Polymerization Shrinkage Strain Research Articles

Factors influencing marginal cavity adaptation of nanofiller containing resin composite restorations

Objectives The aim of this study was to investigate the effects of polymerization contraction, shrinkage stress and Young's modulus of nanofiller containing resin composites on early marginal adaptation of restorations in cavities. Methods Six nanofiller containing and two reference resin composites were studied. Marginal gap widths of restorations in cylindrical 4.2 mm wide and 1.5 mm deep dentin cavities, non-bonded or bonded with a self-etch adhesive, and in Teflon cavities of same dimensions were determined 15 min after irradiation ( n = 8). Polymerization shrinkage strains were measured using the bonded-disk ( n = 8) and a strain gage method ( n = 8). For determination of contraction stress the composites ( n = 10) were bonded to and cured in Araldit molds using a photoelastic method. Flexural moduli of the restoratives were studied according to ISO specification 4049 ( n = 5). Statistical analysis was performed with one- and two-way ANOVA, Kruskal–Wallis ANOVA test and post hoc tests ( p < 0.05). Results Only two nanofiller composites (Kalore, GC, Japan) and Venus Diamond (Heraeus Kulzer, Germany) showed consistently gap-free margins in bonded dentin cavities. The mean gap widths in non-bonded and in Teflon cavities were 6.1–12.8 and 14.1–25.5 μm, and linearly correlated ( r 2 < 0.85). Significant linear relationships were observed between strain, stress and marginal gap widths in non-bonded and Teflon cavities ( p < 0.01). Flexural moduli (15 min) were between 1.66 and 8.63 GPa. Significance Marginal cavity adaptation of restorations in bonded dentin cavities reflects complex interactions between adhesive bonding on the one hand, and polymerization contraction strain, stress and elastic modulus, on the other.

Effect of resin-composite filler particle size and shape on shrinkage–strain

Objectives The aim of this study was to investigate the effect of variations in filler particle size and shape on the polymerization shrinkage–strain kinetics of resin-composites. Methods A model series of 12 VLC resin-composites were studied. The particulate dispersed phase volume fraction was 56.7%: these filler particles were systematically graded in size, and further were either spherical or irregular. The bonded disk method was used to determine shrinkage–strain kinetics. Displacement was recorded following 40 s irradiation (600 mW/cm 2) at 23 °C ( n = 3). All data were captured for 60 min and the final shrinkage–strain calculated. Results For materials with spherical filler, shrinkage–strain was 2.66% (SD 0.18) for those with irregular filler it was 2.89% (SD 0.11). These differences were statistically significant ( p < 0.001): the Scheffé test identified two subsets, those with irregular filler (including materials with a multimodal mix) and those with spherical filler (including materials with a multimodal mix). Additionally, there was a trend for higher shrinkage–strain values with decreasing filler particle size which was apparent for both those composites with spherical filler particles and those with irregular filler particles. For irregular filler particles, linear regression gave a high correlation ( r 2 = 0.99). Significance Statistically significant differences were identified in the shrinkage behavior of resin-composites with differing filler size and shape.

Setting characteristics and cavity adaptation of low-shrinking resin composites

Objectives The aim of this study was to evaluate the setting characteristics of low-shrinking resin composites and examine the possible interactions with curing efficiency and marginal adaptation in dentin cavities. Methods The materials tested were Ceram X Mono/CM, Premise/PR, Clearfil Majesty/CM, ELS/EL, and Filtek Silorane/FS. Polymerization shrinkage strain (% S), strain rate (% S r) and time at maximum strain rate ( t max) were measured using the bonded disk method. Curing efficiency was measured on the top and bottom surfaces of composites with ATR-FTIR spectroscopy. Marginal adaptation was measured in unbonded (%VVF) and bonded (%XVF) specimens by computerized X-ray microtomography (micro-XCT). The % linear length of the interfacial gaps along the cavity margins (%LD) and the maximum gap width (WD max) were calculated under optical microscopy on sectioned specimens. Statistical analysis was performed with one- and two-way ANOVA, Bonferroni's post hoc test and Pearson's correlation coefficient. Results The % S values ranged from 1.34% (FS) to 2.29% (CX), while % S r ranged from 0.06%s −1 (FS) to 0.15%s −1 (CX). %VVF values extended from 1.9% (FS) to 5.3% (CX) and for %XVF from 1.98% (FS) to 3.35% (CX). The values for %LD ranged from 36.52% (FS) to 81.28% (CX). Linear regression showed strong positive correlation for % S r and t max with %VVF ( r 2 = 0.884 and r 2 = 0.927) and also for % S r and t max with %LD ( r 2 = 0.823 and r 2 = 0.869). Significance % S r and t max are more representative than % S in determining the setting pattern of the materials and are strongly correlated to marginal adaptation. The silorane material showed better behavior than the dimethacrylate materials in setting shrinkage and marginal adaptation.

