Outcomes after reoperated medial unicompartmental knee arthroplasties compared with primary total and primary unicompartmental knee arthroplasties: a cohort study based on local Danish databases

BACKGROUND AND PURPOSE: Tibial periprosthetic fractures (PPF), periprosthetic joint infections (PJI), and bearing dislocations (BD) are among the most common short-term complications in medial unicompartmental knee arthroplasty (mUKA). We aim to assess whether patients with these complications have patient-reported outcome measures (PROMs) that differ from patients with primary mUKA, primary total knee arthroplasty (TKA), or after revision TKA . METHODS: This observational study included 74 mUKA patients reoperated for PPF (n = 22), PJI (n = 15), or BD (n =3 7) between January 2018 and January 2023. Comparator groups included 1,940 primary mUKA, 3,485 primary TKA, and 350 reoperated TKA patients. The primary endpoint was Oxford Knee Score (OKS) at 12 months. Missing data was imputed, and multilevel Tobit regression was used to analyze differences in PROMs. RESULTS: At 12 months, reoperated mUKAs had lower PROM scores than primary mUKAs (OKS difference -3.3, 95% confidence interval [CI] -5.0 to -1.5) and TKAs (OKS difference -2.7, CI -4.4 to -0.9) but higher than reoperated TKAs (OKS difference: 3.0, CI 1.1 to 5.0). PPF mUKAs had 12-month scores resembling reoperated TKAs (OKS difference -0.7, CI -3.9 to 2.5). PJI mUKAs and BD mUKAs had 12-month scores resembling primary mUKAs (PJI: OKS difference -2.4, CI -6.2 to 1.5, BD: OKS difference -2.2, CI -4.7 to 0.2) and primary TKAs (PJI: OKS difference -1.7, CI -5.6 to 2.1, BD: OKS-difference -1.6, CI -4.1 to 0.8). CONCLUSION: Patients reoperated for PJI and BD achieved outcomes comparable to primary mUKAs and TKAs, while PPF resulted in scores lower than primary mUKAs and TKAs, comparable to reoperated TKAs.