Comparison of mechanical property and role between enamel and dentin in the human teeth

Dentin Mechanobiology: Bridging the Gap between Architecture and Function

It is remarkable how teeth maintain their healthy condition under exceptionally high levels of mechanical loading. This suggests the presence of inherent mechanical adaptation mechanisms within their structure to counter constant stress. Dentin, situated between enamel and pulp, plays a crucial role in mechanically supporting tooth function. Its intermediate stiffness and viscoelastic properties, attributed to its mineralized, nanofibrous extracellular matrix, provide flexibility, strength, and rigidity, enabling it to withstand mechanical loading without fracturing. Moreover, dentin's unique architectural features, such as odontoblast processes within dentinal tubules and spatial compartmentalization between odontoblasts in dentin and sensory neurons in pulp, contribute to a distinctive sensory perception of external stimuli while acting as a defensive barrier for the dentin-pulp complex. Since dentin's architecture governs its functions in nociception and repair in response to mechanical stimuli, understanding dentin mechanobiology is crucial for developing treatments for pain management in dentin-associated diseases and dentin-pulp regeneration. This review discusses how dentin's physical features regulate mechano-sensing, focusing on mechano-sensitive ion channels. Additionally, we explore advanced in vitro platforms that mimic dentin's physical features, providing deeper insights into fundamental mechanobiological phenomena and laying the groundwork for effective mechano-therapeutic strategies for dentinal diseases.

Unraveling the structural complexity of and the effect of calcination temperature on calcium phosphates derived from Oreochromis niloticus bones

In this study, the interplay between the structural complexity, microstructure, and mechanical properties of calcium phosphates (CaPs) derived from fish bones, prepared at various calcination temperatures, and their corresponding sintered ceramics was explored. Fourier-transform infrared analysis revealed that the calcined powders primarily consisted of hydroxyapatite (HAp) and carbonated calcium hydroxyapatite, with an increasing concentration of Mg-substituted β-tricalcium phosphate (β-TCP) as the calcination temperature was increased. X-ray diffraction patterns showed enhanced sharpness of the peaks at higher temperatures, indicating a larger crystallite size and improved crystallinity. The ceramics exhibited a significantly larger crystallite size and an increased concentration of the β-TCP phase. Rietveld analysis revealed a larger volume of the β-TCP phase in the ceramics than in their calcined powders; this could be attributed to a newly formed β-TCP phase due to the decomposition of HAp. Extended X-ray absorption fine structure analysis revealed the incorporation of Mg in the Ca2 site of HAp, Ca2 site of β-TCP, and Ca5 site of β-TCP, with a higher substitution of Mg in the Ca5 site of β-TCP at elevated temperatures. The mechanical properties of HAp ceramics can be improved by increasing the calcination temperature because of their improved relative density and dense porous structure at elevated temperatures. This comprehensive investigation sheds light on the phase evolution, microstructural changes, and consequential impact on the mechanical properties of CaPs derived from fish bones, thereby facilitating the development of tailored CaP ceramics for biomedical applications.

Microhardness Measurements on Tooth and Alveolar Bone in Rodent Oral Disease Models

The mechanical property, microhardness, is evaluated in dental enamel, dentin, and bone in oral disease models, including dental fluorosis and periodontitis. Micro-CT (µCT) provides 3D imaging information (volume and mineral density) and scanning electron microscopy (SEM) produces microstructure images (enamel prism and bone lacuna-canalicular). Complementarily to structural analysis by µCT and SEM, microhardness is one of the informative parameters to evaluate how structural changes alter mechanical properties. Despite being a useful parameter, studies on microhardness of alveolar bone in oral diseases are limited. To date, divergent microhardness measurement methods have been reported. Since microhardness values vary depending on the sample preparation (polishing and flat surface) and indentation sites, diverse protocols can cause discrepancies among studies. Standardization of the microhardness protocol is essential for consistent and accurate evaluation in oral disease models. In the present study, we demonstrate a standardized protocol for microhardness analysis in tooth and alveolar bone. Specimens used are as follows: for the dental fluorosis model, incisors were collected from mice treated with/without fluoride-containing water for 6 weeks; for ligature-induced periodontal bone resorption (L-PBR) model, alveolar bones with periodontal bone resorption were collected from mice ligated on the maxillary 2nd molar. At 2 weeks after the ligation, the maxilla was collected. Vickers hardness was analyzed in these specimens according to the standardized protocol. The protocol provides detailed materials and methods for resin embedding, serial polishing, and indentation sites for incisors and alveolar. To the best of our knowledge, this is the first standardized microhardness protocol to evaluate the mechanical properties of tooth and alveolar bone in rodent oral disease models.

Three-Dimensional Printed Teeth in Endodontics: A New Protocol for Microcomputed Tomography Studies

This study aimed to describe a support material removal protocol (SMRP) from inside the root canals of three-dimensional printed teeth (3DPT) obtained by the microcomputed tomography (microCT) of a natural tooth (NT), evaluate its effectiveness by comparing the 3DPT to NT in terms of internal anatomy and behaviour toward endodontic preparation, and evaluate if 3DPT are adequate to assess the differences between two preparation systems. After the SMRP, twenty 3DPT printed by PolyJet™ were microCT scanned before preparation and thereafter randomly assigned into two groups (n = 10). One group and NT were prepared using ProTaper Gold® (PTG), and the other group with Endogal® (ENDG). MicroCT scans were carried out after preparation, and the volume increase, volume of dentin removed, centroids, transportation, and unprepared areas were compared. For the parameters evaluated, no significant differences were found between the 3DPT and NT before and after preparation (p > 0.05), and no significant differences were found between the 3DPT PTG group and the 3DPT ENDG group (p > 0.05). It can be concluded that the SMRP described is effective in removing the support material SUP706B™. PolyJet™ is adequate for printing 3DPT. Furthermore, 3DPT printed with high-temperature RGD525™ have similar behaviour during endodontic preparation with PTG as the NT, and 3DPT can be used to compare two preparation systems.

Rapid Deposition of the Biomimetic Hydroxyapatite-Polydopamine-Amino Acid Composite Layers onto the Natural Enamel

In this work, we developed a technology that enables rapid deposition of biomimetic composite films onto natural enamel slices (known as biotemplates). These films are composed of polydopamine (PDA) and nanocrystalline carbonate-substituted hydroxyapatite (nano-cHAp) that have been functionalized with amino acid l-Arginine. We utilized atomic force microscopy (AFM) and scattering scanning near-field optical microscopy (s-SNOM) combined with infrared (IR) synchrotron to achieve nanoscale spatial resolution for both IR absorption and topography analyses. This combined analytical modality allowed us to understand how morphology connects to local changes in the chemical environment on the biotemplate surface during the deposition of the bioinspired coating. Our findings revealed that when using the proposed technology and after the deposition of the first PDA layer, the film formed on the enamel surface nearly covers the entire surface of the specimen whose thickness is larger on the surface of the emerging enamel prisms. Calculation of the crystallinity index for the biomimetic layer showed a multiple increase compared with natural enamel. This indicates regular and dense aggregation of nano-cHAp into larger crystals, imitating the morphology of natural enamel rods. The microhardness of the formed PDA-based biomimetic layer mineralized with nano-cHAp functionalized with amino acid l-Arginine deposited on natural enamel was practically the same as that of natural enamel. The characterization of nano-cHAp-amino acid-PDA layers using IR and Raman microspectroscopy showed that l-arginine acts as a conjunction agent in the formation of mineralized biomimetic composite coatings. The uniformity of the mechanisms of PDA layer formation under different deposition conditions and substrate types allows for the formation of coatings regardless of the macro- and micromorphology of the template. Therefore, the results obtained in this work have a high potential for future clinical applications in dental practice.

Effect of different storage media on elemental analysis and microhardness of cervical cavity margins restored with a bioactive material

Objectives

This study aimed to investigate the elemental analysis and microhardness of a bioactive material (Activa) and marginal tooth structure after storage in different media.

Materials and Methods

Fifteen teeth received cervical restorations with occlusal enamel and gingival dentin margins using the tested material bonded with a universal adhesive, 5 of them on the 4 axial surfaces and the other 10 on only the 2 proximal surfaces. The first 5 teeth were sectioned into 4 restorations each, then stored in 4 different media; deionized water, Dulbecco's phosphate buffered saline (DPBS), Tris buffer, and saliva. The storage period for deionized water was 24 hours while it was 3 months for the other media. Each part was analyzed by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis for different substrates/distances and the wt% of calcium, phosphorus, silica, and fluoride were calculated. The other 10 teeth were sectioned across the restoration, stored in either Tris buffer or saliva for 24 hours or 3 months, and were evaluated for microhardness of different substrates/areas. Data were analyzed using analysis of variance and Tukey's post hoc test.

Results

Enamel and dentin interfaces in the DPBS group exhibited a significant increase in calcium and phosphorus wt%. Both silica and fluoride significantly increased in tooth structure up to a distance of 75 μm in the 3-month-media groups than the immediate group. Storage media did not affect the microhardness values.

Conclusions

SEM-EDS analysis suggests an ion movement between Activa and tooth structure through a universal adhesive while stored in DPBS.

