Translation, Cultural Adaptation, and Validation of the Bulgarian Version of the Forgotten Joint Score-12 (FJS-12) in Patients with Knee Osteoarthritis after Unicompartmental Knee Arthroplasty

Abstract

Knee osteoarthritis is a major cause of disability worldwide. Although commonly used patient-reported outcome measures effectively assess pain and function, they often demonstrate ceiling effects in high-functioning patients, particularly after unicompartmental knee arthroplasty (UKA). The Forgotten Joint Score-12 (FJS-12) was developed to assess joint awareness during daily activities, reflecting a core goal of arthroplasty: a joint that is no longer consciously perceived. The aim of this study was to translate, culturally adapt, and validate the Bulgarian version of the FJS-12 and to evaluate its clinical utility in patients with knee osteoarthritis. A two-phase study design was used. Phase 1 involved forward-backward translation, expert committee review, and pilot testing to produce a culturally adapted Bulgarian version of the FJS-12 in accordance with established guidelines for the cross-cultural adaptation of patient-reported outcome measures (PROMs). Phase 2 consisted of psychometric validation in patients with knee osteoarthritis treated with UKA. Internal consistency was assessed using Cronbach’s alpha, test-retest reliability using the intraclass correlation coefficient (ICC), and construct validity through correlations with established PROMs, including Western Ontario and McMaster Universities (WOMAC) and Knee Injury and Osteoarthritis Outcome Score (KOOS). The Bulgarian FJS-12 demonstrated good comprehensibility and high patient acceptance. The mean FJS-12 score was 79.5±17.0. Internal consistency was high (Cronbach’s alpha = 0.958), and test-retest reliability was excellent (ICC(3,1) = 0.973). No floor or ceiling effect was observed. The FJS-12 showed significant negative correlations with WOMAC pain, stiffness, function, total score. In the subgroup with available KOOS data, significant positive correlations were found with KOOS Symptoms, Quality of Life, and total score. The Bulgarian version of the FJS-12 is a reliable and valid PROM for assessing joint awareness in patients with knee osteoarthritis after UKA. It provides clinically relevant information that complements traditional outcome measures by capturing residual joint awareness not detected by pain- and function-based instruments.