Investigation of mechanical performances and polymerization shrinkage of dual-cured resin composites as core build-up material

This study systematically compared the mechanical performances and polymerization shrinkage of two novel dual-cured resin composites (DCRC) with one conventional packable light-cured resin composite (LCRC) for their application as core build-up material by micro-hardness test, flexural strength test, push-out test, and digital image correlation analysis. The LCRC had a significantly higher micro-hardness (p<0.05) whereas the bond strength demonstrated no difference. The mean values of three materials ranged from 35.16 and 64.82 for the Vickers hardness and from 4.66 MPa to 11.53 MPa for the bond strength. The flexure strength of the three materials was not statistically different from each other. LCRC demonstrated 1.88% of volumetric shrinkage while the two DCRC showed 5.06% and 4.91%, respectively. In general, the DCRC demonstrated a comparable flexural strength and bond strength as the LCRC, however, the significant polymerization shrinkage of DCRC should be emphasized.

The light-curing unit: An essential piece of dental equipment

INTRODUCTION

This article describes the features that should be considered when describing, purchasing and using a light-curing unit (LCU).

METHODS

The International System of Units (S.I.) terms of radiant power or radiant flux (mW), spectral radiant power (mW/nm), radiant exitance or tip irradiance (mW/cm² ), and the irradiance received at the surface (also in mW/cm² ) are used to describe the output from LCU. The concept of using an irradiance beam profile to map the radiant exposure (J/cm² ) from the LCU is introduced.

RESULTS

Even small changes in the active tip diameter of the LCU will have a large effect on the radiant exitance. The emission spectra and the effects of distance on the irradiance delivered are not the same from all LCUs. The beam profile images show that using a single averaged irradiance value to describe the LCU can be very misleading. Some LCUs have 'hot spots' of high radiant exitance that far exceed the current ISO 10650 standard. Such inhomogeneity may cure the resin unevenly and may also be dangerous to soft tissues. Recommendations are made that will help the dentist when purchasing and then safely using the LCU.

CONCLUSIONS

Dental manufacturers should report the radiant power from their LCU, the spectral radiant power, information about the compatibility of the emission spectrum from the LCU with the photoinitiators used, the active optical tip diameter, the radiant exitance, the effect of distance from the tip on the irradiance delivered, and the irradiance beam profile from the LCU.

Depth of cure of high-viscosity bulk-fill and conventional resin composites using varying irradiance exposures with a light-emitting diode curing unit

The purpose of this study was to determine the depth of cure (DOC) of three resin-based composites (RBCs) using varying irradiance exposures with a corded light-emitting diode curing unit. DOCs for Filtek Z250, TPH Spectra, and Tetric EvoCeram Bulk Fill were determined using the International Organization for Standardization (ISO) Standard 4049. The RBCs were light-polymerized using three different power modes and manufacturer-recommended curing times. Irradiance was determined using a spectrometer sensor and the total energy density was calculated for each power mode and concomitant polymerization time. The DOC data were analyzed with a two-way analysis of variance and Tukey's post hoc test. Tetric EvoCeram Bulk Fill produced significantly greater DOCs than TPH and Z250 (P < 0.05) for all three power mode settings. Overall, the DOC of Tetric EvoCeram Bulk Fill was greater than those of TPH and Z250 at all power settings, but the individual RBCs did not show a significant DOC difference among the three power settings (P > 0.05).

Evaluation of effect of different disposable infection control barriers on light intensity of light-curing unit and microhardness of composite - An in vitro study

Aims:

This study evaluated effect of infection control barriers on light intensity (LI) of light-curing unit (LCU) and microhardness of composite.

Materials and Methods:

Four different disposable barriers (n = 30) were tested against the control. LI for each barrier was measured with Lux meter. One hundred and fifty Teflon molds were equally divided into five groups of thirty each. Composite was filled in bulk in these molds and cured without and with barrier. Microhardness was evaluated on top and bottom surface of composite specimen with microhardness testing machine and hardness ratio (HR) was derived.

Statistical Analysis Used:

One-way analysis of variance, Tukey's honestly significant difference test, and paired t-test using SPSS version 18 software.

Results:

All barriers had significantly reduced the baseline LI of LCU (P < 0.0001), but only Cure Elastic Steri-Shield and latex cut glove pieces (LCGP) significantly reduced the microhardness of the composite (P < 0.05). However, HR determined inadequate curing only with LCGP.

Conclusions:

Although entire tested barrier significantly reduced the LI; none, except LCGP markedly affected the degree of cure of the composite.

Effect of Curing Light Barriers and Light Types on Radiant Exposure and Composite Conversion

PURPOSE

Previous research investigated the effects of curing tip barriers on light output and composite properties, but no study has measured the effect of a wide variety of barriers and curing light types on delivered radiant exposure and the resulting composite cure 2 mm below the radiated surface.

