In the frontal plane, the extramedullary tibial guide aims to place the tibial plateau at about 90° to the tibial axis. In UKA, the alignment of the limb (tibiofemoral angle) is independent of the alignment of the plateau. Therefore, any minor angular inaccuracies do not have the same significance as in TKA. Furthermore, the ball-and-socket configuration of the femoromeniscal articulation of the OUKA accommodates a small amount of angulation of the plateau relative to the femoral component without loss of congruence. We have found that ±5° malalignment of the tibial component does not compromise the outcome (Gulati et al., 2009).
Posteroinferior inclination
The natural medial tibial plateau is inclined backwards and downwards in the sagittal plane at a mean angle of 7°, but with a wide range of 0°–15°. As the knee flexes and extends relative to the tibial crest, the femoral condyle moves backwards and forwards on the plateau so alteration of the slope will effect the ligament balance in flexion and extension. We have always implanted the tibial plateau at the mean inclination, irrespective of the knee’s anatomy, and a 7° angle is built into the tibial saw guide. Because the 20° and 110° gaps are measured relative to the cut surface of the tibia, the effects of altering the tibial slope are automatically taken into account when the gaps are balanced.
Mediolateral position
The mediolateral position of the tibial plateau is defined by the vertical saw cut. With the G-clamp and spoon removed and with the knee flexed, the cut is made into the intercondylar eminence, just medial to the apex of the medial tibial spine and is directed towards the ipsilateral Anterior Superior Iliac Spine (ASIS). The direction of the cut will then be in the flexion–extension plane of the knee, the approximate direction of the anteroposterior movements of the mobile bearing. The cut is advanced until the saw blade rests on the guiding surface of the tibial saw guide. When the vertical cut is completed, the transverse cut is carried out, to release the medial tibial plateau.
Choice of plateau size
The tibial component should cover as much of the cut surface as possible to maximise the area available for the transmission of load, and hence minimise the risk of complications such as pain, loosening, or fracture. It is particularly important that the tibial component lies on the posterior and medial cortices (Chau et al., 2009).
It must reach as far as the medial cortex; therefore the implant is chosen from the range of sizes by its width. The plateau can overhang medially by up to 2 mm without risk of soft tissue irritation. (More overhang than this is unnecessary as the next smaller size component is 2 mm smaller.) Some overhang is preferable to underhang which will lead to incomplete coverage.
The tibial component must be placed as far back as possible so that it reaches to the posterior cortex. Full coverage is needed at this site because the bearing slides to, and beyond, the posterior edge of the plateau in flexion. In addition, very high loads may be transmitted across the flexed knee, and so the posterior part of the tibial implant needs to be supported by the posterior tibial cortex. Comparing the sawn plateau with tibial templates of the opposite side guides choice of the optimum component.
The parametric range of sizes cannot always provide a component that satisfies these two criteria and also reaches to the anterior cortex, so that often a small area of the cut surface is uncovered anterolaterally. However if the component is 3 mm or more from the anterior cortex the vertical cut should be repeated 2 mm further lateral so a larger size component can be used. (A larger component will be 2 mm wider and 3 mm longer.)
