Cavity preparation using a superpulsed 9.6-microm CO2 laser--a histological investigation

Automated ablation of dental composite using an IR pulsed laser coupled to a plume emission spectral feedback system

OBJECTIVE

The purpose of this study is to assemble a laser system for the selective removal of dental composite from tooth surfaces, that is feasible for clinical use incorporating a spectral feedback system, a scanning system, articulating arm and a clinical hand-piece, and evaluate the performance of that system on extracted teeth.

METHODS

Ten extracted teeth were collected and small fillings were placed on the occlusal surface of each tooth. A clinical system featuring a CO₂ laser operating at 50 Hz and spectral optical feedback was used to remove the composite. Removal was confirmed using a cross polarized optical coherence tomography (CP-OCT) system designed for clinical use.

RESULTS

The system was capable of rapidly removing composite from small preparations on tooth occlusal surfaces with a mean loss of enamel of less than 20 μm.

CONCLUSION

We have demonstrated that spectral feedback can be successfully employed in an automated system for composite removal by incorporating dual photodiodes and a galvanometer controlled CO₂ laser. Additionally, the use of registered OCT images presents as a viable method for volumetric benchmarking. Overall, this study represents the first implementation of spectral feedback into a clinical hand-piece and serves as a benchmark for a future clinical study. Lasers Surg. Med. 49:658-665, 2017. © 2017 Wiley Periodicals, Inc.

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

CO(2) lasers can be operated at high laser pulse repetition rates for the rapid and precise removal of dental decay. Excessive heat accumulation and peripheral thermal damage is a concern when using high pulse repetition rates. Peripheral thermal damage can adversely impact the mechanical strength of the irradiated tissue, particularly for dentin, and reduce the adhesion characteristics of the modified surfaces. The interpulpal temperature rise was recorded using microthermocouples situated at the roof of the pulp chamber on teeth that were occlusally ablated using a rapidly-scanned CO(2) laser operating at 9.3 μm with a pulse duration of 10 to 15 μs and repetition rate of 300 Hz over a 2 min time course. The adhesion strength of laser treated enamel and dentin surfaces was measured for various laser scanning parameters with and without post-ablation acid etching using the single-plane shear test. The mechanical strength of laser-ablated dentin surfaces were determined via the four-point bend test and compared to control samples prepared with 320 grit wet sand paper to simulate conventional preparations. Thermocouple measurements indicated that the temperature remained below ambient temperature if water-cooling was used. There was no discoloration of either dentin or enamel laser treated surfaces, the surfaces were uniformly ablated, and there were no cracks visible. Four-point bend tests yielded mean mechanical strengths of 18.2 N (s.d. = 4.6) for ablated dentin and 18.1 N (s.d. = 2.7) for control (p > 0.05). Shear tests yielded mean bond strengths approaching 30 MPa for both enamel and dentin under certain irradiation conditions. These values were slightly lower than nonirradiated acid-etched control samples. Additional studies are needed to determine if the slightly lower bond strength than the acid-etched control samples is clinically significant. These measurements demonstrate that enamel and dentin surfaces can be rapidly ablated by CO(2) lasers with minimal peripheral thermal and mechanical damage and without excessive heat accumulation.

Pulpal effects of enamel ablation with a microsecond pulsed lambda = 9.3-microm CO2 laser

BACKGROUND AND OBJECTIVES

In vitro studies have shown that CO2 lasers operating at the highly absorbed 9.3 and 9.6-microm wavelengths with a pulse duration in the range of 10-20-microsecond are well suited for the efficient ablation of enamel and dentin with minimal peripheral thermal damage. Even though these CO2 lasers are highly promising, they have yet to receive FDA approval. Clinical studies are necessary to determine if excessive heat deposition in the tooth may have any detrimental pulpal effects, particularly at higher ablative fluencies. The purpose of this study was to evaluate the pulpal safety of laser irradiation of tooth occlusal surfaces under the conditions required for small conservative preparations confined to enamel.

