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).
Rectangular bar-shaped specimens (25 mm × 2 mm × 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.
While the materials tested differed significantly for all properties ( P < 1 × 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.
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.
The mechanical properties of resin-based composites (RBCs) are influenced by formulation (shade, monomer composition, filler properties), light-curing unit (LCU) (irradiance, spectrum), and irradiation time and technique (“soft-start”, pulsed) . Quartz–tungsten halogen (QTH) LCUs produce light by thermal emission and need a blue filter . The bulbs have a relatively short working life of about 100 h , while the filter and the reflector also degrade over time due to the high operating temperatures . Damage to the fiber-optic bundle due to poor handling and repeated sterilization can also lead to a reduction in output over time. However, it has been found that many practitioners fail to maintain their LCUs and thus preserve the intended output . So-called ‘turbo’ tips for QTH LCUs have been introduced, supposedly to accelerate polymerization, and high-intensity plasma-arc LCUs similarly sold with claims of irradiation times being reduced by as much as 90% . However, while it has been shown that varying the LED tip diameter could influence the depth of cure and Knoop hardness , the use of ‘turbo’ tips has been shown to be associated with increased cuspal deflection in mesio-occlusal-distal cavities , while plasma-arc LCUs may not result in full polymerization at 5 mm depth for the short irradiation times claimed to be adequate by the manufacturers .
Light-emitting diode (LED) devices overcome some of the shortcomings of QTH LCU . The spectrum of gallium nitride blue LEDs corresponds to the activating absorption of the commonly used camphoroquinone photosensitiser (∼400–500 nm), thereby obviating filters . LEDs have an expected lifetime of several thousand hours with little degradation of output . Their markedly lower heat production is expected to improve their operational longevity as well as reduce the temperature rise at the tooth surface during irradiation . It has been demonstrated that the depth of cure , compressive strength , flexure strength , hardness and degree of polymerization achieved with LED LCUs can be at least as good as with QTH LCUs.
The assumption amongst dental LCU manufacturers appears to have been that there is reciprocity between intensity and time such that increasing irradiance allows a corresponding reduction in irradiation time whilst achieving the same values of mechanical properties. But the manufacturer of two LED LCUs, the one with about 4 times the nominal output of the other, has suggested that the recommended irradiation time can be halved when the higher output device is used , suggesting that there is some recognition that reciprocity (which would imply a quarter of the time only is necessary) does not apply.
The purposes of this work were therefore to examine whether the differences between the above LED LCU models had any bearing on the resulting mechanical properties, and likewise the choice of curing tips available for use with them.
Materials and methods
The LED LCUs used were successive models from one manufacturer (Elipar Freelight, Elipar Freelight 2; 3 M ESPE, St. Paul, MN, USA) (F1, F2), with two fiber-optic light guides (‘tips’) of 8 and 13 mm diameter which plugged directly into a port on the body of the device. The output was measured ( n = 7) at the center of the exit window using a radiometer incorporated in another LCU (Optilux 501, Kerr, Orange, CA, USA) for each combination. The adit and exit windows of each light guide were examined with an optical measuring microscope (MM-40, Nikon, Japan) at 10× magnification.
Rectangular bar-shaped specimens (25 mm × 2 mm × 2 mm) of four RBCs (shade A3) ( Table 1 ) were made in an open-ended, knife-edged, split aluminium mould (alloy 6061) at 23 ± 1 °C. The mould was packed with an excess of the RBC, covered with a cellulose acetate strip and a glass microscope slide and a load of 1 kg was applied for 20 s to ensure consistent and reproducible packing of the specimens.
|Material||Code||Filler type||Filler particle size/μm||Filler volume/%||Resin||Recommended irradiation time/s|
|Z100||Z100||Zirconia, silica||0.01–3.5||66||Bis-GMA, TEGDMA||40|
|Filtek Z250||Z250||Zirconia, silica||0.01–3.5||60||Bis-GMA, UDMA, Bis-EMA, TEGDMA||20|
|Filtek P60||P60||Zirconia, silica||0.01–3.5||61||Bis-GMA, UDMA, Bis-EMA, TEGDMA||20|
|Filtek Supreme XT||XT||Zirconia, silica||0.6–1.4||59.5||Bis-GMA, UDMA, Bis-EMA, TEGDMA||20|
After removing the load and microscope slide, the specimens were irradiated for the time specified by the manufacturer ( Table 1 ) at successive overlapping spots as the length of the bar exceeded the LCU tip diameter. Irradiation was first at the center of the specimen. The tip was then moved such that the next irradiated area overlapped a previously exposed area by a quarter of the diameter of the exit window. This was repeated until the entire specimen had been irradiated. This required five and three irradiations for the 8 and 13 mm diameter tips, respectively. This process was facilitated by pre-marking the edge of the acetate strip so that the tip could be placed accurately.
Following irradiation, specimens were removed from the mould, checked for imperfections and any evidently defective replaced, then stored in deionised water in the dark at 37 ± 1 °C for 24 h before testing. In total, 16 groups of 20 specimens (four RBCs, two LCU tip diameters, and two LCUs) were prepared.
A universal testing machine (Instron Model 5565, Instron Ltd., High Wycombe, UK) was used to test specimens in three-point flexure, irradiated side uppermost, using a span of 20 mm at a crosshead speed of 1 mm/min. The three-point flexure strength ( σ 3 ) was calculated from:
where P is the load at fracture, L the span, b the width, and h the thickness of the specimens. Dimensions were measured using a digital micrometer screw gauge reading to 1 μm (Mitutoyo, Kawasaki, Japan). The flexural modulus of elasticity ( E ) was determined from:
where (Δ P /Δ D ) is the gradient of the steepest linear portion of the load–deflection curve.
For each test condition, three further specimens were fabricated as above. Two Vickers indentations (Micromet 5104, Buehler, Lake Bluff, IL, USA) were made at a load of 300 g, applied for 15 s at 23 °C, on each of the non-overlapped (NO) and overlapped (OV) regions for both the top and bottom surfaces of each. Vickers hardness ( H V ) was calculated from:
where P is the load, d the length of the diagonal (in micrometers) and 68° is the pyramid half-angle.
One-, two- and three-way analyses of variance (AoV), as well as regression analyses, were made in software (Sigmaplot 11, Systat Software, Richmond CA, USA) using a critical significance level of P = 0.05, guided as necessary by Bonferroni correction in multiple partial analyses.