Dynamic tensile viscoelastic properties of porcine periodontal ligament

The periodontal ligament plays a significant role in orthodontic and masticatory processes. To explicitly investigate the effects of dynamic force amplitude and frequency on the dynamic tensile properties of the periodontal ligament, in vitro tensile experiments were conducted using a dynamic mechanical analysis at various dynamic force amplitudes across a wide frequency range. Storage modulus, loss modulus, and loss factor values were measured. A Maxwell constitutive model based on modulus was established to describe the dynamic mechanical properties of the periodontal ligament. The results showed that the storage modulus ranged from 29.53 MPa to 158.24 MPa, the loss modulus ranged from 3.26 MPa to 76.16 MPa, and the loss factor values all increased with higher frequencies and higher dynamic force amplitudes. Based on the parameters obtained from the fitting results, it is evident that the short-term response has a more pronounced impact on the elastic response of the periodontal ligament than the long-term response. Increasing the dynamic force amplitude and its frequency amplified the viscous effects of the periodontal ligament and enhanced energy dissipation. The proposed constitutive model further demonstrated that the periodontal ligament acts as a viscoelastic biomaterial. These findings have implications for future research on the periodontal ligament.

Water absorption in artificial composites: Curse or blessing?

OBJECTIVES

This study evaluated the impact of mutable water uptake on the durability of mechanical properties and the long-term reliability of artificial composites.

METHODS

Three resin-based CAD/CAM restorative materials (CRMs) were investigated in three-point bending tests to calculate flexural strength (FS), modulus of elasticity (ME), modulus of resilience (MR), modulus of toughness (MT), and elastic recovery (ER). All specimens (n = 180) were stored under the same conditions and tested in four subsets (n = 15 per material) that were respectively withdrawn after repeated thermocycling (5000 cycles; 5-55 °C, H2O) and repetitive drying (7 d; 37 °C, air). For every specimen, weight differences were determined per storage condition. Likewise, loss tangent data were separately recorded via dynamic mechanical analysis to reliably assess damping characteristics.

RESULTS

Repeated thermocycling always induced weight increase and a concurrent significant loss in all mechanical properties except for MT and ER of a polymethylmethacrylate-based CRM. Drying consistently provoked weight loss and raised mechanical properties to initial values. Weight increase, however, enhanced loss tangent values and accordingly distinct damping characteristics, whereas weight decrease markedly lowered damping properties.

SIGNIFICANCE

Water uptake repeatedly induced a decrease in common mechanical properties but concurrently increased damping behavior. Invertible equilibrium processes were found with no evidence for permanent material degradation.

Mechanical properties of conventional versus microwave-polymerized denture base acrylic resins

STATEMENT OF PROBLEM

New denture base acrylic resins have been introduced that are specifically formulated for microwave polymerization. Microwave polymerization is a time-efficient procedure, but few studies have evaluated how these new acrylic resin formulations compare with conventionally processed acrylic resins.

PURPOSE

The purpose of this in vitro study was to compare the stiffness and strength of denture base acrylic resins formulated for microwave polymerization with conventionally processed acrylic resin.

MATERIAL AND METHODS

Rectangular beams were fabricated from 2 microwave-polymerized denture base acrylic resins, microwave-specific resin (Nature-Cryl MC), resin with the option of microwave polymerization (Diamond D), and a conventionally processed resin as a control (Lucitone 199). Specimens (n=10) were stored in water for 1 week and subjected to a 3-point bend test to determine the flexural modulus (stiffness) and flexural strength before (initial properties) or after 120 000 load cycles. The load cycles, conducted between 5 and 25 N at 2 Hz, simulated 6 months of mastication. Data were analyzed by using 2-way ANOVA, followed by pairwise comparisons (α=.05).

RESULTS

The initial flexural modulus (mean ±standard deviation) was conventionally processed resin, 2.65 ±0.33 GPa; microwave-specific resin, 3.01 ±0.20 GPa; and microwave-option resin, 2.63 ±0.04 GPa. After load cycling, the mean flexural modulus was conventionally processed resin, 2.34 ±0.32 GPa; microwave-specific resin, 2.69 ±0.20 GPa; and microwave-option resin, 1.96 ±0.11 GPa. The initial flexural strength was conventionally processed resin, 77.6 ±11.0 MPa; microwave-specific resin, 83.6 ±3.5 MPa; and microwave-option resin, 78.9 ±2.6 MPa. After load cycling, the mean flexural strength was conventionally processed resin, 68.7 ±9.0 MPa; microwave-specific resin, 73.3 ±3.3 MPa; and microwave-option resin, 65.5 ±3.5 MPa. Resin and loading state significantly affected the stiffness and strength (P<.01); the interaction resin×state was not significant (P≥.558).

CONCLUSIONS

Microwave-polymerized denture base acrylic resins were comparable in stiffness and strength with conventionally processed acrylic resin. All acrylic resins decreased in stiffness and strength after load cycling. The microwave-specific resin was significantly stiffer and stronger than the other denture base acrylic resins, initially and after 120 000 load cycles.

Fracture Resistance of a Bone-Level Two-Piece Zirconia Oral Implant System-The Influence of Artificial Loading and Hydrothermal Aging

Preclinical and clinical research on two-piece zirconia implants are warranted. Therefore, we evaluated the in vitro fracture resistance of such a zirconia oral implant system. The present study comprised 32 two-piece zirconia implants and abutments attached to the implants using a titanium (n = 16) or a zirconia abutment screw (n = 16). Both groups were subdivided (n = 8): group T-0 comprised implants with a titanium abutment screw and no artificial loading; group T-HL was the titanium screw group exposed to hydro-thermomechanical loading in a chewing simulator; group Z-0 was the zirconia abutment screw group with no artificial loading; and group Z-HL comprised the zirconia screw group with hydro-thermomechanical loading. Groups T-HL and Z-HL were loaded with 98 N and aged in 85 °C hot water for 107 chewing cycles. All samples were loaded to fracture. Kruskal-Wallis tests were executed to assess the loading/bending moment group differences. The significance level was established at a probability of 0.05. During the artificial loading, there was a single occurrence of an implant fracture. The mean fracture resistances measured in a universal testing machine were 749 N for group T-0, 828 N for group Z-0, 652 N for group T-HL, and 826 N for group Z-HL. The corresponding bending moments were as follows: group T-0, 411 Ncm; group Z-0, 452 Ncm; group T-HL, 356 Ncm; and group Z-HL, 456 Ncm. There were no statistically significant differences found between the experimental groups. Therefore, the conclusion was that loading and aging did not diminish the fracture resistance of the evaluated implant system.

Advancing 3D Dental Implant Finite Element Analysis: Incorporating Biomimetic Trabecular Bone with Varied Pore Sizes in Voronoi Lattices

The human mandible's cancellous bone, which is characterized by its unique porosity and directional sensitivity to external forces, is crucial for sustaining biting stress. Traditional computer- aided design (CAD) models fail to fully represent the bone's anisotropic structure and thus depend on simple isotropic assumptions. For our research, we use the latest versions of nTOP 4.17.3 and Creo Parametric 8.0 software to make biomimetic Voronoi lattice models that accurately reflect the complex geometry and mechanical properties of trabecular bone. The porosity of human cancellous bone is accurately modeled in this work using biomimetic Voronoi lattice models. The porosities range from 70% to 95%, which can be achieved by changing the pore sizes to 1.0 mm, 1.5 mm, 2.0 mm, and 2.5 mm. Finite element analysis (FEA) was used to examine the displacements, stresses, and strains acting on dental implants with a buttress thread, abutment, retaining screw, and biting load surface. The results show that the Voronoi model accurately depicts the complex anatomy of the trabecular bone in the human jaw, compared to standard solid block models. The ideal pore size for biomimetic Voronoi lattice trabecular bone models is 2 mm, taking in to account both the von Mises stress distribution over the dental implant, screw retention, cortical bone, cancellous bone, and micromotions. This pore size displayed balanced performance by successfully matching natural bone's mechanical characteristics. Advanced FEA improves the biomechanical understanding of how bones and implants interact by creating more accurate models of biological problems and dynamic loading situations. This makes biomechanical engineering better.

Comparison of subcritical growth parameters of a Y-TZP obtained via cyclic or dynamic fatigue tests

OBJECTIVE

The objective of this study was to 1) compare the stress corrosion coefficient (n) of a Y-TZP obtained by two fatigue tests: cyclic and dynamic and 2) evaluate the effect of frequency in the characteristic lifetime and the existence of interaction between the cyclic fatigue and slow crack growth.

