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RESEARCH REPORT |
Department of Restorative and Preventive Dentistry, Westdeutsche Kieferklinik, University of Düsseldorf, Moorenstr. 5, D-40225 Düsseldorf, Germany
* corresponding author, hermann.lang{at}uni-duesseldorf.de
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: root canal treatment • rigidity • deformation • posts
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Endodontic posts are not necessarily expected to increase the stability of endodontically treated teeth, but data about the consequences of the different post-and-scaffold systems on the susceptibility of endodontically treated teeth to fracture are still controversial (Sornkul and Stannard, 1992; Cormier et al., 2001). In most of these studies, the stability of endodontically treated teeth has been investigated by recording the resistance to fracture (Martinez-Insua et al., 1998; Johnson et al., 2000; Pontius and Hutter, 2002). This can predict only failure probabilities, but not the time-course of modifications in deformability or stability, respectively. Such measurements require the use of strain gauges (Reeh et al., 1989; Magne and Douglas, 2000). Some studies have investigated the effect of step-by-step restorative and endodontic treatments on the stability of teeth (Reeh et al., 1989). This study demonstrated a reduction of the stability ("relative stiffness") of the tooth by endodontic preparative treatments. However, the effect was only minor compared with that from preparations of the clinical crown.
The aim of this study was to investigate the influence of preparative procedures at different stages of an endodontic treatment on the rigidity of teeth, and to determine the deformation pattern of tooth roots by interferometry. The hypothesis of the study was that the root would be destabilized by all endodontic preparative steps (Fig. 1
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MATERIALS & METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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).
Endodontic Steps
To establish a baseline for measurements, we determined the deformation of untreated teeth (gr. 1) interferometrically. Thereafter, an access cavity was prepared (Ø 3 mm) by means of a cylindrical diamond bur (No. 806-314-111-544-016, Meissinger GmbH, Düsseldorf, Germany). After residual tissue was removed (extirpation instrument, VDW GmbH, Munich, Germany), the teeth were rinsed with sodium hypochlorite (5%) and, subsequently, with physiological saline (0.9%). We determined the length of the preparation by probing the root canals with a Kerr file (ISO 15) and determined the distance between the access preparation cavity and the reference point, respectively, and the root apex on the outer surface (mean preparation length, 22.9 ± 2.0 mm). We then determined tooth deformation again. In step 3, the root canals were prepared initially by means of Kerr files (K-files, VDW GmbH, Munich, Germany) and widened to ISO 40 (gr. 3). Further determinations of the deformability were performed after widening of the root canals to ISO 60 (gr. 4), 80 (gr. 5), and 110 (gr. 6) in steps 4 to 6. After preparation, the canals were rinsed with saline, and the tooth deformability was determined again.
In step 7, we applied a tapered post preparation to evaluate its influence on tooth deformability. We used the standardized preparation set of a factory-made post system (post bur ISO 110, Komet ER-set, Gebr. Brasseler GmbH, Lemgo, Germany). The preparation depth was two-thirds of the instrumentation length of the respective teeth. We determined deformability again after removing the dentin debris by rinsing with saline. Thereafter, while conserving the preparation depth of the existing tapered preparation, we widened the tapered post preparation to be parallel-sided, in step 8, with a factory-made preparation set (post bur ISO 110, Parapost XP, Coltene/Whaledent GmbH, Langenau, Germany). A final determination of tooth deformability was carried out after another clearance of the prepared canal (Fig. 2).
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) and loaded them with 3.75 N using a force-controlled piezoelectric device (model LE 150/025 Pickelmann, Steinbichler Optotechnik, Neubeuern, Germany). The force was applied at an angle of 135° relative to the tooth’s axis (Loney et al., 1995) with the use of a wedge-shaped slide ram (motion speed resulting in an increase of force of 0.25 N/s). The resulting deformation of the teeth and the roots, respectively, was determined interferometrically (electronic 3D-Speckle-Interferometry [3D-ESPI]). The procedure has been described previously (Lang et al., 2004). Briefly, by illumination of the tooth with an argon laser (512 nm), a speckle pattern characteristic of the position of the tooth is produced. Changing the position (e.g., deformation) changes the speckle pattern and the light intensity. The measurement consists of two recordings (double-exposure technique): recording of the speckle pattern in the unloaded state, and another recording of the modified pattern in the loaded state. Subtracting the light intensities for each speckle point yields an interferogram of the tooth. This interferogram was used to calculate the degree of deformation by image analysis. Measurement error was determined by experimental analysis to be ± 0.025 µm (Fig. 3).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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). Furthermore, any further endodontic step—that is, any further widening of the root canal—caused a distinct increase in tooth deformability (Fig. 4). However, the increase in deformability was non-linear. While the manual widening by means of Kerr files was not accompanied by a significant increase of deformability (p > 0.05), the following treatments resulted in an irregular increase in deformability that was statistically significant:
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).
