Comparison of conversion unicompartmental knee arthroplasties to primary and revision total knee arthroplasties: A gradient of complexity

Background: Unicompartmental knee arthroplasties (UKAs) are often revised to total knee arthroplasty (TKA), but concerns remain surrounding outcomes of UKA revision to TKA. The primary aim of this study was to compare revision rates of conversion UKA to TKA to matched cohorts of both primary and revision TKA procedures. Secondary aims were to compare implants characteristics, intraoperative, and functional outcomes. Methods: This was a single centre cohort study of all consecutive patients who underwent UKA conversion to TKA from 2012 to 2023. Patients undergoing conversion UKA with minimum two year follow-up were included and matched 1:1:1 to patients undergoing primary TKA and aseptic revision TKA, excluding periprosthetic fractures. Indications for surgery, surgical details, and postoperative outcomes, including revision rates, complications, and range of motion (ROM), were collected and compared between groups. Results: One hundred patients underwent conversion of a UKA to TKA that met inclusion criteria and were matched 1:1:1 to primary and revision TKAs (total n = 300). Post-matching, the mean age was 69.0 years and 75% were female. There was no statistically significant difference in revision rates between UKA to TKA (6.0%), primary TKA (1.0%) and revision TKA groups (14.0%), though reoperation rates were higher following UKA revision compared to primary TKA (p = 0.028). UKA conversion cases had longer operative times (103.0min vs. 72.7min, p < 0.001) and more frequent use of tibial augments (38% versus 0%, p < 0.001) and stems (56% versus 3%, p < 0.001) than primary TKA. UKA conversion demonstrated ROM similar to primary TKA (p = 1.000) but superior to revision TKA (p = 0.050). Conclusion: The findings of this study suggest conversion of UKA to TKA is more complex than primary TKA but less than revision TKA. Patients should be advised of higher reoperation rates compared to primary TKA, although similar functional outcomes. Surgeons should be prepared for increased implant complexity in these cases.