Mechanical properties evaluation of two resin-based three-dimensional printing materials and dentine for endodontic training model fabrication: In vitro study

The objective of the study was to compare 4 mechanical properties of two 3-D printing resins with dentine. Four mechanical tests were performed on dentine specimens, VeroClear resin and Splint resin with 15 samples each. Vickers hardness test, samples were subjected to a 300-g load for 10 s. Flexural strength test, 8-millimetre, beam-shaped specimens were tested using the three-point bending method. Drilling force was measured on flat-surface specimens. Intra-canal torque was measured on specimens imitating root canals. The results were analysed using the Kruskal-Wallis and Mann-Whitney tests with adjusted Bonferroni's p-value (p < 0.05). Dentine exhibited the highest mechanical properties in all the tests. VeroClear's flexural strength and drilling force were higher, while the surface hardness was lower than that of Splint resin. The intra-canal torque of dentine and VeroClear resin were not significant and higher than that of Splint resin. VeroClear's mechanical properties were closer to dentine than Splint resin.

Biomechanical performance of resin composite on dental tissue restoration: A finite element analysis

This study investigates the biomechanical performance of various dental materials when filled in different cavity designs and their effects on surrounding dental tissues. Finite element models of three infected teeth with different cavity designs, Class I (occlusal), Class II mesial-occlusal (MO), and Class II mesio-occluso-distal (MOD) were constructed. These cavities were filled with amalgam, composites (Young's moduli of 10, 14, 18, 22, and 26 GPa), and glass carbomer cement (GCC). An occlusal load of 600 N was distributed on the top surface of the teeth to carry out simulations. The findings revealed that von Mises stress was higher in GCC material, with cavity Class I (46.01 MPa in the enamel, 23.61 MPa in the dentin), and for cavity Class II MO von Mises stress was 43.64 MPa, 39.18 MPa in enamel and dentin respectively, while in case of cavity Class II MOD von Mises stress was 44.67 MPa in enamel, 27.5 in the dentin. The results showed that higher stresses were generated in the non-restored tooth compared to the restored one, and increasing Young's modulus of restorative composite material decreases stresses in enamel and dentin. The use of composite material showed excellent performance which can be a good viable option for restorative material compared to other restorative materials.

Wear characteristics of esthetic resin matrix and zirconia reinforced lithium disilicate CAD/CAM materials: two-body wear and surface topography analysis

BACKGROUND

This in vitro study assessed the wear behavior of different CAD-CAM blocks and the abrasion of the enamel antagonist against these materials.

METHODS

64 disk-shaped specimens were prepared from 8 different CAD/CAM blocks as follow: one lithium disilicate glass ceramics block "IPS Emax CAD" as control group, two zirconia reinforced lithium silicate "Vita Suprinity & Celtra DUO," one interpenetrating network ceramic block "Vita Enamic," Three resin-based block composites "Lava Ultimate, Cerasmart & Brilliant-crios" as well as one hybrid nanoceramic "Shofu block HC". All specimens were mounted against canine and tested for two body wear analysis using a chewing simulating loading machine (100,000 cycles, 50 N, 5/55 °C). The amount of wear loss was measured for each specimen using a digital precise scale. Wear area before and after the chewing simulation were evaluated using an optical profilometer. Data analysed using one-way ANOVA test followed by Tukey's post hoc.

RESULTS

The results showed a significantly higher wear loss in resin matrix ceramics in comparison to glass ceramics. However, for tooth wear glass ceramics had significantly higher value than hybrid ceramics.

CONCLUSIONS

Resin based CAD/CAM Blocks gives a superior result when evaluating the wear behavior and its effect on the opposing tooth surface.

Evaluation of one versus two glaze firings on the color stability and mechanical properties of an extrinsically characterized monolithic CAD-CAM lithium disilicate glass ceramic

PURPOSE

To evaluate the effects of 1 versus 2 glaze firings on the color and mechanical properties of an extrinsically characterized lithium disilicate ceramic after thermal cycling, brushing, or both.

MATERIALS AND METHODS

Eighty specimens were divided into 2 groups: 1 glaze firing (GL1) and 2 glaze firings (GL2). Each group was subdivided into 4 groups (n = 10), according to the experimental conditions: thermal-cycling, brushing, thermal-cycling + brushing, and immersion in distilled water (control). Color variation, surface roughness, and Vickers microhardness were analyzed before each designated experiment and after the simulated periods of 2.5, 5, and 10 years. Three-way mixed ANOVA was used for all outcomes, followed by 1-way ANOVA, repeated measures 1-way ANOVA, Bonferroni post hoc test, and t-test to check for statistical differences (α = 0.05).

RESULTS

Thermal cycling generated greater color changes in the GL1 group at 2.5 and 5 years (p < 0.001; p = 0.013). Brushing generated color changes in GL1 at 5 years (p = 0.003) and in GL2 at 10 years (p = 0.017). Regarding surface roughness, the GL1 group suffered alterations in thermal cycling + brushing at 5 years. In the control group, the GL1 group exhibited higher roughness values than GL2 (p < 0.05). Most of the groups experienced an increase in microhardness at 2.5 years (p < 0.05). In the GL1 group, thermal-cycling increased the microhardness at 5 years (p = 0.006); at 5 and 10 years, the GL1 group had a higher microhardness than the GL2 in thermal-cycling + brushing (p < 0.05).

CONCLUSION

Ceramics with 1 glaze firing showed greater color, roughness, and microhardness changes compared to those submitted to 2 firings.

The effect of quaternary ammonium polyethylenimine nanoparticles on bacterial adherence, cytotoxicity, and physical and mechanical properties of experimental dental composites

A significant problem related to the functioning of resin-based composites for dental fillings is secondary or recurrent caries, which is the reason for the need for repeated treatment. The cross-linked quaternary ammonium polyethylenimine nanoparticles (QA-PEI-NPs) have been shown to be a promising antibacterial agent against different bacteria, including cariogenic ones. However, little is known about the properties of dental dimethacrylate polymer-based composites enriched with QA-PEI-NPs. This research was carried out on experimental composites based on bis-GMA/UDMA/TEGDMA matrix enriched with 0.5, 1, 1.5, 2 and 3 (wt%) QA-PEI-NPs and reinforced with two glass fillers. The cured composites were tested for their adherence of Streptococcus Mutans bacteria, cell viability (MTT assay) with 48 h and 10-days extracts , degree of conversion (DC), water sorption (WSO), and solubility (WSL), water contact angle (CA), flexural modulus (E), flexural strength (FS), compressive strength (CS), and Vickers microhardness (HV). The investigated materials have shown a complete reduction in bacteria adherence and satisfactory biocompatibility. The QA-PEI-NPs additive has no effect on the DC, VH, and E values. QA-PEI-NPs increased the CA (a favorable change), the WSO and WSL (unfavorable changes) and decreased flexural strength, and compressive strength (unfavorable changes). The changes mentioned were insignificant and acceptable for most composites, excluding the highest antibacterial filler content. Probably the reason for the deterioration of some properties was low compatibility between filler particles and the matrix; therefore, it is worth extending the research by surface modification of QA-PEI-NPs to achieve the optimum performance characteristics.

Orthodontic Internal Resorption Assessment in Periodontal Breakdown-A Finite Elements Analysis (Part II)

This finite elements analysis (FEA) assessed the accuracy of maximum shear stress criteria (Tresca) in the study of orthodontic internal surface resorption and the absorption-dissipation ability of dental tissues. The present study was conducted over eighty-one models totaling 324 simulations with various bone loss levels (0-8 mm), where 0.6 N and 1.2 N were applied in the intrusion, extrusion, rotation, tipping, and translation movements. Tresca criteria displayed localized high-stress areas prone to resorption for all situations, better visible in the dentine component. The internal resorptive risks are less than external ones, seeming to increase with the progression of the periodontal breakdown, especially after 4 mm. The internal and external surface high-stress areas are strictly correlated. The qualitative stress display for both forces was almost similar. The rotation and tipping displayed the highest resorptive risks for the pulp chamber, decreasing with bone loss. The resorptive risks seem to increase along with the progression of periodontal breakdown if the same applied force is kept. The dentine resemblance to ductile based on its high absorption-dissipation ability seems correct. Tresca seems to supply a better predictability of the prone-to-resorption areas than the other failure criteria.

Comparison of dental flosses - an investigation of subjective preference and mechanical properties

Objective

To investigate the properties (tensile strength, roughness, abrasiveness) of different dental flosses and how these properties relate to subjective preference for floss by users.

Materials and method

Four flosses of differing compositions were selected (polytetrafluoroethylene (PTFE), nylon, silk, and ultra-high-molecular-weight polyethylene (UHMWPE)). Tensile strength (TS) was measured utilising a universal testing machine (total n = 40). Surface roughness (Ra) was measured on 3D reconstructed models of scanning electron microscope and abrasiveness was measured through block-on-ring tests against human enamel. Subjective preference for floss was measured by asking a sample of 16 individuals to use each floss for an 8-day period using a split-mouth design.

