The clinical effects of laser preparation of tooth surfaces for fissure sealants placement: a systematic review and meta-analysis

This systematic review was reported in accordance with recommendations of the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) statement [19]. The research aimed to systematically retrieve and analyze clinical studies assessing the effects of laser preparation compared to other tooth surface pretreatment methods used in fissure sealant placement. The PICO principle, detailed below, was applied during the assessment.

Participants: patients who were caries-free and had untreated premolars and/or molars and/or primary molars suitable for pit-and fissure-sealing.

Interventions: use of lasers as a pretreatment method for pit-and-fissure sealing.

Comparators: use of any other mechanical or chemical preparation for pit-and-fissure sealing, such as acid-etching, enameloplasty or air abrasion.

Outcomes: the retention rate of fissure sealants, the incidence of dental caries, adverse events, clinical time, patient acceptability and anxiety.

The inclusion criteria were: 1) randomized clinical trials; 2) studies using laser and the other pretreatment methods, including phosphoric acid and/or enameloplasty and/or air abrasion to roughen the enamel before sealant application; 3) studies reporting details of the materials and methods, such as tooth preparation designs, isolation methods, materials of the sealants and parameters of the lasers; 5) studies with quantitative clinical outcomes, such as retention rates or the incidence of caries. The exclusion criteria were 1) in vitro or animal studies, case reports, and case series, review articles, or opinion articles; and 2) studies published in other languages besides English.

The following databases were searched from inception to January 2019: Pubmed, Scopus, Medline/EMBASE via OVID and the Cochrane library. The following search terms are used in Pubmed: (“lasers” [MeSH Terms] OR “lasers” [All Fields] OR “laser” [All Fields]) AND “pit” [All Fields] AND (“pit and fissure sealants” [MeSH Terms] OR (“pit” [All Fields] AND “fissure” [All Fields] AND “sealants” [All Fields]) OR “pit and fissure sealants” [All Fields] OR (“fissure” [All Fields] AND “sealant” [All Fields]) OR “fissure sealant” [All Fields]).

ClinicalTrials.​gov (www.​clinicaltrials.​gov) and the International Standard Randomised Controlled Trial Number (ISRCTN) registry (www.​isrctn.​com) were screened to identify unpublished and ongoing trials. Grey literature was also investigated through OpenGrey. In addition, the reference lists of the included studies were manually searched to find any potentially eligible study.

The titles, abstracts and full-texts of potentially relevant studies were read and assessed by two authors (Y.H.Z and Y.W) to judge whether they met the inclusion and exclusion criteria. Unrelated studies were initially excluded based on the titles and abstracts and any conflict was solved by a third author (J.Z).

The characteristics and numerical data in the included studies were extracted independently by two authors (YHZ and YW) using a prepared data collection form. Any disagreement was solved via discussion or by a third author (JZ). The data obtained included follow-up duration, the number of patients, quantity of the sealants, patient age range, drop-out rate of the subjects, the preparation protocol, the laser parameters, the type of sealant, tooth location, retention rate, the incidence of caries, patient acceptance, time consumption and dental anxiety. Retention in this systematic review was defined as completely retained [20].

The risk of bias in the RCTs was assessed according to the Cochrane Handbook for Systematic Reviews of Interventions [21]. Two review authors (YHZ and YW) independently assessed and scored the studies to identify any potential sources of systematic bias. The related risk of bias for each domain was rated at three levels: low risk, high risk, or unclear risk. The comprehensive methodological quality of a study was classified as low risk of bias (six domains assessed as low risk), moderate risk of bias (one or more domains assessed as unclear risk) or high risk of bias (one or more domains assessed as high risk). Disagreements between the authors were resolved through discussion and, if needed, by consultation with another author. Additional risk of bias tables for the included studies are presented.

Statistical heterogeneity was assessed using a Chi-squared test and the Higgins index (I²). Heterogeneity was considered statistically significant for P < 0.1. Meta-analyses were performed when there was little clinical heterogeneity. A random-effects model was used for I² > 50% and a fixed-effect model was used for I² ≤ 50%. Odds ratios (OR) and their corresponding 95% confidence intervals (CI) were calculated. The statistical significance of the hypothesis test was set at a α = 0.05 (two-tailed z tests) All analyses were performed using Revman 5.3 software. The data were summarized qualitatively when a meta-analysis could not be performed.

