Effects of ultrasonic refinement on endodontic access cavity walls: A microcomputed tomography analysis.

OBJECTIVES: The present study aimed in assessing the coronal defects after access cavity finishing and refinement by micro.
METHODS: Access cavities on thirty molars were prepared using a diamond bur. To finish and refine the access cavity, the Endo-Z was used in group 1 (n=15) and Start X 1 in group 2. Preparation time was recorded. A micro-CT scan was done before and after access preparation. Formation and location of the new defects were registered, the extension of defects calculated and the direction of the extension registered, preparation time and surface roughness determined (P < 0.05).
RESULTS: Preparation time was significantly higher with ultrasonics (P <0.001). Internal walls showed smoother surfaces for Endo-Z group. Newly counts and extension length of defects weren't significantly different between groups (P > .05).
CONCLUSION: Ultrasonic tips induced new cracks. Both instruments caused the extension of cracks. Ultrasonic tips requires more time and results in significantly rougher surfaces. Copyright:

INTRODUCTION

The main purpose of endodontic treatment is to eradicate bacteria by cleaning, shaping, and filling the root canal system.[1] An appropriate access cavity (AC) is defined as the opening prepared in a tooth to gain entrance to the root canal system and the most important phase to successfully achieving the above objectives.[23]

In 1957, Richman introduced ultrasonic (US) instrumentation into endodontics,[4] which revolutionized the art of endodontic treatment. Currently, US tips are a basic part of endodontic equipment. They have multiple uses such as access refinement, calcified canal location, pulp stone removal, intracanal obstruction elimination, energy delivery to irrigating solutions, condensation of obturation materials, and root-end surgical preparation.[56] Air-driven burs, usually used in AC preparation, are more effective than piezo-US tips in eliminating volumes of dentin.[4567] Consequently, US tips are more conservative than conventional burs and can be used as an adjunctive tool to refine dentinal walls during AC preparation.[8]

A higher incidence of crack formation was found in dentinal walls of retrograde cavities prepared by US tips compared with those made by burs.[9] Other studies found no significant difference between conventional US tips, diamond-coated US tips, and high-speed stainless-steel burs for the number or type of cracks[10] and no root cracking across the full scale of US power settings used, although chipping of the root-end cavity margins was observed.[11]

Many studies investigated the correlation of dentinal defect (DD) formation in the root dentin during root canal instrumentation[1213] and root-end cavity preparation;[91011] nevertheless, crack formation in coronal enamel and dentin due to AC preparation procedures has not been yet evaluated. Thus, the purpose of the present study was to evaluate the occurrence, location, and extension of coronal extension of dentinal defects (CEDD) after AC refinement using burs and US tips by microcomputed tomography (MCT) and compare the final surface quality of dentinal cavity walls.

MATERIALS AND METHODS

Sample selection and preparation

One hundred molars freshly extracted for periodontal reasons were collected. Teeth were scanned using a cone-beam computed tomography to evaluate the inclusion criteria. Thirty molars with an intact crown, similar coronal volume, and without caries and calcifications were selected and stored in 0.1% thymol solution.

The teeth were embedded in a silicone-based putty material (Zeta plus, Zhermack S.p. A, Rovigo, Italy) up to 2 mm apically to the cementoenamel junction to simulate the supporting tissues. The AC was opened in all teeth a 802/ 0.14 mm round diamond round diamond-coated bur (Edenta AG, Hauptstrasse, Switzerland) under continuous water irrigation until the pulp chamber roof was perforated. Teeth were then distributed into two groups (n = 15) according to the protocol used to complete and refine the AC preparation widening the AC to the periphery, creating tapered, and smooth axial coronally divergent walls and a straight line access to root canal orifices. The Endo Z bur (Dentsply-Maillefer, Ballaigues, Switzerland) was used in Group 1, while the US Start-X tip 1 (Dentsply-Maillefer, Ballaigues, Switzerland) was used in Group 2.

A new bur and US tip were used for each specimen, and the procedures were performed by one trained endodontist under an optical microscope at ×10 (Leica M320, Leica microsystems, Schweiz, Switzerland). The duration of the AC preparation and refinement was measured in seconds for all the specimens tested.

Microcomputed tomography analysis

All specimens were scanned before and after AC preparation to identify CEDDs formation. The samples were scanned using a high-resolution MCT: V |tome|(General Electric, MA, USA), using a 0.60° rotational step and a 360° rotational angle and performing 0.3 steps in randomized movements with a 16.1 μ resolution with the voltage and current of the tube being 120 kV and 180 μA, respectively.

