The hyponychium is a bed of soft tissue that lies just under the free portion of the nail at the end of the finger. It forms an interface with the rest of the nail bed and acts as a barrier to reduce the risk of infection. This component of the nail anatomy is sometimes known as the quick; the saying “cut me to the quick” references the sharp pain people can experience when the hyponychium is cut or disturbed. This tissue is very sensitive to injury.
Nailbeds can provide a potential avenue for infection, allowing pathogens to penetrate and spread into the finger. The hyponychium works as a seal to keep water and particulate materials out of the nail bed, while the surface nail and cuticle cover it to offer protection from above. This tissue is rich in white blood cells to provide an immune boost. If infectious organisms do stray onto the quick, the white blood cells quickly attack them before they have an opportunity to start growing and spreading.
With very short nails, it can be possible to see the hyponychium, a thin band of tissue that tends to be paler than the rest of the skin around the nail. Cutting nails back too far can cut back into the hyponychium. This may cause excessive bleeding due to the rich vascularization of the nailbed, and it can also be extremely painful, as the tissue is so sensitive. Nail cracking and splitting can also irritate this structure by pulling at it and exposing it to the elements.
Disorders of the hyponychium can include infections, skin irritation caused by problems with the nail, and direct injuries like cuts and compression injuries. A dermatologist can evaluate the affected finger to determine the nature of the problem and develop a treatment plan for the patient. Treatments may involve gentle flushing with fluids to clean the site, medications, and bandaging to protect the hyponychium from jarring and jostling, with the goal of minimizing pain for the patient.
This structure can be seen in other animals with nails, such as cats and dogs. Pet owners who trim the claws of their companion animals may notice that clipping too short can elicit a yelp of pain and irritation, and the animal may bleed profusely. Medications like styptic pencils are available to quickly apply to the tip of the nail to seal the injury and stop the bleeding. These can be useful to keep in close proximity during a grooming session in case of an accident.