Periodontology Anatomy - Biologic Width

Biologic width is the distance that is established by the junctional epithelium and connective tissue attachment to the root surface of a tooth. This can also be described as the height between the deepest point of the gingival sulcus and the crest of the alveolar bone. This distance is important to consider when dental restorations are fabricated because they must respect the natural architecture of the gingival attachment in order to avoid harmful consequences. The biologic width is specific to each patient and can range from 0.75-4.3 mm.

According to the 1961 paper by Gargiulo, the mean biologic width was found to be 2.04 mm. From this, 1.07 mm is comprised of the connective tissue attachment and another 0.97 mm is occupied by the junctional epithelium. Because it is nearly impossible to restore a tooth to the precise coronal edge of the junctional epithelium, dentists often remove enough bone to leave 3mm between the restorative margin and the crest of alveolar bone. When restorations do not take these considerations into account and violate biologic width, the following three things can occur:

1. Chronic pain
2. Chronic inflammation of the gingiva
3. Unpredictable loss of alveolar bone

Ferrule effect

In addition to lengthening the crown in order to establish a proper biologic width, the tooth structure should be 2 mm in height to allow for a ferrule effect. A ferrule, as it relates to the teeth, is a band that encircles the external dimension of residual tooth structure. This is similar to the metal bands that exist around a barrel. Sufficient vertical height of the tooth’s structure that will be grasped by the future crown is necessary in order to allow for a ferrule effect of the future prosthetic crown. This has been shown to significantly reduce the incidence of fracture in an endodontically treated tooth. Because a beveled tooth structure is not parallel to the vertical axis of the tooth, it does not fully contribute to ferrule height. A desire to bevel the crown margin by 1 mm requires an additional 1 mm of bone removal during the crown lengthening procedure. Restorations are typically performed without a bevel.

Recent studies have found that, while the ferrule is desirable, it should not be provided at the cost of the existing tooth surface or root structure. It has also been shown that the difference between a successful, long-term restoration and a failed restoration can be as little as 1 mm of additional tooth structure. When encased by a ferrule, provides the tooth a greater level of protection. When it is not possible to predictably create a long-lasting, functional restoration, it may be necessary to consider a tooth extraction.

Crown-to-root ratio

The alveolar bone that surrounds one tooth, naturally surround the adjacent tooth. As a result, when bone is removed for a crown lengthening procedure, it will effectively damage the bony support of the adjacent teeth. It can also unfavorably increase the overall ratio of crown-to-root. In addition, once bone is removed, it is nearly impossible to restore it to its previous levels. In the event a patient wants to consider having an implant placed in the future, there may not be enough bone structure available once a crown lengthening procedure is complete. It is critical for patients to thoroughly discuss all of their treatment plans and goals with their dentist prior to undergoing procedures like crown lengthening.