One speaks of serious and/or complex instability of the knee joint when the knee is multiple directional instabil caused by e.g. major accidents (e.g. ruptured ACL / PCL after a knee dislocation). An ACL absence can cause anterior instability while the absence of the PCL is responsible for a posterior instability and a ruptured medial ligament for medial instability etc.. For each direction there is one or various holding structures. Therefore, in order to treat such a multi directional instability a number of structures and ligaments must be replaced.But first of all a experienced knee specialist is requiered who determines the cause of the instability and the missing structures. Only then, after a detailed treatment planning surgical stabilisation can be successfully addressed.