The effect of low power laser (He-Ne laser) irradiation on the mineralization and alkaline phosphatase (ALP) activity in the tooth germ was examined. Mandibular first molars from 17-day-old mouse embryos were cultured with or without low power laser irradiation (the irradiation time was 5 minutes per day). During 7-day cultivation, ALP activity and calcium content in the tooth germ increased developmentally and formations to predentin, dentin and enamel were observed histologically. In tooth germ cultured with 7 irradiations for 7 days, both ALP activity and calcium content were significantly decreased and formations to predentin, dentin and enamel were inhibited. Similar results were observed in tooth germ cultured for 7 days with 3 irradiations at day-0, 1 and 2. However, in tooth germ cultured for 7 days with 3 irradiations at day-4, 5 and 6, the inhibitory effects of irradiation were not observed. These results suggest that He-Ne laser irradiation suppress tooth formation and mineralization at an early developmental stage.
Recently, it has been demonstrated that the soft and hard tissue can be ablated with minimal thermal damage by Er: YAG laser irradiation. However, there are a few studies about the effects of the Er: YAG laser on the bone tissue. The purpose of this study was to investigate the morphological changes of the bone structure by Er: YAG laser irradiation. Twenty extirpated mandibula were used. Er: YAG laser was irradiated on the buccal surface of the rat's mandibula under the conditions; wavelength; 2.94μm, output; 5mJ/pulse, energy density; 16J/cm2. In the ten of the sample mandibula, a continuous water flow was maintained at the bone surface that allowed a gentle water trickle during laser irradiation. The other half mandibula were irradiated without any such water flow. Histological and scanning electron microscopical studies were perfomed. Er: YAG laser beam produced defect without signs of serious thermal injuries in both cases. Undesirable thermal effects like surface-cracking and carbonization were not obseved. Histological examinations revealed presence of the thin basophilic line at the bottom and along the wall of defects. These was minimal evidence of thermal damage on the surrounding tissue. Combining the Er: YAG laser irradiation with a water flow produced encouraging results. The application of water to the irradiated bone surface increases ablation efficiency with less thermal damage. These results suggest that the Er: YAG laser should be used with a water flow if vital bone are subjected to testing. The mechanism of the Er: YAG laser in this study has not been clarified yet. Therefore more resarch is necessary before Er: YAG laser may be used in the clinical field in the future.
The KrF excimer laser is a high power pulsed laser oscillating in the ultraviolet field and able to destroy or amputate molecules of tissue without thermal damage. In order to evaluate the effects of KrF excimer laser irradiation on human mandibular bone, we examined by visual, pathological and SEM observation. The laser irradiation on human mandibular bone resected by oral and maxillofacial surgery had a pulse energy: 65mJ, spot size: 0.80×0.50mm2, energy density: 16J/cm2, water volume: 6.5ml, assist gas (N2): 301/min, frequency: 80Hz, incision speed: 5, 10, 15mm/min. Visual observation: the incisions were sharp without thermal damage, and the depth of the inicions ranged from 0.68 to 2.53mm according to the incision speed. Pathological observation: the inicisions were almost sharp and there was almost no thermal damage layer. SEM observation: the incision section was slightly wavy, but the incision surface was almost clear at 20 magnification. Crystals formed at the incision surface at all incision speeds, and there was no melting of bone tissue at 5000 magnification.
The purpose of this study was to investigate the effect of Nd: YAG laser irradiation on Streptococcus mutans. Roots of intact bovine incisors were used in this experiment. After cleaning and shaping of the root canal, they were acid-etched for 1 minute and ultrasonically irrigated. The root surface was coated with nail enamel and the samples were sterilized. Each root canal was filled with 50μl of culture medium where S. mutans was incubated at 37°C for 24 hours. One hundred roots were divided into 10 groups according to irradiation energy and addition of evans blue in the culture medium. Non-irradiated samples were used as control. The culture medium in the root canal was irradiated under up-and-down motion between the apical and cervical portion of the root canal with an optical fiber (∅=600, μm) for 30 seconds using Nd: YAG laser (100mJ or 200mJ and 10 or 30 pps). After the irradiation, bacterial samples of 10μl were taken from each root canal and incubated aerobically on Mitis Salivalius Agar at 37°C for 72 hours. The number of colonies on the medium was counted and colony-forming-units (CFUs) /ml were calculated. Thermal change caused by Nd: YAG laser irradiation in the root canal and culture medium was also monitored with a thermal camera. In addition, the irradiated dentin surface of the canal wall was observed with SEM. The results were as follows: 1. In comparison with the control group, the CFUs in 100mJ 10pps, 200mJ 10pps, 100mJ 30pps and 200mJ 30pps irradiated groups without evans blue were reduced to 41%, 37%, 16% and 2%, respectively. Likewise, the CFUs in 100mJ 10pps, 200mJ 10pps, 100mJ 30pps and 200mJ 30pps irradiated groups with evans blue were reduced to 46%, 34%, 3% and 0%, respectively. The bactericidal effect in all experimental groups was statistically significant compared with the control group (t-test, P<0.01). 2. No significant difference was found between the groups with and without evans blue when irradiation energy was the same. 3. The maximum temperature rise in the culture medium was 40°C in accordance with the temperature rise on the root surface of about 5°C under the lasing condition of 200mJ 3Opps. 4. In SEM observation, most part of the lased canal wall surface showed open dentinal tubles, partially closed in other part.