Thermal Effects on Pulp Due to Laser and Handpiece Usage

Written by Christina Penn, Christopher Beninati, Alissa Mariano, Daniel Dooley, Masly Harsono DMD, Ronald Perry DMD, MS and Gerard Kugel DMD, MS, PhD

 

ABSTRACT

OBJECTIVE: The study was designed to compare changes in pulpal temperature during ablation of dental hard tissue while using two established erbium dental laser systems, a new CO2 laser system, and a conventional high-speed handpiece. METHODS: Eighty non-carious human extracted molars were separated into four sample groups of 20 teeth each. Three laser systems were used, respectively, to ablate the occlusal surface of the teeth in three of the groups for 60 seconds each. The high-speed handpiece was used to drill the occlusal surface of the fourth group for 60 seconds. Pulpal temperatures were measured using thermocouples inserted into each tooth’s pulpal chamber prior to ablation.

 

RESULTS: None of the average temperature increases approached the threshold of 5.5°C at which pulpal damage begins. On average, the pulpal temperature of teeth ablated with the Waterlase MD system increased the most (3.56°C). The traditional handpiece caused the lowest average temperature increase (1.57°C), followed by the LightWalker DT system (3.20°C) and the Solea CO2 system (3.30°C).

 

Despite a remarkable decline over the past 40 years, dental decay still affects 60% to 90% of the world’s schoolchildren and almost 100% of adults, according to the World Health Organization Global Oral Health Data Bank and Oral Health/Area Profile Programme.1 Unfortunately, traditional methods of preparing hard tissue have not significantly changed since the late 1950s, when the first high-speed air- turbine drills began replacing belt-driven handpieces.2 Dental drills, which create vibration and make a distinctive noise, often are a prominent factor in many adults’ fear of visiting a dental office. In contrast, lasers can remove dental hard tissue with minimal vibration and noise. Because of the ability to tightly focus the ablation holes, lasers also have the potential to substantially reduce the amount of tissue removed during cavity preparations, facilitating the preservation of healthy tissue.3

 

A number of Er:YAG, Er,Cr:YSGG, and Nd:YAG solid-state lasers have been cleared by the US Food and Drug Administration (FDA) for use in dentistry over the past decade. However, the wavelengths of these lasers are not absorbed efficiently by hydroxyapatite, resulting in slow and inefficient ablation. The wavelengths of these lasers are primarily absorbed by the water content of the tooth.4 This makes these wavelengths inherently inefficient, because the composition of tooth enamel by volume is 12% water and 85% hydroxyapatite.5 In contrast, carbon dioxide (CO2) laser wavelengths, especially at 9.3 μm and 9.6 μm, are highly absorbed by the apatite mineral. Their absorption coefficients are 5,500 cm–1 and 8,000 cm–1 respectively, compared to a range of 480 cm–1 to 800 cm–1 for the erbium lasers.6-10

 

The wavelengths that produce commercially viable cutting speeds range between 9.2 µm and 9.8 µm.6 Recent research has shown that a 9.3-μm CO2 laser can be efficiently absorbed by dental hard tissue and can ablate tooth structure without generating excessive peripheral thermal or mechanical damage or harming the pulp.11,12

 

Although lasers at the 9.3-μm CO2 wavelength up to now have not been commercially available, a new CO2 dental laser system (Solea™, Convergent Dental, www.convergentdental.com) of this type has been developed. The objective of the present study is to compare changes in pulpal temperature during ablation of dental hard tissue while using two established erbium dental laser systems (Er,Cr:YSGG 2.78 μm, Waterlase MD™, Biolase, Inc., www.biolase.com; and Er:YAG 2.94 μm, LightWalker® DT, Fotona, www.fotona.com), the new CO2 system, and a traditional high-speed handpiece (Midwest® High Speed Handpiece, DENTSPLY, www.dentsply.com). These temperature changes will also be compared to the commonly accepted threshold of a 5.5°C rise. A temperature rise of 5.5°C is considered excessive and likely to lead to a loss of pulpal vitality, according to the seminal work of Zach and Cohen.13

 

Materials and Methods

Following IRB approval 80 non-carious human extracted molars with no major decay or defects were cleaned of adherent patient material by scrubbing with detergent and water. They were then immersed in a fresh solution of formalin and stored in containers with a distilled water solution containing thymol (0.1%). The teeth were separated into four sample groups of 20 teeth each. Within each group, each sample tooth was labeled with a number and color-coded to correspond to one of the four systems.

