The measurement threshold and limitations of an intra-oral scanner on polished human enamel

Use of polyvinyl siloxane impressions to monitor sub-5-μm erosive tooth wear on unpolished enamel

STATEMENT OF PROBLEM

Whether polyvinyl siloxane impressions are capable of reproducing 5-μm changes on natural freeform enamel and potentially enabling clinical measurements of early surface changes consistent with wear of teeth or materials is unclear.

PURPOSE

The purpose of this in vitro study was to investigate and compare polyvinyl siloxane replicas with direct measurements of sub-5-μm lesions on unpolished human enamel lesions by using profilometry, superimposition, and a surface subtraction software program.

MATERIAL AND METHODS

Twenty ethically approved unpolished human enamel specimens were randomized to a previously reported cyclic erosion (n=10) and erosion and abrasion (n=10) model to create discrete sub-5-μm lesions on the surface. Low viscosity polyvinyl siloxane impressions were made of each specimen before and after each cycle and scanned by using noncontacting laser profilometry and viewed with a digital microscopy and compared with direct scanning of the enamel surface. The digital maps were then interrogated with surface- registration and subtraction workflows to extrapolate enamel loss from the unpolished surfaces by using step-height and digital surface microscopy to measure roughness.

RESULTS

Direct measurement revealed chemical loss of enamel at 3.4 ±0.43 μm, and the polyvinyl siloxane replicas were 3.20 ±0.42 μm, respectively. For chemical and mechanical loss direct measurement was 6.12 ±1.05 μm and 5.79 ±1.06 μm for the polyvinyl siloxane replica (P=.211). The overall accuracy between direct and polyvinyl siloxane replica measurements was 0.13 +0.57 and -0.31 μm for erosion and 0.12 +0.99 and -0.75 μm for erosion and abrasion. Surface roughness and visualization with digital microscopy provided confirmatory data.

CONCLUSIONS

Polyvinyl siloxane replica impressions from unpolished human enamel were accurate and precise at the sub-5-μm level.

Validation of clinically related aging models based on enamel wear

PURPOSE

Physiological and erosive wear reported in clinical studies were reviewed, and in vitro aging models were developed to simulate and compare the effect of aging on human teeth with the review data obtained from clinical studies.

METHODS

A review of clinical studies and randomized clinical trials that quantify enamel wear was performed in the PubMed database. The first in vitro analysis evaluated the effect of mechanical chewing simulation only. Enamel specimens were aged in the chewing simulator (up to 1.2 million cycles) with two occlusal loads (30 and 50 N). In the second in vitro analysis, specimens were aged in two aging models. The first model (MT) simulated mechanical and thermal oral challenges: MT1- 240,000 chewing and 10,000 thermal cycles, MT2- 480,000 chewing and 20,000 thermal cycles, MT3- 1.2 million chewing and 50,000 thermal cycles. The second model (MTA) simulated mechanical, thermal, and acidic oral challenges as follows: MTA1- 240,000 chewing, 10,000 thermal and 3-h acidic cycles; MTA2: 480,000 chewing, 20,000 thermal and 6-h acidic cycles, MTA3- 1.2 million chewing, 50,000 thermal and 15-h acidic cycles.

RESULTS

The review included 13 clinical studies evaluating tooth wear (eight physiological and five erosive). The results estimated the annual average physiological wear as 38.4 µm (9.37-51). In comparison, the MT1 showed wear of 60 (24) µm. Also, the average annual erosive wear in the literature was 179.5 µm (70-265) compared to MTA1-induced wear of 209 (14) µm.

CONCLUSION

There was wide variation in tooth wear reported in clinical studies, suggesting a critical need for more accurate studies, possibly based on scanning technologies. Despite this, the data reported using the novel aging models are within a range to be considered consistent with and to simulate tooth wear measured in vivo.

Intraoral scanners as tracking devices: A dental protocol for assessing volumetric changes between intraoral scans

Intraoral scanners (IOSs) are digital data acquisition technologies that ease the recording of virtual diagnostic casts. Some IOSs have a specific software tool to assess volumetric changes between 2 scans acquired on the patient at different times. The scans are superimposed and volumetric differences between both meshes are reported. However, these software tools may be limited to scans captured only by the IOS of the same manufacturer. The present manuscript describes a protocol for comparing volumetric changes between 2 scans recorded using any IOS. Additionally, 1 of the scans is divided into 3 sections to minimize the alignment distortion and maximize the evaluation of the volumetric changes.

Quantifying error introduced by iterative closest point image registration

OBJECTIVES

The aim of this paper was to quantify the analysis error introduced by iterative closest point (ICP) image registration. We also investigated whether a subsequent subtraction process can reduce process error.

