Rasch analysis of the forgotten joint score in patients with total hip arthroplasty

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 arthro plasty 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 sub tests) 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.