Partial Knee Replacement

The knee can be considered as having three compartments: the inner side (medial compartment), the outer side (lateral compartment) and the knee-cap articulation (patello-femoral compartment). If only one compartment is worn and if the conditions are right it is possible to replace only the single worn compartment and not the entire knee.

The most commonly worn single compartment of the knee is the medial (or inner) side of the knee. The most widely used partial replacement implant for treatment of this compartment is the Oxford unicompartmental replacement (click to view the website for this implant).

Knee Surgery                   Knee Surgery 

In these x-rays (a front-on and side-on view of the same knee) the inner (medial) part of the knee alone has been replaced.

It is also possible to replace only the outer side (lateral compartment) or only the knee-cap joint (patella-femoral joint).

Knee Surgery               Knee Surgery

In these x-rays only the knee cap has been replaced

Recommended Conditions for Being Able to Have a Partial Knee Replacement Rather Than a Total Knee Replacement
There is some controversy as to who is the optimal candidate for a partial knee replacement but some recommendations are fairly widely accepted by the majority of orthopaedic surgeons:-

  • The compartment to be replaced must have full thickness loss of cartilage - not just partial wear.

  • There must be minimal/no wear in the other compartments of the knee that are not to be replaced.

  • The knee must not be fixed in a bent position.

  • The knee should not be too bowed (varus) or knock-kneed (valgus).

  • The ligaments of the knee should be intact.

  • The knee should not have an "inflammatory arthritis" (such as rheumatoid arthritis).

  • The patient should not be very overweight.

Some surgeons make a decision as to whether to proceed with partial knee replacement or total knee replacement on the basis of plain x-rays. Some surgeons obtain stress x-rays (x-rays taken with a force applied to the knee at the same time) to further assess the wear pattern of the knee. Other surgeons carry out an arthroscopy (key-hole operation) to make their decision. Despite taking such precautions there is a possibility that a surgeon setting out to do a partial knee replacement procedure will have to convert the operation into a total knee replacement (just under 1 in 15 need to be converted). This operation would normally be carried instead of the partial replacement procedure at the same anaesthetic. 

Reasons For Having a Partial Knee Replacement Over a Total Knee Replacement
There is some controversy as to what the difference is in outcome when comparing partial knee replacement to a total knee replacement, but many surgeons believe that a partial knee gives the patient:-

  • Better function

  • A shorter recovery time - less bleeding and less pain

  • A shorter stay in hospital

  • A smaller scar

  • Similar implant survival time - i.e. the partial replacement should last about as long as the total knee replacement

Complications of Partial Knee Replacement
The following are recognised complications of partial knee replacement:-

  • Bleeding

  • Fracture - during the operation or afterwards

  • Infection

  • Nerve injury - causing temporary or permanent numbness and/or weakness

  • Numbness at the front of the knee

  • Ongoing pain

  • Stiffness

  • Damage to the artery at the back of the knee

  • Deep venous thrombosis (clot in the calf)

  • Pulmonary embolus (clot in the lungs)

  • Improperly positioned components leading to early wear and sub-optimal function

Reasons For Failure of a Partial Knee Replacement
The following are recognised reasons for failure of a partial knee replacement:-

  • Loosening of the components

  • Wear of the components

  • Advancing wear and tear changes in the remaining compartments of the knee (i.e. those that have not been replaced)

  • Dislocation of a mobile polyethylene insert (the plastic tray between the two metal implants)

  • Ligament injury