Results

The highest TS was found in UHMWPE floss (194.18± 24.61 MPa) while the lowest TS was found in PTFE floss (11.78± 0.77 MPa). Silk floss had the highest Ra (0.304± 0.025 µm) while PTFE floss had the lowest (0.048± 0.003 µm). In-vitro abrasion testing of the flosses identified no significant differences between the flosses in causing wear on tooth enamel. Subjective ratings of flosses indicated PTFE floss to be most preferred and nylon floss to be least preferred.

Conclusion

There was a difference in subjective preference between dental flosses composed of different materials. The PTFE floss was the overall most preferred while the nylon floss was the least preferred. There was also an association between the mechanical properties and preference for their usage, with PTFE floss being the most preferred but having the lowest surface roughness and tensile strength.

Clinical Relevance

This study compared a wide range of mechanical properties and subject preferences of commercially available dental floss. The results of this study can provide guidance for the recommendation of dental floss for oral hygiene routines.

Calcium phosphate ceramic as a model for enamel substitute material in dental applications

OBJECTIVE

This study aimed to develop enamel substitute material using a mechanochemical technique.

MATERIALS AND METHODS

Hydroxyapatite was synthesized with and without tricalcium phosphate under uniaxial pressing of 10 and 17 MPa (HA10, HA17, BCP10, and BCP17), followed by sintering at 1250 °C for 2 h. Human enamel and dentin blocks were used as control groups. The mechanical properties were determined by compressive strength test and Vickers microhardness. The data were analyzed with one-way ANOVA and LSD post-hoc test (α = 0.05). The phase formation and morphology of the specimens were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM).

RESULTS

HA17 and HA10 had compressive strength values comparable to enamel and dentin, respectively (p > 0.05). The microhardness of all synthesized groups was significantly higher than that of tooth structures (p < 0.05). From the XRD graphs, only the hydroxyapatite peak was observed in the control and HA groups. SEM images showed homogeneous hydroxyapatite grains in all groups, while the BCP groups contained higher porosities.

CONCLUSIONS

Both HA10 and HA17 are suitable for use as the inorganic part of dentin and enamel substitutes.

The luminous transmittance of the quartz-glass fiber posts is superior to glass fiber posts

Fiber-reinforced prefabricated intraarticular posts have gained popularity due to several favorable characteristics for clinical use compared to metallic intraradicular posts.

AIM

To evaluate the light transmission capacity of two types of fiber posts, using two different methods.

MATERIALS AND METHOD

The posts were divided into two groups: experimental group - quartz-glass fiber posts (n=10) and control group - glass fiber posts (n=10). The light transmittance of the samples was compared by means of light intensity test by photographs and ultraviolet-visible spectrophotometer. This test was analyzed by thirds: coronal, middle, and apical. The spectophotometer tested the luminous transmittance along the length of the post. The statistical analysis was conducted with a significance level of 0.05.

RESULTS

Light transmission was 97% on the coronal third, 68% in the middle third, and 27.66% in the apical third in the posts of the experimental group. In the posts of the control group, the light transmission was 95.33% in the coronal third, 80.66% in the middle third, and 41.33% in the apical third. Light transmission was significantly higher in the middle third of the posts of the experimental group when compared to the control group (p<0.05). The luminous transmittance of the posts of the experimental group was 97.4% with wavelengths of 400 nm, 97% at 450 and 500 nm, and 96.9% at 550 nm. In the posts of the control group, the luminous transmittance was 72.3% with wavelengths of 400 nm, 68.6% at 450 nm; 64.6% at 500 nm and 61.5% at 550 nm. The posts of the experimental group demonstrated significantly higher light transmittance than the control group (p<0.001).

Cortical and Trabecular Bone Stress Assessment during Periodontal Breakdown-A Comparative Finite Element Analysis of Multiple Failure Criteria

Background and Objectives: This numerical analysis investigated the biomechanical behavior of the mandibular bone as a structure subjected to 0.5 N of orthodontic force during periodontal breakdown. Additionally, the suitability of the five most used failure criteria (Von Mises (VM), Tresca (T), maximum principal (S1), minimum principal (S3), and hydrostatic pressure (HP)) for the study of bone was assessed, and a single criterion was identified for the study of teeth and the surrounding periodontium (by performing correlations with other FEA studies). Materials and Methods: The finite element analysis (FEA) employed 405 simulations over eighty-one mandibular models with variable levels of bone loss (0-8 mm) and five orthodontic movements (intrusion, extrusion, tipping, rotation, and translation). For the numerical analysis of bone, the ductile failure criteria are suitable (T and VM are adequate for the study of bone), with Tresca being more suited. S1, S3, and HP criteria, due to their distinctive design dedicated to brittle materials and liquids/gas, only occasionally correctly described the bone stress distribution. Results: Only T and VM displayed a coherent and correlated gradual stress increase pattern for all five movements and levels of the periodontal breakdown. The quantitative values provided by T and VM were the highest (for each movement and level of bone loss) among all five criteria. The MHP (maximum physiological hydrostatic pressure) was exceeded in all simulations since the mandibular bone is anatomically less vascularized, and the ischemic risks are reduced. Only T and VM displayed a correlated (both qualitative and quantitative) stress increase for all five movements. Both T and VM displayed rotation and translation, closely followed by tipping, as stressful movements, while intrusion and extrusion were less stressful for the mandibular bone. Conclusions: Based on correlations with earlier numerical studies on the same models and boundary conditions, T seems better suited as a single unitary failure criterion for the study of teeth and the surrounding periodontium.

Physical and mechanical properties of four 3D-printed resins at two different thick layers: An in vitro comparative study

OBJECTIVES

This in vitro comparative study aimed to evaluate the physical and mechanical properties of four 3D-printed resins with two different thickness layers.

METHODS

Four printed resins (VarseoSmile Crown Plus, VSC; NexDent C&B MFH, MFH; Nanolab 3D, NNL; and Resilab 3D Temp, RSL) were printed with 50 µm and 100 µm layer thickness, resulting in 80 bars measuring 25 × 2×2 mm. The specimens underwent a Raman spectroscopy for degree of conversion, confocal laser scanning microscopy for surface roughness (Sa), three-point bending test for flexural strength and elastic modulus, and a Vickers hardness test (VHN). Data was tested for normality using the Shapiro-Wilk, two-way ANOVA, and Tukey test (α = 0.05) for statistical analysis.

RESULTS

The layer thickness affected all performed tests, but the elastic modulus (p < 0.001). Specimens with 100 µm showed, in general, worse results outcomes than those with 50 µm (p < 0.001). However, within the limitations of this comparative in vitro study, it could be concluded that the tested resins and layer thicknesses directly influenced physical and mechanical properties.

SIGNIFICANCE

The physical and mechanical properties of three-dimensional printed restorations can be affected by the layer thickness, which can interfere with the choice of the 3D printing resin for a desired clinical outcome.

Effect of the Abutment Rigidity on the Wear Resistance of a Lithium Disilicate Glass Ceramic: An In Vitro Study

Lithium disilicate (LDS) glass ceramics are among the most common biomaterials in conservative dentistry and prosthodontics, and their wear behavior is of paramount clinical interest. An innovative in vitro model is presented, which employs CAD/CAM technology to simulate the periodontal ligament and alveolar bone. The model aims to evaluate the effect of the abutment rigidity on the wear resistance of the LDS glass ceramic. Two experimental groups (LDS restorations supported by dental implants, named LDS-on-Implant, or by hybrid ceramic tooth replicas with artificial periodontal ligament, named LDS-on-Tooth-Replica) and a control group (LDS-Cylinders) were compared. Fifteen samples (n = 15) were fabricated for each group and subjected to testing, with LDS antagonistic cusps opposing them over 120,000 cycles using a dual axis chewing simulator. Wear resistance was analyzed by measuring the vertical wear depth (mm) and the volume loss (mm3) on each LDS sample, as well as the linear antagonist wear (mm) on LDS cusps. Mean values were calculated for LDS-Cylinders (0.186 mm, 0.322 mm3, 0.220 mm, respectively), LDS-on-Implant (0.128 mm, 0.166 mm3, 0.199 mm, respectively), and LDS-on-Tooth-Replica (0.098 mm, 0.107 mm3, 0.172 mm, respectively) and compared using one-way-ANOVA and Tukey's tests. The level of significance was set at 0.05 in all tests. Wear facets were inspected under a scanning electron microscope. Data analysis revealed that abutment rigidity was able to significantly affect the wear pattern of LDS, which seems to be more intense on rigid implant-abutment supports compared to resilient teeth replicas with artificial periodontal ligament.

Stresses in teeth with External Cervical Resorption defects restored with different Biomimetic cements: A Finite Element Analysis

INTRODUCTION

This study compared the stress distributions in teeth with simulated external cervical resorption defects restored with different restorative materials and identified areas of high stress concentration.

METHODS

A maxillary central incisor created in a scanned model using HyperWorks software served as control. External cervical resorption defects based on Shanon Patel's classification were created (1Bd/2Bd/3Bd) in the scanned model. The defects were restored using Mineral Trioxide Aggregate (MTA), Biodentine, Glass Ionomer Cement (GIC) and Bioaggregate. On all the models a force of 100 N was applied on palatal aspect, 2 mm incisal to cingulum directed at 45° along the long axis of the tooth.

