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J Am Dent Assoc, Vol 131, No 6, 777-785.
© 2000 American Dental Association
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INFORMATICS AND TECHNOLOGY

A LASER-POWERED HYDROKINETIC SYSTEM

FOR CARIES REMOVAL AND CAVITY PREPARATION



JACK HADLEY, D.D.S., DOUGLAS A. YOUNG, D.D.S., M.B.A., LEWIS R. EVERSOLE, D.D.S., M.S.D., M.A. and JEFFREY A. GORNBEIN, D.SC.


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
Background. Laser systems have been developed for the cutting of dental hard tissues. The erbium, chromium: yttrium-scandium-gallium-garnet, or Er,Cr:YSGG, laser system used in conjunction with an air-water spray has been shown to be efficacious in vitro for cavity preparation.

Methods. The authors randomly selected subjects for cavity preparation with conventional air turbine/bur dental surgery or an Er,Cr:YSGG laser-powered system using a split-mouth design. They prepared Class I, III and V cavities, placed resin restorations and evaluated subjects on the day of the procedure and 30 days and six months postoperatively for pulp vitality, recurrent caries, pain and discomfort, and restoration retention. Sixty-seven subjects completed the study.

Results. There were no statistical differences between the two treatment groups for the parameters measured with one exception; there was a statistically significant decrease in discomfort levels for the laser system at the time of cavity preparation for subjects who declined to receive local anesthetic.

Conclusions. The Er,Cr:YSGG laser system is effective for preparation of Class I, III and V cavities and resin restorations are retained by lased tooth surfaces.

Clinical Implications. Hard-tissue cutting lasers are being introduced for use in operative dentistry. In this study, an Er,Cr:YSGG laser has been shown to be effective for cavity preparation and restoration replacement.

Laser devices use different physical media sources to generate a variety of wavelengths that interact with specific molecular components in animal tissues. Each of these wavelengths targets specific tissue components such as melanin, hemosiderin or hemoglobin, extrinsic tattoo materials, and water and other molecules. Lasers have been shown to effectively cut and ablate hard and soft tissues when the appropriate wavelength is selected.1–3

Laser interactions with biocalcified tissues have been studied using a variety of wavelengths. Depending on such laser parameters as pulse duration, repetition rate and fluence, carbon dioxide, or CO2, lasers and neodymium: yttrium-aluminum-garnet, or Nd:YAG, lasers produce such surface changes in enamel as roughness, cratering, cracking, fissuring, melting and recrystallization. In addition, some studies have demonstrated that these lasers can generate markedly elevated surface and pulpal temperatures.4–8 The profound thermal affects and inability to precisely cut biocalcified tissues have eliminated the initial CO2 and Nd:YAG laser systems from consideration as modalities for dental surgery. The argon fluoride:, or ArF:, excimer lasers have been reported to remove dental caries; the ability to effectively cut sound enamel and dentin, however, has not proven to be efficacious.9 Krypton fluoride:excimer lasers emit in the ultraviolet range and have been shown to cut dentin; enamel is resistant to effective ablation.10,11

Previous in vitro studies have shown that CO2 laser irradiation inhibits the progression of carieslike lesions up to 85 percent, which is comparable to a daily application of a sodium fluoride dentifrice.12–15 Subsequent studies have shown similar effects for erbium: yttrium-aluminum-garnet, or Er:YAG, lasers and erbium, chromium: yttrium-scandium-gallium-garnet, or Er,Cr:YSGG, lasers with a 40 percent and 60 percent caries reduction, respectively.16

Recently, laser devices that are more clinically practical and capable of hard-tissue surgery have been introduced. Er:YAG lasers have been shown to cut dental biocalcified tissues effectively,17–20 and clinical trials have demonstrated efficacy for operative dentistry procedures using an Er:YAG device.21,22

