12.1 Introduction

Crown lengthening surgically increases the clinical crown in an incisal-apical dimension for either restorative or esthetic needs or a combination of both. The procedure may include apical repositioning of the gingival margin and osseous contouring. From a restorative standpoint, indications include insufficient clinical crowns for retention, subgingival caries, and subgingival fractures. Esthetically, short clinical crowns and cases of excess gingival display can also benefit from surgical crown lengthening [1, 2]. Case assessment prior to restorative treatment must take into consideration the biologic width and the mucogingival status (Fig. 12.1). Failure to do so can be detrimental to long-term periodontal health, resulting in subsequent inflammation, bone loss, and gingival recession [2, 3].

12.1.1 Biologic Width

Decay or placement of a restorative margin apical to the gingival sulcus risks impingement on the supracrestal fiber attachment and violation of the biologic width. The biologic width refers to the aspect of soft tissue, the dentogingival complex, that is attached to the tooth coronal to the alveolar bone. It is comprised of the connective tissue attachment, the epithelial attachment, and the gingival sulcus (Fig. 12.2) [3, 4]. Early work by Gargiulo et al. [5] on cadaver skulls found average measurements of 0.69 for the sulcus depth, 0.97 mm for the epithelial attachment, and 1.07 mm for the connective tissue attachment. A minimum of 3 mm from the alveolar bone to the restorative margin has been indicated to avoid infringement on the dentogingival complex and maintenance of the biologic width [6]. Kois [3] has expressed that the biologic width “averages” previously noted are quite variable between individuals and among the dentition of the same individual and therefore should be assessed on all included teeth prior to crown lengthening procedures. Additionally, it is more predictable to measure the entire dentogingival complex as a whole as opposed to individual components. This can be done by anesthetizing the patient for comfort and utilizing a periodontal probe to measure from the free gingival margin (FGM) to the osseous crest (so-called bone sounding). The resulting measurements can be categorized into normal, high, and low alveolar crests to further aid in determination of restorative margin location. A normal alveolar crest measures approximately 3 mm on the facial aspect and 3–4.5 mm on the interproximal surfaces. In this case, the restorative margin can safely be placed 0.5–1 mm apical to the FGM or 2–2.5 mm coronal to the osseous crest. In the case of a high alveolar crest, the total dentogingival complex measures less than 3 mm, and therefore the margin should be at, and no more than 0.5 apical to, the FGM. Alternatively, a measurement of greater than 3 mm for the total dentogingival complex is categorized as a low alveolar crest, in which case the margin can be placed more than 1 mm apical to the FGM. The relationship of the FGM to the alveolar crest should be measured prior to restorative preparation and surgical intervention, as well as after crown lengthening healing is completed.

Of critical importance is understanding the risks involved if the biologic width is violated. If crown lengthening is not performed when indicated, the oral tissues will aim to correct for this invasion in an unpredictable and uncontrolled manner. Chronic tissue inflammation can occur, as well as recession and bone resorption, possibly leading to intrabony defects [3].

12.1.2 Mucogingival Considerations

The term mucogingival condition refers to “deviations from the normal anatomic relationship between the gingival margin and the mucogingival junction (MGJ).” Examples include recession, absence or decreased keratinized tissue, and lack of attached tissue [7]. As discussed by Zadeh and Gil in this volume, the etiology of these mucogingival conditions is multifactorial. Factors can include tooth position, orthodontic treatment, gingival biotype, frenum position, vestibular depth, and mechanical insult. A thin gingival biotype is more likely to result in gingival recession versus a thick biotype. Buccally positioned dentition has been associated with thinner labial bone and gingiva and therefore at greater risk of gingival recession as well. Similarly, orthodontic movement in the buccal direction is more likely to cause mucogingival conditions versus that in a lingual direction [8]. Further evidence shows that some toothbrushing factors can be associated with gingival recession, especially in more prone sites (i.e., those with other contributable factors for mucogingival deformities) [8, 9].

Crown lengthening may include gingivectomy, and therefore it is important to understand the gingival condition prior to any surgical intervention. Additionally, the quality and quantity of tissue can contribute to the overall gingival health, especially around restorations.