METHODS

A total of 145 Y-TZP specimens were produced in accordance with the manufacturer's instructions. These specimens, measuring 4.0 × 3.0 × 25.0 mm, were used for dynamic (n = 70) and cyclic fatigue tests (n = 75). The specimens were obtained from CAD/CAM blocks, sectioned, and sintered in a furnace at 1530 °C with a heating rate of 25 °C/min. They were tested in their "as-sintered" form without any additional surface treatment. The fatigue tests were conducted using a four-point bending to obtain the slow crack growth parameters (n). The cyclic fatigue test was also conducted in two frequencies (2 and 10 Hz), using stress levels between 350 and 600 MPa. Data from these tests were analyzed using ASTM C 1368-00 formulas and Weibull statistics. Scanning electron microscope (SEM) was used for fracture surface analysis to identify the origin of the fracture. Critical defect size was measured and used, along with flexural strength values, to estimate fracture toughness. Dynamic fatigue test data were used to obtain subcritical crack growth (SCG) parameters and perform Weibull statistical analysis. The cyclic fatigue data were used in the General Log-linear Model equation using the ALTA PRO software. Data were analyzed using one-way ANOVA followed by Tukey post-hoc tests and Student's t-test at a significance level of p ≤ 0.05.

RESULTS

In the dynamic fatigue test, the values obtained for σfo and n were 667 and 54, respectively. This parameter indicates how the strength of the material diminishes over time due to internal cracks. The Weibull parameters obtained from the same test results were m = 7.9, σ0 = 968, 9 and σ5% = 767, which indicates the reliability of the material. The Weibull parameters obtained by cyclic fatigue were statistically similar for the two frequencies used, the m* was 0.17 (2 Hz) and 0.21 (10 Hz); characteristic lifetimes (η) were 1.93 × 106 and 40,768, respectively. The n values obtained by cyclic fatigue were 48 and 40 at frequencies of 2 and 10 Hz, respectively. There was no effect of the frequency, the stress level or the interaction of the two in the Y-TZP lifetime, when analysed by General Log Linear Model.

SIGNIFICANCE

the n values obtained by cyclic and dynamic fatigue tests showed no statistically significant difference and the effect of frequency in the characteristic lifetime and the existence of interaction between the cyclic fatigue and subcritical growth were not observed in the tested specimens.

The influence of oral cavity physiological parameters: temperature, pH, and swelling on the performance of denture adhesives - in vitro study

BACKGROUND

The various physical and chemical conditions within the oral cavity are hypothesized to have a significant influence on the behavior of denture adhesives and therefore the overall comfort of denture wearers. As such, this study aims to understand the influence of oral cavity physiological parameters such as temperature (17 to 52 °C), pH (2, 7, 10), and denture adhesive swelling due to saliva (20-120%) on the behavior of denture adhesives. This study further aims to emphasize the need for a collective approach to modelling the in-situ behavior of denture adhesives.

METHODS

Rheological measurements were carried out using the Super Polygrip Ultra fresh brand denture adhesive cream to evaluate its storage modulus (G´) and loss modulus (G´´) values at a range of physiologically relevant temperatures, pH values, and degrees of swelling, to represent and characterize the wide variety of conditions that occur within the oral cavity.

RESULTS

Rheological data was recorded with respect to variation of temperature, pH, and swelling. Overall, it can be seen that the physiological conditions of the oral cavity have an influence on the rheological properties of the denture adhesive cream. Specifically, our data indicates that the adhesive's mechanical properties are weakly influenced by pH, but do change with respect to the temperature in the oral cavity and the swelling rate of the adhesive.

CONCLUSIONS

Our results suggest that the collective inter-play of the parameters pH, temperature and swelling ratio have an influence on the behavior of the denture adhesive. The results clearly highlight the need for developing a multi-parameter viscoelastic material model to understand the collective influence of physiological parameters on the performance of denture adhesives. Multi-parameter models can also potentially be utilized in numerically simulating denture adhesives using finite element simulations.

Titanium Alloy Implants with Lattice Structures for Mandibular Reconstruction

In recent years, the field of mandibular reconstruction has made great strides in terms of hardware innovations and their clinical applications. There has been considerable interest in using computer-aided design, finite element modelling, and additive manufacturing techniques to build patient-specific surgical implants. Moreover, lattice implants can mimic mandibular bone's mechanical and structural properties. This article reviews current approaches for mandibular reconstruction, their applications, and their drawbacks. Then, we discuss the potential of mandibular devices with lattice structures, their development and applications, and the challenges for their use in clinical settings.

Fracture Resistance of a Two-Piece Zirconia Implant System after Artificial Loading and/or Hydrothermal Aging-An In Vitro Investigation

The purpose of the present study was to assess the fracture resistance of a two-piece alumina-toughened zirconia implant system with a carbon-reinforced PEEK abutment screw.

METHODS

Thirty-two implants with screw-retained zirconia abutments were divided into four groups of eight samples each. Group 0 (control group) was neither loaded nor aged in a chewing simulator; group H was hydrothermally aged; group L was loaded with 98 N; and group HL was subjected to both hydrothermal aging and loading in a chewing simulator. One sample of each group was evaluated for t-m phase transformation, and the others were loaded until fracture. A one-way ANOVA was applied to evaluate differences between the groups.

RESULTS

No implant fracture occurred during the artificial chewing simulation. Furthermore, there were no statistically significant differences (p > 0.05) between the groups in terms of fracture resistance (group 0: 783 ± 43 N; group H: 742 ± 43 N; group L: 757 ± 86 N; group HL: 740 ± 43 N) and bending moment (group 0: 433 ± 26 Ncm; group H: 413 ± 23 Ncm; group L: 422 ± 49 Ncm; group HL: 408 ± 27 Ncm).

CONCLUSIONS

Within the limitations of the present investigation, it can be concluded that artificial loading and hydrothermal aging do not reduce the fracture resistance of the investigated implant system.

Side switch frequency while masticating different chewing materials, and its relationship with other masticatory behaviors and sensory perceptions

OBJECTIVE

This cross-sectional study aimed to establish normative values for masticatory side switch (MSS) frequency in young Mexican adults and to assess the relationship between various indices and MSS frequency when masticating different chewing materials.

DESIGN

We enrolled 101 dentate adults and performed four masticatory assays that involved masticating different chewing materials (i.e., two-colored chewing gum, sweet cracker, salty cracker, and bread). Participants were asked to eat and swallow these foods and to chew the gum for 40 cycles and the following indices were determined: MSS index (MSSI), unilateral chewing index, chewing cycle duration, and number of cycles before terminal swallowing. The participants then rated perceived flavor intensity, salivary flow, and muscle fatigue during each trial.

RESULTS

The MSSI ranged from 0.03-0.06 (10th percentile) to 0.48-0.54 (90th percentile). A repeated-measures general linear model revealed a mean MSSI value of 0.28 (95 %CI, 0.25-0.30) adjusted by several factors. Male sex, soft food, and the last chewing period were associated with lower MSS frequency. Spearman's test showed a high correlation for the MSSI among the different foods. MSSI correlated negatively with the unilateral chewing index for each chewing material and with number of cycles for the sweet cracker. However, no significant correlation was detected between MSSI and sensory perception.

CONCLUSIONS

In healthy dentate individuals, the mean MSS relative frequency is 25-30 % with an 80-central percentile of 5-50 % of the maximum possible side changes. Lower MSS frequencies were detected in men, when chewing soft food, and during the final chewing period.

The effect of cyclic loading on the fracture resistance of 3D-printed and CAD/CAM milled zirconia crowns-an in vitro study

OBJECTIVES

The aim of this study was to evaluate the effect of cyclic mechanical loading on the fracture resistance of 3D-printed zirconia crowns in comparison to milled zirconia crowns.

MATERIALS AND METHODS

Monolithic zirconia crowns (n = 30) were manufactured using subtractive milling (group M) and 3D additive printing (group P). Nine samples of each group were fractured under one-time loading while the other 6 samples were subjected to cyclic loading for 1.2 million cycles before being subjected to one-time loading until fracture. Scanning electron microscope (SEM) fractographic analysis was carried out on fractured fragments of representative samples.

RESULTS

The mean for fracture resistance of group M was 1890 N without cyclic loading and 1642 N after being subjected to cyclic loading, and they were significantly higher than that of group P (1658 N and 1224 N respectively).

CONCLUSIONS

The fabrication technique and cyclic loading affect the fracture resistance of zirconia crowns. Although the fracture resistance values for the 3D-printed crowns were lower than those of the milled, still they are higher than the masticatory forces and thus could be considered being clinically acceptable.

CLINICAL RELEVANCE

Concerning fracture resistance, 3D-printed crowns can withstand the masticatory forces for the long term without any cracks or failure.

