Estimated coronal plane alignment of the knee phenotype transitions after imageless ROSA-assisted total knee arthroplasty are frequent but not significantly associated with short-term patient-reported outcomes

BACKGROUND: Postoperative changes in coronal plane alignment of the knee (CPAK) phenotype have been reported after total knee arthroplasty (TKA). This study investigated whether estimated CPAK phenotype transitions, including their magnitude and direction, were associated with patient-reported outcome measures (PROMs) after imageless ROSA-assisted TKA. We hypothesised that more extensive phenotype transitions would be associated with worse postoperative PROMs. METHODS: This retrospective single-surgeon study included 231 patients who underwent imageless ROSA-assisted TKA between January 2022 and May 2024. Estimated CPAK phenotypes were derived from short weight-bearing knee radiographs. Patients were classified by postoperative estimated CPAK transition status and further categorised into minor or major change groups by transition magnitude. Exploratory analyses evaluated row and column changes. Outcomes at final follow up (mean 15.7 months) included the Forgotten Joint Score (FJS), Western Ontario and McMaster Universities Osteoarthritis Index, and Visual Analogue Scale. RESULTS: Estimated CPAK phenotype transition occurred in 193/231 patients (83.5%); FJS was 76.8 ± 19.9 in patients with transition and 78.3 ± 16.9 without transition (mean difference -1.42 points, 95% confidence interval -7.19 to 4.61; P = 0.758). No significant PROM differences were detected according to transition magnitude or direction. In multivariable analysis, neither minor nor major change was independently associated with FJS. CONCLUSION: Estimated CPAK phenotype transitions were frequent after imageless ROSA-assisted TKA, but their presence, magnitude and direction were not significantly associated with worse short-term PROMs. Preservation of the estimated preoperative CPAK phenotype may not be necessary for satisfactory short-term outcomes, although confirmation using direct full-length alignment measurements is required.