Authors describe the more important injuries that must be corrected by all means during the operative treatment of the acute instability of the knee joint to prevent later arthrosis. They call attention to the fact that in the treatment of the acute instability of the knee joint the prevention of the arthrosis can be realized only with a reconstructive operation, performed in time and with modern technique. The biomechanical causes of the arthrosis, developing during the persistent instability are analysed. The extraarticular operative methods used by them in chronic cases are described.