Periodontal tissue changes after crown lengthening surgery: A systematic review and meta-analysis

Abstract

Introduction

Crown lengthening is one of the most common periodontal surgical procedures carried out to increase the amount of supragingival tooth structure. There is a lot of literature on crown lengthening surgeries, but very few systematic reviews comparing treated and adjacent sites over a six-month period. The purpose of this systematic review and meta-analysis was to evaluate the outcomes of crown lengthening surgery in terms of changes in periodontal clinical parameters and periodontal tissue stability between treated and adjacent sites.

Methods

Electronic databases were searched up to 28 February 2022 with no restriction on publication status. A manual search of journals was also performed. Predefined inclusion and exclusion criteria were used to select the relevant articles that assessed dimensional changes in periodontal tissues after crown lengthening surgery. The risk of bias was assessed using the JBI critical appraisal checklist. Data meta-analysis was performed using a statistical software program.

Results

A total of 78 studies were identified, of which, four clinical controlled trials containing 182 crown lengthening surgical procedures across 111 participants were included. Meta-analysis showed no statistically significant changes after three or six months in terms of supracrestal tissue attachment levels, bone level and probing pocket depth between treated and adjacent sites. However, clinical attachment level changes were statistically significant, favouring adjacent teeth at six months.

Conclusions

Within the limitation of this systematic review, crown lengthening surgery results in stable periodontal tissues over time according to the acceptable periodontal healing parameters. Further evidence is still required to substantiate these findings.

1. Introduction

Crown lengthening surgery (CLS) is a procedure designed to increase the amount of supragingival tooth structure for restorative and/or aesthetic indications (Gupta et al., 2015, Marzadori et al., 2018, Mugri et al., 2021). It is commonly performed to re-establish and avoid invasion of the supracrestal tissue attachment during restorative procedures. Ideal restoration margins minimise tissue inflammation associated with pathological probing depths and loss of periodontal supporting tissue (Brägger et al., 1992, Carvalho et al., 2020). Teeth with subgingival caries, subgingival tooth fractures, root resorptions or iatrogenic endodontic perforations can benefit from this procedure to improve access and enable placement of restorations which do not invade the supracrestal tissue attachment (Brägger et al., 1992, Bennani et al., 2017, Jepsen et al., 2018).

The dentogingival unit, also known as the biologic width, comprises of junctional epithelium and supracrestal connective tissue attachment. It has been reported that the average measurements for the junctional epithelium and the connective tissue attachment are 0.97 mm and 1.07 mm, respectively (Gargiulo et al., 1961). Therefore, feasibility of performing CLS is highly influenced by the amount of keratinised tissue and supporting alveolar bone (Schmidt et al., 2013, Gupta et al., 2015, Nobre et al., 2017).

CLS can affect not only the areas that undergo the surgical procedure but also the adjacent site as the volumetric changes of the soft and hard tissues occur during the healing period (Nobre et al., 2017). A predictable result after CLS depends on several factors such as periodontal phenotype, healing time and operators’ clinical skills (Lanning et al., 2003, Deas et al., 2004). It has been reported that patients with thin periodontal phenotypes are less resistant to trauma and surgical insults, making them more susceptible to gingival recession (Joshi et al., 2016).

Literature on CLS has reported that there are no statistically significant changes in plaque and gingival indexes or bleeding on probing during follow-ups (Brägger et al., 1992, Arora et al., 2013). Nevertheless, during the first six weeks of healing, the soft tissue margin position showed differences with the majority of the treated sites as they had apical displacement, and some gingival recessions occurred between six and 12 months (Pontoriero and Carnevale, 2001, Arora et al., 2013). Adjacent sites also reported changes, but they were not as pronounced (Pontoriero and Carnevale, 2001, Arora et al., 2013). Changes of the periodontal probing depths were found to be directly correlated with periodontal phenotype. Thicker and flatter phenotypes demonstrated greater tissues rebound than thin, scalloped phenotypes (Arora et al., 2013).

