An indication for functional knee-orthoses is given especially with ACL-insufficiency and after ACL-operations. Analyses of biomechanical examinations demonstrate difficulties in realizing the joint movement of the knee in detail and imitating it with knee-orthoses. An anamnestic investigation and clinical examination of sportsmen and -women, using functional knee-orthoses due to ACL-insufficiency (24 Lenox-Hill, 25 C.Ti., 38 Donjoy), demonstrate problems in practical usage of these orthoses. Problems especially arise from dislocation of the orthoses' joints and a deficient protection against anterior tibial translation. Thus leading to evident giving-way events though using a knee brace in 68% of these cases. In the tested knee-orthotic-devices problems in usage are independent of the kind of mechanical joints of the orthoses. The clinical examination should prove an application of the orthosis without pressure and only little distal dislocation.