BACKGROUND: Skeletal involvement is an uncommon form of extrapulmonary Mycobacterium tuberculosis (MTB) that occurs in 1–3% of the patients. Knee joints may be affected in 8% of cases.

CASE REPORT: We reported a case of TB knee arthritis in a 35-year-old Afghan male who was referred to Kashan Rheumatology Clinic for pain and swelling in the left knee. The patient had no history of fever, chills, weight loss, or anorexia. His chest radiography was normal. The synovial fluid culture was positive for M. tuberculosis. Magnetic resonance imaging (MRI) of the left knee demonstrated a marked joint effusion, chondromalacia in the lateral patellar facet, and edema in the origin of the gastrocnemius muscle. The histopathologic examination revealed multiple granulomas with foci of necrosis.

CONCLUSIONS: This case demonstrated that clinicians should pay particular attention to the possibility of TB as the cause of chronic monoarthritis even when pulmonary involvement is not documented.

Keywords: Arthritis, Knee Joint, Mycobacterium tuberculosis, Arthritis, Infectious, Synovial Fluid, Tuberculosis, Osteoarticular