Knee Arthritis: Partial Knee Replacement Animation

Knee Arthritis: Partial Knee Replacement Animation

Unicompartmental knee arthroplasty, or UKA, is when only one compartment of the knee is replaced due to arthritis. It is also known as a partial knee replacement. This animation shows the replacement of the medial compartment since it is the most commonly replaced of the three compartments in the knee. The cartilage surfaces are replaced with metal pieces, and a plastic insert is placed between them, as shown.

To remove the damaged cartilage from the top of the shinbone, the surgeon places a long guide on the leg and uses a small pin to keep it in place. The flat piece on top of the guide is lined up with the top of the shinbone, and the surgeon uses two different kinds of saws to remove the damaged cartilage. A small amount of bone is removed as well. The surgeon wants to make sure there is no cartilage left. Next, the guide and the damaged cartilage surface are both removed.

Another way this step can be done is with a smaller cutting guide. The small metal hook is placed on the back of the shin bone, and then the white cutting guide is lined up for the saw cuts. This white cutting block helps the surgeon line up these cuts to match the angles of the patient's knee. Three small pins are used to keep the white cutting block in place. There are thin slots in the cutting block to keep the saw blade lined up during the cuts. The surgeon makes the same two saw cuts as the other method, and everything then comes out in one piece.

The next several steps are removing the damaged cartilage surfaces on the end of the thigh bone. Many different cutting blocks and guides are used to make sure these cuts are done smoothly, and that only the correct amount of bone is removed. All of these different guides and measurements help the surgeon decide on the thickness of the metal and plastic pieces to keep the patient's knee alignment as close to normal as possible.

The cuts are done at certain angles so that the metal replacement piece fits like a puzzle. The surgeon will use this spacer block to double-check the gaps when the knee is bent and when it is straight before making this final angle cut. Small pins are used on each cutting guide to stabilize it, keeping the saw blade straight. Two holes are drilled through this guide, which will be used to keep the metal piece on the thigh bone in place. You can see here the pegs on the back of the metal piece for the thigh will fit right into the holes that were just drilled.

At this point, the surgeon will use a metal tray and a plastic insert to double-check the sizes of the final implants that will go on top of the shin. These pieces are called trials. The surgeon will then drill holes on top of the shin so the metal piece can fit in and remain stable. One final hole is made, the guide gets removed and both the bones have been prepared for the final implants.

Here, the tray on the shinbone or tibia is placed first. There are pegs on the back of the tray that fit into the holes that were created. Also, special bone cement is put on the back of the tray to cement it into place. The tray gets tapped into the bone. Going back to the thigh bone or femur, this implant also gets cement put on the back and gets tapped into place. The final plastic insert is put between the metal implants, and the partial knee replacement is complete.