To test the influence of specimen dimensions on light transmission and shrinkage strain properties of curing dental resins.
Materials and methods
Photocurable resin specimens (Bis-GMA/TEGDMA) with aspect ratios (AR) of 2 (4 mm × 2 mm); 4 (4 mm × 1 mm and 8 mm × 2 mm); 8 (8 mm × 1 mm); 12 (AR: 12 mm × 1 mm); and 24 (12 mm × 0.5 mm) were light cured. Light transmission and shrinkage-strain data were recorded throughout, and upper and lower surface hardness measurements were performed following cure.
Light transmission was significantly affected by the specimen aspect ratio even at similar thicknesses ( p < 0.05). By comparing light transmission through a negative control resin without photoinitiator, the lowest AR specimens showed a relative increase in transmission above 100% throughout curing, which was caused by specimen constraint. The extent of lower surface cure (as assessed by increasing hardness) was principally affected by cavity height and decreased for thicker specimens ( p < 0.05). Only the 2 mm thick specimens showed a significantly greater lower to upper hardness ratio with increasing cavity diameter ( p < 0.05). The highest AR specimen showed the greatest lower to upper hardness percentage ( p < 0.05), and was expected since this AR was obtained by reducing the sample thickness to 0.5 mm. Generally, total shrinkage strain increased and shrinkage strain per unit mass decreased with increasing AR.
Specimen constraint in low AR cavities may compromise light transmission as unexpected light intensity variations may occur for low configuration factors, which ultimately affect polymer conversion of light-cured resin-based restorations through depth.
The shrinkage-strain induced during photo-polymerization may be characterized by its magnitude, direction (or vector character of strain) and time dependence. The strain imposed during photo-polymerization is not only influenced by intrinsic material properties such as composition but also shape and the size of the cavity being restored as well as the compliance of the surrounding cavity . When a curing material is bonded by opposing cavity walls, strain becomes restricted and the final stress generated is given by the product of elastic modulus and strain vectors . Not only is the stress magnitude determined by characteristics of composites such as filler content , curing rate and degree of conversion but many authors have also reported stress is governed by the configuration factor ( C -factor) of the cavity; the ratio of bonded to un-bonded surfaces (Eq. (1) ), may also be given by the aspect ratio (diameter/height), and the compliance of the system . In the equation, r is the cavity radius, h is the cavity height and is the cavity diameter.
C f = 2 π r 2 2 π r h = r h = 2 h
Previous investigations have suggested that a C -factor less than 1 results in slower development of shrinkage stress and the composite remains bonded to the cavity walls . The C -factor itself is governed by the geometry of the cavity, i.e. the diameter and the height which both independently influence the generation of shrinkage-stress. An investigation by Watts et al. reported that at a constant height, with diameter variation, a C -factor increase from 0.6 to 6 gave an exact exponential decrease in shrinkage-stress, whereas at constant diameter with height variation, an increase in C -factor from 3 to 100 gave an increase in shrinkage-stress . It is clear that the stress generated during polymerization cannot simply be extrapolated to C -factor alone and is dependent upon several other properties, amongst which material volume and mass, as well as bonding properties to mold or tooth and/or matrix surfaces and the compliance of such will influence the final magnitude of stress .
Light transmission and the number of photons absorbed by the photoinitiator is critically important to the extent of polymerization and is a desired parameter to characterize fully since it ultimately relates to the final mechanical and physical properties of the polymer. Inefficient light transmission and limited cure depths are associated with surface reflection , absorption and interfacial scattering . Upon polymerization, photoinitiator decomposition will reduce the concentration of light absorbers and polymerization shrinkage will reduce the optical path length, which will increase the transmitted light through the sample according to Beer–Lamberts law (Eq. (2) ).
A = ε C l = − log I I 0
where A is the absorbance, l is the optical path length, C and ε are the concentration and molar extinction coefficient of the absorbing species (i.e. the photoinitiator) and I and I 0 are the incident light and transmitted light, respectively.
Varying patterns of light transmission through filled and unfilled photocurable resins have been reported. Harrington et al. demonstrated a method, which allowed the actual consumption of light energy by the photoinitiator to be calculated and showed increasing light intensity with radiation time for commercial composites . Using the same technique, Ogunyinka et al. reported decreasing transmission during polymerization of experimental composite formulations . Shortall et al. showed the effect of monomer and filler composition on light transmission and reported that the refractive index mis-match between filler and resin will affect its light transmission profile where maximum transmission is observed when the refractive index of the resin and filler are equal . We have previously demonstrated the effect of monomer composition on the variation in optical properties and physical thickness change in unfilled resins during irradiation . Although shrinkage strain was dependent upon material composition, cavity constraint is likely to affect shrinkage strain development, which in turn may affect light transmission. Consequently, this study will test the following hypotheses:
Light transmission through curing resins will vary with the specimen aspect ratio at similar thickness.
For samples with increased light transmission, the ratio of lower:upper surface hardness percentage will improve.
Materials and methods
Experimental resins containing a 50:50 mix of bisphenol-A diglycidyl ether dimethacrylate (Bis-GMA) and triethylene glycol dimethacrylate (TEGDMA) were formulated. The photoinitiator system within each experimental resin contained the photoinitiator camphoroquinone (0.2 wt%) and the co-initiator dimethylaminoethyl methacrylate (DMAEMA; 0.8 wt%). A resin that contained no photoinitiator system was employed as a negative control for each of the molds tested. All reagents were provided by Sigma Aldrich, UK and were used as received. Resins were light cured using a quartz tungsten halogen light curing unit (XL2500, 3M ESPE, Dental Products, Seefeld, Germany) in cylindrical cavities placed in black nylon molds having C -factors ( C f : Eq. (1) ) ranging from 1 to 12 and aspect ratios (AR) ranging from 2 to 24; 4 mm × 2 mm ( C f : 1, AR: 2); 4 mm × 1 mm ( C f : 2, AR: 4); 8 mm × 2 mm ( C f : 2, AR: 4); 8 mm × 1 mm ( C f : 4, AR: 8); 12 mm × 1 mm ( C f : 6, AR: 12); and 12 mm × 0.5 mm ( C f : 12, AR: 24; Table 1 ). The diameter of the specimens were chosen to give a range of C -factors/aspect ratios and were restricted to the diameter of the light curing tip (12 mm). The height of the specimens was restricted to a maximum of 2 mm due to limitations of the interferometry technique used to measure shrinkage strain caused by the attenuation of light through the sample thickness. Aspect ratio was always twice that of C -factor ( Table 1 ) and as they both represent the ratio of bonded to non-bonded surfaces, henceforth only aspect ratio will be mentioned. Each specimen ( n = 3) was light irradiated for 60 s (after activation of the curing lamp for 10 s without irradiation of the sample) at an ambient temperature of 23 ± 1 °C. A 10 s non-irradiating lamp activation period was required as fluctuations in light transmission occurred during this initial period. An irradiance of 733 ± 18 mW cm −2 was measured using a handheld digital radiometer (Coltolux light meter, Cuyahoga Falls, OH, USA) held normal to the light curing tip from a 5 mm distance.