Crown lengthening

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

A palatal view of a maxillary premolar during a crown lengthening procedure.

Crown lengthening is a surgical procedure performed by a dentist to expose a greater height of tooth structure in order to properly restore 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 a general dentist may perform this procedure, he or she may refer the procedure to be performed by a periodontist, who will generally be both more experienced as well as specifically trained for this and other types of surgical soft tissue procedures.

Biomechanical considerations

Biologic width

The gingival sulcus (G) is a little crevice that lies between the enamel of the tooth crown (A) and the sulcular epithelium. At the base of this crevice lies the junctional epithelium, which adheres via hemidesmosomes to the surface of the tooth, and from the base of the crevice to the height of the alveolar bone (C) is approximately 2 mm, known as the biologic width.

Biologic width is the distance established by "the junctional epithelium and connective tissue attachment to the root surface" of a tooth. 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.

This distance is 2.04 mm (on average), with 1.07 mm being occupied by the connective tissue attachment and another approximate 0.97 mm being occupied by the junctional epithelium. Because it is impossible to perfectly restore a tooth to the precise coronal edge of the junctional epithelium, the roughly 1 mm depth of the sulcus is often included together with the biologic width when leaving a certain amount of tooth structure remaining, thus establishing a margin of safety. When restorations do not take these considerations into account and violate biologic width, three things tend to occur:

• chronic pain
• chronic inflammation of the gingiva
• unpredictable loss of alveolar bone

Ferrule effect

In addition to removing 2 mm of bone to establish a proper biologic width, another 2 mm should be removed to reveal enough tooth structure to allow for a 2 mm ferrule. 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.

These two X-ray films depict the teeth of the upper right quadrant. In the upper film, there is a tooth, #5, with a large, defective DO composite restoration. The lower film depicts the ideal bone level after a crown lengthening procedure has been completed, as well as the margin of the prosthetic crown in relation to the reduced height of bone. Note that this is a dramatization of the procedure: the lower film is a digital manipulation of the upper film, and not an actual film of the teeth after a crown lengthening procedure and crown cementation have been performed.

Some recent studies suggest that, while ferrule is certainly desirable, it should not be provided at the expense of the remaining tooth/root structure. On the other hand, it has also been shown that the "difference between an effective, long-term restoration and a failure can be as small as 1 mm of additional tooth structure that, when encased by a ferrule, provides great protection. When such a long-lasting, functional restoration cannot be predictably created, tooth extraction should be considered."

Crown-to-root ratio

The alveolar bone surrounding one tooth will naturally surround an adjacent tooth, and removing bone for a crown lengthening procedure will effectively damage the bony support of adjacent teeth to some inevitable extent, as well as unfavorably increase the crown-to-root ratio. Additionally, once bone is removed, it is almost impossible to regain it to previous levels, and in case a patient would like to have an implant placed in the future, there might not be enough bone in the region once a crown lengthening procedure has been completed. Thusly, it would be prudent for patients to thoroughly discuss all of their treatment planning options with their dentist before undergoing an irreversible procedure such as crown lengthening.

Treatment planning

Crown lenghthening is often done in conjunction with a few other expensive and time-consuming procedures who combined goal is to improve the prosthetic forecast of a tooth. If a tooth, because of its relative lack of solid tooth structure, also requires a post and core, and thus, endodontic treatment, the total combined time, effort and cost of the various procedures, as well as the impaired prognosis due to the combined inherent failure rates of each procedure, might combine to make it reasonable to have the tooth extracted. If the patient and the extraction site make for eligible candidates, it might be possible to have an implant placed and restored with more esthetic, timely, inexpensive and reliable results. It is important to consider the many options available during the treatment planning stages of dental care.

References

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