RESULTS

The stresses generated in dentin and cementum is less, with a restorative material having high Young's modulus. For 1Bd defect, MTA and Bioaggregate showed least stresses in dentin and cementum respectively, whereas Biodentine had consistently lower stresses in dentin and cementum both. Larger defects like 2Bd and 3Bd restored with Bioaggregate exhibited minimum stresses in dentin and cementum.

CONCLUSION

Bioaggregate and Biodentine replace dentin with maximum stress and maximum strain. Elastic moduli similar to or higher than dentin are preferred for restoring cervical third resorptive lesions of the tooth.

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.

Recent Methods for Modifying Mechanical Properties of Tissue-Engineered Scaffolds for Clinical Applications

The limited regenerative capacity of the human body, in conjunction with a shortage of healthy autologous tissue, has created an urgent need for alternative grafting materials. A potential solution is a tissue-engineered graft, a construct which supports and integrates with host tissue. One of the key challenges in fabricating a tissue-engineered graft is achieving mechanical compatibility with the graft site; a disparity in these properties can shape the behaviour of the surrounding native tissue, contributing to the likelihood of graft failure. The purpose of this review is to examine the means by which researchers have altered the mechanical properties of tissue-engineered constructs via hybrid material usage, multi-layer scaffold designs, and surface modifications. A subset of these studies which has investigated the function of their constructs in vivo is also presented, followed by an examination of various tissue-engineered designs which have been clinically translated.

Assessment of the Orthodontic External Resorption in Periodontal Breakdown-A Finite Elements Analysis (Part I)

This Finite Elements Analysis (FEA) assessed the accuracy of Tresca failure criteria (maximum shear stress) for the study of external root resorption. Additionally, the tooth absorption-dissipation ability was assessed. Overall, 81 models of the second mandibular premolar, out of a total of 324 simulations, were involved. Five orthodontic movements (intrusion, extrusion, rotation, translation, and tipping) were simulated under 0.6 N and 1.2 N in a horizontal progressive periodontal breakdown simulation of 0-8 mm. In all simulations, Tresca criteria accurately displayed the localized areas of maximum stress prone to external resorption risks, seeming to be adequate for the study of the resorptive process. The localized areas were better displayed in the radicular dentine-cementum component than in the entire tooth structure. The rotation and translation seem prone to a higher risk of external root resorption after 4 mm of loss. The resorptive risks seem to increase along with the progression of periodontal breakdown if the same amount of applied force is guarded. The localized resorption-prone areas follow the progression of bone loss. The two light forces displayed similar extensions of maximum stress areas. The stress displayed in the coronal dentine decreases along with the progression of bone loss. The absorption-dissipation ability of the tooth is about 87.99-97.99% of the stress.

Finite Elements Analysis of Tooth-A Comparative Analysis of Multiple Failure Criteria

Herein Finite elements analysis (FEA) study assesses the adequacy and accuracy of five failure criteria (Von Mises (VM), Tresca, maximum principal (S1), minimum principal (S3), and Hydrostatic pressure) for the study of tooth as a structure (made of enamel, dentin, and cement), along with its stress absorption-dissipation ability. Eighty-one 3D models of the second lower premolar (with intact and 1-8 mm reduced periodontium) were subjected to five orthodontic forces (intrusion, extrusion, tipping, rotation, and translation) of 0.5 N (approx. 50 gf) (in a total of 405 FEA simulations). Only the Tresca and VM criteria showed biomechanically correct stress display during the 0-8 mm periodontal breakdown simulation, while the other three showed various unusual biomechanical stress display. All five failure criteria displayed comparable quantitative stress results (with Tresca and VM producing the highest of all), showing the rotational and translational movements to produce the highest amount of stress, while intrusion and extrusion, the lowest. The tooth structure absorbed and dissipated most of the stress produced by the orthodontic loads (from a total of 0.5 N/50 gf only 0.125 N/12.5 gf reached PDL and 0.01 N/1 gf the pulp and NVB). The Tresca criterion seems to be more accurate than Von Mises for the study of tooth as structure.

Synthesis and Characterization of Zirconia–Silica PMMA Nanocomposite for Endodontic Implants

This study aimed to enhance the mechanical properties of PMMA composites by introducing various types of fillers, including ZrO2, SiO2, and a mixture of ZrO2-SiO2 nanoparticles, which were prepared as prototypes for an endodontic implant. The ZrO2, SiO2, and mixed ZrO2-SiO2 nanoparticles were synthesized using the sol–gel method and the precursors Tetraethyl Orthosilicate, Zirconium Oxychloride, and a mixture of both precursors, respectively. Before polymerization, the as-synthesized powders were subjected to the bead milling process to obtain a well-dispersed suspension. Two scenarios for the fillers were implemented in the preparation of the PMMA composite: a mixture of ZrO2/SiO2 and ZrO2-SiO2 mixed with two different types of silane: (3-Mercaptopropyl) trimethoxysilane (MPTS) and 3-(Trimethoxysilyl) Propyl Methacrylate (TMSPMA). The observation of the characteristics of all of the investigated fillers included the use of a particle-size analyzer (PSA), a Zeta-potential analyzer, FTIR, XRF, XRD, and SEM. The mechanical properties of the MMA composites, as prepared under various scenarios, were observed in terms of their flexural strength, diametrical tensile strength (DTS), and modulus of elasticity (ME). These levels of performance were compared with a PMMA-only polymer. Each sample was measured five times for flexural strength, DTS, and ME. The results showed that the best PMMA composite was SiO2/ZrO2/TMSPMA, as revealed by measurements of the flexural strength, DTS, and ME corresponding to 152.7 ± 13.0 MPa, 51.2 ± 0.6 MPa, and 9272.8 ± 2481.4 MPa, which are close to the mechanical properties of dentin. The viability of these PMMA composites, as measured up to day 7, was 93.61%, indicating that they are nontoxic biomaterials. Therefore, it was concluded that the PMMA composite created with SiO2/ZrO2/TMSPMA can be considered to be an acceptable endodontic implant.

Fracture toughness and hardness of in-office, 3D-printed ceramic brackets

OBJECTIVES

Three-dimensional (3D) printing technology is a promising manufacturing technique for fabricating ceramic brackets. The aim of this research was to assess fundamental mechanical properties of in-office, 3D printed ceramic brackets.

MATERIALS AND METHODS

3D-printed zirconia brackets, commercially available polycrystalline alumina ceramic brackets (Clarity, 3 M St. Paul, MN) and 3D-printed customized polycrystalline alumina ceramic ones (LightForce™, Burlington, Massachusetts) were included in this study. Seven 3D printed zirconia brackets and equal number of ceramic ones from each manufacturer underwent metallographic grinding and polishing followed by Vickers indentation testing. Hardness (HV) and fracture toughness (K1c) were estimated by measuring impression average diagonal length and crack length, respectively. After descriptive statistics calculation, group differences were analysed with 1 Way ANOVA and Holm Sidak post hoc multiple comparison test at significance level α = .05.

RESULTS

Statistically significant differences were found among the materials tested with respect to hardness and fracture toughness. The 3D-printed zirconia proved to be less hard (1261 ± 39 vs 2000 ± 49 vs 1840 ± 38) but more resistant to crack propagation (K1c = 6.62 ± 0.61 vs 5.30 ± 0.48 vs 4.44 ± 0.30 MPa m^1/2 ) than the alumina brackets (Clarity and Light Force respectivelty). Significant differences were observed between the 3D printed and the commercially available polycrystalline alumina ceramic brackets but to a lesser extent.

CONCLUSIONS

Under the limitations of this study, the 3D printed zirconia bracket tested is characterized by mechanical properties associated with advantageous orthodontic fixed appliances traits regarding clinically relevant parameters.

The Effect of Photodynamic Therapy Applied with Different Photosensitizers on Dentin Hardness in Comparison with Conventional Irrigation

Objective: The aim of this study was to examine the effect of photodynamic therapy (PDT) using toluidine blue O (TBO), curcumin (CUR) and methylene blue (MB) photosensitizers on root canal dentin microhardness by comparing it with sodium hypochlorite (NaOCl) + ethylenediamine tetraacetic acid (EDTA). Materials and methods: The root canals of 100 human premolar teeth were shaped by the R25 file (Reciproc; VDW, Munich, Germany). The working length of the teeth was determined by using a #10 K-file, keeping it 1mm shorter than the tooth apex. The R25 file was used to prepare the root canals. After every three pecking motions, irrigation was performed and a total of 10 mm of distilled water was used. The specimens were randomly distributed according to the disinfection method; NaOCl+EDTA, PDT with TBO, PDT with CUR, PDT with MB and distilled water (DS) (n=20). Grooves were prepared on the buccal and lingual surfaces of the prepared teeth, parallel to the long axis of the tooth, without touching the root canals. The roots were divided into two parts by means of a cement spatula placed in these grooves. Root canal dentin microhardness was evaluated by the Vickers test method. Three measurements were made by applying 300 g of force for 15 seconds and the average was calculated. It was recorded as the Vickers hardness value. The data were analyzed by using the one-way ANOVA and Dunnett’s post hoc tests (α=0.05). Results: All photosensitizer groups showed significantly higher microhardness value than the groups of DS and NaOCl + EDTA (p0.05). Conclusions: PDT with CUR obtained the highest radicular dentin microhardness.

