INTRODUCTION

Minimal invasive dentistry has gained popularity with the development of new adhesive systems and technological improvements in cutting tools for tooth preparation. The concept of “extension of prevention” has, as much as possible, left its mark on the preservation of sound tooth structure. Moreover, adhesive dental materials make it possible to conserve tooth structure using minimally invasive cavity preparations by minimizing the requirement for retention and resistance form. The preservation of original tissue is known to enhance the prognosis of the tooth. This new concept has become the guiding factor in cavity preparation.1 Therefore, alternative methods, such as air abrasion, sono-abrasion and lasers, have been suggested for cavity preparation.

Lasers have been used in dentistry for more than 20 years. However, the use of Erbium lasers has recently received a great deal of attention, with the trend moving towards minimum intervention dentistry, as it offers an alternative to conventional bur preparation with minimal tissue loss.2–4 The Erbium: Yttrium-Aluminum Garnet (Erbium:YAG) and Erbium Chromium:Yttrium-Scandium-Gallium-Garnet (Er, Cr:YSGG) lasers present several advantages. These lasers emit energy in the wavelength of 2.94 μm and 2.78 μm, respectively, which coincides with the absorption peak of water and is well absorbed by all biological tissues, including enamel and dentin.5 Once light from the laser is absorbed, it is converted to heat. The overheated water vaporizes and causes micro-explosions that carry away surrounding tooth fragments.6 Thereby, they have several hard tissue applications, such as enamel and dentin etching, caries removal and cavity preparation.27–11

One aspect of patient discomfort frequently noted during treatment with rotary instruments is bone-conducted noise and vibration.12 Local anesthetic is frequently required, which is another aspect of dental treatment that renders patients particularly anxious.13 These factors are eliminated and local anesthesia is rarely needed during laser treatment,14 making the procedure more comfortable for patients.15

The morphology and nature of the prepared tooth surface influences bonding of the adhesive restorative materials that are used to restore the tooth; therefore, prolonging the clinical longevity of restorations. It has been reported that laser irradiated surfaces show a rough, clean surface without the smear layer, with exposed enamel rods and opened dentin tubules.16–17 Therefore, it is reasonable to suppose that these morphologic patterns might be more favorable for the adhesion of restorative materials, far more than bur-prepared cavities that are covered by a smear layer.18–19 Smear layer removal or modification is essential for the formation of a high quality hybrid layer and, thus, optimal adhesion to dentin.20 Although a variety of in vitro studies of the effects of lasers have been reported, the marginal adaptation and bond strength of laser-prepared tooth structure is still a controversial issue in the literature. Several authors have reported lower bonding effectiveness for laser-treated surfaces;21–24 whereas, other authors found similar results compared to bur-cut surfaces.25–26 On the other hand, laboratory results can never completely simulate the complex oral environment. To validate in vitro studies, clinical studies need to be performed. To the best knowledge of the authors of the current study, no clinical study has been conducted that compares the longevity of restorations placed in laser and bur-prepared cavities.

The current study compared the clinical performance of bur- and laser-prepared minimally invasive Class I occlusal resin composite restorations.

METHODS AND MATERIALS

Restorative Procedures

The teeth to be restored were first cleaned with a slurry of plain pumice and water to remove the salivary pellicle and any remaining dental plaque. Isolation was accomplished using cotton rolls. The teeth were randomly assigned to cavity preparation techniques: group I–bur and group II–laser. Each patient received at least two cavities: one was prepared by bur and the other by laser. Distribution of the cavity preparation techniques per tooth was done using a table of random numbers.

Group I: Class I occlusal cavities were prepared using a high-speed diamond bur (835/010-4ML, Diatech Dental AG, Heerbrugg, Switzerland) under constant water cooling.

Group II: Class I occlusal cavities were prepared using an Er, Cr:YSGG laser (Waterlase MD, Biolase World Headquarters, Irvine, CA, USA) emitting photons at a wavelength of 2780 nm. Laser energy was delivered through a sapphire tip, 600 μm in diameter and 6-mm long, bathed in an adjustable air and water vapor. The angulation of the tip was perpendicular to the enamel prisms. The enamel laser irradiation was performed using a focus mode to remove enamel at a setting of 5.5 W, 20 Hz (275mj) with 70% water and 80% air. The pulse duration was 140 μ seconds. The setting was reduced to 4 W, 20 Hz on dentin with 70% water and 80% air.

