Although the histological changes in tooth enamel following Er:YAG laser application have been extensively studied, the acid resistance and remineralization of tooth surfaces after laser application have not been evaluated. In the present study, the tooth surfaces following Er:YAG laser application were observed by quantitative light-induced fluorescence (QLF) to ascertain the effects of laser output, and the following results were obtained. After Er:YAG laser irradiation and high-speed drill preparation, the average fluorescence loss (ΔF) and mineral loss (ΔQ) increased and area decreased in specimens that had been immersed in remineralized solution for four weeks. However, the response of the laser-irradiation side was about three times better than that of the side ground by the high-speed drill. These results suggest that: 1. Er:YAG laser irradiated enamel is effective for remineralization treatment. 2. QLF is a useful observation method for determining the prognosis of tooth surfaces following Er:YAG laser application.
Recently, laser welding is used to connect metal frames instead of soldering in dentistry. However, the changes in the mechanical properties of the connecting area welded by laser irradiation are not clear. This study investigated the characteristics of the melting zone produced by laser irradiation in dental metals. As test materials, pure titanium, Co-Cr alloy and Au-Ag-Pd alloy were used. Board-shaped specimens were prepared by casting and a laser was irradiated to the surface of the specimens under various conditions. After laser irradiation, the diameter of the fusion zone produced by laser irradiation in the surface of the specimens and the depth of the fusion zone in the cross section of the specimens were measured with a stereomicroscope. Furthermore, the Vickers hardness in the fusion zone and in the original casting zone of the specimens were measured. In addition, cross sections of the specimens were observed with SEM and the distribution of components was analyzed with EPMA. The results were as follows: 1. Under the same laser irradiation conditions, pure titanium showed the maximum values of diameter and depth of the fusion zone, followed by Co-Cr alloy and Au-Ag-Pd alloy in that order. 2. The Vickers hardness in the melting zone increased compared with that in the original casting zone in every test metal, however, Au-Ag-Pd alloy showed only a small change. 3. Cross-sectional images of the Co-Cr alloy and Au-Ag-Pd alloy showed an uneven distribution of metal components in the original casting zone, while the distribution was homogeneous observed in the fusion zone.
Low energy lasers are widely used in the treatment of various chronic pain syndromes in dentistry, pain clinics and orthopedics. It is generally agreed that low energy laser irradiation is effective for improving blood circulation, reducing pain and promoting wound healing. Although there have been many fundamental studies on the effects of lasers, the mechanism of laser-induced analgesic effects remains unknown. This report describes the effects of low energy laser irradiation on nerve cells in vitro, and demonstrates the mechanism of analgesic effect. A GaAlAs diode laser (trinpl D, YOSHIDA) and the neurosecretory PC12 cell line were used. After short-term laser irradiation (0.3-0.5 J/cm2) on PC12 cells differentiated by treatment with nerve growth factor, terminals of neurites swelled within a few minutes and the number of synaptic vesicles decreased. After long-term laser irradiation (15.0 J/cm2), synaptic vesicles completely disappeared from the swollen terminals, whereas cell bodies were still well preserved. Some of the cells appeared to be in degeneration and retracted their neurites accompanied with the alteration of F-Actin structures. The laser irradiation induced a temporary increase in intracellular free calcium ion concentration ([Ca2+]i) and enhanced the amplitude of calcium oscillation. Thus, it is clearly demonstrated that the mechanism of laser-induced analgesic effects is that the laser stimulates calcium ion influx and neurotransmitter release, and then degenerates the terminals and processes of neurites, according to the alteration of F-Actin organization. These findings suggest that low energy laser attenuates the sensitivity of nerve cells to painful stimuli and produces analgesic effects.
It has been reported that low-intensity laser treatment has many biological effects, and recent studies have examined the acceleration of bone regeneration by laser as it may hold great potential benefit for clinical therapy in orthopedics and dentistry. However, the biological effects of laser irradiation on bone are not fully understood. In order to use laser therapy for clinical treatment, it is necessary to prove its effects scientifically and to clarify the mechanisms by which it stimulates bone formation. We have been investigating the effects of laser irradiation on bone formation and the stimulatory mechanisms involved by in vivo and in vitro experiments. These results are reported in this review.
Low-power lasers have been reported to significantly enhance the healing rate in both soft and hard tissues. The stimulatory action of laser seems to occur during the proliferative stage of healing by stimulation of prostaglandin E2 (PGE2) and cyclooxygenase-2 (COX-2), which are crucial early mediators in the natural healing process. The effect of Er:YAG laser irradiation on PGE2 production and COX-2 gene expression in human gingival fibroblast in vitro were investigated. Cultured fibroblasts were exposed to low-power Er:YAG laser irradiation with an energy density of 3.37 J/cm2. The amount of PGE2 production was measured by enzyme-linked immunosorbent assay (ELISA). COX-2 mRNA level, which is a critical enzyme for production, was analyzed by reverse transcriptase-polymerase chain reaction(RT-PCR). Er:YAG laser significantly increased PGE2 production by human gingival fibroblasts. COX-2 mRNA, which was hardly detectable in control, increased dramatically after irradiation. COX-2 inhibitor, NS398, completely inhibited the PGE2 synthesis stimulated by Er:YAG laser irradiation. Er:YAG laser irradiation appears to exert its stimulative action on gingival fibroblasts proliferation through the production of PGE2 via the expression of COX-2. This should be considered as one of the important regulatory pathways to accelerate wound healing after Er:YAG laser irradiation.