Another device that emits at a comparable wavelength to the Er:YAG laser is the laser-powered hydrokinetic system, or LPHKS. Although not yet validated by other studies, the proposed mechanism of the LPHKS is that the Er,Cr:YSGG-pulsed laser source delivers photons into an air-water spray matrix with resultant microexplosive forces on water droplets. This process is hypothesized to contribute significantly to the mechanism of hard-tissue cutting.23 The LPHKS with its accompanying air-water spray has been shown to cut enamel, dentin, cementum and bone efficiently and cleanly without deleterious thermal affects on dental pulp.24–26 Comparative quantitation of cutting efficacy measuring hard-tissue ablation in volume per minute has been assessed for dental burs, Er:YAG lasers and Er,Cr:YSGG lasers with air-water spray. For example, in one study the LPHKS has been found to be capable of cutting dentin faster than an Er:YAG device.27 We realize, however, these types of studies must be updated continually as improvements are made in technology.

Lased tooth surfaces have been evaluated for their ability to form adhesion with various bonding agents; shear and tensile strength assays have been used to compare bonding to lased and acid-etched enamel and dentinal surfaces.28 ArF:excimer and Nd:YAG systems have been reported to create a weaker bonding surface than can be achieved with acid etching.29,30 CO2 lasers produce variations in enamel depending on what wavelength is used, but, in general, bonding to these surfaces is superior to that of acid-etched enamel.31–35 Scanning electron microscopy has shown that LPHKS makes clean cuts through enamel and dentin without creating a significant smear layer.36 Resin bonding agents have been shown to bond to LPHKS-cut enamel surfaces with shear strengths comparable with those achieved with bur-cut acid-etched surfaces.37

Thermocouple assessment of LPHKS-cut teeth has shown that in vitro cavity preparations on human teeth and in vivo cavity preparations on anesthetized beagles’ teeth do not cause any adverse pulpal temperature increases.24,38 Histopathologic studies in animals’ and humans’ third molars have disclosed that pulpal tissues underlying deep cavity preparations made with this device do not undergo any pathological changes. No odontoblastic alterations have been noted, nor is there any inflammatory response in the pulp chamber underlying the preparation.37,39

Since the LPHKS has been demonstrated to exhibit efficacy in caries removal and has the ability to precisely and effectively cut enamel and dentin without adverse pulpal thermal or histopathologic responses, its ability to perform operative dentistry on human subjects needed to be investigated in a randomized clinical trial.

We undertook a split-mouth, randomized, single-blind clinical investigation to compare the efficacy and safety of the LPHKS with conventional air turbine/bur dental surgery for caries removal, cavity preparation and restoration retention. In this article, we detail the study parameters, processing variables, final outcome variables, device parameters and statistical methodology.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
Patient-selection criteria. All participants in the study met the following inclusion criteria: being 18 years of age or older; having at least one carious lesion on each of two separate teeth that represented Class I, III or V preparations; and being willing to sign informed consent documents allowing entry into the study as approved by the Institutional Review Board on the use of humans for clinical research at the University of the Pacific. People in all racial and ethnic groups were eligible for entry.

Exclusion criteria included being younger than 18 years of age; being medically ill and not under active medical care or medical control; being pregnant; wearing cardiac rhythm electronic control devices; having nonvital teeth; having caries extending more than two-thirds of the distance through dentin as assessed clinically, radiographically or both; having teeth that required more than one restoration in the same tooth; and having teeth with radiographic evidence of apical radiolucency.

We obtained medical history and physical examination data for all patients who met the inclusion criteria and were enrolled in this study. Subjects received financial remuneration for their participation, which was paid at the final six-month recall visit. A total of 68 subjects participated in the study, providing a total of 75 tooth pairs that met inclusion criteria. Most subjects had a single pair of restorations; some had more than one.