MATERIALS AND METHODS

Six barrier materials and six curing light types were tested. Spectroradiometry was used to measure irradiance with and without barriers for each light type, and radiant exposure values were determined for a commercial camphorquinone-based dental composite material. Composite monomer conversion was measured at 10 minutes following exposure for each light type/barrier condition (N = 5) using infrared spectroscopy. Results were subjected to one-way analyses of variance for radiant exposure and conversion among barrier types within a given curing light: preset alpha 0.05.

RESULTS

All barriers significantly reduced radiant exposure compared with the uncovered tip, but the use of the two polywave LEDs covered with a latex-based barrier demonstrated significantly lower conversion values.

CONCLUSIONS

Although light-curing barriers reduce radiant exposure to a restorative material over a recommended exposure, this reduction is not sufficient to cause significant reduction in composite cure, except when using a latex-based barrier and a polywave LED curing light.

CLINICAL SIGNIFICANCE

Clinicians need to be aware of the possible interaction between curing light barriers and curing light type in order to optimally photocure restorative materials.

Distance and protective barrier effects on the composite resin degree of conversion

Context:

The food wrap films are used to cover the tip of curing light units in order to avoid contamination and prevent damage to the light guide. However, their effects on resin polymerization are not fully known.

Aims:

We investigated the effects on restoration efficiency of a food wrap protective barrier used on the tip of curing light units.

Materials and Methods:

For each treatment, five replications were performed, a total of 60 bovine incisor. The degree of conversion (%DC) of restorations with the composite resin Opallis EA2 was evaluated using 3 curing light devices (Optilux 501, Optilight and Ultra LED) and 2 curing distances (0 and 5 mm). The composite resin was tested for restoration of cavities in bovine crowns. %DC values were measured by the Fourier transform infrared spectroscopy-attenuated total reflectance technique.

Statistical Analysis Used:

The data were analyzed using 3-way ANOVA and Tukey's test.

Results:

Use of the protective film lowered %DC (F = 4.13; P = 0.05), and the effects of curing distance were associated to the curing light device (F = 3.61; P = 0.03).

Conclusions:

The distance from the light curing tip and use of a translucent protective barrier on the light-cure device can both impair composite resin %DC.

Contemporary issues in light curing

This review article will help clinicians understand the important role of the light curing unit (LCU) in their offices. The importance of irradiance uniformity, spectral emission, monitoring the LCU, infection control methods, recommended light exposure times, and learning the correct light curing technique are reviewed. Additionally, the consequences of delivering too little or too much light energy, the concern over leachates from undercured resins, and the ocular hazards are discussed. Practical recommendations are provided to help clinicians improve their use of the LCU so that their patients can receive safe and potentially longer lasting resin restorations.

Effect of a multi-layer infection control barrier on the micro-hardness of a composite resin

Objective

The aim of this study was to evaluate the effect of multiple layers of an infection control barrier on the micro-hardness of a composite resin.

Material and Methods

One, two, four, and eight layers of an infection control barrier were used to cover the light guides of a high-power light emitting diode (LED) light curing unit (LCU) and a low-power halogen LCU. The composite specimens were photopolymerized with the LCUs and the barriers, and the micro-hardness of the upper and lower surfaces was measured (n=10). The hardness ratio was calculated by dividing the bottom surface hardness of the experimental groups by the irradiated surface hardness of the control groups. The data was analyzed by two-way ANOVA and Tukey's HSD test.

Results

The micro-hardness of the composite specimens photopolymerized with the LED LCU decreased significantly in the four- and eight-layer groups of the upper surface and in the two-, four-, and eight-layer groups of the lower surface. The hardness ratio of the composite specimens was <80% in the eight-layer group. The micro-hardness of the composite specimens photopolymerized with the halogen LCU decreased significantly in the eight-layer group of the upper surface and in the two-, four-, and eight-layer groups of the lower surface. However, the hardness ratios of all the composite specimens photopolymerized with barriers were <80%.

Conclusions

The two-layer infection control barrier could be used on high-power LCUs without decreasing the surface hardness of the composite resin. However, when using an infection control barrier on the low-power LCUs, attention should be paid so as not to sacrifice the polymerization efficiency.

Microbial contamination of light curing units: a pilot study

Visible light units (LCU) used in routine dentistry to cure light activated materials may become contaminated with oral micro-organisms. This pilot study was designed to investigate whether the fan, handle and base unit mains on/off button areas of three different designs of quartz tungsten halogen LCUs (3M Unitek 2500, Elipar Highlight, Demetron Optilux 401) were effectively disinfected after use in a dental teaching hospital. Over a period of seven days 52 LCUs were swabbed before clinics in the morning and 28 were swabbed again after clinics in the afternoon. Bacterial contamination was detected on approximately 40% (20/52) of units before use and 64% (18/28) after use: few viable organisms were detected on the fan or handle areas, but many were isolated from the mains button, including Staphylococus aureus. These findings highlight the need for greater awareness of the potential risk of contamination of the base unit and compliance with recommendations to clean and disinfect all areas of the units.