The buccolingual width of each preparation was not greater than one-third the distance between the cusp tips as measured with a periodontal probe. No additional “extension for prevention” and no visible preparation of undercuts was performed after the lesions were completely excavated, if needed. Visual and tactile feedback from an explorer was used to determine the end of caries removal. The enamel was etched for 30 seconds and the dentin for 15 seconds with a 34.5% phosphoric acid etchant (Vocovid, Voco GmbH, Cuxhaven, Germany), then rinsed for 20 seconds with an air-water spray and dried, leaving the dentin slightly moist. The adhesive system, Solobond M (Voco GmbH), was applied to prepared cavities according to the manufacturer's directions. After adhesive curing, all cavities were restored using a nanofilled flowable resin composite, Grandio Flow (Voco GmbH), which was placed in one increment (Table 1). The resin was light-cured for 40 seconds using a halogen light curing unit (Hilux, Benlioglu, Ankara, Turkey) with an output not less than 550mW/cm². No liners or bases were placed. After checking occlusion, finishing was accomplished using contouring and finishing diamond burs (Diatech, Switzerland) at high speed, and polishing was done with polishing discs (Sof-Lex, 3M ESPE, St Paul, MN, USA) and rubber points at low speed under constant water cooling. All the procedures were performed without local anesthesia. All the restorations were done by the same operator (JG) with cotton roll isolation and a chairside assistant.

Table 1 Material, Batch, Composition and Mode of Application of Materials

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At least two restorations were placed in each patient, resulting in a total of 108 restorations. Of the 54 bur-prepared restorations, 31 were placed in upper molars and 23 in lower molars. In laser-prepared cavities, 25 restorations were placed in upper and 29 in lower molars.

Clinical Evaluation Criteria

The restorations were clinically evaluated at baseline, 6, 12, 18 and 24 months using modified Cvar/Ryge criteria (Table 2). For each criteria, Alpha was used to indicate the highest degree of clinical acceptability, Bravo scores represented clinically acceptable scores, while a Charlie score meant a clinically unacceptable score. Evaluation was done by two other independent investigators not involved with the treatment procedures using a mirror, explorer and air stream. The investigators were calibrated to a predetermined level of inter- and intra-examiner agreement at least 95% per single criteria. Any discrepancy between evaluators was resolved at chairside.

Table 2 Modified Cvar/Ryge Criteria

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RESULTS

After 24 months, all patients were available at all the evaluated periods, resulting in a recall rate of 100%. The retention rates of the baseline and re-evaluation after 6 months, 12, 18 and 24 months are shown in Table 3. None of the laser-prepared restorations were lost during the two-year study period, resulting in an excellent retention rate of 100%. Only one restoration was lost in the bur group at two years, resulting in a retention rate of 98.1%. No significant differences in retention rate were observed between the two groups (p=0.500).

Table 3 Results of Clinical Evaluation

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For marginal discoloration, one restoration from the laser-prepared group rated Bravo after 12 months, and no differences were observed between the two cavity preparation techniques (p=0.500). After 18 months, two more restorations, and at the end of 24 months, a total of four restorations rated Bravo in the laser group. Only three restorations from the bur group rated Bravo at the 24-month recall. There were no statistical differences between the two groups at the 18- (p=0.121) and 24-month recall (p=0.500). For both groups, no deep generalized discoloration was detected. Differences in marginal discoloration ratings between each evaluation period were found to be statistically significant for both groups (p=0.017).

In terms of marginal adaptation, two restorations from the laser and two from the bur group rated Bravo at 12 months (p=0.691). No severe margin defects were recorded for both groups either at 18 months (p=0.500) or at the two-year recall (p=0.759). Regarding the marginal adaptation scores in the laser group, differences among all evaluation periods were not statistically significant except for ratings between baseline and 24 months (p=0.016) and 6 months vs 24 months (p=0.016).

With regards to anatomic form, both groups were virtually the same, with 96.3% of the restorations having no anatomic form loss at 18 months (p=0.691) and 94.4% having no anatomic form loss at the end of 24 months (p=0.661). According to the Cochran Q test, statistical differences were found between each evaluation period for the bur group (p=0.040) and no differences were observed in the laser cavity preparation group (p=0.061).

Surface texture and color match was scored as Alpha for all restorations. No secondary caries was detected in association with any restoration after two years in both groups tested. Postoperative sensitivity was absent in all patients. No statistically significant differences were found among the cavity preparation groups in any of the evaluation criteria at all recalls (p>0.05).

DISCUSSION

In light of minimally invasive dentistry, laser technology offers an interesting alternative to the conventional use of diamond burs. In the current clinical study, bur- and laser-prepared occlusal composite restorations were compared and both demonstrated good results. No restorations were lost at 6, 12 and 18 months. As only one restoration from the bur group was lost at the 24-month recall, both groups fulfilled the American Dental Association (ADA) retention criteria.28After two years, the retention rate was 100% for the laser-prepared and 98.1% for the bur-prepared group, thereby satisfying the full acceptance guidelines specified by the ADA.

