About Unicompartmental Knee Replacements

An Oxford unicompartmental knee replacement, made by ZimmerBiomet.

A Unicompartmental Knee Replacement (UKR) is a prosthesis that only covers part of the knee. This is suitable in cases where the rest of the knee is not worn, and the anterior cruciate ligament (ACL) is still intact. At the time of surgery, I will inspect the rest of the knee and very rarely it may be necessary to convert to a total knee replacement during the operation.

A unicompartmental knee replacement is carried out through a smaller incision than a total knee replacement, and the recovery may be slightly quicker than a total knee replacement. Because the cruciate ligaments remain intact, it may also feel more “normal” than a total knee replacement. The vast majority of unicompartmental knee replacements are for the arthritis of the medial (inside) compartment of the knee.

The worn joint surfaces are shaped to accept the new prosthesis. These come in a variety of sizes to ensure an accurate fit. The prosthesis is coated with hydroxyapatite, a component of bone, which allows your bone to grow onto the prosthesis. A polyethylene insert is then placed between the two metal components to ensure a smooth gliding surface.

The aim of a unicompartmental knee replacement is to relieve your pain, while maintaining or improving your movement. You can expect gradual improvement up to 18 months following knee replacement, although the most rapid improvement is between 6 weeks and 6 months.

X-rays showing wear in only the medial compartment of the knee. This is potentially suitable for a UKR.

Prior to surgery, you will have a pre-operative assessment to ensure that you are fit to undergo the operation. You will also see a physiotherapist, who will show you exercises and teach you to use crutches. On the day of the operation, you will be seen by me and by the anaesthetist before surgery. Your leg will be marked and the consent form signed. The operation itself will usually take around 1 hour. The majority of these operations are carried out under a spinal anaesthetic, where the legs are numb, but sedation can also be given to ensure that you are sleepy.

Following surgery, you will be transferred back to the ward. Pain control is prescribed to allow you to undertake your exercises; please alert the nursing staff if more pain relief is required. You will be seen by the physiotherapists to encourage you to move the knee and start mobilising.

Most patients are able to return home within 3 days, once safely mobilising with crutches and able to climb stairs. You will be able to put your full weight through the operated knee. The knee will be somewhat stiff and sore for the first few weeks following surgery and regular pain relief may be required. It is important to keep doing your exercises in these first weeks to ensure a good range of movement of your knee, as well as to decrease the risk of a blood clot. The incision is usually closed with a dissolvable stitch and skin glue, so there are usually no staples or sutures to remove. I encourage patients to keep the waterproof dressing in place for 2 weeks, although showering with the dressing on is permitted.

X-rays demonstrating an Oxford unicompartmental knee replacement

You will be seen by me or my team at the six week mark to ensure that all is progressing well. An X-ray will usually be taken at this visit. If all is well, your next appointment will be at the 6 month mark, with a further X-ray.


Outcomes of Unicompartmental Knee Replacement

All of my joint replacement patients are invited to participate in the National Joint Registry which monitors outcomes of hip and knee replacement. Using this national data, we can predict that approximately 90% of UKRs are still functioning well after 10 years. Should you develop arthritis in the remaining parts of the knee, or develop wear or loosening of the UKR, it is usually possible to exchange the UKR for a standard total knee replacement prosthesis.


Complications of Unicompartmental Knee Replacement

All operations come with attached risks. These can include:

Bleeding – a blood transfusion may be required but this is rare.

Infection – This is a potentially devastating complication and all efforts are taken to prevent this. These include the use of antiseptic skin preparations during surgery, ultra-clean air laminar flow operating theatres with every care taken to maintain sterility during surgery, and the use of antibiotics during and after surgery. Despite this infections can still occur. These may be superficial wound infections which can be successfully treated with antibiotics, but a severe deep infection is likely to need further surgery to wash out the knee, or even to remove and replace the components. The risk of a deep infection is less than 0.5%.

Blood clots – a blood clot in the leg (a deep vein thrombosis; DVT) or in the lung (a pulmonary embolus; PE) is a potentially serious complication. The risk is decreased by giving you blood thinning medication for 14 days following surgery. The most important factor to reduce this risk is getting you mobile as quickly as possible. The risk of a DVT that causes symptoms is approximately 2%; the risk of a PE is 0.5%. If you are unfortunate enough to develop either of these, then blood thinning medication such as warfarin (or a newer alternative) is usually required for 3-6 months.

Knee stiffness – Although relatively uncommon following UKR, the knee may become stiff following surgery. Manipulation of the joint under general anaesthetic to bend the knee and break down scar tissue within the joint may be required. Early and regular exercises can reduce this risk.

Numbness – it is common to have some numbness over the outside of the knee. This is because the small skin nerves running across the knee are cut at the time of surgery. This numbness does not usually cause problems.

Bearing dislocation – the plastic meniscal bearing which glides back and forwards as the knee flexes can very rarely dislocate. If this happens surgery will be required to replace the bearing.

Wear – Modern knee prostheses mean that your joint replacement should last many years. The plastic bearing can wear out but if the metal components are well fixed, the bearing can be replaced. If the remaining joint surfaces become worn and painful, then the knee may need to be revised to a total knee replacement.

Revision – Although generally UKRs have a slightly higher rate of revision compared to total knee replacements, the majority of patients with them do very well. The implant I use has excellent long term results. Should your knee ever need to be revised, it is likely that a standard total knee replacement prosthesis can be used.