Polymerization shrinkage kinetics of dimethacrylate resin-cements

Objectives To determine polymerization shrinkage-strain ( S Y) and shrinkage-stress ( S Z) of six resin-cements and to compare their performance with the aid of degree of conversion (DC) data. Methods Variolink 2 (VL2), Multilink Automix (MA), Multilink Sprint (MS, all Ivoclar-Vivadent), Nexus 2 (NX2), Maxcem (MX, both Kerr) and RelyX Unicem (RX, 3M-Espe) were investigated. MS, MX and RX were self-adhesive; others require a bonding-agent. All measurements were conducted at 23 °C for 60 min ( n = 5), except 80 min for RX, with materials self-cured only (sc) and dual-cured (dc); NX2 and VL2 were additionally light-cured only (lc). S Y was measured by the bonded-disk method [Watts DC, Cash AJ. Determination of polymerization shrinkage kinetics in visible-light-cured materials: methods development. Dent Mater 1991;7(4):281–7; Watts DC, Marouf AS. Optimal specimen geometry in bonded-disk shrinkage-strain measurements on light-cured biomaterials. Dent Mater 2000;16(6):447–51]; S Z by the Bioman instrument [Watts DC, Satterthwaite JD. Axial shrinkage-stress depends upon both C-factor and composite mass. Dent Mater 2008;24(1):1–8 [Epub October 24, 2007]; Watts DC, Marouf AS, Al-Hindi AM. Photo-polymerization shrinkage-stress kinetics in resin-composites: methods development. Dent Mater 2003;19(1):1–11]. Light-cure was achieved by QTH at 500 mW/cm 2. The respective DCs were measured under the same conditions by FTIR–ATR spectroscopy. Data were analyzed by One-Way ANOVA plus Bonferroni test, and by t-test, at p < 0.05. Results DC by self-curing was less than the DC by dual-curing, for all cements. Shrinkage-strain ranged from 1.77 to 5.29% and shrinkage-stress from 3.36 to 10.37 MPa. NX2 and VL2 were not significantly different, when light-cured only. Except for RX, sc and dc shrinkage-strain varied maximally by 0.4%. MX showed the highest S Y, RX the lowest. When sc, RX initially expanded by <0.5% ( t ∼ 5 min). For most materials, S Y correlated with their filler loading. The highest stress with sc was exerted by MX, and when dc by MS, which was not statistically different from MX. Significance Shrinkage data of resin-cements are of intrinsic clinical importance. Self-cure, despite a lower DC, did not necessarily result in a lower S Y compared to dual-cure. S Y-rate and S Z development depend upon cure mode and S Y upon filler fraction.

Polymerization shrinkage strain of interocclusal recording materials

Objectives The aim of this in vitro study was to investigate the polymerization shrinkage behavior and to measure the polymerization shrinkage-strain of interocclusal recording materials. Methods The materials investigated in this study were five polyvinylsiloxane (Imprint™ Bite, Silagum Automix Bite, O-Bite, Blu-Mousse ® Classic and Exabite II), one polyether (Ramitec) and one dimethacrylatebased (Luxabite) materials. The polymerization shrinkage values of ten specimens for each material were measured by the Bonded-disk method at 1, 3, 5, 7 and 10 min after mixing at 37 °C. The amount of shrinkage-strain (%) was derived and all data were statistically analyzed by one-way ANOVA and the multiple comparison Scheffé test ( α = 0.05). Results The representative shrinkage-strain kinetic graphs showed that all specimens shrank immediately, except Luxabite which expanded for the initial few seconds. After that, the shrinkage-strain values increased in the magnitude up to 10 min, but its rate decreased gradually with time. The shrinkage-strain values (0.18 ± 0.03–0.16 ± 0.03%) of O-Bite at 5, 7 and 10 min were significantly lower than the other materials, but Luxabite exhibited the highest values (3.10 ± 0.17–3.30 ± 0.16%). Significance The interocclusal recording materials investigated presented significantly different polymerization shrinkage-strain kinetics and showed dimensional changes even after the setting time indicated by respective manufacturers.

