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. Author manuscript; available in PMC: 2022 Apr 20.
Published in final edited form as: Proc SPIE Int Soc Opt Eng. 2022 Mar 4;11942:1194204. doi: 10.1117/12.2608291

Assessing lesion activity of secondary lesions on extracted teeth by thermal dehydration measurement and optical coherence tomography

Nai-Yuan N Chang 1, Tina Dillas 1, Daniel Fried 1
PMCID: PMC9020384  NIHMSID: NIHMS1797056  PMID: 35450401

Abstract

Secondary caries occurs when leakage in the interfaces between restorative materials and tooth structure allow fluid and bacterial acid infiltration. Thermal imaging coupled with dehydration can be used to measure this increase in fluid permeability for secondary lesions in teeth. Thermal imaging exploits the temperature change due to water evaporation during dehydration to measure the rate of water diffusion from porous lesion areas. Previous in vitro and in vivo thermal imaging studies on enamel and root surfaces have been promising for assessing natural lesion activity. In this study, the rates of dehydration for secondary lesions on extracted teeth were measured. The secondary lesions were also assessed by optical coherence tomography (OCT) and correlated with dehydration rates to determine lesion activity. Future studies with μCT will be used to further confirm lesion severity and structure.

Keywords: lesion activity, secondary caries, optical coherence tomography, thermal imaging

1. INTRODUCTION

With the common use of shade-matched and radiopaque dental restorative materials for replacing tooth structure after cavity preparation, the prevalence of secondary caries lesions has increased in recent years. Microleakage due to maladaptation of bonding materials to tooth structure allows fluids and bacterial acids to infiltrate, leading to demineralization of tooth structure extending beneath the restoration. Clinicians mostly rely on palpation and visual inspection to discern whether dental decay is active or arrested 1. If the lesion appears smooth, dark, and hard, it is assumed to be arrested 2. This method is both subjective and unreliable, as many lesions have been arrested and do not require intervention. However, with tooth-like, radiopaque restorative materials obscuring lesions, it is difficult to identify active lesions with current diagnostic methods. Accurate assessment of lesion activity, depth, and severity is important to determine whether intervention is necessary.

Effective employment of new optical diagnostic technologies that can exploit changes in the light scattering of sound and carious tooth structure, and restorative materials might have great potential for diagnosing the present state of secondary lesions 3. Furthermore, temperature changes in lesion areas during air drying has been exploited to assess lesion activity. Previous studies utilizing thermal dehydration imaging in assessing enamel and root lesion activity both in vivo and in vitro have shown promising diagnostic capability 4-8. The porosity of the outer layers of active lesions has been found to be significantly greater than for arrested lesions. This observation is reflected by larger temperature changes upon dehydration due to the evaporation of water diffusing from pores near the lesion surface 6,9. Further development of thermal dehydration imaging methods is needed for the clinical assessment of secondary lesion activity and to avoid unnecessary cavity preparations.

Recent advances in optical coherence tomography (OCT) have allowed us to quickly capture high-resolution volumetric information and status of teeth non-destructively. When used at near-IR (NIR) wavelengths, OCT is useful in determining whether lesions are active and expanding, partially arrested and undergoing remineralization, or fully arrested and remineralized. This imaging method resolves the reflectivity of each layer of sound, lesion, and restorative material structures, and is able to detect the formation of a zone of increased mineral density and reduced light scattering due to remineralization at the surfaces of lesions. Arrested lesions exhibit a well-defined surface zone of reduced reflectivity that are clearly resolved in OCT images 10,11.

The purpose of this study is to develop a method to assess the activity of secondary lesions by thermal dehydration and OCT. We hypothesize that the depth (LD) and integrated reflectivity (ΔR) of the lesions correlates with the lesion permeability and activity. Regardless of the lesion activity status, we compared the results from thermal dehydration measurements and OCT measurements of each lesion to establish a relationship for further evaluation of lesion severity and activity. Images obtained with μCT will be used to further confirm lesion severity and structure.

