Flexible and elastic porous poly(trimethylene carbonate) structures for use in vascular tissue engineering.

Biocompatible and elastic porous tubular structures based on poly(1,3-trimethylene carbonate), PTMC, were developed as scaffolds for tissue engineering of small-diameter blood vessels. High-molecular-weight PTMC (M(n) = 4.37 x 10(5)) was cross-linked by gamma-irradiation in an inert nitrogen atmosphere. The resulting networks (50-70% gel content) were elastic and creep resistant. The PTMC materials were highly biocompatible as determined by cell adhesion and proliferation studies using various relevant cell types (human umbilical vein endothelial cells (HUVECs), smooth muscle cells (SMCs) and mesenchymal stem cells (MSCs)). Dimensionally stable tubular scaffolds with an interconnected pore network were prepared by particulate leaching. Different cross-linked porous PTMC specimens with average pore sizes ranging between 55 and 116 microm, and porosities ranging from 59% to 83% were prepared. These scaffolds were highly compliant and flexible, with high elongations at break. Furthermore, their resistance to creep was excellent and under cyclic loading conditions (20 deformation cycles to 30% elongation) no permanent deformation occurred. Seeding of SMCs into the wall of the tubular structures was done by carefully perfusing cell suspensions with syringes from the lumen through the wall. The cells were then cultured for 7 days. Upon proliferation of the SMCs, the formed blood vessel constructs had excellent mechanical properties. Their radial tensile strengths had increased from 0.23 to 0.78 MPa, which is close to those of natural blood vessels.

Influence of LED irradiance on flexural properties and Vickers hardness of resin-based composite materials.

OBJECTIVE: To investigate the effect of irradiance through choice of LED light-curing unit (LCU) and fiber-optic tip on the mechanical properties of methacrylate resin-based composites (RBCs). METHODS: Rectangular bar-shaped specimens (25 mm x 2 mm x 2 mm) of four RBCs, irradiated from one side for the manufacturer's recommended times with one of two LED LCUs, and with either 8 or 13 mm tip diameter, were tested in three-point bend for flexural strength and modulus of elasticity; Vickers hardness was measured on top and bottom surfaces, on overlapped and non-overlapped irradiation sites. The effects and interactions of all factors were examined by analysis of variance. RESULTS: While the materials tested differed significantly for all properties (P < 1 x 10(-6)), there was no significant effect for LCU, tip, or irradiance for flexural strength or modulus of elasticity. For hardness, the only significant effect overall was due to irradiance on the bottom surface. CONCLUSIONS: Both LCUs and both tips could be used to give adequate top surface hardness, but the recommended maximum increment thickness is too great for adequate cure at the bottom with the tested LCUs at the recommended times. An extended experimental design would be valuable to test behavior at much elevated irradiances.

Effect of microwave postpolymerization treatment on residual monomer content and the flexural strength of autopolymerizing reline resin.

BACKGROUND: Microwave postpolymerization has been suggested as a method to improve the flexural strength of an autopolymerizing denture reline resin. However, the effect of microwave postpolymerization on the residual monomer content and its influence on flexural strength have not been investigated. OBJECTIVES: This study analyzed the effect of microwave postpolymerization on the residual monomer content and its influence on the flexural strength of an autopolymerizing reline resin (Denture Liner). MATERIALS AND METHODS: A total of 70 specimens (64 Chi 10 Chi 3.3 mm) were polymerized according to the manufacturer's instructions and divided into 7 groups (n = 10). Control group specimens were not subjected to any further processing. Before testing, the specimens were subjected to postpolymerization in a microwave oven using different power (550 and 650 W) and time (3, 4, and 5 min) settings. Two specimens of each group were then manually ground into fine powder and samples extracted from the specimens using reflux method. The samples were then subjected to gas chromatography for residual monomer determination in area%. Eight specimens were subjected to a three-point bending device with a span of 50 mm and crosshead speed of 5 mm/min, and the flexural strength was determined in MPa. Data analyses included Student's t-test and one-way analysis of variance. RESULTS: For the Denture Liner reline resin, the residual monomer content decreased and the flexural strength increased significantly with the application of microwave irradiation using different time/power combinations. The specimens with the lowest residual monomer content were the similar specimens which presented with the highest flexural strength. CONCLUSION: Microwave postpolymerization irradiation can be an effective method for increasing the flexural strength of denture liner (at 650 W for 5 min) by reducing the residual monomer content by further polymerization at free radical sites.

