Correlation between the amount of linear polymerization shrinkage and cuspal deflection

This study evaluated the relationship between the amount of cuspal deflection and linear polymerization shrinkage in resin composites and polyacid modified resin composites (compomers). Materials included were Dyract AP, Compoglass F, Z100, SureFil, Pyramid, Synergy Compact, Heliomolar and Heliomolar HB. To measure polymerization shrinkage, a custom-made linometer (R&B, Daejon) was used. Ten measurements were made for each group, and the amount of linear shrinkage that occurred in 60 seconds was statistically compared by one-way ANOVA analysis and Tukey's test. To measure the cuspal deflection of teeth, standardized MOD cavities were prepared in extracted maxillary premolars. After a self-etching adhesive was applied, the cavities were bulk filled with one of the filling materials. Fifteen teeth were used for each material. Cuspal deflection was measured by a custom-made cuspal-deflection measuring device. One-way ANOVA analysis and Tukey's test were used to determine differences between the materials. The correlation of polymerization shrinkage vs cuspal deflection was analyzed by regression analysis. The amount of polymerization shrinkage from least to greatest was Heliomolar, SureFil < Heliomolar HB < Z100, Synergy Compact < Dyract AP < Pyramid, Compoglass F (p < 0.05). The amount of cuspal deflection from least to greatest was Z100, Heliomolar, Heliomolar HB, Synergy Compact, SureFil, < Compoglass F < Pyramid, Dyract AP (p < 0.05). Both the amount of polymerization shrinkage and cuspal deflection were highly correlated (p < 0.001).     

Dentinal fluid flow and cuspal displacement in response to resin composite restorative procedures.

OBJECTIVES: The aim of this study was to correlate the rate and magnitude of dentinal fluid flow (DFF) with cuspal displacement in response to resin composite restorative procedures. METHODS: Ten extracted human maxillary premolar teeth with an extensive MOD cavity preparation were connected to an automated fluid flow measurement apparatus (Flodec), and a direct current differencial transformer (DCDT) was attached to each cusp. The rate, magnitude and direction of DFF and cuspal displacement were recorded simultaneously in response to each stage of resin composite restorative procedures. RESULTS: Cuspal displacement and DFF in outward and inward directions accompanied each stage of the procedures. Drying produced rapid cuspal contraction. Bonding caused slight cuspal expansion, whereas light curing of resin induced gradual but extensive cuspal contraction, which persisted following light curing. During baseline outward DFF was low and increased slightly during etching. In contrast, substantial outward DFF occurred during drying. Light curing of the bonding agent and the resin composite produced inward DFF. Following light curing, an outward DFF began and continued for at least 15 min. SIGNIFICANCE: The large, rapid fluid movement and cuspal displacement during restoration, and the prolonged outward fluid flow post-curing have implications for post-operative sensitivity. While mechanical stresses within dentin associated with cuspal displacement appear capable of inducing DFF, the net fluid movement is the result of complex interactions either directly or indirectly of several stimuli (thermal, evaporation, osmotic, and possibly mechanical).     

The effect of filling technique on the cuspal strain,polymerization shrinkage stress,enamel crack formation and depth of cure of restored molars

ObjectiveEvaluate the effect of different restorative filling techniques on the residual shrinkage stress (ShrS), cuspal strain (CS), depth of cure (DC), and enamel crack formation (Ec) in molars with MOD restorations.MethodsPost-gel shrinkage, elastic modulus, compressive and diametral tensile strength of the Filtek One Bulk Fill composite were calculated. Sixty molars with MOD preparations were restored using four filling techniques: Bulk; Horizontal; Oblique; Natural enamel and dentin substitution (NEDS) technique. CS was measured using a strain gauge (n = 10). The DC (n = 5) was measured using Knoop hardness. Shrinkage stress/strain was analyzed using 3D finite element analysis. The Ec analysis was carried out by transillumination. Two-way ANOVA with repeated measures and Tukey’s HSD test (α = 0.05) was performed for the CS data. Two-Way ANOVA and Tukey’s HSD test was performed for the DC data (α = 0.05).ResultsCS was higher at the lingual cusp for the horizontal and NEDS technique. No statistical difference was found between the buccal and lingual CS values for the Bulk (p = 0.367) or Oblique techniques (p = 0.192). CS values were lower for the Bulk. More enamel cracks were found for the Bulk. DC was lower at 4 mm regardless the filling technique. The Horizontal showed the highest ShrS values. The Bulk generated the lower ShrS values.SignificanceA Bulk technique caused the lowest shrinkage stress/strain. An Oblique technique yielded the best balance between stress, strain and crack formation. NEDS technique is a good alternative to decrease the number of increments while maintaining the stress levels nearby the Oblique technique.     

Energy dependent polymerization of resin-based composite.

