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Original Research| Volume 1, 100012, 2022

Optical coherence tomography evaluation of deep dentin crack removal techniques

Open AccessPublished:August 12, 2022DOI:https://doi.org/10.1016/j.jfscie.2022.100012

      Abstract

      Background

      This in vitro study used optical coherence tomography for noninvasive evaluation of the effectiveness of current clinical techniques to remove deep coronal dentin cracks.

      Methods

      Standard dentin cracks were induced on the pulpal floor of 40 decoronated, extracted, sound human posterior teeth using a diamond disk, resembling cracks extending from marginal ridges. The specimens were randomly assigned to 1 of 5 treatment groups for crack removal using airborne-particle abrasion or a bur (fissure, small round, medium round, or tapered fine diamond). Optical coherence tomographic scans were obtained before and after the crack removal. Three-dimensional image registration analyzed the amount of dentin removed and the dimensions of cracks initiated or propagated in each treatment group.

      Results

      Particle abrasion resulted in the smallest crack propagation in each dimension, which was significantly different from all bur groups (Mann-Whitney, P < .01). Removed dentin depth and width differed among burs (P < .05). All burs resulted in a degree of crack formation, and there was no difference in crack dimensions among bur groups (P > .05). The amount of removed dentin during crack treatment was the largest in the particle abrasion group, which was significantly different from those of the bur groups (Mann-Whitney, P < .005).

      Conclusions

      Dental burs used with the purpose of removing pulpal floor dentin cracks induced new small cracks and extended existing cracks, and the volume of removed dentin depended on the shape and size of the bur. Air particle abrasion induced the fewest new cracks despite removing larger dentin volume.

      Graphical abstract

      Key Words

      Human teeth, particularly those with an old restoration or under high mechanical stress, may crack under cyclic functional loads over time. These cracks may extend longitudinally into dentin and over the pulp chamber. If not removed, dentin cracks can extend over time, leading to pulpal involvement and possible tooth loss. Practitioners may attempt to remove these cracks using dental burs. In our experiment, each bur type used to remove a large crack induced new smaller cracks, but the volume of removed dentin was significantly different between groups depending on the shape and size of the bur. Air particle abrasion was the only method that induced the least amount of new cracks despite removing a larger volume of dentin. Clinicians must be aware of crack propagation caused by various techniques and use clinical judgment to select an appropriate treatment method to prevent further propagation.

      Introduction

      Mastication and parafunction create stresses and strains that act on the tooth structure. Intracoronal dental restorations variably weaken tooth structure, due to removal of cross bracing, creation of high-stress sharp line angles, retentive grooves or notches, irregular and sharp inconsistent surfaces induced with a dental drill, presence of nonbonded restorative materials, and stress induced by the shrinkage of bonded composite.
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      This study aims to use OCT as an objective diagnostic tool for dentin cracks. There are limited data and no widely accepted clinical protocol for treatment of dentin cracks. This is the first study to our knowledge to use advanced OCT technology for direct assessment of dentin microcracking. The null hypothesis was that there was no difference in the removal of dentin crack among the treatments with 4 types of dental burs and airborne-particle abrasion.

      Methods

      An overview of the experimental design and imaging processes in this study are schematically presented in Figure 1.
      Figure thumbnail gr1
      Figure 1Methodology of experiment. A. Forty posterior teeth randomly assigned to 1 of 5 treatment groups. B. Specimens were decoronated with diamond saw and occlusal and peripheral enamel was removed with a wheel-shaped diamond bur and electric handpiece at 150,000 rpm with water. A diamond disk with a cutting head of 140 μm was used to simulate a coronal crack approximately 0.7 mm in depth. A flame bur was used to create a mesiodistal guide notch. C. The specimens were scanned using Dental SS-OCT (Yoshida). D. Cross-sectional and occlusal pretreatment optical coherence tomography (OCT) were collected. E. Each specimen underwent 1 of the 5 treatments to remove the initial dentin crack. F. Posttreatment OCT scans were collected to evaluate the initial crack treatment. G. Amira 3-dimensional software (Version 6.7; ThermoFisher Scientific) was used to superimpose OCT scans and analyze the dimensions of crack propagation or new crack formation for each treatment.

