X-leggedness is a condition where the legs are upright and together and the knee joints touch but there is still a gap between the ankle joints on both sides. In X-legged patients, the distal end of the knee joint is bent to the outside, hence the scientific name ‘valgus knee’.
Physiological knee valgus exists in children between the ages of 2 and 8 years, with the largest knee valgus occurring between the ages of 2 and 4 years. After the age of 8 years there is little change in the line of force of the lower limb and the line of force returns to neutral.
If the child is too short, has a history of trauma, infection, metabolic disease or bilateral asymmetry, the knee may be pathologically valgus.
The presence of significant knee valgus into adulthood should be taken seriously. More importantly, the force line passing through the lateral aspect of the knee produces greater abnormal stress on the lateral knee joint, which can lead to pain in the lateral aspect of the knee and eventually to lateral osteoarthritis.
Gradual onset of knee valgus in adults is associated with intra-articular wear and tear and is a common manifestation of osteoarthritis, which should also be actively sought.