Tuberculosis of the knee masquerading as pigmented villonodular synovitis: Report of two cases

INTRODUCTION AND IMPORTANCE: The manifestation of tuberculous mimicry is challenging to diagnose, especially in extrapulmonary Tuberculosis (TB). Pigmented villonodular synovitis (PVNS) may mimic tuberculous arthritis of the knee joint. In young, otherwise healthy-appearing individuals, tuberculous arthritis and PVNS in the knee joint could present as monoarticular involvement, with painful swelling of extended duration and limited movement. The core therapies for tuberculous arthritis and PVNS are different. CASE PRESENTATION: There are two patients. First, male 25 years old presented with a painful mass at the left knee of 3-years duration. The second patient was 14 years old boy who presented with a painful mass at his left knee for ten months before being admitted to the hospital. From the physical examination, plain x-ray, and MRI, both patients are highly suggestive of PVNS. However, the histopathology result and microbial culture showed a tuberculous-specific process. CLINICAL DISCUSSION: Because of its clinical manifestations, X-ray, and MRI features, patients were initially suspected of having the diffuse type of PVNS. An improper diagnosis may be due to atypical clinical presentation, wide use of antibiotics, the low specificity of diagnostic tools, and most of all, the clinician not prioritizing the possibility of tuberculous arthritis. Open biopsy and the result of the microbial culture establish the definitive diagnosis of knee tuberculosis. CONCLUSION: In conclusion, the clinical and radiologic findings in TB gonitis and PVNS are occasionally similar. Therefore TB gonitis should be highly suspicious, especially in endemic areas. Histopathology results and mycobacterial culture need to determine the diagnosis.