Biologic width

Crown lengthening
MeSH	D016556
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Crown lengthening is a surgical procedure performed by a dentist to expose a greater amount of tooth structure for the purpose of subsequently restoring the tooth prosthetically. This is done by incising the gingival tissue around a tooth and, after temporarily displacing the soft tissue, predictably removing a given height of alveolar bone from the circumference of the tooth or teeth being operated on. While some general dentists perform this procedure, others frequently refer such cases to periodontists.

Biologic width is the distance established by "the junctional epithelium and connective tissue attachment to the root surface" of a tooth. In other words, it is the height between the deepest point of the gingival sulcus and the alveolar bone crest. This distance is important to consider when fabricating dental restorations, because they must respect the natural architecture of the gingival attachment if harmful consequences are to be avoided. The biologic width is patient specific and may vary anywhere from 0.75-4.3 mm.

Based on the 1961 paper by Gargiulo, the mean biologic width was determined to be 2.04 mm, of which 1.07 mm is occupied by the connective tissue attachment and another approximate 0.97 mm is occupied by the junctional epithelium. Because it is impossible to perfectly restore a tooth to the precise coronal edge of the junctional epithelium, it is often recommended to remove enough bone to have 3mm between the restorative margin and the crest of alveolar bone. When restorations do not take these considerations into account and violate biologic width, three things tend to occur:

In addition to crown lengthening to establish a proper biologic width, a 2 mm height of tooth structure should be available to allow for a ferrule effect. A ferrule, in respect to teeth, is a band that encircles the external dimension of residual tooth structure, not unlike the metal bands that exist around a barrel. Sufficient vertical height of tooth structure that will be grasped by the future crown is necessary to allow for a ferrule effect of the future prosthetic crown; it has been shown to significantly reduce the incidence of fracture in the endodontically treated tooth. Because beveled tooth structure is not parallel to the vertical axis of the tooth, it does not properly contribute to ferrule height; thus, a desire to bevel the crown margin by 1 mm would require an additional 1 mm of bone removal in the crown lengthening procedure. Frequently, however, restorations are performed without such a bevel.

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