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.

Comparative analysis of self-cure and dual cure-dental composites on their physico-mechanical behaviour

BACKGROUND

Clinical practitioners may have become familiar with the rapid transformation of dental composites. However, they may not scientifically understand the factors influencing the mechanical and physical properties. Scientific knowledge of filler-resin interaction can significantly improve clinical understanding of resin composites. Several independent studies have examined the mechanical and physico-mechanical properties of dental resin composites; however, no comprehensive study has examined the influence of fillers and resin materials on the physico-mechanical properties of both self-cure and dual-cure composites.

METHODS

This study performed investigations on the physico-mechanical behaviour of four commercially available dual-cure dental composites (Bioactive, Fill Up!, Surefil One, Cention N) and two commercially available self-cure dental composites (Stela Capsule and Stela Automix). Test specimens for flexural and compressive strength, microhardness, fracture toughness, and hydrolytic behaviour were prepared and tested as per respective standards. The data sets were statistically analysed using one-way ANOVA and Tukey's post-hoc comparison.

RESULTS

There was a substantial variation in flexural strength and modulus values in this study, ranging from 32.0 to 113.4 MPa and 2.36 to 12.07 GPa, respectively. Similarly, there were significant differences in compressive strength between the materials in this study, ranging from 119.3 to 223.5 MPa. The highest fracture toughness value was found to be 1.41 MPa.m0.5 , while the lowest value was 0.43 MPa.m0.5 . Variations in surface microhardness were significant (24.11-68.0 N/mm2 ), which correlated with the filler content. Water sorption and solubility demonstrated high variations among materials, with Surefil One exceeding ISO 4049 thresholds significantly.

CONCLUSIONS

A linear correlation can be established between surface microhardness (HV) and flexural and compressive moduli, as well as filler content (wt.%). However, both flexural and compressive strengths are impacted by the resin's constituent monomers and the resin-filler matrix's cross-linking capability. Additionally, factors such as filler size, shape, and the cross-linking ability of the resin-filler matrix play a crucial role in fracture toughness and the propagation of cracks within the restoration. Also, resin monomers and filler particle size affect the hydrolytic degradation characteristics of composites, which can also affect their mechanical properties. © 2023 Australian Dental Association.

Enhancing the mechanical stability of restored teeth with interfacial cracks: Finite element analysis

OBJECTIVES

This study aims to enhance the mechanical stability of restored molar teeth with class II occlusal-distal (OD) cavities. We seek to achieve this goal through a comprehensive investigation of three primary factors: (1) the choice of restoration material properties, (2) internal cavity geometries, and (3) the impact of double-layered restoration configurations.

METHODS

To achieve our objectives, we initiated by creating two-dimensional (2D) models of restored teeth featuring class II OD cavities, utilizing scanned and segmented images of maxillary molar teeth. We drew 2D profiles of dentine and enamel, which were then imported into finite element analysis (FEA) software. To explore various cavity geometries, we implemented a total of thirteen different designs, encompassing straight, oblique, grooved, curved, and double-layered configurations. We utilized a semi-circular stone to simulate the application of contact load on the restored tooth. We applied identical boundary conditions and contact loading across all models. To assign material properties, we developed a Python code, enabling the automatic assignment of seven elastic moduli ranging from 2 GPa to 26 GPa to the restoration materials. Meanwhile, constant material properties were assigned to the enamel and dentine. In total, we conducted 133 FEA simulations to comprehensively analyse the effects of the aforementioned factors on the strength and performance of restored molar teeth.

RESULTS

Our analysis revealed two key factors significantly influencing the mechanical resistance of treated teeth, particularly in the presence of a crack or debonding: (1) the marginal geometry of the OD cavity and (2) the elastic modulus of the restorative material. However, altering the internal cavity angle and implementing a double-layered restoration did not significantly influence the restored tooth's overall strength and performance in the face of crack or debonding situations.

