Slamming a thumb in a car door is a common and painful crush injury that results in dark discoloration under the nail. This sudden, forceful trauma often causes a deep, throbbing ache. Understanding the biological changes that lead to the blue or black color helps clarify what is happening beneath the surface. This knowledge is important for managing the pain and deciding when professional medical care is necessary.
Why the Nail Turns Blue
The blue or black appearance under the nail is caused by a condition known as a subungual hematoma, which is essentially a bruise beneath the nail plate. The crushing force ruptures the small blood vessels, or capillaries, located in the nail bed tissue. Since the blood is trapped between the hard, unyielding nail and the underlying bone, it pools in that confined space. This trapped collection of blood congeals and darkens, giving the nail its characteristic blue, purple, or black color. The throbbing pain experienced after the injury is a direct result of the increasing pressure exerted by this accumulating blood against the sensitive nail bed.
Immediate Steps for Injury Management
Managing pain and swelling is the primary goal following a crush injury. Immediately elevate the injured hand above the level of your heart to slow blood flow, which reduces swelling and the painful throbbing sensation. Applying a cold compress or ice pack, wrapped in a thin towel, to the thumb for 10 to 20 minutes at a time can also effectively reduce inflammation and numb the area. Over-the-counter nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, can be taken to help reduce both pain and swelling. If the skin near the nail fold is broken, clean the area with mild soap and water and cover any open wounds or lacerations with a sterile, non-stick dressing to prevent infection.
When to Seek Professional Medical Attention
While many small subungual hematomas can be managed at home, certain signs indicate the need for prompt medical evaluation. If the area of dark discoloration covers more than 50% of the nail plate, or if the pain is severe and not relieved by home care steps, a medical professional may need to perform a procedure called trephination. This involves creating a small hole in the nail to drain the trapped blood, which immediately relieves the painful pressure.
You must also seek medical attention if there are any signs of a potential fracture to the underlying bone, such as a visible deformity, an inability to move the joint, or pain that persists intensely beyond a few hours. Damage to the nail bed itself, including deep cuts or lacerations, might require stitches and a repair to the nail bed to ensure proper healing and nail regrowth. Furthermore, watch for signs of a developing infection in the days following the injury, which include increasing redness, warmth, swelling, pus draining from the area, or a fever.
The Healing and Regrowth Process
Once the initial pain subsides, the long-term process of healing and nail regrowth begins. A thumbnail grows at a slow rate, typically taking about four to six months to fully regrow from the base to the tip. The dark, discolored section of the nail will not disappear, but instead will slowly migrate forward as the new, healthy nail grows underneath it.
As the new nail pushes the damaged nail forward, the injured nail plate will often loosen and eventually detach or fall off entirely. This is a natural part of the healing process and should not be forced. If the nail is lost, the exposed nail bed underneath will be sensitive and needs to be protected with a clean, dry bandage while it heals over the course of about a week. Provided the nail matrix, which is the root of the nail, was not severely damaged, the new nail should grow back normally.