Even a minor injury to the finger may cause changes in the nail. The nail may develop a small spot of white discoloration that starts at the injured location and grows out with the nail.
Severe damage to the nail bed (the soft tissue underneath the nail plate that attaches the nail to the finger), particularly from a crush injury, often results in permanent nail deformity. To reduce the risk of a permanent nail deformity, the injury should be repaired immediately, which requires removal of the nail.
(See also Overview of Nail Disorders.)
Blood often collects under the nail (subungual hematoma) immediately after an injury (usually a direct blow, such as with a hammer). The blood appears as a purple-black spot beneath part or all of the nail and causes a great deal of throbbing pain. The doctor can release the blood and relieve the pain by making a small hole in the nail plate (the hard part of the nail). Usually the doctor uses a needle or heated wire (electrocautery device) to make the hole. This procedure is relatively painless and takes only a few seconds.
Because the blood has separated the nail from its bed, the nail usually falls off after several weeks, unless the hematoma is small. A new nail grows below the existing nail and replaces it when fully grown in.
A tumor beneath the nail can cause a similar purple-black spot. However, such a spot appears slowly and not within minutes of an injury and does not grow outward with the nail over time (tumors remain in the same spot under the nail). However, any small hematomas should be watched to make sure that they grow out with the nail.