Multi-response optimisation analysis of material properties in dental restorative composites under the influence of thermal and thermomechanical stimuli - A 3D finite element study.

OBJECTIVES: Restored teeth undergo more damage than intact teeth. Therefore, the scientific investigation of their mechanical and physical behaviour under varying oral conditions is vital. The current study is to numerically investigate the stresses on a class-II mesio-occluso-distal (MOD) restored molar due to thermal and thermomechanical stimuli with varying input properties such as coefficient of thermal expansion and elastic properties. This is performed to optimise the dental restoration material, thereby reducing the stresses and failure of the restoration. METHODS: An upper molar was scanned using µ-CT for segmenting and modelling the enamel and dentine. A class-II MOD cavity was then prepared on the model, after which non-manifold meshing was generated. The coefficient of thermal expansion (CTE) and elastic modulus (E) properties of the restoration were varied from 20 × 10-6 °C-1 to 55 × 10-6 °C-1 and 5 GPa-20 GPa, respectively. After the material properties and boundary conditions were set for the finite element (FE) analysis, the thermal and thermomechanical loading analyses were performed to demonstrate the influence of input parameters on the stress. The maximum values of principal stresses on the restoration-enamel junction and the restoration were evaluated. The results were statistically processed using analysis of variance, response surface methodology (RSM) and optimisation analysis to estimate the most optimum inputs for minimising principal stresses. RESULTS: The study reveals that the location of principal stress occurs at the restoration-enamel junction (REJ) and the restoration changes based on the composite material value of E and CTE due to thermal and thermomechanical stimuli. The REJ showed higher principal stress than restoration during the application of both thermal and thermomechanical stimuli, making it more vulnerable to fracture and failure. Moreover, the study showed non-linear variations in the values and locations of principal stresses due to thermal and thermomechanical stimuli with the change in the property of the restoration composite used. Finally, this study derived an optimised restorative value for CTE and E due to the application of thermal and simultaneous thermal and mechanical stimuli. CONCLUSION: This study highlights the importance of choosing the suitable material properties of the restoration composite by dental clinicians to repair a large class MOD cavity. The findings from this study also suggest that the difference in the values of E and CTE in a dental restoration composite when compared with the enamel causes a lack of uniformity in mechanical and thermal properties, thereby forming stress concentrations at the interfaces. The study establishes two optimised CTE and E values for the MOD restoration composite as 25 × 10-6 °C-1 and 20 GPa and 37 × 10-6 °C-1 and 5 GPa, respectively.

Effect of silane coupling agent and concentration on fracture toughness and water sorption behaviour of fibre-reinforced dental composites.

