Knee Arthrodesis: Understanding Why a Fused Knee Cannot Bend and Its Impact on Mobility

Can you bend a fused knee?

No, a truly fused knee joint, a surgical procedure known as arthrodesis, is designed to eliminate all movement, rendering it permanently rigid and unable to bend.

What is a Fused Knee (Arthrodesis)?

Knee arthrodesis is a surgical procedure that involves permanently joining the bones of the knee joint – the femur (thigh bone) and the tibia (shin bone) – into a single, solid structure. Unlike joint replacement, which aims to restore motion, arthrodesis is performed to eliminate movement entirely.

Key Reasons for Knee Fusion:

• Severe Pain Relief: It is typically considered for patients experiencing severe, debilitating knee pain that has not responded to other treatments, especially when joint replacement is not feasible or has failed.
• Joint Stability: It provides a highly stable limb, which can be crucial in cases of severe joint destruction, chronic infections (e.g., septic arthritis), or extensive bone loss.
• Failed Arthroplasty: In some instances, it may be performed as a salvage procedure following complications from a total knee replacement, such as recurrent infections or severe implant loosening.

The Anatomy of Knee Fusion

To understand why a fused knee cannot bend, it's essential to grasp the anatomical changes involved:

• Normal Knee Joint: A healthy knee is a complex hinge joint, allowing for flexion (bending) and extension (straightening). This movement is facilitated by smooth articular cartilage covering the ends of the femur and tibia, a joint capsule, synovial fluid, and robust ligaments that guide and stabilize the motion.
• During Arthrodesis: The surgeon removes all remaining articular cartilage from the ends of the femur and tibia. The raw bone surfaces are then compressed together, often using internal fixation devices like plates, screws, or intramedullary rods. The goal is to encourage the bones to grow and fuse, forming a single, continuous bone.
• The Result: Once successful fusion occurs, the joint space is obliterated, and the two bones effectively become one. This bony union means there is no longer a hinge mechanism or any remaining joint space for movement.

Why a Fused Knee Cannot Bend

The inability of a fused knee to bend is a direct consequence of the surgical objective and the resulting anatomical state:

• Bony Union: The primary reason is the complete fusion of the femur and tibia. This creates a solid bone bridge across the former joint line, making movement anatomically impossible.
• Absence of Articular Surfaces: The smooth, gliding cartilage that allows bones to move against each other is removed, eliminating the low-friction interface necessary for joint articulation.
• Elimination of Joint Space: The space between the bones, normally filled with synovial fluid, is eradicated as the bones grow together.
• Ligamentous and Muscular Irrelevance for Joint Movement: While the surrounding muscles (quadriceps, hamstrings) and ligaments (ACL, PCL, MCL, LCL) may still be present, they no longer have a functional joint to act upon to produce knee flexion or extension. Their role shifts to stabilizing the fused limb and facilitating movements at adjacent joints.

Functional Implications of Knee Fusion

A fused knee is typically positioned in a slightly flexed angle (often 10-20 degrees) to optimize function for daily activities. A completely straight leg (0 degrees flexion) can be more cumbersome for sitting. Despite this optimized angle, the lack of knee flexion significantly impacts mobility:

• Gait Alterations: Walking becomes a stiff-legged, or "stilt-like," gait. Individuals often compensate by performing a "circumduction" motion, swinging the entire leg outwards in an arc to clear the ground during the swing phase.
• Difficulty with Stairs: Ascending and descending stairs becomes challenging, often requiring the use of handrails or leading with the unaffected leg.
• Sitting and Rising: Sitting in chairs, especially low ones, and getting up from a seated position can be difficult due to the inability to bend the knee.
• Vehicle Entry/Exit: Entering and exiting cars or other vehicles is significantly impacted, often requiring specific techniques and adaptations.
• Bending and Reaching: Activities that require bending down, such as tying shoes or picking up objects from the floor, become much harder and rely heavily on hip and trunk flexibility.

Compensatory Movements and Daily Life

Individuals with a fused knee develop various compensatory strategies to navigate daily life:

• Increased Hip and Ankle Mobility: The hip and ankle joints on the affected side, and the entire lower extremity on the unaffected side, must take on a greater range of motion and load. This can lead to increased stress and potential pain in these joints over time.
• Trunk and Pelvic Movements: Bending from the waist (trunk flexion) and tilting the pelvis become primary methods for approximating movements that would normally involve knee flexion.
• Adaptive Strategies: People learn to adapt by:
  • Using chairs of appropriate height.
  • Modifying dressing techniques.
  • Utilizing assistive devices like long-handled reachers.
  • Adjusting their body mechanics for household tasks.

Rehabilitation and Management

Post-fusion rehabilitation focuses not on restoring knee movement, but on optimizing function with a rigid limb:

• Pain Management: Controlling post-surgical pain and managing any compensatory pain in adjacent joints.
• Strength Training: Strengthening the muscles of the hip, ankle, and core to support the altered gait and compensate for the lack of knee movement.
• Gait Training: Learning to walk efficiently and safely with a stiff leg, often with the guidance of a physical therapist.
• Mobility Aids: Crutches or walkers may be used initially, and some individuals may benefit from custom footwear or orthotics to address leg length discrepancies or gait issues.
• Joint Preservation: Protecting the remaining mobile joints (hips, ankles, spine) from excessive wear due to compensatory movements.

Living with a Fused Knee

While a fused knee presents significant functional limitations, many individuals find that the procedure successfully alleviates chronic pain and provides a stable, weight-bearing limb. Quality of life can improve significantly once adaptations are made and effective compensatory strategies are learned. Ongoing physical therapy, regular exercise focusing on the unaffected joints, and mindful body mechanics are crucial for long-term management and overall well-being.