SIGNIFICANCE

The findings of this study have substantial implications for designing and restoring class II OD cavities to enhance resistance to cracks or debonding. The use of curved marginal geometries in restoration design can significantly improve fracture resistance, with double-curved geometries reducing stress concentrations by approximately 43% compared to straight cavities. These results offer valuable guidance for strengthening the structural integrity of restored teeth, calling for further experimental investigations to explore practical applications and benefits.

Effect of He2+ ion irradiation on the mechanical properties of automated fibre placement (AFP) CF-PEEK thermoplastics composites

Carbon fibre-reinforced polyetheretherketone (CF-PEEK) composites have gained significant usage across diverse industries like automotive and aerospace due to their desirable characteristics. These properties encompass recyclability, low density, high strength, wear resistance and thermal stability. The components made from CF-PEEK composites for space applications will be subjected to a high radiation environment due to the incoming cosmic rays, comprising protons, α particles, electrons, γ rays, etc., once they escape the Earth's atmosphere. The ion irradiation of CF-PEEK is accompanied by radiation-induced effects, which drastically change the structure and properties of irradiated material. Since the resistance of CF-PEEK to radiation damage has not been studied extensively, this study aims to understand the effect of high-energy He2+ ions on the microstructure and properties of CF-PEEK composites manufactured using automated fibre placement (AFP) under different processing conditions. The samples have been radiated with 5 MeV He2+ ions using an energy degrader wheel to create a layer with relatively uniform damage. Then, were characterized using optical and scanning electron microscopy and their hardness was evaluated using nanoindentation. It was observed that, irradiation increases the hardness of the fibres in all cases. Also, fibre orientation affects the hardness in a statistically significant manner in both unirradiated and irradiated conditions.

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 (K_IC) when compared to the 2.0 wt. % and 3.0 wt. % concentration. The K_IC value of dental composites grafted with untreated surface etched glass fibres was less than the K_IC 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.

A biomechanical comparison of posterior fixation approaches in lumbar fusion using computed tomography based lumbosacral spine modelling

Extreme lateral interbody fusion (XLIF) may be performed with a standalone interbody cage, or with the addition of unilateral or bilateral pedicle screws; however, decisions regarding supplemental fixation are predominantly based on clinical indicators. This study examines the impact of posterior supplemental fixation on facet micromotions, cage loads and load-patterns at adjacent levels in a L4-L5 XLIF at early and late fusion stages. CT data from an asymptomatic subject were segmented into anatomical regions and digitally stitched into a surface mesh of the lumbosacral spine (L1-S1). The interbody cage and posterior instrumentation (unilateral and bilateral) were inserted at L4-L5. The volumetric mesh was imported into finite element software for pre-processing, running nonlinear static solves and post-processing. Loads and micromotions at the index-level facets reduced commensurately with the extent of posterior fixation accompanying the XLIF, while load-pattern changes observed at adjacent facets may be anatomically dependent. In flexion at partial fusion, compressive stress on the cage reduced by 54% and 72% in unilateral and bilateral models respectively; in extension the reductions were 58% and 75% compared to standalone XLIF. A similar pattern was observed at full fusion. Unilateral fixation provided similar stability compared to bilateral, however there was a reduction in cage stress-risers with the bilateral instrumentation. No changes were found at adjacent discs. Posterior supplemental fixation alters biomechanics at the index and adjacent levels in a manner that warrants consideration alongside clinical information. Unilateral instrumentation is a more efficient option where the stability requirements and subsidence risk are not excessive.

Design and tribological performance of short S-Glass fibre reinforced biocomposites on the influence of fibre length and concentration

