If transverse ridging of the thumbnails exists without history of injury and without involvement of the other nails, the most probable explanation is the patient's compulsion to traumatize the thumb, usually with another finger of the same hand.
MACAULAY WL. Transverse Ridging of the Thumbnails: "Washboard Thumbnails". Arch Dermatol. 1966;93(4):421–423. doi:10.1001/archderm.1966.01600220033010
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