OBJECTIVES: This paper presents the effect of silane treatment of S-2 Glass fibres on the fracture toughness and water sorption/solubility behaviour of fibre-reinforced flowable dental composites. The effect of epoxy- and methacrylate-based silane coupling agents (SCAs) on the mechanical strength and hydrolytic properties were investigated. The concentration of the selected SCAs on the mechanical and physical properties were investigated. The influence of molecular structure and concentration in the interfacial adhesion at the fibre-matrix interfaces was also studied. METHODS: Short S-2 Glass fibres of 250 µm in length and 5 µm in diameter were etched with acid to remove any impurities and roughen the surface. The acid-etched fibres were silane treated with 3MPS, 3GPS, and 8MOTS at different concentrations by weight (%). The silane-treated fibres were incorporated at 5 % into the dental resin mixture. Untreated fibres were added at 5 % to the dental resin mixture and served as the control group. The physical properties such as water sorption, solubility, and desorption along with mechanical properties such as fracture toughness and total fracture work of the fibre-reinforced dental composites grafted with the above-mentioned SCAs were evaluated. The surface morphology of the fractured surface was studied and analysed. RESULTS: The fracture toughness tests showed that the dental composites grafted with optimum weight per cent (wt. %) concentration of the SCA had a better stress intensity factor (KIC) when compared to the 2.0 wt. % and 3.0 wt. % concentration. The KIC value of dental composites grafted with untreated surface etched glass fibres was less than the KIC values of dental composites grafted with optimum concentrations of 3MPS, 3GPS, and 8MOTS by 81.6 %, 38.6 %, and 110.5 %, respectively. A similar trend was found while investigating the total work of fracture of the dental composites, between optimum concentration, 2.0 wt. % and 3.0 wt. % concentration of respective SCA. The increase in silane concentration also led to an increase in the water sorption/solubility characteristics. The absorption of water was most severe in the fibre-reinforced dental composites without silane treatment (32.9 µg/mm3). The ANOVA results showed that the fibre-reinforced dental composites grafted with 8MOTS at optimum concentration showed an increase in fracture toughness when compared to optimum concentrations of 3GPS and 3MPS by 51.9 % and 15.9 %, respectively. The enhanced mechanical and physical characteristics are due to the increased adhesion between the fibre and silane achieved from the optimum wt. % concentration of 8MOTS. Similarly, dental composites grafted with 8MOTS at optimum concentration showed a decrease in water sorption characteristics when compared to optimum concentrations of 3GPS and 3MPS by 18.2 % and 0.6 %, respectively. The decreased water sorption characteristics at the optimum concentration of 8MOTS could be due to the reduced availability of reactive hydroxyl groups and the hydrophobic characteristics of 8MOTS. SIGNIFICANCE: Silane coupling agents (SCAs) are important components of dental composites. The type and concentration of SCA have a significant effect on material properties. The current study focuses on understanding the effects of different SCAs and wt. % concentrations on the interfacial fracture behaviour and the influence of different SCAs on the water sorption and solubility behaviour of S-2 Glass fibre-reinforced flowable dental composites.

The influence of dental restoration depth, internal cavity angle, and material properties on biomechanical resistance of a treated molar tooth.

OBJECTIVES: To assess the hypotheses that a restored tooth structure for a class II occlusal-distal (OD) cavity can be reinforced by optimizing the cavity geometry and choosing composites with adequate mechanical properties. METHODS: A human maxillary molar tooth was scanned, and segmented. The 2D profiles of dentin and enamel were drawn and imported to ABAQUS software. Eighteen restored tooth models with different cavity occlusal depths (OcDs) and internal cavity angles were developed. A semi-circular stone part was used to apply contact loads to the restored tooth model. After setting up the required interactions and boundary conditions, a written Python code was used to automatically assign a wide range of elastic moduli, from 2 GPa to 26 GPa, to the composite restorations, and assign constant material properties to the enamel and dentine. For simplicity, the behaviour of the mechanical material was postulated homogeneous and elastic, while the FE analyses were linearly carried out in this study. Also, the code enabled the FEA software to conduct the stress analyses, determine maximum principal stresses, and record the obtained results. RESULTS: The internal cavity angle formed between the mesial wall and the pulpal floor of the cavity significantly changed the peak maximum principal stress both in the enamel and restoration. The peak stress concentrations were observed mostly at the enamel-restoration interface, with an almost perpendicular orientation to this interface. Regarding the effect of occlusal cavity depth (OcD), the model with the shallowest cavity (OcD = 1.5 mm) represented greater resistance to applied loads than the model with deeper cavities (OcD = 2.0 mm and OcD 2.5 mm). The composite modulus (CM) in the range of 10-18 GPa reduced the maximum principal stress concentrations in the enamel. The lowest result for maximum principal stress was observed in the model with OcD = 1.5 mm, CM = 10 GPa and internal cavity angles = 100°, which was the strongest model against contact loads. SIGNIFICANCE: Class II OD cavities with optimal geometry have reduced induced stress levels, thus being able to be more mechanically robust against contact load transmitted by a stone. Cavity geometry designs with obtuse (more than 90°) internal cavity angles were significantly efficient in minimizing peak stress concentrations. The results indicated that for the model with obtuse internal cavity angles, choosing a composite with optimised properties can diminish stress, particularly at the tooth-restoration interface. Furthermore, the shallowest the cavity, the sturdier the restoration was, especially when the interface tooth-restoration laid on enamel and not on dentine.

Influence of thermal and thermomechanical stimuli on a molar tooth treated with resin-based restorative dental composites.