There was a large increase in crown height after CLS, which decreased over time due to inflammation as part of the healing response and then stabilised usually between three to six months (Nobre et al., 2017). The extent of tissue bound was affected by various factors such as patient’s age and sex, periodontal phenotype, tooth type, dental arch location, postsurgical flap position, amount of bone reduction and surgical technique (Lanning et al., 2003, Deas et al., 2004, Arora et al., 2013). Bone reduction measured on radiographs was typically recorded to be around 1 to 3 mm after CLS with osseous reduction (Pontoriero and Carnevale, 2001, Arora et al., 2013).

Current literature shows that healing and tissue rebound occurred within the first three months, with the suggestion that stabilisation is gained after 3 months. As there are limited systematic reviews on changes in periodontal tissues beyond three months, the purpose of this systematic review and meta-analysis was to evaluate the outcomes of CLS in terms of changes in periodontal healing parameters and periodontal tissue stability up to six months.

2. Materials and methods

This systematic review was developed following guidelines provided by the Cochrane Collaboration (Higgins et al., 2021) and Preferred Reporting Items for Systematic Reviews and meta-analyses (PRISMA) (Page et al., 2021). The eligibility criteria were defined based on the participant, intervention, comparison, outcome (PICO) framework (Richardson et al., 1995, Higgins et al., 2021). This review was registered in PROSPERO (registration number: CRD42023389496).

2.1. Types of studies

2.1.1. Inclusion criteria

Controlled clinical trials (CCT), case control and case series that reported on periodontal and radiographic changes in sites treated with CLS and adjacent sites. No language restrictions or publication status were employed.

2.1.2. Exclusion criteria

Case reports, in-vitro and animal research, studies providing insufficient data, such as not including adequate clinical measurements and time frame, and surgeries carried out in patients less than 18 years old. Duplicated studies were also excluded.

2.2. Type of participants

Participants that were > 18 years of age requiring CLS procedure.

2.3. Types of interventions

The intervention group involved any CLS procedure. This would include aesthetic or functional crown lengthening with or without osseous surgery. The comparison (control) group involved adjacent sites where CLS was not performed.

2.4. Outcome measures

2.4.1. Primary outcomes

Changes in supracrestal tissue attachment in terms of soft tissue height and stability over time following CLS.

2.4.2. Secondary outcomes

Changes in bone level (BL), coronal crown height, gingival margin, clinical attachment level (CAL) and probing pocket depths (PPD).

2.5. Search strategy

The search protocol followed standard procedures (Faggion et al., 2013, Higgins et al., 2021). The following electronic databases were searched for ongoing, unpublished and published trials up to 28 February 2022: MEDLINE, EMBASE, The Cochrane Central Register of Controlled Trials (CENTRAL), MetaRegister, ClinicalTrials.gov, and the System for Information on Grey Literature in Europe (Table 1). The search was performed independently and in duplicate by two authors (S.S. and A.T.S). A manual search of the last five years of relevant dental journals (Journal of Periodontal Research, Clinical Advances in Periodontics, International Journal of Periodontics and Restorative Dentistry, Journal of Clinical Periodontology, and Journal of Periodontology) and bibliographies of all eligible papers was also carried out for additional studies.

Table 1.

Databases and search terms.

Databases	Keywords
Published studies PubMed (1965 – February 23, 2022)	(Crown lengthening) AND (gingivectomy OR ostectomy OR osteoplasty) AND (biologic* width OR supracrestal tissue attachment OR gingival margin OR clinical attachment level OR pocket depth)
EMBASE via Ovid (1947 - February 23, 2022)	(Crown lengthening).mp. AND (gingivectomy OR ostectomy OR osteoplasty).mp. AND (biological width OR supracrestal tissue attachment OR gingival margin OR clinical attachment OR pocket depth).mp.
Cochrane Central Register of Controlled Trials (CENTRAL) via Ovid (February 23, 2022)	(Crown lengthening).mp. AND (gingivectomy or ostectomy or osteoplasty).mp. AND (biological width or supracrestal tissue attachment or gingival margin or clinical attachment or pocket depth).mp
Unpublished studies ClinicalTrials.gov (February 23, 2022)	(Crown lengthening) AND (gingivectomy OR ostectomy OR osteoplasty) AND (biologic width OR supracrestal tissue attachment OR gingival margin OR clinical attachment level OR pocket depth)

2.6. Selection of studies and data collection

The same authors independently and in duplicate examined the retrieved citations based on the title, abstract, and keywords. Irrelevant papers were excluded. Full texts of the remaining studies were obtained. An eligibility form was used to examine papers for inclusion in the review. Any disagreements were resolved by discussion to reach a consensus or by consultation with a third reviewer (M.A.). In the event of duplicate papers, the one with the most relevant and sufficient information was selected. Reasons for exclusion are reported in Fig. 1.

