I am a rather large woman scheduled to have knee replacement surgery. How do you select the size of the knee replacement, and what happens if one side of the joint is bigger than the other?

Pre-operative planning is an integral part of knee replacement surgery.  As with the clothes you wear, one size does not fit all.
We use X-rays to help determine the size of the components used to replace a knee. There are four parts to a knee replacement. Two metal components are applied to each side of the joint on the femur and tibia bone surfaces. One plastic component is used to resurface the patellar bone and the other plastic piece is an insert that is placed as a bearing surface between the metal components. The components are engineered to allow a large femoral component to be used with a small tibial piece. Measurements from your knee X-ray will give a surgeon an estimate of the size needed for each individual component. Sometimes, custom-made components may have to be made when the bone surfaces are extremely small or large. At times, special metal wedges are used in severe arthritic cases. The actual fine-tuning of size selection is made in surgery. However, careful evaluation of the X-ray before surgery will help the surgeon anticipate your size needs.