The Unpredictability of Seal After Post Space Preparation: A Fluid Transport Study

The Unpredictability of Seal After Post Space Preparation: A Fluid Transport Study

JOURNAL OF ENDODONTICS Copyright © 2001 by The American Association of Endodontists Printed in U.S.A. VOL. 27, NO. 4, APRIL 2001 The Unpredictabilit...

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JOURNAL OF ENDODONTICS Copyright © 2001 by The American Association of Endodontists

Printed in U.S.A. VOL. 27, NO. 4, APRIL 2001

The Unpredictability of Seal After Post Space Preparation: A Fluid Transport Study Itzhak Abramovitz, DMD, Ronit Lev, DMD, Zvi Fuss, DMD, and Zvi Metzger, DMD

(5, 6). Nevertheless the methodology used in these leakage studies has been widely criticized. Air entrapment, absence of or inadequate control groups, semiquantitative data, and inherent external variables in the experimental model were among the prominent shortcomings of these studies. Coronal leakage studies, on the other hand, have shown that when remaining, short, root canal fillings of 5 mm were preserved, leakage did occur (7, 8). Unfortunately none of the above coronal leakage studies have compared the sealing ability of partially removed root canal fillings to that of intact ones. This was most probably due to technical difficulties in running such an assay in teeth with intact fillings; hence the extent of potential seal loss could not be evaluated. A recent modification of the coronal leakage studies made it possible to compare intact root canal fillings and those partially removed to a remaining length of 5 mm. When a pressure-driven assay was used, the seal of 5 mm remaining fillings was clearly inferior to the intact ones (9). A consecutive study has shown that only remaining fillings of 9 mm displayed sealing properties similar to that of intact ones. Shorter fillings were clearly inferior (10). These findings were in agreement with those of Wu et al. (11), who also found that 4 mm remaining root canal fillings were inferior to intact ones. Nevertheless coronal fluid transport studies are not free of drawbacks. The possibility that anatomical, procedural, and operator variables may have played a significant role in coronal quantitative studies, resulting in a high variability of the results, cannot be ignored. Wu et al. (11), using a fluid transport model, have extended the aforementioned approach by using each tooth as its own control. This approach may overcome some of the previously described variables and differentiate between variability due to technical parameters and real variability in sealing ability. The present study was designed to follow and monitor the changes in sealing ability of full length root canal fillings after partial removal of the fillings to a remaining length of 6 mm and then to 3 mm. The same teeth were used throughout the procedure and served as their own controls.

A root canal filling remaining after post space preparation is commonly expected to provide adequate seal. Coronal leakage of 30 endodontically treated teeth was measured before post space preparation using a fluid transport assay. In 10 of these teeth post space was prepared, using a twostep procedure, first to a remaining filling of 6 mm and then to 3 mm, with the leakage studied after each step. In 10 teeth the removal was done in one step to a remaining length of 3 mm. The other 10 teeth, with intact root canal fillings, served as controls and were tested twice for leakage. A significant difference was found between the sealing ability of intact fillings and that of partially removed ones (p < 0.05). The difference between the sealing ability of 3 and 6 mm remaining length group was not statistically significant. The lack of statistical differences between the 6 mm and 3 mm fillings was due to a great variability which existed among the 3 mm remaining fillings. These results suggest that 3 to 6 mm fillings provided a seal inferior to that of intact root canal fillings. Reduction of the fillings to 3 mm resulted in an unpredictable seal.

Restoration of endodontically treated teeth often requires partial removal of root canal filling material to form a post space. Post space preparation is commonly performed using rotary instruments and is done on a separate visit, after complete setting of the sealer (1, 2). The preparation should comply with the mechanical requirements of a post and core restored tooth; nevertheless it should not compromise the apical seal. The current, widely accepted, concept is that a 5 mm remaining root canal filling provides an apical seal that does not differ from that of an intact root canal filling. This concept is based on a selection of in vitro studies (3, 4) that do not directly refer to the commonly practiced technique of partial removal of root canal fillings (1, 2). Studies that addressed this issue may be divided into two groups: (a) apical percolation assays and (b) coronal leakage studies in which the fluid transport method was applied. Some of the in vitro apical percolation studies showed no difference between partial removal of fully set root canal fillings and intact ones

MATERIALS AND METHODS Teeth Thirty straight, intact, single-rooted extracted human teeth were selected from a random collection preserved in 10% buffered 292

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formalin. All teeth were examined for absence of caries or fractures and radiographed to exclude teeth with internal calcifications, internal resorptions, or visible lateral canals. Soft periodontal tissue was removed by hand curettes. The crowns were removed to a uniform remaining root length of 14 mm using diamond highspeed burs with air-water spray coolant. Throughout the experiment teeth were kept at 100% humidity.

