Surgical procedures involving living tissues must be carefully executed to avoid unnecessary damage. The adjacent teeth, soft tissues, and the pulp of the tooth being prepared are easily damaged in tooth preparation. If poor preparation leads to inadequate marginal fit or deficient crown contour, plaque control around fixed restorations will become more difficult. This will impede the long-term maintenance of dental health.
Iatrogenic Damage to the Pulp
Pulpal degeneration occurs during or after tooth preparation, especially complete crown preparation. Great care is also needed to prevent pulpal injuries during fixed prosthodontics procedures. Pulpal degeneration that occurs many years after tooth preparation has been documented [1, 2], particularly when they occur on freshly sectioned dentinal tubules. Prevention of pulpal damage necessitates selection of techniques and materials that will reduce the risk of damage while preparing tooth structure .
Tooth preparations must take into consideration the morphology of the dental pulp chamber. Pulp size, which can be evaluated on a radiograph, decreases with age. Average pulp dimensions have been related to the coronal contour .
The utmost biologic threat happens while inserting into sub gingival boundaries . These boundaries are not as manageable as supragingival or equigingival margins for concluding procedures. In toting up, if the margin is located too far below the gingival tissue crest, it will disturb the gingival attachment apparatus.
Biologic width is the dimension of space that the healthy gingival tissues occupy above the alveolar bone. Gargiulo, Wentz, and Orban's in 1961, were the authors who did the earlier studies on cadavers establishing the dimensions of space required by the gingival tissues . In an average human, the connective tissue attachment occupies 1.07 mm of space above the crest of the alveolar bone, and that the junctional epithelial attachment below the base of the gingival sulcus occupies another 0.97 mm of space higher than the connective tissue attachment. The blend of these two measurements is the “biologic width” (Fig. 3).
Clinically, this information is helpful to identify biologic width violations when the restoration margin is placed 2 mm or less away from the alveolar bone and the gingival tissues are inflamed.
Placement of restoration margins below the gingival tissue crest was decided by restorative considerations. Restorations may need to be extended gingivally.
(1) To make appropriate resistance and retentive form in the preparation,
(2) To make important contour alterations due to caries or other tooth deficiencies, or
(3) To cover the tooth/restoration interface by locating it subgingivally. When the restoration border is placed far below the gingival tissue crest, it will create a violation of biologic width and impinge on the gingival attachment apparatus . Two different responses can be observed from the concerned gingival tissues (Fig. 4).
One likelihood is that gingival tissue recession along with bone loss of an unpredictable nature will occur as the body attempts to recreate space between the alveolar bone and the margin to allow space for tissue reattachment. This occurs most commonly in areas where the alveolar bone surrounding the tooth is very thin in width. Trauma from restorative procedure scan play a key function in causing this delicate tissue to move away. Other factors that may influence the likelihood of recession include
(1) Whether the gingiva is thin and fragileor thick and fibrotic
(2) Whether the gingival form is flat or highly scalloped. It has been found that recession occurs more commonly in highly scalloped thin gingiva than a flat periodontium with thick fibrous tissue .
The more frequent finding with deep margin placement is that gingival inflammation develops and persists even though the bone level appears to remain unchanged. Hence it is important to establish space between the alveolar bone and the restorative margin to restore gingival tissue health. This can be accomplished either by surgery to change the bone level or by orthodontic extrusion to move the restoration margin farther away from the bone level.
Violation of the biologic width becomes of particular concern when considering. If the restoration of a tooth has fractured or if it has been destroyed by caries near the alveolar crest level. Also, esthetic demands often require ‘‘burying’’ of restorative margins subgingivally, which can lead to violation of this space. To avoid deleterious effects various authors have recommended that the restorative margins must be of minimal distances from the bone crest. To permit adequate healing and restoration of the tooth, Ingber et al.  recommended that a minimum of 3mm was necessary from the restorative margin to the alveolar crest.
The periodontium was divided into three dimensions by Maynard & Wilson as: superficial physiologic, crevicular physiologic, and subcrevicular physiologic .
(i) The free and attached gingiva adjacent the tooth is the superficial physiologic dimension, (ii) the gingival crevice – extending from the free gingival margin to the junctional epithelium is the crevicular physiologic dimension. (iii) The subcrevicular physiologic space is similar to the biologic width described by Gargiulo et al., consisting of the junctional epithelium and connective tissue attachment . Maynard &Wilson claimed that the clinician should ‘conceptualize’ all three areas as all three of these dimensions affect restorative management decisions .
In particular the authors claimed that to prevent the placement of ‘permanent calculus’ beyond the crevice margin, placement of margins into the subcrevicular physiologic space should be avoided. Nevins & Skurow stated that when subgingival margins are indicated, the junctional epithelium or connective tissue apparatus must not be disrupted during restorative preparation and impression taking .
It is for the difficult clinician to identify of the sulcular epithelium ends and the junctional epithelium begins and hence the authors recommend to limit the extension of subgingival margin to 0.5–1.0mm. They also emphasized allowing a minimum 3.0 mm distance from the alveolar crest to the crown margin. The free gingival margin has been suggested as the reference point for measurements for margin placement as the biologic width is difficult for the clinicians to picture . Block stated that surgical crown-lengthening procedures are essential when restorative margins end at or near the alveolar crest level. However, it appears that a minimum of 3mm of space between restorative margins and alveolar bone is a dimension that is to adhere to in restorative treatment planning.
In some patients if the margins are located more than 2 mm above the alveolar bone it will violate the biologic width. In 1994, Vacek et al. also examined the biologic width phenomenon . Although their average width finding of 2 mm was the same as formerly reported by Gargiulo et al. (Fig. 5) . They also reported a range of different biologic widths that were specific to some patients. They described biologic widths as narrow as 0.75 mm in some individuals, whereas others had biologic widths as high as 4.3 mm (Fig. 6) .