Among cases of recent or long-standing post-traumatic laxity of the knee ligaments, unstable lateral rotation is due to a well-defined anatomical lesion, viz. Rupture of the medial capsular ligament of the knee. This laxity is demonstrated clinically by the existence of an abnormal postero-anterior movement when the tibia is in lateral rotation. Its surgical treatment, suggested by D. Slocum, consists of displacement of the patella tendon. This rupture may exist alone but, usually, is associated with other ligamentous or meniscal lesions for which further specific treatment should be carried out. Slocum's operation has recently been used in the Strasburg Accident Centre. The early results are encouraging.