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 »  Home  »  Endodontic Articles 4  »  Measurement of strain on tooth roots during post removal with the Eggler post remover
Measurement of strain on tooth roots during post removal with the Eggler post remover
Introduction - Materials and methods.



T. V. Castrisos, J. E. A. Palamara & P. V. Abbott
School of Dental Science, University of Melbourne, Melbourne, Victoria, Australia.

Introduction.
Several techniques are available for the removal of posts from roots and these include the use of ultrasonic vibration and various post removal devices. Most of the literature on post removal discusses the various methods used in the form of clinical case presentations, although some studies have investigated the efficiency of post removal techniques. Buoncristiani et al. (1994) compared the efficiency of ultrasonic and sonic vibration to remove parallel-sided preformed posts and concluded that ultrasonic vibration could remove them in less than 10 min. Berbert et al. (1995) and Johnson et al. (1996) have also examined the influence of ultrasonic vibration on the forces required to remove posts. Berbert et al. (1995) used cast posts/cores and concluded that the force required to remove them after application of ultrasonics for 2 to 5 min was 30% less than the control group with no ultrasonic vibration. Johnson et al. (1996) concluded that 16 min of ultrasonic vibration could significantly reduce the amount of force required to remove parallel-sided preformed posts in extracted human premolars. Yoshida et al. (1997) examined the effect of using one or two ultrasonic tips simultaneously placed on a cast post and core and they reported that the use of two ultrasonic tips was significantly quicker. The use of the Eggler post remover (Automaton-Ventriebs-Gesellschaft, Germany) and the Gonon (Thomas) post remover (Thomas Extracteur De Pivots; FFDM, Bourges, France) have been described by Bando et al. (1985), Stamos & Gutmann (1991) and Machtou et al. (1989), who all described them as highly efficient methods for removing posts.
A survey of the methods used by American endodontists to remove intraradicular posts found that post removal devices such as the Gonon and Eggler post removers were not commonly used (Stamos & Gutmann 1993). The reasons given by the respondents to this survey for not using these devices were that they were too dangerous, could not be universally used, or did not work. In contrast, a recent survey of endodontists in Australia and New Zealand found that the Eggler post remover was the most frequently used post removal device, with 42% of the respondents using it regularly (Castrisos & Abbott 2001). Stamos & Gutmann (1991) stated that the advantages of using these types of post removers were:
  1. conservation of remaining tooth structure
  2. reduced risk of root fracture
  3. reduced risk of root perforation, and
  4. reduced risk of torque forces being placed on the root.
Although several articles have presented case reports about the efficiency of various devices, the safety of these devices has not been established. Altshul et al. (1997) compared the prevalence of dentinal cracks in teeth with cast posts that had been removed by either ultrasonic vibration or the Gonon post removal system. There was a statistically significant difference between the number of dentinal cracks present following ultrasonic removal of posts compared with teeth where the post was not removed. The dentinal cracks were seen more frequently at the cervical level following the use of ultrasonics and the time taken to remove the posts with ultrasonic vibration was significantly longer than with the Gonon system.
As a result of the design and size of the Eggler post remover (Fig. 1), it is usually used to remove posts in anterior teeth. It is considered by most endodontists who use it to be of greater benefit for the removal of cast posts/ cores than ultrasonics (Castrisos & Abbott 2001), where the time required to remove the post is greater.

Figure 1. The Eggler Post Remover.

The Eggler Post Remover

To date, no studies have reported the amount or nature of forces applied to teeth during post removal. However, several methods have been used in various studies to determine the amount of force applied to teeth with various loads during other dental procedures. These methods include two-dimensional and three-dimensional photoelastic stress analysis (Henry 1977, Mentink et al. 1998), finite element stress analysis (Huysmans et al. 1993) and strain gauge measurements. The latter have been used to assess forces applied when obturating root canals using lateral condensation (Saw & Messer 1995, Lertchirakarn et al. 1999), when cementing posts (Ross et al. 1991, Obermayr et al. 1991) and in measuring the reduction in stiffness of endodontically treated teeth (Reeh et al. 1989). The strain gauges used in these studies were applied to the external surface of the root or crown to measure the strain on the tooth.
The aim of this study was to measure the surface strains caused by the application of an Eggler post remover parallel to the long axis of the tooth and to compare them with strains produced by loads applied at a 10 angle to the long axis.