Mechanical and electrochemical characterization of 3D printed orthodontic metallic appliances after in vivo ageing

OBJECTIVES

Three-dimensional (3D) printing technology is a promising technique for fabricating custom orthodontic metallic appliances. Aim of this study was to assess the effect of intraoral aging on the mechanical / electrochemical properties of 3D-printed orthodontic metallic appliances.

METHODS

Twelve molar orthodontic distalization appliances 3D-printed from cobalt chromium (Co-Cr) alloy were retrieved after intraoral use and twenty blocks fabricated under similar conditions were used as control. The samples' microstructural / elemental composition assessment was assessed with SEM/EDS, while their mechanical properties (modulus of elasticity [E_IT], Martens hardness [HM] and the elastic index [η_IT]) were measured by instrumented indentation testing. Finally, the samples' electrochemical features were assessed with a potentiostat-connected cell arrangement in terms of open circuit potential (OCP), corrosion potential (E_z), current density (I₃₀₀) and breaking potential (E_pit). Results were analyzed by t-test / Mann-Whitney test (α = 0.05).

RESULTS

The used Co-Cr alloy was found to have a highly homogenous structure with no significant differences between retrieved and new specimens in HM (4037.7 ± 215.6 vs 4090.9 ± 259.8 N/mm²), E_IT (120.0 ± 13.2 vs 123.8 ± 12.9 GPa), or n_IT (28.4 ± 2.6 vs 28.6 ± 2.9 %) (P > 0.05 in all instances). Metallic surfaces retained the same oxidation tendency and oxide dissolution rate in passive region in both groups (P > 0.05 for OCP, E_z, and I₃₀₀). However, intraorally-aged specimens had a significantly lower breakdown potential due to degraded protection efficacy of surface oxide (P = 0.003 for E_pit).

SIGNIFICANCE

The tested 3D-printed Co-Cr orthodontic appliances present clinically-acceptable mechanical properties that remained unaffected by intraoral ageing, which however degraded the protection of surface oxide against pitting corrosion.

The Effects of Radiotherapy on Microhardness and Mineral Composition of Tooth Structures

OBJECTIVE

 The purpose of this study was to evaluate the microhardness and mineral composition alterations in enamel and dentine after radiotherapy.

MATERIALS AND METHODS

 Forty human maxillary premolar teeth (20 pairs) were assigned to nonirradiated and irradiated groups, the latter irradiated by fractional radiation to achieve a total dose of 70 Gy. Microhardness measurement was performed on a Knoop microhardness tester. Chemical components were analyzed using energy dispersive spectroscopy and Fourier transform Raman spectroscopy. The morphology was observed using a scanning electron microscope. The microhardness data were analyzed using a paired t-tested and one-way repeated analysis of variance (ANOVA), and the mineral composition data using related-samples Wilcoxon signed rank test and related-samples Friedman's two-way ANOVA by ranks.

RESULTS

 The irradiated teeth had a significantly lower microhardness in both enamel and dentine compared with the nonirradiated teeth. The irradiated dentine at 50 μm from the external tooth surface at the cemento-enamel junction showed the lowest microhardness compared with other locations. There was no statistically significant difference in calcium:phosphate ratio and chemical components. There was a reduction in protein:mineral ratio in dentine and at the cemento-enamel junction after irradiation. The irradiated teeth exhibited crack lines at the dentine-enamel junction and in dentine.

CONCLUSION

 Fractional radiation reduced microhardness in both enamel and dentine. The cervical dentine exhibited the highest microhardness reduction compared with other enamel and dentine locations.

Wear Behavior between Aesthetic Restorative Materials and Bovine Tooth Enamel

Tooth enamel wear occurs because of daily mastication and occlusion. This study investigated the wear behavior of bovine teeth against aesthetic restorative materials in vitro. Abrader specimens were fabricated using four tooth-colored restorative materials (zirconia, lithium disilicate glass ceramic, dental porcelain, and resin composite), with bovine tooth enamel as a control. Flattened bovine tooth enamel was used as the substrate specimen. These materials were characterized by Vickers hardness tests and surface roughness measurements. Two-body wear tests between the abrader and substrate specimens were performed, and the worn topographies were evaluated using a contour-measuring instrument and 3D laser microscope. The restorative materials and bovine tooth enamel had similar surface roughness but different hardness and wear behaviors. Bovine teeth showed the largest wear in tooth–tooth contact as the abrader and substrate specimens. Compared to bovine teeth, zirconia, lithium disilicate glass ceramic, and dental porcelain showed greater hardness and less wear on their surfaces, and less substrate wear of the opposite tooth enamel. The lowest hardness resin composite showed intermediate wear on its surface, resulting in the lowest substrate wear. Accordingly, dentists should pay attention to the selection of restorative materials to reconstruct their morphologies owing to different wear behaviors.

Experimental borosilicate bioactive glasses: pulp cells cytocompatibility and mechanical characterisation

AIM

To assess in vitro the effect of two novel phase separated borosilicate glasses (PSBS) in the system SiO₂ -B₂ O₃ -K₂ O-CaO-Al₂ O₃ on dental pulp cells; and to compare their bioactivity and mechanical properties to a conventional fluoroaluminosilicate glass namely FUJI IX.

METHODOLOGY

The cytocompatibility assessment of the two novel borosilicate glasses, one without alumina (PSBS8) and one containing alumina (PSBS16), was performed on cultured primary human pulp cells (hDPCs). Alamar blue assay was used to assess cell metabolic activity and cell morphology was evaluated by confocal imaging. The bioactivity in Stimulated Body Fluid was also evaluated after 1 and 3 weeks of immersion using SEM-EDX analysis. Vickers microhardness and flexural strength were assessed after incorporating the glass particles into a commercial glass ionomer cement liquid containing both polyacrylic and polybasic carboxylic acid.

RESULTS

The data revealed that the two borosilicate glasses enhanced cell viability ratios at all-time points in both direct and indirect contact assays. After 3 days of contact, PSBS8 without alumina showed higher viability rate (152%) compared to the PSBS16 containing alumina (145%) and the conventional glass ionomer particles (117%). EDX analysis confirmed an initial Ca/P ratio of 2.1 for 45S5K and 2.08 for PSBS8 without alumina after 3 weeks of immersion. The cement prepared using PSBS8 showed significantly higher Vickers hardness values (p=0.001) than that prepared using PSBS16 (46.6 vs 36.7 MPa). After 24 hours of maturation, PSBS8 (without alumina) exhibited a flexural strength of 12.9 MPa compared to a value of 16.4 MPa for the commercial control. PSBS8 without alumina had a higher strength than PSBS16 with alumina, after 1 and 7 days of maturation (p=0.001).

CONCLUSIONS

The present in vitro results demonstrated that the borosilicate bioactive glass without alumina enhanced pulp cell viability, spreading and acellular bioactivity better than the conventional glass ionomer cement and the experimental borosilicate glass containing alumina.

Cold plasma enamel surface treatment to increase fluoride varnish uptake

Among the available methods of enamel strengthening, fluoride varnish (FV) treatment has relatively better results. On the other hand, cold plasma technology has shown promising capacities in sterilizing the environment, surface modification, and improving adhesion. Accordingly, this study aimed to increase the adhesion of FV to the enamel surface to prolong the enamel interaction with FV with subsequently increased fluoride uptake by enamel. Emphasizing that the change in adhesion is evidence-based and has not been explicitly measured. For this purpose, we randomly divided twenty bovine teeth into two groups A (consisting of four teeth) and B (composed of four subgroups, each containing four teeth). Samples of group A and one specimen of each subset B investigated the effect of using Helium-DBD (He-DBDJ), Argon (ArJ), and Air-DBD jet on the enamel surface. Other B specimens are devoted to studying the release of FV fluoride ions from processed enamel. Two diagnostic techniques, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), have been utilized to examine the samples' surface morphology and chemical analysis, respectively. Finally, the release of fluoride ions into distilled water was measured by an ion-selective electrode (ISE). SEM images showed that ArJ and Air-DBD significantly damaged enamel hexagonal structures, whereas, in the case of He-DBDJ, the hexagonal structures have only altered from convex to concave. EDX indicated an increase in calcium to phosphorus ratio and the amount of fluoride and sodium uptake on the enamel surface layer in the group processed with He-DBDJ plasma. The latter helps restore the damaged parts of the enamel. Analysis of fluoride released from the FV did not show a significant change owing to plasma processing (P ≤ 0.112). The combination of cold plasma and fluoride varnish treatment on the enamel surface might be considered as a more promising approach to increasing enamel resistance to tooth decay.

