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Unicompartmental Knee Replacement

What is a Knee Replacement?

A joint is a point of contact between two bones, allowing movement and providing support.

A unicondylar knee replacement is used when the arthritis affects just one side of the joint. The other normal areas are preserved and the affected side replaced with an implant made from metal and plastic. The advantage of a unicondylar versus a total knee replacement is a more normal feeling joint.

The knee is the joint formed where the femur (thigh bone) meets the tibia (shinbone). The ends of the bones are covered with cartilage, which acts as a cushion and allows for smooth, gliding movement. The patella (kneecap) is a small bone which runs in a groove on the femur. The function and stability of the knee depends on muscles and ligaments. Flexion (bending) and extension (straightening) are the principal movements of the knee. Knee replacement surgery has become more common, due to advances in the types of implants and materials. The operation can greatly improve quality of life because of pain reduction, increased mobility and the correction of deformity.

When is a unicompartmental knee replacement indicated?

The most common cause of joint problems is arthritis. There are other causes such as congenital abnormalities and deformities due to accidents.

The knee is one of the most complex joints in the body and is a major weight-bearing joint. It allows us to bend, straighten and rotate, while the ligaments give stability and balance. The knees often take the most punishment as a result of arthritis.

Knee conditions tend to be treated conservatively initially. However, many joint problems are progressive and surgery may be inevitable. A unicompartmental knee replacement is indicated when only one side of the joint is affected. The operation is minimally invasive, using a small incision at the front of the knee.

Objectives of knee replacement surgery

The objective of knee replacement surgery is to improve quality of life. It has a high success rate and dramatically improves quality of life by reducing pain, improving mobility and correcting deformities.

Pre-operative assessment

Once the decision has been made to have a knee replacement you will need to attend a pre-operative assessment clinic. Here a registered nurse will give you information about what to expect during your stay in hospital. She will carry out blood tests and/or an ECG and may send you for X-rays. Don’t be alarmed if you need to undergo one or more of these investigations – they are done in your best interest, as the doctors need to know that you are physically able to cope with the surgical procedure.

It is very important to discuss any medication you are taking with the nurse. If you are taking blood-thinners you may need to discontinue them for some time before the operation as they can increase the risk of bleeding and can interfere with your surgery and recovery. You will probably be told not to take aspirin, ibuprofen and all herbal or homeopathic medicines for several days before your operation. You should take your normal medication up to and including the day of surgery unless you have been given specific instructions not to by your anaesthetist or surgeon.

General information

You can expect to be in hospital for two to three days.

Remember to bring the following items with you into hospital:

  • A toiletry bag containing soap, a face cloth/sponge, toothbrush and toothpaste, shampoo and conditioner, deodorant, shaving kit etc
  • Dressing gown
  • Slippers that are easy to put on, preferably non-slip.
  • Spare clothes for ‘dressing practise’ and a set of clothes in which to go home.
  • Books or magazines
  • Reading glasses
  • Relevant telephone contact numbers.


  • Two or three front-fastening, knee length nightgowns. Long ones restrict movement.
  • A few changes of comfortable underwear, but not panty-girdles.


  • Two or three pairs of pyjamas
  • A few pairs of boxer shorts or underpants

Admission to hospital

You will be admitted to hospital on the day of your surgery. Once you have been admitted, you will be re-examined by a nurse and your orthopaedic surgeon, who will mark your knee and ask you to sign a consent form. You will be measured for anti-embolic stockings (TEDs), which promote circulation in the legs and prevent the formation of Deep Vein Thrombosis (DVT) or blood clots.

*It is very important that you tell the anaesthetist any medication that you are currently taking, or have been taking over the previous few days. There is a chance that certain medications can interfere with the outcome of the procedure, so it is important that you disclose the information. This includes:

  • Chronic/repeat medications
  • Hormone replacement therapy
  • Acute medication (medicines prescribed on a one-off basis)
  • Non-prescription drugs such as aspirin, paracetamol or anti-inflammatory drugs.
  • Homeopathic and herbal preparations

Anaesthetic and pain management

You will be seen by the anaesthetist before your operation. He will examine you and discuss the type of anaesthetic that will be used. Quite often a spinal anaesthetic is used. You will not be put to sleep for this, but it ensures that you are pain-free during the operation. For a while afterwards you will have no feeling in your legs and won’t be able to move them, but when the spinal wears off, function and feeling will return. Spinal anaesthesia has the advantage of having a lower risk of developing deep vein thrombosis (DVT).

The anaesthetist will discuss pain management with you, if it has not already been done at your pre-operative assessment.


You will be seen by a physiotherapist, who will assess your physical condition and prepare a programme to assist in your post-operative recovery. You will also be measured for crutches.

During Surgery

Before being taken to the operating theatre you will be asked to change into a rear-fastening gown and the TED stockings. You may be given medication to make you drowsy and more relaxed. Once you have wheeled to the operating theatre, the anaesthetist will administer the anaesthetic and the operation will be performed.