2. MATERIALS AND METHODS

2.1. Sample Preparation

Sixty-three extracted human teeth with suspected secondary caries lesions assessed by clinicians were collected from oral surgeons. These teeth were sterilized with gamma radiation and stored in a 0.1% thymol solution. Each tooth was mounted by adhesives onto black plastic blocks. Per clinical standard, lesions are identified by surfaces. Therefore, a single tooth sample may have multiple lesion surfaces. Each lesion surface was considered for measurement, yielding a total sample size of n=121. Figure 1 illustrates the workflow of the study design.

Fig. 1.

Fig. 1.

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Study workflow schematic.

2.2. Visible Imaging

An USB-powered, Model 5MP Edge AM7915MZT digital microscope from AnMO Electronics Corp. (New Taipei City, Taiwan) equipped with a visible polarizer was used to acquire visible images of all sample surfaces. The digital microscope captures 5 mega-pixel (2952 x 1944) color images. Eight white LED lights contained in the camera illuminate the sample and a single polarization element is utilized to reduce glare.

2.3. Thermal Dehydration Measurements

Samples were stored in a moist environment to preserve internal hydration. Prior to imaging, each sample was imbibed in water before being placed into a custom fabricated sample mount. A computer-controlled air nozzle with a 1 mm aperture and an air pressure set to 25 psi was positioned 5 cm away from the sample surface at a 20° angle. Continuous pressurized air was delivered from the air nozzle to dehydrate the sample at the onset of image capture at approximately 25 frames per second for 60 seconds. For each measurement, the air nozzle and the light source were centered on the region of interest (ROI) that encompasses the entire sample. The dehydration setup was completely automated using LabVIEW software (National Instruments, Austin, TX).

A Model A65 infrared (IR) thermography camera from FLIR Systems (Wilsonville, OR) sensitive from 7.5 – 13 μm with a resolution of 640 × 512 pixels, a thermal sensitivity of 50 mK, and a lens with a 13 mm focal length was used to record temperature changes during the dehydration process. The ambient room temperature, flowing air temperature and water bath temperature were approximately 21°C (294.15 K) and were consistent throughout the experiment. The object emissivity was set to 0.92, and the atmospheric temperature was set to 294.15 K 12. While humidity values were not recorded, every sample was measured under the same conditions, where the relative humidity was set at a default value of 50%. Previous studies have shown the area enclosed by the time-temperature curve, ΔQ, which represents the amount of heat absorbed or lost, can be used as a quantitative measurement of porosity and can be used to discriminate between sound and demineralized tooth structure in vitro 4-6. Thermal images were processed and analyzed using dedicated programs written in LabVIEW and MATLAB (MathWorks, Natick, MA). ΔQ was calculated and recorded.

2.4. Optical Coherence Tomography

An IVS-2000-HR-C OCT system from Santec (Komaki, Aichi, Japan) that utilizes a swept laser source and a handpiece with a microelectromechanical (MEMS) scanning mirror and the imaging optics was used to acquire lesion depth (LD) information. The body of the handpiece is 7 x 18 cm with an imaging tip that is 4 cm long and 1.5 cm across. Complete tomographic images of a volume of 5 x 5 x 5mm are captured in approximately 3 seconds at a wavelength of 1312 nm with a bandwidth of 173 nm with a measured resolution in air of 8.8 μm (3 dB). Measured LDs were divided by 1.6, the refractive index of enamel. The lateral resolution is 30 μm (1/e2) with a measured imaging depth of 5 mm and depth resolution of 5 μm in air. Image analysis and lesion structural measurements were carried out using Dragonfly from ORS (Montreal, Canada). The LD and the integrated reflectivity over the lesion depth (ΔR) were also calculated using built-in functions within Dragonfly environment.

2.5. Data Analysis

Excel (Microsoft, Redmond, WA) and Prism 8 (GraphPad Software Inc., La Jolla, CA) were used for data aggregation, analysis, and graphical presentation.

