Outcomes and short-term survivorship of fixed-bearing all-poly unicompartmental knee arthroplasty. A retrospective single-center study of 42 prostheses with a main follow-up of 32.3 months

PURPOSE: All Poly (AP) tibial unicompartmental knee arthroplasty (UKA) seems to have advantages in comparison with metal-based (MB) UKA, such as greater polyethylene thickness, wear resistance, bone stock preservation and lower implant costs. The objective of this retrospective study was to evaluate the short-term clinical-functional and radiographic results, complications and survivorship of AP UKA performed consecutively with a minimum follow-up of 2 years. METHODS: Retrospective cohort analysis all consecutive knees operated with medial and lateral AP UKAs, including bilateral cases, with a minimum follow-up of 2 years. Demographic data were analyzed at the patient level and clinical-functional results were assessed per knee using the KSS, KOOS and VAS scales. Radiographically, mechanical axis (HKA) correction and the presence of radiolucency > 2mm were evaluated. Complications incidence and prosthetic survivorship was determined. Pre-postoperative comparisons were performed using linear mixed-effects models with random intercept per patient to account for the non-independence of bilateral cases. Subgroup analyses involving fewer than 10 observations were considered exploratory and no formal statistical inference is drawn. A value of p < 0.05 was considered statistically significant with a confidence interval (CI) 95%. RESULTS: 42 AP UKAs were included, 5 lateral (11.9%) and 37 medial (88.1%) in 31 patients, 12 male (38.7%), 68.4 ± 8.9 years old, BMI of 32 (24.6-50.6) and mean follow-up of 32.3 months (24-44). Significant improvement in clinical KSS from 39.9 ± 3.3 to 89 ± 6.1 (Δ 49.1; 95%CI 47.2 to 51.0; p < 0.001), functional KSS from 23.3 ± 2.8 to 93.1 ± 9.3 (Δ 69.9; 95%CI 67.1 to 72.6; p < 0.001), KOOS from 33.0 ± 5.0 to 97.3 ± 6.2 (Δ 64.2; 95%CI 62.0 to 66.5; p < 0.001) and VAS from 9.4 ± 0.6 to 0.7 ± 1.3 (Δ - 8.7; 95%CI - 9.1 to - 8.3; p < 0.001) was observed in all cases. HKA for medial UKA improved from 169.6 ± 4.1 to 175.1 ± 2.5 (Δ 5.5°; 95%CI 4.6 to 6.3; p < 0.001), and for lateral UKA from 189.8 ± 4.3 to 184.6 ± 0.5 (Δ - 5.2°; 95%CI - 8.7 to - 1.7; results are descriptive only due to insufficient sample size). Six complications were observed (14.3%), all of them minor with good evolution. One case (2.4%) required TKA revision due to persistent pain after 2 years postoperatively. Prosthetic survivorship was 97.6% (95%CI 92.2%-100%). CONCLUSION: The AP UKA was associated with favorable clinical-functional and radiographic outcomes, few complications and prosthetic survivorship of 97.6% after an average short-term follow-up of 32.3 months (24-44) in this single-center retrospective series, which included a high proportion of cases with BMI > 30 (54.8%) and > 40 (9.7%), advanced grade 4 osteoarthritis (78.6%) and severe deformities greater than 10º (40.5%) and 15º (19%). Given the observational design, short follow-up, and absence of a control group, these findings should be interpreted with caution. Prospective comparative trials and longer-term survivorship evaluation are needed. LEVEL OF EVIDENCE: IV, retrospective cohort study.