Over all steps of treatment (1–8), tooth deformability increased about three-fold (untreated teeth, 0.24 ± 0.03 µm, to parallel-sided post preparation, 0.73 ± 0.09 µm) (Fig. 3).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS & METHODSRESULTSDISCUSSIONREFERENCES |
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Access preparation (removal of the pulp chamber roof), as well as post preparation, resulted in a significant increase in deformability of the tooth, while the removal of dentin at the wall of the canal without extensive alteration of the root canals’ outline (by manual widening) resulted in no significant increase of deformability. This suggests another important stabilizing factor for the tooth, that is, the natural geometry of the root canal. Considering the destabilizing effects of post preparations, it is noteworthy that there was no difference between the last-used Kerr file and the tapered post bur, in terms of preparation size. Since this instrument is rigid and has no opportunity to adapt to the shape of the root canal, an artificial elongation and straightening of the course of the canal are inevitable. The relatively straight course of the root canal in anterior teeth notwithstanding, there is obviously a significant destabilization of the root after only minor modifications of the root geometry.
This phenomenon is also highlighted when one considers the consequences of the subsequent widening of the tapered post preparation to a parallel-sided shape. We found an irregular removal of hard substance in the root canal—that is, besides the loss of hard substance, there was also a significant modification of the geometry of the root canal. Previous studies considering the micromorphology of root dentin (Hals, 1990; Kishen et al., 2004) have suggested that not only the thickness of the dentin, but also the structure of the inner dentin, has a stabilizing influence on the root. The combined effect of loss of hard substance and non-congruent preparation (that is, non-homogeneous removal of dentin adjacent to the pulp) seems to be responsible for the irregular increase of deformability in the realm of the root and the destabilization of the tooth.
Results regarding the extent to which a (partial) stabilization of the instrumented root with the use of post construction systems (e.g., bonded glass fiber or ceramic posts) is possible have been controversial (Assif et al., 1993; Martinez-Insua et al., 1998; Johnson et al., 2000; Newman et al., 2003). Such bonded posts might decrease the extent of deformation, albeit not necessarily strengthening the root clinically in terms of resistance to fracture. Since the limiting factor for fracture is the remaining tooth hard substance (Sornkul and Stannard, 1992; Magne and Douglas, 2000; Fernandes and Dessai, 2001), systems will presumably prove effective if they (a) afford no or only minimal invasive preparation procedures, (b) correspond to the removed root dentin in terms of their natural structure and function, and (c) are attached firmly (adhesively) to the dentin in the root canal (Cormier et al., 2001; Newman et al., 2003).
In this study, we investigated tooth deformability with a minimal load (3.75 N = 5% of the average masticatory load). Given an adequate sensitivity of the procedure, the advantage of this method is the lower probability of inappropriate results brought about by plastic or irreversible modifications in root dentin (i.e., an increase in deformability caused by microfractures) compared with measurements with mean and maximum loads (70 N). However, the relevance of in vitro studies to the stability and resistance to fracture of endodontically treated teeth is limited to comparative investigations, since (a) they are influenced by the study design chosen (e.g., the direction of load application [Lang et al., 2004]), (b) teeth and their sizes vary, (c) a simulation of in vivo situations is difficult (e.g., protective function of the periodontal ligament under loading), and, most importantly, (d) there is no information about the limit of critical load and deformability.
Basically, any removal of hard substance in the canal increases the deformability of the root. More invasive treatments, such as post preparations, influence the stability of the root considerably, while substance-saving instrumentation results in only minor destabilization if the root canal geometry is preserved. Consequently, a minimally invasive treatment is necessary not only while cavity preparations are being performed, but also while roots are subject to instrumentation.
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ACKNOWLEDGMENTS |
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Received April 27, 2005; Last revision November 20, 2005; Accepted December 19, 2005
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REFERENCES |
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