METHODS

We tested metrology and two 3D inspection software using calibration standards at 0.39 μm, and 2.64 μm and mathematically perfect defects (softgauges) at 2 and 20 μm, on free form surfaces of increasing complexity and area, both with and without registration. Errors were calculated in percentage relative to the size of the defect being measured. Data were analysed in GraphPad Prism 9, normal and two-way ANOVA with post-hoc Tukey's was applied. Significance was inferred at p < 0.05.

RESULTS

Using ICP registration introduced errors from 0 % to 15.63 % of the defect size depending on the surface complexity and size of the defect. Significant differences were observed in analysis measurements between metrology and 3D inspection software and within different 3D inspection software, however, one did not show clear superiority over another. Even in the absence of registration, defects at 0.39 μm, and 2.64 μm produced substantial measurement error (13.39-77.50 % of defect size) when using 3D inspection software. Adding an additional data subtraction process reduced registration error to negligible levels (<1 % independent of surface complexity or area).

CONCLUSIONS

Commercial 3D inspection software introduces error during direct measurements below 3 μm. When using an ICP registration, errors over 15 % of the defect size can be introduced regardless of the accuracy of adjacent registration surfaces. Analysis output between software are not consistently repeatable or comparable and do not utilise ISO standards. Subtracting the datasets and analysing the residual difference reduced error to negligible levels.

CLINICAL SIGNIFICANCE

This paper quantifies the significant errors and inconsistencies introduced during the registration process even when 3D datasets are true and precise. This may impact on research diagnostics and clinical performance. An additional data processing step of scan subtraction can reduce this error but increases computational complexity.

The effect of location/site on polished human enamel after mechanical and chemical wear

OBJECTIVE

To compare profilometry and microhardness of polished occlusal and buccal human enamel following a mechanical and chemical wear regime.

METHODS

Enamel from polished human molars were sectioned into buccal and occlusal surfaces and randomly allocated into two groups (n = 10) and then exposed to 0.3 % citric acid at pH 2.7 for 10, 20, 40 and 60 mins each followed by abrasion with non-fluoridated toothpaste for 240 strokes in a reciprocating brushing machine. A white light profilometer with a spot size of 12 um measured mean step-height following each cycle. Microhardness indentations were conducted following the final cycled 60 mins erosion/abrasion using 0.01, 0.02, 0.1, 0.5 and 2.5 kgf indentation load. Statistical disparity were evaluated using a two-way ANOVA and post-hoc Sidak's multiple comparisons tests at α = 0.05.

RESULTS

After erosion/abrasion, the mean (SD) step-heights on occlusal and buccal surfaces were not significantly different until 60 mins, when occlusal surfaces exhibited greater step-heights, 32.9 µm (2.8) and 31.1 µm (1.8) and p = 0.02, respectively. Buccal and occlusal microhardness was statistically lower following erosion/abrasion at loads of 0.01 kgf (p = 0.0005) and 0.02 kgf (p = 0.0006) but no significant differences were observed in the microhardness between the surfaces at any loads.

CONCLUSION

The occlusal and buccal surfaces were not statistically different for microhardness or step height suggesting the susceptibility to wear is not related to the anatomy and structure of the tooth and is more likely related to other factors such as the environment.

CLINICAL SIGNIFICANCE

The study emphasizes that a notable difference in wear between occlusal and buccal enamel surfaces emerges only after prolonged exposure to simultaneous chemical and mechanical stress. This finding necessitates a preventive dental approach that accounts for both the duration of exposure and environmental factors.

CAD/CAM leucite-reinforced glass-ceramic for simulation of attrition in human enamel in vitro

OBJECTIVE

Investigate attrition simulation using CAD/CAM leucite-reinforced glass-ceramic antagonists on occlusal vs. buccal enamel.

METHODS

Three dental materials with known wear rates (resin-modified glass-ionomer, micro-filled, and fine particle composites) validated the wear simulator (CAD/CAM glass-ceramic antagonists, 200 cycles, 80 N load, deionised water irrigation, 0.7 mm sliding movement). Following this, human molars were sectioned into paired occlusal and buccal polished samples (n = 8/gp). Exposed 1.5 mm Ø enamel areas were subjected to attritional wear with and without pre-immersion in citric acid (5 min, 0.3%, pH 3.8). Profilometry measured step-height enamel wear and surface microhardness at different depths was calculated using Vickers indentation at 0.1 N and 0.5 N loads.

RESULTS

Dental material wear using the CAD/CAM antagonists showed consistency with previous data: mean (SD) resin-modified glass ionomer material loss of 177.77 (16.89) µm vs. 22.15 (1.30) µm fine particle hybrid composite resin wear vs. 13.63 (1.02) µm micro filled composite resin wear (P < 0.001). The coefficient of variation was less than 10%. Following validation, enamel sample wear was significantly increased when attrition was introduced (P < 0.001) independent of buccal vs. occlusal sample location (P < 0.05). Attrition resulted in occlusal wear of 26.1 ± 4.5 µm vs. buccal 26.3 ± 1.2 µm and attrition/erosion resulted in occlusal wear of 26.05 ± 4.46 µm vs. buccal 25.27 ± 1.16 µm. Whereas erosion-alone resulted in occlusal wear of 1.65 ± 0.13 µm and buccal 1.75 ± 0.03 µm. Microhardness testing at different loads revealed significantly greater hardness reductions in occlusal enamel vs. buccal enamel for 0.1 KgF indentations (P < 0.001) whereas in contrast 0.5 KgF indentations showed no differences.

