The mechanical behavior of dentin: importance of microstructure, chemical composition and aging

Dental fracture is one of the three most common forms of failure of restored teeth and the most common cause of tooth loss or extraction in elderly patients -- Previous investigations conducted on aging of hard tissues have identified that there is a considerable reduction in the mechanical properties (i.e. fracture toughness, fatigue and flexural resistance) of dentin with aging and that may predispose tooth fracture -- These declines in properties have been attributed to microstructural and chemical composition changes over time -- However, these aging processes have not been really quantified and related with the changes in mechanical properties -- Accordingly, the aim of this work is to evaluate the aging process of coronal dentin in terms of the evolution of microstructure, changes in chemical composition and mechanical properties from selected age groups (young and old donors) -- The changes in these properties were evaluated in three different regions (outer, middle and inner) in order to identify spatial variations within the crown -- A brief description of the main literature on composition, microstructure and mechanical behavior of dentin is presented in chapter 2 -- An extensive experimental study was carried out in chapter 3 to identify the changes in microstructure of dentin with aging by means of optical and electron microscopy; while changes in chemical composition were analyzed using Raman Spectroscopy to calculate the mineral-to-collagen ratio -- Changes in mechanical properties were measured using Vickers micro-hardness -- Chapter 4 describes the importance of tubule density to the fracture toughness of dentin for young and old donor’s groups -- An approach previously proposed to study the mechanical behavior of porous materials was used to model the fracture toughness of coronal dentin in terms of the tubule characteristics -- Results were then compared with published results from previous studies -- The time-dependent deformation response of dentin was analyzed via spherical indentation experiments at different indentation loads in Chapter 5 -- From the experimental observations was proposed a simple model to describe the time dependent deformation behavior of dentin -- This model was based on previously proposed theories for indentation of time dependent materials, showing that the effective strain rate of dentin depends on its chemical composition (i.e. mineral-to-collagen ratio) and microstructure (i.e. lumen area fraction) -- The descriptions of the model were compared with the experimental results showing good agreement -- The same model was validated with experimental results of aged dentin, finding a low change in the deformation response of dentin with aging, as presented in chapter 6 -- Finally, preliminary results made on the mechanical properties of dentin have shown that the microstructure of aged human dentin can vary depending on the ethnic background of the donor and that this quality is critically important to the mechanical properties of the tissue -- In chapter 7 preliminary results on the comparison between the microstructure, chemical composition and mechanical properties of Colombian, Chinese and American donors is presented -- Finally, conclusions for the study are presented in chapter

Micromorphological and hardness analyses of human and bovine sclerotic dentin: a comparative study

The purpose of this study was to test the hypothesis that both human and bovine sclerotic dentin have similar hardness properties, in addition to similar micromorphological characteristics. Sixteen teeth (8 human and 8 bovine) exhibiting exposed dentin in the incisal edge and showing characteristics typical of sclerosis were used. Vickers surface microhardness testing was conducted. Three areas of the dentin surface of each specimen were selected. All teeth were processed for scanning electron microscopy in order to estimate the amount (in percentage) of solid dentin on the sclerotic dentin surface. The data were compared by Student's t test (α = 0.05). The micromorphological and microhardness data were compared by Pearson's linear correlation test (α = 0.05). The mean percentages of solid dentin of human and bovine sclerotic dentin were similar (human 90.71 ± 0.83 and bovine 89.08 ± 0.81, p = 0.18). The mean microhardness value (VHN) of human sclerotic dentin was significantly higher than that of bovine sclerotic dentin (human 45.26 ± 2.92 and bovine 29.93 ± 3.83, p = 0.006). No correlation was found between the microhardness values and the amount of solid dentin in the sclerotic dentin, irrespective of the species considered (human R² = 0.0240, p = 0.714; bovine R² = 0.0017, p = 0.923; and combined R² = 0.038, p = 0.46). We concluded that although both bovine and human sclerotic dentin present a similar amount of solid tissue, human sclerotic dentin presents higher microhardness than bovine sclerotic dentin.(FAPESP) São Paulo Research Foundation(CAPES) Coordination for the Improvement of Higher Education Personne

