Injuries to the nail root (the matrix that directly underlies the proximal nail fold) and nail bed may be caused by direct trauma. In fact, trauma is the most common cause of nail disorders.

The patient can sometimes be observed performing the repeated action without being aware of it. This is particularly true in children.

A subungal hematoma results from trauma to the nail matrix or nail bed from a single substantial impact (e.g., from a hammer) or repeated minor injury (e.g., tight shoes or sports injuries).

An acute subungual hematoma that results from rapid accumulation of blood under the nail plate can be very painful (Fig. 22.5), whereas small lesions may be painless and go unnoticed for some time.

Chronic, painless subungual hematomas that are not clearly the result of trauma may appear similar to a melanocytic neoplasm. Because the coagulated blood remains until the nail grows out (for 6 to 12 months), the diagnosis may be in doubt for some time and sometimes requires a nail bed or nail matrix biopsy.

Median nail dystrophy results from a compulsive habit of repeated trauma to the proximal nail fold of the thumb, usually inflicted by the nail of the adjacent index finger (Fig. 22.6A,B).

The resultant nail deformity is analogous to an injury to the root of a tree that deforms the growing tree trunk.