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To review the laboratory surface characterization methods utilized to measure dental erosion. Peer-reviewed published scientific articles from Pubmed, Saudi Digital Library, and Google scholar platforms were searched, and only laboratory-based studies were included. Several quantitative and qualitative techniques have been used to assess enamel erosion in the laboratory. This review examines the applications, advantages, and limitations of the various methods currently used to assess dental erosion.
Keywords: Dental erosion, optical coherence tomography, scanning electron microscopy, surface characterization, surface hardness
| Introduction | |  |
Dental erosion is characterized by a loss of surface enamel or dentin from the chemical/acid activity of intrinsic, extrinsic origin, or environmental factors, resulting from microbial action.[1],[2] While the term is widely agreed upon, some researchers concentrate on acid erosion as the critical factor in dental wear's etiology. Erosive wear is the physical effect of a pathological, persistent, sporadic loss of dental hard tissue that is chemically etched away from the tooth surface by acid without bacterial intervention.[1] Dental erosion has been reported as an evolving oral health problem worldwide.[3] The approximate global incidence of dental erosion is 30% in teenagers.[3],[4] Low levels of enamel erosion were observed in 47% of the children, whereas moderate and severe enamel erosion was recorded in 10% and 4%, respectively, in Saudi Arabia.[5] Recognizing risk factors and early detection is essential for preventing and controlling dental erosion.[6] When dental erosion progresses, it can lead to dental hypersensitivity, cosmetic problems, and vertical dimension loss affecting badly oral health-related quality of life.[4],[7],[8] Moreover, dental erosion in the advanced stage may require expensive cosmetic and functional treatment. Dental erosion is multifactorial and is triggered by a dynamic relationship between the environmental conditions and tooth enamel.[9],[10] It highlights the individual's susceptibility to the development of dental erosion.[11] The primary factor is the dietary acids, which are the simplest to regulate.[12],[13] These acids may originate from various sources, including soft beverages and sports/energy such as drinks, teas, berries, fruit oils, alcoholic drinks, vinegar/picked produce, sauces, and confectionery. Similarly, eating habits relating to how these acids are consumed play an essential part. Dietary preferences have steadily modified over the last decades due to shifts in lifestyle, dietary habits, and improved dietary acid availability. Researches have correlated acidic fruits, carbonated beverages, and fruit drinks with dental erosion in children and adolescents. Peer-reviewed published scientific articles from Pubmed, Saudi Digital Library, and Google scholar platforms were searched, and only laboratory-based studies were included. The research proposal was submitted to the research center of Riyadh Elm University (FRP/2021/318), and formal approval was obtained. Methods used to research dental erosion in enamel are based on the stage of erosive lesion under examination, the form of improvement expected in the erosive lesion, and the measurement output of significance. Potential uses, benefits, and drawbacks of current methods concerning the laboratory characterization of dental erosion lesions are discussed in this review.
| Surface Hardness | | |
Surface hardness measurement methods have been widely used to examine the enamel surface pre- and posterosion or equate the enamel's erosion with the protected/reference uneroded enamel. This method is based on the concept of calculating the resistance of the test surface to the penetration of the indentor of the defined sizes, to the specified loading force, and to the indentor dwell time. There are three methods widely used to test the surface hardness of eroded enamel: Knoop microhardness, Vickers microhardness, and nanoindentation. Enamel hardness measurements are a stable tool for assessing surface enamel hardness properties since they are not prone to time-dependent alteration in morphology due to low elasticity and low retraction of enamel.[14]
Surface microhardness
oth Knoop and Vickers surface microhardness techniques use a diamond indenter of proven geometry to generate an indent evaluated for its scale to produce either Knoop hardness number or Vickers hardness number. Knoop microhardness uses a pyramid with a rhomboid base diamond indenter that can penetrate the enamel's surface roughly 1.5 μm when the tetra-pyramidal indenter Vickers can penetrate 5 μm while the regular loading forces of 50–200 g are used.[15] There is no agreement on the loading intensity or indent dwell period used by any of the microhardness methods used to evaluate the erosive lesion, but loading may be done using 1 g up to 1000 g of force with either loading time. There is no agreement as to which indenter is appropriate for testing the consequences of early erosion. However, Knoop microhardness was previously suggested due to its shallow depth of penetration.[12]
Surface nanohardness
Nanoindentation hardness is founded on the same concept as microhardness indentation but on a smaller scale. This procedure utilizes a trigonal pyramidal Berkovich diamond indenter with a maximum duration of 1 μm and loading forces from 0.25 to 50 mN (0.025–5.1 gf) in combination. This technique applies a continually rising load on the sample and the loading is relaxed until partial or complete Relaxation of the sample occurs; this enables the measurement of Young's modulus, nanohardness, time-dependent creep, plastic and elastic force, and fracture strength of the sample. After 2 s of acid exposure, it was confirmed that there is a significant difference in nanohardness in human enamel, and after 5 s of acid exposure, researchers showed that there is a significant difference in nanohardness in bovine enamel owing to the acid exposure.[16]
| Optical Coherence Tomography | | |
Optical coherence tomography (OCT) is a high-resolution, low-coherence interferometric technique using a near-infrared laser (1305 nm) producing surface and subsurface enamel photographs by calculating the strength and time delay of backscattered light from the various layers of enamel. This method is comparable to ultrasound imaging utilized in medical diagnostics; however, light is used instead of sound. Morphological cross-section pictures (B-scans) can be produced using variations in backscattered light (OCT signal) from the demineralized surface of the enamel. The decrease in optical coherence of the backscatter signal due to the eroded enamel's altered optical properties was used to describe the erosive transition. Its usage in the determination of early erosive shift has been restricted because of data analysis difficulties due to the enamel's surface speculation. It may obscure the details of the light scattering inside the initial few micrometers below the enamel's surface. While using different OCT systems of lenses, imaging modes and imaging depth, resolution, field of view, and contrast in enamel should be considered.[17]
