Causes / typical injury mechanism: Genu valgum (knock knees) is characterized by angular deformity of the knees in which the apex of the deformity points inward toward the midline. Below the knees, the legs deviate away from the midline. Genu valgum is a normal stage of growth and development of the lower extremities during childhood (physiologic valgus), with an inward-turning (valgus) alignment considered normal in children between 2 and 5 years old. The valgus deformity is usually maximal at 3-4 years old with an average lateral tibiofemoral angle of 12 degrees in the coronal plane. Subsequently, the valgus alignment typically returns to neutral. Genu valgum rarely worsens after age 7, but the presence of genu valgum beyond 12 degrees persisting into preadolescence often will not correct spontaneously.

Idiopathic genu valgum is a frequently diagnosed growth disorder in adolescence. The condition can be hereditary or develop as a result of genetic disorders or metabolic bone diseases. It may occasionally arise from an injury to the growth area of the tibia (eg, post-traumatic Cozen fracture, distal femoral fracture, or proximal tibial physeal fracture). Obesity may either contribute to pathologic genu valgum or cause gait disturbances resembling knock knees. If the condition does not appear until the age of 6 years or older, this may be a sign of an underlying bone disease such as osteomyelitis or rickets.

Classic history and presentation: Patients with genu valgum often present with concerns about the appearance of their legs, clumsiness, falling due to weakened ligaments supporting the knee, flat feet, and medial foot or knee pain.

Genu valgum persisting beyond childhood may lead to patella tracking issues, walking with a limp or circumduction gait, unbalanced stance, premature osteoarthritis, and/or cosmetic challenges in adults.

Prevalence:

• Age – Genu valgum usually becomes apparent at the age of 2-3 years and may initially increase in severity, self-correcting by age 7. If the condition persists beyond this age, surgical correction may be required.
• Sex / gender – The condition is slightly more prevalent in girls than boys. However, a greater degree of physiologic genu valgum is typical in females, possibly owing to a wider pelvis.

Risk factors:

• Genu valgum may develop as a result of inherited bone or metabolic disorders.
• Obesity can play a role in the etiology of pathologic genu valgum, which worsens with skeletal maturation and increases the risk of osteoarthritis into adulthood.
• Flat feet and external tibial torsion may accompany physiologic valgus, accentuating its appearance.
• The overall incidence of genu valgum is higher in countries where malnutrition is common and infectious diseases result in growth disturbances.

Pathophysiology: The etiology of genu valgum can be separated into bilateral or unilateral.

• Bilateral genu valgum arises from physiologic conditions, such as renal osteodystrophy secondary to renal rickets, or skeletal dysplasias, which can include Morquio syndrome, spondyloepiphyseal dysplasia, or chondroectodermal dysplasia.
• Unilateral genu valgum results from physeal injury due to trauma, infection, or vascular insult, as well as from benign tumors or neoplasms that can include fibrous dysplasia, osteochondromas, or Ollier disease.