Fig. 1.

Flowchart of search strategy and paper selection process.

The following information was collected from the included studies: 1) Study characteristics: title, authors’ names, study location, language of publication, year of publication, published or unpublished data, source of study funding and study design. 2) Participants: demographic characteristics, inclusion/exclusion criteria, number of participants, attrition rate and reasons for dropouts. 3) Interventions: number of sites where CLS was performed. 4) Comparison: number of adjacent sites. 5) Outcomes: Changes in supracrestal tissue attachment, BL, crown length, gingival margin, CAL and PPD. 6) Length of the observation period. Any disagreements between reviewers (S.S. and A.T.S) were resolved by discussion to reach a consensus or by consultation with a third reviewer (M.A.). Corresponding authors of the included studies were contacted for additional information if required (Table 2).

Table 2.

Characteristics of included studies.

	Arora et al. 2013	Deas et al. 2004	Lanning et al. 2003	Shobha et al. 2010
Study design	CCT	CCT	CCT	CCT
Location	Postgraduate Institute of Dental Sciences, Rohtak, Haryana, India	Wilford Hall Medical Center, Lackland Air Force Base, TX, USA	Virginia Commonwealth University, Richmond, Virginia, USA	Ramaiah Dental College, Bangalore, Karnataka, India
Number evaluated (participants/teeth)	53/159	25/86	18/36	15/30
AT	??/53	25/43	18/18	15/15
TT	??/106	25/43	18/18	15/15
Age (years)
Mean	34.5	NR	39.0	NR
Range	18 to 63	NR	28 to 72	15 to 72
Smoking habits N (%)
AT	0	NR	NR	NR
TT	0	NR	NR	NR
Surgical technique	APF with osseous resection	APF with osseous resection	APF with osseous resection	APF with osseous resection
Type of teeth	Incisors, canines, premolars, molars	Posterior teeth	NR	NR
Method of assessment	Periodontal probe* (at six sites) with the aid of stent	Periodontal probe* (at six sites) with the aid of stent	Periodontal probe* (at four sites) with the aid of stent	Periodontal probe* (at four sites) with the aid of stent
Changes in supracrestal tissue attachment (mm)
Three months
AT	−0.51 ± 0.11	NR	−0.29 ± 0.09	0.40 ± 1.06
TT	−0.69 ± 0.10	NR	−0.31 ± 0.12	−0.13 ± 1.06
Six months
AT	−0.30 ± 0.12	NR	−0.15 ± 0.07	0.07 ± 0.80
TT	−0.44 ± 0.12	NR	−0.07 ± 0.09	0.07 ± 1.10

Changes in BL (mm)
Three months
AT	0.99 ± 0.10	NR	2.53 ± 0.78	1.27 ± 0.70
TT	1.17 ± 0.09	NR	3.44 ± 0.58	1.47 ± 0.92
Six months
AT	1.11 ± 0.08	NR	2.54 ± 0.76	0.87 ± 0.64
TT	1.27 ± 0.10	NR	3.50 ± 0.59	1.40 ± 0.99
Changes in crown length/free gingival margin (mm)
Three months
AT	1.51 ± 0.11	1.43 ± 0.96	2.68 ± 0.20	0.93 ± 0.70
TT	1.87 ± 0.10	1.69 ± 1.02	3.07 ± 0.61	2.33 ± 0.72
Six months
AT	1.41 ± 0.88	1.30 ± 0.96	2.82 ± 0.24	0.87 ± 0.83
TT	1.72 ± 0.80	1.57 ± 1.01	3.33 ± 0.15	2.27 ± 0.80
Changes in CAL (mm)
Three months
AT	1.08 ± 0.11	NR	3.07 ± 0.96	0.80 ± 0.77
TT	1.23 ± 0.12	NR	3.75 ± 0.85	1.60 ± 0.99
Six months
AT	1.10 ± 0.10	NR	2.92 ± 0.92	0.80 ± 0.77
TT	1.26 ± 0.12	NR	3.57 ± 0.70	1.33 ± 0.82
Changes in PPD (mm)
Three months
AT	−0.42 ± 0.11	NR	0.24 ± 0.55	−0.20 ± 0.68
TT	−0.63 ± 0.11	NR	0.67 ± 0.51	−0.93 ± 0.88
Six months
AT	−0.31 ± 0.11	NR	−0.04 ± 0.52	−0.13 ± 0.74
TT	−0.45 ± 0.12	NR	0.22 ± 0.50	−0.93 ± 0.88
Follow-up period (months)	6	6	6	6

