High-frequency low-level diode laser irradiation promotes proliferation and migration of primary cultured human gingival epithelial cells

In periodontal therapy, the use of low-level diode lasers has recently been considered to improve wound healing of the gingival tissue. However, its effects on human gingival epithelial cells (HGECs) remain unknown. The aim of the present study was to examine whether high-frequency low-level diode laser irradiation stimulates key cell responses in wound healing, proliferation and migration, in primary cultured HGECs in vitro. HGECs were derived from seven independent gingival tissue specimens. Cultured HGECs were exposed to a single session of high-frequency (30 kHz) low-level diode laser irradiation with various irradiation time periods (fluence 5.7–56.7 J/cm²). After 20–24 h, cell proliferation was evaluated by WST-8 assay and [³H]thymidine incorporation assay, and cell migration was monitored by in vitro wound healing assay. Further, phosphorylation of the mitogen-activated protein kinase (MAPK) pathways after irradiation was investigated by Western blotting. The high-frequency low-level irradiation significantly increased cell proliferation and [³H]thymidine incorporation at various irradiation time periods. Migration of the irradiated cells was significantly accelerated compared with the nonirradiated control. Further, the low-level diode laser irradiation induced phosphorylation of MAPK/extracellular signal-regulated protein kinase (ERK) at 5, 15, 60, and 120 min after irradiation. Stress-activated protein kinases/c-Jun N-terminal kinase and p38 MAPK remained un-phosphorylated. The results show that high-frequency low-level diode laser irradiation promotes HGEC proliferation and migration in association with the activation of MAPK/ERK, suggesting that laser irradiation may accelerate gingival wound healing.     

Effects of high-frequency near-infrared diode laser irradiation on the proliferation and migration of mouse calvarial osteoblasts

Laser irradiation activates a range of cellular processes and can promote tissue repair. Here, we examined the effects of high-frequency near-infrared (NIR) diode laser irradiation on the proliferation and migration of mouse calvarial osteoblastic cells (MC3T3-E1). MC3T3-E1 cells were cultured and exposed to high-frequency (30 kHz) 910-nm diode laser irradiation at a dose of 0, 1.42, 2.85, 5.7, or 17.1 J/cm². Cell proliferation was evaluated with BrdU and ATP concentration assays. Cell migration was analyzed by quantitative assessment of wound healing using the Incucyt^® ZOOM system. In addition, phosphorylation of mitogen-activated protein kinase (MAPK) family members including p38 mitogen-activated protein kinase (p38), stress-activated protein kinase/Jun-amino-terminal kinase (SAPK/JNK), and extracellular signal-regulated protein kinase (ERK)1/2) after laser irradiation was examined with western blotting. Compared to the control, cell proliferation was significantly increased by laser irradiation at a dose of 2.85, 5.7, or 17.1 J/cm². Laser irradiation at a dose of 2.85 J/cm² induced MC3T3-E1 cells to migrate more rapidly than non-irradiated control cells. Irradiation with the high-frequency 910-nm diode laser at a dose of 2.85 J/cm² induced phosphorylation of MAPK/ERK1/2 15 and 30 min later. However, phosphorylation of p38 MAPK and SAPK/JNK was not changed by NIR diode laser irradiation at a dose of 2.85 J/cm². Irradiation with a high-frequency NIR diode laser increased cell division and migration of MT3T3-E1 cells, possibly via MAPK/ERK signaling. These observations may be important for enhancing proliferation and migration of osteoblasts to improve regeneration of bone tissues.     

Microleakage of resin-based sealants after Er:YAG laser conditioning

The aim of this in vitro study was to investigate the effects of Er:YAG laser pretreatment procedures in fissure sealing. The fissures of 90 third molars were prepared in the mesial halves with Er:YAG laser (λ = 2,940 nm, 250 mJ/pulse, 4 Hz, fluence 32 J/cm²) and acid etched. They were randomly assigned to three groups, and the fissures in the distal halves were prepared differently according to the group: acid etching alone, bur and etching or Er:YAG laser alone. The fissures were sealed using Clinpro™ sealant (3M). The extent of microleakage was measured with a digital-image analyzer. The sealants prepared with Er:YAG laser alone displayed greater microleakage than the others (p < 0.05). Er:YAG laser irradiation does not eliminate the need for etching the enamel surface before sealing.     

