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| 2010-22 |
| 22-1 |
ISSN (Print) 1013-9052
EISSN 1658-3558
P.O. Box 52500,
Riyadh 11563,
Kingdom of Saudi Arabia
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| 2009-21-01-09-14-full |
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Evaluation of root-end microcrack formation following retropreparation King Saud University, Riyadh, Saudi Arabia
OBJECTIVE: This study evaluated differences among various ultrasonic instruments in the development of root-end cracks following retropreparation of endodontically treated teeth. MATERIALS and METHODS: Three ultrasonic tips were compared: stainless steel, zirconium nitride and diamond. Fifty-seven single rooted extracted teeth were cleaned, shaped and obturated. Their crowns were removed. A 3 mm resection of the root-tip was completed using a straight fissure bur. The teeth were examined under a light microscope. The teeth that developed cracks after resection were discarded. The teeth were divided into three groups of 19 teeth each and a retropreparation was completed with one of the ultrasonic tips for each group. Teeth were again examined under a light microscope. The photomicrographs of the teeth before and after were compared. RESULTS: Examination of the specimens revealed that in the stainless steel group, 26% (5/19) of teeth developed cracks, in the zirconium nitride group, 10.5% (2/19) of teeth developed cracks and in the diamond group, 10.5% (2/19) of teeth developed cracks. The differences in crack formation among the three groups were not statistically significant. CONCLUSION: The results of the study suggested that more cracks may be evident microscopically in root-ends prepared with stainless steel ultrasonic instruments although this was not statistically significant.
Conventional non-surgical endodontic retreatment is the treatment of choice for endodontic failure.1 However, due to the complexity of the root canal system, inadequate instrumentation and the presence of physical barriers (anatomical barriers, post and core restorations, separated instruments, etc.) ideal goals are often difficult to achieve with an orthograde approach. When circumstances limit the feasibility of non-surgical retreatment, surgical endodontics is the treatment of choice.1
The goal of endodontic surgery is to remove the disease, prevent it from recurring and to facilitate healing.2 Endodontic surgery may include the resection of the involved root, evaluation for possible fractures, evaluation of the obturation material and placement of a root-end filling. This root-end filling should be placed in a preparation that has theoretically incorporated the canal system that was in contact with the periapical tissue. Retropreparation should be parallel to and coincident with the anatomic outline of the pulpal space, have adequate retention form, all isthmus tissue removed, and remaining dentinal walls not weakened.2 Retropreparation should accept filling materials that predictably seal off the root canal system from the periradicular tissues.2
In the past few years, many studies were done comparing ultrasonic tips made of different materials as well as the effect on apical leakage. Brent et al.13evaluated root-end preparations for cracking and described the cavosurface morphology after the use of diamond-coated instruments. The results indicated that the use of diamond-coated instrument for ultrasonic preparation does not result in significant root-end cracking. It was also noted that these instruments resulted in a heavily abraded, debris-covered cavosurface that may affect the apical seal.
Rainwater et al.14 compared stainless steel ultrasonic tips to diamond coated tips and high speed stainless steel burs. They were evaluated using dye and a light microscope. They were unable to correlate the formation dentin cracks or dye leakage to the different instrument designs.
Fifty-seven single rooted freshly extracted teeth were used. Specimens were stored in a humid environment during the study. All crowns were removed with a straight fissure bur in a high-speed handpiece using copious irrigation. Working length was determined by subtracting 1 mm from the length of a #10 hand file just visible out of the apex. The roots were cleaned and shaped using hand files in step-back fashion and obturated with gutta-percha and AH26 sealer using lateral condensation technique. Copious 2.5% NaOCl was used throughout the preparation. During the entire procedure, the roots were held in gauze moistened with saline so as to prevent drying.
Then a 3 mm root resection was completed using a straight fissure bur. The roots were stored in 0.1% methylene blue for 48 hours and were examined using a light microscope at 20X magnification. Teeth with noticeable canal fractures or complete root fractures were discarded and replaced with another tooth free of cracks. Specimens were divided into three groups as follows: Group 1 (19 teeth) -retropreparation completed with two CT-1S stainless steel ultrasonic tips (Analytic Technology Inc, Orange, CA, USA); Group 2 (19 teeth) - retropreparation completed with two KiS 1 zirconium-nitride coated tips (Obtura-Spartan, Earth City, Missouri, USA); Group 3 (19 teeth) - retropreparation completed with two KiS-1D diamond coated tips (Obtura-Spartan, Earth City, Missouri, USA). Approximately one half of each group was prepared with each tip. Images were stored digitally. An ultrasonic unit on medium power with continuous water spray was used. The tip was placed in the center of the gutta-percha and a light back and forth motion was used until the full depth of the tip was reached and no gutta-percha was evident on the walls of the preparation. Then the roots were again placed in methylene blue for 48 hours. The roots were again evaluated microscopically in the same manner as before. A second digital image was stored. The images taken before and after were compared to detect microfractures.
