Unicondylar Knee Replacement


Unicondylar knee replacement is a surgical procedure that involves replacing a part of the knee joint with a prosthesis instead of the entire knee joint as in a total knee replacement. Total knee replacement surgery replaces the ends of the femur (thighbone) and tibia (shinbone) with a metal component, and sometimes even the patella (kneecap) with a plastic prosthesis.

Unicondylar knee replacements have been performed since the early 1970s with mixed success. Over the last 25 years, implant design, instrumentation and surgical techniques have improved markedly, making it a very successful procedure for unicompartmental arthritis. Recent advances allow us to perform this procedure through smaller incisions, and is therefore not very traumatic to the knee, making recovery quicker.


The joint surface is covered by a smooth articular surface that allows pain-free movement in the joint. Arthritis is a general term covering numerous conditions where the joint surface (cartilage) wears out. When the articular cartilage wears out, the bone ends rub against each another, causing pain.


There are numerous conditions that can cause arthritis, and often, the exact cause is never known. In general, but not always, it affects people as they get older (osteoarthritis).

  • Trauma (fracture)
  • Increased stress e.g., overuse, overweight, etc.
  • Infection
  • Connective tissue disorders
  • Inactive lifestyle e.g., obesity, as additional weight puts extra force on your joints
  • Inflammation e.g., rheumatoid arthritis

In an arthritic knee

  • The cartilage lining is thinner than normal or completely absent. The degree of cartilage damage and inflammation varies with the type and stage of arthritis.
  • The capsule of the arthritic knee is swollen.
  • The joint space is narrowed and irregular in outline; this can be seen in an X-ray image.
  • Bone spurs or excessive bone can also build up around the edges of the joint.

The combination of these factors make the arthritic knee stiff and limit activities due to pain or fatigue.


The diagnosis of osteoarthritis is made from history, physical examination and X-rays. There is no blood test to diagnose osteoarthritis (wear and tear arthritis).

Who is suitable for unicondylar knee replacement?

Unicondylar knee replacement can be performed when one compartment is involved clinically and is confirmed on X-ray. The surgery is recommended when pain and restricted mobility interferes with your lifestyle. It is suggested when conservative treatments such as simple analgesics, weight loss, anti-inflammatory medications, modification of your activities, canes or physical therapy fail to relieve your symptoms. You should ideally be over 50 years of age.

Who is not suitable?

Unicondylar knee replacement is not suitable in the following cases:

  • Patients with arthritis that affects more than one compartment
  • Patients with severe angular deformity
  • Patients with inflammatory arthritis e.g., rheumatoid arthritis
  • Patients with an unstable knee
  • Patients who have had a previous osteotomy
  • Patients who are involved in heavy work or contact sports


Your surgeon will send you for routine blood tests and any other investigations prior to your surgery. You will be asked to undertake a general medical check-up with a physician. You should have any other medical, surgical or dental problems attended to prior to your surgery. Make arrangements for help around the house prior to surgery. Cease aspirin or anti-inflammatory medications 10 days prior to surgery as they can cause bleeding. Cease any naturopathic or herbal medications 10 days before surgery. Stop smoking much in advance to avoid complications. 

Day of surgery

You will be admitted to the hospital usually on the day of your surgery. Further tests may be required on admission. You will meet the nurses and answer some questions for hospital records. You will meet your anesthetist, 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 room.

Surgical procedure

Each knee is individual, and knee replacements take this into account by having different sizes for your knee. If there is more than the usual amount of bone loss, sometimes, extra pieces of metal or bone are added. Surgery is performed under sterile conditions in the operating room, under spinal or general anesthesia. Surgery will take approximately two hours. You will be on your back and a tourniquet applied to your upper thigh to reduce blood loss.

The leg is prepared for the surgery with a sterilizing solution. An incision around 7 cm is made to expose the knee joint. The bone ends of the femur and tibia are prepared using a saw or a burr. Trial components are then inserted to make sure they fit properly. The real components (femoral and tibial) are then put into place with or without cement. The knee is then carefully closed, drains usually inserted, and the knee is dressed and bandaged.

Postoperative course

When you wake, you will be in the recovery room with intravenous drips in your arm, a tube (catheter) in your bladder and a number of other monitors to check your vitals. You will usually have a button to press for pain medication called a PCA machine (Patient Controlled Analgesia).

Once stable, you will be taken to the ward. The post-op protocol is surgeon-dependent, but in general, your drain will come out at 24 hours and you will sit out of bed, and start moving your knee and walking within a day or two of surgery. The dressing will be reduced usually on the 2nd post-op day to make movement easier. Your rehabilitation and mobilization will be supervised by a physical therapist.

To avoid lung congestion, it is important to breathe deeply and cough up any phlegm you may have. Your orthopaedic surgeon will use one or more measures to minimize blood clots in your legs with inflatable leg coverings or stockings, and injections administered in your abdomen to thin blood clots or DVTs (will be discussed in detail in the complications section).

