Replica-based inspection of enamel wear microfeatures

Abstract

BACKGROUND:

Surface replication is a nondestructive evaluation technique applied in examining surface wear by recording surface irregularities, especially in conditions when surfaces of interest cannot be further manipulated to fit directly under a microscope to be examined. Enamel is the outermost protective layer of the human teeth and is constantly stressed by mastication forces which results in enamel wear.

OBJECTIVE:

To date, a procedure combining the clinical and microscopic examination of enamel surfaces is absent, which hinders the early diagnosis and comprehension of the wear process.

METHODS:

This study investigated the role of replication sheets in registering microscopic wear on human enamel surfaces by both negative and positive replication techniques.

RESULTS:

The sheets replicated wear features successfully. Sheets were compatible to use with multiple microscopes, with proper preparation, including high resolution microscopes such as the scanning electron microscope and transmitting electron microscope.

Keywords

Surface replication tooth wear dental enamel surface properties electron microscopy

1. Introduction

The numerous etiological factors in the process of wear and their interrelationship make it complex and difficult to define, but there is a need to investigate the process [1]. It is probable that the increase of the natural teeth retention into older ages will result in increased prevalence of the severely worn dentition [2]. The challenges in the clinical management of such patients have aroused considerably. Both the professional and scientific interests in tooth wear have aroused to meet these challenges in the past decade. ESCARCEL, a recent pan-European study amongst 3187 subjects aged 18–35 years, concluded that one in three young adults suffer from tooth wear [3]. In a survey of 200 dental professionals completed in 2013, 84% identified signs of erosive tooth wear on a weekly basis and 86% felt the condition was on the rise. In a study of 1108 adults aged 15–89 years in Tokyo, Japan, 26.1% had signs of erosive wear. Erosive wear, in combination with abrasion and attrition, results in severe loss of tooth tissue [4].

Not only the wear process ranges among patients, but also individually. Wear process range between normal and pathological periods, which complicates the reliable measurement of wear’s initiation and progression. The mechanism of the wear process includes the formation of fatigue microcracks of the worn dental tissues as well as a process of microcutting, i.e. microblading of the surface’s microstructures [5,6].

Revealing the mechanism of surface microcracks (mechanical damage initiation) on the microstructural level or enamel rod level is demanding. If the observation and analysis can be clarified at the microscopic level, the relationship between enamel rods and the structure of hydroxyapatite, which is the basic structure of enamel, and microcracks will be understood. Furthermore, the resistance of enamel according to each age can be known. Understanding the mechanism of wear is crucial in preventive approaches, retaining functional healthy dentition and increasing biomimicry of the restorative dental materials.

Replication is a nondestructive sampling procedure that records and preserves the topography of a prepared surface of interest as a negative relief on a plastic film, i.e. replica. This technique copies the surface and/or wear details for later indirect microscopic examination [7] especially when the surface of interest cannot be further manipulated by cutting, polishing or sectioning for direct microscopy. Replication by sheets has been applied in metallurgical, topographical and tribological studies of surfaces wear since the invention of scanning electron microscopes (SEM). Sheets can replicate the surfaces in either a negative or positive replication [8–10]. The sheet, a ‘negative replica’ of the surface, can be examined directly under a light microscope or covered with a conductive coating and examined by SEM. In a positive replication, the sheet is used as a mold to produce a ‘positive replica’. These replicas can be made by coating negative replicas with conductive coating as carbon or palladium and then dissolving the acetate sheet with acetone. The coating layer replicas are floated over microscopic grids which can be examined in the transmitting electron microscope (TEM). Imaging of the positive replica has the advantage of the absence of inversions and finer details [11]. To date, no material is present to copy surfaces of the human tooth to be examined directly under a microscope unless the tooth is extracted and sectioned which is impractical in the clinical situation. This study investigated the application of replication sheets in the scanning/transmitting electron microscopic examinations of the wear microfeatures on human enamel surfaces.