 

In order to compare the four systems at similar power levels and fluence, the manufacturers’ recommended hard-tissue ablation settings were selected:

 

Solea system:1.6 kHz repetition rate with 94 mJ of pulse energy
Waterlase MD system with Turbo handpiece (2009 model): 30 Hz repetition rate with 233mJ of pulse energy
LightWalker DT system (current model): 50 Hz repetition rate with 160 mJ of pulse energy
Midwest High Speed Handpiece: a size 330 carbide bur

 

(Editor’s Note: The Waterlase MD system used in this study was from 2009. A newer model, the Waterlase iPlus™, was released in 2011.)

 

For each system, the standard (recommended) air/water spray mixture was used. The laser system settings and measured power output for each of the system configurations tested are shown in Table 1. A digital radiograph (Schick CDR Elite Size 2, Sirona Dental, www.schickbysirona.com or Gendex GSX- 700™, Gendex Dental Systems, www.gendex.com) was taken of each sample tooth to identify the pulp- chamber location. A single hole was drilled to allow access to the top of each pulp chamber. A thermocouple (model 5TC-TT-K-36-36, Omega Engineering Inc., www.omega.com) was placed in the top of each pulp chamber and bonded to the tooth using thermally conductive epoxy (Hysol® ES1001™, Henkel Electronic Materials,www.henkel.com). After curing of the epoxy, a second radiograph was taken to verify that no air pockets had formed around the thermocouple and that it was properly positioned. For the laser ablation systems, each sample tooth was placed in a vice holder to ensure a consistent distance (4 mm to 7 mm) away from the laser tip. (The 4 mm to 7 mm range was recommended by the laser manufacturers, who stated there would be minimal fluence changes in this range.) Power readings with a calibrated Ophir 30A-SH laser power meter (Ophir Optronics, www.ophiropt.com) were taken for all systems prior to any tests being run. The occlusal surface of each sample tooth was ablated for 60 seconds using the predetermined laser settings. There were differences in depth among the three systems and the high-speed handpiece. Some units penetrated deeper than others based on such factors as angle of ablation, geometry of the tooth, hardness of lesion, and strength of hydroxyapatite.

 

For the traditional handpiece, each sample tooth was placed in a holder, and the tip of the bur was placed against the occlusal surface of the tooth. Cutting was initiated by stepping on the rheostat foot pedal, and continued for 60 seconds.

 

Pulpal temperatures were measured by attaching the thermocouples to the Apollo IV DT304 Digital Temperature Logger (UEi Test Instruments, www.ueitest.com), which was attached via USB connection to a laptop computer running ApolloDigital software. This enabled recording of real-time data for each sample tooth. The results were sent to the Indiana University School of Medicine, Department of Biostatistics (IndeviaData) for analysis. Pearson coefficient calculations also were performed to analyze the correlation of temperature response over time for each system configuration. The correlation of the temperature measurements of the Solea system with the temperature measurements of the other three systems also was analyzed.

 

Results

All of the systems appeared to effectively ablate all sample teeth, and minimal char was noted. On average, the pulpal temperature of teeth ablated with the Waterlase MD system increased the most (3.56°C). The Midwest High Speed Handpiece caused the lowest average temperature increase (1.57°C), followed by the LightWalker DT system (3.20°C) and the Solea system (3.30°C). The average temperature findings are summarized in Table 2. None of the average temperature increases approached the 5.5°C at which pulpal damage is understood to begin. Figure 1 displays the individual data sets that were compiled to form the average results in Table 2.

 

The Pearson coefficients for the three laser test groups are shown in Table 3. All have a correlation of 0.95 or greater, with respect to time and temperature rise. Table 3 also shows that each of the predicates had a Pearson correlation coefficient of 0.93 or greater when compared to the Solea system. Each individual performance data set significantly correlates to each other, confirming the substantial equivalence of the laser systems with respect to temperature over time.