      Specimen preparation

      Forty extracted human posterior teeth were included and randomly assigned to 1 of the 5 treatment groups (n = 8). Sample size was determined through power analysis based on the pilot study of crack size among the groups. Criteria for teeth selection included classification of posterior teeth with no existing visible crack, caries, or restoration. Teeth were collected from several dentists and oral and maxillofacial surgeons across the Seattle, Washington, area. The teeth were properly stored in 100 mL of distilled water with the addition of thymol crystals to maintain the microhardness throughout the experiment duration.
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      Effect of storage solutions on microhardness of crown enamel and dentin.
      This study was conducted according to the ethical guidelines set by the University of Washington Institutional Review Board, which waived the approval requirement for the use of deidentified human teeth extracted as a part of routine dental treatment.
      At the time of the experiment, the cusps on all teeth were reduced by 1 mm, and a 4 mm wide and 2 mm deep channel was created at midcoronal dentin with 2 proximal boxes using a wheel-shaped diamond bur attached to an electric dental handpiece (Ti-Max Z95L; NSK) at 150,000 rpm under water spray cooling. A guide notch was then created over the pulpal floor dentin extending mesiodistally using the tip of a flame-shaped 30-μm grit fine diamond bur (no. 8862, Komet) as a reference landmark. An initial OCT scan was collected after the placement of the guide notch, which was denoted as the baseline. The OCT scanning procedure is explained in detail below. A diamond disk (Medium HyperFlex Double Sided Diamond Disk; Brasseler) with a cutting head thickness of 140 μm was then used to simulate a mesiodistal dentin crack with an approximate depth of 0.7 mm on the pulpal floor, about 1 mm away from the guide notch. A second OCT scan was collected and denoted as pretreatment OCT (Figure 2).
      Figure thumbnail gr2
      Figure 2A. Occlusal pretreatment optical coherence tomographic (OCT) scan. Note the pretreatment crack was created by diamond disk (finger pointer) and guide notch (arrow pointer). B. Cross-sectional pretreatment OCT scan showing guide notch and pretreatment crack. C. Occlusal posttreatment OCT scan showing guide notch (solid arrow) and treatment site (triangle). D. Cross-sectional posttreatment OCT scan showing guide notch (solid arrow) and treatment site after crack removal with a no. 1156 bur (triangle).

      Experimental groups

      After baseline and pretreatment OCT scans, each experimental group underwent 1 of the 5 treatments aimed at removing or diminishing the baseline simulated crack. The treatment groups were
      • 1.
        Fissure carbide bur (no. 1156; Komet) attached to a high-speed electric handpiece (CA 1:5 L Micro Series; Bien Air) at 150,000 rpm with water spray.
      • 2.
        Round carbide bur no. ½ attached to the high-speed electric handpiece at 150,000 rpm with water spray.
      • 3.
        Airborne-particle abrasion: 53-μm aluminum oxide powder, 0.6-mm diameter nozzle, and up to 5 bar pressure with water flow (Aquacare; Medivance Instruments).
      • 4.
        Round carbide bur (no. 2; Komet) attached to the high-speed electric handpiece at 150,000 rpm with water spray.
      • 5.
        Tapered round-end fine diamond bur (no. 8847; Komet) attached to the high-speed electric handpiece at 150,000 rpm with water spray.
      All treatments were performed by the same calibrated operator (D.H.), who intended to remove the crack completely under simulated clinical conditions with ×3.5 dental loupe magnification and standard illumination. The specimens were kept hydrated at 37 °C in an incubator between the experimental steps. After the treatment in each group was completed, a final OCT scan was obtained and was denoted posttreatment OCT (Figure 2 C, D).