Analysis of images

The acquisition and reconstruction of data were established with the software Datos | × 2.0 (General Electric, MA, USA). The first image analysis was processed by the “VG StudioMax 3.0” software (Volume graphics), with a beam hardening and ring artifact correction of 0%. The voxel number for each scanned tooth was a set of data with 1800 cross-sections per sample. All images from pre and postoperative scans were analyzed by two blinded operators. Images were analyzed from the occlusal surface to the level of the root canal orifices as follows:

Presence of CEDDs: the number of teeth with or without CEDDs and the number of CEDDs present in each tooth were calculated before and after the AC preparation, and percentages were calculated

Location of CEDDs: the location of the CEDDs was registered as buccal, lingual, mesial, or distal

Extension of CEDDs: the extension of the CEDDs was measured by multiplying the number of cross-sections, in which each CEDD appeared by the thickness of the cross-section (0.01 mm). The dimensions of the CEDDs were registered before and after the AC preparation for new and preexisting cracks, thus registering a dimensional expansion for the preexisting cracks. Linear measurements of the crack before and after preparation were registered to quantify the extension.

The teeth were allocated to each group to equally distribute specimens with preoperative presence or absence of CEDDs.

Furthermore, the three-dimensional (3D) reconstructed images of AC were used for the visual evaluation of the surface roughness of the dentinal walls. The surface roughness of the AC walls after the AC preparation procedures was observed for irregularities and striations. A quantitative scale classification containing four scores was used, as previously described (Agarwal et al. 2016): Score 0 represented a very rough surface, Score 1 rough surface, Score 2 smooth surface, and Score 3 very smooth surface. Roughness scoring was evaluated per tooth by two-blinded evaluators. In case of discordance between them, both operators analyzed the images together until full agreement was reached.

Statistical analysis

Statistical analysis was performed using SPSS software (SPSS for Windows Version 20.0; IBM Corp, Armonk, NY). set at P < 0.05. The Fisher exact test was compared the percentage of teeth with or without new CEDDs after AC. The Chi-square test and Fisher's exact test calculated the percentages of new CEDDs and preexisting cracks postoperative expansion. The Kolmogorov–Smirnov test evaluated distribution normality of the dimensions and the duration of the AC preparation procedures between groups. Mann–Whitney test was applied to compare the size of the cracks between the two methods of AC preparation, and Student's t-test was conducted to compare the duration of the AC preparation procedures between the two groups, and to evaluate the effect of the two methods of AC refinement on roughness scoring and equality of the means, Levene's test was finally conducted for the equality of variances.

RESULTS

In the preoperative scans, ten of the thirty teeth analyzed presented preexisting cracks: two teeth presented one crack, three teeth presented 2 cracks, three teeth presented 3 cracks, and two teeth presented 5 cracks [Figures 1 and 2]. The location of preoperative CEDDs and new CEDDs is reported in Table 1. In all cases, the direction of the extension of the preexisting CEDDs was from the occlusal to the apical surface.

Figure 1

Representative axial cuts (a and e) and cross-sections (b-d, f-h) of a tooth without CEDDs before (upper line) and after (lower line) the preparation of the access cavity with ultrasonics showing no new CEDDs formation

Figure 2

Representative axial cuts (a and e) and cross-sections (b-d, f-h) of a tooth with CEDDs before (upper line) and after (lower line) the preparation of the access cavity with Endo-Z bur showing no new CEDDs formation

Table 1

Location of preoperative and new CEDDs and their percentage; number of preoperative CEDDs extended postoperatively and dimension of this extension; dimension of the new CEDDs found postoperatively

Locations of CEDDs	Preoperative	New	Percentage	Extended	Dimension of extension (mm)	Dimensions of new
Mesiobuccal	6	0	20.7	2	0.83 and 2.96
Distal	4	0	13.8	0
Distobuccal	2	2	13.8	0		3 and 3.89
Mesiodistal	4	0	13.8	1	1.66
Mesiolingual	3	0	10.4	1	1.19
Distolingual	2	0	6.9	1	1.13
Buccolingual	2	0	6.9	2	0.74 and 1.14
Distopalatal	2	0	6.9	1	0.91
Mesiopalatal	1	0	3.4	0
Buccopalatal	1	0	3.4	1	1.06
Total	27	2	100.0	9

CEDD: Coronal extension of dentinal defects

No new CEDDs were registered in Group 1 [Figure 2], while new CEDDs appeared in two teeth from Group 2 that did not show these defects in the preoperative scan [Figures 3 and 4]. The percentage of teeth with new CEDDs (0% for bur group and 13.3% for US group) was not significantly different between the two groups (P = 0.483). The extension of the two new CEDDs was occlusoapical and distobuccal.