 

Discussion

The first researchers to investigate the use of lasers for cavity prevention and caries removal concluded that the technology was unworkable, due to the large amount of heat generated locally and the resulting cracking and distortion of the enamel.13 Since then, however, a deeper understanding of the mechanics of hard-tissue ablation has led to the development of lasers that work effectively on the absorbing components of the tissue.12,14

 

The three laser systems compared in the present study differ significantly, with different pulse characteristics and different frequencies. Erbium lasers typically operate most efficiently at very low repetition rates. In order to achieve higher cutting rates, they therefore must deliver a larger amount of energy per pulse (100 mJ to 500 mJ). CO2 lasers have the flexibility to operate efficiently at high repetition rates, allowing them to deliver lower energy per pulse. CO2 laser beams can be scanned to minimize heat accumulation in a given area.12

 

In the present study, variables in cutting included differing occlusal geometries and dehydration levels (molars were extracted at different times). The differences in occlusal geometry to some extent affected the size and/or shapes of the holes drilled to access the roof of the pulp chamber. Enamel thickness averages from 1.28 mm on the mesial surface of the first molar to 1.40 mm on the distal surface of the first molar, and from 1.29 mm on the mesial surface of the second molar to 1.48 mm on the distal surface of the second molar.15 Variations in storage time of extracted molars in thymol solution may affect the temperature changes during ablation.16 However, it was implausible to collect all extracted molars used in the study concurrently to avoid differences in storage time. By averaging the pulpal temperature changes, the authors aimed to standardize data to account for variations in occlusal geometries and dehydration levels.

 

Conclusions

When used to ablate healthy extracted molars, the Solea 9.3-µm CO2 laser operated at standard power settings caused pulpal temperature increases that were substantially equivalent to those caused by the LightWalker DT Er:YAG laser system and slightly less than the pulpal temperature rise caused by the Waterlase MD Turbo Er,Cr:YSGG laser system. Although each of the three laser systems caused pulpal temperatures to increase more than the traditional handpiece, none of the average temperature increases produced by any of the systems approached a rise of 5.5°C, the degree of temperature change known to cause pulpal damage. Despite the differences in beam characteristics, the thermal effects of the three different laser systems on pulpal tissue were equivalent in the sense that they did not harm the pulpal tissue through temperature increases during ablation.

 

Disclosure

Ms. Penn received a research grant for this study from Convergent Dental. The authors report no other affiliations with any of the companies mentioned in this article.

 

References

Petersen PE. World Health Organization. The World Oral Health Report 2003: continuous improvement of oral health in the 21st century–the approach of the WHO Global Oral Health Programme. Community Dent Oral Epidemiol. 2003;31(suppl 1):3-23.
Fried IR laser ablation of dental enamel. In: Featherstone JDB, Rechmann P, Fried D, eds. Proceedings of SPIE. 2000;3910:136. doi:10.1117/12.380820.
Meister J, Franzen R, Forner K, et al. Influence of the water content in dental enamel and dentin on ablation with erbium YAG and erbium YSGG J Biomed Opt. 2006;11(3):34030.
Featherstone JDB, Fried Fundamental interactions of lasers with dental hard tissues. Med Laser Appl. 2001;16(3):181-194.
Fried D, Glena RE, Featherstone JDB, Seka Nature of light scattering in dental enamel and dentin at visible and near-infrared wavelengths. Appl Opt. 1995;34(7):1278-1285.
Fried D, Ragadio J, Akrivou M, et Dental hard tissue modification and removal using sealed transverse excited atmospheric-pressure lasers operating at lambda=9.6 and 10.6 microm. J Biomed Opt. 2001;6(2):231-238.
Fried D, Zuerlein M, Featherstone JDB, et IR laser ablation of dental enamel: mechanistic dependence on the primary absorber. Applied Surface Science. 1998;127-129:852-856.
Zuerlein MJ, Fried D, Featherstone Modeling the modification depth of carbon dioxide laser- treated dental enamel. Lasers Surg Med. 1999;25(4):335-347.

