Biologic width
The biologic width is the distance which is established by the junctional epithelium and the connective tissue attachment to the tooth’s root surface. This may also be used to describe the height between the deepest point of the gingival sulcus and the alveolar bone crest. This distance is a vital consideration in the creation of dental restorations as 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 ranges from about 0.75 to 4.3 mm.
Based on a paper by Gargiulo written in 1961, the mean biologic width was 2.04 mm. Of this amount, 1.07 mm is comprised of connective tissue attachment and another 0.97 mm is occupied by the junctional epithelium. Because it is nearly impossible to perfectly restore a tooth to the exact coronal edge of the junctional epithelium, dentists often opt to remove enough bone needed to maintain 3mm between the restorative margin and the crest of alveolar bone. When the restoration does not account for these considerations and violate the biologic width of the tooth, it can result in the below issues:
- Chronic pain
- Chronic gingiva inflammation
- Loss of alveolar bone
Ferrule Effect
In addition to the lengthening of the crown for the establishment of adequate biologic width, the structure of the tooth should maintain 2 mm in height to allow for a ferrule effect. A ferrule, in relation to the teeth, is a band surrounding the external dimension of the residual tooth structure. This can be compared to the metal bands which surround a barrel. Sufficient vertical height of the tooth, which will be grasped by the future crown, is required to allow for a ferrule effect of the future prosthetic crown. This has shown to substantially minimize the likelihood of a fracture in an endodontically treated tooth. Because the beveled tooth structure is not parallel to the vertical axis of the tooth, it does not fully contribute to ferrule height. As a result, if the goal is to bevel the crown margin by 1 mm, an additional 1 mm of bone removal is required in the crown lengthening procedure. It is common, however, to perform restorations without the need for a bevel.
Some recent studies have suggested that while the ferrule is desirable, it should not be achieved at the expense of the remaining tooth or root structure. Studies have also shown that the difference between successful and failed restorations can differ by just 1 mm of additional tooth structure. When encased by a ferrule, it provides the tooth with a greater level of protection. When it is not possible to create a long-lasting, functional restoration, it may be necessary to consider removing the tooth.
Crown-to-root Ratio
The alveolar bone, which surrounds a tooth, naturally surrounds the adjacent teeth as well. Because of this, when there is removal of bone structure for crown lengthening, it can damage the bony support of the adjacent teeth. This process can also unfavorably increase the crown-to-root ratio. Once the bone is removed, it is nearly impossible to restore it to its previous levels. In the event a patient is considering an implant, there may not be adequate bone structure available once the crown lengthening procedure is completed. It is critical for patients to be thorough in the discussion of their treatment options and goals before undergoing irreversible procedures such as crown lengthening.