Interfacial integrity of bulk-fill resin composite restorations in deep Class-II cavities.

This study evaluated the interfacial integrity of deep (6 mm) Class-II (OM/OD) restorations placed using different bulk-fill resin composites [Filtek™ Bulk Fill Flowable Restorative (BF) and Filtek™ Bulk Fill Posterior Restorative (BP) (3M ESPE, St. Paul, MN, USA)] of different increment thicknesses (2 or 4 mm). BP was used for capping in all cases, while Filtek™ Z250 Universal Restorative (3M ESPE) was used as the control material. Interfacial debonding was measured during curing through acoustic emission (AE), followed by image analysis using micro-computed tomography and scanning electron microscopy. Microhardness testing was also conducted to assess degree of conversion. Depth of cure was adequate in all restorations. Specimens with 4-mm thick first increment of BF, which had a higher shrinkage strain, produced most AE events and debonding. Thus, bulk filling of deep cavities using bulk-fill resin composites with a high shrinkage strain should be avoided.

Stress analysis and risk of failure during clenching in ceramic assembly models: 3-dimensional finite element analysis.

Background/purpose: Clenching is a dental parafunctional disorder that jeopardizes the life of teeth and/or dental prostheses. Computer-aided design and computer-aided manufacturing (CAD/CAM)-fabricated or 3-dimensional-printed dental prostheses are aesthetic, strong, and of good quality, but noticeable damage can still be observed after clenching. Stress analysis of synthetic ceramic assemblies with various parameters was conducted to provide data that may be used to improve the fabrication of CAD/CAM-fabricated dental prostheses. Materials and methods: Abaqus software was used to run the simulations. A total of 96 axisymmetric finite element ceramic assembly models were simulated under 800 N vertical loading and different contact radii (0.25, 0.5, 0.75, 1.0 mm), materials (IPS e.max CAD and Vita Enamic), layer thicknesses and combinations. Results: Four-layered ceramic assembly models produced promising results with the following parameters: contact radius of at least 0.5 mm, total thickness of at least 0.5 mm, and use of IPS e.max CAD as the first layer and Vita Enamic as the second layer without cement. Conclusion: The ideal four-layered assembly model design uses 0.25-mm-thick IPS e.max CAD as its outer layer to simulate enamel binding and 0.25-mm-thick Vita Enamic as its inner layer to imitate the natural tooth. This design may be used as reference for prosthodontic treatment.

Morphology and mechanical performance of dental crown designed by 3D-DCGAN.

OBJECTIVES: This study utilised an Artificial Intelligence (AI) method, namely 3D-Deep Convolutional Generative Adversarial Network (3D-DCGAN), which is one of the true 3D machine learning methods, as an automatic algorithm to design a dental crown. METHODS: Six hundred sets of digital casts containing mandibular second premolars and their adjacent and antagonist teeth obtained from healthy personnel were machine-learned using 3D-DCGAN. Additional 12 sets of data were used as the test dataset, whereas the natural second premolars in the test dataset were compared with the designs in (1) 3D-DCGAN, (2) CEREC Biogeneric, and (3) CAD for morphological parameters of 3D similarity, cusp angle, occlusal contact point number and area, and in silico fatigue simulations with finite element (FE) using lithium disilicate material. RESULTS: The 3D-DCGAN design and natural teeth had the lowest discrepancy in morphology compared with the other groups (root mean square value = 0.3611). The Biogeneric design showed a significantly (p < 0.05) higher cusp angle (67.11°) than that of the 3D-DCGAN design (49.43°) and natural tooth (54.05°). No significant difference was observed in the number and area of occlusal contact points among the four groups. FE analysis showed that the 3D-DCGAN design had the best match to the natural tooth regarding the stress distribution in the crown. The 3D-DCGAN design was subjected to 26.73 MPa and the natural tooth was subjected to 23.97 MPa stress at the central fossa area under physiological occlusal force (300 N); the two groups showed similar fatigue lifetimes (F-N curve) under simulated cyclic loading of 100-400 N. Designs with Biogeneric or technician would yield respectively higher or lower fatigue lifetime than natural teeth. SIGNIFICANCE: This study demonstrated that 3D-DCGAN could be utilised to design personalised dental crowns with high accuracy that can mimic both the morphology and biomechanics of natural teeth.

Contact fracture test of monolithic hybrid ceramics on different substrates for bruxism.