An engineering perspective of ceramics applied in dental reconstructions

The demands for dental materials continue to grow, driven by the desire to reach a better performance than currently achieved by the available materials. In the dental restorative ceramic field, the structures evolved from the metal-ceramic systems to highly translucent multilayered zirconia, aiming not only for tailored mechanical properties but also for the aesthetics to mimic natural teeth. Ceramics are widely used in prosthetic dentistry due to their attractive clinical properties, including high strength, biocompatibility, chemical stability, and a good combination of optical properties. Metal-ceramics type has always been the golden standard of dental reconstruction. However, this system lacks aesthetic aspects. For this reason, efforts are made to develop materials that met both the mechanical features necessary for the safe performance of the restoration as well as the aesthetic aspects, aiming for a beautiful smile. In this field, glass and high-strength core ceramics have been highly investigated for applications in dental restoration due to their excellent combination of mechanical properties and translucency. However, since these are recent materials when compared with the metal-ceramic system, many studies are still required to guarantee the quality and longevity of these systems. Therefore, a background on available dental materials properties is a starting point to provoke a discussion on the development of potential alternatives to rehabilitate lost hard and soft tissue structures with ceramic-based tooth and implant-supported reconstructions. This review aims to bring the most recent materials research of the two major categories of ceramic restorations: ceramic-metal system and all-ceramic restorations. The practical aspects are herein presented regarding the evolution and development of materials, technologies applications, strength, color, and aesthetics. A trend was observed to use high-strength core ceramics type due to their ability to be manufactured by CAD/CAM technology. In addition, the impacts of COVID-19 on the market of dental restorative ceramics are presented

Evaluation of the Biocompatibility and Micromechanical Properties of Experimental Dental Adhesives

The purpose of this study was to investigate the biocompatibility and micromechanical properties of newly synthesized antibacterial monomer and cross-linker functional surfactants into a commercially available dental adhesive (Single Bond, 3M ESPE, Saint Paul, MN, USA) at three concentrations (0.05, 0.1, 0.25 mg/mg). All groups were analyzed by evaluation of micro-tensile bond strength, ultimate tensile strength, cell viability, antibacterial properties, and surface micro-hardness. Scanning electron microscopy (SEM) was used for interfacial characterization. Human extracted molars were used as a substrate for bonding adhesives for the micro-tensile bond strength (MTBS) and scanning electron microscopy (SEM) studies. Twenty resin-dentin beams (0.9 ± 0.1 mm2) per group were evaluated at 24 hrs and 6 mos for MTBS. Slabs of ~1 mm were analyzed in the SEM for surface characterization. For the ultimate tensile strength (UTS), ten hour-glass shaped specimens (10 × 2 × 1mm) per group were tested at 24 hrs, 1 wk, and 6 mos. To evaluate toxicity, four disc-shaped specimens (5 × 2 mm) per group were incubated with human gingival fibroblasts (HGF). Antibacterial properties were evaluated by incubating three disc-shaped specimens (8 × 1 mm) per group with a strain of caries-producing bacteria S. mutans. Within the limitations of this in vitro study, it was concluded that incorporation of antibacterial monomer and cross-linker additives may be a viable option to help increase the longevity of tooth-colored adhesive restorations. Single Bond adhesive modified with 0.1 mg/mg monomer appears to provide the optimal balance for biocompatibility and micromechanical properties

Dentin Remineralization Around Ceramir Restoration

AIM: To determine the efficacy of Ceramir, a modified glass ionomer cement and a surfactant mono-n-dodecyl phosphate in remineralization of dentin around root caries restorations. MATERIALS & METHODS: 45 permanent intact teeth were embedded in self-cured dental acrylic resin to expose buccal or lingual surfaces. The buccal/lingual surfaces were wet ground with carbide paper, final polishing were accomplished with aluminum to obtain highly polished dentin surface. Baseline Knoop micro hardness values were recorded. All specimens were then etched using 37% phosphoric acid for 5 seconds to demineralize dentin and to expose dentin collagen. The Knoop indenter micro hardness measurements were again performed for each sample four indentations in dentin surface within an area of 75 μm. The mean of Knoop microhardness was calculated. Cavities, 6.35 mm width and 3 mm depth were prepared within the etched area of each tooth with round carbide bur. Samples were divided into 4 groups: 2 samples used as control with no restoration, 13 samples were restored with plain Ceramir, 13 sample were restored with Ceramir containing 2% mono-n-dodecyl phosphate restoration and 13 samples were restored with Ceramir containing 5% mono-n-dodecyl phosphate. Samples were stored in SBF a 37° C incubator. Knoop micro hardness values were recorded at a distance of 75 um from the margins of the restoration at 10,20 and 38 day intervals. RESULTS: Knoop hardness of dentin (KHN) was reduced by 33.7% after etching. Knoop hardness of dentin around Ceramir restorations returned to pre-etching levels after 10 days of restoration. There was no statistically significant difference in Knoop micro-hardness (KHN) between the plain Ceramir compared to the Ceramir with surfactant after 10 days. Also, there was no statistically significant difference between the plain Ceramir and Ceramir with 2% surfactant after 20 days. Knoop hardness around cavities restored with Plain Ceramir and Ceramir with 2% Surfactant were significantly higher than around cavities restored with Ceramir with 5% surfactant after 20 and 38 days. CONCLUSIONS: The result of this study shows that Ceramir restorations of dentin lesions lead to remineralization of dentin around the restoration margins in the area of 75 μm where the micro-indentations performed. It shows that addition of 2% surfactant to Ceramir tend to increase the remineralization over time. Toward the end of the observation period samples restored with Ceramir containing 2% surfactant appeared to remineralize at a faster rate than plain Ceramir. On the other hand addition of 5% surfactant was not beneficial as it led to a decrease in the remineralizing effect of Ceramir