Figure 3

Representative axial cuts (a and e) and cross-sections (b, c, e, and f) of a tooth without CEDDs before the preparation of the access cavity with ultrasonics (upper line) showing new CEDD formation after preparation (lower line)

Figure 4

Three-dimensional view of the new crack that appeared in the tooth presented in Figure 3

A total of seven teeth presented an extension of a preexisting microcrack, for a total of nine cracks extended (33.3% of the preexisting cracks): four teeth (26.6% of teeth) in the Endo-Z group (with two teeth presenting two different microcracks extended each for a total of six cracks extended), and three teeth in the US group (20% of teeth). There was no statistically significant difference between the two groups (P = 1.000). The mean size of the extension was also not significantly different between Group 1 (1.35 ± 0.80 mm) and Group 2 (1.18 ± 0.46 mm) (P = 1.000).

The mean, standard deviation, minimum, and maximum duration of the AC procedures are shown in Table 2. They were significantly faster in Group 1 (P < 0.001).

Table 2

Average access cavity procedure time (min: s) in both groups

	n	Mean±SD	Minimum	Maximum
Group 1 (Endo Z)	15	3:23±00:43	2:30	4:30
Group 2 (Start X)	15	14:25±1:42	11:35	17:35

SD: Standard deviation

Percentages of the scores registered in the visual observation and analysis of the 3D images are reported in Table 3. The surface of the AC walls was statistically smoother for the Endo-Z group (P < 0.05). Observed irregularities and surface roughness in the Endo-Z group (mean = 2.40) were fewer than in the US group (mean = 1.33) [Figure 5]. The results showed that. for the US group, the score is close to rough; however, for the BUR group, the score is close to smooth.

Table 3

Surface roughness in both groups

	n	0 (very rough) (%)	1 (rough) (%)	2 (smooth) (%)	3 (very smooth) (%)	Mean±SD
Group 1 (Endo Z)	15	0	6	47	47	2.40±0.63
Group 2 (Start X)	15	7	60	33	0	1.33±0.49

SD: Standard deviation

Figure 5

Access cavity refined by (a) Endo-Z bur and (b) Ultrasonics

DISCUSSION

One of the main factors leading to tooth loss is tooth fracture at any level.[14] Thus, an ideal treatment plan should preserve tooth longevity as long as possible.

Endodontic treatment includes several phases and clinical factors that may play an important role in the durability of teeth. Therefore, many studies have been conducted to evaluate the influence of different instruments and procedures on tooth fracture strength and the possible creation of DD or root fractures.[151617]

During AC preparation procedures, the current tendency is to preserve as much of the sound tooth structure as possible,[18] while being compatible with optimal endodontic procedures. Nevertheless, no studies have investigated the influence of these procedures on the integrity of the residual tooth structure.

In the present study, extracted human molars were used to compare the effect of two different AC preparation procedures on the development of CEDDs by means of a MCT imaging system. Molars were chosen because of their largest occlusal surface, permitting more dentin structure preservation after AC preparation and for a better comparison of the formation or extension of CEDDs.[19]

The results of the present study showed that most of the CEDDs observed after AC preparation procedures were already present in the corresponding preoperative images, except for two CEDDs newly formed in the US group (Group 2). However, this was statistically insignificant in demonstrating the causality of USs in the creation of CEDDs. On the other hand, the extension of already existing CEDDs was noted in both groups with no statistically significant differences.