Relining of prosthesis with auto-polymerizing hard denture reline resins: effect of post-polymerization treatment on flexural strength.

BACKGROUND: It has been suggested that microwave irradiation and prosthesis immersion in hot water after its polymerization may improve mechanical and viscoelastic properties of acrylic resins. PURPOSE: This study was proposed to verify the influence of microwave post-polymerization (PP) treatment over the flexural strength of thermo-polymerizing acrylic resin specimens (QC-20) relined or not with two different composition hard chairside auto-polymerizing reliners [Kooliner (K) and New Truliner (NT)]. MATERIALS AND METHODS: For this study, 50 specimens of 64 x 10 x 3.3 mm were polymerized and distributed into five groups. G1 (control) specimens without relining and PP; G2 specimens relined with K, without PP; G3 specimens relined with NT, without PP; G4 specimens relined with K, with PP (microwave irradiation with 650 W for 5 min); G5 specimens relined with NT, with PP. Tests were performed on a universal testing machine Instron 4411 with compression speed of 5 mm/min. RESULTS: Specimens of K without PP did not show statistically different results (p < 0.05) when compared with control. However, when submitted to PP these specimens showed a significant increase in flexural strength. Specimens of NT showed the lowest flexural strength of all groups, with or without PP when compared with control and K groups. CONCLUSION: Microwave PP (650 W for 5 min) proved to be an effective method of improving the flexural strength of K relined prosthesis. However, it did not seem to affect NT specimens.

Light-dependent regulation of structural flexibility in a photochromic fluorescent protein.

The structural basis for the photochromism in the fluorescent protein Dronpa is poorly understood, because the crystal structures of the bright state of the protein did not provide an answer to the mechanism of the photochromism, and structural determination of the dark state has been elusive. We performed NMR analyses of Dronpa in solution at ambient temperatures to find structural flexibility of the protein in the dark state. Light-induced changes in interactions between the chromophore and beta-barrel are responsible for switching between the two states. In the bright state, the apex of the chromophore tethers to the barrel by a hydrogen bond, and an imidazole ring protruding from the barrel stabilizes the plane of the chromophore. These interactions are disrupted by strong illumination with blue light, and the chromophore, together with a part of the beta-barrel, becomes flexible, leading to a nonradiative decay process.

Effect of microwave disinfection on physical and mechanical properties of acrylic resins.

This study evaluated the effect of microwave energy on the hardness, impact strength and flexural strength of the Clássico, Onda-Cryl and QC-20 acrylic resins. Aluminum die were embedded in metallic or plastic flasks with type III dental stone, in accordance with the traditional packing technique. A mixing powder/liquid ratio was used according to the manufacturer's instructions. After polymerization in water batch at 74 degrees C for 9 h, boiling water for 20 min or microwave energy at 900 W for 10 min, the specimens were deflasked after flask cooling at room temperature, and submitted to finishing. Specimens non-disinfected and disinfected by microwave irradiation were submitted to hardness, impact and flexural strength tests. Each specimen was immersed in distilled water and disinfected in a microwave oven calibrated to 650 W for 3 min. Knoop hardness test was performed with 25 g load for 10 s, impact test was carried out using the Charpy system with 40 kpcm, and 3-point bending test with a crosshead speed of 0.5 mm/min until fracture. Data were submitted to statistical analysis by ANOVA and Tukey's test (alpha=0.05). Disinfection by microwave energy decreased the hardness of Clássico and Onda-Cryl acrylic resins, but no effect was observed on the impact and flexural strength of all tested resins.