2.7. Quality assessment of included studies

The quality of the included studies was evaluated using the Appraisal Checklist for Observational Studies scale (Joanna Briggs Institute) (Munn et al., 2020). The tool consisted of ten questions to assess different domains of the study (inclusion criteria, standardized and reliable method of measuring, valid methods for identification of all participants included, consecutive inclusion of patients, complete inclusion of patients, demographics of patients, clearly reported follow up results, reporting of present sites/clinics demographic information and appropriate statistical analysis). Each question is answered with “yes”, “no” or “unclear”. One point was given for each “yes” answer and the total score ranged between 0 and 10. The studies were categorized as (i) low quality if scored 0 to 3 points, (ii) medium quality if scored 4 to 6 points, or (iii) high quality if scored 7 to 10 points (Table 3 and Fig. 2).

Table 3.

Assessment of risk of bias of the included studies.

	Arora et al. 2013	Deas et al. 2004	Lanning et al. 2003	Shobha et al. 2010
Were there clear criteria for inclusion in the case series?	Yes	Unclear	Unclear	Yes
Was the condition measured in a standard, reliable way for all participants included in the caser series?	Yes	Yes	Yes	Yes
Were valid methods used for identification of the condition for all participants included in the case series?	Unclear	Unclear	Unclear	Unclear
Did the case series have consecutive inclusion of participants?	No	No	No	Yes
Did the case series have complete inclusion of participants?	Unclear	Unclear	Unclear	Unclear
Was there clear reporting of the demographics of the participants in the study?	No	No	No	No
Was there clear reporting of clinical information of the participants?	Yes	Yes	Yes	Yes
Were the outcomes or follow up results of cases clearly reported?	Yes	Yes	Yes	Yes
Was there clear reporting of the presenting site(s)/clinic(s) demographic information?	No	No	No	No
Was statistical analysis appropriate?	Yes	Yes	Yes	Yes
Overall risk of bias	5	4	4	6

Risk of bias assessment: 0–3 low quality; 4–6 medium quality; 7–10 high quality.

Fig. 2.

Quality assessment of the included studies presented with yes (green), unclear (yellow) and no (red) to the following questions:

2.8. Data synthesis

A statistical software program (Review Manager (RevMan) software, version 5.3, The Nordic Cochrane Centre, The Cochrane Collaboration, Copenhagen, Denmark) was used to conduct meta-analyses for studies of similar comparisons reporting the same outcome measures. For example: continuous data, such as changes in crown length, were expressed in mean difference (MD) or standardized mean difference (SMD) and 95 % confidence intervals (CIs). Random-effects model was used to pool the results from more than one study as heterogeneity between studies was expected. With fewer than ten studies, publication bias was not formally assessed because the power to detect publication bias was limited (Higgins et al., 2021). The statistical heterogeneity across different studies was assessed by means of the Cochran’s test for heterogeneity and I² statistic (Higgins et al., 2021). An I² value of > 50 indicated a substantial heterogeneity. The site was considered as the statistical unit of analysis.