Laser irradiation promotes the proliferation of mouse pre-osteoblast cell line MC3T3-E1 through hedgehog signaling pathway

Low-level laser could promote osteoblast proliferation, and it has been applied in clinical practice to promote wound healing and tissue regeneration. However, the mechanism related to laser irradiation remains unclear. This study aimed to investigate the effects of low-level laser irradiation on the cell proliferation and the expressions of hedgehog signaling molecules Indian hedgehog (Ihh), Ptch, and Gli in vitro. In our present study, the MTT method was used to evaluate the effect on cell proliferation of laser irradiation on MC3T3-E1 cells. And cell cycle was examined by flow cytometry. Gene and protein expressions of hedgehog signaling molecules, including Ihh, Ptch, Smoothened (Smo), and Gli, were examined by qRT-PCR and western blot analysis. The results showed that laser irradiation at dosage of 3.75 J/cm² enhances the proliferation of MC3T3-E1 cells compared with control groups (p = 0.00). Moreover, laser irradiation (3.75 J/cm²) increased the cell amount at S phase (p = 0.00). In addition, the expressions of Ihh, Ptch, Smo, and Gli were significantly increased compared to the control during laser irradiation (3.75 J/cm²)-induced MC3T3-E1 osteoblast proliferation. After adding the hedgehog signaling inhibitor CY (cyclopamine), cell proliferation and Ihh, Ptch, Smo, and Gli expressions were inhibited (p = 0.00), and the cell amount at S phase was reduced compared with combination groups (p = 0.00). These results indicated that laser irradiation promotes proliferation of MC3T3-E1 cells through hedgehog signaling pathway. Our findings provide insights into the mechanistic link between laser irradiation-induced osteogenesis and hedgehog signaling pathway.     

Repetitive Er:YAG laser irradiation of human skin: a histological evaluation

BACKGROUND AND OBJECTIVE: Deep coagulation of skin collagen by Er:YAG laser repetitive pulses has been predicted by previous theoretical models and later demonstrated on animal skin. The goal of this study was to determine the effect of repetitive Er:YAG laser pulses on human skin and its response to this treatment. STUDY DESIGN/MATERIALS AND METHODS: Lid skin of six female volunteers with blepharochalasis has been treated with laser at day 0, 7, and 21 before elective surgery-blepharoplasty. The treated skin was excised as part of the procedure and prepared for further histological examination. We used a 2,940 nm Er:YAG laser (Fidelis M320A by Fotona) with 'smooth' mode parameters: fluence from 0.50 to 2.00 J/cm2; six pulses per packet; 550 microsecond/pulse, 250 millisecond/packet; single pass, no overlapping; spot size 5 mm; repetition rate 20 Hz. RESULTS: We observed deep collagen denaturation at laser fluences of 1.25 J/cm2 and over; epidermal damage was proportional to fluence with total coagulation of the epidermal layer at fluences of 1.75 J/cm2 and over. At day 7 after laser treatment we observed a complete regeneration of the epidermal layer and a regeneration zone within the dermis with prominent infiltration of CD68+ monocytes/macrophages. At day 21 after laser treatment we observed collagen remodeling and (myo-)fibroblast proliferation at tissue depths of up to 240 microm. CONCLUSIONS: Repetitive Er:YAG laser irradiation is effective in deep denaturation and remodeling of human skin collagen in vivo, with less epidermal damage compared to standard Er:YAG laser skin resurfacing.     

