Surgeon’s experience influences UKA survivorship: a comparative study between all-poly and metal back designs

Purpose: Concerns exist regarding prosthetic positioning and post-operative limb alignment in unicompartmental knee arthroplasty (UKA). In this study, we hypothesized deviations of the post-op anatomical femoro-tibial angle (aFTA) and the tibial component alignment to be factors influencing UKA survivorship. Moreover, by a comparison between all-poly and metal back UKAs, we hypothesized that prosthetic design plays a role in implant survivorship. Methods: One hundred ninety-five medial UKAs were performed on 176 patients by two experienced surgeons and one low-UKA user. One hundred and forty-seven UKAs were included in the study: 72 all-poly and 75 metal back. Measurements were performed on radiographs: mechanical femoro-tibial angle, Cartier angle, aFTA and tibial posterior slope (PS) on pre-op radiographs; femoral and tibial component varus/valgus, aFTA and tibial component PS on post-op radiographs. Results: At an average follow-up of 61 months (min. 30, max. 107), 147 UKAs were evaluated: The reported survivorship rate was 93.1 %. Eleven implants underwent revision: ten all-poly and one metal back. No differences were reported between the two groups in the radiographic measurements. Significant radiographic differences were reported between revised and not revised UKAs: Revised UKAs were associated with overcorrection of the pre-op Cartier angle and under correction of pre-op aFTA. Most of revised UKAs were performed by the low-volume UKA surgeon. Conclusions: Surgeon’s experience is essential to achieve good results in UKA: Preserving the tibial epiphyseal axis and avoiding excessive or insufficient corrections of the pre-operative limb alignment are predictor of successful replacement, while prosthetic designs, models and fixation geometry do not affect UKA outcome. Level of evidence: IV.