Bow legs (genu varum) is a condition where the legs curve outward, creating a noticeable gap between the knees when the ankles are touching. While common and temporary in infants, its development in older adults is an acquired process signifying significant changes within the knee joint. This adult-onset bowing is a structural deformity resulting from long-term wear and tear, driven primarily by mechanical stress and joint degradation.
The Primary Role of Knee Osteoarthritis
The greatest contributor to acquired bow legs in later life is the progression of osteoarthritis (OA) within the knee’s medial compartment. The inner (medial) side of the knee joint bears a disproportionately large share of the body’s weight during activities like walking. Over decades, the protective articular cartilage on the ends of the femur and tibia in this compartment gradually wears away.
This degradation leads to the collapse of the joint space, causing bone to rub directly against bone. As the inner side compresses, the lower leg’s mechanical axis—the straight line of weight-bearing force from the hip to the ankle—shifts further inward. This shift dramatically increases the load on the damaged medial side, creating a destructive feedback loop that worsens the bowing. Even a small, five-degree varus angle can increase the compressive load on the medial compartment to 90% of the total force borne by the knee. This uneven pressure forces the leg into the characteristic O-shape, a visible symptom of advanced unicompartmental knee degeneration.
Contributing Factors That Accelerate Development
While medial compartment OA is the direct cause, several factors amplify the speed and severity of its progression. Excess body weight is a major accelerator, as obesity significantly increases the compressive force exerted on the medial side of the knee joint with every step. This sustained load accelerates the rate of cartilage breakdown, speeding up joint space collapse.
A history of prior knee trauma or injury also predisposes a person to developing genu varum. Injuries such as meniscal tears or ligament damage destabilize the knee joint, leading to uneven wear patterns that favor medial OA development. A genetic predisposition means some individuals are more susceptible to the mechanical stresses that cause inner joint deterioration. Metabolic bone diseases common in older adults, such as Paget’s disease, can also weaken and reshape the leg bones, directly contributing to the bowing.
Physical Symptoms and Impact on Daily Life
The structural change caused by bow legs leads to physical symptoms that significantly impair mobility. The most common complaint is chronic pain on the inner side of the knee, which intensifies during weight-bearing activities like walking or standing. This pain results directly from bone-on-bone friction and overstressed joint tissues.
The altered mechanical alignment affects gait, often leading to an unstable or waddling walk. In advanced cases, the knee may exhibit a “varus thrust,” where the joint visibly shifts outward with each step due to ligamentous laxity on the outer side. This instability and pain reduce the knee’s range of motion, limiting the ability to fully straighten or bend the leg and perform routine daily tasks.
Diagnosis and Management Approaches
Diagnosis of acquired genu varum begins with a physical examination to observe the outward curvature and assess gait and knee stability. To confirm the diagnosis and plan treatment, a specialist uses standing alignment X-rays of the entire leg, from hip to ankle. These images are essential for precisely measuring the mechanical axis and quantifying joint space narrowing and bone spur formation indicative of OA severity.
Management typically starts with conservative strategies aimed at reducing pain and slowing the deformity’s progression. Non-surgical options include physical therapy to strengthen surrounding musculature, weight loss to decrease the load on the medial compartment, and the use of specialized offloading braces. These braces apply a three-point pressure system to gently push the knee laterally, temporarily shifting the weight-bearing line away from the damaged inner joint.
For severe or progressively worsening cases, surgical intervention is required. Two primary procedures are used: High Tibial Osteotomy (HTO) and Total Knee Replacement (TKR). HTO is a realignment procedure, often preferred for younger, active patients, where a wedge of bone is cut from the tibia to shift the mechanical axis toward the healthier outer (lateral) compartment. TKR is reserved for older patients with extensive, end-stage OA, involving the complete replacement of damaged joint surfaces with metal and plastic components to restore proper alignment and function.