Effect of microwave disinfection on physical and mechanical properties of acrylic resins

This study evaluated the effect of microwave energy on the hardness, impact strength and flexural strength of the Clássico, Onda-Cryl and QC-20 acrylic resins. Aluminum die were embedded in metallic or plastic flasks with type III dental stone, in accordance with the traditional packing technique. A mixing powder/liquid ratio was used according to the manufacturer's instructions. After polymerization in water batch at 74ºC for 9 h, boiling water for 20 min or microwave energy at 900 W for 10 min, the specimens were deflasked after flask cooling at room temperature, and submitted to finishing. Specimens non-disinfected and disinfected by microwave irradiation were submitted to hardness, impact and flexural strength tests. Each specimen was immersed in distilled water and disinfected in a microwave oven calibrated to 650 W for 3 min. Knoop hardness test was performed with 25 g load for 10 s, impact test was carried out using the Charpy system with 40 kpcm, and 3-point bending test with a crosshead speed of 0.5 mm/min until fracture. Data were submitted to statistical analysis by ANOVA and Tukey's test (?=0.05). Disinfection by microwave energy decreased the hardness of Clássico and Onda-Cryl acrylic resins, but no effect was observed on the impact and flexural strength of all tested resins.

Este estudo verificou o efeito da desinfecção por microondas sobre a dureza, impacto e resistência à flexão das resinas acrílicas Clássico, Onda-Cryl e QC-20. Matrizes de alumínio foram incluídas em muflas metálica ou de plástico com gesso pedra tipo III, de acordo com a técnica de inclusão tradicional. A proporção polímero/monômero foi de acordo com as recomendações dos fabricantes. Depois da polimerização em água à temperatura de 74ºC por 9 h, água em ebulição por 20 min ou por energia de microondas a 900 W por 3 min, os corpos-de-prova foram demuflados após esfriamento da mufla em temperatura ambiente e submetidos ao polimento convencional. Testes de dureza, impacto e resistência à flexão foram feitos nos corpos-de-prova submetidos ou não à desinfecção por microondas. Cada corpo-de-prova foi submetido individualmente à desinfecção por microondas em forno doméstico com 900 W de potência por 3 min, imersos em 150 mL de água destilada. O teste de dureza Knoop foi realizado com carga de 25 g por 10 s, a de impacto pelo sistema Charpy com 40 kpcm e o de flexão por três pontos com velocidade de 0,5 mm/min até fratura. Os dados foram submetidos à análise de variância e ao teste de Tukey (?=0,05). A desinfecção simulada por microondas diminuiu a dureza Knoop das resinas Clássico e Onda-Cryl e não promoveu nenhum efeito nas resistências ao impacto e à flexão das resinas estudadas.

Mechanical properties of dual-cured resin luting agents for ceramic restoration.

PURPOSE: The aim of the present study was to evaluate mechanical properties, including surface hardness, flexural strength, and flexural modulus, of two dual-cured resin luting agents [Clearfil Esthetic Cement (CEC) and Variolink II (VLII)] irradiated through four thicknesses of leucite ceramics (0, 1, 2, and 3 mm) and to evaluate their shear bond strength to zirconia ceramic (Cercon) using each ceramic primer. MATERIALS AND METHODS: Knoop hardness was measured on a thin layer of resin luting agent on the ceramic surface. Three-point bending tests were performed after 24 hours of storage at 37 degrees C. Two differently shaped zirconia ceramic specimens with or without sandblasting with alumina were treated with each primer. The specimens were then cemented together with each resin luting agent. Half of the specimens were stored in water at 37 degrees C for 24 hours and the other half were thermocycled 5000 times. RESULTS: VLII revealed statistically higher Knoop hardness and flexural modulus than CEC for each thickness of ceramic. No significant differences in flexural strength were observed between VLII and CEC for each ceramic spacer. Reduction of the mechanical properties with increase of ceramic thickness varied for each property; however, these properties were similar in the two materials. Blasting with alumina was significantly effective for increasing shear bond strength of both resin luting agents before and after thermal cycling. The use of Clearfil Ceramic Primer showed the highest shear bond strength and maintained bond durability after 5000 thermocycles. CONCLUSION: Mechanical properties of CEC dual-cured resin luting agent appear adequate for ceramic restorations.