The search strategy is outlined by a flow diagram and presented in Fig. 1. Initially, 442 published studies were identified through the electronic search and one additional record from other sources. After the removal of duplicates, 224 studies remained. Another 218 articles were removed after analysis of the titles and abstracts, leaving six studies. After evaluating the full texts of the articles in detail, five studies remained eligible for this systematic review. Finally, three studies were included in the meta-analysis.

There were no ongoing studies or grey literature. There was a completed study “The effect of Er:YAG laser on clinical success of a hydrophilic fissure sealant” (NCT03718689) that met the inclusion criteria of this systematic review, retrieved from ClinicalTrials.​gov. We contacted the study director for further information but she refused to supply data. After the electronic searches, the references of the included studies were hand-searched, however, no further applicable studies were found.

Study characteristics and results of the included studies are summarized and presented in Table 1. The included studies were conducted in four countries in subject numbers ranging from 16 to 64. The participants’ ages ranged from six to 38 years old. Follow-up duration ranged from 12 to 36 months, except for one study without a follow-up [26]. Two studies reported drop-out rates according to the follow-up periods [23, 25]. One reported a 17.65% drop-out rate [22] and two reported 0% [24, 26]. Three studies assessed the clinical performance of fissure sealants placed by acid or laser etching and one compared acid etching versus laser combined with acid etching. Four of the studies assessed retention rates [22‐25], two reported the incidence of caries [22, 24] and one evaluated patient acceptance [23]. The study by Shindova et al. evaluated heart rate, oxygen saturation and the subject’s degree of dental anxiety before and after the dental visit [26].

In terms of the methodological characteristics (Table 1), three of the included studies adopted a split-mouth design [22, 23, 25]. All of them used acid-etching as the only comparator. Out of the five studies, two used erbium, chromium: yttrium-scandium-galium-garnet (Er, Cr: YSGG) lasers alone in the intervention group [23, 24], one used carbon dioxide laser [25] and two studies used Er: YAG laser combined with acid etching [22]. Four studies isolated teeth from the oral environment with cotton rolls [22, 24‐26] and the remaining one used a rubber dam [23]. Two studies performed pit-and-fissure sealants on the first permanent molars [22, 23], two were performed on the permanent premolars and molars [24, 25] and one was performed on any intact teeth without caries on the occlusal surface [26].

The laser parameters of the included studies are presented in Table 2. One study used a carbon dioxide laser [25], two studies used Er: YAG [22, 26] and two used Er, Cr: YSGG [23, 24]. The power of the carbon dioxide laser was 5 W and that of the erbium lasers ranged from 0.7 W to 2.0 W. Two studies reported that the exposure time depended on the time needed to guide the laser beam evenly across the pits and fissures to be irradiated [22, 24], one did not report exposure time [26] and the exposure time in the remaining study ranged from 7 to 10 s [23, 25]. One study reported that the energy density was 67 J/cm² [26] and one reported a power density of 530.5 W/cm² [23]. The laser application methods were similar, presenting small differences in the tip-to-tissue distance, tip diameters and angles. The carbon dioxide system used by Walsh was not water-cooled [25], while the erbium laser systems used by other researchers were equipped with water-cooled systems [22‐24, 26].

The risk of bias for the included studies is summarized in Fig. 2 and given in detail in Fig. 3. All the studies had an overall high risk of bias. Because of the enormous difference in instruments and usage, all the studies included presented inevitable high risk in blinding of the intervention operators. Four studies used single-blind designs [22‐25], and the remaining study evaluated the patients’ subjective outcomes and was assessed as high risk in detection bias [26]. Only Karaman reported the use of a random number table to generate random sequences, while the other studies did not refer to allocation concealment [24]. The included studies were considered to be low risk of bias in selective reporting. Four studies showed low risk in attrition bias [23‐26] and the study by Dumurs did not explain the unavailable data [22]. Two studies showed low risk of other bias [24, 25] and the remaining were unclear (Figs. 2 and 3).