Laboratory research can predict the clinical performance of different cavity preparation techniques to a certain extent; however, clinical trials provide better information, and these studies are always needed to confirm these results. Proper comparisons are unavailable due to a lack of studies evaluating the clinical performance of resin composite restorations that are prepared with Er, Cr:YSGG laser.

It might have been expected that the retention rates would be different, since the design and surface finish of the cavity preparation produced by laser differed from that achieved by conventional bur preparation. The morphologic alterations caused by laser irradiation might affect the bond strength of restorative resins.1922 Many studies assessing the bond strength of resin composites to laser-prepared tooth structures have been conducted.21–26 While a decrease in the bond strength of laser-treated surfaces compared with burcut preparations was shown,1621–2329–30 some reported no difference.25–26 These contradictory findings might vary according to the type of laser and the parameters used for cavity preparation.

The microleakage rate for each cavity preparation technique was below the 5% level of ADA requirement for provisional acceptance after six months of clinical service. None of the restorations in the bur group showed discoloration until the 24-month recall and only 1.9% of the laser group had superficial discoloration at 12 months, 5.6% at 18 months and 7.4% at 24-months. No differences between the restorations performed with bur and laser could be detected with regards to marginal discoloration.

In an in vitro microleakage study, laser cavity preparation for Class V cavities resulted in less leakage than cavity preparation with a bur.31 In another study, cavities prepared by Er:YAG laser were found to be capable of decreasing the microleakage of resin composite restorations, and their efficiency was similar to etched bur cavities.32Moldes and others33 compared the degree of microleakage of composite restorations performed by lasers and those by conventional drills associated with two adhesive systems. The enamel margins demonstrated no differences in microleakage for all treatments. The dentin margins presented similar microleakage in cavities prepared with Er:YAG, Er, Cr:YSGG and drill using the etch-and-rinse two-step adhesive system. Khan and others34 compared the resin composite restorations placed in laser- and bur-prepared cavities. They did not observe any difference in terms of microleakage.

It has been reported that a scaly, irregular and rugged appearance of dentin was displayed after Er, Cr:YSGG laser irradiation. The absence of smear layer coverings and exposed dentin tubule orifices were also observed.23 These morphologic features, highly irregular surfaces without a smear layer, seem to be a suitable surface for adhesion. On the other hand, laser irradiation results in collagen fibril fusion and denaturation, closing the interfibriller spaces that might decrease the resin infiltration into the prepared surface and cause inferior adhesion to dentin.

In the current study, no difference was observed between the two cavity preparation methods in terms of marginal discoloration and adaptation. Adjunctive use of acid might cause this result. It has been reported that acid etching of enamel following laser, as a kind of finishing enamel, gave much better results.35–36 Lee and others23 found that Er, Cr:YSGG laser irradiation adversely affected the adhesion of resin to dentin but acid etching following laser irradiation might increase bond strength as much as that of bur-cut/acid-etched dentin. In another study, no difference was noted in the microleakage of composite restorations in Class II cavities between laser and bur preparation if Er, Cr:YSGG laser preparation was associated with acid etching.37 Acid etching following Er, Cr:YSGG laser irradiation could demineralize the inorganic portion of surface dentin and produce a favorable environment for molecular entanglement of polymer chains with collagen fibrils. The widened dentinal tubule orifices might also facilitate the deep infiltration of adhesive.23

Several studies have reported secondary caries as the main reason for restoration failure. It has been reported that Er, Cr:YSGG laser irradiation increases resistance to acid demineralization, thus reducing the acid dissolution of dental hard tissues and playing an important role in the prevention of secondary caries.38

The nano-filled flowable resin composite, Grandio Flow, was used to restore all cavities. The reason for selecting only one resin composite was to solely obtain a comparison of the cavity preparation techniques, not a system that consisted of adhesive and resin composite. Contour and surface texture presented 100% Alpha and they were unchanged from baseline for all restorations, which is more related to the resin composite used for the restoration. In regards to postoperative sensitivity, none of the patients suffered from pain in either group after two years.

CONCLUSIONS

Both cavity preparation methods (bur and laser) performed well clinically and met the ADA full-acceptance criteria for bonded restorative materials after two years. Therefore, lasers can be stated as being an acceptable technique for minimally-invasive resin composite cavity preparations. However, long-term recalls are planned to determine if differences in clinical performance between bur- and laser-prepared groups will occur at later restoration ages.