We randomly selected tooth pairs for treatment A, LPHKS preparations, or for treatment B, air turbine/bur dental surgery. We did this by selecting a sealed envelope designating either treatment A or treatment B and used whichever method was named in the envelope to treat the tooth with the lower tooth number (1–32); the higher-numbered tooth received the other treatment. We obtained both bitewing and periapical preoperative radiographs for diagnostic purposes. We asked subjects if they wanted to have local anesthetic administered preoperatively. If they declined, we gave them the option of receiving local anesthetic intra-operatively to complete the procedure.

Laser-powered hydrokinetic system operating parameters. The LPHKS uses an Er,Cr:YSGG crystal with a photon emission wavelength of 2.78 micrometers. The photons are delivered through a flexible fiber-optic cable and then are focused through a contra-angled handpiece bearing a sapphire tip (Figure 1Go). This tip is bathed in an adjustable air-water spray. This LPHKS device (Millennium System, BioLase Technology Inc.) has been cleared by the U.S. Food and Drug Administration for cavity preparation on adult teeth.



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  		Figure 1. Handpiece and controls for erbium, chromium: yttrium-scandium-gallium-garnet laser-powered hydrokinetic system.

 
The laser parameters are detailed in the boxGo "Operating Parameters of the Laser-Powered Hydrokinetic System." The wavelength of this laser device is absorbed maximally by water molecules and also may target the hydroxyl group in enamel or dentin. Photons are emitted at various power levels regulated by the operator. The energy density and fluence at the tissue interface are directly proportional to the power setting. Effective hard-tissue cutting is achieved at 1 to 1.5 millimeters from the sapphire tip. Defocusing beyond 2 mm from the tissue surface mitigates the cutting effect; thus, the distance between the fiber tip and the tissue inherently regulates the cutting efficacy. We secured U.S. Food and Drug Administration approval to use this specific laser system for the investigation.


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  		OPERATING PARAMETERS OF THE LASER-POWERED HYDROKINETIC SYSTEM.

 
Two operators (J.H. and D.A.Y.) and three blinded evaluators were involved with this study. In this investigation, the operators cut enamel at a power setting of 5.5 to 6.0 watts, and once we reached dentin, we reduced the power to 4.0 to 5.0 W. The cavity preparations tended to follow G.V. Black’s basic outlines with the following modifications: preparation outlines for all classes were conservative, retention points were not placed in dentin, and the enamel cavosurface margins were not flared and beveled.

The operators made the air turbine/bur preparations following the same general guidelines at approximately 20,000 to 24,000 rotations per minute with air-water spray. Then they acid-etched the enamel of all preparations for 10 seconds with 37 percent phosphoric acid before they placed the restorations. Finally, they restored the LPHKS and air turbine/bur preparations with the same type of light-cured hybrid composite resin.

Variables. The boxGo "Procedural and Outcome Variables Assessed" shows the procedural (independent) and outcome (dependent) variables assessed in this study. When the subjects entered the study, we performed clinical examinations to reconfirm that all criteria, both inclusionary and exclusionary, were met, and we began the random selection of treatments.


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  		PROCEDURAL AND OUTCOME VARIABLES ASSESSED.

 
Before the clinical study began, the operators were calibrated by performing in vitro LPHKS preparations on extracted teeth: 10 each of Class I, III and V preparations. For the actual clinical trials, the operators tested both of the sample teeth for vitality with an electric pulp tester. They registered the recordings as vital if the test tooth and its adjacent control tooth responded to electrical stimulation. They registered a nonvital reading if the adjacent control tooth responded, but the test tooth gave no response at maximal stimulation. All teeth entered into the study were vital according to this methodology.

We assessed caries depth clinically and radiographically and dropped only one subject from the study because of carious exposure as determined at the time of operation. The treating operator reviewed all baseline radiographs and clinical characteristics of the carious lesion. At the time of the procedure, the operator recorded a subject’s physical discomfort level using a discomfort scale and reassessed it by telephone 48 hours postoperatively. The discomfort scale was a standard Likert interval from 1 to 5 in which 1 = no discomfort, 2 = mild discomfort, 3 = moderate discomfort, 4 = high level of discomfort and 5 = extreme discomfort.