A model for shrinkage strain in photo polymerization of dental composites

Objectives We formulate a new model for the shrinkage strain developed during photo polymerization in dental composites. The model is based on the diffusion type fractional order equation, since it has been proved that polymerization reaction is diffusion controlled (Atai M, Watts DC. A new kinetic model for the photo polymerization shrinkage-strain of dental composites and resin-monomers. Dent Mater 2006;22:785–91). Our model strongly confirms the observation by Atai and Watts (see reference details above) and their experimental results. The shrinkage strain is modeled by a nonlinear differential equation in (see reference details above) and that equation must be solved numerically. In our approach, we use the linear fractional order differential equation to describe the strain rate due to photo polymerization. This equation is solved exactly. Results As shrinkage is a consequence of the polymerization reaction and polymerization reaction is diffusion controlled, we postulate that shrinkage strain rate is described by a diffusion type equation. We find explicit form of solution to this equation and determine the strain in the resin monomers. Also by using equations of linear viscoelasticity, we determine stresses in the polymer due to the shrinkage. The time evolution of stresses implies that the maximal stresses are developed at the very beginning of the polymerization process. Significance The stress in a dental composite that is light treated has the largest value short time after the treatment starts. The strain settles at the constant value in the time of about 100 s (for the cases treated in Atai and Watts). From the model developed here, the shrinkage strain of dental composites and resin monomers is analytically determined. The maximal value of stresses is important, since this value must be smaller than the adhesive bond strength at cavo-restoration interface. The maximum stress determined here depends on the diffusivity coefficient. Since diffusivity coefficient increases as polymerization proceeds, it follows that the periods of light treatments should be shorter at the beginning of the treatment and longer at the end of the treatment, with dark interval between the initial low intensity and following high intensity curing. This is because at the end of polymerization the stress relaxation cannot take place.

Effect of nanofiller fractions and temperature on polymerization shrinkage on glass fiber reinforced filling material

Objectives The aim was to evaluate the effect of different nanofiller fractions and temperature on polymerization shrinkage strain and degree of monomer conversion of short glass fibers reinforced semi-interpenetrating polymer network (semi-IPN)-polymer matrix composite resin. Methods Experimental composite resin was prepared by mixing 22.5 wt% of short E-glass fibers (3 mm in length) to the 22.5 wt% of resin matrix with various weight fractions of nanofillers (0, 10, 20, 30, 40, 50 wt%) and then 55 wt% of silane treated silica filler were added gradually using high speed mixing machine. Another study group contained composite resin prepared by mixing 22.5 wt% of resin matrix (without nanofillers) to 77.5 wt% of filler particles (without fiber fillers). As control material, commercial particulate filler composite resin was used. The shrinkage strain of the specimens was measured using the bonded-disk technique at 26 and 37 °C with respect to time. Degree of conversion of the experimental composites containing different nanofiller fractions was measured using FTIR spectroscopy. Results ANOVA revealed that fraction of nanofillers and polymerization temperature had significant effect ( p < 0.05) on the shrinkage strain and degree of conversion of the composite resin. Shrinkage strain correlated with nanofiller fraction and polymerization temperature ( r 2 = 0.96 and 0.95). Significance The use of high nanofiller fraction with short fiber fillers and IPN-polymer matrix yielded improved rate of shrinkage strain.

Polymerization shrinkage of experimental short glass fiber-reinforced composite with semi-inter penetrating polymer network matrix

Objectives The aim of this study was to determine the magnitude of short fiber-reinforced composite resin, with a semi-IPN-polymer matrix, on polymerization resin shrinkage-strain, shrinkage stress and marginal microleakage of the restoration. Materials and methods Experimental composite FC resin was prepared by mixing 22.5 wt.% of short E-glass fibers, 22.5 wt.% of IPN-resin and 55 wt.% of silane treated silica fillers using a high speed mixing machine. As control material, commercial particulate filler composite resin (PFC) was used. Polymerization shrinkage-strain and stress of the specimens ( n = 5) were measured using the bonded-disc technique and tensilometer, respectively with respect to time. FC composite and PFC were placed incrementally in class II cavities sized 4 mm × 4 mm × 6 mm ( n = 8/group) using total-etch adhesive system according to manufacturer's instructions. After the class II restorations were completed, the specimens were finished and polished, thermocycled, stained, sectioned, and viewed under a stereo-microscope for leakage at occlusal/enamel and gingival/dentin margins. The data were analyzed using ANOVA. Results ANOVA revealed that restorations made from experimental FC composite had a significantly lower shrinkage stress and microleakage than those made from PFC ( p < 0.05). The data show that gingival margins had higher microleakage than that obtained from occlusal margins of restorations ( p < 0.05). Conclusions The use of short fiber filler with semi-IPN polymer matrix reduced polymerization shrinkage stress and microleakage compared to a conventional restorative composite.

