Genu valgum, commonly called knock-knees, is most often caused by normal childhood development and resolves on its own by around age 7. When it persists beyond that age or appears for the first time later in life, the causes range from nutritional deficiencies and obesity to growth plate injuries, bone infections, and genetic conditions. Understanding which category applies makes a big difference in whether the condition needs treatment or simply time.
Normal Growth Is the Most Common Cause
In young children, knock-knees are a predictable phase of leg development. Babies are typically born slightly bowlegged. Their legs straighten to a neutral position by about age 2, then swing in the opposite direction. Between ages 3 and 4, most children reach peak knock-knee alignment, with their knees angling inward by 10 to 15 degrees. From there, the angle gradually decreases until it stabilizes at a mild 3 to 5 degrees of inward angulation by age 7.
This is the stage that brings most families to a doctor’s office. Children between ages 3 and 5 often look noticeably knock-kneed, and parents understandably worry. But bilateral knock-knees (both legs equally affected) in this age group are almost always physiologic, meaning they’re a normal part of skeletal maturation rather than a sign of disease. No treatment is needed, and the alignment corrects itself.
A simple measurement helps distinguish normal from abnormal. With a child standing with their knees touching, the gap between their inner ankle bones (the intermalleolar distance) between 2.5 and 5 centimeters is considered normal for ages 3 to 7. A gap above 8 centimeters is considered abnormal at any age and warrants further evaluation.
Vitamin D Deficiency and Rickets
When knock-knees have a nutritional cause, vitamin D deficiency is the most frequent culprit. Vitamin D is essential for the body to absorb calcium and phosphorus, the minerals that harden growing bone. Without enough vitamin D, the growth plates near the ends of the thigh and shin bones can’t mineralize properly. The cartilage that should be converting into solid bone instead remains soft and disorganized, a condition called rickets.
Soft growth plates can’t support the mechanical load of a child’s body weight evenly. The result is bending and angular deformity, often showing up as knock-knees or bowlegs depending on the child’s age and which part of the growth plate is most affected. Low calcium also triggers the parathyroid glands to pull calcium from bone, which further weakens the skeleton and worsens the deformity.
The encouraging aspect of nutritional rickets is that the deformities tend to improve once the underlying deficiency is corrected. Restoring adequate vitamin D and calcium levels allows the growth plates to resume normal mineralization, and in many cases the leg alignment gradually straightens as the child grows. Rarer inherited forms of rickets, where the body either can’t activate vitamin D properly or can’t respond to it, tend to cause more persistent deformities because the growth plate defect is harder to fully correct.
Childhood Obesity and Mechanical Overload
Excess body weight places disproportionate stress on the inner (medial) side of growing knees, and research shows a meaningful link between obesity and knock-knees in children. In one study of children and adolescents with idiopathic genu valgum (knock-knees without an obvious underlying disease), 71% were classified as obese, with a BMI above the 95th percentile for their age. Nearly half had a BMI of 30 or higher.
The mechanism appears to involve the growth plates responding to chronic compressive forces. The outer part of the growth plate near the knee may slow its growth rate under sustained pressure, while the inner part continues growing at a normal pace. This asymmetry gradually tilts the shin bone into a more valgus (knock-kneed) position. BMI alone predicted about 10% of the shin bone’s valgus angle in the study, though skeletal maturity was the stronger overall predictor of severity. The concern is that knock-knees caused or worsened by obesity can progress as the child matures, increasing the risk of knee osteoarthritis in adulthood.
Growth Plate Injuries and Infections
The growth plates near the knee are the engine of leg lengthening during childhood. Any disruption to one side of a growth plate can cause the bone to grow unevenly, producing angular deformity over time.
Fractures that cross through the growth plate at the lower end of the thighbone are a well-recognized cause. If the outer portion of the growth plate is damaged and stops growing while the inner portion continues, the result is progressive knock-knee alignment that worsens with each year of remaining growth. Even without a clear fracture, some adolescents develop knock-knees from subtle, repetitive microtrauma to the outer growth plate, possibly compounded by genetic factors that make the growth plate more sensitive to compressive forces.
Bone infections (osteomyelitis) near the knee can cause similar problems. Bacteria damage the growth plate directly, and the inflammatory response compromises blood flow to the area. Reduced blood supply leads to localized death of growth plate tissue and, in severe cases, premature closure of part or all of the plate. When closure is partial, affecting one side more than the other, the bone angles as it grows. These infections can have lasting consequences: once a growth plate closes prematurely, the resulting deformity won’t self-correct and often requires surgical intervention.
Genetic and Skeletal Conditions
Several inherited conditions cause knock-knees as part of a broader pattern of skeletal abnormality. Skeletal dysplasias, a group of genetic disorders affecting bone and cartilage development, frequently produce significant genu valgum because the growth plates throughout the body are structurally abnormal. In these cases, knock-knees tend to be more severe and less likely to resolve without treatment compared to the developmental or nutritional forms.
Inherited forms of rickets also fall into this category. X-linked hypophosphatemia, for example, causes the kidneys to waste phosphorus regardless of vitamin D levels. The resulting growth plate defect is intrinsic rather than purely nutritional, which means deformities tend to persist even with metabolic treatment because the underlying remodeling process remains impaired.
Why Persistent Knock-Knees Matter
When genu valgum doesn’t resolve or continues to worsen past age 7, the altered alignment changes how forces distribute across the knee. The outer compartment of the knee bears more load than it was designed for, accelerating cartilage wear in that area over years and decades.
Knock-knees also create a significant pull on the kneecap. The inward angle of the thighbone relative to the shinbone increases the outward force on the kneecap during activities like squatting, running, or climbing stairs. Research shows that 60% of children evaluated for kneecap instability also have genu valgum, and nearly 1 in 4 of those have high-grade malalignment. The elevated contact pressure on the outer edge of the kneecap joint contributes to pain, cartilage damage, and a substantially higher risk of the kneecap dislocating repeatedly. Correcting the underlying alignment is often necessary before or alongside any kneecap stabilization procedure, because the persistent lateral forces can stretch and weaken surgical repairs over time.
In practical terms, children and teens with persistent knock-knees may notice knee pain during activity, a feeling of the kneecap “shifting” or giving way, or difficulty with prolonged standing and walking. These symptoms are the body’s signal that the malalignment is producing mechanical consequences beyond cosmetic appearance.