Results are summarized in Table 1. In the stainless steel (Group 1), 5 teeth had cracks following ultrasonic preparation. In the zirconium nitride (Group 2), 2 teeth showed evidence of cracking following ultrasonic preparation, and in the diamond coated (Group 3), 2 teeth had cracks.
A chi-square test was conducted to determine if there was a difference in frequency of cracking among the groups. Results were not statistically significant (P = 0.05). However, Group 1 showed a cracking rate of 26% versus 10.5% and 10.5% for Groups 2 and 3, respectively. A post-hoc power analysis indicated that a sample size of approximately 145 would be needed in order to find this size difference to be statistically significant. The photomicrographs taken before and after are shown in Figures 1 through 3. These showed the resected root-ends prior to retropreparation and reveal a smooth dye stained resected root-end surface free of any cracking. Figures 1 and 2 showed the root-ends after retropreparation with evidence of development of cracks. Two types of cracks were observed on the root-end surfaces. Canal cracks were those that originated within the canal and radiated into the dentin. These may be complete, extending from the canal to the external root surface (Fig. 1) or incomplete extending from the canal partially through the dentin. The second type of crack observed were intradentin cracks that were confined to dentin. In several cases, evidence of marginal chipping and gouging of the root-end was also apparent (Fig. 1). Figure 2 is an example where areas of marginal chipping are evident around the lumen and a gouged area is also present that resulted from the ultrasonic tip skidding along the root surface.
This study used instrumented and obturated roots to simulate most clinical situations requiring surgical treatment. Although, Beling et al.16 found no significant difference when endodontically treated and untreated roots were compared with regard to number of fractures after root-end resection or ultrasonic root-end preparation. However, Brent et al.17 used unprepared teeth to evaluate diamond coated ultrasonic instruments and reported no significant root-end fractures.
Upon examination of the images before and after under magnification, cracks were easily identified. Elimination of those roots that had evidence of cracks prior to root-end preparation facilitated their detection after preparation.14, 15
Methylene blue was used for its ability to outline roots, delineate root dentin from bone and to demarcate isthmuses between two canals in a single root. It was found to be particularly useful in identifying fractures in the root-end as well. The use of methylene blue for 48 hours does not have a clinical basis, because during surgery, methylene blue is only in brief contact with the root-end surface. However, the time period has been used successfully in previous studies. 5
Another factor that may influence the results of these studies was wearing of the ultrasonic tips. As the teeth were prepared, wearing of the tip occurred, lowering the cutting efficiency of the instrument. This in turn lead to strain on the root surface and a greater chance for cracking.14 Teeth in a particular group prepared later in the series may be affected by this phenomenon. Rainwater et al.14 and Navarre et al.15 used one tip per group. In the current study, two tips were used per group to lessen this effect. Ideally, a new tip should be used for each retropreparation. This was not feasible clinically.
Propagation of existing cracks not detected after root resection was another issue for consideration. Examination of the root-end after root resection prior to retropreparation allowed the detection of surface cracks but not of cracks below the surface. These undetected cracks may have become worse following retropreparation which affected the results of these studies.
Finally, the handling of the extracted teeth prior to study completion could contribute to variation in the results. Teeth that were allowed to desiccate or that were autoclaved were more prone to cracking. In all of the studies,7-14 the teeth were kept moist at all times, and in an attempt to minimize the effect of previously cracked teeth, roots with evidence of cracking following root-resection were discarded, as previously mentioned.
Cracking of the root-end during retropreparation may jeopardize the seal of the root-end filling, the ability to rid the canal system of bacteria and the overall root-end strength. If there is necrotic debris or bacteria present in the dentin tubules adjacent to the main canal exposed during root resection and retropreparation, these irritants may penetrate the periapical tissues.
The results of the study suggested that more cracks may be evident in root-ends prepared with stainless steel ultrasonic instruments when compared with zirconium nitride or diamond instrument although this finding was not statistically significant. Further studies are needed to evaluate the possible effect of different ultrasonic instruments on the integrity of the resected root-end during endodontic surgery and the effect that this may have on apical leakage.
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