STUDY DESIGN/MATERIALS AND METHODS

Test subjects requiring removal of third molar teeth were recruited and teeth scheduled for extraction were irradiated using a pulsed CO2 laser at a wavelength of 9.3 microm operating at 25 or 50 Hz using a incident fluence of 20 J/cm(2) for a total of 3,000 laser pulses (36 J) for both rates with water cooling. Two control groups were used, one with no treatment and one with a small cut made with a conventional high-speed hand-piece. No anesthetic was used for any of the procedures and tooth vitality was evaluated prior to treatment by heat, cold and electrical testing. Short term effects were observed on teeth extracted within 72 hours after treatment and long term effects were observed on teeth extracted 90 days after treatment. The pulps of the teeth were fixed with formalin immediately after extraction and subjected to histological examination. Additionally, micro-thermocouple measurements were used to estimate the potential temperature rise in the pulp chamber of extracted teeth employing the same irradiation conditions used in vivo.

RESULTS

Pulpal thermocouple measurements showed the internal temperature rise in the tooth was within safe limits, 3.3+/-1.4 degrees C without water cooling versus 1.7+/-1.6 degrees C with water-cooling, n = 25, P<0.05. None of the control or treatment groups showed any deleterious effects on pulpal tissues and none of the 29 test-subjects felt pain or discomfort after the procedure. Only two test-subjects felt discomfort from "cold sensitivity" during the procedure caused by the water-spray.

CONCLUSION

It appears that this CO2 laser can ablate enamel safely without harming the pulp under the rate of energy deposition employed in this study.

Surgical lasers and hard dental tissue

The cutting of dental hard tissue during restorative procedures presents considerable demands on the ability to selectively remove diseased carious tissue, obtain outline and retention form and maintain the integrity of supporting tooth tissue without structural weakening. In addition, the requirement to preserve healthy tissue and prevent further breakdown of the restoration places the choice of instrumentation and clinical technique as prime factors for the dental surgeon. The quest for an alternative treatment modality to the conventional dental turbine has been, essentially, patient-driven and has led to the development of various mechanical and chemical devices. The review of the literature has endorsed the beneficial effects of current laser machines. However utopian, there is additional evidence to support the development of ultra-short (nano- and femto-second) pulsed lasers that are stable in use and commercially viable, to deliver more efficient hard tissue ablation with less risk of collateral thermal damage. This paper explores the interaction of laser energy with dental hard tissues and bone and the integration of current laser wavelengths into restorative and surgical dentistry.

Microstructural changes of enamel, dentin-enamel junction, and dentin induced by irradiating outer enamel surfaces with CO2 laser

The microstructural changes of enamel, dentin-enamel junction (DEJ), and dentin after CO(2) laser irradiation were studied. Buccal enamel surfaces of human third molars about 2 mm from the DEJ were irradiated, and the laser tip was moved 5 mm in a mesiodistal direction with 5 s of irradiation time. The output powers were 2 to 10 W in a continuous mode, and the average doses were approximately from 250 to 1,250 J/cm(2). All specimens were examined by a scanning electron microscope, and heat-transfer simulation was also applied to calculate temperature distribution. Surface ablation of enamel and separation of enamel and dentin along the DEJ were observed when the laser power output exceeded 3 W. Heat-transfer simulation indicated that dentin was prone to store more thermal energy than enamel. In conclusion, operation parameters of the CO(2) laser should be carefully selected to avoid thermal damage to the DEJ.

An Er:YAG laser bone cutting manipulator for precise rotational acetabular osteotomy

Rotational acetabular osteotomy (RAO) has an important advantage in that surgical bony defects are reconstructed with a patients' own tissue. We propose a surgical robot for the RAO using Er:YAG laser irradiating mounted on iliac bone to operate RAO precisely and to reduce recovery and trauma. A water-cooling Er:YAG laser (30 J/cm/sup 2/, l=2.94 mum, 20 Hz, 200 msec) that used optical fiber was operated 4-8 irradiation-overlapping ratio. We kept the distance between the laser and the bone at 0.25 mm using force sensor and spring to maintain effective ablation. Swine scapulae were ablated and performance was evaluated. The manipulator was operated mounting on iliac bone to get a filed position whereby resulting in precise bone cutting. The precision of the manipulator was within 0.3 mm and the efficiency of laser bone ablations per unit time optimized to 0.21 mm/sup 3//secW at the overlapping ratio of the irradiation area was 0.8, meaning a given ablated area was irradiated five times. The troughs showed m charring at this condition and the temperature of the surface was raised to 41.3 degrees C and it lasted only 5 seconds. We are sure that this research will be applied to orthopedics in the near future.