Shade, Aging and Spatial-Dependent Variation of Elastoplastic and Viscoelastic Characteristics in a Dental, Submicron Hybrid CAD/CAM Composite

This article reports the elastoplastic and viscoelastic response of an industrially cured CAD/CAM resin-based composite (Brilliant Crios, Coltene) at different scales, spatial locations, aging conditions, and shading. Mechanical tests were performed at the macroscopic scale to investigate material strength, elastic modulus, fracture mechanisms and reliability. An instrumented indentation test (IIT) was performed at the microscopic level in a quasi-static mode to assess the elastic and plastic deformation upon indentation, either by mapping transverse areas of the CAD/CAM block or at randomly selected locations. A dynamic-mechanical analysis was then carried out, in which chewing-relevant frequencies were included (0.5 to 5 Hz). Characteristics measured at the nano- and micro-scale were more discriminative in identifying the impact of variables as those measured at macro scale. Anisotropy as a function of the spatial location was identified in all shades, with gradual variation in properties from the center of the block to peripheral locations. Depending on the scale of observation, differences in shade and translucency are very small or not statistically significant. The aging effect is classified as low, but measurable on all scales, with the same pattern of variation occurring in all shades. Aging affects plastic deformation more than elastic deformation and affects elastic deformation more than viscous deformation.

Wearables for personalized monitoring of masticatory muscle activity - opportunities, challenges, and the future

Wearable devices are worn on or remain in close proximity of the human body. The use of wearable devices specific to the orofacial region is steadily increasing. Orofacial applications of wearable devices include supplementing diagnosis, tracking treatment progress, monitoring patient compliance, and understanding oral parafunctional behaviours. In this short communication, the role of wearable devices in advancing personalized dental medicine are highlighted with a specific focus on masticatory muscle activity monitoring in naturalistic settings. Additionally, challenges, opportunities, as well as future research areas for successful use of wearable devices for precise, personalized care of muscle disorders are discussed.

Method development for the intraoral release of nanoparticles from dental restorative materials

OBJECTIVE

The aim of this study was the development of a novel in-vitro method to evaluate the intraoral release of wear particles with a diameter< 1 µm from dental restorative materials.

METHODS

Test fixtures for a dual-axis chewing simulator (CS-4.8, SD Mechatronik, Feldkirchen-Westerham, Germany), consisting of three components to mount the specimens and a solvent (distilled water) as well as a zirconia antagonist to transfer the masticatory forces onto the specimen was developed. Ceram.x Spectra™ ST HV (CS) and Filtek™ Supreme XTE (FS) specimens (n = 3) were fixed into the mounts and immersed in 25 ml solvent. All specimens were subjected to 500.000 wear cycles with a load of 49 N. The particle size distribution of the suspensions were examined by dynamic light scattering (DLS). The collected particles were characterised by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). For wear quantification, the surfaces of the specimens were photo-optically scanned and the wear was measured. For the statistical analysis, one-way ANOVA and post-hoc Scheffé tests were applied.

RESULTS

DLS showed particle diameters< 1 µm (CS: 18.06 nm-1.64 µm, FS: 72.30 nm-2.31 µm). SEM/EDS indicated an association between the detected elements and the materials' composition. FS showed significantly higher volume loss (p = 0.007) and maximum depth of the wear profile (p = 0.005) than CS, but no significant differences in the surface loss (p = 0.668).

SIGNIFICANCE

The novel method is able to detect material dependent particles to the size of nanoscale after in-vitro abrasion.

Local Concentrations of TGF-β1 and IGF-1 Appear Determinant in Regulating Bone Regeneration in Human Postextraction Tooth Sockets

Healing after tooth extraction involves a series of reparative processes affecting both alveolar bone and soft tissues. The aim of the present study was to investigate whether activation of molecular signals during the healing process confers a regenerative advantage to the extraction socket soft tissue (ESsT) at 8 weeks of healing. Compared to subepithelial connective tissue graft (CTG), qRT-PCR analyses revealed a dramatic enrichment of the ESsT in osteogenic differentiation markers. However, ESsT and CTG shared characteristics of nonspecialized soft connective tissue by expressing comparable levels of genes encoding abundant extracellular matrix (ECM) proteins. Genes encoding the transforming growth factor-β1 (TGF-β1) and its receptors were strongly enriched in the CTG, whereas the transcript for the insulin-like growth factor-1 (IGF-1) showed significantly high and comparable expression in both tissues. Mechanical stimulation, by the means of cyclic strain or matrix stiffness applied to primary ESsT cells (ESsT-C) and CTG fibroblasts (CTG-F) extracted from the tissue samples, revealed that stress-induced TGF-β1 not exceeding 2.3 ng/mL, as measured by ELISA, in combination with IGF-1 up to 2.5 ng/mL was able to induce the osteogenic potential of ESsT-Cs. However, stiff matrices (50 kPa), upregulating the TGF-β1 expression up to 6.6 ng/mL, caused downregulation of osteogenic gene expression in the ESsT-Cs. In CTG-Fs, endogenous or stress-induced TGF-β1 ≥ 4.6 ng/mL was likely responsible for the complete lack of osteogenesis. Treatment of ESsT-Cs with TGF-β1 and IGF-1 proved that, at specific concentrations, the two growth factors exhibited either an inductive-synergistic or a suppressive activity, thus determining the osteogenic and mineralization potential of ESsT-Cs. Taken together, our data strongly warrant the clinical exploration of ESsT as a graft in augmentative procedures during dental implant placement surgeries.

Influence of loading and aging on the fracture strength of an injection-molded two-piece zirconia implant restored with a zirconia abutment

OBJECTIVE

To investigate the fracture strength and potential phase transformation of an injection-molded two-piece zirconia implant restored with a zirconia abutment after loading and/or aging.

METHODS

Thirty-two two-piece zirconia implants (4.0 mm diameter) restored with zirconia abutments were embedded according to ISO 14801 and divided into four groups (n = 8/group): Three groups were either exclusively hydrothermally treated (group HT; 85°C), dynamically loaded (group DL; 10⁷  cycles; 98 N), or subjected to both treatments simultaneously (group DL/HT). One group remained untreated (group 0). A sample from each group was cross-sectioned and examined by scanning electron microscopy for possible crystal phase transformation. The remaining samples were then loaded to fracture in a static loading test. A one-way ANOVA was used for statistical analyses.

RESULTS

During dynamic loading, three implants of group DL and six implants of group DL/HT fractured at a load of 98 N. The fracture strength of group DL/HT (108 ± 141 Ncm) was significantly reduced compared to the other groups (group 0: 342 ± 36 Ncm; HT: 363 ± 49 Ncm; DL: 264 ± 198 Ncm) (p < .05). Fractures from group 0 and HT occurred at both implant and abutment level, whereas implants from group DL and DL/HT fractured only at implant level. A shallow monoclinic transformation zone of approximately 2 μm was observed following hydrothermal treatment.

CONCLUSIONS

Within the limitations of this study, it can be concluded that dynamic loading and the combination of loading and aging reduced the fracture strength of the implant abutment combination. Hydrothermal treatment caused a shallow transformation zone which had no influence on the fracture strength.

Which dentine analogue material can replace human dentine for crown fatigue test?

OBJECTIVE

To seek dentine analogue materials in combined experimental, analytical, and numerical approaches on the mechanical properties and fatigue behaviours that could replace human dentine in a crown fatigue laboratory test.

METHODS

A woven glass fibre-filled epoxy (NEMA grade G10; G10) and a glass fibre-reinforced polyamide-nylon (30% glass fibre reinforced polyamide-nylon 6,6; RPN) were investigated and compared with human dentine (HD). Flexural strength and elastic modulus (n = 10) were tested on beam-shaped specimens via three-point bending, while indentation hardness (n = 3) was tested after fracture. Abutment substrates of G10, RPN and HD were prepared and resin-bonded with monolithic lithium disilicate crowns (n = 10), then subjected to wet cyclic loading in a step-stress manner (500 N initial load, 100 N step size, 100,000 cycles per step, 20 Hz frequency). Data were statistically analysed using Kruskal-Wallis one-way ANOVA followed by post-hoc comparisons (α = 0.05). Survival probability estimation was performed by Mantel-Cox Log-Rank test with 95% confidence intervals. The fatigue failure load (FFL) and the number of cycles until failure (NCF) were evaluated with Weibull statistics. Finite Element Models of the fatigue test were established for stress distribution analysis and lifetime prediction. Fractographic observations were qualitatively analysed.

RESULTS

The flexural strength of HD (164.27 ± 14.24 MPa), G10 (116.48 ± 5.93 MPa), and RPN (86.73 ± 3.56 MPa) were significantly different (p < 0.001), while no significant difference was observed in their flexural moduli (p = 0.377) and the indentation hardness between HD and RPN (p = 0.749). The wet cyclic fatigue test revealed comparable mean FFL and NCF of G10 and RPN to HD (p = 0.237 and 0.294, respectively) and similar survival probabilities for the three groups (p = 0.055). However, RPN promotes higher stability and lower deviation of fatigue test results than G10 in Weibull analysis and FEA.

