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A hinged-type of metal artificial knee was developed by Walldius, 1,2 and Shiers 3,4 and has been reasonably successful. The experience of many, however, has indicated that such a system tends to have certain faults, which include (1) a metal-to-metal articulation that accumulates metallic debris in the knee, (2) sometimes breakage or loosening of the linking bolt or hinge coupling, (3) a loosening of the stem components in the medullary cavities of the femur and tibia, and (4) a fairly high rate of delayed infection (Young HH: Personal communication). Biomechanically and anatomically, a rigidly hinged prosthetic knee does not allow for reasonable simulation of normal knee motions, including rotation. Thus, stresses are placed on all portions of the components, and fracture or loosening may occur in time. DESIGNFor these reasons, we developed a metal-to-polyethylene total knee arthroplasty. 5 Each of us had his own concept of a nonhinged metal-to-plastic knee system of prostheses. By pooling our knowledge and efforts, the present geometric knee prostheses were and designed, and clinical trials were begun. We have had slightly less than 2 years of experience with the system.The prosthesis for the femoral surface of the knee joint is made of vitallium, and of ultra-high molecular weight polyethylene for the articulating surface of the tibia (Figure 1). The design is such that the collateral and cruciate ligaments can be left intact. The femoral component is slightly smaller than the tibial (by 2 mm in diameter) to allow for rotational movements on extension and flexion of the tibia on the femur. No attempt to duplicate the anatomic structure of the knee is made, although the prostheses are bicondylar. Each unit is held firmly to the femur and tibia by the use of methyl methacrylate. During the course of design and use, many modifications were made. The size of the femoral component has been reduced; the connecting cross bar has been made smaller to accommodate the anterior cruciate ligament on full extension; and the tibial component has had the lips in front and back decreased in height because we found that anterior and posterior stability could be achieved without so much ''cupping.'' Metal markers have been incorporated and placed by the manufacturer so as not to migrate. And a dovetail-type of anterior prolongation has been added to the tibial component to give the prosthesis more stability in flexion. In addition, a smaller size has been made for the smaller-boned patient, usually the woman with rheumatoid arthritis, and larger medial and laleral tibial components are available where there is marked loss or bony substance or either condyle.