Knuckle pads are thickening of the skin over the extensor surface of the proximal interphalangeal joints. Clinical picture, ultrasound imaging, and histopathological examination of the skin biopsy ascertain the diagnosis. In routine practice, two main differential diagnoses are important: knuckle pads vs. pseudo-knuckle pads and idiopathic vs. non-idiopathic forms of knuckle pads.
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