During the operation, the damaged joint is carefully removed and, with the use of precise instruments, the bone ends are shaped exactly to fit the false joint. The new implants are then securely fixed into place. The wound is stitched with a soluble suture placed just under the skin so there are no sutures to remove. The whole procedure takes about one and a half hours

After Surgery

When the procedure has been completed you will be moved into the Post Operative Care Unit (Recovery Ward), where the nurses will monitor your blood pressure and heart rate, and control any pain you have by medication prescribed by the anaesthetist. Once your observations and pain are stable, you will be taken back to the ward, where the nurses will continue to monitor you. If you have any pain, be sure to inform the staff of your discomfort.

You will be given intravenous fluids, which may include antibiotics, prescribed as a measure to prevent infection. You will also have three plastic suction tubes inserted into the wound to drain excess blood from the area. There will be a bulky dressing over the wound, which will be reduced when you begin mobilisation.

Your leg will be positioned in a continuous passive motion (CPM) machine after surgery, to provide knee movement while you are lying in bed. It continuously bends and straightens your knee and may cause discomfort, but will speed up your recovery by preventing stiffening.

You will keep the TED stockings on and you will also receive a daily injection of Clexane to prevent DVT.

Your rehabilitation starts in hospital. A physiotherapist will get you up and show you various stretching and strengthening exercises which are essential for your recovery. You will also be given advice on practical ways to adjust your lifestyle for the first few months after your surgery.

Leaving hospital

You will be discharged from hospital two to three days after surgery. By this time you should be fairly mobile and able to climb stairs with the aid of your crutches. Before you go home, you will be given an appointment to see your orthopaedic surgeon in two weeks time. Your GP will be sent a letter giving him information of the procedure that was performed and your condition on discharge. It is vitally important that you continue with physiotherapy on an out-patient basis, and you will be contacted with regards to an appointment for this. At your two week appointment, your progress will be checked.

At home

Generally, you may drive again six weeks after surgery. Please contact your insurance company to confirm specific details.

It usually takes about three months for the knee to start to feel normal again, though you should find that even on discharge from hospital you can function better than before the operation.

For the first four weeks after the operation you will need to walk with the aid of crutches. After this you will be able to gradually wean yourself off them, going from two crutches to one, and then to a stick, used on the non-operated side.


By six months, you will have completed about 90% of your improvement; but remember, your knee replacement can still improve, albeit slowly for up to 12-18 months.

The long term outcome for your surgery is extremely good. Due to modern replacement implants, more than 90% of replacement joints have a 15 year lifespan.

Infection control

After joint replacement surgery, aggressive antibiotic treatment is recommended. The main reason for this is that an infection can be transmitted from elsewhere in the body directly to the prosthetic implant.

Antibiotic treatment is advisable and highly recommended if you undertake any of the following:

  • Dental procedures
  • Gastro-intestinal/genitor-urinary endoscopic examinations, catheterisation or surgical procedures.
  • Any infectious condition resulting in a bacterial infection
  • Septic lacerations

Where possible, intra-muscular injections or vascular catheterisation in the area of the prosthesis should be avoided.

Why does joint replacement surgery sometimes fail?

There is always the possibility that the body’s self-defence mechanism will reject the ‘foreign body’. As with any transplant or major joint replacement surgery, this must be considered. When this happens, the body rejects the ‘wear particles’ that are produced by the components used in the production of the implants. This causes the implants to become loose and an operation to replace them is needed. Very occasionally the joint will become infected and surgery is usually required to replace the implants.  On very rare occasions the components break and require further surgical invtervention.

Over-activity may cause wear of the implants.  Most patients wil be able to go back to normal activities such as walking, sitting and lying down within a year of surgery.  Any sports or activities which place more strain on the knee replacement such as long distance running, skiing or hiking shuold be approached with caution.  It is advisable to discuss such activities with your doctor.


Infections: contracting an infection is rare.  If it does happen, antibiotics are used to treat the infection or in rare cases of chronic infection, surgery may be required.

Blood clots/DVTs:  This occurs in a small number of patients.  Precautions for this include the spinal anaesthetic, injections when in hospital and tablets to thin the blood, anti-embolic stockings and early mobilisation.

Post-operative bleeding: due to the type and length of the operation, some bleeding is inevitable.  Excess blood is removed via a drain which remains in the wound overnight.  A collection of blood called a haematoma may build up in the wound, which if large enough may need surgical removal.  the need for surgery is extremely rare.

Tips for Daily Activities

During your stay in hospital, a physiotherapist will give you practical tips on how to adjust your lifestyle for the first three months after the operation.

You will have undertaken major surgery and need to take things easy for the first six weeks.  this means:

  • no bathing until the wound is completely healed, usually about 2 weeks after surgery.
  • don't drive until you have been told you can by your doctor.
  • use an armchair with arms that will support your weight
  • stand up on your good leg, don't put your full weight on your operated leg when you stand up
  • don't forget to do your exercises - this is very important for your recover.  If exercising causes alot of pain, ask for more pain medication as the exercises need to be done regularly.

Please contact the hospital at which you had your surgery if you are at all concerned that there is a problem.  In particular, act immediately if you develop a fever, severe pain or significant wound problems. 

Related Advice Articles

An overview of the Knee

An overview of the Knee

The knee is a hinge joint responsible for movement, stability, absorbing shock and weight bearing. Knee injuries are common and can be from muscle imbalance, sports injuries, or degenerative disease.

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