Objectives

Demineralized root dentin and cementum is made up of mostly collagen that shrinks significantly upon dehydration or drying with air. During remineralization mineral is deposited on the outside of the lesion creating a highly mineralized surface layer that inhibits diffusion, arrests the lesion and prevents shrinkage. Previous studies suggest that active root caries lesions manifest shrinkage, while arrested lesions no longer manifest shrinkage upon dehydration. The purpose of this study was to demonstrate that the shrinkage of root caries lesions can be monitored during dehydration using an optical coherence tomography probe suitable for clinical use.

Methods

In this in vitro study the shrinkage of simulated and natural root caries lesions on extracted teeth was measured using a cross polarization optical coherence tomography (CP-OCT) system and a 3D printed appliance with an integrated air nozzle suitable for clinical use. Two methods were employed to assess shrinkage, changes in the position of the lesion surface and changes in the thickness of the lesion.

Results

CP-OCT was successful in measuring a significant (P < 0.05) contraction of the lesion surface, significant decrease in the lesion thickness and increase in the reflectivity per micron upon drying natural lesions on extracted teeth.

Conclusions

In this preclinical study, we have demonstrated that a CP-OCT handpiece modified for infection control with an attached air nozzle suitable for in vivo use can be used to monitor the shrinkage of root caries lesions. In addition, we have developed a new approach to measuring lesion shrinkage with OCT, namely monitoring changes in the lesion thickness as opposed to the position of the lesion surface, that does not require an initial reference position and is more easily implemented in vivo.

The diagnosis of root caries and root fractures is of increasing importance due to our aging population. The objective of this study is to show that optical clearing agents can be used to increase the visibility of subsurface structures in tooth roots to aid in the diagnosis of root caries depth and extension and to differentiate from subsurface defects. Previous studies have shown that high refractive index fluids can increase the depth penetration of optical coherence tomography (OCT) for imaging caries lesions on tooth enamel. Extracted teeth with suspected root caries were imaged with (OCT) with and without the addition of water (W), glycerol (G), and propylene glycol (PG). The reflectance was monitored in sound and demineralized areas before and after application of W, G, and PG. The visibility of subsurface root canals was also measured before and after application of the respective fluids. In addition, shrinkage in lesion areas was measured after lesion dehydration to aid in the assessment of lesion activity. This study shows that optical clearing agents can be used to increase the performance of OCT for the diagnosis of root caries and other defects on root surfaces.

Demineralized root dentin and cementum are mostly collagen that shrinks significantly upon dehydration. Active root caries lesions manifest shrinkage upon dehydration, however during the remineralization of root caries lesions mineral is deposited on the outside of the lesion arresting the lesion and arrested lesions no longer manifest shrinkage upon dehydration. Optical coherence tomography is ideally suited for the measurement of that shrinkage for the assessment of lesion activity. In this study the shrinkage of natural root caries lesions on extracted teeth were measured using a CP-OCT system with a 3D printed appliance with an integrated air nozzle suitable for clinical use.

Demineralized root dentin and cementum are mostly collagen that shrinks significantly upon dehydration. Active root caries lesions manifest shrinkage upon dehydration, however during the remineralization of root caries lesions mineral is deposited on the outside of the lesion arresting the lesion and arrested lesions no longer manifest shrinkage upon dehydration. Optical coherence tomography is ideally suited for the measurement of that shrinkage for the assessment of lesion activity. In this study the shrinkage of natural root caries lesions on extracted teeth were measured using a CP-OCT system with a 3D printed appliance with an integrated air nozzle suitable for clinical use.

Objectives

The purpose of this study was to show that optical coherence tomography (OCT) and thermal imaging can be used to monitor changes in the structure and activity of caries lesions over time after treatment with silver diamine fluoride (SDF).

Methods

Artificial caries lesions were formed on enamel and dentin bovine blocks. Each block was partitioned into five windows with the central three windows exposed to a demineralization solution to create lesions: one sound window served as a sound control (SC), one sound window was exposed to SDF to serve as a test control (SCT), one lesion window served as a lesion control (LC), one lesion window received one application of SDF (L1), while the other lesion window received two applications of SDF (L2). Each window was scanned using OCT before SDF application, and every week subsequently, for 12 weeks after initial SDF treatment. Changes in the mean intensity and the width of the peak of increased reflectivity due to the lesion and SDF along with the intensity at a depth of 180 µm from the surface representing optical penetration through the lesion were monitored. Changes in the heat lost, ΔQ (temperature integrated over time) of each window during drying with air were also monitored using a thermal imaging camera. Transverse microradiography (TMR), and high-resolution microscopy were also used for the analysis of selected samples.