SIGNIFICANCE

Even though dentine analogue materials might exhibit similar elastic properties and fatigue performance to human dentine, different reliabilities of fatigue on crown-dentine analogues were shown. RPN seems to be a better substrate that could provide higher reliability and predictability of laboratory study results.

Fatigue Resistance of 3-Unit CAD-CAM Ceramic Fixed Partial Dentures: An FEA Study

PURPOSE

To estimate the fatigue life of 3-unit molar fixed partial dentures (FPDs) made from two different monolithic ceramic systems, zirconia cercon (ZC) and lithium disilicate (LD). The effect of the connector size on the fatigue resistance of the monolithic FPD was also investigated.

METHODS

Two models for the FPDs were built, a 3-unit all-ceramic and a porcelain-fused-to-metal. The porcelain-fused-to-metal FPD model was used as the control. Actual 3-unit FPDs (replacing the second lower premolar) were constructed using a computer aided design and computer-aided manufacturing (CAD-CAM) system. Finite element analysis (FEA) was executed. A hemispherical indenter was used to simulate occlusal load. The occlusal load phase of the chewing cycle was applied at the premolar pontic.

RESULTS

The failure location for the monolithic FPD was always located at the distal connector. Connector size played a key role in determining the long-term survival of the prosthesis. The fatigue resistance was predicted to be 670 N for the ZC with a connector size 4 × 3 mm, while it was only 226 N for LD. As for porcelain-fused-to-metal (PFM), FEA predicts that fatigue resistance can reach up to 770 N. Under the cyclic load of 670 N, the fatigue life for the zirconia FPD with connector size 4 × 3 mm was 2.23 × 10⁶ cycles while it survived only 3.1 × 10⁵ cycles when the connector was reduced to 3.5 × 2.5 mm. The angle of the oblique load has a significant effect on the stress distribution.

CONCLUSION

3-unit monolithic FPDs made of ZC have superior fatigue performance compared to those made of LD. The fatigue life of the zirconia FPD was about three times longer than that made of LD with a connector size of 4 mm × 3 mm. The survival rates of ZC FPDs are comparable to porcelain-fused-to-metal. A significant reduction in fatigue strength is predicted for reduced connector size. Therefore, it is necessary to establish general guidelines for the minimum connector size.

Biomechanical Modelling for Tooth Survival Studies: Mechanical Properties, Loads and Boundary Conditions-A Narrative Review

Recent biomechanical studies have focused on studying the response of teeth before and after different treatments under functional and parafunctional loads. These studies often involve experimental and/or finite element analysis (FEA). Current loading and boundary conditions may not entirely represent the real condition of the tooth in clinical situations. The importance of homogenizing both sample characterization and boundary conditions definition for future dental biomechanical studies is highlighted. The mechanical properties of dental structural tissues are presented, along with the effect of functional and parafunctional loads and other environmental and biological parameters that may influence tooth survival. A range of values for Young's modulus, Poisson ratio, compressive strength, threshold stress intensity factor and fracture toughness are provided for enamel and dentin; as well as Young's modulus and Poisson ratio for the PDL, trabecular and cortical bone. Angles, loading magnitude and frequency are provided for functional and parafunctional loads. The environmental and physiological conditions (age, gender, tooth, humidity, etc.), that may influence tooth survival are also discussed. Oversimplifications of biomechanical models could end up in results that divert from the natural behavior of teeth. Experimental validation models with close-to-reality boundary conditions should be developed to compare the validity of simplified models.

Site-specific characteristics of bone and progenitor cells in control and ovariectomized rats

One-third of postmenopausal women experience at least one osteoporotic bone fracture in their lifetime that occurs spontaneously or from low-impact events. However, osteoporosis-associated jaw bone fractures are extremely rare. It was also observed that jaw bone marrow stem cells (BMSCs) have a higher capacity to form mineralized tissues than limb BMSCs. At present, the underlying causes and mechanisms of variations between jaw bone and limb bone during postmenopause are largely unknown. Thus, the objective of the current study was to examine the site-specific effects of estrogen deficiency using comprehensive analysis of bone quantity and quality, and its association with characterization of cellular components of bone. Nine rats (female, 6 months old) for each bilateral sham and ovariectomy (OVX) surgery were obtained and maintained for 2 months after surgery. A hemi-mandible and a femur from each rat were characterized for parameters of volume, mineral density, cortical and trabecular morphology, and static and dynamic mechanical analysis. Another set of 5 rats (female, 9 months old) was obtained for assays of BMSCs. Following cytometry to identify BMSCs, bioassays for proliferation, and osteogenic, adipogenic, chondrogenic differentiation, and cell mitochondrial stress tests were performed. In addition, mRNA expression of BMSCs was analyzed. OVX decreased bone quantity and quality (mineral content, morphology, and energy dissipation) of femur while those of mandible were not influenced. Cellular assays demonstrated that mandible BMSCs showed greater differentiation than femur BMSCs. Gene ontology pathway analysis indicated that the mandibular BMSCs showed most significant differential expression of genes in the regulatory pathways of osteoblast differentiation, SMAD signaling, cartilage development, and glucose transmembrane transporter activity. These findings suggested that active mandibular BMSCs maintain bone formation and mineralization by balancing the rapid bone resorption caused by estrogen deficiency. These characteristics likely help reduce the risk of osteoporotic fracture in postmenopausal jawbone.

Cement Choice and the Fatigue Performance of Monolithic Zirconia Restorations

This study investigated the fatigue failure load of simplified monolithic yttria partially stabilized zirconia polycrystal restorations cemented to a dentin-like substrate using different luting systems. Disc-shaped ceramic (Zenostar T, 10 mm Ø × 0.7 mm thick) and dentin-like substrate (10 mm Ø × 2.8 mm thick) were produced and randomly allocated into eight groups, without or with thermocycling (TC=5-55°C/12,000×): "cement" (RelyX Luting 2 - glass ionomer cement [Ion], [Ion/TC]; RelyX U200 - self-adhesive resin cement [Self], [Self/TC]; Single Bond Universal+RelyX Ultimate - MDP-containing adhesive + resin cement [MDPAD + RC], [MDP-AD + RC/TC]; ED Primer II+Panavia F 2.0 - Primer + MDP-containing resin cement [PR + MDP-RC], [PR + MDP-RC/TC])). Each luting system was used as recommended by the manufacturer. Staircase methodology (20 Hz; 250,000 cycles) was applied for obtaining the fatigue failure loads. Fractographic characteristics were also assessed. At baseline, the Ion group presented the lowest fatigue load, although it was statistically similar to the Self group. The resin-based cement systems presented the highest fatigue performance, with the Ion group being only statistically equal to the Self group. Thermocycling influenced the groups differently. After aging, the MDP-AD + RC presented the highest mean, followed by the PR + MDP-RC and Self groups, while the Ion group had the lowest mean. Fractographic analysis depicted all failures as radial cracks starting at the zirconia intaglio surface. The luting system with MDP-containing adhesive applied prior to the resin cement presented the highest fatigue failure load after aging, presenting the best predictability of stable performance. Despite this, monolithic zirconia presents high load-bearing capability regardless of the luting agent.

Universal Chromatic Resin-Based Composites: Aging Behavior Quantified by Quasi-Static and Viscoelastic Behavior Analysis

Universal chromatic dental resin-based composites were recently developed in an attempt to speed up the restoration process with the aim of making it easier for the practitioner to decide on a suitable shade and to avoid time-consuming matching and mixing of materials. The way in which color is created in the analyzed universal chromatic materials is innovative, as it is not only induced by selective light absorption via pigments (Venus Diamond ONE, Venus Pearl ONE), as is usual in regular composites (Charisma Classic, Charisma Topaz, Venus, Venus Diamond), but also by selective light reflection via particularized microstructures (Omnichroma). Material properties were assessed at 24 h post-polymerization and after artificial aging. Flexural strength (n = 20) and modulus were measured in a 3-point-bending test and complemented with fractography and Weibull analysis. Quasi-static (Martens, Vickers, and indentation hardness; elastic and total indentation work; creep, indentation depth) and viscoelastic (storage, loss, and indentation moduli; loss factor) behavior (n = 6) was measured by a depth-sensing indentation test equipped with a DMA module. The nanoscale silica/zirconia polymer core-shell structure in the structural-colored material induces similar or poorer mechanical properties compared with pigment-colored materials, which is related to the higher polymer content. For all materials, aging shows a clear influence on the measured properties, with the degree of degradation depending on the measurement scale.