Endodontic Procedure Working length was determined by introducing a size 10 K file to the apical foramen and subtracting 1 mm. All apical foramina were instrumented by passing a size 25 K file through them to reduce the influence of changes in apical anatomy on the results. Root canals were prepared from crown to apex using GatesGlidden drills, followed by K-files to a size 45 master apical file. EDTA containing lubricant (File-Eze, Ultradent Products, Inc.) was applied, and 2.5% sodium hypochlorite was used as an irrigant. Patency was verified using a size 25 K file. Canals were dried with paper points and filled with gutta-percha points and AH26 sealer, using lateral condensation. Excess material was removed with hot pluggers, followed by vertical compaction using a cold plugger. All teeth were prepared by one operator (R.L.). Each tooth was radiographed from buccal and proximal views to evaluate the quality of the fillings and kept for 30 days at 100% humidity at room temperature before further manipulation.

FIG 1. Modified fluid transport system. The tooth is embedded in a resin cylinder (1), formed in a distal part of a 20 ml Luer-type disposable syringe (2). The coronal and apical orifices of the canal are free of resin. A removable hollow chamber is formed around the coronal part of the tooth using part of a 5 ml Luer-type disposable syringe (3) that is secured in place by two layers of epoxy resin (4). The system is filled with water, and air pressure is applied through the tube (5). Leakage is measured by movement of an air bubble in the pipette (6). A three-way valve (7) is used to introduce water with an air bubble to the distal part of the assembly. The assembled chamber is immersed in water to detect any leakage through connectors.

distilled water and 0.05% sodium Azid (to inhibit microbial growth). On completion of each leakage measurement the chamber was disassembled from the system, opened, and the root submitted to a specified clinical procedure. Then the system was reassembled as previously described and further measurement done.

System Assembly Post Space Preparation Partial removal of root canal fillings was done 30 days after obturation, using Gates-Gliden drills at 4000 rpm. Preparation was done either in a two-step procedure, first to a 6 mm and then to 3 mm remaining root canal filling, or as one step, resulting in 3 mm fillings, as detailed herein.

Modified Fluid Transport Assay A modification of the fluid transport assay (12) allowed easy connection, measurement, disconnection, and reconnection of the root to the system, thus allowing several operational procedures on the same tooth, with leakage measurements between them. Each tooth was embedded in an epoxy resin cylinder, formed in a mold consisting of the end part of a 20 ml Luer-type disposable syringe. Coronal and apical openings of the embedded tooth were kept free of the polymer (Fig. 1). A second hollow cylinder, cut from a 5-ml Luer-type syringe, was adapted to the first resin cylinder and sealed with epoxy resin to form a closed chamber with two Luer-type outlets (Fig. 1). The coronal outlet was fitted to an intravenous infusion tube manifold (Biometrics, Jerusalem, Israel) that allowed application of compressed air (1.2 atm) to the chamber. The apical outlet was connected through a three-way valve to a 200 ␮l glass pipette. Before activation air was removed from the system and it was filled with distilled water using a syringe adapted to the three-way valve. An air bubble was introduced into the micropipette through the three-way valve to serve as a marker for fluid transport. The assembled system was tested for an air-tight seal for 24 hr before its use and then in a water bath containing

The manifold, its annexed chambers and micropipettes were air-vacuumed and filled with distilled water. An air bubble was introduced through the three-way valve and placed at point 0. Air pressure in the manifold of the system was gradually brought to 1.2 atm. The compressed air forced the fluid through the obturated roots from the coronal to the apical direction. Leakage was measured by movement of the air bubble in the pipette and expressed in microliters per 5 minutes. Records were taken at time 0, 10 min, and 1, 6, 10, and 24 hr.