Electron-beam irradiation of experimental denture base polymers.

OBJECTIVE: Since the properties of polymers can be influenced using electron-beam irradiation, the aim of this study was to investigate whether electron-beam post-curing can improve the mechanical properties of experimental denture base polymers. MATERIAL AND METHODS: Rectangular specimens of different experimental polymeric blends were electron-beam irradiated (post-cured) with 25 kGy and 200 kGy using an electron accelerator of 4.5 MeV. Fracture toughness, work of fracture, Vickers hardness and colour changes were measured and compared in non-irradiated specimens. RESULTS: The mechanical properties of all the investigated polymers seemed to benefit from low-energy electron-beam irradiation (25 kGy). Using an energy dose of 200 kGy, all blends showed deteriorated mechanical properties resulting from chain breakage. Nevertheless, all investigated polymers had undesirable colour changes after electron-beam irradiation. CONCLUSIONS: Mechanical properties of experimental polymethyl-methacrylate could be changed using electron-beam irradiation. Because of discolorations caused by the irradiation levels investigated, these levels cannot be recommended for practical applications.

Association of body composition and physical activity with proximal femur geometry in middle-aged and elderly Afro-Caribbean men: the Tobago bone health study.

Osteoporotic fractures are less prevalent in African Americans than in caucasians, possibly because of differences in bone structural strength. Bone structural adaptation can be attributed to changes in load, crudely measured as lean and fat mass throughout life. The purpose of this analysis was to describe the associations of leg lean mass, total body fat mass, and hours walked per week with femoral bone mineral density (BMD) and bone geometry in a cross-sectional sample of 1,748 men of African descent between the ages of 40 and 79 years. BMD, section modulus (Z), cross-sectional area (CSA), and subperiosteal width were measured from dual energy X-ray absortiometry (DXA) scans using the hip structural analysis (HSA) program. Multiple linear regression models explained 35% to 48% of the variance in bending (Z) and axial (CSA) strength at the femoral neck and shaft. Independent of all covariates including total body fat mass, one standard deviation increase in leg lean mass was significantly associated with a 5% to 8% higher Z, CSA, and BMD (P < 0.010) at the neck and shaft. The number of hours walked per week was not a strong or consistent independent predictor of bone geometry or BMD. We have shown that weight is the strongest independent predictor of femur BMD and geometric strength although the effect appears to be mediated by lean mass since leg lean mass fraction and total body fat mass fraction had significant and opposing effects at the narrow neck and shaft in this group of middle aged and elderly men.

Flexural strength of autopolymerizing denture reline resins with microwave postpolymerization treatment.

STATEMENT OF PROBLEM: Microwave postpolymerization has been suggested as a method to improve the mechanical strength of repaired denture base materials. However, the effect of microwave heating on the flexural strength of the autopolymerizing denture reline resins has not been investigated. PURPOSE: This study analyzed the effect of microwave postpolymerization on the flexural strength of 4 autopolymerizing reline resins (Duraliner II, Kooliner, Ufi Gel Hard, and Tokuso Rebase Fast) and 1 heat-polymerized resin (Lucitone 550). MATERIAL AND METHODS: For each material, 80 specimens (64 x 10 x 3.3 mm) were polymerized according to the manufacturer's instructions and divided into 10 groups (n = 8). Control group specimens remained as processed. Before testing, the specimens were subjected to postpolymerization in a microwave oven using different power (500, 550, or 650 W) and time (3, 4, or 5 minutes) settings. Load measurements (newtons) were made at a crosshead speed of 5 mm/min using a 3-point bending device with a span of 50 mm. The flexural strength values were calculated in MPa. Data analyses included 3-way and 2-way analysis of variance and the Tukey Honestly Significant Difference test (alpha = .05). RESULTS: The flexural strengths of resins Duraliner II and Kooliner were significantly increased (P = .0015 and P = .0046, respectively) with the application of microwave irradiation using different time/power combinations. The materials Lucitone 550, Tokuso Rebase Fast, and Ufi Gel Hard demonstrated no significant strength improvement compared to the corresponding control. Only after microwave postpolymerization irradiation for 3 minutes at 550 W did Lucitone 550 show significantly higher flexural strength (P =.001) than Tokuso Rebase Fast and Ufi Gel Hard relining resins. CONCLUSION: Microwave postpolymerization irradiation can be an effective method for increasing the flexural strength of Duraliner II (at 650 W) and Kooliner (at 550 W and 650 W for 5 minutes).

