Knock knees (genu valgum) happen when the knees angle inward and touch while the ankles stay apart, and the causes range from completely normal growth in children to bone, joint, or injury‑related problems in older kids and adults.

What Causes Knock Knees? (Quick Scoop)

1. Normal Growth vs. True Problem

In many young children, knock knees are simply a normal stage of development and not a disease.

  • Toddlers often start slightly bow‑legged, then become more knock‑kneed around ages 3–4, and usually straighten out by around age 7–8.
  • This “physiological” knock knee tends to run in families, is painless, and doesn’t usually need treatment.
  • Doctors become more concerned when it is very severe, appears suddenly, worsens after age 7–8, or only affects one leg.

Think of it like a growth phase: the legs are still figuring out their final alignment.

2. Bone & Growth Plate Problems

Anything that disturbs how the leg bones grow can cause persistent or worsening knock knees.

  • Growth plate injuries
    • Damage to the growth plate at the lower femur or upper tibia (for example after a fracture) can cause one side of the bone to grow slower than the other, tilting the knee inward.
  • Fractures that heal crooked
    • If a thigh or shin bone heals in a slightly twisted or angled position (malunion), it can leave a permanent valgus (inward) deformity.
  • Skeletal dysplasia or irregular bone growth
    • Certain genetic bone growth disorders change bone shape and alignment, leading to knock knees as part of a broader pattern of short stature or limb deformity.

In simple terms: if the “building blocks” (bones and growth plates) don’t grow evenly, the knees can end up leaning inward.

3. Nutritional & Metabolic Causes

Several conditions that weaken bone structure or mineralization can make knock knees more likely.

  • Rickets (most often due to vitamin D deficiency)
    • Soft, poorly mineralized bones can bend under body weight, especially at the knees, producing knock knees.
  • Renal (kidney‑related) bone disease
    • Kidney failure and related problems (renal osteodystrophy, renal rickets) disturb calcium and phosphate balance, weakening bones and causing deformities like genu valgum.
  • Other metabolic bone disorders
    • Conditions affecting how bone turns over or stores minerals can similarly cause valgus deformity.

Here, the “material” of the bone is softer than it should be, so the legs bend under load rather than staying straight.

4. Joint Disease and Arthritis

Problems inside the knee joint itself can also drive the knee inward over time.

  • Arthritis (especially in the knee)
    • When one side of the knee joint wears down more than the other, it can collapse slightly, shifting alignment so the knees angle inward.
  • Inflammatory or degenerative joint conditions
    • Chronic inflammation or cartilage loss changes how forces pass through the joint, gradually increasing valgus.

This is more common in adults, where years of uneven loading and cartilage wear change leg alignment step by step.

5. Infections, Tumors, and Trauma

Less common but important causes include damage from infections or growths in the bone.

  • Bone infection (osteomyelitis)
    • Infection near the growth plate can scar or destroy part of it, leading to asymmetric growth and knock knees.
  • Joint infections
    • Severe infections in or around the knee can injure growing bone and cartilage, again disturbing alignment.
  • Benign bone tumors or cysts
    • These can weaken or distort part of a bone so it bows or angles, creating a valgus deformity.

You can think of these as localized “hits” to the growth system: they don’t affect the whole skeleton but can strongly affect the alignment of one leg.

6. Weight, Muscle Balance, and Lifestyle

Body weight and how you move also influence whether knock knees appear or worsen.

  • Overweight and obesity
    • Extra body weight increases pressure on the knees and can make a mild knock knee more obvious or contribute to progression over time.
  • Muscle imbalances and gait patterns
    • Weak hip and thigh muscles or abnormal walking mechanics might not be a direct “cause,” but they can worsen alignment and symptoms like pain or instability.

In many adults with long‑standing knock knees, the outer side of the knee is overloaded while ligaments on the inner side are stretched, which can lead to pain and joint degeneration.

7. Genetic and “No Clear Cause” Cases

Sometimes knock knees show up even when tests don’t point to a single disease.

  • Familial tendency
    • The pattern can run in families without any obvious underlying illness.
  • Idiopathic genu valgum
    • In some children or adolescents, doctors label it “idiopathic” (no known cause) when growth and lab tests look normal but the alignment is still clearly valgus.
  • Genetic syndromes
    • Certain connective‑tissue or joint laxity conditions (like Ehlers‑Danlos or hypermobility syndromes) can be associated with knock knees in some people.

So not every case will come with a neat label, but doctors still look carefully for red flags that would suggest a hidden condition.

8. When to Worry and What to Do

Most small children with knock knees don’t need treatment and grow out of it; in contrast, adolescents or adults with worsening or painful knock knees should be assessed.

You or a child should see a doctor if:

  • Only one leg is affected, or one side is much worse.
  • The angle is severe, keeps getting worse after age 7–8, or causes tripping and clumsiness.
  • There is pain, swelling, or a history of fracture, infection, or serious illness affecting the bones or kidneys.

Treatment, when needed, can include weight management, exercises and physiotherapy, shoe inserts or bracing in growing children, or corrective surgery for significant deformity or arthritis.

TL;DR:

  • In young kids, knock knees are often just a normal growth phase that improves on its own.
  • Persistent, one‑sided, painful, or worsening knock knees can be caused by growth plate injury, fractures, rickets, kidney‑related bone disease, bone infections, arthritis, obesity, or genetic bone/joint conditions.

Information gathered from public forums or data available on the internet and portrayed here.