Evaluate the effect of environment on post-gel shrinkage (Shr), cuspal strains (CS), microtensile bond strength (μTBS), elastic modulus ( E ) and shrinkage stress in molars with large class II restorations.
Sixty human molars received standardized Class II mesio-oclusal-distal cavity preparations. Restorations were made with two composites (CHA, Charisma Diamond, Heraus Kulzer and IPS Empress Direct, Ivoclar-Vivadent) using three environment conditions (22 °C/50% humidity, 37 °C/50% humidity and 37 °C/90% humidity) simulated in custom developed chamber. Shr was measured using the strain gauge technique ( n = 10). CS was measured using strain gauges. Half of the teeth ( n = 5) were used to assess the elastic modulus ( E ) and Knoop hardness (KHN) at different depths using microhardness indentation. The other half ( n = 5) was used to measure the μTBS. The composites and environment conditions were simulated in a two-dimensional finite element analysis of a tooth restoration. Polymerization shrinkage was modeled using Shr data. The Shr, CS, μTBS, KHN and E data were statistically analyzed using two-way ANOVA and Tukey test (significance level: 0.05).
Both composites had similar Shr, CS, μTBS and shrinkage stress. CHA had higher elastic modulus than IPS. Increasing temperature and humidity significantly increased Shr, CS and shrinkage stress. μTBS were similar for groups with lower humidity, irrespective of temperature, and higher with higher humidity. E and KHN were constant through the depth for CHA. E and KHN values were affected by environment only for IPS, mainly deeper in the cavity. Shrinkage stress at dentin/composite interface had high inverse correlation with μTBS. Shrinkage stress in enamel had high correlation with CS.
Increasing temperature and humidity caused higher post-gel shrinkage and cusp deformation with higher shrinkage stresses in the tooth structure and tooth/restoration interface for both composites tested. The chamber developed for simulating the oral environment conditions will improve the realism of in vitro studies.
Clinical significance Simulating oral temperature and humidity is important to better determine the biomechanical behavior of composite resin restoration. Avoiding high humidity during restorative procedures using rubber dam isolation may reduce cuspal deformation and shrinkage stress and improve the bonding strength of posterior composite restorations.
Despite major developments in new restorative materials over the years, resin composites still experience volumetric reduction due to polymerization shrinkage . Since composites are bonded to the prepared tooth cavities via dental adhesives, the polymerization shrinkage will generate internal stresses that may challenge the mechanical stability of the restoration and potentially lead to failure . Shrinkage causes cuspal deflection , and can result in enamel microcracks , and marginal gaps . Enamel microcracks and marginal gaps may result in post-operative sensitivity, discolored margins, recurrent caries and fractures in the restoration margins , and may lead to misdiagnosis as secondary caries that are the main reasons for replacement of composite restorations .
The process of composite polymerization can be simplified into pre-gel and post-gel phases in relation to the development of elastic modulus . In the pre-gel phase the composite is able to flow, which relieves the stresses within the restoration. After gelation, the developing elastic modulus causes an accumulation of shrinking stresses in the restoration/tooth structure . Mercury dilatometer , and transducer methods have been used to measure polymerization shrinkage. However, only the post-gel component of shrinkage generates the shrinkage stresses. A strain gauge method has been used to determine the post-gel shrinkage . Cuspal deformation can be measured using strain gauges , transducers or surface imaging methods . Besides tooth deformation, the stress during polymerization may also affect the adhesion between composite and tooth substrate .
Interaction between temperature and moisture in the environment can have detrimental effect on polymer-based restorative materials . When the temperature increases, the resin viscosity decreases due to an increase in free volume and molecular mobility which, within limits, increases degree of conversion . Increasing degree of conversion is normally accompanied by increased shrinkage .
Previously, cusp deflection or post-gel shrinkage studies have been conducted at room temperature (23 °C) and humidity (50%). However, the mechanical properties of resin-composites are influenced not only by their chemical composition, but also by the environment to which they are exposed . It is therefore important to simulate environmental conditions in vitro with versatility to obtain standardized post-gel shrinkage and cusp deformation for different temperatures and humidity conditions. The aim of this study was to develop and test a new chamber for measuring post-gel shrinkage and cuspal deflection of molars restored with two composite resins under different environmental conditions. The null hypothesis was that the restorative materials and environment conditions would not affect mechanical properties, cuspal deformation, bond strength, and shrinkage stress in restored molars.