Experimental Design Each tooth was first tested with its intact 14-mm root canal filling. Teeth were then disassembled and randomly divided into 3 groups of 10 teeth. GROUP A: TWO-STEP POST SPACE PREPARATION Root canal fillings were partially removed to a remaining length of 6 mm. Chambers were reassembled and leakage studied as described. At the end of this step chambers were disassembled, and root canal fillings were further removed to a remaining length of 3 mm, followed by leakage measurement.

GROUP B: ONE-STEP POST SPACE PREPARATION Root canal fillings were partially removed, in one step, to a remaining length of 3 mm, and served as controls to examine the influence of the extra step in group A on leakage.

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GROUP C: INTACT ROOT CANAL FILLINGS REMEASURED Root canal fillings were not removed and were subjected again to leakage measurements. They served as controls to examine the possible influence of repeated measurements on the results.

Statistical Analysis Results were expressed in microliters per 5 minutes and statistically analyzed using analysis of variance and covariance for repeated measurements.

RESULTS Intact Root Canal Fillings When all 30 intact root canal fillings were tested for 24 hr, before any post space preparation, an average leakage rate of 0.052 (⫾0.031) ␮l/5 min was recorded. Partial Removal of Root Canal Fillings Leakage through roots in which post spaces were prepared was significantly higher than that of the same roots before that procedure (p ⬍0.05). Higher leakage was found in 3 mm remaining root canal fillings, compared with 6 mm filling: an average of 0.174 (⫾0.175) vs. 0.100 (⫾0.042) ␮l/5 min. Because a very high variability in results existed in the 3 mm group, this difference was not significant.

Repeated Measurements To test whether the repeated measurements had influence on outcome, intact root canal fillings of control group C were retested. A significant decrease in leakage in the second test was recorded: an average of 0.043 (⫾0.033) ␮l/5 min vs. 0.064 (⫾0.034) ␮l/5 min, for the first and second measurements, respectively (p ⬍0.05). Three Millimeter Root Canal Fillings Three millimeter remaining root canal fillings that resulted from the two-step post space preparation did not significantly differ from those produced by a one-step preparation to the same length. High variability in leakage was recorded among roots in both types of 3 mm remaining root canal fillings (Fig. 2).

DISCUSSION Post space preparation should allow a remaining root canal filling that provides an adequate seal. It is commonly accepted that a remaining root canal filling of 3 to 5 mm provides such a seal (5, 6). However this recommendation was based mainly on apical percolation studies and should be re-evaluated. The methodology of apical percolation studies has been extensively criticized. First the clinical relevance of the apical percolation concept is questionable (13). Coronal leakage, on the other

FIG 2. Variability in the seal of 3 mm root canal fillings. (A) Each group of 3 bars represents the average 24 hr leakage in the same individual root with intact, 14 mm, root canal filling, after partial removal of the filling to 6 mm and then to 3 mm. Although some of the fillings maintained their seal, others (A1, A3, A5) lost their seal. (B) Each pair of bars represents the average 24 hr leakage of an individual root before and after partial removal of the root canal fillings to 3 mm, in one step. Some of the fillings maintained the seal whereas others (B2, B3, B6, B9) lost it.

hand, is more likely to occur in different clinical situations (14). Therefore in the present study we used a modified fluid transport system to test the seal against coronal leakage provided by intact root canal fillings that were later partially removed to a remaining length of 6 and 3 mm, respectively. Active pressure leakage assays have proven to be superior to passive leakage ones, due to their ability to overcome air entrapment that may mask the true leakage pattern (15). Additionally, when used with radioactive tracers, they are more sensitive and are able to detect minute differences between samples (9) and enhance leakage demonstration (10). The fluid transport model is an example of an active pressure leakage assay that does not require the sacrifice of the tested specimen. Furthermore it allows repeated measurements of the same specimen, before as well as after procedural manipulation. By that it may diminish the effect of variables, such as anatomical variations, that may otherwise influence the results (12). Embedding each tooth in a cylindrical resin block made it possible to use teeth with any anatomy, thus avoiding premachining of the teeth or ligatures as proposed by Wu et al. (16). We have recently applied this concept also to test fluid transport through furcations of multirooted molars (17). This modification properly

