4.1. Quality of the Studies
The quality of the included studies was presented in Figure 2. All of the studies were considered to have moderate risk of bias. Unclear information about ‘random sequence generation’, ‘blinding of personnel’ and ‘allocation concealment’ accounted for the main risk factors. Although all studies were conducted randomly, only 1 study mentioned the randomization method adopted in the experiment. Computer programs, random tables or other randomization way are neccessray to balance the assignment to experimental and control groups. Besides, the importance of allocation concealment and blinding techniques must be emphasized. Improper blinding and allocation concealment may exaggerate the effect of experimental treatments, causing bias. The quality of the included studies is insufficient so that it may reduce the reliability of conclusion drawn in our study.
4.2. The effect of Er,Cr:YSGG laser on microleakage of restorations
With the advance of dental materials, composite resin has replaced amalgam restoration gradually. While polymerization shrinkage, the major defect of composite materials, can cause a marginal gap at tooth-restoration interface, which affects the long-term success of restorations. The phenomenon that bacteria, liquids, molecules or ions pass through the marginal gaps, also known as microleakage, is often regarded as the most important factor resulting in secondary caries and pulpal infection[8, 20]. From this viewpoint, many suggestions have been proposed to reduce microleakage such as the use of low shrinking resins and adequate preparation of the tooth. Currently, the most common way for cavity preparation is still the use of burs. While the generated heat and pressure may cause pain and pulpal damage during cavity preparation, which always trouble doctors and patients. For years, numberous new techniques have been developed as alternatives to traditional bur, amongst which the application of erbium family lasers have obtained wider and wider acceptance. Owing to its specific mechanism, Er,Cr:YSGG laser can cut enamel and dentine effectively, while produce less vibration, sharp noise and pain for patients during cavity preparation. Also, negative thermal effect on pulp and surrounding tissue can be prevented effectively with the use of water mist spray. Some investigations have indicated that the Er,Cr:YSGG preparations benefited the bonding process of adhesives and reduced the microleakage[8, 23]. However, there are still disputes about the promising efficacy of Er,Cr:YSGG preparations on microleakage. Some studies found there was no difference between cavities prepared by bur and Er,Cr:YSGG[5, 15, 24]. And some even found Er,Cr:YSGG had an higher microleakage scores. Up to now, the results have not been evaluated systematically. Thus, we aimed to figure out whether the Er,Cr:YSGG laser can reduce the microleakage of restorations or not, when used to prepare cavities.
Generally, microleakage test can be conducted in vitro and in vivo, but in vitro studies are more precise and easier to employ. Several measurments have been widely used to evaluate microleakage including dye penetration test, scanning electron microscope, chemical tracers, air pressure and neutron activation. Among them, dye penetration is the most common way for easy conduction, low price and safety. Thus, we only included in vitro studies using dye penetration for microleakage evaluation in this systematic review.
As shown in our results, it had no significant difference between Er,Cr:YSGG and bur preparations on microleakage. With high absorbance in water and hydroxyapatite, the Er,Cr:YSGG laser beam can heat the water content of dental hard tissue and cause a microeplosion in water particles. The process of laser ablation produces an irregular, rough and moist dental surface in appearance with exposed dentinal tubles, intact enamel rods and no smear layer. Considering these microscopic changes, some studies declared that Er,Cr:YSGG preparations may be more suitable for adhesion to restorations. However, there were some studies indicating that the enamel melting, minimal cracking and acid-resistant surface created by Er,Cr:YSGG irradiation during preparation may have negative effects on the bonding process of adhesives, especially for the ultra-mild self-etch adhesives[27-29]. Additionally, the ablation of dentine can fuse the collagen fibrils and reduce interfibrillar spaces, thus limiting the penetration of adhesives and resins. Based on previous fingdings, we would hazard a guess that the two opposite effects may just cancel each other out and finally present a similar microleakage trend with burs. On clinical conditions, bonding to dentine is more challenging because of the higher water and orangic matter. However, as shown in our study, no significant difference was observed between the cavities prepared by lasers and those by burs in terms of microleakage rate, either on enamel or dentine margins. It should be noted that there is clearly a huge heterogeneity among included studies and our conclusions should be taken cautiously. This heterogeneity can be explained by the variability in tooth type, cavity type, restoration materials, adhesives and the irradiation parameters. For example, some studies used conventional methacrylate-based microhybrid composite for restorations, while some used low-shrinking composite resin systems, which may interfere with results and account for the inconsistent conclusions[4, 9].
4.3. The effect of acid etching on microleakage after laser preparations
Dental adhesives can be classified into 2 categories based on the way they act with smear layer. Etch-and-rinse adhesives need a prior acid etching, while self-etch adhesives contain acidic functional monomers which can remove the smear layer selectively. In recent years, there has been a growing debate on the use of acid etching after laser preparations. Previous studies observed that the prior acid etching improved the bond strength of self-etch and etch-and-rinse adhesives following laser preparation[32, 33]. Whereas some authors found acid etching did not affect the adhesions in laser-prepared cavities, even weaken the bonding effectiveness of self-etch adhesives[34, 35]. Based on these studies, acid etching may reduce the hydroxyapatite content of dental tissue, thereby weakening the chemical bonding of adhesives[12, 36]. In our study, self-etch adhesives in combination with prior acid etching show less microleakage rate than those without acid etching on enamel and dentine margins, but the result was insignificant for etch-and-rinse adhesives. The following explanations may account for this result. According to many researches, lack of water in dental tissue after laser preparation may affect the etching capacity of self-etching adhesives, thus limiting its penetration through laser-modified layer. While additional acid etching can remove this layer, increase surface wettability and benefit the hybrid layer formation, all of which are helpful in reducing microleakage in laser-prepared cavities. As reported by Obeidi et al., the etching time should be prolonged to 40 seconds but not to 60 seconds to improve the bond strength of self-etch adhesives in Er,Cr:YSGG-prepared tooth.
4.3. Limitations of this study
This systematic review and meta-analysis have some limitations. Firstly, we did not fully consider the effect of different restoration materials on microleakage. With inconsistent shrinkage rates, different resin-based systems may interfere with the results of further microleakage evaluation and cause bias.
Secondly, selecting an effective adhesive is also important to restorations. The applied adhesive systems varied a lot among studies including different self-etch and etch-and-rinse adhesives. Some authors revealed self-etch and etch-and-rinse adhesives had similar microleakage values in Er,Cr:YSGG laser preparations. While the impact of different adhesives on microleakage need to be demonstrated further. Moreover, the difference of tooth type, cavity type and irradiation parameters may also have an influence on microleakage. Although some studies found the morphology of primary teeth prepared by Er:YAG laser was similar to that of permanent teeth, the primary teeth often had a higher degree of mineralization, which may result in different reaction to laser. The efficacy of laser depends on the its parameters such as spot size, emission model and pulse duration. But the laser parameters applied in experiments were different and some studies even lacked a detailed description of the laser imformation.
4.4. Directions for future research
Based on the limitations mentioned above, the following suggestions were proposed to indicate directions for future research. Firstly, more RCTs should be strictly conducted according to the Cochrane’s risk of bias criteria. Secondly, researchers should report more information of the laser parameters applied in the study, and it is worthy to explore the best parameters of Er,Cr:YSGG laser for cavity preparation. Thirdly, further investigations should focus on the long-term effect of laser preparations on restorations.