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Patient Info

Unicondylar Knee Replacement

Introduction

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, see HTO vs UKR vs TKR in the FAQ section). Each procedure has its pros and cons. TKR can be applied successfully to nearly any situation whereas UKR, like HTO has very specific indications, outside of which TKR 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. By the same token UKR does not correct malalignment, does not rebalance or substitute for ligament deficiencies and therefore is applicable to very specific situations only. 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.  

The procedure is usually recommended for older patients who suffer from pain and loss of function from arthritis and have who have failed to achieve satisfactory relief from other more conservative methods of therapy. Age by itself is no barrier to UKR. UKR can be done in younger adults if the symptoms are severe enough and no other effective medical or surgical treatment option exists.

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.

Fig 1. UKR. Frontal projection.

Fig 2. UKR. Side projection.

Diagnosis

The diagnosis of osteoarthritis is made on history, physical examination & X-rays.

There is no blood test to diagnose Osteoarthritis (wear & tear arthritis).

Benefits of UKR

The decision to proceed with UKR surgery is a cooperative one between you, your surgeon, family and your local doctor.

The benefits following surgery are relief of symptoms of arthritis. These include:

  1. Severe pain that limits your everyday activities including walking, shopping, visiting friends, getting in and out of chair, gardening, etc.

  2. Pain waking you at night.

  3. Prior to surgery you will usually have tried some conservative treatments such as simple analgesics, weight loss, anti-inflammatory medications, modification of your activities, canes, or physical therapy.

Once these have failed it is time to consider surgery. Most patients who have UKR are between 50 to 70 years, but each patient is assessed individually and patients as young as 20 or old as 90 are occasionally operated on with good results.

Pre-Operation

UKR is not a minor procedure and should be undertaken under optimal conditions. This means that your general health needs to be assessed and any correctable problems dealt with prior to surgery. In most circumstances this will involve blood tests, an ECG and sometimes, an echocardiogram. Ideally this should be done by your GP prior to you being referred for major surgery. This is especially important if you have a known heart condition and have not been seen by your cardiologist in the recent past. Dr McEwen will organize much of the appropriate testing to be conducted prior to your consultation if not already done. This is very important to minimize surgical risk to yourself. Take note of the following:

  • High Blood Pressure (Hypertension) should be controlled and stable.

  • Diabetes should be controlled and stable with blood sugar levels checked regularly.

  • Any cardiac symptoms (chest pains, shortness of breath, palpitations) should be investigated and treated.

  • Sleep apnoea should be investigated and treated.

  • BMI should be less than 35. BMI is a measure of obesity and is calculated by dividing weight in kg by the square of height in meters. For example someone who weighs 100kg and is 1.89m tall will have a BMI 100/1.89x1.89 = 100/3.57 = 28. The normal range for a BMI is 20-25. A BMI over 35 is associated with an increased risk of serious complications and Dr McEwen will apply this as a cutoff for UKR. If your BMI is over 35 you should lose weight before seeking a UKR. This should be done with the aid of a dietician and exercise physiologist.

  • Smoking is very dangerous when combined with major surgery. Smoking must be ceased. Dr McEwen will not undertake a UKR if you're smoking.

Many medications have an effect on bleeding, blood clotting and wound healing. Blood thinning medications (Aspirin, Clopidogrel, Warfarin, Pradaxa) will need to be ceased at least 5 days prior to surgery. Dr McEwen will give specific instructions regarding this. Seek clarification well before surgery if unsure.

Cease any naturopathic or herbal medications 10 days before surgery.

Look after your skin. The skin on the leg being operated on must be free of cuts, scratches, grazes and bites.

Day of Your Surgery

You will be admitted to the hospital either the evening before or on the day of your surgery depending on what time your surgery is scheduled for.
Blood tests may be required on admission
You will meet the nurses and answer some questions for the hospital records
You will meet your Anaesthetist, who will ask you a few questions
You will be given hospital clothes to change into and have a shower prior to surgery
The operation site will be shaved and cleaned
Approximately 30 minutes prior to surgery you will be transferred to the operating theatre complex

Surgical Procedure

Surgery is performed under sterile conditions in the operating room under spinal or general anaesthesia. You will be positioned on your back and a tourniquet applied to your upper thigh but used for only a brief period to reduce injury to the thigh muscles. The surgery takes approximately two hours.