At the time of cavity preparation, we performed a blind evaluation to assess adequacy of caries removal, preparation outline form and restoration placement. The variables were evaluated as acceptable or unacceptable on a two-point scale. In the event of an unacceptable outcome, we repeated the procedure until an acceptable level was achieved. Acceptability was based on the following criteria:

– caries removal—no yellow or brown discoloration of dentin or marginal enamel, no soft enamel or dentin found using compression with a dental explorer;
– preparation—no unsupported enamel, no pulp exposure (Figure 2Go);
– restoration placement—preparation totally filled and restored, no marginal flashing present as assessed by dental explorer, color selection matched, surface finish smooth and nonporous.



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  		Figure 2. Laser-powered hydrokinetic system Class I preparation in a maxillary premolar.

 
We conducted a second blinded discomfort assessment immediately after the procedure.

At the 30-day recall, we evaluated restoration retention on a four-point scale in which 1 = retained no breakdown, 2 = retained with marginal breakdown, 3 = partial loss, 4 = total loss. In addition, we assessed vitality measures and discomfort levels as previously specified. At the final six-month recall appointment, we evaluated the test teeth for restoration retention and integrity, marginal recurrent caries, vitality and discomfort levels (Figure 3Go).



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  		Figure 3. Cervical Class V restoration on teeth nos. 8 (laser-powered hydrokinetic system–treated) and 9 (air turbine/bur–treated) six months postoperatively.

 
At the time of the procedure, the operator recorded a subject’s physical discomfort level using a discomfort scale and reassessed it by telephone 48 hours postoperatively.

Statistical analysis. We computed the McNemar {chi}2 statistic and corresponding P value in all cases in which there were nonzero (paired) differences between the LPHKS and air turbine/bur treatments.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSION
 REFERENCES
 
From the total of 75 tooth pairs entered into the study, three patients failed to return for recall visits, and another six did not return for the final six-month assessment. The results, therefore, are based on 66 pairs for which complete data were available. Thirty-seven percent of the subjects were women, and the subjects’ ages ranged from 20 years to 84 years (median 41 years). Of the tooth pairs evaluated, 45 percent were Class I, 1.5 percent were Class III, and 53 percent were Class V preparations. Of these restorations, 51 percent were facial, 3 percent were occlusofacial, 1.5 percent were lingual, 1.5 percent were mesial, and 42.4 percent were occlusal.

The evaluators found all outcomes in this study to be comparable when collating variables for LPHKS treatment with those of air turbine/bur dental surgery treatment for caries removal, cavity preparation and restoration placement. All restorations were retained without recurrent caries and without loss of restorative materials, and all treated teeth remained vital at the six-month termination assessment. Table 1Go shows the data for outcome variables, excluding discomfort levels, and shows no statistical differences between the groups. We found that both the LPHKS and air turbine/bur methods were equally adequate for caries removal and preparation outline and were acceptable for restoration finish.


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  		TABLE 1 COMPARISON OF TREATMENT METHODS BY OUTCOME VARIABLE (N = 66).

 
The discomfort scale analysis is shown in Table 2Go. Intra-operative discomfort assessment indicated that 98.5 percent of LPHKS-treated teeth had no discomfort, and 1.5 percent (one tooth) had moderate discomfort. Among the air turbine/bur-treated teeth, 87.9 percent had no discomfort, and 12.1 percent had some degree of discomfort—four teeth had mild discomfort, and three teeth had moderate discomfort. We found these data to be significant (McNemar {chi}2 = 7.118, P = .0078).


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  		TABLE 2 PERCENTAGE OF TEETH WITH NO DISCOMFORT AT EACH ASSESSMENT.

