Koilonychia and clubbing are two nail abnormalities that cause bent fingernails, and they are symptoms of underlying medical conditions, explains Healthline. Koilonychia may be a sign of heart disease, iron deficiency, lupus, hemochromatosis or Raynaud's disease. Clubbing may develop due to inflammatory bowel disease, cardiovascular disease, liver disease or AIDS.
Koilonychia causes the nails to bend outward, creating a spoon-like appearance. With this condition, the nails are so curved that they can hold a drop of liquid, explains Healthline. Clubbing causes the nails to bend downward and curve around the fingertips. Clubbing slowly develops over the course of many years, according to Mayo Clinic.