Validation and Reliability of the Vietnamese Pain Self-Efficacy Questionnaire in Knee Osteoarthritis

Inclusion Criteria: Participants will be eligible for inclusion if they meet all of the following conditions: * Age ≥ 50 years. * Diagnosis of knee osteoarthritis (OA) according to the criteria of the American College of Rheumatology (ACR), confirmed by: 1. application of ACR criteria by the investigator at the time of enrollment; or 2. a prior diagnosis documented in medical records by a rheumatologist or musculoskeletal specialist. * Presence of chronic pain related to knee OA, with current pain intensity ≥ 1 on the Visual Analog Scale (VAS). * No acute flare of knee pain within the past 2 weeks. * Ability to perform activities of daily living (with or without assistance). * Ability to read, understand, and complete questionnaires in Vietnamese. * Willingness to participate and provide informed consent. Exclusion Criteria: Participants will be excluded if they meet any of the following conditions: * Diagnosis of severe psychiatric disorders (e.g., schizophrenia, severe anxiety disorders). * Presence of cognitive impairment that interferes with the ability to comprehend or answer the questionnaire. * Diagnosed with systemic inflammatory diseases such as rheumatoid arthritis, systemic lupus erythematosus, or gout. * Presence of other chronic pain conditions (unrelated to knee OA) that are predominant and significantly affect mobility and daily functioning. * History of intra-articular injection or knee surgery within the past 3 months. * Questionnaire (PSEQ) with ≥ 20% of items missing or invalid.