Fibre-reinforced biocomposites usage has gained prominence over the past decade. Although higher fracture toughness was observed when fibres were added to biocomposites, material degradation could occur due to filler and fibre content intolerance in the biocomposite matrix. Optimisation of resin-fibre-filler ratios helps in increasing the tribological performance of high load-bearing applications. However, the tribological performance is less understood due to limited in-vitro studies on the effect of fibre microstructures. A comprehensive investigation of the reciprocating and rotary wear behaviour of different compositions was carried out by varying fibre (0%, 5%, 10% and 15%) to particulate filler (40%, 45%, 50%, and 55%) weight fractions. The investigation aimed to identify the optimal composition of fibre-reinforced biocomposites based on the in-vitro ball-on-disc reciprocating and rotary wear tests in the presence of modified Fusayama solution. The cross-sectional areas of wear tracks were analysed using laser microscopy and scanning electron microscopy techniques to assess the surface morphology and subsurface damage of the wear tracks on biocomposites and the antagonist. The numerical results were statistically analysed using two-way ANOVA followed by a posthoc Tukey's test (p = 0.05). The results showed a combination of adhesive, abrasive and fatigue wear for all the tested Groups. The friction coefficient had a longer transient period for the 5 wt% and 10 wt% Groups. Based on the surface roughness, coefficient of friction, SEMs, specific wear rate, and ease of manufacturing, the threshold limit for fibre loading was found to be 10 wt%. The rotary test had a considerably lower specific wear rate compared to the reciprocating test. Fibre weight fraction was found to be the influencing factor of the abrasive wear behaviour compared to fibre length for the tested Groups.

Measuring compressive loads on a 'smart' lumbar interbody fusion cage: Proof of concept

There are several complications associated with lumbar interbody fusion surgery however, pseudarthrosis (non-union) presents a multifaceted challenge in the postoperative management of the patient. Rates of pseudarthrosis range from 3 to 20 % in patients with healthy bone and 20 to 30 % in patients with osteoporosis. The current methods in post-operative follow-up - radiographs and CT, have high false positive rates and poor agreement between them. The aim of this study was to develop and test a proof-of-concept load-sensing interbody cage that may be used to monitor fusion progression. Piezoresistive pressure sensors were calibrated and embedded within a polyether ether ketone (PEEK) interbody cage. Silicone and poly (methyl methacrylate) (PMMA) were inserted in the graft regions to simulate early and solid fusion. The load-sensing cage was subjected to distributed and eccentric compressive loads up to 900 N between synthetic lumbar vertebral bodies. Under maximum load, the anterior sensors recorded a 56-58 % reduction in pressure in the full fusion state compared to early fusion. Lateral regions measured a 36-37 % stress reduction while the central location reduced by 45 %. The two graft states were distinguishable by sensor-recorded pressure at lower loads. The sensors more effectively detected left and right eccentric loads compared to anterior and posterior. Further, the load-sensing cage was able to detect changes in endplate stiffness. The proof-of-concept 'smart' cage could detect differences in fusion state, endplate stiffness, and loading conditions in this in vitro experimental setup.

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, -CH_2, 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.

Finite element modeling of temporal bone graft changes in XLIF: Quantifying biomechanical effects at adjacent levels

Extreme lateral interbody fusion allows for the insertion of a large-footprint interbody cage while maintaining the presence of natural stabilizing ligaments and the facets. It is unclear how the load-distribution mechanisms through these structures alter with temporal changes in the bone graft. The aim of this study was to examine the effects of temporal bone graft changes on load distribution among the cage, graft, and surrounding spinal structures using finite element analysis. Thoracolumbosacral spine computed tomography data from an asymptomatic male subject were segmented into anatomical regions of interest and digitally stitched to generate a surface mesh of the lumbar spine (L1-S1). The interbody cage was inserted into the L4-L5 region during surface meshing. A volumetric mesh was generated and imported into finite element software for pre-processing, running nonlinear static solves, and post-processing. Temporal stiffening was simulated in the graft region with unbonded (Soft Callus, Temporal Stages 1-3, Solid Graft) and bonded (Partial Fusion, Full Fusion) contact. In flexion and extension, cage stress reduced by 20% from the soft callus to solid graft state. Force on the graft was directly related to its stiffness, and load-share between the cage and graft improved with increasing graft stiffness, regardless of whether contact was fused with the endplates. Fused contact between the cage-graft complex and the adjacent endplates shifted load-distribution pathways from the ligaments and facets to the implant, however, these changes did not extend to adjacent levels. These results suggest that once complete fusion is achieved, the existing load paths are seemingly diminished.

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.

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.

