Mechanics of the knee and problems in reconstructive surgery

Static force transmission at the knee is analysed using measurements from radiographs showing the position of the line of body weight and also the bones of the knee in their correct orientation during function. With this technique it is possible to suggest values for a variety of forces acting at the knee. During function the degree of knee flexion is not as important as the angle that the thigh makes with the vertical. The tension in the extensor mechanism is not the same above and below the patella. Failure to recognise these two features results in fundamental errors. The patella, the effects of patellectomy and of forward displacement of the attachment of the patellar ligament are discussed. The importance of the transmission of force in the coronal plane is emphasised with particular reference to total knee replacement. It is suggested that small errors of geometry, as seen in the anteroposterior radiograph, can produce large changes of load.