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isolates the lateral root walls and allows assembly and disassembly of teeth before and after partial removal of root canal fillings. Additionally it may reduce potential strains induced in teeth by tight ligatures, as well as reduce the danger of microcracks that may develop when machining a tooth to a cylindrical shape (16). Finally, the proposed modification reduces operator variability due to differences in assembly and reassembly of the system. It allows continuous assurance of proper isolation that is otherwise impossible. Using this modified fluid transport system we were able to measure leakage in intact root canal fillings that were exposed coronally. Our finding is in agreement with that of Yared et al. (18), who have found fluid transport of 0.34 ␮l/5 min through intact 10 mm root canal fillings and with Wu et al., who showed leakage of more than 20 ␮l/24 hr (⫽ 0.07 ␮l/5 min) through intact control 10 mm root canal fillings using the fluid transport model (11). The smaller average leakage in our study (0.052 ␮l/5 min) may be attributed to longer fillings (14 mm) and to better lateral isolation used in the present modified transport model. The assumption that 3 to 5 mm of remaining root canal filling provides adequate seal was partially undermined by Rhom et al. (7) and Fuss et al. (8), who have shown that remaining root canal filling of 5 to 6 mm leaked, once exposed to a long duration coronal leakage assay. However absence of comparison to full length root canal fillings, as controls, in these studies made the evaluation of the extent of deterioration of the seal impossible. In the present study we compared the sealing ability of each partially removed filling to the sealing ability of the same filling before its partial removal. We found significant loss of sealing ability after partial removal of the fillings. This observation supports our prior observations (9, 10) that remaining root canal fillings of 5 mm provided a seal inferior to that of full length ones. Our present results carry this concept one step further: once root canal fillings were removed to a remaining length of 3 mm, a wider range of leakage was observed. Comparison of sealing ability of each root canal filling to its own control (Fig. 2, A and B) revealed that the short root canal fillings were unpredictable: some maintained a relatively adequate seal, whereas others lost their sealing ability. A similar phenomenon was also observed by Wu et al. (11), who showed that whereas half of their partially removed root canal fillings kept their sealing ability, the other half deteriorated and demonstrated leakage of up to more than 100 ␮l/hr, which is equivalent to more than 8.3 ␮l/5 min in our study. It seems that the apical part of a laterally condensed root canal filling is the least filled and most techniquesensitive (19). It is conceivable that if that sensitive part of filling is left alone the seal will vary. Occasionally, a good seal may be obtained but in other cases the seal may be faulty. These findings substantially differ from those of most apical percolation studies, which confined the leakage measurement to the apical area rather than to the entire root canal filling. It must be emphasized that even a good root canal filling, such as those performed under in vitro optimal conditions, may not be able to prevent leakage without additional proper coronal seal. In the present study partial removal of root canal filling was done after the full setting of the cement, as clinically practiced by the vast majority of the dentists (1). The unpredictability of the seal of the short root canal fillings presents a major disadvantage of this method when compared with the alternative of immediate partial removal of a root filling using a hot plugger (9, 10). The operator is unable to improve the remaining root canal filling, if desired, by supplementary condensation or compaction, as can be done in the

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latter method. In a clinical situation, when a filling of 3 mm remains after delayed partial removal, one should be very skeptical about the ability of the remaining root canal filling to provide a proper seal, should coronal leakage take place (20). Fogel (20) and Wu et al. (11) studied the inability of posts to seal the canal and concluded that their seal was also unpredictable regardless of post type or lueting material. Therefore improving the seal of both root canal fillings and post and cores is advisable, if long-term prevention of recontamination is desired. Studies with this goal in mind are currently in progress in our laboratory. This study was part of the requirements for the DMD degree of Ronit Lev and was conducted at The Alpha Omega Research Laboratories, The Goldschleger School of Dental Medicine at Tel Aviv University. Dr. Abramovitz is a graduate student, Department of Endodontology at the Goldschleger School of Dental Medicine at Tel Aviv University; Dr. Lev was a DMD student; Dr. Fuss is affiliated with the Department of Endodontology; Dr. Metzger is associate professor, Departments of Restorative Dentistry and Oral Biology, and Director of Research Laboratories at The Goldschleger School of Dental Medicine at Tel Aviv University, Tel Aviv, Israel. Address requests for reprints to: Dr. Itzhak Abramovitz, The Goldschleger School of Dental Medicine, Tel Aviv University, Ramat Aviv, Tel Aviv 96978, Israel.

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