To reduce the effect of stresses due to volumetric shrinkage the authors propose an incremental technique for placing composite restorations.
The goal of the method is to reduce the volume of the resin that is polymerized and eliminate a stress singular point in the resin that is positioned at the geometric center of the cavity. This is achieved by a two step type incremental technique. In the first step the resin is placed in the cylindrical cavity with a metal pin embedded in the middle of the composite restoration. After polymerization, the metal pin is removed and the cylindrical hole is filled with the second layer of composite. Finally, the second layer in the center of the composite restoration is polymerized.
This study confirmed that the proposed incremental type placement technique reduces marginal debonding.
The main hypothesis is that the elimination of a stress singular point at the center of the restoration results in the reduction of stresses at tooth–composite interface and therefore improve the marginal adaptation (reduces length of the contraction gap at tooth–composite interface).
The polymerization of light-activated dental composites is accompanied by volumetric shrinkage and shrinkage stress, resulting in the development of tensile and/or shear stresses at the cavo-restoration interface. When shrinkage stress overcomes the bond strength to the cavity walls, the marginal seal of adhesive composite restoration is lost, resulting in the formation of a contraction gap at the tooth–composite interface . Once the bond between restoration and tooth is broken, microleakage of oral fluids, bacteria, molecules, and ions occurs at the cavo-restoration interface resulting in marginal staining, postoperative sensitivity, the development of marginal caries, and consequent pulp pathology . All current resin composites shrink by about 2–4% and as a consequence polymerization shrinkage stress puts the bond under severe tension, which may lead to immediate failures of adhesive joint or tooth deformation and cohesive failures within the restorative material or tooth structure . The polymerization contraction behavior of composite resins is dependent on material properties, the curing method, the restorative technique and cavity configuration or “ C -factor” which is defined as the ratio between the bonded and unbonded resin composite surface areas. Three-dimensional, fully-bonded cavities have the highest C -factor, where polymerization stresses may be greater .
Recently, Ferracane proposed different clinical methods to reduce the polymerization shrinkage stress, such as altered light-curing mode and speed , flowable resin liner application, and an incremental filling technique. Flowable composites are low-viscosity resin-based restorative materials with a reduced filler content, compared to non-flowable composites. On the basis of a low filler content, flowable composites are less rigid and have a modulus of elasticity lower than conventional hybrid composites. Flowable composites have been proposed as an elastic intermediate layer underneath conventional composites , fissure sealants and restorative materials for small cavities. On the other hand, some flowable composites have high polymerization shrinkage values due to their low filler content, resulting in high shrinkage stresses .
The incremental filling techniques (horizontal, vertical, oblique) are capable of reducing the shrinkage stresses due to a reduction in the volume of the composite material and “ C -factor” at placement , but it is not yet clear what incremental filling technique is the most appropriate one. However, no method has proved to be completely successful in reducing shrinkage stress during the photo-polymerization period.
In this research the hypothesis tested was, that reducing shrinkage stresses can be achieved by an incremental type placement technique consisting in a reduction of volume of the resin that is polymerized and elimination of a stress singular point in the resin.
Material and methods
Twenty intact non-carious, non-restored human third molars were stored in 0.5% chloramine solution at 4 °C and used within 1 month after extraction. The teeth were cleaned with scalers and polished with pumice. The buccal enamel was ground using a model trimmer under running water, to expose a flat dentin surface and then finished with wet SiC 600-grit paper. Standardized cylindrical cavities (3.5 mm diameter, 1.5 mm deep) with all margins in dentin were prepared using diamond bur (107–126 μm, No: 806 314110524014 NTI-Kahla, Gmbh, Germany) in a high speed hand-piece with copious air–water spray. Fine grained diamond burs (40 μm, No: 806 314110514014, NTI-Kahla, Gmbh, Germany) were used for finishing the preparations. A new bur was used after every other preparation. The cavities were evaluated using an optical microscope at 40× magnification to check for any imperfect finishing lines and/or any visible pulp exposure.
