Unicompartmental Knee Replacement

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Some patients that are at the stage when they are considering joint replacement may have arthritis that is quite localised to one area, but with the rest of the knee in good condition.

If these people are young and highly active, then a tibial osteotomy (HTO) may be the best choice.

However, in lower demand patients, we can replace the diseased part of the knee and retain the rest (fig 1).

The advantages of this over a total knee replacement (TKR) are that it is a much smaller operation, patients recover quicker, and it feels much more like a normal knee.
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Fig 1: Unicompartmental knee replacement.

Advantages over an HTO, are that it is much quicker to recover from, more predictable short and medium term result, and it does not change the shape of the leg. Disadvantages include less reliable long term result than a TKR.


Cause of arthritis of the knee
Unlike arthritis in the hip, the majority of knee arthritis is caused by injury to the structures within the knee, or wearing out of the cartilage. There are other secondary causes such as infection, malalignment, trauma, dysplasia, inflammatory disease, avascular necrosis, and knee disease often runs in families suggesting an inherited problem.

The decision to undergo total knee replacement always remains with the patient. The surgeon makes sure of the diagnosis and advises the patient of the implications and possible complications. Pain and loss of function are the most important indications for joint replacement surgery.
Different people have different pain thresholds and also respond to painful diseases differently. We often try and quantify pain by the number of painkilling tablets or analgesics patients take each day, what the pain stops the patient from doing, and if their sleep is affected. If this pain cannot be controlled by other measures surgery is indicated.
Loss of function
Once again there is no absolute guideline as this is a very individual interpretation based on the patient’s own expectations of mobility and function.

Both of these factors affect the quality of life of the patient, at home and at work. When the quality of life is affected to an unacceptable level by the disease, and not controlled by other not surgical measures, then surgery should be considered.

Implant survival

Joint replacement was previously reserved for elderly patients because we know that over 80% of implants will survive 20 years, but much less will survive in the more active and demanding younger patients.

There is no doubt that joint replacement can make an enormous positive difference in quality of life in younger patients, and should not be denied to them. However, the likelihood of further re-do surgery is inevitable, and this presents potential future problems for these patients.

Contraindications to UKR

The arthritis must be limited to one side of the knee (usually the inside part), the knee needs to have a good range of movement, and the ligaments need to be intact. Assessment of these factors is usually done with an arthroscopy. Some patients may be at very high risk of complications, and some may need special facilities such as ICU. The presence of active infection is a contraindication to any knee replacement.


Knee replacements are very reliable at reducing the pain of arthritis, and providing a stable knee for daily activity. Survival is about 85% at 10 years for UKR, compared to 95% for TKR, and 70% for HTO.

Possible Complications Include

Infection <1%
Organisms are usually introduced onto the prostheses at the time of surgery from the patients skin, or just after surgery from wound problems. Occasionally infection can spread from a distant site to a well functioning prosthesis. Once established, infection is hard to eradicate without removal of the prostheses. Extensive surgery is usually required usually in 2 stages, and results can be poor.
Thrombosis/Pulmonary embolus 1%
Thrombosis in the deep veins of the leg (DVT) are common after hip or knee surgery, however rarely do these cause any problem and they need no specific treatment. About 1% of patients will have a DVT that requires treatment either because of calf pain, or when the thrombosis has spread into the thigh. About 1 in 1000 patients will have a thrombosis that will travel up into the lung (PE) where it can cause serious problems including death.

All patients are assessed for risk preoperatively. Routine preventative management is by minimising operative time, keeping the patient hydrated, regional anaesthetic (if possible), foot/calf pumps to circulate the blood, and early mobilisation. We do not believe that heparin significantly reduces the risk of thrombosis, and definitely causes wound problems. However, the hospital policy is that we use this drug. We treat high-risk patients with warfarin for 6 weeks after surgery.
Anaesthetic complications <5%
Complications may result from the anaesthetic and stress of the surgery. Patients are carefully assessed preoperatively in order to try and minimise these risks, but patients with some medical problems such as diabetes and heart conditions will be at higher risk. Risk can be further minimised by optimising preoperative health, such as stopping smoking, reducing excessive weight, balanced diet etc.
It is very rare for our patients to require transfusion after this operation (<1%), and at Fremantle, we have published results showing one of the lowest transfusion rates in the World.
Death (very rare)
Stiffness (<5%)
Stiffness after knee replacement is strongly associated with stiffness before the operation. Careful examination and investigation is required as there are many causes of this that can be hard to define. Secondary causes include: infection, component malposition, allergy to metal, muscular problems, and hip disease. Occasionally it is amenable to a manipulation under anaesthesia, but often requires specialist assessment and revision surgery.
Continued pain (<5%)
A few patients continue to experience pain after UKR. Again, this needs careful evaluation by a specialist to look for a treatable cause.
Anterior numbness
All approaches to the knee for this surgery require sacrifice of a small nerve at the front of the knee that supplies the feeling to the skin on the outside of the knee. This area will be numb after the surgery, and as nerves grow back into the area, patients may feel pins and needles, or sometimes pain. This can be dealt with by massaging the area, which just helps 're-educate' the nerves. This process usually takes about three months, after which time an area of numbness may remain.
If a UKR fails, then conversion to a TKR is usually straightforward in experienced hands.
The moving parts of all prostheses wear, causing bone loss and loosening of prostheses. Modern materials are harder wearing so that wear is becoming less of a problem.
Fracture <1%
This is rare, but more common in complex operations, deformity or revision operations. Most can be dealt with during surgery, but occasionally they are only picked up after surgery and may require intervention.
Vascular injuries<1 in 1000
Penetration or incision of an artery or vein.
Nerve injuries<1 in 1000

Activity after UKR

Walking and normal daily activity is encouraged after UKR, but impact sport is probably damaging. If you are to have any invasive procedures after a UKR such as dental work, bowel surgery etc. please inform your treating professional that you have a joint replacement, as you may need to be given antibiotics to protect the prosthesis from infection. The highest risk for this is within the first three months.

We provide the full range of treatments for arthritic disease of the knee including, arthroscopic treatments, meniscal surgery, osteotomy, unicompartmental knee replacements, patellofemoral replacement, total knee replacement, and revision surgery.
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