Biomechanical Comparison of Six Different Root-Analog Implants and the Conventional Morse Taper Implant by Finite Element Analysis

Taper implants differ greatly from anatomical teeth in shape. In this study, seven three-dimensional finite element models were established, including a conventional taper implant and six root-analog implants with different root numbers and shapes. Vertical, horizontal, and oblique instantaneous loads of 100 N were applied to the models to obtain stress distribution in the implant, mucosa, cortical bone, and cancellous bone. ANSYS was used to perform the analysis under hypothetical experimental conditions. We find the stresses in all the implants and surrounding tissues varied by loading direction, the sequence of stress magnitude is vertical load, oblique load, and then horizontal load. The maximum stress values in root-analog implants were significantly less than in the taper implant. Moreover, stress distribution in the former was equalized contrary to the concentrated stress in the latter. Root-analog implants with different root geometry also revealed a pattern: stresses in multiple-root implant models were lower than those in single-root implants under the same load. The implant with a long and rounded root distributed the stress more uniformly, and it was mainly concentrated on the implant itself and cancellous bone. However, the opposite effect was observed in the short implant on mucosa and cortical bone. The root geometry of anatomical teeth can modify their functions. A uniform-shaped implant can hardly meet their functional requirements. Thus, the root-analog implant could be a possible solution.

Comparison of modern light-curing hybrid resin-based composites to the tooth structure: Static and dynamic mechanical parameters

The study aims to compare the way modern resin-based composites (RBCs) respond to mechanical stress related to the tooth structure they are designed to replace. Eight representative light-cured RBCs, including ormocers, giomers, RBCs with nano and agglomerated nanoparticles, prepolymerized, or compact fillers, were selected. Flexural strength, FS and modulus/E, were measured in a three-point bending test. A fractographic analysis determined the origin of fracture. The quasi-static (indentation hardness/H_IT , indentation modulus/E_IT ) and viscoelastic (storage modulus/E', loss modulus/E″, loss factor/tan δ) behavior was assessed by a depth-sensing indentation test equipped with a dynamic-mechanical analysis module. One and multiple-way analysis of variance (ANOVA), Tukey honestly significant difference (HSD) post-hoc tests (α = 0.05), and Weibull statistics were applied. Parameter material exhibited the highest effect on E (p < .001, η_P ²  = .857), followed by FS (η_P ²  = .729), and the strain (η_P ²  = .553). Highest material reliability was identified in the RBCs with nano and agglomerated nanoparticles. The most frequent type of failure originated from volume (81.3%), followed by edge (10.6%), and corner (8.1%) flaws. Enamel evidenced three times higher H_IT , E_IT , and E' values as RBCs and dentin, and the smallest deviation from ideal elasticity. Ormocers exhibited the highest damping capacity, followed by the RBCs with prepolymerized fillers. Damping capacity and static mechanical properties are mutually exclusive. Analyzed RBCs and the tooth structure are better adapted to the relevant frequency for chewing than for higher frequencies. RBCs are comparable to dentin in terms of their mechanical performance, but apart from the damping behavior, they are far inferior to enamel. Damping ability of analyzed material could be exploited for correlation with the clinical behavior.

The effect of bolus size on masticatory parameters at swallowing threshold in children using a hard, solid, artificial test food

The effect of different bolus sizes on food breakdown has been studied in adults, but not in children. The objective of this study was to study median particle size (MPS) and other parameters of masticatory function at swallowing threshold (ST) in 8–10‐year‐old‐children with two different bolus sizes. A randomized crossover trial was undertaken in 89 eight to ten‐year‐old children. The study was performed with informed consent and ethical approval. The artificial test food used was made of a condensation silicone (Optosil Comfort) following a standardized protocol. Two bolus sizes (three or four quarters of a 20‐mm diameter, 5‐mm thick tablet) were randomized to avoid an order effect and tested in different sessions. Variables were: MPS (X₅₀) at ST, number of cycles until ST, sequence and cycle duration as well as cycles/g. Comparisons were performed with paired t and Wilcoxon tests, regressions and correlations were run. Cutoff for statistical significance was .05. Statistically significant differences were found for all variables; X₅₀ (2.5 ± 0.8 vs. 2.8 ± 0.7 mm, p < .001), cycles until ST (38 vs. 40, p = .022), sequence (25 vs. 27 s, p = .003), and cycle duration (650 vs. 683 ms, p = .015) and cycles/g (27 vs. 21 cycles/g, p < .001), three or four quarters, respectively. In conclusion, in children, as in adults, chewing on a bigger bolus size leads to a larger MPS (X₅₀) at ST. When chewing on a larger bolus the number of cycles increases, but not enough to swallow the same particle size since the number of cycles/g is less with a bigger bolus size.

ActiSight: Wearer Foreground Extraction Using a Practical RGB-Thermal Wearable

Wearable cameras provide an informative view of wearer activities, context, and interactions. Video obtained from wearable cameras is useful for life-logging, human activity recognition, visual confirmation, and other tasks widely utilized in mobile computing today. Extracting foreground information related to the wearer and separating irrelevant background pixels is the fundamental operation underlying these tasks. However, current wearer foreground extraction methods that depend on image data alone are slow, energy-inefficient, and even inaccurate in some cases, making many tasks-like activity recognition- challenging to implement in the absence of significant computational resources. To fill this gap, we built ActiSight, a wearable RGB-Thermal video camera that uses thermal information to make wearer segmentation practical for body-worn video. Using ActiSight, we collected a total of 59 hours of video from 6 participants, capturing a wide variety of activities in a natural setting. We show that wearer foreground extracted with ActiSight achieves a high dice similarity score while significantly lowering execution time and energy cost when compared with an RGB-only approach.

Investigation of the Damping Capabilities of Different Resin-Based CAD/CAM Restorative Materials

The aim of the present study was to evaluate and quantify the damping properties of common resin-based computer-aided design and computer-aided manufacturing (CAD/CAM) restorative materials (CRMs) and assess their energy dissipation abilities. Leeb hardness (HLD), together with its deduced energy dissipation data (HLD_dis), and loss tangent values recorded via dynamic mechanical analysis (DMA) were determined for six polymer, four composite, and one ceramic CRM as well as one metal. Data were statistically analyzed. Among resin-based CRMs, the significantly highest HLD_dis data were detected for the fiber-reinforced composite FD (p < 0.001) directly followed by the filler-reinforced Ambarino High Class (p < 0.001). The significantly lowest HLD_dis values were observed for the polymer-based CRM Telio CAD (p < 0.001). For loss tangent, both PEEK materials showed the significantly lowest data and the polymer-based M-PM the highest results with all composite CRMs in between. HLD_dis data, which simultaneously record the energy dissipation mechanism of plastic material deformation, more precisely characterize the damping behavior of resin-based CRMs compared to loss tangent results that merely describe viscoelastic material behavior. Depending on material composition, resin-based CRMs reveal extremely different ratios of viscoelastic damping but frequently show enhanced HLD_dis values because of plastic material deformation. Future developments in CAD/CAM restorative technology should focus on developing improved viscoelastic damping effects.

Home-Based Monitoring of Eating in Adolescents: A Pilot Study

Objectives: To investigate eating episodes in a group of adolescents in their home-setting using wearable electromyography (EMG) and camera, and to evaluate the agreement between the two devices. Approach: Fifteen adolescents (15.5 ± 1.3 years) had a smartphone-assisted wearable-EMG device attached to the jaw to assess chewing features over one evening. EMG outcomes included chewing pace, time, episode count, and mean power. An automated wearable-camera worn on the chest facing outwards recorded four images/minute. The agreement between the camera and the EMG device in detecting eating episodes was evaluated by calculating specificity, sensitivity, and accuracy. Main results: The features of eating episodes identified by EMG throughout the entire recording time were (mean (SD)); chewing pace 1.64 (0.20) Hz, time 10.5 (10.4) minutes, episodes count 56.8 (39.0), and power 32.1% (4.3). The EMG device identified 5.1 (1.8) eating episodes lasting 27:51 (16:14) minutes whereas the cameras indicated 2.4 (2.1) episodes totaling 14:49 (11:18) minutes, showing that the EMG-identified chewing episodes were not all detected by the camera. However, overall accuracy of eating episodes identified ranged from 0.8 to 0.92. Significance: The combination of wearable EMG and camera is a promising tool to investigate eating behaviors in research and clinical-settings.

Evaluation of the damping capacity of common CAD/CAM restorative materials

OBJECTIVES

To evaluate and quantify the damping capacities of common CAD/CAM restorative materials (CRMs) and to assess their energy dissipation abilities by comparing loss tangent and Leeb hardness data.

METHODS

Leeb hardness (HLD), together with its deduced energy dissipation data (HLD_dis), and loss tangent values recorded via dynamic mechanical analysis (DMA) were determined for 4 ceramic, 13 composite, and 2 polymer-based CRMs as well as 1 metal. For Leeb hardness, ten indentations per material were performed on two separate specimens (12.0 × 12.0 × 3.5 mm³) after water storage (24 h; 37.0 ± 1.0 °C). For DMA, ten specimens (16.00 × 4.00 × 1.00 mm³ ± 0.05 mm) per material were investigated in distilled water (37.0 ± 0.5 °C) with a dynamic force of 1 N at 1.5 Hz. Each data set was analyzed using two-way analysis of variance (ANOVA) with material type and material nested in material type as factors. Post-ANOVA contrasts were performed using a Bonferroni adjustment for multiple comparisons (α = 0.05). Correlations between different parameters were tested (Pearson, α = 0.05).