      OCT scanning and image analysis process

      3D OCT scans were obtained from the occlusal surface of the specimens over a 10- × 10-mm window by using a prototype swept-source OCT system (Yoshida Dental) with the center and range wavelengths of 1,310 nm and 140 nm, respectively, and a frequency of 50 kHz. The system optical resolution was 11 μm in depth, for a medium with a hypothetical refractive index of n = 1. The OCT handheld scanning probe was placed at a predetermined distance from the specimen, projecting the scanning beam to the experimental surface. A 3D scan with this configuration can be obtained in less than 4 s over an optical depth of 8 mm and 400 × 400 × 2048 voxel data.
      The 3D data set was then imported into the analysis software (Amira Version 6.7). The pretreatment and posttreatment scans for each specimen were aligned and registered in 3D manually by visualizing the guide notch and then fine-tuned using the image registration function of the software to enable accurate comparisons between the scans at each spatial location (Figure 2).
      The comparisons were performed on 5 standard regions of interest (ROIs) on each specimen, defined as the dentin area on cross-sectional slices 0.5 mm through 1 mm apart covering the mesiodistal length of the initial crack. In total, 7 variables were measured for each specimen from the superimposed OCT scans.
      To ensure standardization of the initial simulated crack among the groups, the initial crack depth and initial crack width were measured using the software on the central cross-sectional ROI.
      For comparison of the amount of dentin structure removed with each treatment method, the removed dentin depth and removed dentin width variables were measured through the subtracting comparison of the registered pretreatment and posttreatment scans at each ROI.
      Dentin crack initiation and propagation index (DCIPI) was defined as the dimension of the longest new or propagated crack observed after removal of the initial (pretreatment) crack in the ROI in each orientation: DCIPIMD (X, Mesiodistal), DCIPIBL (Y, Buccolingual), and DCIPIDepth (Z, axiopulpal). If no crack was observed in the ROI, a value of 0 was recorded.

      Statistical analysis

      The frequency of posttreatment crack observation among groups was compared using the Pearson χ2 test. All measured variable data were subjected to normality tests using Kolmogorov-Smirnov Z to determine the appropriate statistical comparison method. Accordingly, 1-way analysis of variance was used to analyze the initial crack depth and initial crack width across groups.
      Removed dentin depth, removed dentin width, DCIPIMD, DCIPIBL, and DCIPIDepth were compared by Kruskal-Wallis tests among all groups, followed by the Mann-Whitney U test for pairwise comparisons with Bonferroni correction. All analyses were carried out using SPSS 16.0 software (SPSS) at a preadjustment significance level of α = 0.05.