High-intensity pulsed laser irradiation accelerates bone formation in metaphyseal trabecular bone in rat femur

 Low-energy laser irradiation has positive effects on bone fracture healing, osteoblast proliferation, bone nodule formation, and alkaline phosphatase activity. However, the mechanism by which low-energy laser irradiation affects bone is not clearly known. It was recently found that light at a low radiation dosage is absorbed by intracellular chromophores. High-intensity pulsed laser irradiation can produce acoustic waves in the target surface by rapidly heating the tissue. We considered that the acoustic waves induced by high-intensity pulsed laser irradiation, in addition to the photochemical effects that are induced, accelerate bone formation. To clarify whether high-intensity pulsed laser irradiation accelerates bone formation, we investigated bone formation in the irradiated femur of rat, using histomorphometric analysis. Rat femurs were irradiated with a Q-switched Nd: YAG laser, which has a wavelength of 1064 nm, under two conditions: once a day, with the average fluence rate set at 100 mW/cm² (LA1), and twice a day, i.e., every 12 h, with the average fluence rate set at 50 mW/cm² (LA2). The mean bone volume and mineral apposition rate in the LA1 group were significantly higher than those in the nonirradiated group (control). These values were highest for the LA2 group, and were about 1.52 and 1.25-fold those of the control, respectively. These data demonstrated that the number of pulses, rather than the intensity of the laser irradiation, affects bone formation. Thus, this study indicated that high-intensity pulsed laser irradiation accelerates bone formation in the metaphysis. This bone formation induced by high-intensity pulsed laser irradiation might be due to laser-induced pressure waves. Received: May 3, 2002 / Accepted: August 8, 2002 Offprint requests to: T. Ninomiya     

Er:YAG laser therapy for peri-implant infection: a histological study

The purpose of this study was to evaluate the effects of Er:YAG laser on degranulation and implant surface debridement in peri-implant infection. The peri-implant infection was experimentally induced in dogs, and the treatment was performed using an Er:YAG laser or a plastic curet. Animals were sacrificed after 24 weeks, and undecalcified histological sections were prepared and analyzed. Degranulation and implant surface debridement were obtained effectively and safely by Er:YAG laser. Histologically, a favorable formation of new bone was observed on the laser-treated implant surface, and the laser group showed a tendency to produce greater bone-to-implant contact than the curet group. These results indicate that the Er:YAG laser therapy has promise in the treatment of peri-implantitis. Contract grant sponsor: 21^st Century Center of Excellence Program for Frontier Research on Molecular Destruction and Reconstruction of Tooth and Bone in Tokyo Medical and Dental University and a Grant-in-Aid for Scientific Research(c)(2) (No. 16592064) (A.A.), Ministry of Education, Culture, Sports, Science and Technology of Japan.     

Effects of Er:YAG laser on bacteria associated with titanium surfaces and cellular response in vitro

This in vitro study examined (a) the anti-bacterial efficacy of a pulsed erbium-doped yttrium aluminum garnet (Er:YAG) laser applied to Streptococcus sanguinis or Porphyromonas gingivalis adhered to either polished or microstructured titanium implant surfaces, (b) the response of osteoblast-like cells and (c) adhesion of oral bacteria to titanium surfaces after laser irradiation. Thereto, (a) bacteria adhered to titanium disks were irradiated with a pulsed Er:YAG laser (λ?=?2,940 nm) at two different power settings: a lower mode (12.74 J/cm² calculated energy density) and a higher mode (63.69 J/cm²). (b) After laser irradiation with both settings of sterile titanium, disks were seeded with 10⁴ MG-63 cells/cm². Adhesion and proliferation were determined after 1, 4, and 24 h by fluorescence microscopy and scanning electron microscopy. (c) Bacterial adhesion was also studied on irradiated (test) and non-irradiated (control) surfaces. Adhered P. gingivalis were effectively killed, even at the lower laser setting, independent of the material’s surface. S. sanguinis cells adhered were effectively killed only at the higher setting of 63.69 J/cm². Laser irradiation of titanium surfaces had no significant effects on (b) adhesion or proliferation of osteoblast-like MG-63 cells or (c) adhesion of both oral bacterial species in comparison to untreated surfaces. An effective decontamination of polished and rough titanium implant surfaces with a Er:YAG laser could only be achieved with a fluence of 63.69 J/cm². Even though this setting may lead to certain surface alterations, no significant adverse effect on subsequent colonization and proliferation of MG-63 cells or increased bacterial adhesion was found in comparison to untreated control surfaces.     