OBJECTIVES: In-vivo experimental techniques to understand the biomechanical behavior of a restored tooth, under varying oral conditions, is very limited because of the invasive nature of the study and complex tooth geometry structure. Therefore, 3D-Finite element analyses are used to understand the behavior of a restored tooth under varying oral conditions. In this study, the distribution of maximum principal stress (MaxPS) and the location of MaxPS on a restored tooth using six different commercially available dental resin composites under the influence of thermal and thermomechanical stimuli are performed. METHODS: An intact tooth was scanned using µ-CT and segmented to obtain separate geometric models of the tooth, including enamel and dentine. Then, a class II mesial-occlusal-distal (MOD) cavity was constructed for the tooth model. The restored tooth model was further meshed and imported to the commercial Finite Element (FE) software ANSYS. Thermal hot and cold stimuli at 50 °C and 2 °C, respectively, were applied on the occlusal and lingual surface of the tooth model with the tooth's ambient temperature set at 37 °C. A uniform loading of 400 N was applied on the occlusal surface of the tooth to imitate the masticatory forces during the cyclic thermal stimuli. RESULTS: The results of this study showed that the restorative materials with higher thermal conductivity showed a lower temperature gradient between the restoration and enamel, during the application of thermal stimuli, leading to a higher value of MaxPS on the restoration. Moreover, on applying thermal stimuli, the location of MaxPS at the restoration-enamel junction (REJ) changes based on the value of the coefficient of thermal expansion (CTE). The MaxPS distribution on the application of simultaneous thermal and mechanical stimuli was not only dependent on the elastic modulus of restorative materials but also their thermal properties such as the CTE and thermal conductivity. The weakest part of the restoration was at the REJ, as it experienced the peak stress level during the application of thermomechanical stimuli. SIGNIFICANCE: The findings from this study suggest that restorative materials with lower values of elastic modulus, lower coefficient of thermal expansion and higher values of thermal conductivity result in lower stresses on the restoration. The outcomes from this study also suggest that the thermal and mechanical properties of a restorative material can have a considerable effect on the selection of restorative materials by dental clinicians over conventional restorative materials.

Influence of silane coupling agent on the mechanical performance of flowable fibre-reinforced dental composites.

OBJECTIVES: This experimental investigation explored the optimisation of silane treatment of surface-modified S-2 Glass fibres in restorative dental composites for improved mechanical performance. The influence of optimum amount of silane to improve the interfacial adhesion at the fibre-matrix interfaces and its effect on the mechanical properties of the restorative composites were explored. METHODS: S-2 Glass fibres of 5 µm diameter and 250 µm length were surface modified using the acid etching technique. The etched fibres were then treated with either 3-methacryloxypropyltrimethoxysilane (3-MPS), 3-Glycidoxipropyltrimethoxysilane (3-GPS) or 8-methacryloxyoctyltrimethoxysilane (8-MOTS) at varying molar % / wt% concentrations. Fibres that were not silanised with any silane coupling agents were used as the control sample. The silanol content of each mixed silane was observed using Fourier transform infrared (FT-IR) spectroscopy analysis. Fibres (5 wt%) with optimised molar% / wt% silane coupling concentration were added to UDMA/TEGDMA dental resin. Mechanical properties such as flexural strength, flexural modulus, and the breaking energy of the materials were evaluated using a comprehensive experimental programme. RESULTS: FTIR spectrum of glass fibre silanised with each silane coupling agent revealed many peaks from 3800 to 1400 cm-1, indicative of -CH3, -CH2, and CO bonding, suggesting the proper silanization of the fibre. The contact angle test revealed that optimum wt% concentration of 3-MPS, 3-GPS and 8-MOTS were 0.5%, 0.8% and 1.4% respectively. The flexural strength of the fibre-reinforced with optimum concentration of 3-MPS (DC-3-MPS_0.5%) increased by 7.0% compared to those of the 2 wt% concentration of 3-MPS fibre-reinforced composite (DC-3-MPS_2.0%). While the flexural strength of optimum concentration 8-MOTS grafted dental resin composites (DC-8-MOTS_1.4%) were 9.9% higher than that of 2 wt% concentration 8-MOTS grafted dental resin composite (DC-8-MOTS_2.0%) and the flexural strength of optimum concentration of 3-GPS (DC-3-GPS_0.8%) was 7.5% higher when compared to that of 2 wt% concentration 3-GPS grafted dental resin composites (DC-3-GPS_2.0%). A concurrent trend was found while investigating the fracture behaviour of the dental composite with optimum wt% concentration of each silane coupling agent against its corresponding higher wt% concentrations. The ANOVA results showed that the optimum fibre-reinforced dental composites grafted with 8-MOTS showed better mechanical behaviour when compared to 3-GPS and 3-MPS. SIGNIFICANCE: The interfacial adhesion between the fibre and the resin due to silane coupling agents has helped to improve the mechanical properties of the fibre-reinforced dental composite. This is the first experimental study to provide a thorough investigation into the significance of the optimal use of silane coupling agents to treat the S-2 Glass fibres and subsequently the influence on the mechanical performance of the fibre-reinforced flowable dental composites.