A few methodological differences are worth noting. First, regarding the outcomes of the present studies, split-mouth design, in which the effects of inter-subject variation can be minimized when the individual is self-matched or self-controlled, is more suitable than parallel design for clinical trials conducted in the oral cavity [27, 28].

Caries risk may influence the retention rate and the incidence of caries. In high-risk children, the enamel of the occlusal pit and fissures has already undergone tissue structural changes that might have altered the properties of the enamel surface before sealant application. It may also contain higher proportions of organic material that might hinder the acid penetration into the deeper layers and prevent homogeneous etching of the enamel, hampering resin impregnation and resulting in shorter resin tags [29]. According to Oulis et al. [30], children with a high baseline caries risk showed lower sealant retention rates and higher occlusal caries prevalence following fissure sealants loss compared to those of moderate and low-risk status. The concept of risk-based sealant application can form the basis of a rationale for and effects of sealant placement and specific considerations like tooth morphology, caries history, fluoride history and oral hygiene can be assessed by an experienced clinician for the indication of sealant placement [31, 32]. In the included studies, the study by Durmus et al. was conducted in subjects with high caries risk [22], while patients in the other studies had low caries risk. All individual caries risk was determined at the initial visit with simple indicators and was not reported at the follow-up visits, so changes and the effect of any changes were unclear. Under these circumstances, split-mouth designs may be recommended. Karaman [24] used a parallel design, while others adopted a split-mouth design. In addition, the study by Shindova et al. adopted a parallel design for the subjective parameters of dental anxiety measurement which did not eliminate the effect of individual variation on the surveyed outcomes, affecting the accuracy of the conclusion.

Second is the choice of tooth location. According to a systematic review and meta-analysis from Papageorgiou and previous studies, premolars had better sealant retention rates than that of the first permanent molars. The number of sealants, the complexity of the access to the tooth surface and isolation and variations in the morphology and microscopic structure of the enamel may contribute to these outcomes [33]. In half of the studies that recorded retention rates, sealants were placed on the first permanent molars [22, 23], while the others were placed on permanent premolars and molars [24, 25]. Karaman [24] found no statistical differences between the retention rates of the premolars and molars, while Walsh [25] showed that all the sealants lost were on molars. Location of the loss may be a factor affecting retention rates.

Third is the single evaluation criterion for patient acceptance. Patient acceptance and dental anxiety are subjective perceptions. Thus, rating scales were formulated to quantify their perceptions. Kumur et al. used a visual analogue scale in which pediatric patients were asked to mark their discomfort on a 100 mm line with extreme ends denoting no discomfort to worst possible discomfort during the procedure. According to Naegeli et al. [34], verbal rating scales, visual analogue scales, numeric rating scales, and graphical scales can all be reliable and valid response options in pediatric populations. However, the current empirical basis was insufficient to draw firm conclusions and to make differentiated recommendations. In children between the ages of six and 18, these instruments showed various outcomes. Therefore, diversified evaluations related to subjective parameters should be considered.

Lastly, the lack of comparison between lasers and other preparation methods, the comparison of different kinds of lasers to other chemical or mechanical preparation methods, such as enameloplasty and air-polishing systems, and research on primary molars was a shortcoming of this review. Enameloplasty with burs (fissurotomy shallow taper fissure (STF) and narrow taper fissure (NTF) burs, small one-quarter round burs) conducted with a high- or slow-speed handpiece, has been demonstrated in several in vivo and in vitro studies as a simple, cost-effective and easily accessible method resulting in acceptable sealant retention and reduced microleakage [35‐41]. The air abrasion system uses a high-speed stream of purified aluminum oxide particles or bioactive glass delivered by air pressure and has been reported to be an effective method to prepare enamel before the application of the sealant in vitro, which needs more research to confirm its clinical effects [42‐44].

In our study, we regarded lasers as a whole to analyze the effect of this kind of intervention. As Karaman speculated, different outputs and experimental designs might result in conflicting findings [24]. Subgroup analysis could not be performed, which means that the effects of laser parameters on outcomes are still unclear.