| Tandem Scanning Confocal Microscopy | | |
Tandem Scanning Confocal Microscopy was first introduced for use in Dentistry by Watson and Boyde, 1985. The word confocal applies to an aperture in the objective lens's focal plane in both the microscope's illumination and imaging pathways. Only light within the lens's focal plane will travel through the aperture, and any stray light is blocked, creating high-resolution confocal microscopic images. The scanning disk enables the side-scanning of the sample to generate incredibly detailed images. The surface or subsurface of the enamel can be represented, and the thickness of the optical portion can be changed according to the Z-axis movement of the objective lens. Usually, Tandem Scanning Confocal Microscopy can penetrate 100 μm into enamel and dentin or can be used to obtain precise surface reflection images using green or blue light.[18]
| Focus Variation Three-Dimensional Microscopy | | |
Focus variation three-dimensional (3D) microscopy is a noncontact optical microscope image acquisition technique that uses the concept of focusing variation by holding the object stationary under the objective lens and rotating the lens in the z-axis using a piezo-control mechanism to or from the sample. Multiple images are captured by altering the position of the z-axis lens across the sample, and a 3D z-stack consisting of multiple images is generated, which can then be examined for the changes in surface shape and texture using specialized software. Focus variation 3D microscopy was used to measure the changes in surface roughness in acid erosion using traditional beverages, fruit juices and was also used to quantify changes in surface shape enamel.[19]
| Quantitative Light Fluorescence Microscopy | | |
Teeth have traditionally been seen to display auto-fluorescence when irradiated with blue-green light (~ 470 nm) and fluorescence with a wavelength of (~540 nm). It is caused by chromophores present in the organic portion of dentin and specific dentinoenamel junction The extent of fluorescence of the tooth is linked to the enamel's mineral content, with the demineralized areas becoming darker as the amount of fluorescence is lost due to the mineral depletion of the overlying enamel.[12] Quantitative light fluorescence microscopy was used to study carious enamel lesions and advanced dental erosion lesions. It has been shown that the quantitative light fluorescence microscopy can identify statistically significant fluorescence losses (1.28%) after 10-min erosion of normal human enamel samples and any successive 20-min erosive challenge. This was closely associated with both OCT results and surface microhardness values, which decreased with increased erosion.[12]
| Scanning Electron Microscopy | | |
Scanning electron microscopy utilizes an electron beam scanned over the material's surface to create an extremely accurate image that can be used to observe nano-and micro-scale surface morphology. Scanning electron microscopy was used to distinguish the various surface characteristics of enamel in conjunction with other measurement methods such as contact profilometry and surface microhardness. Qualitative differences in enamel samples could be visualized under scanning electron microscopy. Polished human enamel exhibited a characteristic keyhole pattern with eroded enamel prism cores and raised interprismatic enamel regions. Further, a scanning electron microscope was used to evaluate the differences between the natural and polished human enamel surface related to erosive behavior.[18]
| Atomic Force Microscopy | | |
Atomic force microscopy is one of the most precise measuring devices used to research surface structure, form, and morphological changes in enamel and dentin after nanometre resolution erosion. This is a scanning probe microscope technique that runs a stylus connected to a cantilever and an optical sensor over the sample's surface and calculates the forces between the stylus and the sample. As the stylus slides over the surface, the cantilever is deflected up and down due to the surface features present. The cantilever deflection is quantified by reflecting the optical sensor laser beam according to the photodetector's cantilever deflection. Atomic force microscopy has been used to demonstrate human enamel's erosive behavior after exposure to various acidic beverages. Similarly, atomic force microscopy was used to characterize very early erosion in polished enamel in a gastro-oesophageal reflux disease.[20]
| Profilometry | | |
Profilometry (surfometry) has been widely used to characterize the loss of enamel caused by erosion. The machine uses a tiny metal stylus with a diameter of 20 mm, which scans the enamel sample at a pace of around 10 mm/min to determine the impact on the part of the erosive agent. Simultaneously, another part of the sample surface is covered by adhesive tape or nail polish. The unprotected surface is subjected to erosive chemicals, offering a clear contrast between handled and secured regions. The sample surface is scanned before and after erosion, and the amount of material lost can be determined from the trace formed. Moreover, a cast made of the eroded enamel can be used to measure the cast. Profilometry was used to determine the erosive capacity of different in vitro products and clinical trials.[21],[22]
| Conclusion | | |
Several quantitative and qualitative laboratory methods currently available to measure dental erosion. However, every method has a specific application, benefits, and limitations. Micro indentation, profilometry, microradiography, and scanning electron microscopy are the most common methods to measure dental erosion. Atomic force microscopy imaging can enhance the measurement methods of quantitative analysis of dental erosion. Quantitative light fluorescence microscopy has shown promising usage in dental erosion studies.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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Correspondence Address:
Sultan S Al-Shamrani,
Department of Restorative Dentistry, College of Dentistry, Riyadh Elm University, Namuthajiya Campus, P. O Box: 84891, Riyadh 11681
Kingdom of Saudi Arabia
 Source of Support: None, Conflict of Interest: None DOI: 10.4103/ijpcdr.ijpcdr_3_21
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