Tuberculosis of the knee as a great mimicker of inflammatory arthritis: a case report

Case presentationIn January 2015, a 36-year-old man with good past health presented to a hospital in Hong Kong with intermittent low-grade fever and left knee effusion.Physical examination revealed mild effusion, erythema, warmth and tenderness over the left knee, with 10˚ flexion contracture and flexion range up to only 70˚.The levels of inflammatory markers (Creactive protein [CRP], erythrocyte sedimentation rate [ESR], and antinuclear antibodies [ANA]) were elevated.Single-attempt arthrocentesis on the affected knee yielded 1 mL of yellow fluid, subsequently negative for Gram stain and culture only.No obvious abnormalities were observed on plain radiograph (Fig 1a), but magnetic resonance imaging (MRI) in June 2015 demonstrated synovial thickening, bone marrow oedema and subtle cortical erosion at the lateral femoral condyle (Fig 1b).Infection could not be excluded.Nonsteroidal anti-inflammatory drugs were prescribed for symptomatic control.The recurrent left knee effusion persisted despite treatment, but no further attempts at arthrocentesis were made until 2018.In view of the joint stiffness, recurrent knee effusion and persistently elevated levels of inflammatory markers, the patient was referred to a rheumatologist.A working diagnosis of atypical rheumatoid arthritis (RA) was made despite negative testing of anticyclic citrullinated peptide antibody and rheumatoid factor.Sulphasalazine was started in September 2015.Due to persistent knee inflammation, intraarticular steroid injection was given in November 2015 with limited effect.Methotrexate and leflunomide were prescribed in escalating doses.The patient was simultaneously followed up by the orthopaedic department where analgesics and physiotherapy were prescribed.Interval MRI in December 2017 showed diffuse synovial thickening, multifocal erosive changes and bone marrow oedema in the proximal tibia, reported to be in keeping with RA.Due to progressive worsening of his knee, the patient attended the private sector and was prescribed golimumab biologics in February 2018.