The bones have to be cut very precisely so that the components fit perfectly and straighten the leg. An error of even 1 mm may be critical because the knee has no ball and socket mechanism to compensate for error. The precision is achieved with jigs fitted to the bones during the operation. The jigs, which have slots to direct the cutting blades, are removed when the cuts have been made. Because people come in different shapes and sizes, so do knee prostheses.
Most designs have five different sizes of all three components.
The thickness of the plastic bearing is also variable and as many as 10 different thicknesses may be available for each size of prostheses. When there is a very gross bone loss, extra pieces are added, and the theatre sister must have 70-100 different items available when a knee replacement is performed. When the bone has been measured and cut to shape a trial prosthesis, an exact replica of the real thing, is put in position and the joint tested to be sure that it is stable and moves correctly. The trial components are then removed, final adjustments made, the bone surfaces cleaned thoroughly and the real prosthesis fixed in position with a cement prepared by mixing a powder and liquid together. The wound is then closed and dressed.