Investigation of the effect of GaAs laser therapy on cervical myofascial pain syndrome.

Low-energy laser therapy has been applied in several rheumatoid and soft tissue disorders with varying rates of success. The objective of our study was to investigate the effect of laser therapy on cervical myofascial pain syndrome with a placebo-controlled double-blind prospective study model. It was performed with a total of 53 patients (35 females and 18 males) with cervical myofascial pain syndrome. In group 1 (n = 23), GaAs laser treatment was applied over three trigger points bilaterally and also one point in the taut bands in trapezius muscle bilaterally with a frequency of 1000 Hz for 2 min over each point once a day for 10 days during a period of 2 weeks. In group 2 (n = 25), the same treatment protocol was given, but the laser instrument was switched off during applications. All patients in both groups were instructed to perform daily isometric exercises and stretching just short of pain for 2 weeks at home. Evaluations were performed just before treatment (week 0), immediately after (week 2), and 12 weeks later (week 14). Evaluation parameters included pain, algometric measurements, and cervical lateral flexion. Statistical analysis was done on data collected from three evaluation stages. The results were evaluated in 48 patients (32 females, 16 males). Week 2 and week 14 results showed significant improvement in all parameters for both groups. However, comparison of the percentage changes both immediately and 12 weeks after treatment did not show a significant difference relative to pretreatment values. In conclusion, the results of our study have not shown the superiority of GaAs laser therapy over placebo in the treatment of cervical myofascial pain syndrome, but we suggest that further studies on this topic be done using different laser types and dosages in larger patient populations.

Shortwave diathermy and prolonged stretching increase hamstring flexibility more than prolonged stretching alone.

STUDY DESIGN: A randomized, counterbalanced 2x3x5 repeated-measures design. OBJECTIVE: To compare changes in hamstring flexibility after treatments of pulsed shortwave diathermy and prolonged stretch, sham diathermy and prolonged stretch, and control. BACKGROUND: Heat and stretch techniques have been touted for years. To date, the effect of shortwave diathermy and hamstring stretching has not been studied. Because diathermy heats a large area and penetrates deep into the muscle, use of this device prior to or during hamstring stretching may increase flexibility. METHODS AND MEASURES: Thirty college-age students (mean age, 21.5 years) with tight hamstrings (inability to achieve greater than 160 degrees knee extension at 90 degrees hip flexion) participated. Subjects were assigned to 1 of 3 groups: diathermy and stretch, sham diathermy and stretch, and control). Range of motion was recorded before and after each treatment for 5 days and on day 8. A straight leg-raise stretch was performed using a mechanical apparatus. Subjects in the diathermy-and-stretch group received 10 minutes of diathermy (distal hamstrings) followed by 5 minutes of simultaneous diathermy and stretch, followed by 5 minutes of stretching only. Subjects in the sham-diathermy-and-stretch group followed the same protocol, but with the diathermy unit turned off. Subjects in the control group lay on the table for 20 minutes. Data were analyzed using an ANOVA and post hoc t tests. RESULTS: Mean (+/- pooled SE) increases in knee extension after 5 days were 15.8 degrees 2.2 degrees for the diathermy-and-stretch group, 5.2 degrees +/- 2.2 degrees for the sham-diathermy-and-stretch group, and -0.3 degrees +/- 2.2 degrees for the control group. Seventy-two hours after the last treatment, the diathermy-and-stretch group lost 1.9 degrees +/- 2.2 degrees, the sham-diathermy-and-stretch group lost 3.0 degrees +/- 2.2 degrees, and the control group changed -0.4 degrees +/- 2.2 degrees. CONCLUSION: These results suggest that hamstring flexibility can be greatly improved when shortwave diathermy is used in conjunction with prolonged stretching.

