A hairline fracture involving the occlusal surface of the crown of a tooth often extending into the root. The crack may involve the pulp or be confined to enamel and dentine. It is most common in heavily restored teeth but may occur following biting on a foreign body such as lead shot or bone. The symptoms are often poorly localized, making diagnosis problematical. It is often characterized by severe pain on occlusal contact, due to the flexure of the tooth, which usually lasts for the duration of the contact only; there may also be pain in response to thermal stimuli.