 
When a blinded examiner assessed these same measures immediately postoperative, he found that while the differences were not identical, they were similar. Among LPHKS-treated teeth, 98.5 percent were without discomfort; among the air turbine/bur-treated teeth, 90.9 percent were without discomfort. Discomfort level assessments at 48 hours, 30 days and six months revealed that 100 percent of teeth in both groups had no discomfort.
All restorations were retained without recurrent caries and without loss of restorative materials.

At the day-30 assessment, we scored restoration retention for the LPHKS-treated teeth at 1.0 for all teeth, indicating restoration retention without material breakdown or loss. A single tooth in the air turbine/bur group had marginal breakdown. We replaced this restoration on day 30. At the six-month recall, we observed restoration retention in both groups and recorded a recurrent caries rating of 1.0 that indicated no evidence of marginal or gross caries for both treatment groups. All teeth remained vital in both groups.


   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSION
 REFERENCES
 
The study design for this clinical trial was statistically strong. By using a split-mouth randomized design, we eliminated intrasubject variables, which allowed us to test the efficacy of the two treatments in an uncomplicated fashion. With the exception of discomfort levels, we found no significant differences among variables assessed in this clinical study. Caries removal and Class I, III and V cavity preparations were all performed adequately with both devices.

Discomfort and pain are prominent factors in dental phobias. Although histologic and thermal effects on pulp tissue data have been gathered previously in animal studies, it was not possible to obtain symptomatological data from the animals. Therefore, we assessed the pain levels of human subjects using the Likert scale. Intraoperative discomfort levels indicated a higher prevalence of discomfort among the air turbine/bur–treated teeth than among the LPHKS-treated teeth. Only one subject indicated intraoperative discomfort in an LPHKS-treated tooth. Discomfort remained at baseline levels in both groups at the 30-day and six-month assessment even in those instances in which discomfort initially was reported during and immediately after cavity preparation.

At the termination recall appointment at six months, we found that blind assessment variables were the same for both treatment groups and no recurrent caries at the margins of the preparation sites were indicated. All teeth remained vital, and no restorations were lost or fractured about the margins. Restoration retention in both groups may be attributed to the fact that we used conservative preparation outlines and acid etching to improve bond strength.

Previous in vitro and in vivo studies using animal models have shown biological safety and clinical efficacy for the LPHKS in operative dentistry.25 This water-cooled system allows for dental surgery without heat transfer to the dental pulp. Thermal measurements and histologic studies have shown minimal thermal changes and normal pulpal responses, respectively, to cavity preparations made with this device.26,38

We did not undertake histologic or thermocouple assessments of human pulpal tissue in this clinical trial. In recent studies, we found no histologic evidence of pulpal necrosis or inflammation among treated human teeth in vivo when the LPHKS device was operated correctly with an interface of air-water spray between the sapphire tip and tooth surface.39

Lastly, our clinical findings support laboratory results on bonding of restorative resins,35 as all teeth prepared with the LPHKS retained the restorative materials for the duration of the study.


   CONCLUSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
REFERENCES
 
We believe that the LPHKS is an efficient, effective, precise and safe device for the removal of caries and for the preparation of tooth structure for the placement of dental restorative resin. It also offers an alternative to the vibratory and auditory irritation that attends conventional air turbine/bur dental surgery.


   FOOTNOTES
 

Dr. Gornbein is a professor, Department of Biomathematics, Center for Health Sciences, University of California, Los Angeles.


This study was sponsored by a grant from BioLase Technology Inc., San Clemente, Calif.


Dr. Hadley is a professor, Department of Radiology, and the director, Emergency Clinic, University of the Pacific, School of Dentistry, San Francisco.


Dr. Young is an associate professor, Department of Diagnosis and Management, University of the Pacific, School of Dentistry, San Francisco.


Dr. Eversole is a professor, University of the Pacific, School of Dentistry, 2155 Webster St., San Francisco, Calif. 94115. Address reprint requests to Dr. Eversole.


   REFERENCES
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 

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