Materials and methods.
Twenty extracted single-rooted human teeth were obtained from the clinics of the Royal Dental Hospital of Melbourne. At all times prior to and during the experiment, the teeth were kept moist by either storing them in phosphate buffered saline or by covering them with gauze moistened in phosphate buffered saline. Radiographs were taken from the buccal and mesial directions to ensure that only a singleroot canal was present and all the teeth were examined under a microscope ( 16 magnification) to ensure they were free of fractures and cracks. The crowns of the teeth were removed with a diamond bur at a level 1 mm coronal to the labial cemento-enamel junction and the teeth were then measured. The mesio-distal dimensions of the teeth at the cemento-enamel junction ranged from 4.2 mm to 5.9 mm and each tooth was checked to ensure that the repeller arms of the Eggler post remover could be rested on the root surface.
The root canals of each tooth were negotiated and instrumented up to a size 40 Hedström file at a length that was 1 mm short of the apical foramen. The apical third of each canal was flared with the step-back technique and the coronal third was flared with sizes 2 and 3 Gates Glidden burs. Whilst being instrumented, the canals were irrigated with 1% sodium hypochlorite between each file size. They were then filled with guttapercha and AH26 (Dentsply DeTrey GmbH, Konstanz, Germany) using lateral condensation; a post space was provided in each tooth to a length of 10 mm by removing gutta-percha with a heated plugger. A minimum of 4 mm of gutta-percha remained in each tooth. Post spaces were further prepared with Parapost drills (Coltene Whaledent, Konstanz, Germany), ensuring that at least 1 mm of dentine remained on both the mesial and distal sides of the root canals at the apical extent of the post holes. The dimensions of the post holes were verified radiographically.
The teeth were divided into two groups, with approximately equal distribution of root lengths and diameters based on visual inspection. The post hole preparations were then finalized: in group 1, the post holes were flared with Gates Glidden burs to leave a thickness of 1 mm of dentine coronally on the buccal, lingual, mesial and distal sides; in group 2, post holes were prepared so as to maintain 2 mm of dentine on each side of the root face. The final shape of each post was parallel-sided for the apical half with a tapered coronal half to accommodate the thickness of coronal dentine required for each group. Direct patterns for cast posts/cores were formed using Parapost burn out points and GC Pattern Resin (GC Corporation, Tokyo, Japan). The core was cuboid-shaped with dimensions of 2 mm 2 mm and a height of 3 mm to allow placement of the Eggler post remover. The post/ cores were cast in a non-precious alloy and cemented with zinc phosphate cement (SS White, Gloucester, UK) which was allowed to set for 1 week prior to removal of the posts with the Eggler post remover.
Strain gauges (EA-06–125BT-120, Micro-Measurements Group Inc., Raleigh, NC, USA) were trimmed to fit the root surface. Four strain gauges were placed vertically 1.5 mm below the coronal root surface after each tooth had been lightly scraped on the buccal, lingual, mesial and distal surface to facilitate their placement. Pilot studies indicated that this was the most appropriate site for placement of the strain gauges so that some extra dentine could be removed for the second stage of the experiment when the posts were removed at a 10 angle and also to allow clearance between the point of loading and the wires of the strain gauge. The root surface was etched with 37% orthophosphoric acid for 30 s, washed with water for 15 s and dried with air. The strain gauges were cleaned with chloroform, primed with a catalyst and attached to the root surface with cyanoacrylate adhesive (M-Bond 200, Micro-Measurements Group Inc., Raleigh, NC, USA). Excess cyanoacrylate adhesive was removed by applying pressure to the surface of the strain gauges until adhesion occurred, leaving a thin layer of cyanoacrylate that did not interfere with the measurement of strain. The strain gauges, solder contacts and roots were covered with silicone (Dow Corning 3140 RTV Coating, Dow Corning Corp., Midland, MI, USA) to protect the strain gauges from moisture. The strain gauges were then connected to a data acquisition board (AT-MID-16E-2, National Instruments Corporation, Austin, TX, USA) and the data was stored on a computer using process control software (LabVIEW 4.0, National Instruments Corporation, Austin, TX, USA). Four separate channels were used to allow simultaneous measurements from each of the strain gauges and the measurements were made continuously, whilst removing the posts with the Eggler post remover. Throughout the experiment, one tooth was used as the experimental tooth, and a second tooth, which had been prepared in the same manner, served as a compensator for apparent strain that may arise from temperature fluctuations and electrical heating.

Figure 2. Application of the Eggler Post Remover to the core section of a cast post/core with strain gauges attached to the external root surfaces

Application of the Eggler Post Remover to the core section of a cast post/core with strain gauges attached to the external root surfaces

Figure 3. The Eggler Post Remover in use. Note the repeller arms have splayed as they are pushed against the mesial and distal surfaces of the root face which has caused the left repeller arm to almost slide off the tooth.

The Eggler Post Remover in use

In order to remove the post, the forceps of the Eggler device were placed over the buccal and lingual surfaces of the core and tightened with the large inner wheel (Fig. 1). The repeller arms were then lowered by tightening the small thumbscrew at the top of the device. As the repeller arms were lowered, they contacted the mesial and distal tooth structure and the forceps moved away from the tooth as the post was removed (Figs 2,3).
Strain measurements were initially recorded when the cast post/cores were removed from the teeth in a direction that was parallel to the long axis of each tooth and then again when removed at an angle of 10 to the long axis. After the initial removal of the posts, any retained luting agent was cleaned from the posts and from within the post spaces by scraping it off with a probe and an excavator. The teeth were examined under magnification (16 ) to ensure complete removal of cement and for any evidence of fractures or cracks. The posts were recemented after a minimum period of 24 h and the teeth were kept moist in phosphate buffered saline during this time. The height of the mesial dentine of each tooth was reduced so that when the Eggler post remover was applied to the cores, it was at a 10 angle to the long axis of each tooth. The 10 angle was measured with a protractor following application of the Eggler post remover. Then the posts/cores were removed from the teeth again and strain gauge measurements were recorded. The teeth were examined once again under magnification to determine whether there were any fractures or cracks.

Statistical analysis.
Strain measurements were recorded continuously on the buccal, lingual, mesial and distal surfaces of the tooth whilst each post was being removed. The maximum strains recorded when removing the post along the long axis and at a 10 angle to the long axis within the same group were compared using the Wilcoxon Signed Rank test. Comparisons were made between group 1 and group 2 for removal of the post along the long axis and at the 10 angle to the long axis using the Mann–Whitney U -test. All comparisons were made with a 95% level of confidence to determine statistical significance and data were analysed using the SPSS statistical software program ( SPSS for Windows 6.1.31995, SPSS Inc, Chicago, IL, USA).