The effect of laser irradiation Michael Medical
L. Myers,
on oral tissues
DMD*
(‘allege ( f Georgia, School of Dentistq,
Augusta, (:a
Lasers have been proposed for numerous dental applications. Research on laser irradktion of enamel h.as demonstrated structural changes that resulted in a decrease in acid dissolution of the enamel. Dentin irradiation produced changes in surface morphology that improved bonding of restorative resins. This article reviews the studies of the effects of laser irradiation on oral tissues. (J PROSTHET DENT 1991:66::395-7.3
T
he idea of usmg a laser for dental applications has been considered for over 25 years. Lasers have been evaluated for a variety of uses which include both hard and soft tissue applicationis. The earliest reference in the dental literature can be credited to Stern and Sognnaes’ in which laser irradiation of enamel and dentin was evaluated. The laser was invented by Maiman in 1960 and it promis,ed to have many research and therapeutic applications. The term laser is an acronym for light amplification by stimulated emissi’an (if radiation (LASER). This occurs in the laser when atoms are excited and interact to release photons of light with the same wavelength and spatial coordination. The wavelength of the emitted radiation is dependent on the type of laser and the specific lasing medium u,sed. The light is focused by lenses to produce an intense b’eam. The eff’ect of the laser on the target tissue is dependent on several factors, primarily the wavelength of the laser and the absorption characteristics of the particular tissue. Other fa.ctors would include the amount of power used, length of exposure, sharpness of focus, and distance to t.he object. Lasers that have been investigated for dental use include ruby, argon, neodymium yttrium-aluminum-garnlet (Nd:YAG). helium-neon (He:Ne), and carbon dioxide (CO*) lasers.
SOFT
TISSUE
APPLICATIONS
Numerous soft tissue applications have been proposed for lasers. Helium-neon or “soft” lasers have been suggested for treatment of oral ulcerations, periodontitis, pericoronitis, and hypersensitivth dentin. There have been claims for improved wound healing and a decrease in edema following t,heir use. However, several studies of t,he He:Ne low-energy lasers have placed doubt on the effectiveness of this type of laser. Surinchak et a1.2 and Hunter et al.3 used He:Ne lasrrs at 632.8 mm and low-energy densities (1 to 2 J!cm-2i but found no difference in wound healing as compared .,vith conventional surgery.
Presented
bei’ore the .4( ademy of Denture
Prosthetics
meeting,
Corpus Christi, Tex. aAssociate Professor, 10/l/30546
THE
JOURNAI.
OF
Dwartment
P’ROSTHETIC
of Restorative
DENTISTRY
Dentistry.
In contrast, the CO2 laser has been used extensively for soft tissue surgical applications. CO2 lasers are well suited for soft tissue surgery because the light energy of the CO2 laser is highly absorbed by water. When a CO2 laser is used on mucosa, vaporization of cells occurs because of the high water content of the cells. With the beam focused to a narrow surface, incisions can be made. A wider focus can be used to produce more extensive tissue destruction or removal. The primary use of CO2 lasers in soft tissue surgery is for cutting, hemostasis, coagulation of bleeding tissues, and sterilization of the operative site. Drug-induced hyperplasias, such as dilantin hyperplasia of the gingiva, seem particularly suited to surgical treatment with the CO2 laser. The excess gingival tissue can be removed with a reduced amount of bleeding. Carbon dioxide lasers have been used for resection of tumors in the head and neck region. They have also been used successfully for incisional and excisional biopsies. Rossman et al.” used a CO:! laser on the gingival tissues of cynomolgus monkeys to determine the histologic effects of the CO2 laser. The laser was used with a lo-watt power setting and beam duration of 0.5 second to de-epithelialize the attached gingiva without affecting the underlying connective tissue. This allowed for an extremely controlled method of surgical removal of tissue. Miserendino5 reported the use of a CO2 laser for periapicai surgery for a failing root canal filling. He made a scalpel incision and reflected a flap to expose the periapical abscess. He then used a CO2 laser in a continuous mode at 3 watts with I- to 2second applications to produce hemostasis of the granulomatous tissue. This tissue was then vaporized with the laser before placement of a retrograde filling. He cited as advantages of this technique the ability of the laser to maintain hemostasis and the potential for sterilization of the infected root surface.
