The clinical success of the indirect restoration requires a precise working model and thus depends on the accuracy of the final impression. An accurate impression is one of the primary determinants for a precisely fitting indirect restoration. Taking a precise impression is one of the most challenging steps in restorative dentistry, yet it is the foundation and the key to the longevity of indirect restorations. 

The need to retract the gingiva for taking impressions is universally acknowledged. The complexity of the procedure arises from a need to provide adequate horizontal space for the impression materials. Some authors have determined that the horizontal space must be between 0.2 and 0.4 mm. Gingival retraction during any stage of restoration, no matter the technique, may cause a lesion to form on the periodontal tissue. Periodontal tissue with normal healing capacity will repair itself. 

Healthy periodontal tissue is a prerequisite for the success and accuracy of the final impression. Inflammation of gingival tissues before taking the impression can complicate the procedure.  Bleeding and moisture from the crevicular fluid can displace impression material, resulting in voids and rounded, indistinct finish lines that can cause an inaccurate cast and an improperly fitting definitive restoration. Furthermore, if a subgingival margin is placed in the presence of inflammation, there is a potential risk of gingival recession and exposure of the restorative finish line. Therefore, the soft tissue must be properly managed.

The preoperative considerations during initial therapy are to control and eliminate all sources of irritation and inflammation. This can be accomplished by control of plaque-related etiologies and/or correction of restorative contributing factors. Unfortunately, this may require delaying the impression procedure after tooth preparation to allow for the improvement in the soft tissue condition. 

The provisional restoration is an essential component of this initial therapy and can improve the quality of the impression. It preserves the position, form, and color of the gingiva and maintains periodontal health before impression-taking and while the definitive restoration is being fabricated.

Countless techniques and materials have been developed over the years to improve the accuracy and predictability of this challenging task. Some exciting developments have recently emerged alongside the growing application and widespread use of CAD/CAM technology. Improved intraoral optical scanners may be able to simplify or someday even eliminate the mechanical aspect of taking an impression with impression material and tray. This new digital technology will continue to transform digital intraoral scanning into a routine procedure in more dental offices in the near future.

Management of soft tissue during the preparation and impression-taking stages requires an understanding of the gingival tissue architecture. The most important determining factor in predicting how the tissue will respond to preparation and impression techniques begins with the relationship of the free gingival margin to the osseous crest. Preoperative recordings of facial and interproximal bone height and determination and preservation of the biologic width can provide predictable post restorative gingival margin levels and periodontal health.

A very important trend in dentistry nowadays is represented by the increased attention paid to the connection between periodontal and prosthetic treatments. This connection is the weak point and the place where patients most often notice that “ the tooth is a fake ”. 

Violation of the biological width

With regard to the interaction of restorations to the periodontium, the most important region is that portion between the tooth, the crestal bone, and the cervical margin of the restoration. In this region, the biological components responsible for the maintenance of periodontal homeostasis are inserted. The connective insertion and the junctional epithelium together compose the biological width, defined as the height of the gingival tissue to the tooth inserted coronally to the bone crest. The dimensions of each of the components of the biological width were determined from findings of histological studies, with the following average values:  junctional epithelium (0.97 mm) and connective insertion (1.07 mm) with the sum of 2.04 mm. The gingival sulcus is 0.69 mm by the way. These values are determined biologically, ranging between individuals, between teeth, and between different surfaces of the same tooth.

The adherence of these structures to the dental tissue - through dentogingival fibers to the connective tissue attachment and through hemidesmosomes to the junctional epithelium - results in an organic sealing around the tooth neck. This allows for the host to maintain periodontal health against continuous bacterial invasion. The gingival sulcus, on the other hand, does not present any adherence to the tooth surface and therefore may be broken during restorative procedures, without prejudice to the periodontal homeostasis.

Since its average length is only 0.69 mm, it has been suggested that - if necessary - the preparations may be extended up to 0.5 mm within the sulcus. Therefore,  the margin is restricted to the intrasulcular area without compromising its biological seal. However, when the biological width is invaded and consequently the dentogingival attachment is compromised whether by pathological (e.g. carious lesion), traumatic (e.g. fractures), or iatrogenic processes (e.g. inappropriate placement of the preparation margins), there is a disruption of the organic seal allowing bacteria and their byproducts to reach the underlying connective tissue which can result in inflammation. Since the biological width is violated, the body itself is responsible for promoting resorption of the bone crest to allow apical migration of its components.

Violation of that space by restorations impinging on the biological width has been associated with gingival inflammation, discomfort, gingival recession, alveolar bone loss, pocket formation, and the like. There is a great body of evidence that the destruction of the periodontal attachment device is preceded by inflammatory changes in the region of the margin. Regarding the appearance of the gingival tissue, one should observe not only its shade - this varies greatly between individuals - but the chromatic pattern in an attempt to detect localized changes. The correction of the biological distances whether by a surgical or a non-surgical approach and the elimination of the inflammation are essential conditions for the implementation of a restorative treatment that will not interfere with the local homeostasis of the periodontium. To have a harmonious and successful long-term restoration, it is advocated to keep 3 mm of sound supracrestal tooth structure between bone and prosthetic margins, which allows for the reformation of the biological width plus sulcus depth. This can be achieved surgically (crown lengthening) or orthodontically (forced eruption) or by a combination of both.

To be continued...

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