2. Materials and methods

2.1. Samples preparation for impact sliding wear testing (ISWT)

This study methodology is a sequel and adaption of a previously published article [12]. Twelve flat human enamel specimens were embedded in acrylic resin within acrylic rings. The surfaces of the rings were polished to produce enamel windows with a glossy finish. Samples were then subjected to enamel-to-enamel impact and sliding wear testing (ISWT) conditions (ISWT K655-07; Tokyo Gaiken, Tokyo, Japan). The test was set to direct a vertical enamel hammer, a 5 mm dome-shaped enamel specimen attached to a metal stylus, to drop from a height of 1 mm with a force of 30 N. The hammer then slides 1 mm horizontally while contacting the enamel window and back to its position, before rising up to the start of the cycle. A total of 20,000 cycles were performed with a frequency of 0.23 Hz. The entire impact sliding test took place below the water level in the tank of the ISWT machine to prevent burning of enamel due to friction heat generation [13]. The water temperature and level were checked for heating and evaporation at 37 °C. During the test, samples were covered by a slurry of PMMA powder (non-plasticized, polymethylmethacrylate; Techpolymer MBX-50, Sekisui Plastics, Tokyo, Japan) mixed with tap water at a 1:1 weight ratio to mimic a food bolus during the wear test. At every 5000 cycles, enamel samples were rechecked to prevent displacement. The slurry was washed off and renewed.

2.2. Samples preparation after wear testing (ISWT) and replication of sheets

After ISWT, all samples were washed with distilled water for 20 s and dried with gentle air spray without polishing. Samples were then immersed in a hydrochloric acid solution (0.005 M for 10 s), washed for 60 s and then stored back in distilled water for 15 min to halt any remaining effect of the HCl solution. For surface replication, samples were re-dried by gentle air spray. Two drops of acetone were placed on each enamel surface, 1 cm × 1 cm pieces of replication sheets (replicating material AGG255, Cellulose Acetate, 35 μm, 150 mm × 100 mm; Agar Scientific, EM, Tokyo, Japan) were placed immediately over the specimen surfaces. Acetone was allowed to evaporate for 10 min. The sheets were then gently stripped off the dry specimen’s surface and fixed upside down on a glass slab by scotch tapes around the corners. The process of sheet replication was conducted three times. The first set of sheets was marked as the ‘extraction set’ while the second and third sets were marked as ‘proper sets’. Samples were then stored back in distilled water to prevent excessive dehydration.

2.3. Imaging of the samples and sheet replicas

Both samples and their corresponding sheet replicas were examined with multiple microscopic techniques. A three-dimensional (3D) colored laser microscope and profilometer (VK 250 series; Keyence, Tokyo, Japan) were used to examine the enamel window and wear zone shapes/measurements, obtain the profile of the surface and automatically compute the surface’s microroughness parameters. The parameters computed were Sa (arithmetical mean height of the surface; the difference in height of each point compared to the center plain of the surface), Sz (the sum of the largest peak height value and the largest pit depth value within the defined area of a surface) and Sdr (the percentage of the definition area’s additional surface area contributed by the texture as compared to the planar definition area). A 3D image buildup of the samples and sheets was constructed with the same microscope. For SEM imaging, sheets were carefully cut smaller, fixed on SEM stumps by double-sided carbon tape, directly spattered with gold-palladium (40 nm) and imaged by SEM (H-4500; Hitachi High-Technologies, Tokyo, Japan). For TEM imaging, the required areas were cut out from the sheets and laid upside down onto microscopic grids and spattered by palladium (20 nm). After spattering, grids were placed on a wire mesh standing in a dish of acetone, with the acetone’s level just touching the bottom of the mesh. After 1 h, the grids were removed from the mesh and allowed to dry for 15 min prior to placement in TEM (H-600; Hitachi High-Technologies, Tokyo, Japan) for fine details examination. Each sample/replica had a set of multiple microscopic images to capture and analyze most of the various microfeatures of wear.

2.4. Statistical analysis

The data obtained by computerized measurement of the samples group and sheet replica group comprised enamel window microroughness measurements of Sa, Sz and Sdr, wear zone cross-sectional area, and total enamel window volume. Due to the small sample size, absence of both homogeneity of variance and normality (Sharpio-Wilk test) in the results, non-parametric statistical analysis was conducted. Wilcoxon signed-rank test was conducted to detect differences between the two groups with the significance level set at 0.05 by a computer software (SPSS Statistics for Windows, version 24; IBM, Armonk, NY, USA). Spearman’s rank correlation coefficient was conducted to assess the correlation of the measurements between the two groups. In addition, Bland-Altman analysis [14] was conducted to evaluate the sample-replica reliability of each measurement. The compatibility between sheets and samples exists if the relative error is within ± 20% at 75% or more of the whole data [15].

