Cross-Cultural Adaptation and Validation of Knee Quality of Life 26-Item Questionnaire into Spanish in Patellofemoral Pain

Background/Objectives: Patellofemoral pain (PFP) is a persistent knee disorder that impacts quality of life (QoL). Although holistic assessments are needed, no specific tools exist regarding QoL in PFP. The Knee Quality of Life 26-item questionnaire (KQoL-26), covering physical and psychosocial dimensions, could be useful but has not been adapted for Spanish speakers. This study aimed to cross-culturally adapt KQoL-26 into Spanish and assess its psychometric properties regarding PFP, including validity and reliability. Methods: This cross-cultural adaptation followed COSMIN recommendations. The cultural and linguistic adaptation of KQoL-26 included 104 subjects with PFP, involving 145 affected knees. Inclusion criteria: symptomatic PFP, aged between 16 and 55, and being native Spanish speakers. Validity (structural, construct, and external), reliability (internal consistency, test–retest, and agreement), discriminant ability, and feasibility were assessed. Results: Factor Analysis identified five dimensions. Convergent validity demonstrated a strong correlation between KQoL-26 and Knee Injury and Osteoarthritis Outcome Score for Patellofemoral pain and osteoarthritis, Kujala Score, Fulkerson Scale, and the Physical Component Summary of 12-item Short Form, while weak correlations with descriptive data supported discriminant validity. The questionnaire demonstrated high internal consistency (Cronbach’s α = 0.949) and test–retest reliability (ICC = 0.949), with adequate agreement parameters. No floor or ceiling effects were observed. Regarding feasibility, KQoL-26 was understandable and completed by all participants. Conclusions: The Spanish KQoL-26 is a valid, reliable, and feasible QoL assessment tool for Spanish speakers with PFP. Its use may support both clinical decision-making and research in this population group by providing a comprehensive assessment of patient-reported outcomes.