3. Results

3.1. Characteristics of the study settings

Seventy-five papers were initially identified, of which 65 studies were excluded after the title and abstract screening (Fig. 1). Therefore, 10 papers met the eligibility criteria (Herrero et al., 1995, Sonick, 1997, Lanning et al., 2003, Deas et al., 2004, Shobha et al., 2010, Ganji et al., 2012, Arora et al., 2013, Ribeiro et al., 2014, Bhat et al., 2015, Aroni et al., 2019, González-Martín et al., 2020, Cortellini et al., 2021, Altayeb et al., 2022). The manual search of relevant journals and reference lists produced further three studies (Deas et al., 2004, Shobha et al., 2010, Arora et al., 2013). Of the 13 eligible studies, only four studies (Lanning et al., 2003, Deas et al., 2004, Shobha et al., 2010, Arora et al., 2013) were included in this systematic review and meta-analysis as the remaining nine studies either had insufficient data (Sonick, 1997, Ganji et al., 2012, Aroni et al., 2019), uninterpretable data (Herrero et al., 1995) or no control group (Ribeiro et al., 2014, Bhat et al., 2015, González-Martín et al., 2020, Cortellini et al., 2021, Altayeb et al., 2022). Of the four included studies, two were conducted in India (Shobha et al., 2010, Arora et al., 2013), and the others in the United States of America (Lanning et al., 2003, Deas et al., 2004). All four studies were CCTs. Three of the studies were conducted at a university setting (Lanning et al., 2003, Shobha et al., 2010, Arora et al., 2013) while the remaining study was done at an air force medical treatment facility (Deas et al., 2004). None of the four studies mentioned any funding for their studies. Three studies reported participants’ age ranges: 18 to 63 (SD 34.5) years old (Arora et al., 2013), 28 to 72 (SD 39.0) years old (Lanning et al., 2003), 15 to 72 years old with no mean age reported (Shobha et al., 2010). In total, 111 participants with 311 teeth were included in this systemic review. Of these, CLS was carried out on 182 teeth using apically positioned flaps (APF) with osseous reduction, while 129 adjacent teeth acted as control sites.

3.2. Characteristics of participants at baseline

3.2.1. Inclusion criteria of the selected studies

>18 years old (Arora et al., 2013). Aged between 15 and 72 years (Shobha et al., 2010). Plaque index (PI) and gingival index (GI) < 1 (Arora et al., 2013). No clinically significant systemic disease and not requiring antibiotic prophylaxis (Arora et al., 2013). No active orthodontic treatment (Arora et al., 2013). Absence of attachment loss or history of periodontitis (Arora et al., 2013). Absence of pathological tooth mobility or furcation involvement (Arora et al., 2013). Patients with adequate width of attached gingiva (Shobha et al., 2010). Patients requiring CLS for delayed passive eruption, retentive crown preparations, subgingival caries or crown margins or restorations, root fractures, root perforations, gummy smile, gingival margin discrepancies, and short clinical crowns with high lip line (Lanning et al., 2003, Shobha et al., 2010).

3.2.2. Exclusion criteria of the selected studies

Patients who have contraindications to periodontal surgery (Shobha et al., 2010, Arora et al., 2013). Patients with missing teeth immediately adjacent to the teeth requiring CLS (Arora et al., 2013). Pregnant and lactating females (Arora et al., 2013). Smokers (Arora et al., 2013). Grade II/III mobile teeth (Shobha et al., 2010). Periodontal pockets ≥ 4 mm (Shobha et al., 2010). Bone loss (Shobha et al., 2010). Unrestorable teeth (Shobha et al., 2010). Molars without adequate periodontal support and furcation involvement (Shobha et al., 2010). Two studies stated no exclusion criteria (Lanning et al., 2003, Deas et al., 2004).

3.3. Characteristics of the interventions

All four studies had APF with osseous resection for CLS. All four studies recorded baseline preoperative clinical measurements and follow-up measurements at three and six months (Lanning et al., 2003, Deas et al., 2004, Shobha et al., 2010, Arora et al., 2013).

