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Minimally invasive surgical techniques in clinical crown lengthening : A systematic review

The surgical procedure for the clinical crown lengthening, aims for an aesthetic or functional form to compensate for the reabsorption of the alveolar bone tissue caused by the invasion of the biological space. The conditions that must be taken into account during surgical planning are related to an excessive gingival exposure, altered passive eruption and lack of height of the dental element for restorative purposes. The objective of this systematic review was to evaluate in the scientific literature the minimally invasive surgical techniques used to clinical crown lengthening. A systematic review of the literature was performed using the databases, Science Direct, Embase, Cochrane Collaboration Library, and PubMed/MEDLINE. The search strategy provided a total of 157 studies. After selection, five articles met all the inclusion criteria and were included in this systematic review. Studies have shown that conventional surgical techniques for clinical crown lengthening are presented as an effective approach that promotes good aesthetic and functional results. However, they have some limitations. After evaluating the minimally invasive surgical techniques included in this systematic review, it was possible to infer that they presented satisfactory results in the soft and hard tissue contouring, with no complications and dissatisfaction being observed by the patients. Research, Society and Development, v. 9, n. 10, e579108256, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i10.8256 3


Introduction
The periodontium consists of structures that support and involve the dental element.
The vitality of periodontal tissues is extremely important for dental elements with or without restorative treatment. The surgical procedure for the clinical crown lengthening aims for an aesthetic or functional form to compensate for the resorption of the alveolar bone tissue caused by the invasion of the biological space. Often the indication for this surgical technique is related to restorative procedures, in which the presence of subgingival cavity margins caused by caries, imperfect amelogenesis, fracture and dental malformation are usually found. (Tomar et al., 2013;Ebersole et al., 2016;Lavu et al., 2019).
The width of the keratinized gingiva is of fundamental importance to keep the periodontium healthy. The biological space corresponds to the groove epithelium, junctional epithelium and conjunctive insertion, showing an average value equivalent to 2.75 mm and ranging between 2.16 and 3.34 mm. The conditions that must be taken into account during the surgical planning of the clinical crown lengthening are related to the presence of excessive gingival exposure, altered passive eruption (alveolar crest equal to or less than 2 mm of the cementum-enamel junction) and lack of height of the tooth for restorative purposes (Tomar et al., 2013;Ryder., Couch., & Chaffee, 2018;Lavu et al., 2019).
The forms of treatments proposed to perform the clinical crown lengthening, include procedures such as soft tissue gingivectomy, flap repositioned apically or flap repositioned apically with bone contour. The literature also points out other types of approaches, such as exposing the crown through orthodontic dental extrusion, using the Er:YAG laser and performing piezosurgery that provides a precise cut of bone tissue, without causing damage to adjacent soft tissues (Gaspirc., & Skaleric, 2007;Tomar et al., 2013;Lavu et al., 2019;Worthington et al., 2019). Thus, it is observed that the surgical procedures used to restore the contour of the soft and hard tissues of the dental elements, present several advantages. Research, Society and Development, v. 9, n. 10, e579108256, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i10.8256 5 However, it is not yet fully clear which surgical techniques provide less postoperative morbidity, being classified as minimally invasive. Therefore, the objective of this systematic review was to evaluate in the scientific literature the minimally invasive surgical techniques used to clinical crown lengthening.

Materials and Methods
This study followed the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) Statement (Moher et al., 2009).

Information of sources and search strategies
The following review question was developed according to population, intervention,

Eligibility criteria and Study selection
The reviewer independently screened and assessed potential articles. Studies that did not fulfill the inclusion criteria were excluded. In the first stage, the titles and abstracts of all retrieved reports were screened for potentially eligible studies. The full text articles of the previously identified studies were then examined in detail according to predefined eligibility criteria for inclusion in the qualitative review. Disagreements were solved by discussion between the authors. Eligibility criteria: the included studies were randomized clinical trials Research, Society and Development, v. 9, n. 10, e579108256, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i10.8256 6 that examined different minimally invasive surgical techniques in clinical crown lengthening.
Exclusion criteria: animal studies, in vitro studies, opinion articles, letters to the editor, review articles, interviews, updates, abstracts and unpublished studies were excluded.

Data extraction/analysis
The review authors independently screened the articles for data extraction. Any disagreements were resolved by discussion. In order to increase the strength of the present systematic review the studies that were included underwent a quality assessment following the recommendations of the Consolidated Standards of Reporting Trials (CONSORT) statement (Moher., Liberati., & Tetzlaff., 2001). The CONSORT tool uses a systematic approach based on 7 specific criteria which are: (1) sample size calculation (minimum number of participants required to detect a significant difference among compared groups); (2) randomization and allocation concealment methods; (3) clear definition of inclusion and/or exclusion criteria; (4) complete follow-up; (5) experimental and control groups comparable at study baseline; (6) presence of masking; and (7) appropriate statistical analysis.

Study selection and characteristics
A search strategy developed in this systematic review identified a total of 157 studies located in validated databases. After a screening through the reading of two titles and summaries and excluding duplicate articles, 36 studies were considered potentially eligible and fully qualified. After further analysis, five articles published between 2013 and 2017 attended to all the inclusion criteria and were selected for the systematic review (Nethravathy., Vinoth., & Thomas, 2013;Ribeiro et al., 2014;Paolantoni et al., 2016;Chen et al., 2016;Koppolu et al., 2017). The flowchart applied for the triage process and selection of items can be found in Figure 1.
According to the CONSORT evaluation criteria, all studies presented clear definition of inclusion and/or exclusion criteria, complete follow-up and appropriate statistical analysis.