The effect of dental restoration geometry and material properties on biomechanical behaviour of a treated molar tooth: A 3D finite element analysis.

OBJECTIVES: To test the hypothesis that restoration of class II mesio-occlusal-distal (MOD) cavities can be strengthened through judicious choice of restoration geometry and material properties. METHODS: An intact extracted human maxillary molar tooth was digitized, segmented, reconstructed, and four 3D restored tooth models were developed with four different restoration geometries: one straight, one single-curved, and two double-curved. Stress analysis was conducted for representative loading using finite element analysis, and maximum principal stresses were determined at the dentine-enamel and restoration-enamel junctions. A range of restorative material elastic moduli (5-80 GPa) and Poisson's ratios (0.25-0.35) were studied. Vertical loads of 400 N were applied on occlusal points, while the roots of the molar teeth, below the crevices, were supported in all directions. All the materials were modelled as homogeneous, isotropic, and elastic. RESULTS: The maximum principal stresses at the restoration-enamel junctions were strongly dependent on the MOD restoration geometries. Peak stresses occurred along the palatal surface of the restoration rather than the opposite buccal surface. Double-curved restorations showed the lowest peak stress at restoration-enamel junctions. Choice of the mechanical properties of restorative material in the range of 5-35 GPa further reduced stress concentrations on the enamel. SIGNIFICANCE: Class II MOD restorations may be stronger if designed with double-curved marginal geometries that can reduce stress concentrations. Designs with convex and concave geometries were particularly effective because they reduced stress concentrations dramatically. Results suggest that relatively minor changes to the geometry of a restoration can have a substantial effect on stress at the restoration-enamel junction and motivate future experimental analysis.

Evaluation of depth-wise post-gel polymerisation shrinkage behaviour of flowable dental composites.

Short fibre reinforced flowable dental composites are gaining acceptance over particulate filled composites due to their competence to impart improved physio-mechanical properties and capability to prevent crack propagation. However, limited research exists to assess their overall post-gel shrinkage behaviour, which is an important factor to determine marginal seal around restoration and hence its longevity. In this paper, depth-wise post-gel shrinkage strain and the resulting factors such as degree of conversion and rheological behaviour of flowable fibre reinforced composite (FRC) containing 5% weight fraction of 5 µm diameter, 350 µm length S-Glass fibres in UDMA/TEGDMA mixture along with 50% strontium filler particles were investigated. Post-gel shrinkage strain was measured using an array of optical fibre Bragg grating sensors (FBGs) of diameter 250 µm and length 1 mm each embedded at three different depths (depth 0 mm, depth 2.5 mm and depth 5 mm from curing light tip) within the flowable dental composite samples. The rheological behaviour during the polymerisation process was carried out using dynamic oscillatory tests. To evaluate the conversion of CC during polymerisation, degree of conversion tests were conducted by using FTIR spectroscopy. The results obtained for FRC samples were further compared with that of particulate filled composite (PFC) samples, with 55% strontium filler particles only within the same resin system. The relationship between post-gel shrinkage strain at different depths, rheological behaviour and degree of conversion was also explored. The experimental results from the sensor embedded materials suggested that the post-gel shrinkage strain was higher at the top surface (depth 0 mm) and was 50% more than at the bottom surface (depth of 5 mm) for dental FRC as well as PFC samples. Further, similar flow behaviour and not significant different (p<0.05) degree of conversion (DC), post-gel shrinkage strain for dental PFC and FRC composites was observed, establishing a convincing positive relationship between all the key factors and further implying that replacement of fibres with fillers did not affect the overall post-gel polymerisation shrinkage behaviour in dental composites. This investigation has also demonstrated that fibre optic sensors-based shrinkage measurements can be an ideal technique to evaluate post-gel shrinkage performance of dental resins with PFCs or FRCs.