      Results

      The initial crack depth and initial crack width across groups were 700 through 800 μm and 200 μm, respectively, and are presented in Figure 3. One-way analysis of variance showed that both variables were not significantly different among groups with P = .104 and P = .082, respectively.
      Figure thumbnail gr3
      Figure 3Bar chart comparing the initial size of crack in μm by treatment group on the category axis. There was no statistically significant difference among the groups for each variable (P > .05).
      The representative OCT image results of all groups are presented in Figure 4. In each OCT panel, 2 images are included; the grayscale image represents the pretreatment scan, which included a guide notch in the center and the induced crack, and the colored super imposing scan represents the posttreatment scan. Overall, the frequency of the observed dentin cracks on the ROIs were 32 in the no. 1156, ½ round, and no. 8847 groups; 30 in no. 2 round; and 22 in the particle abrasion group. Pearson χ2 showed a significant difference in frequency of the posttreatment crack observation among groups with χ24 (N 200) = 9.77, P < .05.
      Figure thumbnail gr4
      Figure 4A. No. 1156 exhibiting one of the deepest dentin cracks (rectangle) induced across treatment groups. Crack dimensions (dentin crack initiation and propagation index [DCIPI]Depth × DCIPIBuccolingual × DCIPIMesiodistal) are 1,661 μm × 38 μm × 74 μm. B. In no. 1156 treatment, new crack propagating from the base treatment site (solid arrow) measured 426 μm × 100 μm × 100 μm. Note the new extending crack is in proximity with the pulpal horn (finger pointer) away from the guide notch (arrow). C. ½ round carbide treatment showing a new crack (117 μm × 55 μm × 55 μm) extending laterally from treatment site. D. Particle abrasion treatment, with small crack extending occlusogingivally from treatment site with dimensions of 114 μm × 80 μm × 100 μm. Note the difference in depth and width removal compared with bur treatment groups. E. New crack created from treatment site of no. 2 round, starting occlusally and traveling interproximal. Crack dimensions were 1,198 μm × 69 μm × 73 μm. F. Dentinoenamel junction (finger). No. 8847 resulted in a new crack (1,228 μm × 99 μm × 460 μm) created from base of treatment site propagated toward the gingiva.
      In the no. 1156 group (Figure 4A), the posttreatment scan illustrated complete removal of the original crack. However, a deep coronal crack was created during the removal process in this specimen. This new crack extended from the treatment site (left side of the guide notch) toward the gingival portion of the specimen. DCIPIDepth of this crack was measured to be 1,661 μm. Figure 4B illustrates a more comprehensive analysis of the no. 1156 treatment group with the average DCIPIDepth being 247 μm. Specifically, Figure 4B exhibited a crack propagating from the base of the treatment site toward the pupal horn with a DCIPIDepth of 426 μm. The ½ round representative specimen (Figure 4C) displayed a posttreatment crack extending in the occlusogingival direction. The dimensions of the crack (DCIPIDepth × DCIPIBL × DCIPIMD) were approximately 426 μm × 100 μm × 100 μm.
