Dissolution studies of bovine dental enamel surfaces modified by high-speed scanning ablation with a lambda = 9.3-microm TEA CO(2) laser

BACKGROUND AND OBJECTIVES: Previous studies have demonstrated that lasers can be used to modify the chemical composition of dental enamel to render the mineral phase more resistant to acid dissolution with minimal peripheral thermal damage. Transverse excited atmospheric (TEA) CO(2) lasers tuned to the strong mineral absorption of hydroxyapatite (HAP) near lambda = 9 microm are well-suited for the efficient ablation of dental hard tissues if the laser-pulse is stretched to greater than 5-10 microseconds to avoid plasma shielding phenomena. Moreover, TEA CO(2) lasers can be operated at very high repetition rates and are inherently less expensive and more versatile than Er:YAG and Er:YSGG solid-state lasers. In this study a lambda = 9.3-microm TEA CO(2) with a pulse duration of 8 microseconds and a repetition rate of 300 Hz was used to uniformly treat bovine enamel surfaces at ablative irradiation intensities. We hypothesized that a uniform surface layer of modified enamel of improved crystallinity and CaP phase composition would be formed with an enhanced resistance to acid-dissolution in the ablated areas at higher scanning rates used with the water spray. Such a modified layer of enamel formed at the base and walls of a cavity preparation under the irradiation conditions employed in this study have the potential to inhibit secondary caries under sealants and restorations. STUDY DESIGN/MATERIALS AND METHODS: The surfaces of bovine enamel blocks (3 x 3 mm(2)) were rapidly scanned across the laser beam at rates of 2, 3, and 6 mm/second with and without a water-spray at an incident fluence of 30 J/cm(2). The resistance to acid dissolution was evaluated using controlled surface dissolution experiments on laser-irradiated and control samples. RESULTS: The groups irradiated at a fluence of 30 J/cm(2) with a repetition rate of 300 Hz and a high scan rate of 6 mm/second with and without water-cooling significantly reduced the overall surface dissolution rates (P < 0.001). At low scan rates (2-3 mm/second) excessive heat deposition resulted in the formation of an outer layer of asperities containing non-apatitic CaP phases that were more susceptible to acid-dissolution. At a scanning rate of 6 mm/second even without the water spray a layer of purer phase HAP was formed without thermal damage, indicating that a high scanning rate can be used to avoid excessive thermal damage during ablation. The best results (80% inhibition) were attained for the higher scanning speed 6-mm/second combined with a water spray. CONCLUSION: This study demonstrates that an enamel surface with enhanced resistance to acid dissolution is produced after ablation with lambda = 9.3-microm TEA CO(2) laser pulses delivered at high-repetition rates if sufficiently high scanning rates are used with or without a water-spray.     

Influence of an optically thick water layer on the bond-strength of composite resin to dental enamel after IR laser ablation

BACKGROUND AND OBJECTIVES: Several studies of hard tissue ablation with Er:YAG lasers have shown that the addition of an optically thick water layer ( approximately 1 mm) added to the surface of dental enamel before each incident laser pulse, profoundly influences the rate and efficiency of ablation and the resulting surface morphology. The objective of this study was the determination of laser parameters which result in clinically useful bond strengths without the need for phosphoric acid etching. The hypothesis to be tested was that laser irradiation through a relatively thick layer of water would result in a surface to which composite could be bonded with bond strength similar to surfaces etched with phosphoric acid. This hypothesis is predicated on the assumption that the water prevents the formation of non-apatite calcium phosphate phases on the enamel surface. MATERIALS AND METHODS: In this study, a calibrated syringe pump and a motion control system were used to uniformly treat flat enamel surfaces using free-running Er:YAG laser pulses with and without water, and 9.6 mum CO(2) laser pulses on a dry surface for comparison. The rate of water delivery that resulted in the most efficient ablation was determined by profiling the resulting laser incisions using optical coherence tomography. In addition, enamel surfaces of 5 x 5 mm(2) were uniformly treated and the resulting surface morphology was examined using synchrotron radiation-fourier transform infrared spectroscopy (SR-FTIR), and optical and electron microscopy. The influence of the modified surface morphology on the adhesion of composite resin was investigated. RESULTS: The shear-bond strength of composite bonded to enamel surfaces irradiated at intensities clinically relevant for caries removal approached values measured for conventional acid etching when the water delivery rate was optimized. CONCLUSIONS: This study demonstrates that composite restorative materials can be directly bonded to laser prepared surfaces without the necessity of further surface preparation and acid etching and that the addition of a thick water layer ( approximately 1 mm) prevents the formation of undesirable CaP phases that compromise adhesion to restorative materials. 2003.     

Non-destructive assessment of inhibition of demineralization in dental enamel irradiated by a lambda=9.3-microm CO2 laser at ablative irradiation intensities with PS-OCT

BACKGROUND AND OBJECTIVE: Polarization sensitive optical coherence tomography (PS-OCT) has great promise for the non-destructive assessment of the efficacy of anti-caries agents such as fluoride and thermal laser treatments on enamel surfaces. The purpose of this study was to demonstrate that PS-OCT can be used to measure demineralization in craters/incisions prepared in enamel by a CO(2) laser operating at the high irradiation intensities required for cavity preparations. MATERIALS AND METHODS: Incisions in bovine enamel surfaces were produced by a CO(2) laser used with a water spray. The laser was operated at lambda=9.3 microm with a pulse duration of 15 micros and an incident fluence of 20 J/cm(2). The laser treatments were also combined with topical fluoride treatments. A PS-OCT system operating at 1,310 nm was used to acquire polarization resolved images of six areas including sound and laser-ablated+topical fluoride treated zones on each sample. After imaging the teeth, they were sectioned and the thin sections were examined with polarized light microscopy (PLM) and transverse microradiography (TMR). The integrated reflectivity and lesion depth derived from the PS-OCT scans, the integrated mineral loss and depth measured using TMR and the lesion depth measured with PLM were acquired for each area on the fifteen samples for comparison. RESULTS: The integrated reflectivity and depth in the areas treated by the laser and fluoride were significantly lower (P<0.05) than for the untreated enamel. Similar results were observed for TMR and PLM. CONCLUSIONS: These results suggest that PS-OCT has great potential for the non-destructive "in vivo" assessment of the inhibition of demineralization by lasers at ablative irradiation intensities with and without topical fluoride application.     