Results

The reflectivity and optical penetration of sound and lesion areas of enamel and dentin manifested significant changes in OCT images after SDF application. Thermal imaging showed significant differences in ΔQ indicative of permeability changes in the sound and lesion areas of enamel and dentin after SDF application.

It is challenging to identify demineralized areas of root lesions due to cervical erosion, calculus formation, and heavy staining of dentin. We have found that root caries can be imaged with extremely high contrast at short wavelength IR (SWIR) wavelengths beyond 1500-nm. Lasers are well suited for the selective removal of caries lesions from tooth surfaces. A CO₂ laser operating at a wavelength of 9.3-μm was combined with a thulium-doped fiber laser operating at 1880-nm for the selective removal of root caries lesions from extracted teeth. Serial SWIR reflectance images at 1880-nm were used to guide the CO₂ laser for image-guided laser ablation. Cross polarization optical coherence tomography (CP-OCT) was used to assess the initial depth of the lesions before removal and assess the volume of sound and demineralized tissue removed by the CO₂ laser. With this image-guided approach, we believe we can achieve highly selective lesion removal and minimal damage to surrounding sound tissues.

Near Infrared Reflectance (NIR) is a new imaging technology that detects dental caries (decay) on tooth occlusal surfaces and in the interproximal contact sites between teeth. Conventional techniques, mostly dental x-rays, do not provide the high sensitivity and specificity at the vulnerable pits and fissure regions. The contrast of demineralization on tooth surfaces changes with increasing severity and the magnitude of that change with depth depends on the wavelength. The purpose of this study is to determine how the contrast changes with depth as a function of wavelength. Demineralization of varying depth was produced in 1.5 × 1.5 mm exposed windows after 1, 2, 3, 4, and 5 days of exposure to a demineralizing solution at pH 4.5. Lesions were imaged at 405, 630, 850, 1300, 1460, 1535, 1675, and 1950-nm with multiple imaging systems. The highest lesion contrast was measured at 1950-nm.

This clinical study tested cross-polarization optical coherence tomography (CP-OCT) monitoring of erosive tooth wear (ETW). Twenty participants completed a 14-day/arm, 3-arm crossover study simulating different ETW severities. Participants received two enamel specimens (per arm) and were randomized to: severe (s-ETW, lemon juice/pH:2.5/4.25%wt/vol citric acid), moderate (m-ETW, grapefruit juice/pH:3.5/1.03%wt/vol citric acid), and non-ETW (water). Enamel thickness was measured with CP-OCT (day[D] 0, 7, 14) and micro-computed tomography (μ-CT; D14). Enamel surface loss was determined with CP-OCT and optical profilometry (OP; D7, D14). CP-OCT showed higher enamel surface loss for D14 than D7 for m-ETW (P = .009) and s-ETW (P = .040) and differentiated severity at D14 (s-ETW > non-ETW, P = .027). OP was able to differentiate surface loss between days (D7 < D14, P < .001) for m-ETW and s-ETW, and ETW severity effect after 7 and 14 days (non-ETW < m-ETW < s-ETW, P < .001). At D14, CP-OCT and μ-CT were positively correlated (r = .87, ICC = .62). CP-OCT showed potential as a tool for clinical ETW monitoring.

New imaging technologies are needed for the clinical assessment of lesions on root surfaces. It is not sufficient to simply detect caries lesions; methods are needed to assess lesion depth, structural composition and activity to determine if chemical intervention has the potential to be effective and if remineralization has occurred. Lesions were monitored using CP-OCT during lesion dehydration to assess the lesion structure and any shrinkage. Thermal imaging at 6-10 μm wavelengths and short wavelength-IR imaging at 1450-1750-nm were used to monitor thermal emission during lesion dehydration to assess lesion activity. Imaging probes were custom fabricated for clinical use. We present the first clinical results of a small feasibility study employing CP-OCT, thermal and SWIR imaging to assess lesion activity in vivo on thirty test subjects with suspected root caries lesions.