Enamel reduction and the Tarnow rule for biological width

The general expectation of orthodontic treatment is to deliver an aesthetic and predictable dental experience to the patient as well as the orthodontist. However, much of this biological impediment is difficult to achieve considering the need for maintenance of the distance from the interproximal bone crest to the contact point and the presence or absence of the interproximal papilla that influences this change.

On a related note, it is worth mentioning the immensely intimate relationship between periodontal health and the restoration of teeth. For controversial orthodontic procedures in line with enamel reduction, gingival health is of utmost importance. This is where we come across a peculiar dental term called ‘biologic width’.

What is biologic width?

The concept of biologic width was first put forth by Gargiulo, Wentz, and Orban. They described it as the distance between the apical end of the gingival sulcus and the crest of the alveolar bone. Later on, Cohen coined the term ‘biologic width’ when he described the space over the tooth surface, occupied by the connective tissue and epithelial attachments, to be equivalent to the distance between the bottom of the gingival sulcus and the alveolar bone crest.

Biologic width is defined as the dimensions of soft tissue which is attached to the portion of the tooth coronal to the crest of the alveolar bone. In other words, it is the average combined histological width of connective tissue attachment along with junctional epithelium (approximately estimated to be 2.04 mm)

Upon further cadaver studies, it was found that while biologic width was similar on all the teeth, it varied between individuals. Some can have biologic widths as small as 0.75 mm while others as tall as 4 mm, but statistically the majority followed the 2 mm average measurement.

Why is biologic width important?

Tarnow and his colleagues found that for the gingival tissue to assume complete filling of the interdental space, the distance from the contact point to the alveolar crest should not exceed 5 to 5.5 mm. In fact, they recorded that the greater this distance, the more significant the loss of alveolar height.

Biologic width is important for many reasons, primarily for the clinician to determine the position of restorative margins and their impact on post-surgical tissue position. If a restoration, like an implant, is placed beyond the tissue level in a way that invades the biologic width, funnel-shaped bone resorption as well as gingival inflammation can make way.

Therefore, as a rule of thumb, follow the rules listed below when determining biologic width in restorative dentistry:

• If the gingival sulcus is equal to or less than 1.5 mm, place the margin 0.5 mm below the gingival tissue crest.
• If the gingival sulcus is more than 2 mm, margins of restoration are prepared 0.7 mm, subgingivally.
• If the gingival sulcus is more than 2 mm, in an esthetic zone from the vestibular side, a gingivectomy is recommended. Also, the margins of restoration should be prepared 0.5 mm, subgingivally.

The health of the periodontal tissues will depend entirely on the design of the restorations placed. It is thus of paramount importance that the clinician respects the biologic width and places restorations within the established margins.