What exactly happens during a total knee replacement surgery? How does the surgeon remove the existing knee joint components and replace them with a prosthetic implant? The surgery begins with an incision into the tissue to allow access to the patella (kneecap) and quadriceps muscle (on top of the upper leg). The quadriceps must then be moved in some manner to reveal the femur and tibia. The damage to those bones are then cut and smoothed to allow for installation of the implant components. After installation, the incisions are repaired.
Your surgeon has a few options in a how to go about the joint replacement surgery though. You will want to discuss the various approaches with your surgeon to understand exactly why a particular approach is used for your surgery.
The Incision and Joint-Revealing Approaches
In most instances, incisions are made straight down the center of the leg, sometimes making a curved deviation around the patella or knee cap. On occasion, other changes may be made necessary by the presence of previous surgical scars.
Once the skin incision is made, the surgeon will then go on to follow the natural cleavages between muscle bodies and ligaments. This is accomplished by opening up these divisions with blunt dissection, meaning there is no actual cutting of muscle involved. Once the areas are opened up and the joint capsule is revealed, this (the capsule) is opened with a scalpel. The knee is place in a bent position and an incision is made as described above. After the incision is made, the quadriceps muscle and the patella are moved and held aside with retractors to reveal the knee joint.
There are a variety of modifications to this basic incision. The differences are relatively technical.
After this initial procedure, a surgeon has several possible approaches to complete the replacement. Each approach has advantages and disadvantages.
Medial Parapatellar Approach: With a medial parapatellar incision, the surgeon will cut the quadriceps tendon above and around the inside (medial) of the patella. The patella is generally flipped in this approach to allow access to the knee joint. The tendon is repaired at the end of the procedure. Some are of the opinion that the tendon heals faster than the muscle belly of the vastus medialis oblique (VMO), which is part of the quadriceps muscle.
Mid-Vastus Approach: The mid-vastus approach cuts into the VMO muscle belly and around the inside of the knee cap, instead of cutting the tendon. Some feel that leaving a large portion of the VMO intact will help patellar tracking and strength. The patella may or may not be flipped. The muscle belly is repaired at the end of the procedure.
Sub-Vastus Approach: The sub-vastus approach does not involve cutting the VMO muscle at all. Instead, the muscle is elevated and the patella is not flipped. The incision extends around the inside of the knee cap. The patella is typically not flipped with this approach. This approach may be difficult in patients with large VMO muscles.
Quad-Sparing Approach: The incision in the quad-sparing approach cuts only the inside of the knee cap. This approach requires special side cutting instruments and is less common than some of the other approaches.
Lateral Parapatellar Approach: Another relatively uncommon approach is the lateral parapatellar approach. In this approach, the incision extends around the outside (lateral) of the knee cap.
Any of these total knee replacement approaches can provide good results. Patients should discuss the approaches with their surgeon to understand the mechanics of the surgery but are probably best advised to allow the surgeon to perform the approach the surgeon recommends and is most comfortable with.
After the Incision
Once he has access, your surgeon will will then begin to trim the damaged surfaces at the ends of the femur and the tibia (or shin bone) using special cutting guides which accurately shape the end of the femur so that it accurately fits into the chosen knee prosthesis. The tibia is cut straight across and a depression made in the center. If any knee ligaments have contracted because of pain or mechanical deformity, the surgeon may release them to allow them to improve functionality.
The prostheses are then inserted and secured with some bone cement. The surgeon will then test and balance the joint to assure proper placement and alignment.
If the patella requires resurfacing, the surgeon will then insert a domed ‘button’ made of polyethylene (plastic) that will mimic the original knee cap.
Increasingly, total knee replacement surgery is performed with the assistance of computer-driven robotics, helping the surgeon navigate the joint more precisely, using smaller incisions and getting a better fit between implant and bone.
image credit: Flood