Total knee arthroplasty (TKA) is a successful operation. Knee osteoarthritis is an increasing healthcare burden and the number of arthroplasty procedures implanted each year is estimated to increase exponentially over the next 10 years. At present in the UK, over 90% of these procedures are performed as total knee replacements mirroring similar findings in other countries.1 Over the last 10 years, there has been increased interest in partial knee replacement as an alternative treatment option for knee osteoarthritis. Although each of the three compartments of the knee can be treated with partial replacement, the commonest form of partial replacement involves the medial side of the joint (Fig. 1). The indications for medial unicompartmental knee arthroplasty (UKA) have been defined more clearly and many patients who undergo TKA do meet the criteria for a UKA procedure. This article will outline the argument for increased use of UKA on the medial side of the knee in place of total knee replacement. Figure 1 (A) An anteroposterior radiograph of a knee demonstrating anteromedial osteoarthritis. This pattern of disease is the commonest indication for UKA (B). In building the case for the use of unicompartmental knee replacement, four important areas should be considered: (i) the immediate patient benefits; (ii) the long-term clinical outcome of UKA implants; (iii) the number of patients who are eligible for the procedure; and (iv) the health economics of UKA use.