Zuerlein MJ, Fried D, Featherstone JDB, Seka Optical properties of dental enamel in the mid-IR determined by pulsed photothermal radiometry. IEEE Journal of Selected Topics in Quantum Electronics.1999;5(4):1083-1089.
Staninec M, Darling C, Goodis H, et Pulpal effects of enamel ablation with a microsecond pulsed λ=9.3-μm CO2 laser. Lasers Surg Med. 2009;41(4):256-263.
Nguyen D, Chang K, Hedayatollahnajafi S, et High-speed scanning ablation of dental hard tissues with a λ = 9.3 μm CO2 laser: adhesion, mechanical strength, heat accumulation, and peripheral thermal damage. J Biomed Opt. 2011;16(7):071410. doi:10.1117/1.3603996.
Zach L, Cohen Pulp response to externally applied heat. Oral Surg Oral Med Oral Pathol. 1965;19:515-530.
Seka W, Featherstone JDB, Fried D, et Laser ablation of dental hard tissue: from explosive ablation to plasma-mediated ablation. In: Wigdor HA, Featherstone JDB, White JM, Neev J, eds. Lasers in Dentistry II. Proceedings of SPIE. 1996;2672:144-158. doi: 10.1117/12.238763.
Stroud JL, English J, Buschang, Enamel thickness of the posterior dentition: its implications for nonextraction treatment. Angle Orthod. 1998;68(2):141-146.
Goodis HE, Marshall GW Jr, White The effects of storage after extraction of the teeth on human dentine permeability in vitro. Arch Oral Biol. 1991;36(8):561-566.

 


High-speed Scanning Ablation of Dental Tissues with a 9.3-μm CO2 laser: Heat Accumulation and Peripheral Thermal Damage Study Summary

By: Daniel Nguyen, Michal Staninec, Chulsung Lee, and Daniel Fried, University of
California, San Francisco

Overview

The purpose of the study is to determine whether a 9.3-μm mechanically scanned CO2 laser operated at high laser pulse repetition rates can safely ablate enamel and dentin without excessive heat accumulation and peripheral thermal damage. To this end, three tests performed on samples derived from non-carious extracted molars were conducted to determine the heat accumulation, adhesive bond strength and four-point bend measurements.

 

Background

Several studies have demonstrated that CO2 lasers operating λ=9.3 and 9.6-μm wavelengths, which are strongly absorbed by hydroxyapatite in dental hard tissues, are ideally suited for the efficient ablation of dental caries and for surface treatments to increase the resistance to acid dissolution. The primary concern when operating lasers at high pulse repetition rates is increased potential for peripheral thermal damage due to heat accumulation from multiple laser pulses delivered in rapid succession. This heat accumulation can be offset by rapid scanning the laser beam over the area of ablation and by use of a water-spray. The effect of peripheral thermal damage on adhesion has been studied with a wide range of results on a variety of lasers and post ablation surface treatments. In a previous study, it was observed that high bond strengths were attainable by using a 9.3-μm CO2 lasers (1).

 

Testing Methods Heat Accumulation

For this study, two groups were studied, containing 12 and 16 samples per group. Pulpal temperatures were recorded using micro thermocouples situated at the pulp chamber roof, which were occlusally ablated using a rapid-scanning, water-cooled 9.3 µm CO2 laser over a two minute time period. The laser was operated at a pulse repetition rate of 300 Hz, and two single pulse energy levels were used, 14 mJ for the first group and 22 mJ for the second.

 

Shear-bond Test

The adhesive strength of dental composite to laser treated enamel was determined via a single plane shear-bond test with samples being divided into 3 groups: ablated/non-etched (n=10), ablated/acid-etched samples (n=8) and control samples (n=9) prepared only by 320 grit wet sanding. Bonding resin was applied to all the blocks in two coats, dried, and cured for 10 seconds prior to bonding with composite. The modified single plane shear test assembly (SPSTA) followed the procedure used by Sheth et al. and Watanabe et al. Two aligning plates were used to connect the SPSTA to an Instron testing machine, which recorded measurements in kilograms with the crosshead speed set to 5 mm/ min. When the two plates separated, the force level was recorded.

 

Four-point Bend Measurements

Beams (1 × 1 × 9 mm) of dentin were used in the dentin mechanical strength study. Two groups were studied, with 10 samples per group. The mechanical strength of facially ablated dentin (n=10) was determined via four-point bend test and compared to control samples (n=10) prepared with 320 grit wet sand paper to simulate conventional preparations.