HARD
TISSUE
APPLICATIONS
In his early work in 1964 and 1966, Stern1q6 irradiated enamel with a ruby laser. He reported that the enamel was vaporized when a high-energy ruby laser pulse of 1 msec’s duration was used. This produced craters in the enamel that were characterized by melting and recrystallization of
395
MYERS
the enamel. There was also a decrease in the permeability of the enamel because of the fusing of the surface. Vahl’ used electron microscopy and x-ray diffraction to demonstrate ultrastructural and crystallographic changes resulting from the heat caused by laser irradiation. The use of a ruby laser as a cutting tool for hard structures was investigated by Goldman et al8 and Gordon.g However, they found that the cutting of enamel required high energies ( lo4 J/cme2) and long interaction times that produced extreme local heat, which resulted in cracking of the enamel. This problem, not to mention the potential of adverse pulpal effects, ended investigations of the ruby laser for dental applications. Other studies have focused on the use of an Nd:YAG laser. Yamamoto and OoyalO and Yamamato and Satol’ demonstrated the potential to fuse enamel pits and fissures and also make the enamel surface more resistant to subsequent acid dissolution due to a decrease in enamel permeability. Most of the early studies (1965 to 1970) used high-energy visible light laser radiation that could produce structural and chemical changes, but they also produced undesirable morphologic effects (crazing, pitting, and cracking) because of the high heat. Then it was discovered that low-energy pulses produced by a CO2 laser could produce some of these same structural and chemical effects. Stern et a1.,12 Kantola et a1.,i3and Lenz et a1.14did studies with this type of laser and found increased resistance to acid dissolution of enamel and partial inhibition of artificial caries formation. Stern et all2 found that the CO2 laser, which produces radiation in the infrared region, was 10 times more effective at a given energy density than the ruby laser, which emits radiation in the visible light region. They suggested that the CO2 laser, because of its wavelength, was more efficiently absorbed by the enamel. Nelson et a1.15proposed a reason for the efficiency of the CO:,laser for irradiation of enamel and dentin. Enamel and dentin contain carbonated apatite, which has absorption bands in the infrared wavelength range because of phosphate, carbonate, and hydroxyl groups in the crystal structure. For this reason, enamel absorbs very little light in the visible light region. However, the wavelength of radiation produced by the carbon dioxide laser is in the infrared region (9.3 to 10.6 pm), which is efficiently absorbed by the apatite crystals of dental hard tissues. In other words, these wavelengths coincide with the frequencies of maximum absorption of energy of the apatite. This energy is then converted into heat energy to produce the changes in the apatite. The earliest lasers were continuous wave lasers. Later developments led to the use of pulsed lasers. A pulsed laser provides a way for increasing the peak power density while keeping the pulse energy density constant. The result is that the effect of this type of radiation (fusing, melting, and recrystallization of enamel) is confined to a 5 to 10 pm
396
thin layer at the surface. It should not affect the deeper layers of dentin and should not affect the pulp.16 Nelson et a1.15investigated the effect of a pulsed CO2 laser on artificial caries formation in enamel. Noncarious crowns of molars and premolars were sectioned from their roots and painted with acid-resistant varnish on the surface, leaving two 1 X 3 mm rectangular windows. The teeth were then cut in half so that one window from each tooth could be used as an unlased control and the other window would be irradiated with the laser. The laser used in this study was a tunable CO2 laser. It produced 100 to 200 msec pulses with a maximum pulse energy of 5 J. Four different wavelengths were chosen by use of an optical laser spectrum analyzer. The wavelengths used were 9.32 pm, 9.57 pm, 10.27pm, and 10.59 pm. A concave mirror was used to produce a laser beam 2 to 5 mm in diameter with a pulse energy density of 10 to 50 J/cmm2.The enamel surface was lased at 0.67 Hz for 1, 3, or 10 minutes. After laser treatment, the control and lased specimens were placed in an artificial caries solution with a pH of 5 for 2 to 14 days. Following the emersion period in the artificial caries solution, the control and lased specimens were prepared for cross-sectional microhardness testing. A Leitz microhardness tester (E. Leitz, Inc., Rockleigh, N.J.) was used to determine hardness profiles beginning at a depth of 25 pm from the surface and continuing through the lesion into sound enamel underneath. The change in hardness was used to calculate the percent of mineral loss (demineralization) of the enamel lesion. The result of this study was that the laser treatment inhibited the formation of artificial carious lesions in enamel from 25 % to 50%. However, some wavelengths and energy densities were more effective than others. At the lower energy density of 10 J/cmm2 the laser line (wave number) of 1073 cm-’ was most effective. At the higher energy density of 50 J/cmm2, the 1073 cm-’ line was also the most effective. All four wavelengths showed greater inhibition of lesion formation at the higher energy density than the lower energy density. SEM evaluation of the enamel surfaces made it possible to examine the morphologic changes that occurred as a result of the laser treatment.“, l8 The enamel surface was roughened to varying degrees depending on the wavelength and energy density used. The higher energy levels produced a melted surface that, although irregular, was characterized by a fused surface. Cross sections of these zones revealed that the effect of the laser extended approximately 5 Mm below the enamel surface. Polarized light examination of the lesions were also performed. They confirmed the inhibition of lesion formation from the laser treatment. The depth of the surface melt was also visible on the polarized light sections. Observations of distinct changes in surface morphology of dentin surfaces that had been irradiated with the carbon dioxide laser led to another study by Cooper et al.lg On a microscopic level the lased dentin surface appeared very
SEPTEMBER
1991
VOLUME
66
NUMBER
3
LASER
IRRAD‘.41‘10\;
OF
)RAL
TISSaUES
irregular. It was thought that this surface would be more desirable for bonding of composite resin filling materials. Teeth were cbliqu’ely sectioned to expose a dentinal surface of 1 cm in diameter. Iialf of the teeth were irradiated with a COs laser. The wavelength chosen, based on previous findings, was the 9.3 ! pm wavelength (1073 cm-r wave number). Thirty pulses delivered over a period of 40 seconds produced IO to .‘:OJ/cm-’ energy doses. After an application of demin adhesive, a cylinder of composite resin was bonded IX) each dentin surface. The samples were tested for shear strength of the bonded composite resin to the prepared surfsces by use of an Instron testing machine with a 5 kg load delivtred at a crosshead speed of 0.01 cm/ min. Comparison of composite retsin bond strengths of lased dentin to unlased dentin indicated a threefold increase in bond strength to the lased surface. The unlased dentin had a bond of 25 kg/c& as compared with 77 kg/cm2 for the dentin surface that had been prepared with the laser irradiation SEM examination reve,aled significant differences in the lased dentin surface as compared to normal dentin. Localized melting and recrystallization produced fungiform projectons on the dentin surface. Examination of the fracture interfaces sh:rwed that the composite resin had filled the spaces between the dentinal projections. Failures occurred most often within the dentin, indicating that the bond of the resin was stronger than the dentinal projections. Laser irradiation of the dent n produced a surface morphology that significantly improved the mechanical bond of the composit.e resin. The resin adapted to the spaces and undercuts between the dentinal projections in a manner similar to that of acid etching techniques of enamel. This showed the potential of laser irradiation of dentin to improve composite resin bond strengths. Other applications that have been successfully demonstrated include fusing hydroxyapatite into pits and fissures of teeth,‘O debridment of incipient caries21 sealing dentin walls for root canal procedures, (and fusing enamel, dentin, or hydroxyapatite in root apices’s! *s
There are many aspects of laser technology and application that need further refinement and research. The ability to direct the beam in the oral cavity with the degree of precision required for hard tissue procedures must be improved. Other major limitation of lasers for dental use include cost acd size of laser units. Determination of optimum lasing protocols and histologic studies are needed to assess the long-term effects of ls.ser irradiation. The development of practical deiivery systems and investigations of
THE
JOURNAL
OF
PROSTHETIC
DENTISTRY
other laser modalities are required if lasers are to be used routinely for hard tissue applications in the clinical practice of dentistry.
