<i>Mycobacterium Intracellulare</i> Infection of the Metacarpophalangeal Joint in a Patient With Rheumatoid Arthritis: A Case Report

<b>Background:</b> Nontuberculous <i>Mycobacterium</i> (NTM) infections affecting musculoskeletal structures are rare, particularly in patients with well-controlled rheumatoid arthritis (RA). This case is reported to highlight the potential risk of focal tenosynovitis due to <i>Mycobacterium intracellulare</i> following intra-articular glucocorticoid injection. <b>Case presentation:</b> A 79-year-old man with well-controlled RA developed tenosynovitis with bone destruction in the right index finger metacarpophalangeal joint following a single intra-articular injection of triamcinolone acetonide. Despite antibiotic treatment, the condition progressively worsened. Synovectomy revealed <i>Mycobacterium intracellulare</i> infection involving both flexor tendons, joint space, and bone marrow. The patient regularly engaged in gardening activities without protective gloves. <b>Conclusion:</b> This case highlights the importance of considering NTM infection in the differential diagnosis of persistent monoarthritis that worsens after intra-articular glucocorticoid injection, especially in patients with exposure risk factors such as gardening.