A lot of the long-term results of knee replacements depend on how much work you put into it following your operation.

You will usually remain in the hospital for 3-5 days. Depending on your needs, you will return home or proceed to a rehabilitation facility. You will need physical therapy on your knee following surgery. You will be discharged on a walker or crutches, and usually progress to a cane at six weeks. When you go home you need to take special precautions around the house to make sure it is safe. You may need rails in your bathroom or to modify your sleeping arrangements, especially if you need to climb a lot of stairs.

Bending your knee is variable, but by 6 weeks it should bend to 90 degrees. The goal is to obtain 110-115 degrees of movement. Once the wound is healed, you may shower. You can drive at about 6 weeks, once you have regained control of your leg. You should be walking reasonably comfortably by 6 weeks.

More physical activities, such as sports, previously discussed, may take 3 months to be able to do comfortably. Your sutures are sometimes dissolvable, but if not, are removed at approximately 10 days.

You will usually have a 6-week check up with your surgeon, who will assess your progress. You should continue to see your surgeon for the rest of your life to check your knee and take X-rays. This is important, as sometimes, your knee can feel excellent, but there can be a problem only recognized on X-ray.

You are always at risk of infections, especially with any dental work or other surgical procedures where germs (bacteria) can get into the blood stream and find their way to your knee. If you have any unexplained pain, swelling or redness, or if you feel generally poor, you should see your doctor as soon as possible.

Advantages and disadvantages

The decision to proceed with knee replacement surgery is a cooperative one between you, your surgeon, family and your local doctor. The benefits of surgery include:

  • Smaller incision
  • Not as much bone removed
  • Shorter hospital stay
  • Shorter recovery period
  • Blood transfusion rarely required
  • Better movement in the knee
  • Feels more like a normal knee
  • Less need for physical therapy
  • Able to be more active than after a total knee replacement

The big advantage is that, if for some reason it is not successful or fails many years down the line, it can be revised to a total knee replacement without difficulty.

The benefits following surgery include relief from the following:

  • Severe pain that limits your everyday activities, including walking, shopping, visiting friends, getting in and out of a chair, gardening, etc.
  • Pain waking you at night
  • Deformity, either bowleg or knock knees
  • Stiffness


It is not quite as reliable as a total knee replacement in completely treating pain. The long-term results are not quite as good as a total knee replacement.

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, specific to the knee

Medical complications

Medical complications include those of the anesthetic 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 attack, stroke, kidney failure, pneumonia, bladder infection
  • Complications from nerve blocks, such as infection or nerve damage

Serious medical problems can lead to ongoing health concerns, prolonged hospitalization and rarely, death.

Local Complications


Infection can occur with any operation. In the knee, this can be superficial or deep. Infection rates are approximately 1%. If it occurs, it can be treated with antibiotics but may require further surgery. Very rarely, your knee may need to be removed to eradicate infection.

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 surgeon.

Fractures or breaks in the bone

Fractures or breaks can occur during surgery or afterwards, if you fall. To repair these, you may require surgery.

Stiffness in the knee

Ideally, your knee should bend beyond 100 degrees, but on occasion, it may not bend as well as expected. Sometimes, manipulation is required. This means going to the operating room where the knee is bent for you while you are under an anesthetic.


The plastic liner eventually wears out, usually after 10 to 15 years, and may need to be changed.

Wound irritation or breakdown

The operation will always cut some skin nerves, so you will inevitably have some numbness around the wound. This does not affect the function of your joint. You may also experience aches around the scar. Vitamin E cream and massaging can help reduce this.

Occasionally, your body can react to the sutures or a wound breakdown that may require antibiotics, or rarely, further surgery.

Cosmetic appearance

The knee may look different than it was.

Leg length inequality

This is also due to the fact that a corrected knee is straighter and is unavoidable.


This is an extremely rare condition where the ends of the knee joint lose contact with each other, or the plastic insert loses contact with the tibia or the femur.

Patella problems

The patella (kneecap) can dislocate, i.e., move out of place, and can break or loosen.

Ligament injuries

There are a number of ligaments surrounding the knee. These ligaments can be torn during surgery, or break or stretch any time afterwards. Surgery may be required to correct this problem.

Damage to nerves and blood vessels

Rarely, these can be damaged at the time of surgery. If recognized, they are repaired, but a second operation may be required. Nerve damage can cause a loss of feeling or movement below the knee, and can be permanent.

Discuss your concerns thoroughly with your orthopaedic surgeon prior to surgery.


Surgery is not a pleasant prospect for anyone, but for some people with arthritis, it could mean the difference between leading a normal life and 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 is ultimately an informed decision between you, your surgeon, family and medical practitioner.

Although most people are extremely happy with their new knee, complications can occur. 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.