OBJECTIVES: To determine the minimum thickness required for a monolithic hybrid ceramic crown on different substrates (soft vs stiff) used in posterior dentition for bruxism. METHODS: 80 polymer-infiltrated ceramic networks Vita Enamic (PICN VE) disc specimens with four different occlusal thicknesses (0.8, 1.2, 1.6 and 2.0 mm), were produced using a computer-aided design/manufacturing system, and cemented on a stiff (zirconia) or soft (polyamide) substrate of 4-mm thickness. The ten specimens, in soft or stiff groups, were subjected to compressive loading by a MTS machine until fracture or maximum load (4500 N) was reached. The unbroken specimens were examined using optical coherence tomography. Eight axisymmetric finite element models and eight 3D models comprising the four different occlusal thicknesses and two substrates under different vertical loads and sliding movements were constructed. The maximum principal stress was selected to evaluate the stress distribution in this study. RESULTS: The fracture resistance of the specimens was significantly different between the two substrates (P < 0.001). Fracture resistance was positively associated with specimen thickness (r = 0.597 and 0.896 for the soft and stiff substrate respectively). Specimens on the soft substrate had lower fracture loads, whilst cone cracks were observed in unbroken samples on different soft/stiff substrate prior to final fracture. The finite element analysis confirmed that samples on the stiff substrate had lower maximum principal stress values than those on the soft substrate. For the maximum principal stress not to exceed the flexural strength of PICN VE, a stiff substrate and minimum thickness of 2.0 mm are required for the prostheses. SIGNIFICANCE: A minimum 2.0 mm thick, stiff substrate was needed for bruxism as shown by the effect of high/large chewing force on the posterior dentition of monolithic PICN VE crowns.

Sorptivity of water in enamel for categorizing caries lesions.

OBJECTIVE: The aim of this research was to determine the association between sorptivity of water and the state of mineralization in carious enamel of different stages of severity. METHODS: As a preliminary work, water droplets of 1.5 µL were placed on the surfaces of hydroxyapatite (HA) discs of different densities. The water droplet profile was dynamically recorded every second over a period of 10 s using a contact angle meter to determine the relationship between sorptivity and density. To measure and calculate sorptivity on enamel surfaces, varnish was painted on the labial surface of 96 extracted caries-free human teeth, leaving two 1.4 ± 0.1 mm diameter circular exposed test sites. The specimens were randomly divided into 6 groups (n = 16) and subjected to 0(G0), 7(G7), 14(G14), 21(G21), 28(G28) and 35(G35) days of pH cycling, respectively. A 0.7 µL water droplet was placed on each exposed site and Optical Coherence Tomography was used to measure its height every 10 seconds for 2 min. Sorptivity was computed by considering sorption equations and Washburn's analysis of capillary kinetics and correction for evaporation was also performed. Micro-Computed Tomography scans of the specimens were obtained and delta Z (ΔZ) is the parameter used to measure mineral loss. ΔZ at 10 µm (ΔZ10) and 50 µm (ΔZ50) from the surface were calculated. One-way ANOVA and Post-hoc Tukey tests were used to compare sorptivity between groups and bivariate correlations were used to analyze the association between sorptivity and ΔZ. RESULTS: Sorptivity was found to be inversely and linearly correlated with HA density with R2 value of 0.95. With enamel, there is a general trend of increase in mean sorptivity from G0 to G35, except for a decrease in G21. The same trends were observed for both ΔZ10 and ΔZ50. The decrease in sorptivity in G21 coincided with the presence of a surface hypermineralized layer in G21 samples. Post-hoc Tukey showed significant differences in mean sorptivity between G0 and G14, G0 and G21 as well as G14 and G21. Post-hoc Dunnett's T3 showed significant differences for ΔZ10 between G0 and G14 as well as G14 and G21. Significant correlation between mean sorptivity and ΔZ10 was detected with Pearson correlation coefficient of 0.461. For ΔZ50, post-hoc Tukey showed significant differences between G0 and G14 but no significant difference was detected between G14 and G21. No correlations were detected between mean sorptivity and ΔZ50. SIGNIFICANCE: Sorptivity was found to be inversely and linearly correlated with HA density with R2 value of 0.95. With enamel, there is a general trend of increase in mean sorptivity from G0 to G35, except for a decrease in G21. The same trends were observed for both ΔZ10 and ΔZ50. The decrease in sorptivity in G21 coincided with the presence of a surface hypermineralized layer in G21 samples.

A finite element analysis of ceramic restorations in endodontically treated premolars.

PURPOSE: To investigate the level and distribution of stresses in endodontically treated maxillary premolar teeth restored using various cavity designs of bonded all-ceramic restorations. The hypothesis tested was that the various all-ceramic approaches, including incorporating a pulp chamber extension in the restoration, had no influence on the stresses in the restored tooth unit. METHODS: Finite element packages Patran and Abaqus were used for the stress analysis. The cavity designs investigated include: (1) inlay (I); (2) inlay with palatal cusp coverage (IPC); (3) onlay (O); (4) inlay with pulp chamber extension (IPE); (5) inlay with palatal cusp coverage and pulp chamber extension (IPCPE); and (6) onlay with pulp chamber extension (OPE). RESULTS: In each case, tensile stresses were found to be concentrated subjacent to the occlusal fossa. Peak tensile stress and peak shear stress values along the tooth/restoration interface for IPC, O IPCPE and OPE cavity designs were found to be associated with the axiogingival line angle. Overall, the order of the various forms of restoration investigated in terms of the maximum principal stress (from greatest to lowest) was as follows: IPE > IPCPE > OPE > I > IPC > O.