Main results
In the study by Nethravathy, Vinoth and Thomas (2013) the sample is subdivided into three groups according to the treatment carried out for the clinical crown lengthening: (1) gingivectomy; (2) repositioned apically; (3) surgical extrusion using periotome. For the authors, the surgical extrusion technique using periotome can be used in succession, especially in the anterior region, where aesthetics are of great concern to the patient. the dental papilla preservation procedure in the crown lengthening. In the dental papilla preservation group in crown lengthening, there was also a significant increase in aesthetic evaluation by patients. Research, Society andDevelopment, v. 9, n. 10, e579108256, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i10.8256 In the study by Chen et al. (2016) the patients selected for the study had as reasons for the clinical crown lengthening, the presence of caries (22 patients), dental fracture (6 patients) and subgingival prosthetic preparation (4 patients) in the posterior region. The authors noted that clinical crown lengthening using the Er:YAG laser is effective and less traumatic compared to the traditional surgical procedure. Clinical crown lengthening procedures are often performed to provide access for the treatment of subgingival cavities, fractures or defective restorations. Using a minimally invasive procedure through the use of the Er:YAG laser can bring more advantages to the dental surgeon and patients than traditional approaches, avoiding bleeding, suturing and scar formation and allowing a less traumatic treatment for the tissue.
To Koppolu et al. (2017) who also made use of the surgical laser aiming at the clinical crown lengthening, the surgical lasers really show efficacy in such procedure, with a lower rate of bleeding and painful post-surgical symptoms. Therefore, they have some clinical relevance and can provide a precise and safe treatment modality for the patient, with a superior aesthetic result, satisfying the patient's aesthetic demands in smile corrections, and being a less traumatic procedure.

Discussion
The procedures used to perform the increase of the clinical crown lengthening have often been used to favor the return of the aesthetics and functionality of the teeth, maintaining the harmony of the stomatognathic system. To perform this procedure, the amount of bone tissue to be removed must be previously evaluated, using periapical radiographs and probing the site (Kalaivani et al., 2015;Lavu et al., 2019).
The distance between the alveolar crest and the enamel-cement junction is of fundamental importance to determine the choice of the surgical technique to be used. In cases in which the periodontium is thin or intermediate, osteotomy should be performed using only manual instruments, as the risks of tissue laceration are greater. Micro-chisels are the recommended instruments, and they must be positioned through the gingival sulcus, eliminating the need to lift a flap (Shobha et al., 2010;Ganji., Patil., & John, 2012). These findings corroborate the study by Ribeiro et al. (2014) in which it was observed that the use of minimally invasive techniques for the aesthetic clinical crown lengthening without the flap using instruments such as micro-chisels, promoted better results in relation to the correction of the gingival smile. Research, Society and Development, v. 9, n. 10, e579108256, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i10.8256

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The bone contouring procedures used in surgical procedures to clinical crown lengthening are efficient when associated with the accomplishment of an apically positioned flap. The apically repositioned flap eliminates the periodontal pocket, protecting the inserted gingiva and providing a bone contour similar to the healthy structure (Ganji., Patil., & John., 2012;Patil et al., 2016). However, in the study by Nethravathy, Vinoth and Thomas (2013) it was observed that the surgical extrusion technique using periotome presented better results when compared to the use of the flap repositioned apically to promote an increase of minimally invasive clinical crown lengthening, especially in the anterior region.
The interdental gingival papilla plays an important role in maintaining the aesthetics of dental elements and in protecting adjacent soft tissues. The absence of papillae causes the presence of black spaces that compromise the health of periodontal tissues, which can cause diastemas in the anterior region and aesthetic dissatisfaction related to the smile (Mohan et al., 2013;Lavu et al., 2019). Paolantoni et al. (2016) performed different procedures to increase the clinical crown lengthening seeking to promote the preservation of the dental papilla of the upper anterior teeth, taking into account its aesthetic and functional importance. However, it was observed that the use of the minimally invasive surgical technique exhibited a lower loss of interproximal clinical insertion, leading to greater patient satisfaction.
The mechanisms of action of lasers in hard and soft dental tissues depend directly on the absorption of light by the chromophore in the target tissue. Er:YAG lasers are a noninvasive modality, effective to promote bone recontouring in surgical procedures for clinical crown lengthening (Elavarasu,, Naveen., & Thangavelu., 2012;Deeb et al., 2019). These findings corroborate the studies by Chen et al. (2016) and Koppolu et al. (2017) in which it was observed that the use of the Er:YAG laser was less traumatic for dental hard and soft tissues when compared to the traditional surgical procedure. In addition, surgical lasers promoted a lower rate of bleeding during the operation and less painful postoperative symptoms.

Conclusion
Studies have shown that conventional surgical techniques to clinical crown lengthening are presented as an effective approach that promotes good aesthetic and functional results. However, they have some limitations related to bleeding and painful symptoms in the postoperative period. After evaluating the minimally invasive surgical techniques included in this systematic review, it was possible to infer that they presented satisfactory results in the soft and hard tissue contouring, with neither complications or dissatisfaction being observed by the patients.
It is important to note that in clinical crown lengthening, correct planning must be carried thoroughly, individually analyzing each case to be operated. In addition, the use of Er:YAG lasers has shown promise and in the near future new studies are expected that better elucidate the parameters of laser use in periodontal surgical procedures.