Translation, cultural adaptation, validity, and reliability of the Japanese version of the constant score: A cross-sectional study.

Background: The Constant Score (CS) is one of the most widely used shoulder function assessment instruments. However, a culturally and linguistically adapted Japanese version has not yet been established. This study aimed to translate and culturally adapt CS into Japanese (CS-J) and evaluate its reliability and validity.

Methods: The CS was translated into Japanese following internationally accepted cross-cultural adaptation guidelines. Psychometric properties of the CS-J were evaluated in 64 patients with shoulder dysfunction. Internal consistency was assessed using Cronbach's alpha. Test-retest reliability was examined using intraclass correlation coefficients (ICC). Construct validity was confirmed by exploratory factor analysis (EFA), and concurrent validity was tested against SPADI, QuickDASH, and SF-12 scores.

Results: The CS-J demonstrated excellent internal consistency (Cronbach's alpha = 0.91) and test-retest reliability (ICC = 0.91). The EFA revealed a two-factor structure corresponding to the subjective and objective components of the CS. Significant correlations were found between CS-J and SPADI (r = -0.82), QuickDASH (r = -0.82), SF-12 Physical Component Summary (r = 0.72), and Mental Component Summary (r = 0.52) scores, supporting concurrent validity.

Conclusions: The CS-J has acceptable reliability and validity, making it a useful tool for evaluating shoulder function in Japanese patients. The standardized adaptation of the CS-J enables more accurate clinical assessments and facilitates international research collaboration.