Partial Knee Replacement for localised arthritis

Patella femoral replacement
Unicondylar replacement

The knee is made up of three joints: the medial (or inner side) joint, the lateral (outer side) joint and the patellofemoral (kneecap) joint which is located at the front of the knee. Each individual compartment of the knee can wear out and cause arthritis pain. When this process becomes very severe, replacement surgery is required. It may be possible to undertake a partial knee replacement of any of these three compartments and, occasionally, more than one.  Traditionally, when more than one compartment is involved your doctor will recommend total knee replacement.

Patellofemoral replacement

This is specialist surgery for which Bristol has a national and international reputation. It is performed for isolated disease of the patellofemoral joint (where the kneecap joins the front of the thigh bone) and involves two small components being inserted into the front of your knee. It is carried out as an open surgical procedure requiring an in-patient stay. The main advantage to this type of replacement surgery is that the healthy medial and lateral joints are retained so your knee will feel ‘more normal’ and achieve an excellent range of movement after surgery compared to total knee replacement. Because of its selective nature, this surgery is sometimes performed in younger patients than those we would normally select for total knee replacement.

Medial Unicondylar knee replacement

The medial (inner) joint is the most common joint of the knee to wear out. There is a specific pattern of wear which affects the front inner aspect of your knee; it is also known as anteromedial arthritis. 


  • Pain made worse by activity or standing
  • Stiffness
  • Reduced function
  • Most people feel a very localised pain which can often be ‘pointed out’ with a finger. Sometimes the pain moves up the thigh or down the leg on the inner aspect.


When the disease becomes sufficiently severe and in certain patients after consideration of osteotomy, it may be necessary to perform a partial medial knee replacement.

Surgery is performed in a minimally invasive manner with small incisions to reduce post-operative pain and immobility. The procedure can be guided by computer navigation systems in specific cases. Most patients are in hospital for 2-4 days and are able to walk with one crutch or stick when they go home. At 4-6 weeks, the majority of patients should expect a significant improvement in function. This contrasts with total knee replacement, which often takes up to three months for patients to feel they have only just ‘broken even’.

There are two main types of partial knee replacement in the medial joint which use either a mobile or a fixed bearing device. A mobile bearing unicondylar replacement has three component parts: a femoral component made of metal to resurface the medial femoral condyle (inner aspect of your thigh bone at the knee); a tibial base plate made of metal to resurface the medial tibial plateau (inner aspect of the top of your leg bone in your knee); a meniscal bearing made of polyethylene (a type of plastic) which acts as the shock absorber between the two metal components. In a fixed bearing design, the plastic component is ‘fixed’ in the tibia (leg bone) and the femoral component slides on this. Both types work well and there is very little difference in the early outcome of both designs. Your specialist will discuss the options with you in clinic.

Lateral Unicondylar partial knee replacement

This condition is rarer than medial arthritis, but occasionally patients do have an isolated wear pattern in the lateral (outer aspect) of their knees. 


  • Lateral (outer) sided pain
  • The pain getting worse descending stairs or going down hills
  • A feeling of instability, again particularly on descending hills or stairs


Lateral unicondylar replacement is carried out using a minimally invasive surgical approach. In the majority of cases this means the incision can be smaller, in addition to reduced damage to the underlying muscles, with the overall effect of reducing patient pain and allowing an earlier return to activity.  There are two basic designs – fixed or mobile – which are very similar to the medial joint as described above with just a few subtle differences.

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