The effect of oxalic acid incorporation on the setting time and strength of a glass-ionomer cement.

Oxalic acid and its metal oxalate salts have been used extensively in dentistry in a range of applications: as desensitisers, in cavity preparation, and as bonding agents. This study investigated the influence of oxalic acid upon the working time, initial setting time, 24-h hardness and compressive strength of a glass-ionomer cement. Conventional glass-ionomer liquids were prepared from polyacrylic acid, tartaric acid, water, and oxalic acid at concentrations of 0-7% w/w. Liquids were dosed into capsules with a commercial glass-ionomer powder, activated and mixed. The resultant pastes were assessed for working time, initial setting time, 24-h hardness and 24-h compressive strength. Liquids containing 0.5-1% oxalic acid lengthened the working time and initial setting time. At concentrations greater than 2%, both working and initial setting times decreased with increasing oxalic acid. Surface hardness values using liquids with 3% and 7% oxalic acid were less hard than the control. Compressive strength was unchanged over the concentrations tested. Oxalic acid may be a useful reaction modifier in glass-ionomer systems. It accelerated the setting reaction without affecting strength, but was limited to low concentrations because of its relatively poor solubility in water.

The effects of boric acid and phosphoric acid on the compressive strength of glass-ionomer cements.

OBJECTIVES: Both boric acid (H3BO3) and phosphoric acid (H3PO4) are components of dental cements, commonly incorporated into glass (as ingredients in the melt) and occasionally added to the powder or liquid components. This study investigated the effect of boric acid addition to an experimental glass-ionomer powder and the effect of phosphoric acid addition to a glass-ionomer liquid on the 24-h compressive strength. METHODS: Boric acid powder was added in various concentrations to an experimental glass-ionomer powder and, separately, phosphoric acid was added to an experimental glass-ionomer liquid. Powders and liquids were dosed into capsules at various powder:liquid ratios and cements thus formed were assessed for 24-h compressive strength. RESULTS: Incorporation of boric acid in glass-ionomer powder resulted in a pronounced decrease (p < 0.05 at 1% boric acid) in compressive strength. Addition of phosphoric acid produced initially stronger cements (up to 13% increase at 1% phosphoric acid) before also declining. SIGNIFICANCE: The incorporation of less than 2% w/w phosphoric acid in glass-ionomer liquids may improve cement strengths without compromising clinical usefulness. The incorporation of boric acid in glass-ionomer cements is contraindicated.

The effect of ytterbium fluoride and barium sulphate nanoparticles on the reactivity and strength of a glass-ionomer cement.