Light-attenuating effect of dentin on the polymerization of light-activated restorative resins.

The light-attenuating effect of dentin on the mechanical properties of light-activated composite resins was evaluated using a simple experimental filter. The filter was designed to simulate the light transmittance and light diffusion characteristics of 1.0-mm thick dentin. The depth of cure, surface hardness, and flexural strength for 13 shades of three light-activated restorative resins were examined. These resins were cured either using direct irradiation with a light source, or indirect irradiation through the filter. The attenuation of light intensity by 1.0-mm thick dentin reached 85-90% in the 400-550 nm wavelength region. For all materials, the values of depth of cure, surface hardness on the top and bottom surfaces, and flexural strength of specimens irradiated indirectly through the simulated 1.0-mm thick dentin filter decreased by 37-60%, 16-55%, 50-83%, and 44-82% in comparison with those by direct irradiation, respectively. Recovery from mechanical properties' reduction was achieved when materials were irradiated 1.5-4 times longer than the standard irradiation time.

Effect of extracorporeal shock waves on callus formation during bone lengthening.

The effects of extracorporeal shock waves (ESWs) on callus formation during bone lengthening were studied in 25 female Japanese white rabbits. Bone lengthening of 9.8 mm was obtained over 2 weeks using the Orthofix M-100 bone fixator. ESWs were applied 3 weeks after surgery. Pins were removed 7 weeks after surgery, and specimens were prepared after the animals were killed at 9 and 24 weeks. The shock wave setting used was 0.42 mJ/mm(2) with a pulse interval of 2 Hz; 3000 shots each were applied to the central and peripheral areas. The specimens were evaluated using radiography, bone mineral density (BMD) measured by dual energy absorptiometry (DXA), and a three-point bending test to evaluate mechanical strength. Histological examination was performed on the lengthened portion. Radiographs and histological observations revealed no apparent fractures in nonlengthening tibias at the shock wave energy densities used. Radiographic observations revealed no apparent differences between the control group and the ESW group. BMD measurements by DXA revealed significantly increased bone mass in the ESW group 9 weeks after surgery. At 24 weeks after surgery the mean BMD had decreased to 25% and 15% of the values at 9 weeks in the control and ESW groups, respectively. The three-point bending test revealed no significant differences between the groups. Histological observations revealed significant capillary formation and osteoblasts and chondrocytes in the bone marrow as well as bridging of newly formed trabeculae 2 weeks after the bone was lengthened. At 4 weeks after treatment, observations included parts of the lengthened portion with no cortex or immature bone. At 9 weeks after surgery, cortex formation and a normal medullary cavity were clearly observed in the control group, whereas observations in the treated group included areas of the lengthened portion with no cortex and formation of immature trabecular structures and increased cancellous bone in the center of the lengthened portion. At 24 weeks after surgery, more prominent cortex formation and fatty marrow were observed in the ESW group than in the control group.

The effect of light intensity on double bond conversion and flexural strength of a model, unfilled dental resin.