 

Results

It was found that laser-ablated surfaces were smooth and highly uniform. No visible discoloration or charring indicative of thermal damage was observed on either the enamel or dentin surfaces under both macroscopic and microscopic inspection.

 

The maximum temperature recorded during ablation remained below the ambient temperature of 21°C for all samples in both groups. Heat accumulation measurements indicated that the mean temperature after laser ablation of tooth samples with a pulse energy of 14 mJ (Group TC20) was 17.6 ± 0.9°C (n=16), with a mean change in temperature of 2.0 ± 0.6°C. With a laser pulse energy of 22 mJ (Group TC30), the mean temperature after ablation was 19 ± 0.9°C (n=12), with a mean change in temperature of 3.2 ± 0.8°C.

 

The bond strengths achieved for both enamel and dentin after laser irradiation and acid etching were very high, near 30 MPa. The shear-bond testing yielded mean bond strengths of 31.2 ± 2.5 MPa (n=8) for the ablated/ acid-etched samples (Group LESB), 5.2 ± 2.4 MPa (n=10) for ablated/non-etched samples (Group LSB) and 37.0 ± 3.6 MPa (n=9) for the control (Group CSB). ANOVA with Tukey post-test indicated that there was a significant difference between all the groups (P < 0.05).

 

The four-point bend tests yielded that bending strength was not significantly different for the laser irradiated samples which indicates that the minor thermal effects from the laser does not compromise the mechanical strength.

 

In conclusion these results suggest that dental hard tissues can be rapidly ablated with a mechanically scanned 9.3 µm CO2 laser at high pulse repetition rates without excessive heat accumulation in the tooth or peripheral thermal damage that produce no significant reduction in the tissue’s mechanical strength or a large reduction of adhesive strength to restorative materials.

 

For the complete study, contact Convergent Dental

 

1. Hedayatollahnajafi S, Staninec S, Watanabe L, Lee C, Fried D. Dentin bond strength after ablation using a CO2 laser operating at high pulse repetition rates. Lasers in Dentistry VX. 2009; Vol. 7162:F1–F7.


Pulpal Safety during Ablation of Tooth Occlusal Surfaces Using a CO2 Laser with a 9.3- Micrometer Wavelength

By: Michal Staninec, Cynthia L. Darling, Harold E. Goodis, Daniel Pierre, Darren P. Cox, Kenneth Fan, Michael Larson, and Daniel Fried (UCSF)

Overview

A new, conservative approach to restorative dentistry has emphasized the importance of early treatment of pits and fissures by means of micropreparation of such sites. When decay is highly localized, however, removing caries without damaging the surrounding healthy tissue can be difficult to accomplish.

 

Background

Although CO2 lasers have been used for soft-tissue surgical procedures for three decades and today are the most common lasers found in clinics, early experimentation with CO2 lasers operated at 10.6 µm for hard-tissue ablation resulted in cracking and charring of the surrounding enamel, dentin, and bone. However, recent studies using pulsed 9.3-9.6 µm CO2 laser pulses of sub- millisecond duration have demonstrated efficient ablation of dental hard tissue with no excessive peripheral thermal and mechanical damage. The peak absorption of dental hard tissues occurs near 9.3 and 9.6 µm. At those wavelengths, the incident laser light is absorbed at a depth of less than 1-2 µm. For ablating enamel, dentin, and bone, pulse durations near 10-20 microseconds has been shown to be optimal.

 

Testing Methods

To evaluate the effect on tooth pulps of 9.3µm CO2 laser irradiation of occlusal surfaces, a recent study irradiated third molar teeth scheduled for extraction using a 9.3 µm CO2 laser.* Occlusal surfaces were irradiated for 1 minute at 50 hz and 2 minutes at 25 Hz using an ablative fluence of 20 J/cm2 and 12-15 mJ per pulse. The same total number of laser pulses were delivered to each tooth for both repetition rates, namely 3,000 pulses for a total energy of approximately 36-45 J. After extraction, both short- and long-term (90-day) effects on the teeth were observed, and micro- thermocouple measurements were used to estimate the potential temperature rise in the pulp chamber of the extracted teeth. Results were compared to those for two control groups (one with no treatment and one with a small cut made with a conventional high-speed handpiece.)