Dimensional stability of short fibre reinforced flowable dental composites

Fibre-reinforced dental composites are proven to have superior mechanical properties in comparison with micro/nano/hybrid filled composites. However, the addition of small quantities of short glass fibres could affect the dimensional stability of the restoration both during initial stages as well as through the life of the restoration. This in-vitro study aims at evaluating the physical properties of short S-Glass reinforced flowable dental composites. Two S-Glass short fibre-particulate reinforced (5 wt% of aspect ratios 50 and 70) and one particulate only reinforced flowable dental composites were prepared with UDMA-TEGDMA based dental monomer systems. Samples were photopolymersied for 60 s and stored in distilled water at 37 °C for 24 h before testing. Depth of cure (through-thickness microhardness), volumetric shrinkage (Archimedes technique), polymerisation stress (cantilever based tensometer), curing exotherm (thermocouple), water sorption and solubility (ISO 4049) and thermal expansion coefficient (dilatometer) were determined. The test results were statistically analysed using one-way ANOVA (p < 0.05). Depth of cure increased by 41%, volumetric shrinkage increased by 8.3%, shrinkage stress increased by 37.6%, exotherm increased by 20.2%, and thermal expansion reduced by 6.4% while water sorption and solubility had a negligible effect with the inclusion of short glass fibres. The study demonstrates that within the same organic resin system and quantity, a small replacement of fillers with short fibres could significantly affect the dimensional stability of the composite system. In conjunction with mechanical properties, this study could help clinicians to gain confidence in fibre reinforced dental composite restorative system.

Evaluation of rheological behaviour of flowable dental composites reinforced with low aspect ratio micro-sized glass fibres

OBJECTIVE

Experimental investigation is carried out to determine the flowability and stickiness of the developed composite material for dental restoration containing low aspect ratio (AR ≤ 100) surface treated micro-sized glass fibres.

METHODS

Specimens are manufactured by mixing low AR (50/70/100) micro-sized glass fibres with two different weight fractions (5%/10%) into UDMA/TEGDMA based resin. Particulate filler composite (PFC) containing 55% glass fillers is used as the control group. Dynamic oscillatory strain sweep tests are conducted to analyse the linear viscoelastic behaviour. Solid-to fluidic transition behaviour of dental composites is also calculated in terms of flow and yield stresses. Furthermore, the oscillatory frequency sweep tests are conducted at three different strains (0.5%, 5% and 50%) resembling the positioning of unset paste onto restorations for different real-life clinical situations. Additionally, stickiness of dental composites with handling instrument (steel) and dentine covered with bonding agent is also evaluated.

RESULTS

The results suggested the all the FRC groups exhibited non-Newtonian, shear-thinning behaviour. It is further established that inclusion of 5% of 50/70AR fibres into dental composites does not affect the flowability. Simultaneously, stickiness with dentine covered with bonding agent is more for these two compositions as compared to that of handling instrument (steel).

SIGNIFICANCE

This study suggest that visco-elastic properties of dental composites are greatly affected by the type of filler (spherical shaped particulate fillers or rod-shaped fibres) as well as fibre weight fraction/fibre AR. This phenomenon can be attributed to the varying interactions between micro-sized fibres of different AR/weight fraction, particulate fillers and monomers.

Acoustic emission and finite element study on the influence of cusp angles on zirconia dental crowns

OBJECTIVE

The effect of cusp angle on the load-carrying capacity and failure behaviour of BionZ Diamond zirconia crowns is carried out using experimental and numerical investigations.

METHODS

The experimental program using monolithic crowns were divided into three groups (n = 14) for three cusp angles, 60, 80 and 120 degrees and were tested to failure under the static load. A 2-channel acoustic emission (AE) system was used to monitor the failure process while the piezo sensors were attached to the rigid stainless-steel jig for recoding the AE events. Load-displacement and AE response were simultaneously monitored until failure of specimens. Parametric AE analysis was conducted for the factors such as amplitude, energy released, signal duration and cumulative counts, for each AE signal. Fast Fourier transform (FFT) was conducted to assess the frequency at failure. Linear finite element analysis (FEA) was carried out using commercial software Ansys Workbench 19.1 to present the stress distribution and failure modes. Post-failure surface morphology study was carried out using scanning electron microscopy (SEM) and statistical analysis was performed using Weibull analysis.