Therefore, the choice of laser parameters in clinical studies has usually referred to those used in previous in vitro studies. For example, Walsh et al. [25] stated that they based the carbon dioxide laser dosimetry on laboratory studies of enamel surface changes and bond strength, as well as on data for pulpal safety. These kinds of choices might also affect the clinical effects of laser preparation. The study by Dumurs et al. [22] used Er: YAG laser at a wavelength of 2.94 μm, 2 W power, 120 mJ energy output and 10 Hz frequency. Mahtab et al. [45] found no difference in the proportion of microleakage between conventional acid etching and Er: YAG laser surface pretreatment before sealant resin was applied to teeth with fluorosis, where the parameters were 30 mJ, 20 Hz, 6 W initially and 120 mJ, 10 Hz, and 1 W on the enamel margins. Unal et al. [46] evaluated the Er:YAG laser as an alternative to acid etching for the application of fissure sealants in pediatric dentistry and found that the Er: YAG laser with 4 W power and 200 mJ energy output resulted in higher microtensile bond strength than the 2 W, 100 mJ Er: YAG laser in permanent teeth. However, in a study of primary teeth, Unal et al. [47] reported that higher microleakage values were found in groups with Er: YAG laser etching (3.25 W, 150 mJ, 25 Hz and 5 W and 20 mJ, 25 Hz) compared to conventional acid-etching. Microleakage in the 3.25 W group and the 5 W group was not significantly different. Kumar et al. [23] used a lower output (1.5 W) to etch enamel, per Berks and colleagues, and observed that Er, Cr: YSGG laser irradiations using 1 W, 1.5 W or 2 W produced etching patterns similar to those produced by etching with acid in the scanning electron microscope (SEM) study. While Vijayan et al. [48] found that the enamel surfaces etched by Er, Cr: YSGG laser at 1.5 W/20 Hz, 2 W/10 Hz and 2 W/20 Hz, 2 W/20 Hz provided the best results with consistency in tag lengths and other studies found that it provided adequate bond strength. Moreover, in vitro studies have shown that the same parameters could result in different outcomes and different parameters could result in similar outcomes. So, while in vitro studies can predict clinical outcomes, real-life performance should be evaluated by clinical studies. More elaborately designed laser dosimetry studies should be conducted in future research.

It should be noted that the use of laser preparation involves some difficulties for the operator. Clinicians must use the correct angle and tip during preparation. A smaller tip may be useful to reduce errors but are associated with problems, such as the need to ensure an equal level of ablation at the same speed, which requires more time and energy compared to larger tips [14]. Thus, theoretical parameters do not always represent actual emissions that impact clinical effects in patients.

One of our included studies evaluated the clinical effects of Er:YAG laser as an adjunct to acid preparation, showing that it was superior to acid preparation at the end of 18 months. The retention rate for fissure sealants in the laser plus acid group was significantly higher than that in the acid-etched group at 12 (P = 0.0161) and 18 (P = 0.0227) months. These results agreed with an in vivo study in which Brugnera and colleagues [49] demonstrated that the retention rate of sealants applied after carbon dioxide laser plus acid-etching was significantly better than that of acid-etching preparation. However, the latter study was excluded from this review because it did not describe the randomization process. In the Durmus study, the incidence of caries in the acid-etched group was 22% (n = 18) versus 10% (n = 8) in the laser plus acid group at 18 months. The difference in caries development between the groups was not significant (P > 0.05). In vitro studies showed no statistical difference between the acid and laser combined method and the acid enamel conditioning method [18, 50]. From this perspective, laser combined with acid preparation was probably superior to acid preparation. More high-quality studies should be conducted to confirm this conclusion.

The economic aspect of sealant application should also be considered. Sealant application has to remain simple, rapid and affordable to be used as a prophylactic measure. Although laser combined with acid etching improved the retention of sealants compared to acid etching alone, the cost-benefit ratio of the extra time and cost of the equipment and materials required may not add value [14].

Considering the limited evidence in the present studies, more clinical research is needed to confirm the role of laser preparation in pit-and-fissure sealants in the future. They should be of high methodological quality, design and conduct. Comparison of retention rates, and the incidence of caries, as well as consideration of caries risk, the economic impact and the appropriate choice of study subjects and their psychological evaluation, between laser preparation and other chemical or mechanical preparation methods may further our understanding of the role of lasers as an enamel conditioning procedure before sealant placement.