RESULTS

HLD_dis data revealed the significantly highest damping capacity for metal and the lowest values for ceramics with composites and polymers in between. However, for loss tangent, the metal together with lithium disilicate glass-ceramics exhibited the lowest damping effects and polymer materials the highest results with composites likewise in between. A strong dependency of the loss tangent results on the filler content of the investigated CRMs was indicated (r = - 0.822, p < 0.001), while a positive and only moderate correlation between loss tangent and HLD_dis was observed (r = 0.565, p < 0.001), which conversely revealed a very strong correlation (r = 0.911, p < 0.001) if the metal was excluded from the calculation.

CONCLUSIONS

Although HLD_dis and loss tangent values both allowed a distinct differentiation of the damping capabilities of various CRMs and the respective material types, HLD_dis data appeared to more accurately describe the damping capacity of CRMs as the energy dissipation mechanism of permanent plastic material deformation, that is commonly observed for metals and some composite-based CRMs, is equally captured. This finding could be particularly interesting for the future development of new CRMs with improved mechanical properties as HLD_dis data determination in principle is a very efficient and simple technique to entirely specify unknown damping capacities of materials.

Cyclic fatigue tests on non-anatomic specimens of dental ceramic materials: A scoping review

The aim of the present scoping review was to identify and discuss the methods, testing parameters, and characteristics used to induce cyclic fatigue on non-anatomic dental ceramic specimens. In vitro studies written in English which evaluated commercially-available non-anatomic dental ceramic specimens subjected to mechanical cyclic fatigue were selected. The search was performed in the PubMed, Scopus and Web of Science databases. The initial search yielded 1,636 articles, of which 81 were included. Based on the collected data, most of the included studies evaluated dental ceramic specimens cemented to supporting substrate (n= 42; 51.9%); used step-stress (n= 35; 42.2%) accelerated fatigue test, loading frequencies above 10 Hz (n= 31, 35.6%), stainless steel (n = 28, 32.6%) load applicator with spherical shaped tip 40 mm diameter (n= 25, 30.9%); applied only axial loads (n= 77, 95.1%); and considered a wet testing environment (n= 65, 78.3%). The definition of test geometry, method, and testing parameters must be cautiously considered according to the study objective and the scenario that is simulated. Accelerated fatigue tests, load frequencies up to 20 Hz, a 40 mm stainless steel spherical load applicator and a wet testing environment are the major common defined parameters presented in the existing literature. More studies exploring the influence of such factors on fatigue mechanism are necessary.

Fracture and viscoelastic behavior of novel self-adhesive materials for simplified restoration concepts

OBJECTIVE

The aim of the study was to offer a comparative perspective on the mechanical and viscoelastic behavior of currently developed materials for simplified restoration concepts. These materials have not yet been clearly assigned whether they are complex hybrids of already known material categories or new material classes.

METHODS

A dual-cured, bulk-fill, bioactive resin-based composite (alkasite), a resin-modified glass ionomer cements (RM-GIC) with novel polymerizable acid polymers, and a glass ionomer cement (GIC) with improved adaptation to an acidic environment were compared with regard to their macro-mechanical parameters (3-point bending test, 3-PBT), fracture mechanism, quasi-static and viscoelastic behaviour (instrumented indentation test, IIT), morphology and structural appearance of the filler system (SEM analysis). The influence of surface finishing was quantified on the outcome of the 3-PBT, while the influence of aging and frequency was monitored on the outcome of the IIT. One and multiple-way analysis of variance (ANOVA) with Tukey honestly significant difference (HSD) post-hoc tests (α = 0.05) and Weibull analysis were applied.

RESULTS

Surface finishing strongly influenced the outcome of the 3-PBT for RM-GIC and GIC but not of the alkasite. The highest material reliability (Weibull parameter m) was found with the alkasite, irrespective of the curing mode. Ground specimens showed decrease reliability, except for the alkasite in the light-cured mode. The predominant failure mode originated from sub-surface defects (52.5%), followed by corner (25%), edge (18.1%), and crack arrest (4.4%). The effect of the parameter material on the quasi-static outcome of the IIT was highest on elastic/plastic parameters (p < 0.001; e.g. elastic indentation work, η_P² = 0.875), was moderate on the Martens Hardness (η_P² = 0.420), and was low on the Vickers hardness (η_P² = 0.218). The viscoelastic parameters, in particular the loss factor (tan δ) allow a clear documentation of the ongoing acid-base setting reaction during aging of one month, which was more pronounced in the GIC than in the RM-GIC. The decrease in tan δ with aging for GIC and RM-GIC reflects the maturation process and increased brittleness, while the increase in tan δ with aging reflects the polymer plasticization in the polymer-based alkasite.

CONCLUSIONS

The mechanical and viscoelastic behavior depending on surface refinement, aging and frequency clearly allow to classify the currently developed materials for simplified restoration concepts into known material categories such as RBCs (alkasite), RM-GIC or GIC.

Three-Dimensional Finite Element Investigation into Effects of Implant Thread Design and Loading Rate on Stress Distribution in Dental Implants and Anisotropic Bone

Variations in the implant thread shape and occlusal load behavior may result in significant changes in the biological and mechanical properties of dental implants and surrounding bone tissue. Most previous studies consider a single implant thread design, an isotropic bone structure, and a static occlusal load. However, the effects of different thread designs, bone material properties, and loading conditions are important concerns in clinical practice. Accordingly, the present study performs Finite Element Analysis (FEA) simulations to investigate the static, quasi-static and dynamic response of the implant and implanted bone material under various thread designs and occlusal loading directions (buccal-lingual, mesiodistal and apical). The simulations focus specifically on the von Mises stress, displacement, shear stress, compressive stress, and tensile stress within the implant and the surrounding bone. The results show that the thread design and occlusal loading rate have a significant effect on the stress distribution and deformation of the implant and bone structure during clinical applications. Overall, the results provide a useful insight into the design of enhanced dental implants for an improved load transfer efficiency and success rate.

Microstructural dependence of mechanical properties and their relationship in modern resin-based composite materials

OBJECTIVE

The aim of the study was to determine the influence of the microstructure on the mechanical behavior of resin-based composites (RBC) with little variation in chemical composition, and to verify a possible mutual dependency.

METHODS

Four RBCs with similar chemical composition of fillers (Ba-Al-B-F-Si-glass, SiO₂), but with different mean (0.7 µm to 1.8 µm) and maximum particle size values (5 µm to 20 µm) were selected. Fracture toughness/K_IC was evaluated in an NTP (Notchless Triangular Prism) test. Flexural strength/FS and modulus/E were measured in a 3-point-bending test. A fractographic analysis determined the fracture mode and origin. Quasi-static and viscoelastic parameters were assessed by a depth-sensing indentation test equipped with a DMA-module. One and multiple-way analysis of variance (ANOVA), Tukey honestly significant difference (HSD) post-hoc tests (α = 0.05), and Weibull statistics were applied.

RESULTS

The influence of the microstructure and the chemical composition of the organic matrix is weighted differently in the measured properties. The inorganic filler amount influences highest K_IC (η_P²=0.813), followed by FS (η_P²=0.768) and E (η_P²=0.611). An optimized filler distribution with a high proportion of larger fillers up to 20 µm contributes to high FS and E data. The reliability of the material (Weibull parameter m) did not depend on the origin of fracture. Smaller inorganic fillers and the addition of 1% larger pre-polymer fillers improve the fracture toughness. Analysed RBCs were well adapted to chewing frequencies.

CONCLUSIONS

The properties of RBCs can be tuned by little variation in microstructure and monomer matrix.

CLINICAL SIGNIFICANCE

Reverse engineering of clinically successful materials can be used to better design future RBCs.

The extended present: an informational context for perception

Several previous authors have proposed a kind of specious or subjective present moment that covers a few seconds of recent information. This article proposes a new hypothesis about the subjective present, renamed the extended present, defined not in terms of time covered but as a thematically connected information structure held in working memory and in transiently accessible form in long-term memory. The three key features of the extended present are that information in it is thematically connected, both internally and to current attended perceptual input, it is organised in a hierarchical structure, and all information in it is marked with temporal information, specifically ordinal and duration information. Temporal boundaries to the information structure are determined by hierarchical structure processing and by limits on processing and storage capacity. Supporting evidence for the importance of hierarchical structure analysis is found in the domains of music perception, speech and language processing, perception and production of goal-directed action, and exact arithmetical calculation. Temporal information marking is also discussed and a possible mechanism for representing ordinal and duration information on the time scale of the extended present is proposed. It is hypothesised that the extended present functions primarily as an informational context for making sense of current perceptual input, and as an enabler for perception and generation of complex structures and operations in language, action, music, exact calculation, and other domains.

