Bow legs, medically known as genu varum, describe a condition where an individual’s legs bow outward, creating a noticeable gap between the knees when the ankles are together. While often associated with infants due to their developmental stage, this alignment can also develop or become more pronounced in older age. The progression of genu varum in older adults is typically a result of various underlying medical conditions and cumulative stresses on the lower limbs.
Understanding Adult Bow Legs
Adult bow legs, or genu varum, present as an outward curving of the legs at the knee, meaning the lower leg angles inward in relation to the thigh’s axis, giving the limb an archer’s bow appearance. In adults, genu varum signifies a structural deviation from normal bone alignment.
Over time, this altered alignment can become more noticeable or symptomatic, often leading to discomfort on the outside of the knees. The anatomical changes characterizing adult genu varum involve a misalignment where the mechanical axis of the lower limb passes medial to the center of the knee, increasing force across the inner (medial) tibiofemoral compartment. This uneven weight distribution and increased pressure on the medial side of the knee joint can lead to pain, instability, and progressive degeneration of the joint over time.
Primary Medical Causes in Older Age
Osteoarthritis, particularly medial compartment osteoarthritis, is a significant cause of bow legs in older adults. This condition involves the wearing away of cartilage and surrounding bone within the knee joint, which can lead to a bow-legged deformity if the wear is more pronounced on the inner side of the knee. The progression of osteoarthritis in the medial compartment can directly alter the knee’s angle, causing the leg to bow outward. As the cartilage thins and bone-on-bone contact increases, the leg’s alignment changes, placing even more stress on the already damaged medial compartment, thus accelerating the arthritic process.
Paget’s disease of bone is another condition that can cause bow legs to develop in older age. This metabolic disease disrupts the body’s normal bone recycling process, where old bone tissue is replaced by new. In Paget’s disease, this remodeling goes into overdrive, producing new bone that is less organized and weaker than healthy bone. This rapid, chaotic bone rebuilding can lead to bone deformities and fractures, and in the legs, it can cause the bones to bend and result in bow-leggedness.
Osteomalacia, often referred to as adult rickets, is a bone-softening disorder that can also contribute to genu varum. This condition typically results from a prolonged deficiency of vitamin D, which is necessary for proper calcium and phosphate absorption and bone mineralization. When bones lack sufficient minerals, they become soft and weak, increasing the risk of pain, muscle weakness, and bone deformities, including bowed legs.
Factors Contributing to Progressive Bow Legs
Long-term mechanical stress on the knee joints significantly contributes to the progression of bow legs in older individuals. When the lower limb’s alignment causes the mechanical axis to pass medial to the knee’s center, it increases the load on the medial compartment of the joint. This sustained, uneven distribution of force can accelerate the wearing away of cartilage and bone on the inner side of the knee, worsening an existing varus deformity.
Obesity also plays a role in the progression of bow legs by placing excessive weight and pressure on the knee joints. The increased load from excess weight can exacerbate existing conditions like medial compartment osteoarthritis, accelerating cartilage degeneration and worsening the bowing.
Previous knee injuries, such as meniscus tears or ligament damage, can further contribute to the development and progression of genu varum. The menisci are crucial for weight distribution and shock absorption within the knee joint, and their injury, particularly medial meniscus tears, can lead to uneven load bearing. Severe varus knee osteoarthritis is frequently accompanied by medial meniscus tears, and the loss of the meniscus’s cushioning function can accelerate cartilage breakdown and worsen the varus alignment. Ligament damage, especially to the lateral collateral ligament (LCL), can also destabilize the knee joint, contributing to abnormal forces that encourage bowing.
General age-related wear and tear on cartilage and bone also contributes to the progression of genu varum. As people age, articular cartilage, the protective tissue covering bone ends in joints, thins and can lose water content, making it more susceptible to damage. While cartilage itself lacks nerves, the underlying bone and surrounding joint tissues can become painful as cartilage wears away, leading to a “bone on bone” scenario. This age-related deterioration of joint tissues, coupled with the ongoing mechanical stress of a bowed leg, can perpetuate a cycle of increasing deformity and pain.