An Enamel Based Biopolymer Prosthesis for Dental Treatment with the Proper Bond Strength and Hardness and Biosafety

Although dental prosthesis materials such as metal alloys, ceramics, and cured resin composite have long been utilized to restore teeth, their bond strength and hardness values are not well matched to human enamel. Prosthesis detachment and opposing enamel wear are major concerns in restorative dentistry. An experimental biopolymer, hybridized enamel, was synthesized and utilized as a dental prosthesis to compare hardness and tensile bond strength (TBS) with those of commercial materials. Vickers hardness (VHN) with a 100 g loading for 15 s at eight indentations on each specimen (n = 20) was measured. TBSs between prostheses and two types of resin luting agents (n = 10), Super-Bond C&B and All-Bond2 + Duo-Link, were tested. Fractured surfaces and the luting resin-prosthesis interface were examined under a stereomicroscope or a scanning electron microscope (SEM). Statistically significant differences in the TBS and hardness were revealed (p < 0.05). The experimental biopolymer provided a hardness value comparable with human enamel and the highest TBS for both luting agent types. The SEM micrograph demonstrated a honeycomb-like pattern interface between the experimental biopolymer and luting resin. These results suggest that this experimental biopolymer may be a better restorative material to protect from natural enamel loss from tooth reduction or attrition and prevent prosthesis detachment during mastication.

Characterization of various acrylate based artificial teeth for denture fabrication

Acrylic resins-based artificial teeth are frequently used for the fabrication of dentures has and contribute a very strong share in the global market. However, the scientific literature reporting the comparative analysis data of various artificial teeth is scarce. Focusing on that, the present study investigated various types of commercially available artificial teeth, composed of polymethyl methacrylate (PMMA). Artificial teeth are characterized for chemical analysis, morphological features, thermal analysis, and mechanical properties (surface hardness, compressive strength). Different types of artificial teeth showed distinct mechanical (compression strength, Vickers hardness) and thermal properties (thermal gravimetric analysis) which may be attributed to the difference in the content of PMMA and type and quantity of different fillers in their composition. Thermogravimetric analysis (TGA) results exhibited that vinyl end groups of PMMA degraded above 200 °C, whereas 340-400 °C maximum degradation temperature was measured by differential thermal analysis (DTA) for all samples. Crisma brand showed the highest compressive strength and young modulus (88.6 MPa and 1654 MPa) while the lowest value of Vickers hardness was demonstrated by Pigeon and Vital brands. Scanning electron microscope (SEM) photographs showed that Crisma, Pigeon, and Vital exhibited characteristics of a brittle fracture; however, Artis and Well bite brands contained elongated voids on their surfaces. According to the mechanical analysis and SEM data, Well bite teeth showed a significantly higher mechanical strength compared to other groups. However, no considerable difference was observed in Vickers hardness of all groups. Graphical abstract.

Wear Behavior of Occlusal Splint Materials Manufactured By Various Methods: A Systematic Review

PURPOSE

To systematically review studies on various materials and methods used for wear testing of occlusal devices and their antagonists in vitro and in vivo.

METHODS

An electronic search in OVID, Web of Science, PubMed and Scopus was conducted using the following terms (MeSH words) with any synonyms and closed terms: "Splint*" OR "occlusal splint*" OR "night guard" OR "occlusal device" OR "occlusal devices" OR "deprogrammer" OR "bite splint" OR "bite plane" OR "orthotic appliance*" OR "orthotic devices" AND "wear" OR "two-body wear" OR "three-body wear" OR "tooth wear" OR "wear measurement*" OR "wear behaviour" OR "wear behavior" OR "abrasion" AND "Polymethyl Methacrylate" OR "PMMA" OR "acrylic resin*" OR "dental material*" OR "dental enamel" OR "CAD" OR "CAM" OR "PEEK" OR "material* testing". Database search was limited to English-language publications and published between 2001 and 1st of September 2021. A further hand search was done to ensure all materials were captured.

RESULTS

After the removal of duplicates, 115 studies were identified, and 11 were chosen for review. Studies showed that the lowest volumetric loss was observed in PEEK occlusal device materials, whereas heat-cure, CAD-milled, and 3D printed occlusal device materials had no significant difference in wear. Vacuum-formed materials showed the highest wear among all groups. Testing parameters were found to be inconsistent across all studies.

CONCLUSION

There is a need for standardization of in vitro and in vivo wear measurement and testing protocols as this study revealed a wide variety of testing protocols which potentially could influence the outcome. Polishing procedures are required for the material. Limited studies are available on 3D printed occlusal device materials and would therefore require further investigation, especially on printing build angles and settings. Further clinical studies would be advantageous to provide guidance on the selection of the best occlusal device material that would last the longest without remake.

Additively Manufactured Dental Crown with Color Gradient and Graded Structure: A Technique Report

To assess the feasibility of manufacturing a dental crown with internal color gradient and graded structure design using additive manufacturing technology, a mandibular first molar was prepared and a monolayer dental crown with 1.5 mm uniform thickness was designed in a dental software (STL_C1 ). The monolayer crown design was sliced into multiple layers of 0.1 mm thickness and a design for a multilayer crown was obtained (STL_C2 ). A multilayer crown was manufactured with gradient color and graded structure using a material jetting printer. Different materials with different colors and properties were used and mixed in different ratios during manufacturing to achieve the prospected design. The feasibility of manufacturing such a crown was reported. This report confirms that multilayer dental crowns with internal gradient color and graded structure are possible when using a multimaterial jetting printer.

Influence of Preparation Design and Restorative Material on Fatigue and Fracture Strength of Restored Maxillary Premolars

STATEMENT OF PROBLEM

Extensive carious lesions and/or large preexisting restorations possibly contribute to crack formation, ultimately resulting in a fracture that may lead to the loss of a tooth cusp. Hence, preparation design strategy in conjunction with the restorative material selected could be influential in the occurrence of a cuspal fracture.

PURPOSE

The purpose of this in vitro study was to evaluate the fatigue behavior and fracture strength of maxillary premolars restored with direct composite and indirect ceramic inlays and overlays, with different preparation depths in the presence or absence of cuspal coverage, and analyze their failure types.

METHODS AND MATERIALS

Sound maxillary premolars (N=90; n=10) were divided into nine groups: group C: control; group DCI3: direct composite inlay 3 mm; group DCI5: direct composite inlay 5 mm; group ICI3: indirect ceramic inlay 3 mm; group ICI5: indirect ceramic inlay 5 mm; group DCO3: direct composite overlay 3 mm; group DCO5: direct composite overlay 5 mm; group ICO3: indirect ceramic overlay 3 mm; group ICO5: indirect ceramic overlay 5 mm. In indirect ceramic, lithium disilicate restoration groups, immediate dentin sealing was applied. After restoration, all specimens were tested in fatigue (1,200,000 cycles, 50 N, 1.7 Hz). Samples were critically appraised, and the specimens without failure were subjected to a load to failure test. Failure types were classified and the data analyzed.

RESULTS

Zero failures were observed in the fatigue testing. The following mean load to failure strengths (N) were recorded: group ICO5: 858 N; group DCI3: 829 N; group ICO3: 816 N; group C: 804 N; group ICI3: 681 N; group DCO5: 635 N; group DCI5: 528 N; group DCO3: 507 N; group ICI5: 482 N. Zero interaction was found between design-depth-material (p=0.468). However, significant interactions were found for the design-depth (p=0.012) and design-material (p=0.006). Within restorations at preparation depth of 3 mm, direct composite overlays obtained a significantly lower fracture strength in comparison to indirect ceramic onlays (p=0.013) and direct composite inlays (p=0.028). In restorations at depth 5 mm, significantly higher fracture load values were observed in indirect ceramic overlays compared with the inlays (p=0.018). Indirect ceramic overlays on 3 mm were significantly stronger than the deep inlays in ceramic (p=0.002) and tended to be stronger than the deep direct composite inlays. Severe, nonreparable fractures were observed with preparation depth of 5 mm within ceramic groups.

CONCLUSIONS

The preparation depth significantly affected the fracture strength of tooth when restored with either composite or ceramic materials. Upon deep cavity preparations, cuspal coverage proved to be beneficial when a glass ceramic was used as the restorative material. Upon shallow cavity preparations, a minimally invasive approach regarding preparation design used in conjunction with a direct composite material was favorable.

Biomechanics of juvenile tyrannosaurid mandibles and their implications for bite force: Evolutionary biology

The tyrannosaurids are among the most well-studied dinosaurs described by science, and analysis of their feeding biomechanics allows for comparison between established tyrannosaurid genera and across ontogeny. 3D finite element analysis (FEA) was used to model and quantify the mechanical properties of the mandibles (lower jaws) of three tyrannosaurine tyrannosaurids of different sizes. To increase evolutionary scope and context for 3D tyrannosaurine results, a broader sample of validated 2D mandible FEA enabled comparisons between ontogenetic stages of Tyrannosaurus rex and other large theropods. It was found that mandibles of small juvenile and large subadult tyrannosaurs experienced lower stress overall because muscle forces were relatively lower, but experienced greater simulated stresses at decreasing sizes when specimen muscle force is normalized. The strain on post-dentary ligaments decreases stress and strain in the posterior region of the dentary and where teeth impacted food. Tension from the lateral insertion of the looping m. ventral pterygoid muscle increases compressive stress on the angular but may decrease anterior bending stress on the mandible. Low mid-mandible bending stresses are congruent with ultra-robust teeth and high anterior bite force in adult T. rex. Mandible strength increases with size through ontogeny in T. rex and phylogenetically among other tyrannosaurids, in addition to that tyrannosaurid mandibles exceed the mandible strength of other theropods at equivalent ramus length. These results may indicate separate predatory strategies used by juvenile and mature tyrannosaurids; juvenile tyrannosaurids lacked the bone-crunching bite of adult specimens and hunted smaller prey, while adult tyrannosaurids fed on larger prey.