      Figure 4D presents the particle abrasion treatment, which resulted in a wide and deep area of dentin removal and fewer smaller posttreatment cracks. The no. 2 round treatment group, represented by Figure 4E, exhibited a generally wider but shallower area of dentin removal, but posttreatment cracks were still visible in this group. A posttreatment crack originated near the occlusal surface and propagated toward the buccal surface. Dimensions of crack were 1,198 μm ×119 μm ×189 μm. The no. 8847 treatment displayed a narrower, but deeper treatment site compared with the no. 2 round. Figure 4F exhibits a crack propagating in the occlusoginvigal direction with approximate dimensions of 1,228 μm × 99 μm × 460 μm, respectively. Table 1 is a comprehensive analysis of DCIPI including DCIPIMD, DCIPIBL, and DCIPIDepth. Overall, the largest cracks were in the axiopulpal depth dimension (DCIPIDepth), followed by the mesiodistal dimension (DCIPIMD). Kruskal-Wallis analysis showed a significant difference among groups in all crack variables (P < .05).
      Table 1Comprehensive analysis of treatment groups including removed depth, removed width, and DCIPI
      DCIPI: dentin crack initiation and propagation index.
      .
      VariableGroupsMean (SD)95% CIMean Rank
      Mean Rank: Kruskal-Wallis mean rank. Different superscript letters in the column indicate statistically significant difference between the groups Mann-Whitney U pairwise comparisons (P < .05).
      Removed DepthNo. 1156314.5 (285.6)221.9 to 407.199.15a
      ½ round160.9 (183.5)98.8 to 22359.14b
      Particle abrasion1,123.7 (338.5)1,015.4 to 1,231.9169.28c
      No. 2 round164.2 (124.6)122.0 to 206.369.11b
      No. 8847180.9 (127.5)140.1 to 221.777.02b
      Removed WidthNo. 1156852.7 (157.1)802.4 to 902.9118.32a
      ½ round407.6 (81.7)381.5 to 433.723.38b
      Particle abrasion1,174.9 (288.0)1,082.8 to 1,267.0161.95c
      No. 2 round991.1 (223.6)919.6 to 1,062.6138.70d
      No. 8847561.4 (74.0)537.7 to 585.160.15e
      DCIPIMesiodistalNo. 115694.2 (74.6)70.3 to 118.0104.70a
      ½ round88.5 (72.0)65.1 to 111.8100.10a
      Particle abrasion53.1 (66.5)31.8 to 74.470.30b
      No. 2 round127.3 (117.6)89.7 to 164.9114.48a
      No. 8847122.0 (119.3)83.9 to 160.2110.42a
      DCIPIBuccolingualNo. 115665.1 (51.2)48.7 to 81.5107.48a
      ½ round54.6 (35.5)43.3 to 66.098.00a
      Particle abrasion32.8 (34.0)21.9 to 43.769.45b
      No. 2 round67.8 (54.4)50.4 to 85.2112.35a
      No. 884764.6 (43.1)50.7 to 78.6112.33a
      DCIPIDepthNo. 1156247.4 (312.8)147.4 to 347.4110.20a
      ½ round186.4 (212.2)118.5 to 254.299.88a
      Particle abrasion86.8 (94.6)56.5 to 117.066.98b
      No. 2 round273.0 (326.5)168.5 to 377.4109.95a
      No. 8847257.3 (276.5)168.9 to 345.7115.50a
      DCIPI: dentin crack initiation and propagation index.
      Mean Rank: Kruskal-Wallis mean rank. Different superscript letters in the column indicate statistically significant difference between the groups Mann-Whitney U pairwise comparisons (P < .05).
      The dimensions of the removed dentin structure as a result of crack treatment were largest in the particle abrasion group as exhibited by the average depth removal of 1,124 μm and an average width removal of 1,017 μm (Table 1). The average ½ round treatment dimensions were 139 μm × 460 μm, which was the smallest volume of removed dentin across treatments. Both the ½ round and no. 2 round had a similar depth removal of approximately 139 μm (Table 1).
      The mean DCIPIMD ranged from 53 μm to 127 μm across treatment groups. The largest average DCIPIMD was 127.3 μm in the no. 2 round treatment. In contrast, particle abrasion exhibited the smallest average DCIPIMD of 53.1 μm, which was significant compared with all bur groups with P <.005. No other DCIPIMD comparisons were significant (P > .05).
      The no. 2 round resulted in the largest mean DCIPIBL of 68 μm. In contrast, the smallest mean DCIPIBL was 33 μm shown by the particle abrasion treatment. The average DCIPIBL produced by particle abrasion was nearly one-half that of the bur treatments, excluding ½ round. The DCIPIBL in the particle abrasion group was statistically significantly different from all other groups with P < .005.
