• Due to infection with Borrelia burgdorferi, a spirochete transmitted by Ixodes tick; joints affected weeks to years later
• Causes chronic arthritis of large joints with pannus formation in 10%
• Also affects skin, heart, nervous system
• Precise pathophysiology of disease unknown, but involves intricate network of vector, bacterial and host factors (Clin Vaccine Immunol 2008;15:21)
• "Amber theory": disease due to infection of structures close to joint space, followed by entry of nonviable bacteria and debris into joint space, eliciting inflammatory response (Clin Rheumatol 2012;31:989)

Erythema migrans skin lesion

Lyme arthritis of right knee

• Chronic papillary synovitis with hyperplasia, fibrin deposition, mononuclear cell infiltrates, onion skin thickening of arterial walls; resembles rheumatoid arthritis
• Perineuritis may be present

Borrelia burgdorferi

Borrelia spirochetes identified on silver stain

Joint capsule, elbow

Chronic lyme disease - perineuritis

• Silver stain highlights bacteria

• Surface fibrin-like material, thickened synovial lining cell layer, evidence of vascular injury, Borrelia-like structures in synovial membranes and some synovial fluid cell samples (Hum Pathol 1996;27:1025)

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