Age effect on presence, susceptibility and treatment of erosive tooth wear

Indiana University-Purdue University Indianapolis (IUPUI)Erosive tooth wear (ETW) is a growing dental condition often associated with aging. This in-vitro project comprised three studies aiming to investigate the impact of tooth age on ETW susceptibility and prevention. In the first study, un-identified extracted premolars were collected and had their ages estimated using validated dental forensic methods. The premolars were examined to investigate the relationship between age and presence and severity of ETW, as well as other main dental-hard tissues conditions. ETW, dental caries, fluorosis, extrinsic staining and tooth color were evaluated using established clinical indices. In the second study, the tooth age impact on ETW susceptibility and response to preventive treatments (Sn+F, NaF, and de-ionized water control) were evaluated using representative samples from the initial study. Enamel and dentin specimens were prepared and subjected to daily erosion-treatmentremineralization cycling procedure. Surface loss (SL) was determined during and after the cycling, by optical profilometry. Similar protocol was adopted in the third study with the addition of toothbrushing abrasion to the model, in order to explore the interplay between age and toothpaste abrasivity on erosion-abrasion development. SL was measured during and after the erosion-toothbrushing-remineralization cycling. The relationships between age and the investigated variables were assessed using linear regression models. In conclusion: 1. The presence and severity of ETW, dental caries, and extrinsic staining increased with age, while of enamel fluorosis decreased. Tooth also showed to be darker with age. 2. Susceptibility of enamel and dentin to demineralization increased with age. Sn+F showed the highest anti-erosive efficacy, and was not affected by age. NaF showed lower efficacy on dentin, which increased with age. 3. Enamel and dentin SL increased with toothpaste abrasivity level. Dentin SL also increased with age. Age effect on enamel SL was observed only with low abrasive toothpaste. Age-related changes on enamel and dentin affected ETW development.2020-05-1

Tooth-bleaching procedures and their controversial effects: A literature review

AbstractAimThis review article will help clinicians improve their understanding of the history of bleaching procedures, bleaching types, components, mechanisms, and their effects on soft tissue, tooth structures, resin composite, and bonding.MethodsThe controversial issues about bleaching procedures and their effects are reviewed. Additionally, the consequences of pre- and post-bleaching on the bonding potential of composite resin restorations to tooth structure are discussed.ConclusionThe overall goal of the paper is to help reduce risks for patients

Conventional, Speed Sintering and High-Speed Sintering of Zirconia: A Systematic Review of the Current Status of Applications in Dentistry with a Focus on Precision, Mechanical and Optical Parameters.

The aim of this systematic review was to provide an overview of the technical and clinical outcomes of conventional, speed sintering and high-speed sintering protocols of zirconia in the dental field. Data on precision, mechanical and optical parameters were evaluated and related to the clinical performance of zirconia ceramic. The PICOS search strategy was applied using MEDLINE to search for in vitro and in vivo studies using MeSH Terms by two reviewers. Of 66 potentially relevant studies, 5 full text articles were selected and 10 were further retrieved through a manual search. All 15 studies included in the systematic review were in vitro studies. Mechanical, precision and optical properties (marginal and internal fit, fracture strength and modulus, wear, translucency and opalescence, aging resistance/hydrothermal aging) were evaluated regarding 3-, 4- and 5-YTZP zirconia material and conventional, high- and high-speed sintering protocols. Mechanical and precision results were similar or better when speed or high-speed sintering methods were used for 3-, 4- and 5-YTZP zirconia. Translucency is usually reduced when 3 Y-TZP is used with speed sintering methods. All types of zirconia using the sintering procedures performed mechanically better compared to lithium disilicate glass ceramics but glass ceramics showed better results regarding translucency

Dental prostheses mimic the natural enamel behavior under functional loading: A review article

SummaryAlumina- and zirconia-based ceramic dental restorations are designed to repair functionality as well as esthetics of the failed teeth. However, these materials exhibited several performance deficiencies such as fracture, poor esthetic properties of ceramic cores (particularly zirconia cores), and difficulty in accomplishing a strong ceramic–resin-based cement bond. Therefore, improving the mechanical properties of these ceramic materials is of great interest in a wide range of disciplines. Consequently, spatial gradients in surface composition and structure can improve the mechanical integrity of ceramic dental restorations. Thus, this article reviews the current status of the functionally graded dental prostheses inspired by the dentino-enamel junction (DEJ) structures and the linear gradation in Young's modulus of the DEJ, as a new material design approach, to improve the performance compared to traditional dental prostheses. This is a remarkable example of nature's ability to engineer functionally graded dental prostheses. The current article opens a new avenue for recent researches aimed at the further development of new ceramic dental restorations for improving their clinical durability