In some instances, a knee joint may develop arthritis in only one compartment – i.e. only one of the medial, lateral or patella-femoral compartments, with otherwise normal surfaces in the remaining two compartments. If there is such localised arthritis, it may be possible in some patients to replace only that portion of the knee which is damaged, leaving the normal remaining two compartments untouched. This is known as a unicompartmental knee replacement (UKR). Most commonly this is done for the medial compartment, although the patello-femoral and lateral compartments can be done.

Whilst technically demanding, in young, fit patients they can provide an excellent range of pain-free movement and very rapid recovery as the operation is usually done through a smaller incision than a conventional total knee replacement and less tissue is disturbed. The design philosophy is that there is less bone removed during a UKR and so, in theory, it should leave more bone for any subsequent revision surgeries, such as conversion to a total knee replacement if other compartments wear out in the meantime, or if the UKR fails/wears out.

The drawbacks of this procedure is that it is not suitable for all patients, specifically if there are symptoms from more than one region in the knee, if there is significant deformity or stiffness, or if there is an inflammatory cause of arthritis (e.g. Rheumatoid or sero-negative arthritis) rather than the usual osteo-arthritis. Despite the design philosophy of maintaining bone stock, in some instances it can be difficult to revise a unicompartmental knee replacement to a total knee replacement, as there can be more bone loss than anticipated associated with removing the unicompartmental knee replacement components.