The effect of fiber orientation on the polymerization shrinkage strain of fiber-reinforced composites

The aim of this study was to characterize the linear polymerization shrinkage strain of glass fiber-reinforced composite (FRC) according to the fiber orientation. Test specimens (n=5) (10.0 x 10.0 x 1.5mm) were prepared from different brands of photopolymerizable resin-preimpregnated FRC; unidirectional continuous FRC, experimental random-oriented FRC, and bidirectional continuous FRC. As control materials, particulate filler composite resin and unfilled dimethacrylate monomer resin were used. Two uniaxial strain gages (gage length 2mm) were used to measure shrinkage strains in two directions: longitudinally and transversally to the fiber direction. The uncured composite or resin was placed on top of the strain gages, covered with a separating sheet and a glass plate, and irradiated for 40s with a light-curing unit. The shrinkage strain was monitored for 300 s. ANOVA and Tukey's posthoc test were used at a significance level of 0.05. ANOVA revealed that orientation of fiber and brand of material had a significant effect (P<0.05) on shrinkage strain. The unidirectional FRC revealed no shrinkage longitudinally to the fiber direction, whereas the shrinkage occurred transversally to the fiber direction. Particulate filler composite resin and unfilled resin revealed equal shrinkage strain in both of the measured directions. Anisotropic nature of FRC exists with regard to polymerization shrinkage strain. The variation of polymerization shrinkage strains of FRC compared to those of particulate filler composites and unfilled resin might be important for future clinical applications.

Low shrinkage light curable nanocomposite for dental restorative material

The aim of this study was to develop a low shrinkage visible light curable nanocomposite dental restorative material without sacrificing the other properties of conventional materials. This nanocomposite was developed by using an epoxy resin 3,4-epoxycyclohexylmethyl-(3,4-epoxy)cyclohexane carboxylate (ERL4221) matrix with 55% wt of 70-100 nm nanosilica fillers through ring-opening polymerization. GPS (gamma-glycidoxypropyl trimethoxysilane) was used to modify the surfaces of silica nanoparticles. The nanocomposite was shown to exhibit low polymerization shrinkage strain, which is only a quarter of currently used methacrylate-based composites. It also exhibited a low thermal expansion coefficient of 49.8 microm/m degrees C which is comparable to that of the methacrylate based composites (51.2 microm/m degrees C). The strong interfacial interactions between the resin and fillers at nanoscales were demonstrated by an observed high strength and high thermal stability of the nanocomposite. A microhardness of 62 KHN and a tensile strength of 47 MPa were reached. A high degree of conversion ( approximately 70%) can be obtained after less than 60 s of irradiation upon the nanocomposite. A transmission electron microscope (TEM) study of the nanocomposite showed no aggregation of fillers. Comparable results to the methacrylate based composites were obtained from the one day MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide) cytotoxicity test. The developed epoxy resin based nanocomposite demonstrated low shrinkage and high strength and is suitable for dental restorative material applications.

Low‐shrinkage dental restorative composite: Modeling viscoelastic behavior during setting

Much attention has been directed toward developing dental direct restorative composites that generate less shrinkage stress during setting. The aim of this study was to explore the viscoelastic behavior of a new class of low-shrinkage dental restorative composite during setting. The setting behavior of an experimental oxirane composite has been investigated by analyzing stress-strain data with two-parametric mechanical models. Experimental data were obtained from a dynamic test method, in which the setting light-activated composite was continuously subjected to sinusoidal strain cycles. The material parameters and the model's predictive capacity were analyzed with validated modeling procedures. The light-activated oxirane composite exhibited shrinkage delay and low polymerization shrinkage strain and stresses when compared with conventional light-activated composite. Noise in the stress data restricted the predictive ability of the Maxwell model to the elastic modulus development of the composite only. Evaluation tests of their potential as restorative material are required, to examine if the biocompatibility and mechanical properties after setting of oxirane composites are acceptable for dental use.