Comparative analysis of root surface smear layer removal by different etching modalities or erbium:yttrium–aluminum–garnet laser irradiation. A scanning electron microscopy study

The purpose of this study was to evaluate the effect of erbium:yttrium–aluminum–garnet (Er:YAG) laser (2.94 μm) irradiation on the removal of root surface smear layer of extracted human teeth and to compare its efficacy with that of citric acid, ethylenediamine tetra-acetic acid (EDTA), or a gel containing a mixture of tetracycline hydrochloride (HCl) and citric acid, using scanning electron microscopy (SEM). Thirty human dentin specimens were randomly divided into six groups: G1 (control group), irrigated with 10 ml of physiologic saline solution; G2, conditioned with 24% citric acid gel; G3, conditioned with 24% EDTA gel; G4, conditioned with a 50% citric acid and tetracycline gel; G5, irradiated with Er:YAG laser (47 mJ/10 Hz/5.8 J/cm²/pulse); G6, irradiated with Er:YAG laser (83 mJ/10 Hz/10.3 J/cm²/pulse). Electron micrographs were obtained and analyzed according to a rating system. Statistical analysis was conducted with Kruskal–Wallis and Mann–Whitney tests (P < 0.05). G1 was statistically different from all the other groups; no statistically significant differences were observed between the Er:YAG laser groups and those undergoing the other treatment modalities. When the two Er:YAG laser groups were compared, the fluency of G6 was statistically more effective in smear layer removal than the one used in G5 (Mann–Whitney test, P < 0.01). Root surfaces irradiated by Er:YAG laser had more irregular contours than those treated by chemical agents. It can be concluded that all treatment modalities were effective in smear layer removal. The results of our study suggest that the Er:YAG laser can be safely used to condition diseased root surfaces effectively. Furthermore, the effect of Er:YAG laser irradiation on root surfaces should be evaluated in vivo so that its potential to enhance the healing of periodontal tissues can be assessed.     

Calcitonin, sodium alendronate and high intensity laser in the treatment of traumatized teeth: a preliminary study

The aim of this study was to evaluate the influence of erbium:yttrium–aluminum–garnet (Er:YAG) laser compared with traditional treatment on dentin permeability to calcitonin and sodium alendronate. Forty bovine roots were sectioned and divided into eight groups. Groups 1 and 2 (G1/G2) were immersed in saline solution; G1T/G2T were immersed in ethylene diamine tetra-acetic acid plus sodium lauryl ether sulfate (EDTA-T) and sodium hypochlorite (NaOCl); G1I/G2I were irradiated with Er:YAG laser (2.94 μm, 6 Hz, 40.4 J/cm²); G1TI/G2TI were immersed in EDTA-T, NaOCl and subjected to Er:YAG irradiation. After 4 h the radioactivity of the saline solution was measured. Statistical analysis revealed a significant difference (P < 0.05) when the groups treated with EDTA-T and NaOCl followed by Er:YAG laser irradiation were compared with the groups treated with EDTA-T only and with the groups that received no treatment. Er:YAG laser associated with traditional procedures significantly increased the diffusion of calcitonin and sodium alendronate through dentin. All groups showed calcitonin and sodium alendronate diffusion.     

Shear bond strength of self-etching adhesive systems to Er:YAG-laser-prepared dentin

This study was conducted to compare the shear bond strengths of composite resin bonded to Er:YAG laser or bur-prepared dentin surfaces using three self-etching adhesive systems. The occlusal surfaces of 120 human third molars were ground flat to expose dentin. The dentin was prepared using either a carbide bur or an Er:YAG laser at 350 mJ/pulse and 10 Hz (fluence, 44.5 J/cm²). Three different self-etching adhesive systems were applied: iBond™, Xeno III™ and Clearfil SE Bond™. Rods of composite resin were bonded to dentin surfaces and shear bond tests were carried out. Both dentin surfaces after debonding and resin rods were observed using a scanning electron microscope. When the Xeno III™ was used, no difference was observed on shear bond strength values when bur and Er:YAG laser were compared. When using iBond™ and Clearfil SE Bond™, bond strength values measured on Er:YAG-laser-prepared surfaces were lower than those observed on bur-prepared surfaces. The absence of smear layer formation during the preparation of the dentin by the Er:YAG laser did not improve the adhesion values of self-etching adhesive systems.     

