Mycobacterium Intracellulare Infection of the Metacarpophalangeal Joint in a Patient With Rheumatoid Arthritis: A Case Report.

Background: Nontuberculous Mycobacterium (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 Mycobacterium intracellulare following intra-articular glucocorticoid injection. Case presentation: 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 Mycobacterium intracellulare infection involving both flexor tendons, joint space, and bone marrow. The patient regularly engaged in gardening activities without protective gloves. Conclusion: 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.