REFERENCES 1. Stern RH, Sognnaes RF. Laser beam effect on dental hard tissues. J Dent Res 1964;43:873. 2. Surinchak dS, Alago ML, Bellamy RF, Stuck BE, Belkin M. Effects of low-level energy lasers on the healing of full-thickness skin defects. Lasers Surg Med 1983;2:267-74. 3. Hunter J, Leonard L, Wilson R, Snider G, Dixon J. Effects of low energy laser on wound healing in a porcine model. Lasers Surg Med 1984;3:285-90. 4. Rossman 5.4, Gottlieb S, Koudeika BM, McQuade MJ. Effects of COe laser irradiation on gingiva. J Periodontol 1987;58:423-5. 5. Miserendino LJ. The laser apicoectomy: endodontic application of the CO2 laser for periapical surgery. Oral Surg 1988;66:615-9. 6. Stern RH, Sogannaes RF, Goodman F. Laser effect on in vitro enamel permeability and solubility. J Am Dent Assoc 1966;73:838-43. 7. Vahl d. Electron microscopical and X-ray crystallographic investigations of teeth exposed to laser rays. Caries Res 1968;2:10-8. 8 Goldman L, Gray JA, Goldman J, Goldman B, Meyer R. Effect of laser beam impacts on teeth. J Am Dent Assoc 1965;70:601-6. 9 Gordon TE. Single-surface cutting of normal tooth with ruby laser. J Am Dent Assoc 1967;74:398-402. 10 Yamamoto H, Ooya K. Potential of yttrium-aluminum-garnet laser in caries prevention. J Oral Pathol 1974;3:7-15. I1 Yamamoto H, Sato K. Prevention of dental caries bv Nd:YAG laser irradiation. J Dent Res 1980;59:2171-7. 12 Stern RH, Vahl J, Sognnaes RF. Lased enamel: ultrastructural observations of pulsed carbon dioxide laser effects. .J Dent Res 1972;51:45560. 13 Kantola S, Laine E, Tarna T. Laser-induced effects on tooth structure. VI. X-ray diffraction study of dental enamel exposed to a COe laser. Acta Odontol Stand 1973;31:369-79. 14 Lenz P, Glide H, Walz R. Studies on enamel sealing with the CO:! laser. Dtsch Zahnarztl 2 1982;37:469-78. 15 Nelson DGA, Shariati M, Glena D, Shields CP, Featherstone JDB. Effect of pulsed low energy infrared laser irradiation on artificial carieslike lesion formation. Caries Res 1986;20:289-99. 16 Boem RF, Chen MJ, Blair CK. Temperatures in human teeth due to laser heating. Am Sot Mechan Engineering 1975;75-WA BIO-8. 17 Nelson DGA, Jongebloed WL, Featherstone JDB. Laser irradiation of human dental enamel and dentine. N Z Dent J 1986;82:74-7. 1X Nelson DGA, Wefel JS, Jongebloed WL, Featherstone JDB. Morphology. histology and crystallography of human dental enamel treated with pulsed low energy infrared laser radiation. Caries Res 1987;21:411-26. 19 Cooper LF, Myers ML, Nelson DGA, Mowery AS. Shear strength of composite bonded to laser-pretreated dentin. J PROSTHETDE~VT 1988;60:4.5-9. 20 Stewart L, Powell GL, Wright S. Hydroxyapatite attached by laser: a potential sealant for pits and fissures. Oper Dent 1985;10:2-5. 21 Myers TD, Myers WD. The use of a laser for debridement of incipient caries. J PROSTHETDENT 1985;53:776-9. 22 Zakariasen KL. McMurray M, Patterson K, Dederich D, Tulip J. Apical leakage associated with lased and unlased apical plugs. J Dent Res 1986;65:253. 22 Dederich DN, Zakariasen KL, Tulip J. Effects ofcontmuous-wave CO2 laser on tanal-wall dentin. J Dent Res 1986;65:253. Reprmt rrqwais tit DR. MICHAEL L. MYERS ScH001,oF DENTI.~TRY MEUICAI~ COLLEGE OF GEORGIA Armrs?~. GA X191’
397
L. Myers,
on oral tissues
DMD*
(‘allege ( f Georgia, School of Dentistq,
Augusta, (:a
Lasers have been proposed for numerous dental applications. Research on laser irradktion of enamel h.as demonstrated structural changes that resulted in a decrease in acid dissolution of the enamel. Dentin irradiation produced changes in surface morphology that improved bonding of restorative resins. This article reviews the studies of the effects of laser irradiation on oral tissues. (J PROSTHET DENT 1991:66::395-7.3
T
