What is a "knee replacement"?
Sometimes also called "resurfacing", this operation involves the removal of the damaged, worn ends of the thigh (femur), and leg (tibia) bones that make up the knee joint, as well as the back of the kneecap (patella). The bone ends are precisely reshaped and recapped with metal surfaces, with special plastic liners added as a bearing surface that promotes low friction gliding between metal and plastic.
How is it done?
Basically, this resurfacing project is stepwise precision carpentry. After an incision is made, the surgeons remove the damaged portions and surfaces of the bones using guides or jigs, and oscillating saws. Precise amounts of bone are removed, usually between 2 and 12 mm. The ends of the bones are shaped to accommodate the prosthesis to be used. Trial, or test prosthetic components are then used to evaluate the fit, range of motion, stability, alignment, and balance of the soft tissues. When all is satisfactory, the final prosthetic components are cemented into place, and the surgical wound is closed.
Current trends: smaller incisions and computer-assisted, "navigated" knee replacement surgery...
How large is the incision?
The photos below show a somewhat larger than normal exposure, in order to show the component parts. In other words, the incision shown is longer than what is typically used. The smallest incision possible, yet practical, is ideal. The incision must be big enough for the surgeons to be able to see what they are doing. The current trends are for smaller incisions, to reduce scarring, stiffness, and weakness, and to allow a faster recovery.
How is a computer helpful?
When knee replacements were first done, bone cuts were made by eye. Next came guides and jigs, placed on or in the bones to more precisely define the cuts. Now, computers, or navigation systems are used to assists with the placement of the cutting guides, for even more precision. The computers rely on the input of landmark mapping data done with an electronic stylus to create a 3D program of the limb. The theory is that a more precise set of bone cuts may facilitate better replacement component alignment, and hopefully, enhance the longevity of the replacement. This theory is not yet proven, but is intuitive. This photo shows the computer camera system and video monitor used during the procedure.
Left knee incision during knee exposure and with knee extended (straight) shows wear behind patella (reflected backward on the right) and in trochlear femoral groove (in center of the photo).
Further exposure with knee flexed (bent) shows a very common wear pattern on the medial (inner) condyle (knuckle) of the femur with complete loss of surface cartilage adjacent to tip of retractor. The patella (right side, still flipped), has been prepared for placement of a plastic prosthesis (see below), as the worn cartilage has been resected, and anchor holes have been drilled.
Left knee: making surface bone cuts with oscillating saw from above, removing damaged bone from the femur, using a cutting guide. The soft tissues are carefully protected with retractors.
Left knee: a cutting guide is prepared for removing damaged bone from the top of the tibia. The saw will be used through the slot near the top and front of the guide. The long guide with T-handle above goes down the length of the tibia to help establish satisfactory alignment for a precise cut.
Left knee in extension: the final prosthesis in place. The round white plastic disk is the back of the flipped patella, the silver component caps the lower end of the femur, and the white tray below the femur is the combined plastic-metal component on the upper end of the tibia. All components have been cemented with a very thin layer bone cement.
Left knee flexed: after complete resurfacing "total knee replacement". The wound is closed, and rehabilitation can begin.