3. Results

3.1. Wear features on the enamel surfaces

Wear features were induced on the enamel samples after ISWT (Fig. 1). All samples showed fine scratching marks of the contacting slurry particles with disappearance of the glossy mirror-like finish. The initial impact zone suffered the most extensive loss of substance forming pits and craters. The sliding movement caused the formation of a shiny wear zone surrounding the pits/craters. Microcracks were radiating from the crater corners or surrounding the wear area. Furthermore, enamel of the impacting hammers showed multiple wear features such as fractures, pitting, ploughing, chipping and microcracks.

Fig. 1.

Wear microfeatures on enamel surfaces after ISWT. A 3D image of the enamel hammer. The impaction and sliding caused wear with multiple features on the enamel dome i.e. fractures, pitting and chipping. B A scanning laser microscopic image of the enamel dome (upper sample/impact hammer) representing some of the wear features. C 3D image of an enamel window after ISWT with a depth indicator of the reference plain. D A scanning laser microscopic image of wear features, a shiny appearance of the wear zone with the presence of a crater in the initial contact zone, scratching marks, and microcracks extending from the wear zone.

3.2. 3D colored laser microscope/profilometer images

Sheets replicated the features of wear such as scratching marks, microcracks, and craters. However, sheets replica inverted images of the replicated enamel surfaces (Fig. 2). The inversion was either vertical or horizontal, which depended on the fixation of the sheets on the glass slaps. The inversion was also present in the profile images with depths of the wear features registered as heights in the sheets’ images. Many images had hazy circles on the surface. The clarity of those circles changed with different focus depths. Surface roughness measurements and analysis of the dimensions of the image were conducted (Fig. 3).

Fig. 2.

Inversion between the specimens’ images and their corresponding sheets. The sheet images were the reverse of the specimen images in both directionality and topography. In directional inversion, features of the replica will be on the opposite side of a reference point; left positions will become right or upper positions will become lower. In topographical inversion, the changes occurred in depth measurements as depths on the specimen’s surface became heights on the sheet surfaces, the crater area (marked with arrows) on the specimen change of the depth’s indicator color. Images were taken by a 3D laser microscope.

Fig. 3.

Multiple analysis of the enamel wear microfeatures. Images of the enamel surfaces and sheet replicas were analyzed by the computerized scanning colored laser microscope and profilometer. The upper image represents a microcrack extending on the enamel surface. A Depth measurement of the microcrack on the enamel surface. B Width measurement of the microcrack on a replica image. C Relationship of the microcrack to enamel microstructures and surface topographic image of the microcrack pathway.

3.3. SEM images

SEM was conducted after the preparation of samples and sheets. Microcracks and fine scratching marks were clearly visible as whitish shiny lines. This was not the case when viewing wear zones boundaries (Fig. 4). On the other hand, craters inside the wear zone were viewed successfully. Some tearing was noticed in the sheets’ deepest portions of the craters or deep microcracks. SEM imaging on high magnifications with secondary electrons, i.e. high mode, had to be conducted in less than 60 s to prevent burning of the sheets.

Fig. 4.

Scanning electron microscope (SEM) imaging of different wear features. A Colored laser microscope of wear of an enamel sample with presence of scratching marks and microcracks. B SEM image of the same sample; scratching marks and microcracks are shown as whitish shiny lines with faint boundaries of the wear zone. C SEM image of the sheet replica of the same wear; the wear zone boundaries are absent with clear microcracks as whitish lines with reduced lengths. (White arrows indicate microcracks, black arrows indicate wear zone boundaries.)

3.4. TEM images

TEM was conducted after sheet preparation. Microcracks were clearly visible on the TEM images as dark wavy lines following the pathway of interrod spaces. Faint marks of the enamel rod boundaries were present. Some particles were caught off the surface and appeared on the images as dark areas (Fig. 5). Craters and wear zones were impractical to be imaged with TEM due to the limited size of the grids and superimposition of the radiopaque microscopic grids’ bars.

Fig. 5.

Transmitting electron microscope (TEM) imaging of the wear features. A Scanning electron microscope (SEM) of a replication sheet. B TEM image of area 1 on the SEM image; the scalloping of the microcrack is shown as a wavy dark line, small white arrows indicate faint replicas of enamel rods. C TEM image of area 2 on the SEM image; the discontinuation of microcrack through enamel structure is captured. The image of some particles caught off the surface in the process of replication are presented as dark marks.

