May 15, 2022
Each tooth in the oral cavity is surrounded by the gums when seen clinically. However, the gums around the tooth are not restricted to the portion that we see with our naked eye. There are two parts of the gums: supragingival region and subgingival region.
The supragingival region starts from the upper margin of the gums till its attachment at the lowermost portion of the crown of the tooth. Below this attachment starts the subgingival region. The part of the supragingival region that attaches to the lower portion of the crown is called the junctional epithelium.
Starting from the junctional epithelium till further below up to the highest portion of the underlying bone is called the connective tissue attachment. The total width covered by the junctional epithelium and the connective tissue attachment is called the biological width.
The biological width plays an important role in protecting the underlying ligament and bone of the tooth from microbial attack. It should thus be preserved during any restorative or surgical procedure. No specific number can be assigned to the dimension of the biologic width because it is different around each tooth. It can vary from 1.7 mm to 2.40 mm.
Clinical evaluation of biological width
Biological width can be assessed in a patient with the help of an instrument known as a probe. A probe features markings at specific intervals. The procedure of evaluating biological width is called bone sounding. First, the tissues of the concerned region are anaesthetised. When the probe is inserted between the gum and the tooth, the end of the probe hits the bone. The marking on the probe is then measured. It is later subtracted from the length of the visible gap between the tooth and the gingiva. This gives the biological width. An adequate measure would range between 3 to 4.5 mm.
At times the dentist may even take a radiograph to evaluate the biological width. The technique that dentists use for this is called parallel profile radiograph.
Margins and biological width
The biological width forms the terminal margin of most treatments that involves placement of a crown, taking an impression or preparing the tooth for a prosthesis. During these procedures, if the biological width is violated, the following things are likely to happen:
• An already infected tooth will be susceptible to further infection.
• The tooth will be susceptible to plaque accumulation.
• There will be acceleration of resorption (eating away) of the underlying bone of that tooth.
• There are high chances of the development of a lesion with respect to that particular tooth.
Factors that lead to biological width violation
Overhanging restorations
These include restorations that are not shaped according to the tooth's anatomy and thus irritate the gingival tissues. This causes the gingival tissues to recede.
Restoration margins
In cases where the restorative margins are placed subgingivally, there is a good chance that biological width has already been violated. These restorative margins should be at or above the gingival margin. They are made for the tooth to receive a prosthesis.
Open contacts
Upon placement of a crown or any other prosthesis, if the gap between the treated tooth and the adjacent tooth remains open, food impaction can cause the gingiva to inflame and recede.
In most cases of biological width violation, the bone sounding will give a measure of less than 2 mm. In these cases, correction can be done through certain orthodontic and surgical techniques.
Crown Lengthening
Crown lengthening is done by removing a portion of gums and exposing a previously unexposed part of the crown. It can also be done by incising a part of the gums, raising a flap of it, and suturing it at a new position such that the crown appears longer.
Orthodontic Extrusion
This is done by incising a portion of gingival tissue (fibrotomy) at regular intervals while simultaneously extruding the crown with the help of an orthodontic appliance. The movement can be rapid or slow. This technique helps in achieving the needed crown to root ratio of the tooth.
Biological width is an important landmark for evaluating the prognosis of a tooth both before and after the treatment. An understanding of the biological width for the patient will help him or her understand the longevity of his or her corrective treatment better.