Systematic review of chewing simulators: Reality and reproducibility of in vitro studies

Additive manufacturing of ceria and yttria incorporated toughened monolithic zirconia dental ceramic crowns: In vitro simulated aging behavior

STATEMENT OF PROBLEM

The comprehensive characterization of additively manufactured zirconia-based dental prostheses can promote widespread clinical application. However, simulated in vitro analysis of the aging behavior is lacking.

PURPOSE

The purpose of this in vitro study was to assess the simulated in vitro durability of monolithic transformation toughened additively manufactured zirconia-based restorations with different compositions to predict the clinical reliability depending on their ceramic composition.

MATERIAL AND METHODS

Crowns were 3-dimensionally (3D) printed by using a combination of custom-made stereolithography and a laser polymerized digital light processing process with high solid content slurries with suitable photo-interactive polymers. The main characteristics tested for mechanical behavior (structural reliability and flexural strength) were overall toughness and fatigue limits. Combinations of chemical compositions including yttria and ceria additives and processing conditions including pressing and sintering temperatures were applied to transform custom stereolithography and digitally light activated polymerized green parts to high strength and toughened ceramic crowns. The fluctuations in strength and toughness of as-sintered parts before and after physical thermocycling, physiochemical hydrothermal aging, and mechanical mastication simulation were studied via statistical methods (ANOVA) to indicate variable dependencies (α=.05).

RESULTS

Near theoretical density as high as 99.1%, minimum surface porosity as low as 0.25%, medium translucency, and high contrasts were achieved. The high original hardness near 19 GPa, a toughness of 6 to 7 MPa.m1/2, and 1300 MPa flexural strength with 95% confidence interval in as-sintered specimens satisfied the requirements for crowns. The simultaneously yttria and ceria stabilized systems should be able to resist low-temperature degradation aging with decreases as small as 2% in flexural strength and near 25% in fatigue fracture limits. The structure and process dependency of the mechanical properties of flexural strength (P<.020), hardness (P<.030), and modulus of elasticity (P<.020) were statistically significant while the toughness showed significant dependency (P≤.001).

CONCLUSIONS

The 3D printed posterior crowns with enhanced mechanical properties and augmented simulated in vitro durability can be manufactured by adding tetragonal phase stabilizer oxides (ceria and yttria) to zirconia-based ingredients. The combination of both oxide stabilizers in the additive manufacturing of crowns is a significant approach to improving clinical performance, enhanced toughness, and fatigue limit before and after physicomechanical, mechanochemical, and thermocyclic aging analysis.

Temporomandibular joint biomechanics and equine incisor occlusal plane maintenance

In equine dentistry, the physiological incisor occlusal surface is visually perceived as a plane with a distinct inclination to the head's coronal plane, extending rostro-ventrally to caudo-dorsally. To better understand the formation of this inclined plane and its connection to dental wear, we investigated the hypothesis that it arises from masticatory movements and the considerable distance between mandibular articular heads and the incisor occlusal surfaces, acting as the three points of support for the mandibles. Leveraging data from a large-scale clinical study involving static and dynamic orthodontic measurements in horses, we approximated the mandibular movement range where incisor occlusion and dental wear occur. By introducing and testing a segment coordinate system, we explored possible angular deviations from the occlusal plane caused by mandibular roll and pitch rotations during two lateral mandibular movement patterns, protrusion and retrusion. Theoretical biomechanical calculations and simulations confirmed the visual perception of the incisor occlusal surface as a plane. To further examine our assumptions, we employed a simple mechanical simulator to assess incisor normal occlusion and provoked malocclusions (diagonal, smile, and frown bite) by modifying temporomandibular joint (TMJ) movement patterns. The results from clinical investigations were corroborated by both the theoretical analysis and mechanical simulations, strengthening our understanding of the biomechanical basis behind the physiological incisor occlusal plane maintenance in horses. These findings have significant implications for equine dental health and contribute to a thorough understanding of TMJ dynamics.

Impact of Contracted Endodontic Access Cavities on the Fracture Resistance of Endodontically Treated Teeth After Mechanical Aging by Simulated Chewing Forces

INTRODUCTION

This in vitro study aimed to comparatively evaluate the fracture resistance of contracted endodontic cavities (CECs) versus traditional endodontic cavities (TECs) in mandibular molars after subjecting the samples to a chewing simulator.

