Knock knees, known medically as genu valgum, describe a lower limb alignment where the knees angle inward, causing them to touch or nearly touch when a person stands with their feet apart. This alignment shifts the mechanical axis of the leg, meaning the line of weight bearing travels outside the center of the knee joint. While this condition is often a normal, temporary phase in the development of young children, its persistence or its development in adulthood indicates a structural change that warrants investigation. Understanding the cause of adult-onset or persistent genu valgum is important because the condition places abnormal stress on the knee, potentially leading to pain and joint degeneration over time.
Developmental Factors That Persist
The most common reason an adult has knock knees is that the condition originated in childhood and never spontaneously corrected itself. This is often referred to as persistent physiological genu valgum. Most children exhibit a valgus alignment peak around the age of three to five, but the legs typically straighten into normal alignment by age seven or eight. When this natural self-correction fails to occur, the underlying skeletal structure remains angled into adulthood.
For many adults with this lifelong alignment, the cause is classified as idiopathic, meaning there is no known specific disease or injury responsible for the structural deviation. Instead, these cases are likely influenced by a genetic predisposition that dictates bone shape and growth patterns. The inherited anatomy of the femur (thigh bone) and tibia (shin bone) can result in a fixed valgus angle that is simply the person’s natural, albeit non-standard, skeletal structure. While the alignment itself is stable, the long-term presence of this structural difference means that the mechanical forces across the knee joint are perpetually uneven.
Acquired Causes from Injury and Wear
The development of knock knees later in life, known as secondary or acquired genu valgum, is most often a consequence of mechanical wear or trauma to the knee joint. A significant factor is the progression of osteoarthritis (OA), a degenerative joint disease. OA alters the knee’s alignment asymmetrically, particularly when cartilage on the outer (lateral) compartment wears down faster than the inner compartment. The resulting loss of cushioning material causes that side to collapse, pulling the leg into a valgus alignment.
Traumatic injuries can also directly cause or contribute to a progressive valgus deformity. A fracture of the femur or tibia near the knee joint may heal in a slightly misaligned position if not perfectly set. Even minor malunion can shift the mechanical axis, leading to a gradual worsening over years. Furthermore, significant ligamentous injury, such as damage to the medial collateral ligament (MCL) or anterior cruciate ligament (ACL), causes chronic joint instability. Over time, this instability allows the knee to drift inward under load, contributing to a progressive valgus deformity.
Excessive weight bearing is a major exacerbating factor in acquired genu valgum, particularly with underlying joint issues. Obesity significantly increases the compressive forces transmitted through the knee joint, which accelerates the degenerative process of osteoarthritis. This increased stress rapidly worsens uneven cartilage wear and contributes to the progressive collapse of the joint space. The pressure placed on the compromised joint speeds up the rate at which a mild alignment issue progresses into a more severe valgus deformity.
Systemic Conditions and Bone Health
Less common causes of adult knock knees are systemic diseases that disrupt normal bone metabolism and structure. Rickets, caused by a severe deficiency of Vitamin D, calcium, or phosphate, primarily affects children but its skeletal effects can persist into adulthood. Resolved childhood rickets leaves bones weaker and softer, leading to permanent deformities like genu valgum as the legs bear weight. In adults, a similar condition called osteomalacia, characterized by poor bone mineralization, can cause existing bones to soften and bend under pressure, leading to a progressive valgus alignment.
Kidney disease can also affect bone health through renal osteodystrophy. When the kidneys fail to properly regulate calcium and phosphorus levels, the resulting mineral imbalance interferes with bone maintenance, leading to structural weaknesses and deformities. Paget’s disease of bone disrupts the body’s normal bone remodeling cycle, causing new bone tissue to be abnormally shaped and structurally weak. This irregular bone turnover, which primarily affects older adults, can cause the leg bones to bow or angle abnormally, leading to a valgus deformity.
Rare genetic disorders that affect skeletal development can also manifest as knock knees. These conditions include various forms of skeletal dysplasia or fibrous dysplasia, where normal bone is replaced by abnormal, fibrous tissue. The resulting structural weakness and abnormal growth within the bone’s architecture can lead to a severe and persistent genu valgum deformity. These systemic and metabolic causes are often characterized by bilateral, or two-sided, involvement, distinguishing them from the unilateral deformities frequently caused by trauma.
Recognizing Symptoms and Seeking Evaluation
Adult knock knees often become noticeable when the altered biomechanics begin to cause symptoms that interfere with daily life. The most common complaint is pain, typically around the knee joint, but discomfort can also radiate to the hips, ankles, or feet as the body attempts to compensate for the misalignment. This structural issue places abnormal tension on the ligaments and muscles, leading to chronic joint stiffness and instability during movement. An altered gait, or walking pattern, is also a frequent symptom, as the body struggles to efficiently swing the misaligned limb.
Any adult who notices a recent inward angulation of the knees, a progression of an existing alignment, or the onset of persistent joint pain should seek a professional evaluation. Consulting with an orthopedic specialist is the appropriate next step to determine the specific cause and severity of the genu valgum. The specialist will typically use standing, full-length X-rays to accurately measure the mechanical axis of the leg and assess the degree of joint space narrowing. A precise diagnosis is necessary to understand whether the condition is a stable, lifelong alignment or a progressive problem resulting from trauma, degeneration, or a systemic disease.