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Azerbaycan Saytlari

 »  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
Discussion - References.



Discussion.
The greatest strains measured were on the mesial and distal root surfaces where the repeller arms of the Eggler post remover contacted the root. There was considerable variation in strain from a tensile value of 214 strain to a compressive strain of 5406 strain. The strains measured at the root surface were below the slightly greater than 1% strain required to exceed the compressive elastic limit where fracture may occur (Waters 1980).
Posts were cemented and then removed twice for each tooth – first along the long axis of the root (0 ) and then at a 10 angle to the long axis. Post removal was performed in this order because there were concerns that root fractures may occur when the posts were removed at 10 to the long axis of the tooth and this was confirmed with four teeth during the second stage of the experiment. The teeth were examined under a microscope ( 16) for evidence of any fractures or cracks prior to recementing the post, but none were seen. However, it is recognized that multiple cementation and removal of each post, and the order in which the posts were removed, could possibly have had deleterious effects on the tooth roots but this could not be determined in the current study. The same teeth were used at 0 and then at 10 in an attempt to provide a more accurate comparison of the strains produced by loading the roots at the two different angles.
Four teeth (20% of cases) suffered damage when the Eggler post puller was not used correctly – that is, at an angle of 10 to the long axis of the tooth root. Only one of these was a root fracture that clinically would have rendered the tooth unrestorable and necessitated extraction. The other three teeth had small (1–2 mm) sliver fractures of dentine where the repeller arms of the Eggler post remover contacted the teeth. These findings emphasize the need to be careful to ensure that the Eggler post remover is applied parallel to the long axis of the tooth before applying any force.
There were several factors that may have contributed to these fractures. First, there was an uneven application of load by the repeller arms during post removal, as demonstrated by the strain measurements in the teeth that fractured where there was a difference between the distal and mesial surfaces with compressive strain values ranging from 490 up to 3481. Secondly, it was observed that the distance between the two repeller arms increased as the load applied to the root surface was increased (Fig. 3). This action introduces lateral shear forces on the mesial and distal root surfaces, which may increase the risk of having small fractures as was seen at the site of application of the repeller arms. Fractures may occur since dentine is weaker under shear or tensile loading than under compression (Waters 1980). In the region where the repeller arms contacted the cut root faces, it is possible that point, tensile or shear stresses may have initiated cracks or exceeded the ultimate strength of dentine to cause these fractures.
One tooth had an oblique fracture on the buccal root surface, which is typical of most vertical root fractures. The reason for this has not been established, although it could be related to the stress patterns that developed within the tooth. The tooth that fractured exhibited tensile stresses on the buccal surface and the fracture line progressed in a distal direction as it moved apically, which was opposite to the direction in which the post was being pulled out of the canal. The fracture also may have been related to the amount of remaining dentine thickness, as shown by Tjan & Whang (1985), who measured the forces required to fracture roots when removing cast posts/cores with different thickness of buccal dentine. They reported that there was no difference in the load required for fracture, but the type of failure that occurred varied according to the thickness of remaining dentine. Tjan & Whang (1985) concluded that roots with less than 1 mm of dentine remaining were more prone to fracture than those with 2 mm or more of buccal dentine. In the current study, the tooth that had an oblique fracture had only 1 mm of dentine remaining coronally.
The Eggler post remover is designed so that the gripping forceps engage the core portion of the post/core in order to pull the post out of the root canal. The repeller arms partially surround the gripping forceps and push against the root face to provide the load to remove the post. The distance between the repeller arms is 3.2 mm and hence the Eggler device can not be used on teeth with a mesio-distal distance between the adjacent teeth that is less than this. In addition, the mesio-distal dimension of the tooth root will determine how the repeller arms contact the surface of the root. In narrow teeth the repeller arms may only contact the outer portion of the cut root face, whilst in wider teeth the entire cross sectional area of the repeller arm could be in contact with the cut root face. In narrow teeth, the surface area through which the load is applied will be decreased and most of the load will be applied through the outer dentine of the root face (Fig. 4), which may increase the risk of root fracture occurring.
When the posts were removed along the long axis of the teeth, the mesial and distal strains were equal in the majority of cases, but in seven teeth there was a large discrepancy. This may have been due to incorrect alignment of the post remover along the long axis of the tooth in these specimens. Since the alignment of the post remover is determined by the positioning of the forceps on the core and if it is placed at an angle to the long axis of the tooth, then one of the repeller arms will contact the tooth before the other, which results in a greater load being applied through that arm. Another possibility is that some excess localized strain may develop under one of the repeller arms. When the posts were removed at a 10 angle to the long axis of the tooth, the mesial strain was higher than the distal strain in seven teeth in group 1 and eight teeth in group 2, indicating that there was uneven loading through the repeller arms.
Whilst using the Eggler device in this study, it was noted that there is a discrepancy between the width of the cross bar that connects the two repeller arms (2.7 mm) and the dimensions of the gripping forceps (3.2 mm) that the repeller arms partially surround (Fig. 2). Hence, as the repeller arms are lowered to contact the tooth, they splay out and will contact the tooth at divergent angles. This introduces a lateral component to the load applied to the tooth which may have contributed to the occurrence of the dentinal ‘sliver’ fractures seen in this study. This problem is more likely to occur in narrow teeth where only the outer dentine is contacted by the repeller arms. The splaying of the Eggler’s repeller arms during use could be rectified with some simple modifications to the design of the device. Tensile forces were found on the buccal or lingual surfaces of seven teeth in group 1 and two teeth in group 2 when the posts were removed parallel to the long axis of the tooth. The vertical tensile strain on the buccal or lingual surfaces was most likely associated with principal compressional strains in the horizontal direction. Horizontal compressional strains on the buccal and lingual surfaces are generated when the mesial and distal surfaces are pushed out. This may have occurred when the repeller arms of the Eggler post puller splayed apart during use.

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