The materials used in this study and their composition are listed in Table 1 . A light-cure flowable composite (Tetric EvoFlow, Ivoclar, Vivadent AG, Schaan/Liechtenstein, shade A2) and a single-component self-etch dental adhesive (AdheSE ONE VivaPen, Ivoclar, Vivadent AG, Schaan/Liechtenstein) were used. The adhesive system as well as the filling material, were handled according to the manufacturer’s instructions.
|AdheSE ONE VivaPen||Self-etching adhesive||Ivoclar, Vivadent AG, Schaan/Liechtenstein||Bis-acrylamide derivative
|Tetric EvoFlow||Flowable composite||Ivoclar, Vivadent AG, Schaan/Liechtenstein||Bis-GMA, uretane dimethacyrilate, decandiol dimethacyrilate 37.6 wt.%
Barium glass filler, ytterbium trifluoride, mixed oxide, highly dispersed silicon dioxide 41.1 wt.%
Prepolymer 20.4 wt.%
Additives, catalyst stabilizers 0.9 wt.%
Pigments <0.1 wt.%
Self-etching dental adhesive was applied to each cavity and continuously brushed for 30 s. Excess material was dispersed with a strong air-stream until there was no movement of the material and light-cured for 10 s with a LED curing light (Smartlite IQ 2, Dentsply, Caulk, DE Milford, Serial No. B21581). Subsequently, with regard to the different resin composite filling techniques, the teeth were randomly divided into two groups ( n = 10): bulk – a and incremental – b .
Group a: bulk filling technique . Composite (Tetric EvoFlow, Ivoclar, Vivadent AG, Schaan/Liechtenstein) was placed in one bulk increment with a syringe and light-cured for 20 s and additionally for 20 s to make curing time identical (total 40 s).
Group b: incremental technique . Composite (Tetric EvoFlow, Ivoclar, Vivadent AG, Schaan/Liechtenstein) was placed in two consecutive increments. First, cavities were bulk filled with syringe and a metal pin ( 1.1 mm) was embedded in composite resin approximately through the center of the cavity. The first increment was light-cured for 20 s and the metal pin removed from the composite resin. The second increment inside the cylindrical hole was placed with a syringe (cannulla 0.8 mm), and light-cured for 20 s (total 40 s).
All photo-polymerizing steps were performed with a light guide held perpendicularly and within 4 mm of the cavity floor . Throughout the study, the light output from the curing unit was verified by a built-in radiometer. Immediately after the filling procedure, the excess resin was removed by gentle wet grinding with SiC papers through 1000-grit until the cavity margins were exposed and then rinsed with copious amounts of water to remove the polishing surface debris. The restored teeth were stored in a container with 100% relative humidity for 24 h to prevent dehydration.
Evaluation of the marginal adaptation and SEM observation
Both an epoxy replica and the original teeth were viewed using SEM. The impressions were made and epoxy replicas were prepared for the computer-assisted quantitative margin analysis under a scanning electron microscope (JEOL, JSM-6460LV, Tokyo, Japan). Images were taken with magnification of 25× for the evaluation of the entire cavity margins, then marginal adaptation was evaluated along the resin–dentin interfaces at a magnification of 200× . The following evaluation criteria were applied to the marginal quality, continuous margin : continuous transition between restoration and tooth hard substance; non-continuous margin : marginal debonding; marginal gap. The distance of continuous margins in relation to the entire length of the particular dentin-restoration margin was calculated and expressed as a percentage . Additional observations at magnification up to 1000× were taken for the examination of questionable marginal areas. The original teeth specimens were observed under SEM using high magnification up to 4000×, for the interface typical failure between composite restoration and dentin surface.
Statistical analysis and model
Statistical differences were calculated using the non-parametric Mann–Whitney U -test. The level of statistical significance was defined as p ≤ 0.05.
For the model, a visco-elastic constitutive equation was used to describe stresses induced by volumetric contraction. It was observed that volumetric contraction changes with time . We consider a composite without the pin ( Fig. 1 a ) and with pin ( Fig. 1 b).
It was assumed that the composite is in a plane stress state . Thus, we assume that shear stresses at the lower part of the composite restoration are small and do not influence the stresses in the bulk of the composite. Relevant equilibrium equations may be reduced to:
subject to the following boundary conditions
σ r ( r = R ) = T a , for case ( a ) ; σ r ( r = R ) = T a , σ r ( r = r 0 ) = 0 , for case ( b )
where σ r is the radial component of the stress, r is the polar coordinate (see Fig. 2 ), R and r 0 are the outer and inner radii, respectively, prime denotes differentiation with respect to r , i.e., (⋅)′=d(⋅)/dr
( ⋅ ) ′ = d ( ⋅ ) / d r
is the adhesive stress at the cylindrical boundary of the restoration.
Solution for the case of composite with and without pin, read
σ r = T 0 , case ( a ) ; σ r = r 0 2 T 0 R 2 − r 0 2 1 − r 0 2 r 2 , case ( b )
The component of the displacement vector in the radial direction becomes