Critical configuration analysis of four methods for measuring polymerization shrinkage strain of composites

Objectives. To compare four methods for measuring polymerization shrinkage strain of composites and to develop a rational basis for comparing data from different methods and laboratories. Methods. Dilatometry, modified bonded disk, strain gage, and a new linear transducer method were used to measure polymerization shrinkage strain of a model composite under similar irradiation conditions. The resin consisted of an untinted resin (50:50 BISGMA/TEGDMA, 0.7% CQ, 0.35% DMAEM, 0.05% BHT) filled with 5% fumed silica and 67 wt% untreated hybrid filler. Specimens ( n=10) were exposed for 60 s at 600 mW/cm 2 and then monitored for 300 s. Specimen volumes were 8 mm 3 for the strain gage method, 25 mm 3 for the linear transducer and dilatometer methods and 43 mm 3 for the bonded disk method. The degree of constraint applied to the specimens by each method was calculated and compared. Shrinkage strain values at 60 and 300 s were tested for significance at p=0.05 using one-way ANOVA and Tukey's test. Results. Shrinkage strain magnitudes at 60 and 300 s for the four methods were significantly different ( p<0.01). The modified bonded disk method measured the highest shrinkage value and exhibited the highest degree of specimen constraint. There was a 5 s delay after light activation before strain was detected by the strain gage. Significance. Specimen constraint differed in all four methods and was linearly correlated with shrinkage strain magnitude when the degree of constraint was less than 42%.

Prediction of composite elastic modulus and polymerization shrinkage by computational micromechanics

Objectives. The objective of this study was to simulate the elastic modulus and polymerization shrinkage of a light activated polymer matrix composite using a generalized method of cells (GMC) micromechanics model. Two hypotheses were tested: (1) the micromechanics model provides estimates of elastic modulus vs filler fraction with greater accuracy than the rule of mixtures, Hashin-Shtrikman and phenomenological models; (2) Micromechanics Analysis Code/Generalized Method of Cells accurately simulates experimental benchmarks of polymerization shrinkage strain. Methods. The study applied mathematical algorithms to a representative volume element of a model polymer composite to yield value estimates of the elastic modulus and contraction strain. Mechanical properties of the composite constituents were derived from thermomechanical and dynamic mechanical analysis of BisGMA and TEGDMA filled and unfilled resins. Data from the micromechanics model were compared to results of other analytical methods as well as experimental benchmarks. Results. Predictions of elastic modulus vs filler fraction from the micromechanics model provided greater accuracy than the rule of mixtures and the Hashin-Shtrikman models. Predictions of polymerization shrinkage strain were within 13% of experimental values. Significance. The elastic micromechanics model presented accurately predicted elastic modulus and polymerization shrinkage strain as a function of filler fraction, superior to other analytical methods.

Polymerization shrinkage-strain kinetics of temporary crown and bridge materials

Objective. The purpose of this study was to measure the polymerization shrinkage kinetics of four commercially available polymer-based temporary crown and bridge materials, including the effect of ambient temperature. Methods. Three dimethacrylate-based materials and one monomethacrylate-based material were investigated. The polymerization shrinkage-strains were measured by using the Bonded-disk method with initial specimen temperature at both 23 and 37 °C, with values particularly noted at 5, 10, and 120 min after mixing. Five recordings were taken for each material. The progress of the setting reaction and its temperature-dependence were evaluated by the kinetic curves, and net shrinkage and total shrinkage (inclusive of expansion magnitude) of each material were compared by independent sample t-test and one-way ANOVA. Results. Most shrinkage occurred in the first 10 min after mixing although there was an early expansion especially with the monomethacylate in the first 5 min. At 120 min, the net shrinkage-strain at 23 and 37 °C of the materials used in this test ranged from 3.54 to 4.13%. The fastest setting dimethacrylate-based material and the monomethacrylate-based material showed higher shrinkage-strain than other materials. No significant differences of net shrinkage-strain were found between 23 and 37 °C, but higher shrinkage rates were measured at 37 °C than at 23 °C. Significance. The Bonded-disk method is a suitable method for measuring temperature-dependence of shrinkage-strain of polymer-based temporary materials. The dimethacrylate-based materials are preferable to monomethacrylates for temporary restoration as judged by the magnitude of polymerization shrinkage-strain, the majority of which is apparent within 10 min from the start of mixing and may affect the clinical outcome.

