Subungal exostosis is a bony projection, or bone spur, that forms underneath the big toenail most often but can be any toe. It usually occurs on the big toe as a result of injury or trauma such as crushing the toe. Subungual exostoses are painful because the affected area on the outside of the big toe is subjected to pressure from walking and footwear. Surgical removal of the projection is usually recommended and necessary. Enchondromas also develop in the bone underneath the toenail; these are small, benign growths composed of cartilage.