RESULTS

All the samples in three different groups have failed at the mid-line, splitting the zirconia crowns into two equal pieces. The load to failure was directly proportional to the cusp angle in crowns; 120° group had the highest load-carrying capacity of 2.93 ± 0.26 kN while 60 and 80° groups had a failure load of 2.46 ± 0.53 and 2.52 ± 0.16 kN, respectively. Parametric AE analysis revealed that the failure was instantaneous and 60-degree samples had higher AE signature. FE analysis showed the crack initiation at the occlusal surface of the crown which is in agreement with the SEM images. A close agreement of results for the load and stress distribution from FEA complemented with the experimental study.

SIGNIFICANCE

Optimisation of cusp-angle could help clinicians to accurately design the monolithic zirconia crown focussing on maximum load-carrying capacity, increasing the restoration life.

Smart orthopaedic implants: A targeted approach for continuous postoperative evaluation in the spine

Real-time health monitoring systems are emerging in diverse medical fields, tracking biological and physiological signals for direct feedback to the user. Orthopaedics is yet to adapt to innovative trends in health monitoring. Despite an evident entry point during orthopaedic surgeries, clinicians remain unable to objectively examine the structural integrity and biomechanics in the operated region through implantable sensors. As such, postoperative advice can be non-specific and poorly guided. This perspective discusses the clinical need for load-sensing implants that address biomechanical postoperative monitoring, taking the example of spinal interbody cages. Research has attempted to establish sensing approaches in different orthopaedic settings; however, they fail to meet mechanical sensing requirements or lack in vivo translatability, especially in the spine. Polymeric flexible sensors and Microelectromechanical Systems (MEMS) have favourable attributes aligned to the required features for in vivo load-sensing, although these approaches are yet to be tested extensively in orthopaedics. While inductive powering is promising, wireless energy transfer and telemetry are areas of ongoing research. This perspective proposes a thorough understanding of the relevant biomechanics to identify the pertinent sensing parameters, concurrent treatment of sensing and powering aspects, and utilisation of energy harvesting for sensing and data transmission. While sensing advancements have contributed to the rise of real-time health monitoring in other fields of medicine, orthopaedics has so far been overlooked. It is the application of these innovations that will lead to the development of a new generation of 'smart' implants for continuous postoperative evaluation.

3D printed nanocomposites using polymer grafted graphene oxide prepared by multicomponent Passerini reaction

The surface of commercial graphene oxide was modified with polymers using Passerini reaction, which enhances the compatibility between nanoparticles and 3D printing resin.

Influence of Surface Treatment on the Interfacial and Mechanical Properties of Short S-Glass Fiber-Reinforced Dental Composites

The influence of interfacial shear strength (IFSS) between processed short S-glass fibers (250 and 350 μm in length, 5 μm in diameter) and the dental resin (a mixture of urethane dimethacrylate and triethylene glycol dimethacrylate monomers) on the mechanical properties has been studied experimentally. The surface profile of short S-glass fibers was modified using a selective atomic level metal etching process and simple silanization process to enhance the interfacial properties. The S-glass fibers were etched in acid solutions to increase the surface roughness and selectively remove Al³⁺ and Mg²⁺ ions, which promoted the mechanical and chemical interfacial bonding reactions. The single glass fiber tensile and microdroplet pull-out tests were performed to investigate the effects of interfacial properties on the flexural strength of the resultant composites. The surface modified S-glass fibers showed an increase of 11-40% in IFSS compared to untreated glass fibers. Composites reinforced with 350 μm length glass fibers (AR-70), which were treated in piranha solution for 4 h, showed the highest improvement in overall mechanical properties, flexural strength (34.2%), modulus (9.7%), and breaking energy (51.9%), compared to the untreated fiber-reinforced composites. The modified Lewis-Nielsen equation was developed using the effective fiber length factor to accurately predict the modulus of the short fiber-reinforced composites and validated with experimental results.