Rat bite sign | Radiology Signs

The rat bite sign, also called rat bite erosions, occurs due to chronic tophaceous gout, a crystal-induced arthropathy characterized by deposition of monosodium urate crystals in and around joints. On radiographs, this leads to punched-out lytic bone erosions with sclerotic, well-defined margins and characteristic overhanging edges of new bone. These erosions typically involve juxta-articular bone regions, often seen in the first metatarsophalangeal joint. Clinically, the rat bite sign is associated with chronic gout presenting with joint inflammation, tophi formation, and episodic arthritis.

Why is it called so?

The term “rat bite” refers to the appearance of the bone erosions resembling small, irregular bites taken out of the bone cortex, similar to the damage caused by a rat’s gnawing. The name emphasizes the sharply defined, punched-out lytic defects with overhanging sclerotic edges that look like marginal “bites” on imaging.

Pathophysiology

Monosodium urate crystals deposit in periarticular soft tissues and synovium, invoking a chronic inflammatory response that leads to tophus formation. These tophi erode adjacent bone by direct pressure and enzymatic bone destruction, producing sharply marginated lytic lesions with surrounding bone sclerosis and new bone formation at margins. The overhanging edges reflect reactive bone growth displaced away from the original bone surface as a response to erosion.

Alternative names: Martel sign, G sign

Other associated named signs: None specific, though tophus-related dense soft tissue swelling may be observed around affected joints.