For a UKR to work well the replaced part of the knee must match the unreplaced part. This means that the geometry of the replacement needs to match closely the prearthritic geometry of the damaged side, that the prosthetic and native joint lines match up with respect to level and inclination and that the ligaments are under normal tension. This can be done in 2 ways:

  1. Conventional Instrumentation (Old School). Mechanical jigs are used around or through the centres of the femur and tibia. Cutting blocks are mounted on these jigs and the worn ends of the bones removed by cutting through these blocks.

  2. Computer Assisted Surgery (The Gold Standard). Position sensors are attached to the bones and used to teach a computer about the specific anatomy of the knee in a process called registration. A customized plan is developed to achieve optimal sizing, positioning and alignment. This plan is then executed using the computer to position the cutting blocks and to check that each step has been completed as per the plan. Used to its full extent Computer Assisted Surgery is a very powerful tool that predictably leads to better sizing, positioning and alignment compared to conventional instruments.

Dr McEwen uses Computer Assisted Surgery for all UKRs.

Accurate sizing, positioning and alignment make a successful UKR a possibility but constitute only one part of the formula for success. The remainder of the formula is ensuring that pain and swelling don’t get in the way of movement. Many things contribute to this including:

  1. Spinal anaesthesia. A spinal anaesthetic lasts for 4-7 hours, is usually combined with a light general anaesthetic and has several advantages:

    1.  It allows the patient to wake without pain and time for secondary pain relief measures to kick in before the limb becomes sensate again.

    2. It dramatically reduces the amount of anaesthetic medicine used so problems like nausea, vomiting, drowsiness and disorientation are much less likely to occur.

    3. It allows the anaesthetist to the keep the blood pressure low and even so there is no need to use a tourniquet for long periods (see below).

  2. Limiting Tourniquet Time. A tourniquet prevents blood loss during the operation but causes pain, swelling and wasting of the thigh muscles. If the blood pressure is kept low and even a tourniquet is used for short periods only or not at all without increasing blood loss. Pain is reduced and control of the knee by the thigh muscles is better and recovers faster.

  3. Minimal Incision Muscle Sparing Approach. There are several ways to surgically enter the knee. Each has its pros and cons. Muscle sparing approaches do less damage to the quadriceps so control of the knee is better and recovers faster.

  4. Periarticular Injection. The lining of the joint (synovium) is very sensitive and is the source of much of the pain, swelling and scarring after UKR. A periarticular injection of painkillers and anti-inflammatory drugs into the synovium and other tissues around the knee during the UKR reduces pain, swelling and scarring and is particularly effective when combined with a spinal anaesthetic and a cryocuff.

  5. Cryocuff. A cryocuff is a mobile, patient operated ice delivery system that reduces pain and swelling. The cryocuff is applied to the knee after wound closure and will be operational long before the spinal anaesthetic wears off.

  6. Multimodal Analgesia. A combination of drugs is used to manage pain and ensure reasonable sleep after the UKR.

Dr McEwen employs all these strategies routinely. All UKR patients will discharge with a cryocuff which is returned after 2 weeks. The vast majority of UKR patients will walk the day of surgery and walk out of hospital within 24-48 hours. This does not mean that there will be no pain or swelling but that pain will be manageable and much less likely to interfere with movement.

Bilateral UKR. Knee osteoarthritis frequently affects both knees. Having both knees replaced at the same time has many advantages but is not applicable to all situations. The vast majority of patients having both knees replaced will walk the day of surgery and walk out of hospital within 2-3 days. 

Post-Operative

Getting over major surgery without any problems requires an active approach to prevent problems rather than reacting to problems once they occur. The rule with prevention is “the sooner the better”. Therefore there are a few things the patient should actively take responsibility for:

  1. Ice therapy. Turn over the water in the cryocuff at least hourly unless sleeping.

  2. Triflo. Use your incentive spirometer as soon as possible. 10 good hard efforts every hour. This prevents pneumonia.

  3. Blood Clot Prevention. Drink plenty of fluids. Do calf pump exercises when lying or sitting. Be prepared to be out of bed a few hours after surgery. Being up quickly is the most important.

  4. Pain Control. Don’t let pain ramp up. As soon as you are aware that pain is increasing let the nursing staff know. Much less painkiller is required to keep pain away compared to beating severe pain into submission.

The nursing staff will assist you with these issues but remember to actively take control of your own well-being.

