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Partial Knee Replacement

What is a Partial Knee Replacement?

Partial or unicompartmental Knee Replacement (UKR) is a surgical procedure for replacing a single worn articulation in a knee. The knee has three separate articulations:

  1. The medial tibiofemoral articulation (between thigh and shin bone on the inside of the knee)

  2. The lateral tibiofemoral articulation (between thigh and shin bone on the outside of the knee)

  3. The patellofemoral articulation (between the kneecap and thigh bone).

Many arthritic conditions initially affect only one of these articulations. Malalignment (bowed legs) is a potent cause of osteoarthritis that causes one tibiofemoral articulation to be more heavily loaded than the others. Consequently the overloaded compartment begins to wear out while the others are normal. Osteonecrosis and post-traumatic arthritis are other arthritic conditions that often affect only a single articulation. Unicompartmental Knee Replacement (UKR) is a possible treatment option in such a situation. Any one of the three articulations can be replaced whilst leaving the undamaged compartments alone.

Generically speaking, unicompartmental knee arthritis can be treated with UKR, a TKR (Total Knee Replacement) or an HTO (High Tibial Osteotomy)(see the TKR and HTO sections for information on these procedures. Each procedure has its pros and cons. TKR can be applied successfully to a broader array of  situations whereas UKR, like HTO has very specific indications, outside of which TKR or no surgery at all is a better option. The universal reliability of TKR comes at a cost so it’s important to understand the differences.

TKR is an extremely powerful technique that allows your surgeon to alter many things in an arthritic knee. Malalignment can be corrected (a bowed leg is straightened), ligament imbalance can be corrected, joint lines can be altered, failed ligaments compensated for and inflamed synovium removed completely. To use a building analogy, a TKR is like pulling an old building down to its foundations and starting with a clean slate. The down side is that, for those with unicompartmental arthritis, most of what is being removed is normal including the Anterior Cruciate Ligament, the meniscus on the undamaged side and large areas of articular cartilage. UKR removes only what is damaged and leaves everything else alone. UKR is done through a smaller incision, is generally a much smaller operation, carries less risk, has a faster recovery and has much more normal mechanics when compared to TKR but, paradoxically, is technically more difficult because the replacement must match what is left behind or the replaced side of the knee fights with the unreplaced side. This is known as a kinematic conflict. To use the building analogy again, a UKR is like a renovation where the new area must match the existing building for everything to function well as a whole. Survivorship (how long will it last) also differs between UKR and TKR with TKR being the more durable option albeit at a cost. A UKR can be revised to a TKR if it fails.

Like TKR, UKR lasts best in older age groups because they take fewer steps but UKR is generally done on younger adults who are chasing a very high level of function and in whom a TKR is the last resort.

A typical unicompartmental knee replacement replaces one condyle on the end of the femur (thigh bone) and one condyle on the opposing surface of the tibia (shin bone) with metal. A plastic bearing is inserted between. The other areas of the knee are left alone including the Anterior Cruciate Ligament.


UKR Surgery

Much of what happens prior to, during and after a UKR is very similar to TKR. For more information on this go to the TKR page (Be sure to go to the Making a Fast recovery Section and watch the video). The big difference between the two is the comparative ease of recovery after UKR. In particular, the return of movement is easier and very predictable. The amount of work involved in the recovery is much less, especially if both knees are done. A UKR can be done as a day case or with a single night in hospital .

The National Joint Replacement Registry reports on UKR as it does TKR and the annual reports are open access online at

Robot Assisted Partial Knee Replacement

Robot assisted partial knee replacement has been around since around 2013. Robots at this point in time are not intelligent and all the decisions regarding the execution of the surgery are made by the surgeon. This is done by acquiring a CT scan of the knee prior to surgery. This is used to create a virtual model of the knee and then a virtual knee replacement is done on the virtual model. Ligament tension is then measured and added into the virtual model which then results in a definitive plan for sizing and positioning of the implant. The robotic arm is driven by the surgeon rather than being autonomous but works with the surgeon to produce very accurate removal of bone so that there is subsequently very accurate positioning of the implant. If you want more information regarding robot assisted partial knee replacement then hit he button to the link below.

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