseous membrane. The amount of movement depends on the length of the fibers. Examples include the distal tibiofibular joint, which allows very little movement, and the interosseous membrane between the radius and ulna, which allows for slight rotation.
• Cartilaginous Joints: In these joints, bones are united by cartilage.
  • Synchondroses: Joints where bones are joined by hyaline cartilage. Most synchondroses are temporary and ossify with age (e.g., epiphyseal plates in growing bones). The first sternocostal joint (between the first rib and the sternum) is a permanent synchondrosis, allowing no movement.
  • Symphyses: Joints where bones are joined by a pad of fibrocartilage. These joints typically allow for limited movement, rather than being completely immobile. Examples include the pubic symphysis and the intervertebral discs.

Pathological Joint Immobility: Ankylosis

Beyond natural anatomical classification, a joint can become fixed and unable to move due to pathological conditions. This state is known as ankylosis. Ankylosis results from the abnormal stiffening and immobility of a joint due to disease, injury, or surgery.

• Causes of Ankylosis:
  • Inflammatory Arthritis: Conditions like rheumatoid arthritis or ankylosing spondylitis can lead to chronic inflammation, causing cartilage destruction and subsequent fusion of the joint surfaces.
  • Infection: Severe joint infections can damage cartilage and bone, leading to fusion.
  • Trauma: Fractures involving joint surfaces or severe ligamentous injuries can result in abnormal bone growth or fibrous tissue formation that limits or eliminates movement.
  • Degenerative Conditions: While often causing pain and limited movement, severe osteoarthritis can, in rare cases, lead to bony fusion.
  • Genetic Factors: Some rare genetic disorders can predispose individuals to joint fusion.
• Types of Ankylosis:
  • Fibrous Ankylosis: The joint space is filled with fibrous connective tissue, severely restricting but not always completely eliminating movement.
  • Bony Ankylosis (Synostosis): The most severe form, where the bones forming the joint fuse completely, resulting in a solid bone formation across the joint space, eliminating all movement.

Surgical Intervention: Arthrodesis (Joint Fusion)

In some clinical scenarios, a joint is intentionally "fixed" or fused through a surgical procedure called arthrodesis. This is a deliberate intervention to eliminate movement in a joint, typically performed to alleviate severe pain or provide stability.

• Purpose of Arthrodesis:
  • Pain Relief: Often performed in joints severely damaged by arthritis or trauma where other treatments have failed. By eliminating movement, the source of pain (bone-on-bone friction, inflammation) is removed.
  • Stability: To stabilize a joint that is chronically unstable due to neurological conditions, ligamentous laxity, or severe deformity.
  • Deformity Correction: To correct severe joint deformities that impair function.
• Procedure Overview: During arthrodesis, the cartilage surfaces of the joint are removed, and the bones are then permanently joined together, often using bone grafts, plates, screws, or rods. The goal is to encourage the bones to grow together, forming a single, solid bone mass. Common sites for arthrodesis include the spine (spinal fusion), ankles, wrists, and small joints of the feet.

Differentiating True Fixation from Muscular Stabilization

It is important to distinguish between true joint fixation (immobility) and the concept of muscular stabilization or "bracing." While muscles can create significant stiffness and limit unwanted movement around a joint, they do not inherently "fix" a joint in the same way that anatomical structures, pathological processes, or surgical fusions do.

• Joint Stability vs. Immobility: Muscles work dynamically to control movement, absorb forces, and maintain joint alignment. They provide stability by contracting, often in a co-contraction pattern (simultaneous contraction of opposing muscle groups) to create stiffness.
• Co-contraction and Bracing: In exercise, "bracing" or creating core stiffness involves co-contraction of abdominal and back muscles to protect the spine and enhance force transfer. While this reduces unwanted motion, the joint itself remains anatomically capable of movement if the muscular contraction ceases. This is a temporary, active form of stiffness, not a permanent structural fixation.

Implications for Movement and Health

Whether a joint is naturally immobile (synarthrosis), pathologically fused (ankylosis), or surgically fixed (arthrodesis), the consequence is a significant impact on an individual's movement capabilities.

• Functional Limitations: Immobile joints can limit range of motion, affect gait, impair fine motor skills, and impact the ability to perform daily activities, exercise, and occupational tasks.
• Compensatory Movements: The body often adapts by increasing movement at adjacent joints or developing compensatory movement patterns, which can sometimes lead to overuse injuries or pain in those areas.

Conclusion

The "fixation of a joint so it cannot move" encompasses several distinct concepts: the natural design of synarthrotic joints (like skull sutures), the pathological condition of ankylosis (bony or fibrous fusion due to disease or injury), and the intentional surgical procedure of arthrodesis (joint fusion). While muscular co-contraction can create temporary stiffness and stability, it does not result in the permanent structural immobility defined by true joint fixation. Understanding these distinctions is fundamental for anyone involved in exercise science, rehabilitation, or health care.