OBJECTIVE: This study explores the relationship between the extent of polymerization and the radiant energy (dose) applied during the photopolymerization of resin-based composites.METHOD: FTIR was used to measure the 5-min and 24-h conversion of four resin-based composites prepared in a thin film and polymerized under conditions of decreasing intensity and a constant exposure time (30s) using a tungsten halogen curing light. The measured conversion was obtained over a wide range of applied radiant energy. Additionally, samples for two of the materials were polymerized at various intensities and exposure times such that the dose remained constant. This process was performed at four dose levels representing approximately 75% of the conversion range.RESULTS: The curing profiles (percent conversion versus applied radiant energy) depict a gradual decrease in conversion with decreasing energy followed by a rapid descent. Though there are differences in the maximum conversion attained between the materials, when conversion is represented as a fractional conversion relative to the maximum 24-h value, their 5-min and 24-h curing profiles appear quite similar. Additionally, very similar conversion was measured when the films were exposed using equivalent doses providing evidence for a reciprocal relationship between irradiance (power density) and exposure time. For the 24-h measurements, statistical equivalence (Fishers protected LSD at the 0.05 level) was noted for most of the combinations of exposure time and power density within a given dose. Generally, the exceptions occurred with the shortest exposure times.SIGNIFICANCE: A reciprocal relationship between exposure time and power density adds significance to the study of conversion as a function of the total applied dose. This relationship establishes the curing profile as a universal correlation between exposure time and power density.     

Effectiveness of polymerization of a prosthetic composite using three polymerization systems.

STATEMENT OF PROBLEM: Although properties of laboratory-polymerized composite materials are influenced by the type of polymerizing unit, little information is available regarding the comparison between use of a high-intensity light source and application of secondary heat treatment. PURPOSE: This study examined properties of a prosthetic veneering composite polymerized with 3 polymerizing systems to evaluate the effects of varying polymerization modes on hardness, solubility, and depth of cure. MATERIAL AND METHODS: A composite material designed for a prosthetic veneer (Conquest Crown and Bridge) was polymerized using 3 methods: (1) exposure in the proprietary photopolymerizing unit with 2 halogen lamps (Cure-Lite Plus), followed by heating in an oven (Conquest Automatic Curing Unit); (2) exposure in a photopolymerizing unit with a xenon stroboscopic light source (Dentacolor XS); and (3) exposure in a photopolymerizing unit with 2 metal halide lamps (Hyper LII). Knoop hardness, water solubility, and depth of cure were determined for groups of 5 specimens, according to standardized testing methods. Data were compared using analysis of variance and the Duncan new multiple range test (P <.05). RESULT: The hardness number generated with the metal halide unit was statistically greater than those produced by the other 2 methods, and material component released into water was minimal when the material was exposed with the metal halide unit (P <.05). Among the 3 photopolymerizing units, the metal halide unit consistently exhibited the greatest depth of cure. CONCLUSION: Certain properties generated with the use of the high-intensity polymerizing unit exceeded those obtained from a proprietary system that requires a postheat treatment.     

A mathematical analysis of shrinkage stress development in dental composite restorations during resin polymerization.

OBJECTIVE: To derive an analytical solution of shrinkage stresses in a simplified Class-I composite restoration using a visco-elastic material model. METHODS: Simplified, multi-layer, circular plane models were used to represent different sections of a tooth with a Class-I restoration: one section is close to the top occlusal surface and the other is at a deeper location of the restoration. The sections are therefore subjected to different stress states, i.e., plane-stress and plane-strain, respectively. The analytical solution obtained was compared with the numerical results from finite element analysis. A sensitivity study was then carried out to examine the relative influence of geometric and material parameters on the shrinkage stress development. RESULTS: The analytical solution for the shrinkage stress agrees reasonably well with the numerical results given by finite element analysis of more realistic geometries. The result shows that the residual stresses deep inside the restoration are much higher than those at the occlusal surface. This is because material at the former location is subjected to a stress state similar to that of equi-triaxial tension, which limits stress relaxation through viscous flow. However, a stress concentration exists at the restoration margin on the occlusal surface. Sensitivity analysis indicates that the most important factor in shrinkage stress development is material shrinkage, and the second most important factor is Young's modulus. Viscosity and polymerization rate only affect the residual stresses at the surface. The size of the restoration had relatively little influence on the residual stress development. On the other hand, increasing the enamel thickness increases the stresses inside the restoration but not those at the occlusal surface. SIGNIFICANCE: A visco-elastic solution for the shrinkage stresses developed in a simplified Class-I restoration during polymerization has been derived. The solution allows the influence of several geometric and material parameters on shrinkage stress development to be examined readily. It also provides a benchmark test for more elaborate numerical schemes before they are used to analyse more complicated cases.     