3. RESULTS AND DISCUSSIONS

An example of thermal dehydration measurements for a lesion surface are shown in Fig. 2. Relative ΔQ values are represented by the difference between the areas enclosed by the dehydration curves and their respective maximum y-intercept. The integrated heat map allowed the operator to designate the respective control and lesion ROIs. Across all the lesion surfaces, the dehydration curves typically exhibited an initial drop in temperature followed by a slow recovery to the ambient temperature. An overall average dehydration curve was not derived for every lesion as each lesion behaves differently. ΔQ values for each lesion were further modified by subtracting the respective control ΔQ for each tooth from the lesion ΔQ, which is represented as ΔQL-C.

Fig. 2.

Fig. 2.

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Thermal dehydration analysis results for one of the lesions. A) Visible image. B) Thermal image at onset of dehydration. C) Thermal image at the end of dehydration. D) Integrated thermal emissivity change presented as a heat map. E) Heat map with ROIs (blue = control ROI, red = lesion ROI). F) Thermal image at the end of dehydration with ROIs. G) Thermal dehydration curves for control ROI (blue) and lesion ROI (red); ΔQ is area enclosed by the curve and the respective y-intercept line.

Sample OCT C- and B-scans for a lesion surface are shown in Figure 3. Using previously developed processing workflow within the ORS Dragonfly environment thresholding was performed as image segmentation to determine the lesion depth (LD), and the integrated reflectivity (ΔR). As expected, ΔR was higher in the lesion area compared to the sound control area.

Fig. 3.

Fig. 3.

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OCT scan of the same sample depicted in Fig. 2. Top: C-scan of the tooth surface with lesion. Middle: B-scan of lesion. Bottom: B-scan of lesion with segmentation.

Figure 4 shows the correlation between ΔQL-C and lesion depth and integrated reflectivity as measured with OCT, respectively. Both plots presented a generally weak positive trend of increasing permeability, or increasing ΔQL-C, with increasing lesion depth or integrated reflectivity. However, the correlation was statistically significant between ΔR and ΔQL-C (r = 0.295, p < 0.05), but not significant between LD and ΔQL-C (r = 0.04, p > 0.05). Although the correlation between LD and ΔR was statistically significant (r = 0.312, p < 0.05). The variability of permeability seems to be greater with increasing ΔR. An explanation for this might be the inherent variation of the lesion structure and orientation. Lesions with larger surface area with less demineralization may have similar ΔR’s compared to lesions with smaller surface area but greater demineralization. Dehydration results appeared to correlate more closely with ΔR than with LD. A reason for this could be that deeper lesions may not be represented as accurately by dehydration values. Water loss for some samples may require more extensive dehydration than was performed here. Alternatively, high subsurface reflectivity due to variable demineralization throughout the whole lesion might have obscured the true lesion depth, contributing to greater variation in the dehydration values for similar lesion depths.

Fig. 4.

Fig. 4.

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Correlation plots. Top: between lesion depth (LD) and integrated reflectivity (ΔR) (r=0.312, p<0.05). Middle: between LD and thermal dehydration measurement (ΔQL-C) (r=0.04, p>0.05). Bottom: between ΔR and ΔQL-C (r=0.295, p < 0.05).

In summary, the results suggest that the permeability increased with increasing lesion depth, but more significantly with reflectivity. Furthermore, small increases in integrated reflectivity led to large permeability changes. Future imaging of these samples with μCT will provide more insight and provide more evidence to validate the results here. A full publication will present additional data and analysis from μCT indicating the existence of the highly mineralized transparent surface layer (TSL) within these lesions. The results will further determine the validity of thermal permeability measurements as a method to assess secondary lesion activity and severity.

4. ACKNOWLEDGMENTS

The authors acknowledge the support of NIH/NIDCR Grants F30-DE027264 and R01-DE027335. The authors also thank Yihua Zhu for his contribution.

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