A fingernail or toenail can be injured by a blow to the nail or by closing the finger or toe in a door or drawer. This kind of trauma commonly results in blood under the nail, a condition called subungual hematoma. Nails also can be accidentally torn or split, or a splinter can get under the nail.
Repeated trauma to toenails, caused by ill-fitting shoes, can lead to deformities in the nails. The deformities may resemble a fungal infection; nails can be thickened or discolored and can lift away from the nail bed, which causes cosmetic concerns.
Habits such as nail biting and cuticle biting also can cause trauma to the nails. Nail biting is a main cause of acute paronychia, a type of infection in which bacteria get underneath the tissue at the side of the nail and cause swelling and irritation. Repetitive picking at your thumb's nail and cuticle with your index finger can produce a line of horizontal hatch marks in the middle of the nail. In extreme cases, the nail can split.
A subungual hematoma appears dark red under the nail. The finger or toe is painful and may throb.