he idea of usmg a laser for dental applications has been considered for over 25 years. Lasers have been evaluated for a variety of uses which include both hard and soft tissue applicationis. The earliest reference in the dental literature can be credited to Stern and Sognnaes’ in which laser irradiation of enamel and dentin was evaluated. The laser was invented by Maiman in 1960 and it promis,ed to have many research and therapeutic applications. The term laser is an acronym for light amplification by stimulated emissi’an (if radiation (LASER). This occurs in the laser when atoms are excited and interact to release photons of light with the same wavelength and spatial coordination. The wavelength of the emitted radiation is dependent on the type of laser and the specific lasing medium u,sed. The light is focused by lenses to produce an intense b’eam. The eff’ect of the laser on the target tissue is dependent on several factors, primarily the wavelength of the laser and the absorption characteristics of the particular tissue. Other fa.ctors would include the amount of power used, length of exposure, sharpness of focus, and distance to t.he object. Lasers that have been investigated for dental use include ruby, argon, neodymium yttrium-aluminum-garnlet (Nd:YAG). helium-neon (He:Ne), and carbon dioxide (CO*) lasers.
SOFT
TISSUE
APPLICATIONS
Numerous soft tissue applications have been proposed for lasers. Helium-neon or “soft” lasers have been suggested for treatment of oral ulcerations, periodontitis, pericoronitis, and hypersensitivth dentin. There have been claims for improved wound healing and a decrease in edema following t,heir use. However, several studies of t,he He:Ne low-energy lasers have placed doubt on the effectiveness of this type of laser. Surinchak et a1.2 and Hunter et al.3 used He:Ne lasrrs at 632.8 mm and low-energy densities (1 to 2 J!cm-2i but found no difference in wound healing as compared .,vith conventional surgery.
Presented
bei’ore the .4( ademy of Denture
Prosthetics
meeting,
Corpus Christi, Tex. aAssociate Professor, 10/l/30546
THE
JOURNAI.
OF
Dwartment
P’ROSTHETIC
of Restorative
DENTISTRY
Dentistry.
In contrast, the CO2 laser has been used extensively for soft tissue surgical applications. CO2 lasers are well suited for soft tissue surgery because the light energy of the CO2 laser is highly absorbed by water. When a CO2 laser is used on mucosa, vaporization of cells occurs because of the high water content of the cells. With the beam focused to a narrow surface, incisions can be made. A wider focus can be used to produce more extensive tissue destruction or removal. The primary use of CO2 lasers in soft tissue surgery is for cutting, hemostasis, coagulation of bleeding tissues, and sterilization of the operative site. Drug-induced hyperplasias, such as dilantin hyperplasia of the gingiva, seem particularly suited to surgical treatment with the CO2 laser. The excess gingival tissue can be removed with a reduced amount of bleeding. Carbon dioxide lasers have been used for resection of tumors in the head and neck region. They have also been used successfully for incisional and excisional biopsies. Rossman et al.” used a CO:! laser on the gingival tissues of cynomolgus monkeys to determine the histologic effects of the CO2 laser. The laser was used with a lo-watt power setting and beam duration of 0.5 second to de-epithelialize the attached gingiva without affecting the underlying connective tissue. This allowed for an extremely controlled method of surgical removal of tissue. Miserendino5 reported the use of a CO2 laser for periapicai surgery for a failing root canal filling. He made a scalpel incision and reflected a flap to expose the periapical abscess. He then used a CO2 laser in a continuous mode at 3 watts with I- to 2second applications to produce hemostasis of the granulomatous tissue. This tissue was then vaporized with the laser before placement of a retrograde filling. He cited as advantages of this technique the ability of the laser to maintain hemostasis and the potential for sterilization of the infected root surface.