3.5. Microcracks surface measurements of samples/sheets images

Wilcoxon signed-rank test showed no significant differences (P = 0.875) in the wear zone dimensions between the samples and their corresponding replicas. The surface microroughness of the replication sheets was significantly higher than that of the samples in all parameters of Sa (P = 0.028), Sz (P = 0.003) and Sdr (P = 0.002). Spearman’s rank correlation coefficient (Table 1) showed similar findings as the parameters of the replica group were significantly related to those of samples in cross-sectional area measurements only (r = 0.888, P = <0.001). Bland-Altman analysis (Fig. 6) showed that the reliability of sheet replication in the measurement of the cross-sectional area of the wear zones only compared with corresponding replicas.

Table 1
Spearman’s rank correlation coefficient between replicas and samples

r P

Enamel window Sa 0 1

Sz 0.385 0.217

Sdr 0.007 0.983

Wear area Sa 0.413 0.182

Sz −0.287 0.366

Sdr 0.077 0.812

Cross-sectional area 0.888 <0.001

Surface area 0.028 0.931

		r	P
Enamel window	Sa	0	1
	Sz	0.385	0.217
	Sdr	0.007	0.983
Wear area	Sa	0.413	0.182
	Sz	−0.287	0.366
	Sdr	0.077	0.812
Cross-sectional area		0.888	<0.001
Surface area		0.028	0.931

Fig. 6.

Bland-Altman plot of cross-sectional measurement of wear zones’ areas. The large dashed line denotes the mean of difference and the small dashed lines denote 95% limits of agreement (±2 SD of difference).

4. Discussion

The aim of this study was to investigate the use of replication sheets in the microscopic examination of enamel wear. Sheets replicated surface wear microfeatures as scratches, wear craters, and furrows successfully. Sheets registered wear zones/areas successfully and no differences were detected in the actual measurement of the zones on the samples’ surfaces directly (P = 0.875). This was confirmed by the Spearman’s correlation test (r = 0.888, P = <0.001) and Bland–Altman analysis, as sheets showed reliability and correlation in the measurements of these areas. On the contrary, microroughness registration was unsuccessful as the parameters of the sheets’ microroughness were significantly greater than enamel samples’ surfaces.

The microscopic imaging of enamel samples and their replicas showed the presence of wear-induced microfeatures on the initially glossy surfaces. These features comprised polished wear zones, stripes, striations in addition to various furrows of variable sizes, fine scratch marks, microcracks and pits [16]. This is consistent with a previous study [17] which reported striations on the occlusal surface as the main feature of occlusal microwear and pits were present in the force application points with microcracks propagating from these zones.

In this study, it was necessary to clear and enhance the surface topography of the surfaces to be examined without compromising the micro-details of the wear. This micro-damage can occur even with minimal mechanical preparation as polishing. Chemical treatment by acid etching enhances the topography of enamel, changing it from a low-reactive surface to a surface that is more susceptible to adhesion, increasing the surface energy so low viscosity fluids, such as acetone, are attracted to the interior of microporosity [18]. In a pre-study with a trial/error approach, immersion in HCl of 0.005 M for 10 s was sufficient to clean and enhance the surface topography of the enamel surfaces to be examined without compromising the micro-details of the enamel wear.

Repeating the replication twice decreased the amount of impurities and artifacts between the extraction and proper replication sheets. Some studies used the initial extraction sheet as a source of capturing microorganisms especially in studying bacterial contamination of enamel cracks [19]. In this study, some particles were extracted from the sample surfaces and were revealed by TEM. Unfortunately, detecting the nature of these particles was not an objective of the study protocol. Hazy circles on the sheets appeared on the initial scanning laser microscope imaging. These circles did not hinder the registration of the wear features of the enamel surfaces. Changing the focus depth on the microscope made the circles disappear. These circles are caused by the roughness of the back surface of the acetate sheets [10] and their interference with the light penetration. Coloring the back of the sheets in black could counter this effect.

In SEM imaging, microcracks appeared as shiny whitish lines since electrons will concentrate in these projections on the sheets’ surfaces. Our findings indicated that the use of secondary electrons is useful in examining fine-linear details rather than wide-flat features. Wear zones were not detected on the SEM images of sheets or specimens. This can be attributed to the instrumentation of SEM on feature visibility and effects of magnification levels in most microwear studies [20]. SEM has limitations in defining surface topography, as the electron beam technique does not allow visualization of 3D surface texture as its contrast relies on the different emissions of electrons, which cannot give contrast on flat homogeneous surfaces. For finer details examination, the sheet can be used as a mold to produce a ‘positive replica’ [11]. In this study the positive replicas were made by palladium spattering as sheets could not bear the heat of the high voltage of the TEM. This procedure limited the examination of the wear area to the dimensions of the microscopic grid (a 2 mm circular brass grid) hindering the registration of crater details. In this study, SEM was preferred over TEM as an imaging technique of sheet replicas as TEM required more sophisticated preparations. Increasing the number of steps of replication might cause less completeness of registration, decreased accuracy, reduced field of imaging, and complicated preparations. The examination of the replicas by means of a light or laser microscope was sufficient for a direct, simple examination of the wear features.