In addition, the CEDDs observed in the preoperative scan were detected for all tooth surfaces with predominance for the mesiobuccal side. According to De-Deus et al., the vertical root fracture may develop as a result of a fracture initiated at the crown.[20] This could be explained by the occlusoapical direction of the extension of the CEDDs reported in our study. Thus, roots vertical fracture may be a consequence of the extension of coronal micro-cracks. In addition, PradeepKumar et al. showed in their MCT study that DD are more common in the endodontically treated teeth of older patients (40–70 years).[21]

No previous authors compared the parameters found in the present study; however, several studies highlighted the impact of US on the formation of DD in retrograde cavities. Saunders et al. suggested that US use may increase the incidence of cracking in retrograde cavities,[22] while Layton et al. demonstrated a significantly greater number of cracks after root resection and US preparation of retrograde cavities in comparison to teeth with only root resection.[23] In contrast, Beling et al. found no statistical increase in the number of cracks after US preparation of the root.[24] This is in accordance with several other studies that found no significant difference between stainless-steel, diamond-coated, and zirconium-coated tips.[112526]

In the current study, MCT was used to evaluate the CEDDs before and after the preparation of the AC. This technique allows a 3D nondestructive evaluation of the teeth before and after the experimental procedures, and preoperative DD can be detected.[27] As a consequence, samples can be examined by evaluating hundreds of sections to determine the location of the fracture, thereby avoiding damage to the tooth structure and obtaining reliable results.[162829] This is a methodological difference from previously conducted studies using destructive experimental models.[2830] In fact, the formation of DD in the retrograde cavities was evaluated by the use of dyes[22] and the visualization of images using scanning electron microscopy.[9] The DD after root canal shaping was evaluated by teeth sectioning and microscopic observation.[30] These methods proved unsatisfactory, especially because they did not allow preoperative visualization of the DD, while showing bias since the cutting of the teeth itself could create cracks.

The results of the present study also showed that the average duration of the AC preparation procedures with burs was significantly shorter than with US, which is in agreement with the results from Gutmann et al. showing that root-end cavity preparation with US required more time and effort compared to the use of a bur.[31] On the other hand, Tobón-Arroyave et al. have shown that DD can occur independent of the thickness of the dentinal walls and can be associated with the extended ultrasound preparation time required for the removal of the root filling when preparing a root-end cavity.[32] Thus, the extended time of the procedures may have also influenced the results of the present study.

In addition, the surface condition of the walls during AC preparation was significantly smoother for the Endo-Z bur group than for the US group. The striations, especially at the level of the canal entrances, for the Endo-Z group, can be caused by the tip of the bur, although this point is noncutting. Moreover, other studies have shown that surfaces prepared with carbide burs are smoother than those prepared with diamond-coated US tips.[33]

In the present study, the extracted teeth were mounted in silicone-based material to simulate the surrounding in vivo structures. However, this may have not been adequate for absorbing all of the vibrations created during the procedures that may have caused the appearance of newly formed CEDDs or the extension of preexisting cracks. In fact, it has been suggested that the presence of the periodontal structure around the teeth may help to better absorb US energy,[34] therefore leading to the possibility of forming fewer CEDDs. Furthermore, a major disadvantage of the extracted teeth is that the samples may be subject to undue stress during the extraction procedure and may exhibit cracks before being used as samples.[35] In addition, the time elapsed since the extraction and the lack of knowledge about the patient's previous circumstances of dentition such as occlusal dysfunctions or trauma, tooth group, dentin structure, and the age and sex of the patient may influence the results.[27]

De-Deus et al. demonstrated that DD is mainly due to storage conditions and not to extraction phenomena.[20] Regarding the storage conditions of the present study, the extracted teeth were stored in a 0.1% thymol solution, as previously done in other studies.[16] The conditions of storage during experimental procedures may affect the occurrence of DD because of tooth dehydration.[12] In addition, microcracks in dried dentin slices can develop spontaneously and be misinterpreted as procedural cracks.[27] Previous studies reported that the spread of DD continued at the root slices even after 1 month of storage,[36] although no stress was applied; therefore, the samples were kept moist during all experimental procedures.[37] It has been proven that the detection of DD is influenced by the moisture content of dentin when evaluated by MCT, but scanning should be performed on dried specimens for better identification of DD since the formation of new DD during dry periods up to 24 h was not confirmed.[38]

CONCLUSIONS

More newly formed cracks were registered in the US group than in the bur group, and both instruments may cause the extension of preexisting CEDD, with no significant difference between groups. Further studies need to be conducted on a larger scale to confirm the results obtained in this study. The use of cadaver teeth, whose origin and storage conditions are known, would allow a better approach to the clinical situation and limit the confounding factors.

The clinical relevance of this study suggests that the use of US during AC preparation procedures may be considered safe, but the limitations of its use must always be taken into consideration. In fact, the use of US tips requires significantly more time compared to the conventional bur and is not justified to refine the cavity walls, as smoother surfaces were found in the bur group.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.