METHODS

A total of 24 freshly extracted human mandibular molars were included in the study. Teeth with intact crowns and mature root apices that were devoid of caries, attrition, restorations, and cracks were selected and randomly assigned to 3 groups (n = 8) as follows: Group 1: TECs, Group 2: CECs, and Group 3: intact teeth (control group). After endodontic treatment, the teeth were restored with EverX bulk-fill composite and layered occlusally with nanohybrid composite SolareX and subjected to a chewing simulator where 240,000 masticatory cycles were simulated, which translates to 1 year of clinical function. The teeth were then subjected to static loading in a universal testing machine and the maximum load to fracture and the pattern of failure (restorable/unrestorable) were recorded. Data were evaluated with analysis of variance and the Tukey post hoc test for multiple comparisons.

RESULTS

The CEC group had higher fracture resistance when compared with the TEC group; however, the difference was not statistically significant. The fracture resistance of the samples in the control group was statistically higher than those in the experimental groups (P < .005).

CONCLUSIONS

There was no difference observed in the fracture resistance of mandibular molars with TECs and CECs subjected to masticatory loading.

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.

Wear effects between polymethyl methacrylate occlusal splints and opposing dentin surfaces during bruxism mimicking events

OBJECTIVES

To compare the surface wear rate between polymethyl methacrylate (PMMA)-based occlusal splints and opposing dentin-exposed teeth in bruxism simulating models.

MATERIALS AND METHODS

PMMA-based occlusal splints and extracted premolars were tested on a chewing stimulator at 30,000 or 60,000 cycles. Dentin wear was measured under a stereomicroscope and PMMA wear was determined with an optical profilometer. In addition, wear surface topography was assessed and quantified by scanning electron microscopy (SEM).

RESULTS

Wear rate of PMMA was significantly greater (11 times) than that of dentin specimens at 60,000 cycles, though these findings were not observed at 30,000 cycles. When comparing wear rates within each group at different duration cycles, PMMA surfaces exhibited an average wear rate 1.4 times higher with high duration cycles, while dentin surfaces displayed a marginal decrease in wear. In SEM micrographs, PMMA surfaces displayed more wear abrasion lines with higher duration cycles. However, dentin surfaces did not exhibit major differences between low and high duration cycles.

CONCLUSION

Wear rate on PMMA-based occlusal splints remarkably increases upon high chewing cycles mimicking bruxism comparing with the rate on dentin. Hence, it is reasonable for bruxing patients to wear single-arch PMMA-based occlusal splints to protect opposing dentin-exposed teeth.

Reliability and lifetime of lithium disilicate, 3Y-TZP, and 5Y-TZP zirconia crowns with different occlusal thicknesses

OBJECTIVES

To assess the probability of survival of crowns made with a 3Y-TZP, a 5Y-TZP, and a lithium disilicate.

MATERIALS AND METHODS

CAD-CAM premolar crowns with occlusal thicknesses of 1.0 mm or 1.5 mm and cemented on a metal dye. Step-stress accelerated life testing (SSALT) was performed to access the use level probability Weibull curves and reliability were calculated for the completion of a mission of 100,000 cycles at 300 to 1200 N.

RESULTS

All ceramics showed a high probability of survival (87-99%) at 300 N, irrespective of thickness. 3Y-TZP shows no significant decrease in the probability of survival up to 1200 N (83-96%). Lithium disilicate presented lower reliability than zirconia under the 600 N mission. 5Y-TZP showed lower reliability than 3Y-TZP at 1200 N. There was no significant difference in the Weibull modulus (3.23-7.12). 3Y-TZP had the highest characteristic strength (2483-2972 N), followed by 5Y-TZP (1512-1547 N) and lithium disilicate (971-1154 N).

CONCLUSION

Zirconia ceramics have a high probability of survival (up to 900 N-load), while lithium disilicate survives up to 300 N load only, irrespective of the thickness (1.0 mm or 1.5 mm).

CLINICAL RELEVANCE

The probability of survival of posterior crowns made with zirconia ceramics resists extreme loads while glass ceramics resist normal chewing loads. In addition, crowns with thinner occlusal face showed sufficient mechanical behavior.