3.4. Characteristics of outcome measures

Changes in supracrestal tissue attachment: Three studies (Lanning et al., 2003, Shobha et al., 2010, Arora et al., 2013) reported changes in supracrestal tissue attachment with two follow-up time points (3 and 6 months) following the CLS. The supracrestal tissue attachment levels did not change significantly during this period (three months: MD −0.12; 95 % CI −0.27 to 0.03, P = 0.13, Fig. 3a(i); six months: MD −0.03; 95 % CI −0.23 to 0.18; P = 0.78; Fig. 3a(ii). Substantial heterogeneity was detected at 3- (Chi2 = 17.42, df = 2 (P = 0.0002); I2 = 89 %) and six-month analysis (Chi2 = 42.80, df = 2 (P < 0.0001); I2 = 95 %).

Fig. 3a.

Comparison: crown lengthening: treated teeth versus adjacent teeth Primary outcome: (i) changes in supracrestal tissue attachment at three months. (ii) changes in supracrestal tissue attachment at six months. SE: standard error; IV: inverse variance; CI: confidence interval; τ: Kendall tau; z: z test.

3.4.1. Changes in bone level

Three studies (Lanning et al., 2003, Shobha et al., 2010, Arora et al., 2013) reported on changes in bone levels; however, the meta-analysis did not show any statistically significant differences between treated and adjacent teeth at three (MD 0.41; 95 % CI −0.06 to 0.88; P = 0.09; Fig. 3b(i) and six months (MD 0.52; 95 % CI −0.03 to 1.07; P = 0.07; Fig. 3b(ii). Substantial heterogeneity was detected at three (Chi2 = 10.10, df = 2(P = 0.006); I2 = 80 %) and six months (Chi2 = 13.84, df = 2 (P = 0.001); I2 = 86 %).

Fig. 3b.

Comparison: crown lengthening: treated teeth versus adjacent teeth Secondary outcomes: (i) changes in bone level at three months. (ii) changes in bone level at six months. (iii) changes in crown length/free gingival margin at three months. (iv) changes in crown length/free gingival margin at six months. (v) changes in CAL at three months. (vi) changes in CAL at six months. (vii) changes in PPD at three months. (viii) changes in PPD at six months. SE: standard error; IV: inverse variance; CI: confidence interval; τ: Kendall tau; z: z test.

3.4.2. Changes in crown length/free gingival margin

All four studies (Lanning et al., 2003, Deas et al., 2004, Shobha et al., 2010, Arora et al., 2013) reported on changes in crown lengths/free gingival margins with statistically significant differences were observed at three (MD 0.54; 95 % CI 0.21 to 0.87; P = 0.001; Fig. 3b(iii) and six months (MD 0.54; 95 % CI 0.23 to 0.85; P = 0.0006; Fig. 3b(iv). Substantial heterogeneity was noticed in both analyses.

3.4.3. Changes in clinical attachment level

Three studies (Lanning et al., 2003, Shobha et al., 2010, Arora et al., 2013) reported on changes in CAL but no statistically significant difference was observed at three months (MD 0.46; 95 % CI −0.01 to 0.94; P = 0.06; Fig. 3b(v), while a statistically significant difference was observed at six months (MD 0.36; 95 % CI 0.01 to 0.70; P = 0.04; Fig. 3a and Fig. 3b(vi). Moderate to substantial heterogeneity was noticed in both analyses.

3.4.4. Changes in probing pocket depths

The changes in PPD were reported in three studies (Lanning et al., 2003, Shobha et al., 2010, Arora et al., 2013) at two time points; however, the differences between treated and adjacent teeth were not statistically significant at three (MD −0.13; 95 % CI −0.64 to 0.37; P = 0.60; Fig. 3b(vii) and six months (MD −0.15; 95 % CI −0.55 to 0.24; P = 0.44; Fig. 3b(viii). Substantial heterogeneity was noticed in both analyses.

3.5. Risk of bias

All included studies scored medium on all domains. Overall risk of bias judgement was considered “medium to high” due to issues related to measurement of outcomes and influence of confounding factors (Table 3). The JBI critical appraisal checklist for case series for systematic reviews was used to complete the bias risk assessment (Munn et al., 2020). All four studies were judged to be at medium risk of bias; however, they all provided standardized measurement in a reliable way for all participants, had clear clinical information on participants including their follow-up results, and applied appropriate statistical analysis. Two of the studies (Lanning et al., 2003, Deas et al., 2004) had unclear inclusion (Fig. 2), and no study had the funding information which may have been another source of potential bias.