An acoustic emission study on interfacial debonding in composite restorations.

OBJECTIVE: This paper studied in vitro the effect of the C-factor on interfacial debonding during curing of composite restorations using the acoustic emission (AE) technique. Finite element (FE) analyzes were also carried out to evaluate the interfacial stresses caused by shrinkage of the composite resin in restorations with different C-factors. MATERIALS AND METHODS: Twenty extracted third molars were divided into 4 groups of 5. They were cut to form Class-I (Groups 1 and 2) and Class-II (Groups 3 and 4) cavities with different C-factors. The average C-factors of the four groups were 3.37, 2.88, 2.00, and 1.79, respectively. The cavities were then applied with an adhesive and restored with a composite, which was cured by a halogen light for 40 s. A 2-channel AE system was used to monitor the interfacial debonding, caused by shrinkage stress, between the tooth and restoration through an AE sensor attached to the surface of the specimen. Recording of the AE started at the same time as curing of the composite and lasted 10 min. Simplified FE models were used to evaluate the interfacial stresses in restorations with different C-factors, with a thermal load (temperature decrease) being applied to the composite resin to simulate its shrinkage. RESULTS: The mean and standard deviation of the total number of AE events for the four groups were 29.6±15.7, 10.0±5.8, 2.6±1.5, and 2.2±1.3, i.e. the number of AE events increased with an increase in the C-factor. The FE results also showed that, the higher the C-factor of the restoration, the higher the interfacial tensile stress between the tooth and restoration. SIGNIFICANCE: From the results of the AE tests and FE simulations, it can be concluded that, the higher the C-factor, the higher the shrinkage stress and the more likely is interfacial debonding.

Non-destructive examination of interfacial debonding using acoustic emission.

OBJECTIVES: This study aims to assess the viability of using the acoustic emission (AE) measurement technique to detect and monitor in situ the interfacial debonding in resin composite restorations due to build-up of shrinkage stresses during polymerization of the composite. MATERIALS AND METHODS: The non-destructive testing technique that measures acoustic emission (AE) was used to detect and monitor the interfacial debonding in resin composite during curing of the composite. Four groups of specimens, n=4 each, were tested: (1) intact human molars with Class-I cavities restored with the composite Z100 (3M ESPE, USA); (2) intact human molars with Class-I cavities restored with the composite Filtek™ P90 (3M ESPE, USA); (3) ring samples prepared from the root of a single bovine tooth and 'restored' with Z100; (4) freestanding pea-size specimens of Z100 directly placed on the AE sensor. The restorations in Groups (1)-(3) were bonded to the tooth tissues with the adhesive Adper™ Scotchbond™ SE Self-Etch (3M ESPE, USA). The composites in all the specimens were cured with a blue light (3M ESPE, USA) for 40s. The AE signals were recorded continuously for 10 min from the start of curing. Non-destructive 3D imaging was performed using X-ray micro-computed tomography (micro-CT) to examine the bonding condition at the tooth-restoration interface. RESULTS: The development of AE events followed roughly that of the shrinkage stress, which was determined separately by the cantilever beam method. The number of AE events in the real human tooth samples was more than that in the ring samples, and no AE events were detected in the pea-size specimens placed directly on the AE sensor. The number of AE events recorded in the specimens restored using Z100 was more than that found in specimens restored with Filtek P90. The micro-CT imaging results showed clear interfacial debondings in the tooth specimens restored with Z100 after curing, but no clear debonding was found in the P90 specimens. CONCLUSIONS: The AE technique is an effective tool for detecting and monitoring in situ the interfacial debonding of composite restorations during curing. It can potentially be employed to evaluate the development of shrinkage stress and the quality of interfacial bonds in teeth restored with different composite materials, cavity geometries, and restorative techniques.

Fracture simulation of restored teeth using a continuum damage mechanics failure model.

OBJECTIVE: The aim of this paper is to validate the use of a finite-element (FE) based continuum damage mechanics (CDM) failure model to simulate the debonding and fracture of restored teeth. MATERIALS AND METHODS: Fracture testing of plastic model teeth, with or without a standard Class-II MOD (mesial-occusal-distal) restoration, was carried out to investigate their fracture behavior. In parallel, 2D FE models of the teeth are constructed and analyzed using the commercial FE software ABAQUS. A CDM failure model, implemented into ABAQUS via the user element subroutine (UEL), is used to simulate the debonding and/or final fracture of the model teeth under a compressive load. The material parameters needed for the CDM model to simulate fracture are obtained through separate mechanical tests. The predicted results are then compared with the experimental data of the fracture tests to validate the failure model. RESULTS: The failure processes of the intact and restored model teeth are successfully reproduced by the simulation. However, the fracture parameters obtained from testing small specimens need to be adjusted to account for the size effect. The results indicate that the CDM model is a viable model for the prediction of debonding and fracture in dental restorations.