SIGNIFICANCE

Wear simulation with CAD/CAM glass ceramic antagonists produced consistent wear in dental materials and human enamel, regardless of enamel surface origin. Lighter (0.1 KgF) hardness testing of occlusal vs. buccal origin revealed damage to the mechanical integrity of the superficial worn enamel.

Impact of citric acid exposures on the erosion susceptibility and microhardness of anatomically different enamel surfaces

OBJECTIVE

To determine the indentation-load dependency of the observed changes in surface microhardness of occlusal and buccal enamel polished surfaces before and after a dietary acid exposure.

METHODS

Enamel specimens (n = 20) were obtained from previously extracted human molars. The samples were divided into two groups, buccal and occlusal (n = 10). Both groups were exposed to 0.3% citric acid at pH 2.7 over different time periods. The mean 3D step-heights defined by ISO 5436-1 standard of the erosion scar was calculated using a white light profilometer. Vickers microhardness testing was carried out to discriminate the surface mechanical changes and different loading regimens used to provide an estimate of depth related changes to the enamel sub-surface. Differences between groups were assessed using two-way ANOVA and post-hoc Sidak's multiple comparisons tests at α = 0.05.

RESULTS

A two-way ANOVA identified that erosion duration significantly influenced step-height (p < 0.01). The factorial analysis also identified a significant increase in step-height on occlusal surfaces compared to buccal surfaces (p < 0.01). At 5 min the mean 3D step-heights µm ± (SD) on the occlusal surface was 3.6 ± (0.65) and on the buccal surface was 3.1 ± (0.60) which increased after 60 mins to 32.9 ± (3.43) and 30.1 ± (3.34), respectively. In contrast to the non-eroded surface, increasing the indentation load on eroded enamel resulted in a significantly increased HV (p < 0.01) suggesting a significant modification to the enamel's response to an increasing indentation load.

SIGNIFICANCE

Occlusal surfaces following acid exposure were more prone to erosion scar relative to buccal surfaces in vitro. Different loading regimens estimated the hardness of the erosive lesion from the outermost surface through the unaffected enamel.

Comparison between intraoral scanning and direct visual analysis for the assessment of developmental defects of enamel

OBJECTIVE

To compare direct visual analysis (DVA) and intraoral scanning (IOS) for the assessment of developmental defects of the enamel (DDE).

METHODS

Thirty-nine extracted permanent human teeth with DDE were selected by an experienced examiner and digitised using IOS. The scanning was recorded using the OBS Studio software parallel to the IOS software to obtain a coloured high-definition MP4 file of the process. Two other experienced, blinded, and calibrated examiners randomly analysed the same teeth through DVA and IOS. A third examiner resolved any disagreements between the two examiners. Descriptive statistics were used to analyse the frequencies of the scores. Cohen's kappa test was used to determine whether the DVA scores were different from those assigned using IOS. Spearman's test was used to verify non-random examiner errors. The Chi-square test was used to compare score frequencies. Statistical significance was set at p <0.05.

RESULTS

Scores indicating more severe and extended DDE (p <0.05) were more frequently assigned with IOS than with DVA (IOS: 25.64%, 25.64%, 38.46%, and 35.90% between one-third to two-third of the lingual, occlusal, mesial, and distal surfaces, respectively; vs. DVA: 10.26%, 7.69%, 15.38%, and 10.26% for the respective aforementioned tooth surfaces). Contrarily, 'no visible enamel defect' was significantly less assigned for IOS than for DVA (IOS: 15.38%, 43.59%, 35.90%, 15.38%, and 17.95% for buccal, lingual, occlusal, mesial, and distal surfaces, respectively; vs. DVA: 38.46%, 66.67%, 56.41%, 51.28%, and 43.59% for the respective aforementioned tooth surfaces). Kappa agreement ranged from fair to moderate when comparing DVA and IOS; the correlation between both methods was positive, indicating that the examiners assigned the scores properly and the differences arose from employing different methods.

CONCLUSION

The assessment of DDE differed depending on the method used. IOS scores indicated more severe and extended DDE than DVA scores. Clinical investigation is the next step in validating the use of IOS for DDE diagnosis.

CLINICAL SIGNIFICANCE

This study showed that DDE can be assessed differently using IOS. It is clinically relevant as it directly affects the determination of the severity of the defect and dental treatment planning.