Effect of Simulated Mastication on Structural Stability of Prosthetic Zirconia Material after Thermocycling Aging

Recent trends to improve the aesthetic properties-tooth-like color and translucency-of ceramic dental crowns have led to the development of yttria-stabilized zirconia (Y-TZP) materials with higher stabilizer content. These 5Y-TZP materials contain more cubic or t' phase, which boosts translucency. The tradeoff as a consequence of a less transformable tetragonal phase is a significant reduction of strength and toughness compared to the standard 3Y-TZP composition. This study aims at determining the durability of such 5Y-TZP crowns under lab conditions simulating the conditions in the oral cavity during mastication and consumption of different nutrients. The test included up to 10,000 thermal cycles from 5 °C to 55 °C "from ice cream to coffee" and a chewing simulation representing 5 years of use applying typical loads. The investigation of the stress-affected zone at the surface indicates only a very moderate phase transformation from tetragonal to monoclinic after different varieties of testing cycles. The surface showed no indication of crack formation after testing. It can, therefore, be assumed that over the simulated period, dental crowns of 5Y-TZP are not prone to fatigue failure.

Wear Properties of Conventional and High-Translucent Zirconia-Based Materials

This study investigated the two-body wear resistance of a first generation 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP), a second generation 3Y-TZP, a third generation 4 mol% yttria partially stabilized zirconia (4Y-PSZ), a 5 mol% yttria partially stabilized zirconia (5Y-PSZ), and a type III gold alloy (Aurocast 8), performed using opposing antagonistic cusps made out of the same material. Eight cylindrical specimens were prepared for each material (n = 8) for a total of forty specimens (N = 40). Conical cusps were fabricated for each material. Each cylinder−cusp pair was arranged inside a two-axis chewing simulator over up to 360,000 loading cycles. The wear resistance was analyzed by measuring the vertical substance loss (mm) and the volume loss (mm3). The antagonist wear (mm) was recorded before and after the wear test to evaluate the linear difference. Statistical analysis was performed using one-way analysis of variance (ANOVA); multiple comparisons were performed according to Tukey’s method. No statistically significant differences (p > 0.05) among the first generation 3Y-TZP, second generation 3Y-TZP, and 4Y-PSZ wear were found. 5Y-PSZ showed statistically significant higher wear compared to other the zirconias. Aurocast 8 displayed the highest values in terms of vertical wear, antagonist cusp wear, and volumetric loss. Although still not statistically comparable, the wear behavior of the latest 5Y-PSZ was the closest to the widely recognized gold standard represented by the type III gold alloy.

Evaluation of a Method to Determine Wear Resistance of Class I Tooth Restorations during Cyclic Loading

The aim of this study was the development of a test regime to determine the wear resistance and predict the clinical performance of conventional glass ionomer cement (GIC) restorations in Class I tooth cavities. Cavities were prepared in excised human teeth and restored using three conventional glass ionomer restorative materials: DeltaFil, Fuji IX GP and Ketac Universal. The restored teeth were mechanically and thermally stressed using a chewing simulator with a maximum number of 1,200,000 load cycles. Besides determining the number of cycles achieved, the abrasion volume after termination of the chewing simulation was calculated using µCT images. All teeth restored with DeltaFil reached 1,200,000 cycles without any restoration failure. Only 37.5% of the restorations each with Ketac Universal and Fuji IX GP were able to achieve the maximum cycle number. A significant lower abrasion volume for restorations with DeltaFil compared to Ketac Universal (p = 0.0099) and Fuji IX GP (p = 0.0005) was found. Laboratory chewing simulations are a useful tool to study basic wear mechanisms in a controlled setting with in-vivo related parameters. DeltaFil shows an improved wear resistance compared to other conventional GICs, indicating the high potential of this material for long-lasting Class I restorations.

Functional Role of AKNA: A Scoping Review

Human akna encodes an AT-hook transcription factor whose expression participates in various cellular processes. We conducted a scoping review on the literature regarding the functional role of AKNA according to the evidence found in human and in vivo and in vitro models, stringently following the "PRISMA-ScR" statement recommendations.

METHODS

We undertook an independent PubMed literature search using the following search terms, AKNA OR AKNA ADJ gene OR AKNA protein, human OR AKNA ADJ functions. Observational and experimental articles were considered. The selected studies were categorized using a pre-determined data extraction form. A narrative summary of the evidence was produced.

RESULTS

AKNA modulates the expression of CD40 and CD40L genes in immune system cells. It is a negative regulator of inflammatory processes as evidenced by knockout mouse models and observational studies for several autoimmune and inflammatory diseases. Furthermore, AKNA contributes to the de-regulation of the immune system in cancer, and it has been proposed as a susceptibility genetic factor and biomarker in CC, GC, and HNSCC. Finally, AKNA regulates neurogenesis by destabilizing the microtubules dynamics.

CONCLUSION

Our results provide evidence for the role of AKNA in various cellular processes, including immune response, inflammation, development, cancer, autoimmunity, and neurogenesis.