4. Discussion

There are only a few systematic reviews on the biological impact on treated and adjacent teeth at three and six months post-CLS. The present systematic review found only four studies comparing teeth that underwent CLS to adjacent sites. They included measurements at baseline and at three and six months post-operatively (Lanning et al., 2003, Deas et al., 2004, Shobha et al., 2010, Arora et al., 2013). These studies were controlled clinical trials and used the APF surgical technique with osseous reduction. Therefore, we aimed to compare the changes in the clinical parameters of supracrestal tissue attachment, BLs, crown height/free gingival margin, CAL, PPD, between treated and adjacent sites at three and six months.

This systematic review and meta-analysis did not show any statistically significant differences between treated and adjacent sites in terms of supracrestal tissue attachment, BL and PPD at both three and six months. There was substantial heterogeneity detected in supracrestal tissue attachment and BLs, and moderate to substantial heterogeneity in the PPD changes. Statistically significant changes occurred with treated teeth in crown height/free gingival margin in comparison to the adjacent teeth at both time points. This finding is expected in periodontal procedures such as CLS, as increased clinical crown height is the surgical aim. The changes in CALs between treated and adjacent teeth were comparable at three months whereas the changes at six months were less for adjacent sites.

Greater changes were observed with treated sites compared to adjacent site in all clinical parameters. This systematic review found that the supracrestal tissue attachment level increased significantly at the treated sites between three and six months after CLS (Lanning et al., 2003, Shobha et al., 2010). Gain of supracrestal tissue attachment was also observed at adjacent sites, in line with another review which suggested coronal proliferation of supracrestal tissues during the healing phase (Gupta et al., 2015).

There were also greater changes in bone levels at the treated sites compared to the adjacent sites; however, the bone levels remained stable between three and six months. Another earlier systematic review has also shown that both treated and adjacent sites experience changes in bone levels (Nobre et al., 2017). In the current systematic review, there were different levels of bone removal. Two studies removed less than 2 mm of alveolar bone (Shobha et al., 2010, Arora et al., 2013), while another study removed slightly more (Lanning et al., 2003). While it is unclear in terms of reasons for this difference due to the lack of sufficient information on the inclusion criteria of these studies, it is likely that decisions were made according to the individual needs. It is important that sufficient osseous reduction is carried out as unnecessary soft tissue rebound can occur and thus not meeting the clinical requirement (Shobha et al., 2010, Majzoub et al., 2014, Nobre et al., 2017)(Deas et al., 2004). This current systematic review also found that there was a decrease in PPD after CLS while an increase in CALs at both treated and adjacent sites were observed. However, the CALs were relatively similar between three and six months, suggesting that the tissue stability was already gained in the earlier stage of healing.

4.1. Heterogeneity

In this systematic review, only CCT fulfilled the selection criteria. All five of the clinical parameters assessed showed moderate to substantial heterogeneity. Sources of heterogeneity could be related to differences in adjacent site, treated sites, osseous reduction, amount of crown lengthening required and clinicians’ surgical skill.

4.2. Limitations

Due to the strict inclusion criteria, there were only four studies included in the current systematic review and all studies used APF with osseous reduction (Lanning et al., 2003, Deas et al., 2004, Shobha et al., 2010, Arora et al., 2013). Therefore, the generalizability of findings may be limited when applied to other CLS techniques. All four studies showed a moderate risk of bias. This systematic review also included both anterior and posterior sites for analysis, which can differ considerably in its clinical management, and therefore the results should be interpreted with some caution.

5. Conclusion

Within the limitations of this systematic review, CLS using APF with osseous reduction offers periodontal tissue stability over time. No significant changes were observed in supracrestal tissue attachment, bone level, and periodontal probing depth at three and six months between treated and adjacent sites, while crown height was significantly higher in treated sites at both time points. Clinical attachment level was not significantly different between treated and adjacent sites at three months but was significantly lower for treated sites at six months. Further evidence is still required to substantiate the current findings.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.