This article reviews the diagnosis and treatment of cracked teeth, and explores common clinical examples of cracked teeth, such as cusp fractures, fractures into tooth furcations, and root fractures. This article provides alternative definitions of terms such as cracked teeth, complete and incomplete fractures and crack lines, and explores the scientific rationale for dental terminology commonly used to describe cracked teeth, such as cracked tooth syndrome, structural versus nonstructural cracks, and vertical, horizontal, and oblique fractures. The article explains the advantages of high magnification loupes (×6–8 or greater), or the surgical operating microscope, combined with co-axial or head-mounted illumination, when observing teeth for microscopic crack lines or enamel craze lines. The article explores what biomechanical factors help to facilitate the development of cracks in teeth, and under what circumstances a full coverage crown may be indicated for preventing further propagation of a fracture plane. Articles on cracked tooth phenomena were located via a PubMed search using a variety of keywords, and via selective hand-searching of citations contained within located articles.
Keywords: Crack propagation, cracked tooth syndrome, microscopes, tooth fractures
Cracked teeth are generally diagnosed by visually observing (ideally using microscopes)if a tooth is cracked. A dentist often diagnoses a crack by observing a crack line, which is a line segment from the perimeter of a fracture plane, such that this line segment is also located on a tooth surface that a dentist can observe. Observation of the crack line does not necessarily indicate the fracture plane size and shape. If a direct restoration is observed, such that a crack line is observed to be contiguous with the restoration margin, it may be prudent to remove the restoration to observe the full extent of the crack line underneath the restoration.
The most commonly fractured teeth are the mandibular molars, perhaps due to pointy, protruding maxillary molar palatal cusps occluding powerfully into the mandibular molar central grooves. Maxillary premolars, which often have steep inclines on nonfunctional cusps that result in torque forces during mastication, are more likely to crack than mandibular premolars, which experience mostly compression forces due to opposing teeth occluding into the mandibular premolar buccal cusps. A posterior tooth with a class II restoration may be the source of discomfort if it is the only remaining (or restored) posterior tooth in that quadrant, or if it shows signs of occlusal trauma or an abfraction lesion
A furcation stress plane is a stress plane, such that complete fracture of this plane would result in one tooth root becoming completely disconnected from another root. To completely disconnect one root from another, a fracture plane must fracture tooth structure, that is inferior to the pulp chamber floor, that helps to connect those two roots, and also fracture tooth structure superior to the pulp chamber floor, within the buccal and/or lingual and/or mesial and/or distal and/or occlusal walls of coronal tooth structure, that also helps to connect the two roots. Accordingly, a furcation stress plane consists of two subset planes. One subset plane exists inside the volume of tooth structure that is superior to the pulp chamber floor. Another subset plane exists inside a horizontal cross sectional volume that cuts completely through the tooth that is approximately bounded occlusally by the curved surface that is the floor of the pulp chamber, and is also approximately bounded apically by a second curved surface that forms the external underside of the tooth between the tooth roots. Complete fracture of a furcation stress plane fractures tooth structures within, occlusal to and apical to the pulp chamber floor that if intact, would connect one root to another root Observation of a furcation fracture in an asymptomatic tooth (or in a tooth within a quadrant where the patient feels sensitivity but is not sure which tooth in the quadrant is causing the sensitivity) is possibly an indication that the tooth is necrotic.
A cuspal stress plane is a stress plane, the perimeter of which is located approximately apical to one cusp, or apical to two or more connected cusps, where the axial aspect of the stress plane may intersect the pulp chamber walls or roof (but does not intersect the pulp chamber floor), and where the lateral aspect of the stress plane intersects the external buccal or lingual tooth surface, and possibly a root, and may be located subgingivally. A cuspal stress plane can be caused by torque forces resulting from occlusion into steep cuspal inclines of cusps located occlusal to the cuspal stress plane. Complete fracture of a cuspal stress plane results in one or more cusps breaking off a tooth Complete fracture of a cuspal stress plane that has a subgingival aspect may result in a tooth that cannot be restored without the restoration showing permanent periodontal inflammation at the sub-gingival aspect of the fracture.