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Objective: To assess the internal construct validity, including local independence, unidimensionality, monotonicity, and invariance, reliability, and targeting of the Forgotten Joint Score within the Rasch Measurement Theory framework., Design: Cross-sectional study., Patients: A total of 111 patients with total hip arthroplasty at least 3 months after surgery., Methods: The Forgotten Joint Score was submitted to each subject during their rehabilitative treatment in an Italian centre and then to Rasch analysis., Results: The base Rasch analysis showed a satisfactory fit to the model with strict unidimensionality and no differential item functioning. However, monotonicity (11 out of 12 items showed disordered thresholds) and local independence were  violated. After rescoring 10 items and creating 5 subtests to account for local dependence, the scale satisfied all the other Rasch model requirements (i.e. invariance, local independence, monotonicity, unidimensionality, and multi-group invariance), with reliability indexes (> 0.850) for measurement at the individual level and proper targeting. A raw-score-to-measure conversion table was provided., Conclusion: After structural (i.e. collapsing items categories) and non-structural (i.e. creating subtests) strategies, the Forgotten Joint Score satisfied the measurement requirements of the Rasch model, and it can be used in patients with total hip arthroplasty in clinical and research settings.

Total hip arthroplasty (THA) surgeries are increasing; to assess quality of life after THA, an instrument that considers patient's perspective on surgical outcomes is necessary. The objective of this study is to assess the psychometric properties of the Italian version of the Forgotten Joint Score (FJS-I) in patients with THA. The FJS-I was administered to 111 patients with THA, as well as the Western Ontario and McMaster Universities (WOMAC), Numerical Pain Rating Scale (NPRS), and the EuroQol 5D-5L (EQ-5D-5L). Structural validity [confirmatory factor analysis (CFA)], internal consistency (Cronbach's alpha), test-retest reliability [intraclass correlation coefficient (ICC 2,1 )], measurement error [standard error of the measurement (SEM)], and construct validity (hypothesis testing with correlation of the WOMAC, NPRS, and EQ-5D-5L) were assessed. In addition, the minimal detectable change (MDC) was computed. The result of CFA confirmed the one-factor structure. Internal consistency was supported (α = 0.944). A high test-retest reliability (ICC = 0.958; 95% confidence interval, 0.914-0.980) was found with an SEM and an MDC of 5.3 and 16.6 points, respectively. The a-priori hypotheses were fully met, determining the construct validity to be satisfactory. Psychometric properties of the FJS-I were confirmed, and it can be used for single-person assessment. Further research is suggested to refine its structural validity., (Copyright © 2022 Wolters Kluwer Health, Inc. All rights reserved.)