Effects of composite restorations on resistance to cuspal fracture in posterior teeth

A clinically acceptable MOD cavity preparation was used instead of a jig-mounted slot preparation in this investigation. Each preparation was proportional to the tooth dimension. The design of the testing instrument ensured that forces applied to the specimens were applied to tooth structure and not to the restorative material. Prepared unrestored teeth were weaker than restored teeth. No significant difference was noted in fracture resistance between teeth restored with amalgam and with composite resin. The method used in this study is replicable, allowing the possibility of comparison studies by using identical procedures. Future research will focus on issues of polymerization method and modulus of elasticity and their effects on fracture resistance of remaining tooth structure.     

The influence of a packable resin composite,conventional resin composite and amalgam on molar cuspal stiffness

Packable resin composites may offer improved properties and clinical performance over conventional resin composites or dental amalgam. This in vitro study examined the cuspal stiffness of molars restored with a packable resin composite, a conventional posterior microfilled resin composite and amalgam. Forty-eight intact caries-free human third molars were distributed into four treatment groups (n=12) so that the mean cross-sectional areas of all groups were equal. Standardized MOD cavity preparations were made and specimens restored using one of four restorative materials: (1) a spherical particle amalgam (Tytin); (2) Tytin amalgam with a dentin adhesive liner (OptiBond Solo); (3) a conventional microfilled posterior resin composite (Heliomolar); (4) a packable posterior resin composite (Prodigy Posterior). Cuspal stiffness was measured using a Bionix 200 biomaterials testing machine (MTS). Specimens were loaded vertically to 300 N at a crosshead speed of 1.0 mm/minute. Stiffness was measured at 10 intervals: (1) prior to cavity preparation (intact); (2) following cavity preparation, but before restoration; (3) seven days after restoration; then (4) 1, 2, 3, 4, 5, 6 and 12 months after restoration. All specimens were stored at 37 degrees C in deionized water throughout the study and thermocycled (5 degrees/55 degrees C; 2000 cycles) monthly for 12 months. Repeated Measures ANOVA revealed significant differences among treatment groups over time (p<0.0001). Cavity preparation reduced cuspal stiffness by more than 60%. At 12 months, the cuspal stiffness of restored teeth was, on average, 58% that of intact specimens. Neither the packable nor the conventional resin composite increased cuspal stiffness over that of amalgam.     

Effects of polymerization contraction in composite restorations.

Post-gel polymerization contraction of resin composite induces contraction stresses at the composite-tooth bond and in surrounding tooth structure. Strain gauges have been shown to be an effective method for measuring linear post-gel polymerization contraction of composites. A new model was developed in which the composite sample was bonded to and circumscribed by an acrylic ring. The model simulates a composite restoration surrounded by dentine. A strain gauge measured the deformation of the ring while a second strain gauge simultaneously recorded the dimensional change of the sample. Stresses placed on the acrylic ring as a result of polymerization contraction of the composite were calculated, based on the strains on the ring and the ring's material properties. Four composites (Heliomolar, Vivadent, Tonawanda, NY, USA; Herculite XR, Kerr Manufacturing Co., Romulus, MI, USA; P-50, 3M Co., St Paul, MN, USA; Silux Plus 3M Co.) were evaluated for polymerization contraction strain and stress on the surrounding acrylic ring during polymerization. At the end of the 60 s light application, Heliomolar demonstrated significantly lower post-gel contraction (0.12 per cent, P less than 0.05) when compared to the other materials. When the strain reached an equilibrium at the end of 14 min Heliomolar continued to demonstrate lower post-gel contraction, however this was not statistically significant at P less than 0.05. When the contraction stress on the surrounding acrylic ring was considered, P-50 rapidly developed and produced the largest stress values (1.7 MPa) at the end of the light application while Heliomolar produced the lowest stress values (0.3 MPa). These values, however, were not significantly different when evaluated statistically.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of different composite restoration techniques on cuspal deflection: an in vitro study

Cuspal deflection produced by polymerization shrinkage was measured after using different composite restoration techniques. This study included 30 healthy premolars embedded in acrylic resin connected to a system that simulated intrapulpal pressure. A small ball was attached to each cuspal vertex as a reference point for intercuspal distance measurements. A large mesio-occlusal cavity was cut in each premolar. All premolars were treated with the same adhesive (ScotchBond) and composite (Tetric Ceram). The teeth were randomly distributed among three study groups: Group 1, filled with two horizontal increments; Group 2, filled with two horizontal increments, the first up to half the cavity height and light cured using a transparent plastic cone (Cerana), and the second filling the remainder of the cavity and Group 3, filled in the same way as Group 2, except that the first increment only filled one-third of the cavity height. The intercuspal distance was measured before beginning the restoration and immediately after polymerization of the first and second increments. Under the experimental conditions used, none of the filling techniques utilized avoided the cuspal deflection phenomenon. Polymerization of the final increment, which binds occlusal enamel in the buccal-lingual plane, was the main cause of cuspal deflection and produced a statistically significant reduction in intercuspal distance vs the baseline measurement in the three study groups. The global deflection ranged from 4 microm to 6 microm, depending on the filling technique used, although the differences among techniques did not reach statistical significance (p<0.05).     