Laser-induced temperature changes in dentine

OBJECTIVE

The purpose of this work is to study the temperature rise and potential thermal damage caused during ablation of human dentine using a super pulsed carbon dioxide laser of 9.6-microm wavelength, equipped with a water-cooling spray and scanner system.

BACKGROUND DATA

There have been no reports on thermal effects of super pulsed CO2 laser of 9.6 microm wavelength on human dentine recently.

MATERIALS AND METHODS

Two different types of samples were investigated to yield data most consistent with a typical clinical situation. Human dentine slices and crown segments were studied at a drilling depth of 1.0 +/- 0.1 mm and 2.5 +/- 0.5 mm, respectively. A control group treated with a conventional hand piece was compared to four laser groups with settings varying from 2 to 8 W.

RESULTS

In the laser group demonstrating the highest elevation in temperature of the four studied, dentine slices lased at 2 W for 15 sec showed a mean temperature rise of less than 1.68 degrees C at an ablation rate of 0.86 +/- 0.08 mm. Conventional drilling with a comparable ablation rate of 0.76 +/- 0.59 mm resulted in a mean rise of 2.87 degrees C. The laser groups of crown segments revealed a constant decrease in temperature. SEM observations were lacking the typical morphological changes seen in earlier studies, specifically extensive melting, charring or cracking.

CONCLUSION

A maximum rise of mean temperature to 1.68 degrees C in closest vicinity to the pulpal chamber and the morphological unaltered dentine surfaces demonstrate the safe and tissue preserving character of the superpulsed 9.6 microm CO2 laser. The laser caused an even lower temperature rise than conventional drilling. Moreover, the laser showed acceptable efficacy with ablation rates that did not significantly differ from the conventional dental drill.

Wavelength and average power density dependency of the surface modification of root dentin using an MIR-FEL

BACKGROUND AND OBJECTIVES

Surface modification of root dentin by mid-infrared (MIR) pulsed-laser irradiation is one of the candidates for a novel, non-invasive treatment to prevent root surface caries. To modify root dentin effectively and non-invasively it is essential to estimate quantitatively and qualitatively the laser parameters, such as the wavelength and power density, required for surface modification. The key aspect is to bring about effective surface modification of the root dentin while minimizing the unwanted removal of the underlying dentin.

STUDY DESIGN/MATERIALS AND METHODS

Using a tunable, MIR Free Electron Laser with lambda = 8.8-10.6 microm, we have investigated macroscopically the extent of the surface modification (morphological and chemical changes) of root dentin. We have obtained experimental results related to the ablation depth, the MIR absorption spectrum, and the elemental chemical composition.

RESULTS

The observations showed that the surface modification of root dentin was inclined toward well-recrystallized HAp-like material, leading to an increase in the acid resistance and dentinal tubule sealing. The laser parameters, at which efficient surface modification without enhanced ablation occurred, were estimated to be approximately in the wavelength region around lambda = approximately 9.0 or approximately 9.7 microm and in the average power density region of approximately 10-20 W/cm2 (resulting in total energy density and peak power density regions of approximately 1-2 kJ/cm2 and approximately 0.67-1.2 kW/cm2).

CONCLUSIONS

The surface modification of root dentin strongly depends on the laser parameters applied. We conclude that the optimum wavelengths for laser treatment of root surface caries are lambda = approximately 9.0 or approximately 9.7 microm, corresponding to the absorption peak due to P-O stretching.