Polymers for conventional, subtractive, and additive manufacturing of occlusal devices differ in hardness and flexural properties but not in wear resistance

OBJECTIVES

To investigate the wear resistance of polymers for injection molding, subtractive and additive manufacturing of occlusal devices in comparison with enamel antagonist wear and material properties (i.e., hardness, flexural strength, and flexural modulus).

METHODS

Injection molding was compared with milling and the additive technologies stereolithography, low force stereolithography, and digital light processing. For each material, eight specimens were produced for wear measurements. Extracted human premolars served as indenters. All samples were subjected to two series of a 2-body wear test consisting of 200,000 circular loading cycles with an applied load of 1) 20 N and 2) 50 N in a thermocycling environment (5/55 °C, 30 s, 3860 cycles, H₂O). Wear resistance was characterized by means of maximum depth and volume of the resulting traces. In addition, enamel wear of the indenters and Vickers hardness, flexural strength, and flexural modulus of the polymers were determined. Wear was statistically analyzed with linear general models for repeated measures and material properties with one-way ANOVA with post-hoc Tukey-HSD tests.

RESULTS

Wear of the antagonists was not influenced by the material (P ≥ 0.343). Likewise, no differences in wear resistance were found between materials after cyclic loading with 20 N or 50 N (P ≥ 0.074). Material properties investigated revealed decreased values for the resins for the additive manufacturing with the exception of flexural strength of one material.

SIGNIFICANCE

Within the limitations of this in-vitro study, arylates for conventional, subtractive, and additive manufacturing of occlusal devices differ in material properties but not in wear resistance and antagonist wear.

Effect of differences in the type of restoration and adhesive resin cement system on the bonding of CAD/CAM ceramic restorations

The effect of differences in the type of restoration and adhesive resin cement system on the bonding of CAD/CAM ceramic restoration after cyclic loading was examined quantitatively and qualitatively. Seventy-two human maxillary first molars were divided into three restoration groups: MOD-inlay, MODP-onlay, and crown. Immediate dentin sealing was applied to the exposed dentin of all prepared specimens. The 24 specimens of each restoration group were further divided into another three groups, and a different adhesive resin cement system was applied to each group for cementation. All restoratives were fabricated from feldspathic-ceramic-blocks and cemented with each adhesive resin cement system according to the manufacturer's instructions. The microtensile bond-strength was measured after cyclic loading and was not significantly affected by differences in the type of restoration or adhesive resin cement system. However, the type of restorations and adhesive resin cement systems did show significant differences in terms of the bonding reliability.

Fracture micromechanics of human dentin: A microscale numerical model

In the present study, we investigate the effects of microstructural morphology and heterogeneity on the initiation and propagation of microcracks in dentin. We create 2D pre-cracked models of human dentin at the microscale level and use a brittle fracture framework of the phase-field method to analyze the crack growth. We discuss the influence of the microstructural features on crack deflection, microcracking, and uncracked ligament bridging through various regions in dentin. The results demonstrate that the difference between the critical energy release rates of peritubular (PTD) and intertubular dentin (ITD) has considerable impacts on microcracking. Our simulations reveal that tubules surrounded by PTDs play an important role in the crack deflection. Our results also indicate that the toughness of dentin increases from the inner to outer dentin. In conclusion, the findings in our study provide valuable insights into the fracture behavior in various regions of dentin.

Characterisation of mechanical and surface properties of novel biomimetic interpenetrating alumina-polycarbonate composite materials

OBJECTIVE

To determine the mechanical and surface characteristics of two novel biomimetic interpenetrating phase alumina-polycarbonate (Al₂O₃-PC) composite materials, comprising aligned honeycomb-like porous ceramic preforms infiltrated with polycarbonate polymer.

METHOD

Two composite materials were produced and characterised. Each comprised a porous structure with a ceramic-rich (polymer-poor) top layer, graduated through to a more porous ceramic-poor (polymer-rich) bottom layer. In addition, pure polycarbonate and dense alumina specimens were subjected to the same characterisation namely: density, compression, three-point bend, hardness, surface loss and surface roughness testing. Scanning electron microscopy and micro computerised tomography were employed for structural examination.

RESULTS

Three-dimensional aligned honeycomb-like ceramic structures were produced and full interpenetration of the polymer phase was observed using MicroCT. Depending on the ceramic volume in the initial aqueous ceramic suspension, the density of the final interpenetrating composites ranged from 2.64 to 3.01g/cm³, compressive strength ranged from 192.43 to 274.91MPa, flexural strength from 105.54 to 148.47MPa, fracture toughness from 2.17 to 3.11MPa.m^½, hardness from 0.82 to 1.52GPa, surface loss from 0.71 to 1.40μm and surface roughness, following tooth brushing, from 0.70 to 0.99μm. Composite specimens showed characteristic properties part way between enamel and polycarbonate.

SIGNIFICANCE

There was a correlation between the initial solid ceramic loading in the aqueous suspension, used to produce the porous ceramic scaffolds, and the subsequent characteristic properties of the composite materials. These novel composites show potential as aesthetic orthodontic bracket materials, as their properties fit part way between those of ceramic, enamel and polycarbonate.

Raman spectroscopy for early detection and monitoring of dentin demineralization

Early detection of dental caries and variations in composition/structure of both enamel and dentin represents an important issue in modern dentistry. Demineralization has been associated to teeth discoloration, development of caries, and formation of cavities.

OBJECTIVE

In this study, we systematically monitored the processes of demineralization/remineralization in dentin samples by means of three different spectroscopic techniques, namely, Raman spectroscopy, X-Ray Photo-electron spectroscopy (XPS), and X-Ray Diffractometry (XRD).

METHODS

Bovine dentin samples were first exposed to acidic solutions and their structure systematically monitored as a function of time and pH. Then, the samples were rinsed in artificial saliva to simulate remineralization.

RESULTS

The above three spectroscopic techniques provided quantitative structural information spanning from the nanometer to the millimeter scale of sample penetration depth. An irreversible level of demineralization was reached when dentin was exposed to pH 2 beyond a time threshold of 6h, successive treatments with artificial saliva being unable to restore the mineral fraction. On the other hand, short-term treatments at pH 5 and long-term treatments at pH 6 could partially or completely recover the dentin structure within one week of remineralization treatment.

SIGNIFICANCE

Two specific Raman parameters, namely, the bandwidth of the symmetric phosphate-stretching signal and the mineral-to-matrix intensity ratio, showed strong correlations with XPS and XRD data, and matched laser microscopy observations. Such correlations open the path to apply Raman spectroscopy in monitoring dentin demineralization in vivo and provide quantitative working algorithms for the prevention of oral caries.

Comparative evaluation of three different toothpastes on remineralization potential of initial enamel lesions: A scanning electron microscopic study

Background

The early enamel lesions are reversible as it is a process involving mineral transactions between the teeth and saliva.

Aim

To evaluate the efficiency of three different tooth pastes on remineralization potential of initial enamel lesions using Vickers Micro hardness Test and Scanning electron microscopy.

Materials and Methods

Artificial carious lesions were prepared in human enamel with demineralizing solution. The treatment agents included were Colgate sensitive plus^® toothpaste, Regenerate enamel science™ toothpaste, BioRepair^® toothpaste and control as Deionized water. All the samples were subjected to treatment solutions as per the pH cycling model for 12 days to simulate the daily oral environment's acid challenge. The remineralization parameters-surface hardness and surface roughness of enamel blocks were evaluated with Vickers indenter and Scanning electron microscope respectively. Statistical Analysis: ANOVA test was used to check mean differences between the groups. Post hoc analysis was done using Tukey's post hoc test. SEM images were graded according to Bonetti et al grading criteria.

Results

As per statistical analysis, maximum remineralization of enamel blocks occurred after applying Colgate Sensitive Plus^® tooth paste followed by BioRepair^® tooth paste and Regenerate enamel Science™ toothpaste. Least remineralization potential was shown by control group.

Conclusion

Colgate sensitive plus tooth paste with Pro Argin™ formula can be regarded as a potential remineralising agent. It can be concluded as a noninvasive means of managing early enamel carious lesions.

Biological molecules in dental applications: hyaluronic acid as a companion biomaterial for diverse dental applications

Objectives

The application of hyaluronic acid (HA) in dental treatments is relatively new, and modified-HA products can be vastly different from each other. This study aims to provide a basis for bridging specific characteristics of HA with its potential applications in dental treatments, evaluating and comparing different types of HA products and for future research on HA applications in dentistry.

Data sources

Information from the existing literature on HA applications has been cited.

Study selection

Furthermore, this study is specifically oriented to provide oral health care providers with a scientific basis for HA use along with the clinical aspects of HA.

Conclusions

Outcomes from existing and future studies cannot be generalised for HA use in dental applications. Therefore, we have proposed a scheme to bridge HA specific characteristics to its applications in dental treatments and compare different HA products used for the same clinical application to identify the most suitable one.

Clinical significance

Highlighting the use of HA in dental treatments and providing a basis for developing new methods, protocols, and products specifically oriented for dentistry.

