Knee Osteoarthritis: Understanding Genu Varum (Bow-Leggedness) and Its Management

What is the most common knee deformity in osteoarthritis?

The most common knee deformity associated with osteoarthritis (OA) is genu varum, commonly known as "bow-leggedness," which involves an outward angulation of the lower leg relative to the thigh, placing increased stress on the medial (inner) compartment of the knee.

Understanding Osteoarthritis (OA) of the Knee

Osteoarthritis is a chronic, progressive joint disease characterized by the breakdown of articular cartilage, the smooth, slippery tissue that covers the ends of bones and allows them to glide over each other with minimal friction. In the knee, OA primarily affects the tibiofemoral joint (between the shin bone and thigh bone) and the patellofemoral joint (between the kneecap and thigh bone). As the cartilage erodes, the underlying bone can thicken and form bone spurs (osteophytes), leading to pain, stiffness, swelling, and reduced range of motion.

The Most Common Deformity: Genu Varum (Bow-Leggedness)

Genu varum, or a varus deformity, is the hallmark angular deformity seen in advanced knee OA. It is characterized by:

• Medial Compartment Collapse: The vast majority of knee OA cases, particularly those that are primary (not caused by specific injury or other disease), predominantly affect the medial compartment of the knee. This means the cartilage and meniscal tissue on the inner side of the knee joint degenerate more rapidly.
• Joint Space Narrowing: As the medial cartilage wears away, the joint space on the inner side of the knee narrows significantly.
• Tibial Adduction and Femoral Abduction: This leads to the lower leg (tibia) moving inward relative to the thigh (femur), creating a visible "bow-legged" appearance where the knees angle outward.
• Increased Stress: The altered alignment shifts the mechanical axis of the limb, concentrating even more weight-bearing stress on the already compromised medial compartment, accelerating cartilage loss.

While genu valgum (knock-knees), where the knees angle inward, can also occur with OA, it is far less common in primary knee OA and is more often associated with specific conditions like rheumatoid arthritis or previous trauma. The biomechanical forces typically lead to medial compartment degeneration in the general population, making genu varum the predominant deformity.

The Biomechanics of Genu Varum in OA Progression

The development of genu varum is not merely a cosmetic change; it plays a critical role in the progression and symptoms of knee OA.

• Vicious Cycle: The varus alignment creates a biomechanical imbalance that exacerbates the very condition causing it. The increased compressive forces on the medial compartment lead to further cartilage breakdown, which in turn worsens the varus deformity, creating a self-perpetuating cycle of degeneration.
• Altered Gait: Individuals with genu varum often develop a characteristic "thrusted" or "waddling" gait as they attempt to shift their weight away from the painful inner knee. This altered gait pattern can place additional stress on other joints, such as the hips and ankles, and increase energy expenditure during walking.
• Muscle Imbalances: The altered alignment can lead to muscle imbalances, with certain muscle groups becoming overactive or underactive in an attempt to stabilize the joint. For instance, the adductor muscles of the thigh may become tighter, further pulling the leg into a varus position.
• Pain and Instability: The excessive loading and joint incongruity contribute significantly to pain and a sensation of instability within the knee.

Clinical Implications and Management

Identifying and managing genu varum is crucial in the comprehensive treatment of knee OA.

• Diagnosis:
  • Physical Examination: A physical assessment will reveal the characteristic bow-legged appearance and often tenderness over the medial joint line.
  • Radiographs (X-rays): Standing X-rays are essential to assess the degree of joint space narrowing, bone spur formation, and the precise mechanical axis of the limb, quantifying the varus deformity.
• Non-Surgical Management:
  • Weight Management: Reducing body weight significantly lessens the load on the knee joint, slowing progression and reducing pain.
  • Targeted Exercise: Strengthening the quadriceps (especially the vastus medialis obliquus), hip abductors, and core muscles can improve joint stability and alignment. Flexibility exercises for hamstrings and hip adductors are also important. Gait training and balance exercises can improve functional mobility.
  • Orthotics and Bracing: Lateral wedge insoles can help shift the load away from the medial compartment to the lateral (outer) compartment. Unloader braces are designed to provide external support and reduce stress on the affected compartment.
  • Pain Management: Non-steroidal anti-inflammatory drugs (NSAIDs), corticosteroid injections, and hyaluronic acid injections can help manage symptoms, though they do not correct the deformity.
• Surgical Considerations:
  • High Tibial Osteotomy (HTO): For younger, active individuals with isolated medial compartment OA and significant varus deformity, HTO is a surgical procedure that involves removing a wedge of bone from the upper tibia to realign the leg, shifting the weight-bearing axis to the healthier lateral compartment.
  • Total Knee Arthroplasty (TKA): In cases of severe, widespread OA and significant deformity that profoundly impacts quality of life, total knee replacement surgery is often the most effective solution. The surgeon will correct the varus deformity during the implantation of the prosthetic joint components.

Prevention and Proactive Strategies

While not all OA is preventable, proactive measures can help mitigate risk and manage progression:

• Maintain a Healthy Weight: Excess body weight is a major modifiable risk factor for knee OA.
• Regular, Appropriate Exercise: Engage in low-impact activities like swimming, cycling, and walking to maintain joint health, muscle strength, and flexibility without overloading the joints.
• Address Biomechanical Issues: Early assessment and correction of gait abnormalities or muscle imbalances can prevent excessive stress on the knee.
• Listen to Your Body: Pay attention to knee pain and seek medical advice early to manage symptoms and slow disease progression.

Conclusion

Genu varum, or bow-leggedness, stands as the most common and clinically significant angular deformity in knee osteoarthritis, largely due to the predominant involvement of the medial compartment. Understanding its biomechanical implications is vital for both conservative and surgical management strategies. By addressing this deformity through a combination of weight management, targeted exercise, supportive orthotics, and, when necessary, surgical intervention, individuals can effectively manage their OA symptoms, slow disease progression, and improve their overall quality of life.