Clinical studies have revealed that low-energy laser irradiation facilitates alleviation of pain or wound healing. However, basic studies have demonstrated that irradiation using low-energy laser such as He-Ne laser modulates various biological processes. Some reports found that He-Ne laser irradiation activated the cell cycle and stimulated cell proliferation activity, while others suggested that it indicated inhibitory behavior. Further elucidation of the photo-chemical effects of low-energy laser irradiation on cultured cells is required. The present report reviews our studies on the effects of low-energy laser irradiation with He-Ne laser. We have also been examining the effects of He-Ne laser irradiation on the proliferation activity of rat bone-marrow-derived osteoblastic cells cultured on TiO2 discs. The results suggest that He-Ne laser irradiation stimulates both the proliferation and differentiation capacity of the cells. Therefore, He-Ne laser irradiation around an implant hole prior to pegging an implant body might promote healing of bone tissue and contribute to an initial fixation of a dental implant, leading to an increase in the graft survival rate. We also examined the effects of He-Ne laser irradiation on the growth of mouse colonic-carcinoma-derived tumor cells. The results showed that brief irradiation stimulated proliferation of the cells, whereas prolonged irradiation caused cytostatic activity of the cells. These studies revealed that the effects of low-energy laser irradiation on various cells differ according to the kind of laser, irradiation time, and amount of energy as well as the kind of cells. Further studies on the photo-chemical effects of low-energy laser irradiation are necessary.
Rheumatoid arthritis (RA) is a systemic autoimmune disorder that involves inflammation and pain of joints and has been treated by steroid hormones. However, long-term use of steroid often causes side effects. Low-level laser irradiation (LLLI) is being evaluated for treating RA, and no side effects have been reported. LLLI reduced inflammation in collagen-induced RA rats. Linearly polarized infrared light irradiation (IPIL) and steroid reduced the expression of IL-8 in human RA synoviocyte MH-7A cells. GeneChip analysis coupled with a signal pathway database showed that steroid randomly altered the expression of many genes including unwanted genes, whereas IPIL reduced genes which are useful for anti-inflammation.
Mesenchymal stromal cells (MSCs) are multipotent cells present in adult bone marrow that replicate as undifferentiated cells and can differentiate to lineages of mesenchymal tissues. Here, I show that laser irradiation can direct the osteogenesis of MSCs by altering the intracellular localization of the circadian rhythm protein Cryptochrome (CRY). After laser irradiation (wavelength: 405 nm) to MSCs, circadian rhythm proteins CRY1 and PER2 were immunostained and histochemical stainings for osteogenic differentiation were observed. Laser irradiation promoted osteogenesis of MSCs, induced the translocation of CRY1 and PER2 proteins from the cytoplasm to the nucleus, and decreased mCRY1 mRNA levels quantified by real-time PCR. Since the timing of nuclear accumulation of clock proteins constitutes an important step in the transcription-translation feedback loop driving the circadian core oscillator, laser irradiation could provide a simple and effective technology for clock protein localization and turnover. However, I believe that the mechanism of cell differentiation by laser irradiation is not so simple. In this article, I propose that cell differentiation is regulated by intracellular photoreceptors, such as CRY, cytochrome c oxidase, and porphyrin, and reactive oxygen species (ROS), etc. after laser irradiation. The detailed mechanism should be clarified by using molecular biology based techniques.
Recently, osteocytes embedded in mineralized matrix have attracted much research attention as they are thought to translate mechanical loading into biochemical signals that affect bone (re) modeling. In order to understand the biological mechanisms by which osteocytes control bone formation and resorption under mechanical force, we conducted a study in the light of Wolff's Law and Frost's mechanical thresholds theory. The findings indicate that mechanical stress can be used for bone regeneration therapy. We have demonstrated that carbon dioxide laser irradiation induces bone formation, and that laser irradiation is a kind of mechanical force and so could be used for bone regeneration therapy. Here, we review how laser irradiation influences bone metabolism via osteocytes on the basis of Wolff's Law and Frost's mechanostat thresholds theory.
Reparative dentin is formed in the dental pulp in response to various external stimuli such as caries and abrasion. Carbon dioxide laser has been used to accelerate the formation of reparative dentin on the exposed pulp surface. However, the mechanism by which carbon dioxide laser irradiation stimulates mineralization in direct pulp capping treatment is not fully understood. We examined the effects of carbon dioxide laser irradiation on mineralization in rat dental pulp cells. Rat dental pulp cells were irradiated with a carbon dioxide laser at an output power of 2 W for 20, 40, and 60 s and were cultured in media containing ascorbic acid and β-glycerophosphate. Cell viability was examined 24 h after laser irradiation by a modified MTT assay. Alizarin Red S staining was performed 10 days after laser irradiation. The amounts of collagen secreted from the cells after irradiation were quantified following Sirius Red staining. The expression levels of collagen type I and HSP47, collagen-binding stress protein, were analyzed by real-time PCR. HSP47 protein expression was examined by Western blotting. The cell viability was not affected by laser irradiation at 2 W for up to 40 s. However, it was significantly decreased by 20% at 60 s. The amount of mineralization after 10 days of irradiation at 2 W for 40 s was significantly increased in comparison to the other conditions. The extracellular collagen production was significantly increased, by 73% on day 2 and 38% on day 4, after laser irradiation. Although collagen type I gene expression was not changed by laser irradiation, HSP47 gene and protein expression was induced within 12 h and 24 h, respectively. These results suggested that carbon dioxide laser irradiation stimulated mineralization in dental pulp cells by increasing HSP47 expression.