 

Results

After 2 minutes of laser irradiation with water spray, the tooth temperatures rose by only 1.7°C (± 1.6°C). Their mean temperature was 7°C below the ambient temperature and well below the 5.5°C above ambient temperature at which pulpal inflammation has been shown to occur. Even without water cooling, the mean temperature rise above ambient temperature was only 3.3°C. None of the control or treatment groups showed any deleterious effects, and none of the 29 test subjects felt any pain or discomfort after the procedure.

 

Conclusion

The results indicate that this 9.3µm CO2 laser can ablate enamel safely without harming the pulp.

 

For the complete study, contact Convergent Dental

 


Fundamental Interactions of Lasers with Dental Hard Tissues

By: John D.B. Featherstone and Daniel Fried, Department of Preventative &
Restorative Dental Sciences, UCSF

 

Introduction

The authors’ studies have demonstrated that treatment of enamel with a carbon dioxide laser can markedly inhibit subsequent caries progression. During irradiation, heat causes carbonate loss from the carbonated hydroxyapatite mineral, converting it into a low solubility hydroxyapatite-like calcium phosphate. The studies show it is possible to produce a laser that can selectively remove carious tissue and protect against future caries within the ablated walls. The purpose of the paper is to review fundamental aspects of hard tissue interactions with particular emphasis on the prevention of progression of dental decay.

 

Background

Numerous investigators have successfully shown that lasers effectively absorb components of hard tissue

[1, 2]. Laser interactions fall into three major categories, namely, 1) interaction with the mineral, 2) interaction with the protein or 3) interaction with the water. If detection of early decay is of interest, the laser wavelength must be such that the light will scatter in the carious region or have altered fluorescence properties that can be detected by instruments. If caries removal is of interest, the wavelength must be such that there is a major interaction with the components of the decay to lead to its ablation. In case of caries prevention, the laser interaction will most likely need to change the mineral from its acid soluble form to a much less soluble form.

 

Testing Methods

The paper reviews extensive studies conducted in the laboratory in regards to laser interactions such as absorption coefficients, temperature studies, caries prevention, and scattering/reflection.

 

Intra-oral Studies Caries Inhibition

Featherstone and coworkers [3] have published a study in which enamel blocks were irradiated in the laboratory and then placed in the mouths of human volunteers for 4 weeks simulating exposure to natural sources of erosion and decay in the mouth. The aim of the study is to use an intraoral model to determine whether caries inhibition due to laser irradiation, similar to our laboratory studies is observed in the human mouth. Samples were assessed by microradiography to compare the mineral loss before and after   treatment and derive a net change in mineral value. This study was designed only to show the effect of the laser treatment on inhibition of demineralization, and not the potential combined effect of laser and  fluoride treatment. Laboratory studies have indicated an additive effect of the two treatment modes on caries inhibition and/or remineralization [4].

 

Results

Extensive laboratory work has led to the choice of a range of laser conditions that can be used to treat enamel to make it resistant to dissolution by acids in the dental caries process. With a careful choice of laser parameters that are based upon fundamental knowledge of laser/hard tissue interactions, it is possible to selectively remove carious tissue and protect the preparation from further caries progression. These studies have demonstrated that treatment of enamel by carbon dioxide laser (9.3–9.6 μm) irradiation can markedly inhibit subsequent caries progression.

For the complete study, contact Convergent Dental

 

1 FRIED D: IR laser abalation of dental enamel. Lasers in Den- tistryVI, SPIE, Bellingham, WA 3910: 136–148
2 SEKA W, FEATHERSTONE JDB, FRIED D, VISURI SR, WALSH JT: Laser ablation of dental hard tissue: from explosive ablation to plasma-mediated in Lasers in Dentistry II. Vol. 2672 SPIE, Bellingham, WA 1996. 144–158
3 FEATHERSTONE JDB, FRIED D, GANSKY SA, STOOKEY GK, DUNIPACE AJ: Effect of carbon dioxide laser treatment on lesion progression in an intra-oral model. Lasers in Dentistry SPIE, Bellingham (WA) 2001. 4249–22: 87–91
4 PHAN ND, FRIED D, FEATHERSTONE JDB: Laser–induced transformation of carbonated apatite to fluorapatite on bovine Lasers in Dentistry V. SPIE, Bellingham, WA 1999. 3593: 233–240