Selective removal of residual composite from dental enamel surfaces using the third harmonic of a Q-switched Nd:YAG laser

BACKGROUND AND OBJECTIVE: Conventional methods of residual composite removal after the debonding of orthodontic brackets involve the use of abrasives that damage the underlying enamel. The objective of this study was to demonstrate that 355-nm laser pulses with a pulse width of 10 ns are well suited for the removal of composite through selective laser ablation. STUDY DESIGN/MATERIALS AND METHODS: The residual composite remaining on the surface of extracted human third molars and bovine incisors was removed using multiple laser pulses from the third harmonic (355-nm) of a Q-switched Nd:YAG laser. RESULTS: There is selective ablation of composite from the enamel surface without any discernable damage to the underlying enamel. CONCLUSION: This study demonstrates that 355-nm, 10 ns laser pulses can be used for the selective ablation of dental composite without thermal or mechanical damage to the underlying enamel.     

Calcium solubility of dental enamel following sub-ablative Er:YAG and Er:YSGG laser irradiation in vitro

BACKGROUND AND OBJECTIVE: The objective of the present study was to investigate the effect of sub-ablative Er:YAG (lambda = 2.94 microm) and Er:YSGG (lambda = 2.79 microm) laser radiation on the acid solubility of dental enamel. The influence of fluoride application prior to laser irradiation was additionally evaluated. STUDY DESIGN/MATERIALS AND METHODS: To this end, 294 enamel specimens were prepared from bovine teeth and divided into 14 groups of 21 specimens each. The enamel samples were irradiated in their groups with the Er:YAG and the Er:YSGG laser, using energy densities of 4, 6, and 8 J/cm(2) in each case. Irradiation was additionally repeated in the same way on specimens, which had previously been immersed in 1% sodium fluoride solution for 15 minutes. One group was left untreated and served as a control group. A further group was not irradiated, but only immersed in the 1% fluoride solution for 15 minutes. The enamel specimens were demineralised for 24 hours in an acetate buffer solution. The calcium content in the demineralisation solution was subsequently determined with the aid of atomic absorption spectrometry. RESULTS: The results indicate a decline in calcium solubility after laser irradiation. Compared to the control group, a 20% lower calcium content was detected in the demineralisation solution after irradiation with the Er:YSGG laser at 8 J/cm(2). The difference between the laser-irradiated groups and the untreated control group was, however, not statistically significant. A significantly lower calcium content was found in the demineralisation solution after fluoridation of the specimens. Additional laser radiation had no further effect on this result. CONCLUSIONS: In summary, it can be stated that, although the erbium laser wavelengths apparently have the potential to increase acid resistance, their application solely for caries prevention would not appear to be sensible under the prevailing conditions.     

Selective ablation of surface enamel caries with a pulsed Nd:YAG dental laser

BACKGROUND AND OBJECTIVE: High intensity infrared light from the pulsed Nd:YAG dental laser is absorbed by carious enamel and not absorbed by healthy enamel. Consequently, this system has potential for selective removal of surface enamel caries. Safety and efficacy of the clinical procedure was evaluated in two sets of clinical trials at three dental schools. Selective ablation was evaluated with FTIR spectroscopy. STUDY DESIGN/MATERIALS AND METHODS: Carious lesions were randomized to drill or laser treatment. Pulp diagnosis, enamel surface condition, preparations, and restorations were evaluated by blinded evaluators. In Study I, surface caries were removed from 104 third molars scheduled for extraction. One-week post-treatment teeth were evaluated clinically, extracted, and the pulp was examined histologically. In Study II, 90 patients with 462 lesions on 374 teeth were randomized to laser or drill and followed for 6 months. RESULTS: Pulsed Nd:YAG laser removal of surface enamel caries was demonstrated to be both safe and effective. Caries were removed in all conditions. There were no adverse events and both clinical and histological evaluations of pulp vitality showed no abnormalities. A significantly greater number of preparations in the drill groups vs. laser groups entered dentin (drill = 11, laser = 1, P = 0.007). CONCLUSION: The more conservative laser treatment removed the caries but not the sound enamel below the lesion. The pulsed Nd:YAG dental laser was found to be both safe and effective for surface caries removal.     

Adhesion of composite resins to enamel irradiated by the Er:YAG laser: application of the ultrasonic scaler on irradiated surface

BACKGROUND AND OBJECTIVE: The clinical use of the Er:YAG laser in cavity preparation has become widespread. With respect to cavities prepared with the Er:YAG laser, restorations using composite resins and glass ionomer cements are common. The bond strength of current resin bonding systems to ground enamel is 14-30 MPa, whereas the strength to Er:YAG laser-irradiated enamel, regardless of the use or non-use of phosphoric acid etching is less than 10 MPa. STUDY DESIGN/MATERIALS AND METHODS: In this investigation, an ultrasonic scaler was tested as a technique to increase the bond strength of composite resins to the Er:YAG irradiated enamel surface. RESULTS: The application of the ultrasonic scaler significantly increased bond strength. CONCLUSIONS: The tensile bond strength in the groups treated with the ultrasonic scaler exhibited approximately twice the strength observed in groups treated with laser irradiation alone.