HARD
TISSUE
APPLICATIONS
In his early work in 1964 and 1966, Stern1q6 irradiated enamel with a ruby laser. He reported that the enamel was vaporized when a high-energy ruby laser pulse of 1 msec’s duration was used. This produced craters in the enamel that were characterized by melting and recrystallization of
395
MYERS
the enamel. There was also a decrease in the permeability of the enamel because of the fusing of the surface. Vahl’ used electron microscopy and x-ray diffraction to demonstrate ultrastructural and crystallographic changes resulting from the heat caused by laser irradiation. The use of a ruby laser as a cutting tool for hard structures was investigated by Goldman et al8 and Gordon.g However, they found that the cutting of enamel required high energies ( lo4 J/cme2) and long interaction times that produced extreme local heat, which resulted in cracking of the enamel. This problem, not to mention the potential of adverse pulpal effects, ended investigations of the ruby laser for dental applications. Other studies have focused on the use of an Nd:YAG laser. Yamamoto and OoyalO and Yamamato and Satol’ demonstrated the potential to fuse enamel pits and fissures and also make the enamel surface more resistant to subsequent acid dissolution due to a decrease in enamel permeability. Most of the early studies (1965 to 1970) used high-energy visible light laser radiation that could produce structural and chemical changes, but they also produced undesirable morphologic effects (crazing, pitting, and cracking) because of the high heat. Then it was discovered that low-energy pulses produced by a CO2 laser could produce some of these same structural and chemical effects. Stern et a1.,12 Kantola et a1.,i3and Lenz et a1.14did studies with this type of laser and found increased resistance to acid dissolution of enamel and partial inhibition of artificial caries formation. Stern et all2 found that the CO2 laser, which produces radiation in the infrared region, was 10 times more effective at a given energy density than the ruby laser, which emits radiation in the visible light region. They suggested that the CO2 laser, because of its wavelength, was more efficiently absorbed by the enamel. Nelson et a1.15proposed a reason for the efficiency of the CO:,laser for irradiation of enamel and dentin. Enamel and dentin contain carbonated apatite, which has absorption bands in the infrared wavelength range because of phosphate, carbonate, and hydroxyl groups in the crystal structure. For this reason, enamel absorbs very little light in the visible light region. However, the wavelength of radiation produced by the carbon dioxide laser is in the infrared region (9.3 to 10.6 pm), which is efficiently absorbed by the apatite crystals of dental hard tissues. In other words, these wavelengths coincide with the frequencies of maximum absorption of energy of the apatite. This energy is then converted into heat energy to produce the changes in the apatite. The earliest lasers were continuous wave lasers. Later developments led to the use of pulsed lasers. A pulsed laser provides a way for increasing the peak power density while keeping the pulse energy density constant. The result is that the effect of this type of radiation (fusing, melting, and recrystallization of enamel) is confined to a 5 to 10 pm
396
thin layer at the surface. It should not affect the deeper layers of dentin and should not affect the pulp.16 Nelson et a1.15investigated the effect of a pulsed CO2 laser on artificial caries formation in enamel. Noncarious crowns of molars and premolars were sectioned from their roots and painted with acid-resistant varnish on the surface, leaving two 1 X 3 mm rectangular windows. The teeth were then cut in half so that one window from each tooth could be used as an unlased control and the other window would be irradiated with the laser. The laser used in this study was a tunable CO2 laser. It produced 100 to 200 msec pulses with a maximum pulse energy of 5 J. Four different wavelengths were chosen by use of an optical laser spectrum analyzer. The wavelengths used were 9.32 pm, 9.57 pm, 10.27pm, and 10.59 pm. A concave mirror was used to produce a laser beam 2 to 5 mm in diameter with a pulse energy density of 10 to 50 J/cmm2.The enamel surface was lased at 0.67 Hz for 1, 3, or 10 minutes. After laser treatment, the control and lased specimens were placed in an artificial caries solution with a pH of 5 for 2 to 14 days. Following the emersion period in the artificial caries solution, the control and lased specimens were prepared for cross-sectional microhardness testing. A Leitz microhardness tester (E. Leitz, Inc., Rockleigh, N.J.) was used to determine hardness profiles beginning at a depth of 25 pm from the surface and continuing through the lesion into sound enamel underneath. The change in hardness was used to calculate the percent of mineral loss (demineralization) of the enamel lesion. The result of this study was that the laser treatment inhibited the formation of artificial carious lesions in enamel from 25 % to 50%. However, some wavelengths and energy densities were more effective than others. At the lower energy density of 10 J/cmm2 the laser line (wave number) of 1073 cm-’ was most effective. At the higher energy density of 50 J/cmm2, the 1073 cm-’ line was also the most effective. All four wavelengths showed greater inhibition of lesion formation at the higher energy density than the lower energy density. SEM evaluation of the enamel surfaces made it possible to examine the morphologic changes that occurred as a result of the laser treatment.“, l8 The enamel surface was roughened to varying degrees depending on the wavelength and energy density used. The higher energy levels produced a melted surface that, although irregular, was characterized by a fused surface. Cross sections of these zones revealed that the effect of the laser extended approximately 5 Mm below the enamel surface. Polarized light examination of the lesions were also performed. They confirmed the inhibition of lesion formation from the laser treatment. The depth of the surface melt was also visible on the polarized light sections. Observations of distinct changes in surface morphology of dentin surfaces that had been irradiated with the carbon dioxide laser led to another study by Cooper et al.lg On a microscopic level the lased dentin surface appeared very
SEPTEMBER
1991
VOLUME
66
NUMBER
3
LASER
IRRAD‘.41‘10\;
OF
)RAL
TISSaUES
irregular. It was thought that this surface would be more desirable for bonding of composite resin filling materials. Teeth were cbliqu’ely sectioned to expose a dentinal surface of 1 cm in diameter. Iialf of the teeth were irradiated with a COs laser. The wavelength chosen, based on previous findings, was the 9.3 ! pm wavelength (1073 cm-r wave number). Thirty pulses delivered over a period of 40 seconds produced IO to .‘:OJ/cm-’ energy doses. After an application of demin adhesive, a cylinder of composite resin was bonded IX) each dentin surface. The samples were tested for shear strength of the bonded composite resin to the prepared surfsces by use of an Instron testing machine with a 5 kg load delivtred at a crosshead speed of 0.01 cm/ min. Comparison of composite retsin bond strengths of lased dentin to unlased dentin indicated a threefold increase in bond strength to the lased surface. The unlased dentin had a bond of 25 kg/c& as compared with 77 kg/cm2 for the dentin surface that had been prepared with the laser irradiation SEM examination reve,aled significant differences in the lased dentin surface as compared to normal dentin. Localized melting and recrystallization produced fungiform projectons on the dentin surface. Examination of the fracture interfaces sh:rwed that the composite resin had filled the spaces between the dentinal projections. Failures occurred most often within the dentin, indicating that the bond of the resin was stronger than the dentinal projections. Laser irradiation of the dent n produced a surface morphology that significantly improved the mechanical bond of the composit.e resin. The resin adapted to the spaces and undercuts between the dentinal projections in a manner similar to that of acid etching techniques of enamel. This showed the potential of laser irradiation of dentin to improve composite resin bond strengths. Other applications that have been successfully demonstrated include fusing hydroxyapatite into pits and fissures of teeth,‘O debridment of incipient caries21 sealing dentin walls for root canal procedures, (and fusing enamel, dentin, or hydroxyapatite in root apices’s! *s
There are many aspects of laser technology and application that need further refinement and research. The ability to direct the beam in the oral cavity with the degree of precision required for hard tissue procedures must be improved. Other major limitation of lasers for dental use include cost acd size of laser units. Determination of optimum lasing protocols and histologic studies are needed to assess the long-term effects of ls.ser irradiation. The development of practical deiivery systems and investigations of
THE
JOURNAL
OF
PROSTHETIC
DENTISTRY
other laser modalities are required if lasers are to be used routinely for hard tissue applications in the clinical practice of dentistry.