The need for keratinized and/or attached gingiva for periodontal health is somewhat controversial in the literature. It is well-documented that areas of little to no keratinized tissue are able to be maintained and provide support over long periods of time. Nonetheless, this outcome is only possible with excellent oral hygiene and regular professional maintenance [8, 9]. This is highlighted in a split mouth long-term study [10]. Areas of little to no attached gingiva were either augmented with a free gingival autogenous graft or left alone, and not all of the patients received professional maintenance. Over time, patients who followed good oral hygiene and received maintenance showed adequate health in treated sites, as well as those that were not treated. In patients who did not follow maintenance protocols, the non-augmented sites resulted in increased inflammation and recession compared to augmented sites. Overall, the general consensus is that keratinized tissue deficiency predisposes to the development of gingival recession and inflammation [8]. It is suggested that 2 mm of keratinized gingiva, with 1 mm being attached, is needed for optimal health [9, 11]. Therefore, the keratinized and attached tissue should be assessed prior to crown lengthening procedures.

Furthermore, the role of tissue around restorative margins has been evaluated in the literature. Studies have compared two groups, one with a wide zone (greater than or equal to 2 mm) of keratinized gingiva and the other with a narrow zone (less than 2 mm) of keratinized gingiva [12, 13]. In the presence of subgingival restorations, the amount of inflammation was significantly increased in those with a narrow zone versus a wide zone of keratinized tissue. Another study was completed on dogs, where steel bands were placed subgingivally, and sites with adequate widths of keratinized gingiva were compared to those with inadequate keratinized gingiva [14]. Sites with inadequate keratinized tissue showed gingival inflammation in addition to loss of gingival tissue. Later work has confirmed that restorative margins placed subgingivally lead to early gingival recession and attachment loss, and recession is more likely in areas of narrow gingiva [15]. Systematic reviews and position papers have confirmed the negative impact on gingival health that intrasulcular margins can have, especially in the presence of minimal or no attached gingiva. Gingival augmentation is indicated in those sites planned for intrasulcular restorative margins [8, 10, 15, 16]. Some authors even advocate for a minimum of 5 mm of keratinized tissue (3 mm attached and 2 mm free) at those sites [17]. Therefore, prior to restorative treatment, the biologic width and the mucogingival state should be evaluated. As discussed, violation of biologic width has been shown to lead to unpredictable bone loss and recession. Crown lengthening procedures to provide restorative access should consider the biologic width of each tooth before and after surgery (Fig. 12.3). Additionally, the amount of keratinized and attached tissue, and the presence of mucogingival deformities, should be noted prior to surgical intervention and restoration placement. Thin gingival biotype and minimal attached gingiva can result in gingival inflammation and recession defects.

12.1.3 Functional Crown Lengthening

At its essence, functional crown lengthening is a resective procedure undertaken to so that sound tooth structure can be exposed to support a new restoration and to re-establish a biologic width at a more apical position than prior to the surgical intervention. Initially proposed by D.W. Cohen in 1962, current protocol involves judicious removal of surrounding hard and soft tissue structures, so that the resulting tooth exposure is approximately 4 mm superior to the osseous crest. This amount of tooth exposure is required to allow re-establishment of the biologic width and to facilitate the ideal preparation of the tooth, ferrule, and marginal seal [3, 18–20].

Rosenberg et al. [21] noted that there are several indications for functional crown lengthening in the dentoalveolar complex. These include:

1. (a)

   Tooth decay which compromises the gingival sulcus and connective tissue attachment and/or is invading the biologic width

    
2. (b)

   Tooth fracture which compromises the gingival sulcus and connective tissue attachment and/or is invading the biologic width, with adequate remaining tooth structure, periodontal attachment, and supporting alveolar bone

    
3. (c)

   Teeth with excessive retrograde wear where crown lengthening is required for adequate seating and retention of a full coverage restoration

    
4. (d)

   Teeth, due to super-eruption, which have insufficient interocclusal space for requisite restorative dentistry

    
5. (e)

   Altered passive eruption, where the gingival margin is coronal to the CEJ and the osseous crest is approximate to or at the CEJ (Fig. 12.4a, b)

    
6. (f)

   External root resorption involving the dental structures adjacent to the gingival margins and/or the osseous crest

    

An adjunctive or ancillary treatment modality to functional crown lengthening is the use of orthodontics for forced eruption. Orthodontic forces may be utilized to either slowly or rapidly erupt the tooth in an occlusal or incisal direction in an attempt to bring either the osseous crest and underlying periodontal structures more coronally [22] or to extrude the tooth from the dentoalveolar complex so that the fracture or caries is exposed. Subsequent surgical re-establishment in an apical direction of the periodontal complex may or may not be required. Further discussion of this treatment modality can be found in the chapter by Schmerman and Obando in this volume.