      The greatest range of DCIPIDepth was displayed in the no. 1156 treatment, with crack depth varying from 147 μm through 374 μm. In comparison, particle abrasion that exhibited the narrowest range from 57 μm through 117 μm. However, across all treatment groups, mean DCIPIDepth ranged from 87 μm through 273 μm. The largest average DCIPIDepth (273 μm) was produced by the no. 2 round bur, whereas the smallest average DCIPIDepth (87 μm) occurred in the particle abrasion group. DCIPIDepth in each bur treatment group was significantly different from the particle abrasion (P < .005).
      A summary of all statistical analyses is presented in Table 2. In addition, each variable data are presented in the form of box plots in the Appendix A figures.
      Table 2Statistical analysis of P values from the pair comparisons using the Mann-Whitney U test.
      VariableGroupsNo. 1156½ RoundParticle AbrasionNo. 2 Round
      Removed Depth½ round0.000
      indicates statistical significance with P < .05.
      NA
      NA: not applicable.
      NANA
      Particle abrasion0.000
      indicates statistical significance with P < .05.
      0.000
      indicates statistical significance with P < .05.
      NANA
      No. 2 round0.002
      indicates statistical significance with P < .05.
      0.2330.000
      indicates statistical significance with P < .05.
      NA
      No. 88470.048
      indicates statistical significance with P < .05.
      0.0740.000
      indicates statistical significance with P < .05.
      0.74
      Removed Width½ round0.000
      indicates statistical significance with P < .05.
      NANANA
      Particle abrasion0.000
      indicates statistical significance with P < .05.
      0.000
      indicates statistical significance with P < .05.
      NANA
      No. #2 round0.011
      indicates statistical significance with P < .05.
      0.000
      indicates statistical significance with P < .05.
      0.002
      indicates statistical significance with P < .05.
      NA
      No. 88470.000
      indicates statistical significance with P < .05.
      0.000
      indicates statistical significance with P < .05.
      0.000
      indicates statistical significance with P < .05.
      0.000
      indicates statistical significance with P < .05.
      DCIPIMesiodistal
      DCIPI: dentin crack initiation and propagation index.
      ½ round0.609NANANA
      Particle abrasion0.004
      indicates statistical significance with P < .05.
      0.010
      indicates statistical significance with P < .05.
      NANA
      No. 2 round0.3240.2140.002
      indicates statistical significance with P < .05.
      NA
      No. 88470.5880.4600.002
      indicates statistical significance with P < .05.
      0.460
      DCIPIBuccolingual½ round0.475NANANA
      Particle abrasion0.003
      indicates statistical significance with P < .05.
      0.010
      indicates statistical significance with P < .05.
      NANA
      No. 2 round0.6840.2460.002
      indicates statistical significance with P < .05.
      NA
      No. 88470.7080.2330.001
      indicates statistical significance with P < .05.
      0.233
      DCIPIDepth½ round0.369NANANA
      Particle abrasion0.001
      indicates statistical significance with P < .05.
      0.004
      indicates statistical significance with P < .05.
      NANA
      No. 2 round0.9610.4050.002
      indicates statistical significance with P < .05.
      NA
      No. 88470.5950.1790.000
      indicates statistical significance with P < .05.
      0.179
      indicates statistical significance with P < .05.
      NA: not applicable.
      DCIPI: dentin crack initiation and propagation index.