REFERENCES 1. Stern RH, Sognnaes RF. Laser beam effect on dental hard tissues. J Dent Res 1964;43:873. 2. Surinchak dS, Alago ML, Bellamy RF, Stuck BE, Belkin M. Effects of low-level energy lasers on the healing of full-thickness skin defects. Lasers Surg Med 1983;2:267-74. 3. Hunter J, Leonard L, Wilson R, Snider G, Dixon J. Effects of low energy laser on wound healing in a porcine model. Lasers Surg Med 1984;3:285-90. 4. Rossman 5.4, Gottlieb S, Koudeika BM, McQuade MJ. Effects of COe laser irradiation on gingiva. J Periodontol 1987;58:423-5. 5. Miserendino LJ. The laser apicoectomy: endodontic application of the CO2 laser for periapical surgery. Oral Surg 1988;66:615-9. 6. Stern RH, Sogannaes RF, Goodman F. Laser effect on in vitro enamel permeability and solubility. J Am Dent Assoc 1966;73:838-43. 7. Vahl d. Electron microscopical and X-ray crystallographic investigations of teeth exposed to laser rays. Caries Res 1968;2:10-8. 8 Goldman L, Gray JA, Goldman J, Goldman B, Meyer R. Effect of laser beam impacts on teeth. J Am Dent Assoc 1965;70:601-6. 9 Gordon TE. Single-surface cutting of normal tooth with ruby laser. J Am Dent Assoc 1967;74:398-402. 10 Yamamoto H, Ooya K. Potential of yttrium-aluminum-garnet laser in caries prevention. J Oral Pathol 1974;3:7-15. I1 Yamamoto H, Sato K. Prevention of dental caries bv Nd:YAG laser irradiation. J Dent Res 1980;59:2171-7. 12 Stern RH, Vahl J, Sognnaes RF. Lased enamel: ultrastructural observations of pulsed carbon dioxide laser effects. .J Dent Res 1972;51:45560. 13 Kantola S, Laine E, Tarna T. Laser-induced effects on tooth structure. VI. X-ray diffraction study of dental enamel exposed to a COe laser. Acta Odontol Stand 1973;31:369-79. 14 Lenz P, Glide H, Walz R. Studies on enamel sealing with the CO:! laser. Dtsch Zahnarztl 2 1982;37:469-78. 15 Nelson DGA, Shariati M, Glena D, Shields CP, Featherstone JDB. Effect of pulsed low energy infrared laser irradiation on artificial carieslike lesion formation. Caries Res 1986;20:289-99. 16 Boem RF, Chen MJ, Blair CK. Temperatures in human teeth due to laser heating. Am Sot Mechan Engineering 1975;75-WA BIO-8. 17 Nelson DGA, Jongebloed WL, Featherstone JDB. Laser irradiation of human dental enamel and dentine. N Z Dent J 1986;82:74-7. 1X Nelson DGA, Wefel JS, Jongebloed WL, Featherstone JDB. Morphology. histology and crystallography of human dental enamel treated with pulsed low energy infrared laser radiation. Caries Res 1987;21:411-26. 19 Cooper LF, Myers ML, Nelson DGA, Mowery AS. Shear strength of composite bonded to laser-pretreated dentin. J PROSTHETDE~VT 1988;60:4.5-9. 20 Stewart L, Powell GL, Wright S. Hydroxyapatite attached by laser: a potential sealant for pits and fissures. Oper Dent 1985;10:2-5. 21 Myers TD, Myers WD. The use of a laser for debridement of incipient caries. J PROSTHETDENT 1985;53:776-9. 22 Zakariasen KL. McMurray M, Patterson K, Dederich D, Tulip J. Apical leakage associated with lased and unlased apical plugs. J Dent Res 1986;65:253. 22 Dederich DN, Zakariasen KL, Tulip J. Effects ofcontmuous-wave CO2 laser on tanal-wall dentin. J Dent Res 1986;65:253. Reprmt rrqwais tit DR. MICHAEL L. MYERS ScH001,oF DENTI.~TRY MEUICAI~ COLLEGE OF GEORGIA Armrs?~. GA X191’
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