Unicompartmental knee replacements have not performed as well in the lateral compartment as in the medial. This may be because the tibial components have flat or slightly concave surfaces which match the medial plateau but not the convex lateral plateau. The aim of this study was to find the optimal radius for a convex lateral tibial component. Twelve normal lateral tibial plateau were retrieved at knee replacement, and their surface contour in their mid sagittal plane was determined. The optimal circle was fitted and its radius measured. A series of different shaped tibial components were superimposed. From published information about the position of the femoral condyle relative to the tibia in different degrees of flexion, the flexion gap at these angles was determined. The average radius of the lateral tibial plateau was 40 mm. However, as the surface was polyradial it was not clear if this average radius would be optimal. In full flexion, a flat tibial plateau distracted the knee by 8 mm (p