Microtensile bond strength of resin composite to dentin treated with Er:YAG laser of bleached teeth

The objective of this study was to evaluate the influence of Er:YAG laser (λ = 2.94 μm) on microtensile bond strength (μTBS) and superficial morphology of bovine dentin bleached with 16% carbamide peroxide. Forty bovine teeth blocks (7 × 3 × 3 mm³) were randomly assigned to four groups: G1- bleaching and Er:YAG irradiation with energy density of 25.56 J/cm² (focused mode); G2 - bleaching; G3 - no-bleaching and Er:YAG irradiation (25.56 J/cm²); G4 - control, non-treated. G1 and G2 were bleached with 16% carbamide peroxide for 6 h during 21 days. Afterwards, all blocks were abraded with 320 to 600-grit abrasive papers to obtain flat standardized dentin surfaces. G1 and G3 were Er:YAG irradiated. Blocks were immediately restored with 4-mm-high composite resin (Adper Single Bond 2, Z-250-3 M/ESPE). After 24 h, the restored blocks (n = 9) were serially sectioned and trimmed to an hour-glass shape of approximately 1 mm² at the bonded interface area, and tested in tension in a universal testing machine (1 mm/ min). Failure mode was determined at a magnification of 100× using a stereomicroscope. One block of each group was selected for scanning electron microscope (SEM) analysis. μTBS data was analyzed by two-way ANOVA and Tukey test (α = 0.05). Mean bond strengths (SD) in MPa were: G1- 32.7 (5.9)^A; G2- 31.1 (6.3)^A; G3- 25.2 (8.3)^B; G4- 36.7 (9.9).^A Groups with different uppercase letters were significantly different from each other (p < .05). Enamel bleaching procedure did not affect μTBS values for dentin adhesion. Er:YAG laser irradiation with 25.56 J/cm² prior to adhesive procedure of bleached teeth did not affect μTBS at dentin and promoted a dentin surface with no smear layer and opened dentin tubules observed under SEM. On the other hand, Er:YAG laser irradiation prior to adhesive procedure of non-bleached surface impaired μTBS compared to the control group.     

Effects of an Er:YAG laser on mitochondrial activity of human osteosarcoma–derived osteoblasts in vitro

The aim of the present study was to investigate the collateral damage of an Er:YAG laser on bone cells in vitro using a special application tip designed for treatment of periimplantitis. Before laser irradiation, SaOs-2 osteoblasts (2×10⁴ cells) were inoculated into 96-well tissue culture plates and incubated for 48 h under standardised conditions. A total of 120 cell cultures were irradiated with an Er:YAG laser using a cone-shaped quartz glass fibre tip at energy settings of 40, 60, 80 and 100 mJ at 10 Hz (energy densities of 5.08, 7.62, 10.16 and 12.7 J cm^–2) for 10 s. Each energy setting was used at a distance of 1, 2 and 3 mm between the application tip and the bottom of the culture plate. Following irradiation, mitochondrial activity of the cells was measured using a luminescent cell viability assay. After laser irradiation, mitochondrial activity of SaOs-2 osteoblasts was significantly reduced when compared with nonirradiated cells (P<0.001), dependent on the energy setting used and the distance between the application tip and the bottom of the culture plate. Mitochondrial activity increased significantly with decreasing energy settings and increasing distances (P<0.001). Within the limits of the present study, it was concluded that an Er:YAG laser, used with a cone-shaped glass fibre tip designed for treatment of periimplantitis, has detrimental effects on mitochondrial activity of SaOs-2 osteoblasts in vitro at energy settings of 40, 60, 80 and 100 mJ (10 Hz).     

Histopathological and Electrophysiological Study of CO2, Er:YAG, and CO2 + Er:YAG Laser Effect on Peripheral Nerve

Background  

The purpose of this study was to investigate the morphological, histopathological, and electrophysiological changes of peripheral nerve after CO₂ (carbon dioxide), Er:YAG (erbium:yttrium aluminum garnet), and CO₂ + Er:YAG laser irradiation. There have been no comparative reports on CO₂, Er:YAG, and CO₂ + Er:YAG laser effects on peripheral nerve.     

Fibroblast growth factor-2 is an immediate-early gene induced by mechanical stress in osteogenic cells.