OBJECTIVE: Two visible light sources (tungsten-quartz-halogen and xenon-arc plasma) with vastly different intensities (200 and 1800 mW/cm(2)) but similar spectral outputs, were used to examine the effects of light intensity on conversion and flexural strength of a model dental resin formulation (75/25wt% bis-GMA/TEGDMA). METHODS: The exact same polymer samples were used to correlate double bond conversion (measured with near-IR spectroscopy) to flexural strength, both immediately after light exposure and after storage. RESULTS: In general, polymers which were irradiated with the high light intensity source exhibited greater double bond conversion. However, increasing the light intensity also increased the maximum temperature reached during polymerization. Therefore, the greater double bond conversion was caused by a combination of both photo and thermal effects. Regardless of the light intensity, a single linear relationship existed between conversion and final flexural strength (measured 4 days after cure) over the conversion range analyzed (50-80%). However, deviations from linearity were noted in several samples that were tested immediately after exposure. SIGNIFICANCE: These findings illustrate that light intensity does not affect the final flexural strength of a dental resin as long as the final conversions are similar.

Polymerization of resin composite restorative materials: exposure reciprocity.

OBJECTIVE: To examine whether there is reciprocity between irradiation time and irradiance with regard to the mechanical properties of filled, resin composite restorative materials (RCs). METHODS: Four visible light-cured RCs, all of shade A3, were used: Heliomolar Radiopaque (HR) and Tetric Ceram (TC) (Ivoclar, Schaan, Liechtenstein), Filtek Z250 (FZ) (3M, St Paul, MN, USA) and Prodigy condensable (PR) (Kerr, Orange, CA, USA). Bar specimens (1.0 x 1.5 x 16.0 mm(3)) were cured at irradiances (I) ranging from 25-1500 mW/cm(2) and irradiation times (t) of 1-3000 s. Six specimens at 250 combinations of t and I were prepared and stored in artificial saliva of pH 6, at 37 degrees C for 7d before performing three-point bend tests for flexural strength (F), flexural modulus (E) and total energy to failure (W). Contour plots of property value vs. t and I on log scales were prepared. RESULTS: The contour plots showed three regions: unset at low I.t, a plateau corresponding to more or less full property development, and connecting ramp. The boundary between the plateau and the ramp suggests the minimum acceptable exposure. No practical lower limit to irradiance was detected, but there may be no benefit from increasing I beyond about 1,000 mW/cm(2). The slopes of the contours in the log-log plots provided a test of the hypothesis of reciprocity. These slopes were approximately -1.5 for HR, TC and PR; and approximately -1 for FZ, compared with the expected value of -1. The general hypothesis therefore fails. The existence of localized maxima in property values is further evidence of that failure, even for FZ. SIGNIFICANCE: Dentists may use any lamp, including LED sources, and attain satisfactory results providing irradiation time is long enough. Manufacturers ought to supply a graph indicating the minimum acceptable exposure for each product for specified curing lamps. Calculations based on total energy delivered to guide irradiation protocols are invalid and do not recognize product behavior.

Irradiance effects on the mechanical properties of universal hybrid and flowable hybrid resin composites.

OBJECTIVES: A potential problem with high-intensity lights might be failure of polymer chains to grow and cross-link in a desired fashion, thereby affecting the structure and properties of the polymers formed. The purpose of this study was to evaluate mechanical properties of resin composites polymerized using four different light-curing units. METHODS: A conventional quartz-tungsten-halogen (QTH) light, a soft-start light, an argon-ion laser, and a plasma-arc curing light were used to polymerize disk-shaped (9.0mm diameter x 1.0 mm high) and cylinder-shaped (4mm diameter x 8 mm high) specimens of a universal hybrid and a flowable hybrid composite. Biaxial flexure strength, fracture toughness, hardness, compressive strength, and diametral tensile strength were determined for each composite. RESULTS: The use of the plasma-arc curing light, a high-intensity light, resulted in significantly lower hardness for the universal hybrid composite compared with the hardness obtained using the conventional QTH and the soft-start units. Hardness was the only mechanical property that was adversely affected by the use of a high-intensity light. SIGNIFICANCE: High-intensity lights might affect some resin composite mechanical properties, but this effect cannot be generalized to all resin composites and all properties.