In vitro characterization of two laboratory-processed resin composites

Purpose. To compare various characteristics of two new-generation laboratory-processed resin composites ( BelleGlass HP/SDS-Kerr and Sinfony/3M-ESPE). The properties evaluated were degree of CC conversion, microhardness, roughness, biaxial flexural strength and polymerization shrinkage-strain. Materials and methods. All specimens were subjected to a first and a second polymerization cycle according to the manufacturers' instructions. The degree of CC conversion (DC) was recorded on rectangular (3×2×0.5 mm 3) specimens ( n=3) by FT-IR micromultiple internal reflectance spectroscopy immediately after each of the two polymerization cycles. Twenty cylindrical specimens (10×2 mm 2) of each material were prepared for surface microhardness ( n=10, VHN, 200 g load, 20 s) and surface roughness ( n=10, Ra) measurements. The biaxial flexural strength and stiffness were determined on disk-shaped ( n=8, 15×0.7 mm 2) specimens loaded to fracture at 1 mm/min crosshead speed. The polymerization shrinkage-strain was calculated with the bonded-disk method. All values were statistically analyzed by Student's unpaired t-test ( p<0.05). Results. The second polymerization cycle significantly increased the degree of CC conversion for both materials ( p<0.05). BelleGlass HP exhibited significantly higher degree of CC conversion, surface microhardness, surface roughness, biaxial flexural strength and stiffness values compared to Sinfony ( p<0.05). Significance. Several differences exist between the materials although both products are recommended for the same clinical applications.

Polymerization shrinkage-strain and microleakage in dentin-bordered cavities of chemically and light-cured restorative materials

Objectives: The aim of this study was to evaluate in vitro the relationship between polymerization shrinkage and microleakage in dentin-bordered restorations. Methods: Four light-cured restorative materials in combination with their respective dental bonding agents (DBA) were investigated: Tetric Ceram/Syntac classic (Vivadent), Solitaire/Gluma Solid bond (Heraeus Kulzer), Definite/Etch & Prime 3.0 (Degussa), Solitaire 2/Gluma Solid bond (Heraeus Kulzer). The chemically cured resin Degufill sc microhybrid (Degussa) in combination with ART Bond (Coltène) was also included. Polymerization shrinkage of the restorative materials was measured using three different methods (dilatometer, linometer, buoyancy method) and analyzed with ANOVA. For the determination of microleakage, caries-free human molars were embedded in acrylic resin and subsequently abraded with a wet abrasion machine to produce four level dentin surfaces. One hundred sixty cavities (3 mm diameter/1.5 mm deep) were randomly assigned to four groups of equal size. The groups were restored without (group 1 and 2) and with DBA (group 3 and 4), and either not subjected (group 1 and 3) or subjected (group 2 and 4) to 2000 cycles from 5–55°C. Each group was further divided into five material subgroups of eight cavities each. Microleakage was determined using a dye penetration test assessed at depths of 200, 400 and 600 μm into the fillings. Data were analyzed with the Kruskal–Wallis and the Mann–Whitney test. Results: All three methods of measuring polymerization shrinkage (PS) generated the same, statistically secured ranking for the four light-cured restorative materials: PS Definite<PS Tetric Ceram<PS Solitaire 2<PS Solitaire. In the microleakage study, only a few statistically significant differences were observed. Etch & Prime 3.0/Definite in group 3 and Solid Bond/Solitaire 2 in group 4 tended to exhibit the least microleakage. Correlation coefficients between aggregated shrinkage and microleakage data were 0.3 for group 3 and −0.2 for group 4. Significance: The results do not suggest any correlation between polymerization shrinkage and microleakage in dentin of direct adhesive restorations.

Effect of adhesive layer properties on stress distribution in composite restorations—a 3D finite element analysis

Objectives: Teeth, adhesively restored with resin-based materials, were modeled by 3D-finite elements analysis that showed a premature failure during polymerization shrinkage and occlusal loading. Methods: Simulation of Class II MOD composite restorations with a resin bonding system revealed a complex biomechanical behavior arising from the simultaneous effects of polymerization shrinkage, composite stiffness and adhesive interface strain. Due to a polymerization contraction, shrinkage stress increases with the rigidity of the composites utilised in the restoration, while the cusp movements under occlusal loading are inversely proportional to the rigidity of the composites. The adhesive layer's strain also plays a relevant role in the attenuation of the polymerization and occlusal loading stresses. Results: The choice of an appropriately compliant adhesive layer, able to partially absorb the composite deformation, limits the intensity of the stress transmitted to the remaining natural tooth tissues. For adhesives and composites of different rigidities, FEM analysis allows the determination of the optimal adhesive layer thickness leading to maximum stress release while preserving the interface integrity. Application of a thin layer of a more flexible adhesive (lower elastic modulus) leads to the same stress relief as thick layers of less flexible adhesive (higher elastic modulus).