Fifteen minutes of physiological MS induces FGF-2 in osteogenic cells. Here, we show that MS induced proliferation in both MC3T3-E1 and BMOp cells isolated from Fgf2(+/+) mice; Fgf2(-/-) BMOp cells required exogenous FGF-2 for a normal proliferation response. The induction of fgf-2 is mediated by PKA and ERK pathways. INTRODUCTION: Mechanical stress (MS) induces gene expression and proliferation of osteogenic MC3T3-E1 cells. We have previously shown that physiological levels of MS in MC3T3-E1 cells causes extracellular signal-regulated kinase (ERK)1/2 phosphorylation. Here we evaluate the induction and importance of fibroblast growth factor-2 (FGF-2) for MS-induced proliferation. MATERIALS AND METHODS: We characterized the MS induction of fgf-2 using a 15-minute pulse of 120 mustrain and studied the stability of fgf-2 message using actinomycin D. Bone marrow stromal cells (BMOp) isolated from Fgf2(-/-) and Fgf2(+/+) mice were used to study the importance of FGF-2 in MS-induced proliferation. RESULTS: We found that the induction of fgf-2 by MS is dependent on both protein kinase A (PKA) and ERK pathways. MS transiently induces fgf-2 within 30 minutes. FGF-2 receptor (FGFR2) was also significantly increased within 1 h. All three isoforms of FGF-2 (24, 22, and 18 kDa) were significantly increased by MS. The MS-mediated increase of fgf-2 mRNA was caused by new synthesis and not stabilization. Pretreatment of MC3T3-E1 cells with cycloheximide showed that the induction of fgf-2 did not require new protein synthesis. Pretreating MC3T3-E1 cells with the mitogen-activated protein kinase (MAPK)/ERK kinase 1/2 (MEK1/2) inhibitor, U0126, or H-89, a PKA inhibitor, significantly inhibited the induction of fgf-2, showing that mechanical induction of fgf-2 is dependent on ERK and PKA signaling pathways. The downstream consequence of a single 15-minute stress pulse was a 3.5-fold increase in cell number in MC3T3-E1 compared with growth in nonstressed control cells. In studies using bone marrow osteoprogenitor cells (BMOp) isolated from Fgf2(+/+)and Fgf2(-/-) mice, we found that FGF-2 was necessary for a full proliferative response to MS. CONCLUSIONS: These studies show that FGF-2 is an immediate-early gene induced by MS, and its expression is mediated by both the PKA and MAPK signal transduction pathways. FGF-2 was required for a full proliferative response.     

Histological examination of experimentally infected root canals after preparation by Er:YAG laser irradiation

The influence of Er:YAG laser irradiation on periodontal tissues along the root surface and apical region during root canal preparation was histologically evaluated using experimentally infected root canals of rats. Eighty experimentally mesial infected root canals of mandibular first molars in rats were divided into four groups. In three groups, root canals were irradiated using an Er:YAG laser at 2 Hz with 34, 68, or 102 mJ/pulse for 30 s. Non-irradiated canals served as controls. The influence of laser irradiation on periodontal tissues along the root surface and apical area was evaluated histologically under light microscopy at 0 (immediately after), 1, 2, 4, and 8 weeks after irradiation. At all periods, no inflammation or resorption on the root surfaces caused by laser irradiation was observed in any cases in the control or 34 mJ/pulse-irradiated groups. However, mild to severe inflammation with resorption of root surfaces was observed in some cases in the 68- and 102-mJ/pulse-irradiated groups. No significant difference was apparent between control and laser-irradiated groups at the apical area for all experimental periods (p > 0.05). These results suggest that thermal influences on periodontal tissues of experimentally infected root canals during root canal preparation by Er:YAG laser irradiation are minimal if appropriate parameters are selected. Er:YAG laser irradiation is thus a potential therapy for human infected root canals.     