Materials and methods
Two resin composites (IPS, IPS Empress Direct, Ivoclar-Vivadent and CHA, Charisma Diamond, Heraeus Kulzer) and with a self-etching adhesive system (Clearfil SE Bond, Kuraray) were tested in this study ( Table 1 ). The composites used in this study were chosen because they have different monomer compositions and both are regularly used by clinicians. The post-gel shrinkage and cusp deformation were tested at (a) 22 ± 2 °C and 50 ± 3% relative humidity, representing laboratory temperature and humidity; (b) 37 ± 2 °C and 50 ± 3% relative humidity, simulating the use of rubber dam with absolute isolation of the operatory area; and (c) 37 ± 2 °C and 90 ± 3% relative humidity, simulating the oral environment without rubber dam.
|Dental composites||wt.%||Filler type||Matrix||Manufacturer|
|Charisma Diamond||77||Ba, Al, F glass (0.02–2.0 μm) and colloidal silica (0.02–0.07 μm)||Bis-GMA and TEGDMA||Heraus Kulzer, Hanau, Germany|
|IPS Empress Direct||81.2||Barium, alumina, fluorosilicate glass, barium glass filler, mixed oxide, and ytterbium trifluoride||Bis-GMA and UDMA||Ivoclar Vivadent, Schaan, Liechtenstein|
Development of environmental chamber
Post-gel shrinkage and cusp deformation measurements were carried out in a chamber to simulate different environments developed at the Dental School of the Federal University of Uberlandia. The principal components of the chamber were two precision mechanical micrometers and one dial indicator with a 0.01 mm accuracy (Mitutoyo Am. Corp., Mississauga, Ontario, Canada), a rod for supporting the light source, a fixed stainless steel table (25 mm × 25 mm × 5 mm − length × width × height), a mobile table coupled to the two precision micrometers and a metal holder to keep the sample stable. Placing the chamber over a plane and flat surface avoided visual imperfections due to inclination.
The chamber is enclosed with acrylic panels (70 cm in length × 50 cm in width × 50 cm in height) to allow control of the temperature and relative humidity. The temperature control consisted of an electrical heater, sensor element and digital controls to keep the temperature within a set range. The humidity was controlled by a water spray system, which was activated automatically to maintain a preset humidity. The precision of the environmental controls were ±2 °C temperature and ±4% relative humidity.
The base of the chamber comprised of a specimen fixation to stabilize the tooth during restorative procedure. Strain gauges were attached to the external cusp surfaces for measuring cuspal deformation. For post-gel shrinkage measurement, an accessory was developed with strain gauge and a light-cell attached for monitoring light intensity during composite shrinkage.
Post-gel shrinkage measurement (Shr)
Composite post-gel shrinkage was determined using the strain gauge method . A Teflon mold (2 mm × 2 mm × 1 mm) was used to standardize the volume of the composite resin samples before placing them ( n = 10) on top of a biaxial strain gauge (CEA-06-032WT-120, Measurements Group, Raleigh, NC, USA), and shaped into a hemisphere approximately 1.5 mm high and 3–4 mm wide. The perpendicular strains were averaged since the material properties were assumed homogeneous and isotropic in the plane of the strain gauge on a macro scale. The composite was light-cured using a quartz–tungsten–halogen (QTH) unit (XL2500, 3M ESPE, St Paul, MN, USA) with the light tip placed at 1 mm distance from the surface of the composite. The diameter of the light guide was 8 mm. The radiant exposure (measured with Demetron Kerr; Orange, CA, USA) was 24 J/cm 2 (550 mW/cm 2 × 40 s). A strain conditioner (2101A Series, Micro Measurements Group) converted electrical resistance changes in the strain gauge to voltage changes through a quarter-bridge circuit with an internal reference resistance. Microstrain resulting from polymerization shrinkage was recorded for 10 min, starting from the beginning of photo-activation. The post-gel shrinkage value at 10 min was used in the finite element analysis. The mean shrinkage strain, which is the linear shrinkage, was converted to percentage and multiplied by three to obtain the volumetric shrinkage.
Cuspal strain measurement (CS)
The experimental design is shown on Fig. 1 . Sixty extracted intact, caries-free human third molars were used with approval from the University Ethics Committee in Human Research. The teeth were selected to have an inter-cuspal width within a maximum deviation of 10% of the determined mean. The measured inter-cuspal width varied between 5.06 and 6.02 mm. The teeth were embedded in a polystyrene resin (Cristal, Piracicaba, SP, Brazil) up to 2.0 mm below the cervical line to simulate alveolar bone . The teeth were cleaned using a rubber cup and fine pumice water slurry and distributed into six groups of ten teeth. The teeth were restored and used for cuspal deflection measurement using strain gauges, and afterwards for the measurement of Knoop hardness (KHN) and elastic modulus ( E ). All restored teeth had Class II cavities with 4.5 mm inter-cuspal width and 5 mm depth, prepared with a diamond bur (#3099, KG Sorensen, Barueri, SP, Brazil) in a high-speed handpiece with copious air-water spray using a cavity preparation machine . This machine consisted of a high-speed handpiece coupled to a mobile base. The mobile base moves vertically and horizontally with 3 precision micrometric heads (152-389; Mitutoyo Sul Americana Ltda, Suzano, SP, Brazil), attaining a 0.002-mm level of accuracy.