      Discussion

      Microcracks within the coronal tooth structure may propagate, possibly resulting in tooth fracture.
      • Lynch C.D.
      • McConnell R.J.
      The cracked tooth syndrome.
      ,
      • Cameron C.E.
      Cracked-tooth syndrome.
      ,
      • Ellis S.G.S.
      Incomplete tooth fracture: proposal for a new definition.
      These fractures may result in painful and costly procedures including extraction, root canal therapy, and crowns.
      • Hasan S.
      • Singh K.
      • Salati N.
      Cracked tooth syndrome: overview of literature.
      ,
      • Banerji S.
      • Mehta S.B.
      • Millar B.J.
      Cracked tooth syndrome, part 2: restorative options for the management of cracked tooth syndrome.
      The recommended method for removal of dentin crack is drilling it out; however, crack propagation has been a noted concern because of the heat generated and mechanical stress imposed on the tooth structure.
      • Majd B.
      • Majd H.
      • Porter J.A.
      • Romberg E.
      • Arola D.
      Degradation in the fatigue strength of dentin by diamond bur preparations: importance of cutting direction.
      ,
      • Majd H.
      • Viray J.
      • Porter J.A.
      • Romberg E.
      • Arola D.
      Degradation in the fatigue resistance of dentin by bur and abrasive air-jet preparations.
      We used OCT for the first time for comprehensive analysis of dentin microcracks. OCT is a noninvasive cross-sectional imaging technique that detects backscattered light along with echo time delay.
      • Arola D.D.
      • Gao S.
      • Zhang H.
      • Masri R.
      The tooth: its structure and properties.
      OCT can detect cracks due to the contrast in light scattering caused by dentin and enamel cracks. This method has been validated by several studies.
      • Arola D.D.
      • Gao S.
      • Zhang H.
      • Masri R.
      The tooth: its structure and properties.
      ,
      • Dao Luong M.N.
      • Shimada Y.
      • Turkistani A.
      • Tagami J.
      • Sumi Y.
      • Sadr A.
      Fractography of interface after microtensile bond strength test using swept-source optical coherence tomography.
      ,
      • Segarra M.S.
      • Shimada Y.
      • Sadr A.
      • Sumi Y.
      • Tagami J.
      Three-dimensional analysis of enamel crack behavior using optical coherence tomography.
      Although our OCT study was conducted in vitro, evidence for its intraoral application is growing.
      • Espinoza K.
      • Hayashi J.
      • Shimada Y.
      • Tagami J.
      • Sadr A.
      Optical coherence tomography for patients with developmental disabilities: a preliminary study.
      Other diagnostic assays, such as microcomputed tomography, are not appropriate alternatives due to the lack of resolution for fine cracks.
      • Clark D.J.
      • Sheets C.G.
      • Paquette J.M.
      Definitive diagnosis of early enamel and dentin cracks based on microscopic evaluation.
      Similarly, SEM requires slicing of specimen and eliminates baseline comparisons. In fact, the formation of microcracks as a result of dentin mechanical removal has been a theoretical notion based on the observation of dentin fatigue strength reduction after treatment rather than direct observations since SEM has been inconsistent in producing measurable results.
      • Majd B.
      • Majd H.
      • Porter J.A.
      • Romberg E.
      • Arola D.
      Degradation in the fatigue strength of dentin by diamond bur preparations: importance of cutting direction.
      ,
      • Majd H.
      • Viray J.
      • Porter J.A.
      • Romberg E.
      • Arola D.
      Degradation in the fatigue resistance of dentin by bur and abrasive air-jet preparations.
      ,
      • Sehy C.
      • Drummond J.L.
      Micro-cracking of tooth structure.
      One limitation of our study was the creation of a simulated crack instead of a naturally occurring crack under fatigue loading; however, creating standard dentin natural cracks to compare across groups would be challenging. The results presented in Figure 3 suggested that the simulated cracks were comparable at the baseline, enabling comparison among crack removal methods. Simulated cracks have more dentin loss and are possibly blunter at the crack tip than natural cracks. It was speculated that natural cracks would propagate even further under vibration owing to a sharper crack tip.
      Electric handpieces were primarily used by the operator throughout the duration of the experiment. Less crack propagation was expected compared with conventional air-driven turbines because of less vibration at higher torques. The literature has also reported on the effect of bur selection on dentin surface roughness. In general, finer grits are expected to create a smoother surface.
      • Majd B.
      • Majd H.
      • Porter J.A.
      • Romberg E.
      • Arola D.
      Degradation in the fatigue strength of dentin by diamond bur preparations: importance of cutting direction.
      ,
      • Majd H.
      • Viray J.
      • Porter J.A.
      • Romberg E.
      • Arola D.
      Degradation in the fatigue resistance of dentin by bur and abrasive air-jet preparations.
      In our study, standard steel burs, carbide burs, and fine-grit diamonds were included. Although there were differences in the amount of dentin removal with each bur that could be attributed to the geometry of the burs, these treatment groups were not statistically significant when analyzing dentin crack initiation and propagation or DCIPI. It was reported that the direction of cutting in relation to the orientation of dentin was an important factor in the possible propagation of the cracks.
      • Majd B.
      • Majd H.
      • Porter J.A.
      • Romberg E.
      • Arola D.
      Degradation in the fatigue strength of dentin by diamond bur preparations: importance of cutting direction.