Physiotherapy begins the day of surgery and will be a daily event until discharge. The aim is to have you independently mobile and self caring as quickly as possible. Canadian crutches are used for balance, not to take weight as immediate full weightbearing is the goal. Elderly patients tend to be better with a walking frame for balance.

The criteria for discharge are a dry wound, good pain control with oral medications, no high temperatures and a pass mark from the physiotherapist including on stairs. Most will achieve this within 24 hours. Make sure you have your discharge envelope with scripts, information sheets and appointment card when you leave. These information sheets are important and should be read. They are available on this website as well. Hit the Patient Info tab on the home screen and click on Patient Information Pamphlets.

Remember that swelling is the enemy of movement. Use the cryocuff and keep the leg up. Lots of small walks around the house followed by periods of elevation and ice is the correct approach.

Sleep in whatever position is most comfortable. There is no need to sleep only on your back with the knee straight.

The wound should be clean and dry. Notify Dr McEwen immediately if this is not the case.

You will have a postoperative appointment scheduled for 2 weeks after the surgery. Sutures will be removed at this time. Paul Parker, a very senior and experienced musculoskeletal physiotherapist will see you at the same time so leave plenty of time for this. There is no need for supervised physiotherapy between discharge from hospital and your appointment with Paul.

Sleep disturbance is very common after UKR and tends to persist long after walking is easy. Sleep in whatever position is most comfortable, use ice, and use the provided sleeping tablets and painkillers if needed (as long as they are causing no side effects).

Risks and Complications

As with any major surgery, there are potential risks involved. The decision to proceed with the surgery is made because the advantages of surgery outweigh the potential disadvantages

It is important that you are informed of these risks before the surgery takes place

Complications can be medical (general) or local complications specific to the Knee

Medical complications include those of the anaesthetic and your general well being. Almost any medical condition can occur so this list is not complete. Complications include:

  • Allergic reactions to medications

  • Blood loss requiring transfusion with its low risk of disease transmission

  • Heart attacks, strokes, kidney failure, pneumonia, bladder infections Complications from nerve blocks such as infection or nerve damage

  • Serious medical problems can lead to ongoing health concerns, prolonged hospitalization or rarely death

Local Complications

  • Infection. Infection can occur with any operation. In the knee this can be superficial or deep. Superficial infections are treated with antibiotics. Deep infections always require surgical treatment as well as antibiotics. Deep infection is an uncommon but very serious complication and many measures are employed during the course of having a UKR to prevent it. In most cases of deep infection the joint can be salvaged but in certain circumstances the UKR will need to be removed and another implanted after the infection is cured.

  • Blood Clots (Deep Venous Thrombosis).These can form in the calf muscles and can travel to the lung (Pulmonary embolism). These can occasionally be serious and even life threatening. If you get calf pain or shortness of breath at any stage, you should notify your doctor. As with infection, many measures are employed to prevent this complication. These include compression stockings, calf compressors, IV fluids, blood thinning medications but by far the most important is early mobility.

  • Stiffness in the Knee (Arthrofibrosis). As discussed above implant positioning and control of pain and swelling are important if the UKR is to bend well. Other factors including how well the knee bent before surgery and diligent attention to physiotherapy and home exercise are also important.

  • Neurovascular injury (Damage to nerves and blood vessels). Some of the major nerves and vessels that pass the knee are within a centimetre of the surgical envelope of a UKR. Great care is taken to avoid damage to these important structures.

  • Wear. The plastic liner eventually can wear out over time. Improvements in the mechanical properties of the plastic have extended the useful lifespan of a UKR and in many cases the unreplaced sections of the knee will wear before the implant.

  • Numbness. The skin on the outside edge of the incision will be numb after the surgery. Most people are very aware of this in the first 6 months after surgery. The sensation of numbness fades with time but probably never returns absolutely to normal.

Summary

Surgery is not a pleasant prospect for anyone, but for some people with arthritis, it could mean the difference between leading a normal life or putting up with a debilitating condition. Surgery can be regarded as part of your treatment plan—it may help to restore function to your damaged joints as well as relieve pain.

Surgery is only offered once non-operative treatment has failed. It is an important decision to make and ultimately it is an informed decision between you, your surgeon, family and medical practitioner.

Although most people are extremely happy with their new knee, complications can occur and you must be aware of these prior to making a decision. If you are undecided, it is best to wait until you are sure this is the procedure for you.

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