OBJECTIVES: While clinical advantages of glass-ionomers include fluoride release and radiopacity, disadvantages include low strength, slow initial setting times and opacity. The addition of nanoparticles, in particular those containing fluoride and cross-linkable ions, may mitigate the disadvantages while further improving the advantages. This investigation evaluated the effects of the addition of ytterbium fluoride (YbF3) and barium sulphate (BaSO4) on the strength and reactivity of a commercial glass-ionomer cement. METHODS: YbF3 and BaSO4 nanoparticles were incorporated into the powder component of Riva SC (SDI Ltd., Bayswater, Australia) at 1, 2, 5, 10, 15, and 25% by weight. Capsules were assembled at a powder:liquid ratio of 2.9:1, activated and mixed, and the resultant pastes evaluated for working time, initial setting time, 24-h surface hardness and 24-h compressive strength. RESULTS: Working and initial setting times were reduced with the addition of YbF3. Addition of BaSO4 at low concentrations reduced working and initial setting times, but further addition delayed the setting reaction. Compressive strength decreased with the addition of either YbF3 or BaSO4, while surface hardness was slightly but insignificantly higher at 1-2% nanoparticles and then decreased with increasing nanoparticle concentrations. SIGNIFICANCE: Nanoparticles modified the setting characteristics, strength and surface hardness of a commercial glass-ionomer cement, and may be useful for refining the handling characteristics of these materials. Further improvements in powder blending may result in more significant improvements in mechanical properties.

The effect of mixing time on the handling and compressive strength of an encapsulated glass-ionomer cement.

OBJECTIVES: The effects of varying mixing times on the properties of glass-ionomer cements have been poorly investigated, and many clinicians are uninformed about the potential changes in handling achieved by this. This study aims to explore the effects of mixing time variation. METHODS: An experimental glass-ionomer system was dosed into a capsule, activated, and triturated for varying lengths of time, from 2 to 14 s. Measurements were made of working time, initial setting time, compressive strength and compressive modulus, and a subjective assessment of handling conducted. RESULTS: The working time and initial setting time decreased as mixing time increased, while compressive strength and compressive modulus increased to a maximum at 12 s mixing time. The material essentially pre-gelled after 14 s of mixing, which resulted in breaking of the gel matrix and poorer properties. SIGNIFICANCE: Clinicians should feel confident, within limits, of varying mixing times of glass ionomer cements to improve properties or to slow the reaction.

The effect of particle size distribution on an experimental glass-ionomer cement.

OBJECTIVES: The role of particle size and size distribution of glass powders in glass-ionomer cements (GICs) has been largely overlooked, being limited to demonstrations of the classical inverse size-strength relationship. This study investigated variation in properties of an experimental glass-ionomer cement when a combination of large ('Powder A') and small ('Powder B') particles was used. METHODS: Large- (mean size 9.60mum) and small-particle (3.34mum) glass powders were blended in various proportions and mixed with powdered polyacrylic acid to make a range of glass-ionomer powders. These powders were mixed with a glass-ionomer liquid (SDI Ltd, Australia) at powder to liquid ratios of 2:1, 2.5:1, and 3:1, and the resultant cements evaluated for working time, setting time, clinical handling, and compressive strength. Results were analysed by ANOVA. RESULTS: An increased proportion of smaller particles corresponded to higher strengths, and an increased proportion of larger particles with a decrease in viscosity of the unset cement. When 20-30% by weight of small particles was used, the paste demonstrated a peak in cohesion and working time, with a viscosity similar to commercial restorative GICs. SIGNIFICANCE: Optimisation of particle sizing and distribution may thus lead to glass-ionomer cements with improved clinical handling characteristics and greater strength, which may increase the longevity of the restoration.

Ion leaching of a glass-ionomer glass: an empirical model and effects on setting characteristics and strength.

The release of ions from a glass-ionomer glass, which in the polyacid matrix effects the cross-linking and setting of a cement, can be modelled and initiated by acid-treatment in a dilute acid. This study examined the effect of time of acetic acid leaching on the working time, setting time, and strength of a model GIC. A reactive fluoride glass was immersed in hot acetic acid for 0 (control), 5, 15, 35, 65, 95 and 125 min, filtered and dried. The glass was mixed with an experimental GI liquid in a capsule system and the mixed pastes assessed for working and initial setting time. Compressive strength testing was undertaken according to ISO9917:2003. Immersion time had a significant effect on both working and setting time of the resultant pastes only up to 65 min of immersion, and corresponded with a thin-film ion diffusion model. Compressive strength did not vary significantly with immersion time. The glass-ionomer setting reaction can be conveniently retarded by immersion of the powder in acetic acid, without affecting strength. A reactivity model was developed, whereby the effects of various changes to the leaching process may be usefully examined.