Standing long-leg X-rays allow the weightbearing mechanical axis to be drawn from hip to ankle, and reveals that there is a varus deformity in the one knee. That is, the axis passes normally through the middle of the joint in one knee, but is medial to the joint in the abnormal knee (red arrow).
In early cases it may be apparent without X-rays, because of a thrust of the joint to the outer side when walking (varus thrust).
Varus malalignment and its association with impairments and functional limitations in medial knee osteoarthritis. Lim BW, Hinman RS, Wrigley TV and Bennell KL. Arthritis Rheum. 2008 Jul 15;59(7):935-42.
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