Investigation of polymerisation shrinkage strain, associated cuspal movement and microleakage of MOD cavities restored incrementally with resin-based composite using an LED light curing unit

OBJECTIVES: To investigate the polymerisation shrinkage strain, associated cuspal movement, degree of conversion (DC) and cervical gingival microleakage of mesio-occlusal-distal (MOD) cavities restored with four resin-based composite (RBC) filling materials placed incrementally using a light emitting diode (LED) light curing unit (LCU). METHODS: Standardised extensive MOD cavity preparations on extracted teeth were performed on 40 sound upper premolar teeth. Restoration of the teeth involved the placement of RBCs in eight increments with the appropriate bonding system before irradiation using an LED LCU. Buccal and palatal cusp deflections at each stage of polymerisation were recorded using a twin channel deflection measuring gauge. Following restoration, the teeth were thermocycled, immersed in a 0.2% basic fuchsin dye for 24 h, sagittally sectioned and examined for cervical microleakage. The DC was determined using a Fourier transform infra-red (FT-IR) spectrometer. RESULTS: No significantly difference (P=0.677) in cuspal movement was recorded for Z100 (13.1+/-3.2 microm) compared with Filtek Z250 (8.4+/-3.5 microm), P60 (7.3+/-3.8 microm) and Admira (6.7+/-2.7 microm). The LED LCU deflections were compared with a halogen LCU used in a conventional (Fleming GJP, Hall D, Shorthall ACC, Burke FJT. Cuspal movement and microleakage in premolar teeth restored with posterior filling materials of varying reported volumetric shrinkage values. Journal of Dentistry, 2005;33:139-146) and soft-start mode (Fleming GJP, Cara RR, Palin WM, Burke FJT. Cuspal movement and microleakage in premolar teeth restored with posterior filling materials cured using 'soft-start' polymerization. Dental Materials, 2006, , in press) and a significant reduction in cuspal movement was identified for curing type and material type (P<0.001 and P=0.002, respectively). No significant differences were noted between the four RBC materials investigated when the DC or microleakage scores were examined for the LED LCU. SIGNIFICANCE: It would appear that irradiation of RBCs using the LED LCU offered a significant reduction in associated cuspal movement in large MOD cavities. However, the microleakage scores following polymerisation were significantly increased with dye penetration into the pulp chamber from the axial wall evident in teeth restored with the LED LCU.     

Assessment of polymerization contraction stress of three composite resins.

OBJECTIVES: The purpose of this study was to measure the development of contraction stress of three composite resin restorative materials during photo-polymerization: a micro-hybrid composite (Filtek Z250, 3M ESPE, St. Paul, MN, USA); a nano-filled composite (Filtek Supreme, 3M ESPE, St. Paul, MN, USA); and a low-shrinkage composite (AElite LS, Bisco Inc., Schaumburg, IL, USA). METHODS: Curing shrinkage stress was measured using a stress-analyzer. Composites were polymerized with a halogen-curing unit (VIP, Bisco Inc., Schaumburg, IL, USA) for 40 s. The contraction force (N) generated during polymerization was continuously recorded for 150 s after photo-initiation. Contraction stress (MPa) was calculated at 20, 40, 60 and 150 s. Data were statistically analyzed. RESULTS: The low-shrinkage composite AElite LS exhibited the lowest stress values compared to other materials (p<0.05). Statistical analysis did not show significant differences between Filtek Z250 and Filtek Supreme. SIGNIFICANCE: The low-shrinkage composite showed lower contraction stress than micro-hybrid and nano-filled composite. Ideally, non-shrinking resins would represent the ultimate solution to overcome polymerization contraction and stress-related problems.     

Curing light performance and polymerization of composite restorative materials.

The majority of modern composite restorative materials require light activation for polymerization. Variables affecting light energy absorption by the composite have been examined for their effect on the polymerization contraction. Since the polymerization contraction is closely associated in a complex way to the degree of cure of the restoration, this parameter served as an empirical indicator for the extent of polymerization. Variables included the composite shade, distance between the light source and composite sample, and light intensity. Three resin composites are evaluated. Post-gel polymerization contraction was evaluated using a strain gauge method. Curing light intensity diminished rapidly for distances greater than 2 mm between the tip of the light guide and material surface. A linear relationship was demonstrated between polymerization contraction and light intensity. The polymerization contraction of a microfilled composite and posterior composite, using a constant curing time and light intensity, decreased linearly with increasing sample thickness. Less than optimal light output of the curing light source can be compensated by increasing application time within reasonable limits.