Biologic width is a term that refers to the attachment of tissues, connective tissue and epithelium, to the tooth surface. Maintaining this dimension of attachment when performing restorations is necessary to maintain healthy periodontal tissues. In a study by Garguilo and colleagues, this attachment dimension was found to be 1.07mm for connective tissue fibers, 0.97mm for epithelium attachment and 0.67mm for sulcus depth. These dimensions make up the biologic width.
When a restoration has to be placed below the gum, it often violates the biologic width of health. When we use a bur on the tooth below the gingiva we often remove the organized tissues of the periodontium which lead to inflammation and disease.
Multiple studies have shown that subgingival margins are associated with gingival inflammation. Studies by Maynard and Wilson have shown that violating the biologic width will lead to progressive inflammation and loss of connective tissue fiber attachment to the tooth.
In short, the restoration margin needs to be at least 3 mm from bone or a crown lengthening needs to be performed.