Low-level 809 nm GaAlAs laser irradiation increases the proliferation rate of human laryngeal carcinoma cells in vitro

The aim of the study was to investigate the effect of low-level 809 nm laser irradiation on the proliferation rate of human larynx carcinoma cells in vitro. Epithelial tumor cells were obtained from a laryngeal carcinoma and cultured under standard conditions. For laser treatment the cells were spread on 96-well tissue culture plates. Sixty-six cell cultures were irradiated with an 809 nm GaAlAs laser. Another 66 served as controls. Power output was 10 mW(cw) and the time of exposure 75–300 s per well, corresponding to an energy fluence of 1.96–7.84 J/cm². Subsequent to laser treatment, the cultures were incubated for 72 h. The proliferation rate was determined by means of fluorescence activity of a redox indicator (Alamar Blue Assay) added to the cultures immediately after the respective treatment. The indicator is reduced by metabolic activity related to cellular growth. Proliferation was determined up to 72 h after laser application. The irradiated cells revealed a considerably higher proliferation activity. The differences were highly significant up to 72 h after irradiation (Mann–Whitney U test, p < 0.001). A cellular responsiveness of human laryngeal carcinoma cells to low-level laser irradiation is obvious. The cell line is therefore suitable for basic research investigations concerning the biological mechanisms of LLLT on cells.     

Er:YAG (λ=2.94 µm) Laser Etching of Dental Enamel as an Alternative to Acid Etching

. Acid etching is widely used in clinical dentistry to facilitate the mechanical retention of resin-based materials to teeth, in particular enamel surfaces. Several laser systems have been developed with the aim of modifying dental hard tissues and the Er:YAG (λ=2.94 μm) laser may offer a possible alternative to the acid etching technique. This study compares the shear bond strengths of composite beads attached to sound enamel surfaces prepared using either (a) no etching (negative control), (b) acid etching (positive control) or (c) Er:YAG laser etching, either with or without water, at one of three fluences: 15 J/cm², 18 J/cm² or 24 J/cm². A histological appraisal was also conducted using environmental scanning electron microscopy (ESEM) techniques. The mean shear bond strength for acid-etched enamel was 16.6 MPa (SD 4.4, n=10), whereas the best laser-etched mean bond strength obtained was 11.5 MPa (SD 4.1, n=11) using a fluence of 24 J/cm² with water. These values were significantly greater than those obtained for the negative control (no etching) of 4.4 MPa (SD 0.9, n=8). There was a significant positive correlation between the etching fluence and the shear bond strength, but pitting of the enamel surface at fluences above 25 J/cm² limited the maximum fluence for etching purposes. Although Er:YAG laser etching enhanced the retention of a resin-based material to an enamel surface when compared to a negative control, the mean shear bond strengths were significantly lower than those obtained using conventional acid etching. The optimal laser etching parameters in this study were shown to be 24 J/cm² in conjunction with water. Paper received August 1999; accepted after revision December 1999.     

In vitro inactivation of endodontic pathogens with Nd:YAG and Er:YAG lasers

Both Nd:YAG and Er:YAG lasers have been suggested as root canal disinfection aids. The aim of this in vitro study is to compare both wavelengths in terms of irradiation dose required for microbial inactivation, to quantify these irradiation doses and to investigate the influence of certain (laser) parameters on the antimicrobial efficacy. Agar plates containing a uniform layer of Enterococcus faecalis, Candida albicans or Propionibacterium acnes were mounted perpendicularly underneath the laser handpieces (5?mm spot). The Er:YAG laser was operated in single-pulse mode. Pulse energies of 40–400?mJ and pulse lengths of 100, 300, 600, and 1,000?μs were tested. After incubation at 37°C for 48?h, growth on the plates was scored. The pulse energy yielding complete absence of growth over the entire spot area was taken as the total inhibition threshold (TIT). TITs were determined for every species and pulse length. The Nd:YAG laser was operated with pulse trains because single pulses were ineffective. Output power was 15?W and frequency was 100?Hz. Spots were irradiated for 5–120?s. After incubation, the diameters of the inhibition zones were measured. For the Er:YAG laser, TITs varied between 100 and 210?mJ, and differed significantly between species and pulse lengths. Using Nd:YAG irradiation, TITs were around 5,300?J/cm² for C. albicans and 7,100?J/cm² for P. acnes. No inhibition was observed for E. faecalis. Er:YAG irradiation was superior to Nd:YAG in inactivating microorganisms on agar surfaces.