Impact of ultra-fast (3 s) light-cure on cell toxicity and viscoelastic behavior in a dental resin-based composite with RAFT-mediated polymerization

OBJECTIVE

The aim of the study was to determine the effects of ultra-fast (3 s) light-curing on the viscoelastic behaviour at clinically relevant frequencies, and cell toxicity, in a resin-based composite (RBC) with reversible addition-fragmentation-chain transfer (RAFT) mediated polymerization.

METHODS

Three different protocols were used to cure cylindrical samples (height = 4 mm, ϴ = 5 mm), including ultra-fast (3s) cure with high radiant emittance, 10 s and 20 s cure with moderate radiant emittance. The properties of the light curing device were evaluated in all curing protocols by spectrophotometry up to an exposure distance of 10 mm. The light transmission through the samples was determined in real-time with the same spectrophotometer. Absorbance was calculated as a function of wavelength. The quasi-static (indentation hardness/H_IT, indentation modulus/E_IT) and viscoelastic (storage modulus/E', loss modulus/E″, loss factor/tan δ) material behavior was determined in an instrumented indentation test with a DMA (Dynamic Mechanical Analysis) module for 10 frequencies (0.5-5 Hz) by profiling the center of the samples in 330 μm steps from top to bottom. Cellular toxicity on human gingival fibroblast (HGF-1) was assessed using a WST-1 colorimetric assay after incubation time of up to 3 months. One and multiple-way analysis of variance (ANOVA) with Tukey honestly significant difference (HSD) post-hoc tests (α = 0.05) were applied.

RESULTS

The irradiance transmitted through a 4 mm high sample was less than 7% of the incident irradiance, and the absorbance was similar for all curing protocols, showing a decrease with wavelength. Similar quasi-static and viscoelastic parameters were observed regardless of the curing protocol. H_IT increased slightly and E_IT, E', E″ and tan δ decreased with frequency. Occasionally, slightly higher confidence intervals were observed for the ultra-fast curing group, which were related to a potential accumulation of stress. The curing protocol had no effect on cell viability (p = 0.326) but the eluate age (p < 0.001, η_P² = 0.879) did. None of the groups showed cell toxicity at any point in time with respect to the corresponding negative control.

CONCLUSIONS

The ultra-fast curing with high irradiance induced no cell toxicity and an equivalent viscoelastic behavior as with conventional curing protocols in a RAFT-modified RBC.

NeckSense: A Multi-Sensor Necklace for Detecting Eating Activities in Free-Living Conditions

We present the design, implementation, and evaluation of a multi-sensor, low-power necklace, NeckSense, for automatically and unobtrusively capturing fine-grained information about an individual's eating activity and eating episodes, across an entire waking day in a naturalistic setting. NeckSense fuses and classifies the proximity of the necklace from the chin, the ambient light, the Lean Forward Angle, and the energy signals to determine chewing sequences, a building block of the eating activity. It then clusters the identified chewing sequences to determine eating episodes. We tested NeckSense on 11 participants with and 9 participants without obesity, across two studies, where we collected more than 470 hours of data in a naturalistic setting. Our results demonstrate that NeckSense enables reliable eating detection for individuals with diverse body mass index (BMI) profiles, across an entire waking day, even in free-living environments. Overall, our system achieves an F1-score of 81.6% in detecting eating episodes in an exploratory study. Moreover, our system can achieve an F1-score of 77.1% for episodes even in an all-day-long free-living setting. With more than 15.8 hours of battery life, NeckSense will allow researchers and dietitians to better understand natural chewing and eating behaviors. In the future, researchers and dietitians can use NeckSense to provide appropriate real-time interventions when an eating episode is detected or when problematic eating is identified.

Comparison of dynamic mechanical properties of dentin between deciduous and permanent teeth

Purpose: Even though differences between deciduous and permanent dentin have been widely studied, their dynamic mechanical behavior has never been compared. The objective of the present study was to quantify the differences between deciduous and permanent dentin under cyclic mechanical loading, which is similar to masticatory stress.Materials and Methods: Deciduous and permanent teeth, respectively from children (9 ~ 12 years old) and young people (18 ~ 25 years old), were wet-sectioned perpendicular to the longitudinal axis and the central specimens of coronal dentin were evaluated by nanoscopic dynamic mechanical analysis (nanoDMA).Results: The average storage, loss, and complex moduli, as well as the hardness of deciduous dentin were significantly (p < 0.05) lower than those of permanent dentin. Moreover, the tan δ value of permanent dentin was significantly (p < 0.05) lower than that of deciduous dentin across the loading frequency range, indicating that viscoelastic behavior and loss of elastic energy were significantly reduced in the stiffer permanent dentin. All the nanoDMA responses showed a significant influence of the dynamic loading frequency (p < 0.05): Both deciduous and permanent dentin showed reduced viscoelasticty with increased loading frequencies.Conclusions: Compared with deciduous dentin, permanent dentin exhibits higher stiffness with reduced energy loss during deformation, and therefore superior mechanical characteristics for the mastication process.

Relationship between chewing features and body mass index in young adolescents

BACKGROUND

Behavioural aspects of chewing may influence food intake, nutritional status and in turn body weight.

OBJECTIVES

The current study aimed to study chewing features in adolescents as they naturally occur in home-based settings, and to test for a possible association with weight status.

METHODS

Forty-two adolescents (15.3 ± 1.3 years) were recruited (21 with healthy-weight/21 with overweight). Using a smartphone-assisted wearable electromyographic device, the chewing features of each participant were assessed over one evening, including the evening meal, in their natural home setting.

RESULTS

The mean (±SD) for chewing pace was 1.53 ± 0.22 Hz, chewing power 30.1% ± 4.8%, number of chewing episodes 63.1 ± 36.7 and chewing time 11.0 ± 7.7 minutes. The chewing pace of the group with overweight was slower than that of healthy weight (-0.20 Hz; 95% CI, -0.06 to -0.33; P = .005) while their chewing time was shorter (-4.9 minutes; 95% CI, 0.2-9.7; P = .044). A significant negative correlation was observed between BMI z-score and chewing pace (R = -.41; P = .007), and between BMI z-score and chewing time (R = -0.32; P = .039).

CONCLUSION

The current study suggests that adolescents who are overweight eat at a slower pace for a shorter period of time than their counterparts who are a healthy weight. This unexpected finding based on objective data appears to conflict with existing questionnaire findings but provides impetus for further work testing the effectiveness of changing eating behaviour as a weight-management intervention in youth.

Comparative Analysis of Static and Viscoelastic Mechanical Behavior of Different Luting Material Categories after Aging

The longevity of indirect restorations is primarily determined by the appropriate selection of the luting material. The function of a luting material is to seal the restoration and hold it in place for the time required for service. The mechanical behavior of luting materials and in particular their aging behavior, therefore, play a decisive role. The study provides a comparative analysis of the static and dynamic mechanical behavior of the most commonly used luting material categories-zinc phosphate cement, glass-ionomer cement, resin-modified glass-ionomer cement, resin-based composites, and self-adhesive resin-based composites-and their aging behavior. It also takes into account that luting materials are viscoelastic materials, i.e., materials that respond to external loading in a way that lies between an elastic solid and a viscous liquid. Flexural strength and modulus were determined in a three-point bending test followed by fractography analysis. The quasi-static and viscoelastic behavior was analyzed by a depth-sensing indentation test provided with a dynamic mechanical analysis (DMA) module at 20 different frequencies (1-50 Hz). The fracture toughness was evaluated in a notchless triangular prism (NTP) test. Material type exhibits the strongest influence on all measured properties, while the effect of aging becomes more evident in the material reliability. The variation of the viscoelastic parameters with aging reflects cement maturation or polymer plasticization.

Frequency-related viscoelastic properties in high translucent CAD-CAM resin-based composites

OBJECTIVE

The study aims to quantify the viscoelastic properties of representative dental CAD/CAM resin-based composites (RBC) and to determine the effects of loading frequencies on the viscoelastic material response in comparison to clinically established CAD/CAM glass ceramics.

METHODS

Eight RBCs, one leucite-reinforced, and one lithium disilicate glass-ceramics were selected. The quasi-static (indentation hardness H_IT, indentation modulus, E_IT) and viscoelastic (storage modulus E', loss modulus E″, loss factor tan δ) material behavior was monitored by a depth-sensing indentation test equipped with a DMA module. A low-magnitude oscillating force was therefore superimposed onto a quasi-static force (F_max = 1000 mN) at 20 different frequencies in the range 1-50 Hz. One and multiple-way analysis of variance (ANOVA), the Tukey honestly significant difference (HSD) post-hoc tests (α = 0.05), and a Pearson correlation analysis were used for data analysis.