      This is because dentin is an anisotropic substrate with varying dentinal tubule orientation and collagen-mineral content.
      • Majd H.
      • Viray J.
      • Porter J.A.
      • Romberg E.
      • Arola D.
      Degradation in the fatigue resistance of dentin by bur and abrasive air-jet preparations.
      It appears that cracks propagate regardless of the bur type at the pulpal floor dentin where a major dentin crack had previously developed under a wedging force. This observation is also in line with the literature that a reduction in the dentin resistance to fatigue crack growth was observed in deeper dentin, as deeper dentin is more brittle.
      • Majd B.
      • Majd H.
      • Porter J.A.
      • Romberg E.
      • Arola D.
      Degradation in the fatigue strength of dentin by diamond bur preparations: importance of cutting direction.
      ,
      • Majd H.
      • Viray J.
      • Porter J.A.
      • Romberg E.
      • Arola D.
      Degradation in the fatigue resistance of dentin by bur and abrasive air-jet preparations.
      ,
      • Ivancik J.
      • Neerchal N.K.
      • Romberg E.
      • Arola D.
      The reduction in fatigue crack growth resistance of dentin with depth.
      The initial preparation of sound dentin did not create as many cracks as was caused by attempts to remove the simulated cracks, based on OCT imaging. This observation indicates the particular challenge associated with treating a preexisting crack in deep dentin.
      The particle abrasion treatment resulted in significantly smaller cracks across every dimension (DCIPIDepth × DCIPIBL × DCIPIMD) compared with bur treatments, suggesting that particle abrasion may not induce the same mechanical stress as rotary instruments. This result rejected our null hypothesis of no difference in crack removal across treatment groups. In addition, the frequency of cracks and treatment dimensions were different among the groups. The technique in our study is a new implementation of particle air abrasion with improved control due to smaller nozzle size as well as an adjustable simultaneous water spray. Although the crack frequency and size was diminished by with particle abrasion, the dimensions of removed dentin were much greater than with bur treatments, which indicates less control while removing the initial dentin crack compared with bur treatments. The studies on the dentin fatigue strength indicated that both bur and air abrasion treatments resulted in reduced fatigue strength of dentin, which is in line with the observation of crack formation in all groups.
      • Majd B.
      • Majd H.
      • Porter J.A.
      • Romberg E.
      • Arola D.
      Degradation in the fatigue strength of dentin by diamond bur preparations: importance of cutting direction.
      ,
      • Majd H.
      • Viray J.
      • Porter J.A.
      • Romberg E.
      • Arola D.
      Degradation in the fatigue resistance of dentin by bur and abrasive air-jet preparations.
      Our study has limitations. Dentin properties in extracted teeth could depend on various factors and may not necessarily represent the behavior of dentin in a vital tooth or under different clinical situations. Thin grooves were used to simulate cracks, and true cracks were not used. We were unable to create true standardized cracks, ultimately settling on the use of a thin diamond disk, with the assumption that the disk would leave a comparatively blunt crack that would lessen the effect of bur and handpiece vibration rather than amplify it. The investigations of an appropriate treatment method for dentin cracks should continue. The use of carbon dioxide lasers in the dental field has been a growing concept in dental hard- and soft-tissue treatment.
      • Bakry A.S.
      • Takahashi H.
      • Otsuki M.
      • Sadr A.
      • Yamashita K.
      • Tagami J.
      CO2 laser improves 45S5 bioglass interaction with dentin.
      ,
      • Lin C.P.
      • Lee B.S.
      • Kok S.H.
      • Lan W.H.
      • Tseng Y.C.
      • Lin F.H.
      Treatment of tooth fracture by medium energy CO2 laser and DP-bioactive glass paste: thermal behavior and phase transformation of human tooth enamel and dentin after irradiation by CO2 laser.
      Our research will continue to examine crack formation in dentin after various potential crack treatments such as laser treatment and application of bioactive materials. The prototype OCT system used in our study is capable of intraoral imaging; therefore, clinical analysis of these dentin cracks is a possibility. OCT along with sophisticated image analysis is opening a new horizon in research on the formation, propagation, and treatment of cracks in dentin.

      Conclusions

      Each bur type used throughout this experiment induced new cracks, but the volume of removed dentin was significantly different among the groups depending on the shape of the bur, a variable that should be considered for clinical situations. Air particle abrasion induced the least amount of new cracks despite removing larger dentin volume and should be given consideration when applicable. Clinicians must be aware of crack propagation caused by various techniques and use clinical judgment to select an appropriate treatment method to prevent further propagation.

      Acknowledgments

      The authors acknowledge Dr James Haberman, Dr Michael Wasson, Dr Craig Neal, and Dr Galia Leonard (Seattle, WA) for supporting human teeth collection for the experiment; Dr Grant Chyz (Seattle, WA) for advice on specimen preparation and support with air-abrasion equipment; and Yoshida Dental (Tokyo, Japan) for providing the optical coherence tomographic system. Figure 3 was created with Biorender.com. The authors acknowledge the help and support of University of Washington School of Dentistry Office of Research and Summer Undergraduate Research Fellowship program director, Dr Whasun O. Chung.

      Supplemental Data

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