A previous study [21] reported that the use of an optical instrument is the most appropriate examination method of surface roughness in metallo-biomaterials. This method does not require any particular preparation of the surfaces nor will it cause any deterioration. Since surface topography is 3D in nature, the measurement of 3D surface topography can represent more realistic, natural characteristics of a surface [22]. Surface parameters should be chosen which can both quantify surface roughness and provide information on the shape of the surface under investigation [23]. Measurement of the surface texture by a non-contact laser-scanning microscopy can reveal a more detailed definition of the surface texture than the examination under SEM [22,24]. Accordingly, this study assessed the surface microroughness by automatic computing of multiple parameters: Sa, Sz and Sdr using a 3D scanning laser microscope and profilometer, a procedure with both quantitative and qualitative approaches. Although our findings indicated that replication sheets were unable to register microroughness of enamel samples, the Bland-Altman analysis showed that the sheet evaluation has reliability for cross-sectional area measurements compared to the measurements on enamel samples directly. The reliability was evaluated based on the relative error [15]. However, tolerance range of agreement is not defined for this statistical method [25]. Some 3D construction images had blackened voids. These voids were reported [26,27] to represent torn parts of the sheets due to the entrapment of the replication materials in undercuts or deep portion of the surface of interest. Replication by sheets requires further improvements in registering 3D complex features of the surface, accuracy of microroughness data in sheets’ replication, and sheets’ ability to penetrate deeply in the surface’s details.

Replication by sheets is the most common nondestructive surface testing (NDT) technique [27]. It is considered simple, inexpensive, durable, and SEM compatible since sheets can be spattered directly with conductive coatings [19,27–29]. SEM constitute a tremendous tool in scientific studies of tooth wear and has been described as a more sophisticated alternative method in monitoring the progression rate of wear. Thus, SEM should be incorporated in clinical practice [30]. By combining sheet replicas and SEM imaging, changes in enamel wear could be detected at shorter time intervals. Applying cellulose cement instead of acetone to the examined surface [28] or pre-wetting cellulose sheets with acetone might be considered as an alternative to the direct application of acetone intraorally. This might widen the scope of sheet replication towards clinical applications.

Multiple replicas can be easily produced from one original dental crown impression. Sheet replicas are durable, capable of capturing fine details, and are very robust. Replicas can be mailed easily to other researchers for records, comparative studies [31], and sequential documentation [29]. The production of permanent records for future studies is of a considerable concern due to the extended nature of dental wear. These properties are quite valuable in the process of early detection of enamel microcracks/wear, indirect microscopic examination of teeth, sequential recording of wear progress, and mass reservation of patient records.

5. Conclusions

Within the scope of this study, the following conclusions can be drawn: (1) Enamel wear can be inspected by sheet replication, and (2) the verification of microwear details, replica flaws and the true samples features depended on the microscopic examination. The registration of complicated 3D details such as microroughness requires further improvements.

Footnotes

Acknowledgements

This work was partially supported by the Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT grant no. 19K19259 to K.W, 2019).

Conflict of interest

None to report.

Appendix

A schematic representation of the replication process and post-replication preparation of the sheets. When the sample surface is wet with acetone, the replication sheet will soften and flow inside the sample surface’s features. After the acetone is evaporated, the sheets solidify registering the details of the replicated surface. A Sheets can be fixed on a glass slap around the corners by tape and immediately examined under a light microscope. B For scanning electron microscopes (SEM) microscopic examination, sheets are fixed by double sided carbon tape on SEM stage, spattered with a conducting coting as Gold/Platinum (Au/Pt spattering) and examined under SEM. C For transmitting electron microscope (TEM) imaging, the area of interest on the sheets are cut smaller, spattered by Palladium or Carbon evaporated. The coated sheets are placed over the TEM grid and placed over a metallic mesh on an acetone bath. The acetone surface is just touching the lower surface of the metallic mesh. After 2 h, sheets are dissolved and the grid can be placed in TEM to examine the coating replica.

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