RESULTS

The quasi-static parameters increased with the crystalline phase in glass ceramics and with the amount of inorganic filler in RBCs. The tan δ, which is related to the damping capacity of a material, increased with the increasing amount of glass phase in glass ceramics or with the amount of organic phase in RBCs. A pronounced influence of the frequency on the measured parameters and their patterns of variation was observed. H_IT was up to ten time higher in glass ceramics compared to RBCs and highest at the lowest frequency (1 Hz). Parameters E_IT and E' differ less and were lowest at the lowest frequency. E″ distinguished three different patterns of variation with frequency. The tan δ decreased rapidly with frequency in glass ceramics, while the decrease in RBCs was gradually. Frequency influenced stronger tan δ (p < 0.001, η_P² = 0.85), followed by E' (p < 0.001, η_P² = 0.773), E_IT (p < 0.001, η_P² = 0.772), and E″ (p < 0.001, η_P² = 0.714), and less H_IT (p < 0.001, η_P² = 0.384).

CONCLUSIONS

All materials sowed viscoelastic behavior related to their microstructure and the internal friction created by grain or interphase boundary relaxation. RBCs have better damping capabilities over a wider frequency range. The deviations from the ideal elasticity were significantly lower in the glass ceramics than in the RBCs.

Effect of cyclic loading on reverse torque values of angled screw channel systems

STATEMENT OF PROBLEM

The angled screw channel concept has become popular. However, research is lacking as to how reverse torque values of nonaxially tightened implant crowns compare with axially tightened cement-retained crowns restored on angle-correcting abutments when subjected to long-term cyclic loading.

PURPOSE

The purpose of this in vitro study was to evaluate the ability of different 25-degree angled screw channel hexalobular systems to apply the target torque value on their screws, the effect of cyclic loading on their reverse torque values, and their survival compared with crowns cemented on conventional 0-degree screw channel abutments.

MATERIAL AND METHODS

A total of 28 implants were divided into 4 groups. Twenty-one angled screw channel crowns were fabricated at a 25-degree angle correction by using angled titanium (Ti) bases by 3 manufacturers DY (Dynamic Tibase), DE (AngleBase), and ASC (Angulated Screw Channel) (n=7). The fourth group, UB (Universal Base, Control), had cement-retained crowns with 25-degree custom-milled, angled zirconia abutments that were cemented onto their respective Ti bases (n=7). All implants were embedded in epoxy resin blocks and tightened to manufacturer recommended values: 35 Ncm for ASC, UB, and DE and 25 Ncm for DY. Initial torque values (ITV1) were recorded. After 24 hours, the reverse torque values (24hr-RTV1) were recorded. A new set of screws was then used for each group, and the initial torque values (ITV2) were recorded. Specimens were loaded at 2 Hz for 5 million cycles under a 200-N load, and reverse torque values (RTV2) were recorded. ANOVA (α=.05) was used to compare differences in the means of deviation of initial torque values and means of reverse torque values followed by a Tukey-Kramer post hoc analysis (α=.05). Preload efficiency was calculated for each system (RTV2/ITV2), and a survival analysis was performed by using the Lifetest procedure.

RESULTS

A significant difference in the means of deviation of initial torque values of the groups with 25-degree torque application (DY, DE, and ASC) was found when compared with UB at 0 degrees. ASC and DE had lower initial torque values than UB (P<.001 and P=.003 for ASC ITV1 and ITV2, P<.001 and P=.006 for DE ITV1 and ITV2). A significant difference was found in mean reverse torque values both for after 24 hours and after cyclic loading among all groups (P<.001). A significant difference was found between mean reverse torque values before and after cyclic loading for each group (P<.001). Preload efficiency was 43.8% for DY, 46.8% for DE, 54.2% for ASC, and 48.5% for UB. No significant difference was found in the time-to-failure survival among groups (P>.05).

CONCLUSIONS

The hexalobular system of DY delivered comparable initial torque values to its target value at 25 degrees, similar to how UB (control group) delivered at 0 degrees. ASC and DE scored lower initial torque values than their target value compared with UB. The DY abutment, which had a lower manufacturer recommended torque value, had lower reverse torque values compared with those of other groups. Time-to-failure survival of all groups was similar. Fractures at the zirconia to titanium base connection were seen with ASC crowns.

"Automatic Ingestion Monitor Version 2" - A Novel Wearable Device for Automatic Food Intake Detection and Passive Capture of Food Images

Use of food image capture and/or wearable sensors for dietary assessment has grown in popularity. "Active" methods rely on the user to take an image of each eating episode. "Passive" methods use wearable cameras that continuously capture images. Most of "passively" captured images are not related to food consumption and may present privacy concerns. In this paper, we propose a novel wearable sensor (Automatic Ingestion Monitor, AIM-2) designed to capture images only during automatically detected eating episodes. The capture method was validated on a dataset collected from 30 volunteers in the community wearing the AIM-2 for 24h in pseudo-free-living and 24h in a free-living environment. The AIM-2 was able to detect food intake over 10-second epochs with a (mean and standard deviation) F1-score of 81.8 ± 10.1%. The accuracy of eating episode detection was 82.7%. Out of a total of 180,570 images captured, 8,929 (4.9%) images belonged to detected eating episodes. Privacy concerns were assessed by a questionnaire on a scale 1-7. Continuous capture had concern value of 5.0 ± 1.6 (concerned) while image capture only during food intake had concern value of 1.9 ±1.7 (not concerned). Results suggest that AIM-2 can provide accurate detection of food intake, reduce the number of images for analysis and alleviate the privacy concerns of the users.

Influence of bolus size and chewing side on temporomandibular joint intra-articular space during mastication

Previous studies suggested that, during mastication, magnitude and location of mechanical load in the temporomandibular joint (TMJ) might depend on chewing side and bolus size. Aim of this study was to dynamically measure the TMJ space while chewing on standardized boluses to assess the relationship among minimum intra-articular distances (MID), their location on the condylar surface, bolus size, and chewing side. Mandibular movements of 12 participants (6f, 24±1y.o.; 6 m, 28±6y.o.) were tracked optoelectronically while chewing unilaterally on rubber boluses of 15 × 15 × 5, 15 × 15 × 10, and 15 × 15 × 15 mm³ size. MID and their location along the main condylar axis were determined with dynamic stereometry. MID were normalized on the intra-articular distance in centric occlusion. Repeated measures ANOVA (α = 0.05) showed that MID were smaller on the balancing (0.74±0.19) than on the working condyle (0.89±0.16) independently of bolus size (p < 0.0001). MIDs did not differ between 5 and 10 mm bolus thicknesses (0.80±0.17) but increased for 15 mm (0.85±0.22, p = 0.024) and were located mostly laterally, close to the condylar center. This study confirmed higher reduction of TMJ space on the balancing than on the working condyle during mastication. Intra-articular distances increased significantly for the greatest bolus thickness. Loaded areas were located laterally, for both working and balancing joint.

Effect of resin coating on highly viscous glass ionomer cements: A dynamic analysis

OBJECTIVES

This study determined the effects of self-adhesive resin coatings on viscoelastic properties of highly viscous glass ionomer cements (HVGICs) using dynamic mechanical analysis.

MATERIALS AND METHODS

The HVGICs evaluated were Zirconomer [ZR] (Shofu), Equia Forte [EQ] (GC) and Riva [RV] (SDI). Sixty specimens (12mm x 2mm x 2mm) of each material were fabricated using customized Teflon molds. After initial set, the specimens were removed from their molds, finished, measured and randomly divided into 3 groups of 20. Half the specimens in each group were left uncoated while the remaining half was covered with the respective manufacturers' resin coating. The specimens were subsequently conditioned in distilled water, artificial saliva or citric acid at 37°C for 7 days. The uncoated and coated specimens (n=10) were then subjected to dynamic mechanical testing in flexure mode at 37°C with a frequency of 0.1 to 10Hz. Storage modulus, loss modulus and loss tangent data were subjected to normality testing and statistical analysis using one-way ANOVA/Scheffe's post-hoc test and Ttest at significance level p<0.05.

RESULTS

Mean storage modulus ranged from 1.39 ± 0.36 to 10.80 ± 0.86 GPa while mean loss modulus varied from 0.13 ± 0.03 to 0.70 ± 0.14 GPa after conditioning in the different mediums. Values for loss tangent ranged from 39.4 ± 7.75 to 213.2 ± 20.11 (x10 -3 ). Significant differences in visco-elastic properties were observed between mediums and materials. When conditioned in distilled water and artificial saliva,storage modulus was significantly improved when ZR, EQ and RV were uncoated. Significantly higher values were, however, observed with resin coating when the materials were exposed to citric acid.

CONCLUSION

The visco-elastic properties of HVGICs were influenced by both resin coating and chemical environment.