Acoustic diagnosis of elastic properties of human tooth by 320 MHz scanning acoustic microscopy after radiotherapy treatment for head and neck cancer

BACKGROUND

On the elastic profiles of human teeth after radiotherapy for head and neck cancers, generation of dental complications, which may bring several side effects preventing the quality of life, has not well clarified. Thus, we aimed to show the applicability of using 320 MHz Scanning Acoustic Microscopy (SAM) in the evaluation of the tooth damage acoustically at the micrometer level following radiation therapy, and also in the determination of the safe dose limits to impede severe dental damage.

METHODS

This prospective study was performed by SAM employed at 320 MHz by an azimuthal resolution of 4.7 μm resolving enamel and dentin. A total of 45 sound human third molar teeth collected between September 2018 and May 2019 were used for the acoustic impedance measurements pre- and post irradiation. Nine samples for each group (control, 2 Gy, 8 Gy, 20 Gy, 30 Gy and 60 Gy) were evaluated to acquire the acoustic images and perform a qualitative analysis. Scanning Electron Microscopy (SEM) images were obtained to establish a relationship between micromechanical and morphological characteristics of the teeth. Statistical analysis was conducted using the Student t-test succeded by Mann-Whitney U investigation (p < .05), while SEM images were assessed qualitatively.

RESULTS

The analysis included 45 sound teeth collected from men and women 18 to 50 years old. Post irradiation micromechanical variations of human teeth were significant only in the radiation groups of 30 Gy and 60 Gy compared to pre-irradiation group for enamel (7.24 ± 0.18 MRayl and 6.49 ± 028 MRayl; p < 0.05, respectively). Besides, the teeth subjected to radiation doses of 20, 30 and 60 Gy represented significantly lower acoustic impedance values relative to non-irradiated group for dentin (6.52 ± 0.43 MRayl, 5.71 ± 0.66 MRayl and 4.82 ± 0.53 MRayl p < 0.05), respectively.

CONCLUSIONS

These results are evidence for a safe acoustic examination device which may be a useful tool to visualize and follow the safe dose limits to impede severe dental damage through the radiation therapy treatment for head and neck cancers.

Dissimilar sintered calcium phosphate dental inserts as dentine substitutes: Shear bond strength to restorative materials

The application of sintered calcium phosphate dental inserts in the central part of tooth cavities can reduce amount of embedded dental composite and polymerization shrinkage of final dental fillings. The aim of this study was to analyze comparatively physico-chemical and mechanical properties of dental inserts and shear bond strength (SBS) between three dissimilar hydroxyapatite-based dental inserts and different restorative materials, after application of different clinical protocols. Starting from two different hydroxyapatite nano powders and nanostructured stabilized zirconia (YSZ), monophasic two-step sintered dense HAp inserts (TSSHAp), biphasic single-step sintered controlled porous inserts (HAp/TCp), and single-step sintered reinforced HAp/YSZ inserts were processed and characterized. Obvious differences in the microstructure of inserts surface were visualized after etching with 37% phosphoric acid. Fracture toughness of sintered inserts was ranged between 1.01 and 1.85 MPam^1/2 (maximum value in the case of HAp/YSZ), while hardness values were in the range of 3.71-5.22 GPa (maximum value in the case of TSSHAp). Acid etching before application improved SBS between inserts and Maxcem compared to direct cement application. TSSHAp and HAP/TCp inserts showed comparable and relatively high SBS values, certainly higher compared to HAp/YSZ inserts. Slightly higher SBS values were measured in the case of TSSHAp insert group, and the highest mean SBS value of 18.51 MPa was determined between TSSHAp inserts and Filtek Z250_SBU following the "self-etch" protocol.

The bulk compressive creep and recovery behavior of human dentine and resin-based dental materials

OBJECTIVE

To evaluate and compare the viscoelastic properties of dentine and resin-based dental materials by bulk compressive test and the Burgers model.

MATERIALS AND METHODS

Sound dentine, three resin composites as well as a resin-based cement were prepared into cylindrical specimens (n = 8). A bulk compressive creep test was applied with a constant load of 300 N (23.9 MPa) for 2 h, followed by another 2 h recovery. The maximum strain, creep stain, percentage of recovery and permanent set was measured using a linear variable displacement transducer. The viscoelastic properties were characterized via the Burgers model, and the instantaneous elastic, viscous as well as elastic delayed deformation were separated from the total strain. Data were analysed via ANOVA (or Welch's Test) and Tukey (or Games-Howell Test) with a significance level of 0.05.

RESULTS

Sound dentine presented the lowest maximum strain, creep strain, permanent set and the highest percentage of recovery, followed by 3 resin composites with comparable parameters, while the cement showed a significantly higher maximum strain, permanent set and lower percentage of recovery (p < 0.001). The Burgers model presented acceptable fits for characterization viscoelastic processes of both dentine and resin-based dental materials. Viscous and elastic delayed strain of dentine was significantly lower than those for tested materials (p < 0.001) with the highest instantaneous elastic strain percentage. Similar viscous and delayed strain was found among the 4 resin-based materials (p > 0.05).

SIGNIFICANCE

Sound dentine exhibited superior creep stability compared to resin-based dental materials. The viscous deformation in sound dentine could be ignored when loading parallel to dentine tubules.

Occlusal load modelling significantly impacts the predicted tooth stress response during biting: a simulation study

Computational models of the masticatory system can provide estimates of occlusal loading during (static) biting or (dynamic) chewing and therefore can be used to evaluate and optimize functional performance of prosthodontic devices and guide dental surgery planning. The modelling assumptions, however, need to be chosen carefully in order to obtain meaningful predictions. The objectives of this study were two-fold: (i) develop a computational model to calculate the stress response of the first molar during biting of a rubber sample and (ii) evaluate the influence of different occlusal load models on the stress response of dental structures. A three-dimensional finite element model was developed comprising the mandible, first molar, associated dental structures, and the articular fossa and discs. Simulations of a maximum force bite on a rubber sample were performed by applying muscle forces as boundary conditions on the mandible and computing the contact between the rubber and molars (GS case). The molar occlusal force was then modelled as a single point force (CF1 case), four point forces (CF2 case), and as a sphere compressing against the occlusal surface (SL case). The peak enamel stress for the GS case was 110 MPa and 677 MPa, 270 MPa and 305 MPa for the CF1, CF2 and SL cases, respectively. Peak dentin stress for the GS case was 44 MPa and 46 MPa, 50 MPa and 63 MPa for the CF1, CF2 and SL cases, respectively. Furthermore, the enamel stress distribution was also strongly correlated to the occlusal load model. The way in which occlusal load is modelled has a substantial influence on the stress response of enamel during biting, but has relatively little impact on the behavior of dentin. The use of point forces or sphere contact to model occlusal loading during mastication overestimates enamel stress magnitude and also influences enamel stress distribution.

Contemporary full-mouth rehabilitation using a digital smile design in combination with conventional and computer-aided design/manufacturing restorative materials in a patient with bruxism: A case report

RATIONALE

Full-mouth rehabilitation of patients with bruxism and severely worn dentition poses a great challenge to clinicians. Several treatment planning methods and restorative materials are used to treat tooth wear in modern dentistry. Clinicians should be able to select the most suitable treatment planning methods and materials for individual patients depending on their specific situation.

PATIENT CONCERNS

A 47-year-old male was referred for evaluation of a severely worn dentition.

DIAGNOSES

Clinical and radiographic evaluation revealed tooth wear in the entire dentition. The interocclusal distance at rest was 4 mm, and the patient had a parafunctional habit of bruxism.

INTERVENTIONS

A digital smile design was used to formulate a treatment plan. Full-mouth rehabilitation was performed using a combination of conventional and digital materials and methods.

OUTCOMES

The full-mouth restoration showed satisfactory functions and esthetics. No complications were observed in the restorations, supporting tissues, and temporomandibular joints during 2-year follow-up.

LESSONS

In clinical practice, it is important to determine the optimal combination of the available methods for treatment planning. This case report details the formulation of a unique treatment plan for the dental rehabilitation of a severely worn out dentition, which is considered challenging due to the limitations imposed by biological tissues and restorative materials. The use of conventional and digital tools for treatment planning, patient education, and treatment execution was demonstrated.

Numerical Analysis of a Dental Zirconium Restoration and the Stresses That Occur in Dental Tissues

When it is about restorative dental materials, aesthetics is traditionally preferred. This has led to the selection of materials very visually similar to the enamel, but unfortunately, their mechanical properties are not similar. This often translates into disadvantages than advantages. In the present work, a comparison is made of the stresses that occur during dental occlusion (dental bit) in a healthy dental organ and those that are generated in a dental organ with a dental zirconium restoration. Numerical simulation was carried out by means of the Finite Element Method, in computational biomodels, from Cone-Beam Tomography, to obtain the stresses generated during dental occlusion. It was found that the normal and von Mises stresses generated are substantially greater in the molar with restoration compared to those produced in the healthy molar. In addition, the normal function of the enamel and dentin to disperse these stresses to prevent them from reaching the pulp is altered. Therefore, it is necessary to analyze the indiscriminate use of this restoration material and consider other aspects, in addition to aesthetics and biocompatibility for the choice of restorative materials such as biomechanical compatibility.