The national registers have shown that the re-revision rate after a UKA to UKA revision is higher than a UKA to TKA revision. Therefore, the general recommendation is that UKA should be revised to TKA. However, there are certain circumstances when a UKA to UKA revision should be considered as the patient recovers quicker, with less morbidity and a better functional outcome. These include replacing a bearing for a dislocation; a lateral or medial UKA for disease progression; and implanting a new component for loosening with minimal bone loss.
The results of conversion of OUKA to TKA have been reported in a number of studies and are variable (Berend et al., 2009; Martin et al., 1995; Saldanha et al., 2007; Wynn Jones et al., 2012). If there is a mechanical cause for the failure, such as disease progression, component loosening, recurrent dislocation, or damage to deep fibres of the MCL, and there is not substantial bone loss, the conversion to a primary TKA is straightforward. The tibial resection should be at the level of the top of the medial defect. The remaining defect, which is contained, can be filled with cement or bone graft from resected bone. A 14 or 16 mm tibial bearing is usually needed. The results tend to be as good as those of a primary TKA.
If there is no mechanical cause for the pain then, although the conversion to a primary TKA is straightforward, the results are poor. The typical case is a patient with early arthritis and partial thickness cartilage loss who is treated with a UKA. The UKA does not relieve the pain and the surgeons misinterpret the physiological radiolucency as indicative of loosening. At revision surgery, although the tibia is secure, it is easily removed when hit hard so it is recorded as ‘loosening’ in the registry. The conversion to TKA does not relieve the pain and further re-revision may be done. The message is clear – do not implant an OUKA for partial thickness disease and do not revise unless there is a definite mechanical problem.
If there is severe bone loss, for example following tibial plateau fracture, a two stage revision for infection, a deep tibial resection or gross ligament instability, then a revision TKA with stems, augments and increased constraint will be necessary. The results of this type of surgery may be similar to the results of revision TKA.
