[Constrained knee prostheses]

There is great controversy over constrained knee prostheses, both from a clinical and a theoretical point of view. Hinged prostheses show disadvantages such as the large implant volume, a potentially higher risk of infection, and long intramedullary stems, which are difficult to remove in cases of revision surgery. However, there are advantages as well such as simple operative procedure for implantation, primary stability without need for soft tissue balancing, and greater femorotibial congruency based on the monocentric axes for flexion/extension. Clinical long-term results remain contradictory. Implantation of a constrained prosthesis makes sense as a primary implant in elderly patients, for severe defects, deformities, and instabilities, or as a revision implant.