Periodontal Anatomy – Biologic width
The biologic width is the distance which is created by the junctional epithelium and connective tissue attachment to the tooth’s root surface. The concept of the biologic width was first published by Ingber JS, Rose LF and Coslet JG. in 1977. This was based on cadaver measurements by Gargiulo, Wentz and Orban who did not use the biologic width term in their article which was written in 1961. The term biologic width was first used by Dr. D. Walter Cohen at the University of Pennsylvania in 1962. In more simple terms, the biologic width is the height between the gingival sulcus’ deepest point and the crest of the alveolar bone. This distance is especially important when dental restorations are being created. Dental restorations must respect the natural architecture of the gingival attachment in order to avoid negative consequences. The biologic width is unique to each patient and will typically vary from about 0.75-4.3 mm.
Based on the paper by Gargiulo written in 1961, the mean biologic width was 2.04 mm. Of this, the connective tissue attachment is 1.07 mm and about 0.97 mm is occupied by the junctional epithelium. Because it is virtually impossible to create a perfect restoration of a tooth to the precise coronal edge of the junctional epithelium, the recommendation is to remove enough bone to provive 3 mm between the restorative margin and the crest of alveolar bone. When a restoration consider this and violates the biologic width, it often results in the following issues:
- Chronic pain and discomfort
- Chronic inflammation of the gingiva
- Unpredictable loss of alveolar bone
In addition to crown lengthening for establishing a proper biologic width, a 2 mm height of the structure of a tooth should also be available to allow for a ferrule effect. A ferrule, as it relates to the teeth, is a band which encircles the external dimension of residual tooth structure. This can be thought of similarly to the metal bands which surround a barrel. Adequate vertical height of tooth structure, which will be grasped by the future crown, is required to allow for a ferrule effect of the future prosthetic crown. Honoring these dimensions has demonstrated the ability to significantly reduce the cases of fractures in a tooth which is endodontically treated. Because a beveled tooth structure is not parallel to the tooth’s vertical axis, it does not correctly contribute to ferrule height. As a result, the desire to bevel the crown margin by 1 mm requires an additional 1 mm of bone removal in the procedure for lengthening the crown. However, in most cases restorations do not include this type of bevel.
Based on data from recent studies, while a ferrule is truly desirable, it should never be provided at the expense of the remaining structure of the tooth or the root. Data has also demonstrated that the difference between an effective, long-term restoration and a failed restoration may be as little as 1 mm of additional tooth structure. When it is encased by a ferrule, this additional small structure provides the tooth with additional protection. When it is not possible to predictably create a long-lasting, functional restoration, the dentist should consider extracting the tooth.
The alveolar bone which surrounds one tooth, naturally surrounds the adjacent tooth. When bone is removed for a crown lengthening procedure, it will inevitably damage the bony support of the adjacent teeth to some extent. The removal of bone will also unfavorably increase the crown-to-root ratio. Once the bone is removed, it is nearly impossible to restore it to the previous levels. When patients are considering an implant, there may not be adequate bone needed to support the implant once a crown lengthening procedure is performed. As a result, patients should thoroughly discuss their options with the dentist prior to undergoing a procedure such as crown lengthening, which cannot be reversed.