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Table of Contents   
ORIGINAL RESEARCH  
Year : 2012  |  Volume : 23  |  Issue : 5  |  Page : 613-616
Evaluation of resistance to displacement of metal posts with different lengths


1 Department of Prosthodontics and Periodontology, Piracicaba Dental School, State University of Campinas, Piracicaba, Brazil
2 Department of Restorative Dentistry, Piracicaba Dental School, State University of Campinas, Piracicaba, Brazil
3 Department of Dental Materials, Piracicaba Dental School, State University of Campinas, Piracicaba, Brazil
4 Department of Dental Materials and Prosthodontics, Ribeirão Preto Dental School, University of São Paulo, Ribeirão Preto, SP, Brazil
5 Department of Dental Materials and Prosthodontics, University Cruzeiro of Sul (Unicsul), Passo Fundo, RS, Brazil

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Date of Submission08-Jul-2009
Date of Decision15-Nov-2010
Date of Acceptance28-Aug-2011
Date of Web Publication19-Feb-2013
 

   Abstract 

Aims and Objectives: to evaluate the resistance to displacement of metal intraradicular retainers of different lengths by means of the shear test. Material and Methods: Thirty-six maxillary central incisors were cross-sectioned at 16 mm from the root apex, endodontically treated and randomly divided into three groups that were treated as follows: G1 (control) preparation of 2/3 (10.6 mm) of the total root length; G2: preparation of 1/2 (8 mm) of the total root length and G3: preparation of 1/3 (5.3 mm) of the total root length. After canal preparation, a matrix was made of the coronal and radicular portions using Duralay to obtain metal intraradicular retainers. These retainers were cemented with zinc phosphate cement and subjected to the compression shear test in a Universal Test Machine (EMIC DL 2000) at a crosshead speed of 0.5 mm/minute. The results were subjected to statistical analysis by analysis of variance (ANOVA) and Newman- Keuls, which showed statistically significant difference (P < 0.01). Results: The means in Newtons and their respective standard deviations were: G1 = 972.05 (±81.36); G2 = 921.15 (±112.25); G3 = 686.66 (±113.02). Conclusion: It could be concluded that metal retainers of 2/3 and 1/2 the length of the root portion showed higher resistance to displacement values when compared with the group that had been prepared for 1/3 of the root length.

Keywords: Endodontically treated teeth, metal post, post length, resistance to displacement

How to cite this article:
Farina AP, Cecchin D, Spazzin AO, Pires-de-Souza FP, Dartora NR, Mesquita MF. Evaluation of resistance to displacement of metal posts with different lengths. Indian J Dent Res 2012;23:613-6

How to cite this URL:
Farina AP, Cecchin D, Spazzin AO, Pires-de-Souza FP, Dartora NR, Mesquita MF. Evaluation of resistance to displacement of metal posts with different lengths. Indian J Dent Res [serial online] 2012 [cited 2020 Aug 10];23:613-6. Available from: http://www.ijdr.in/text.asp?2012/23/5/613/107351
Endodontically treated tooth restoration is a challenge to modern dentistry. These teeth are normally more fragile due to structural loss caused by carious lesion, trauma and the pulp devitalization process during the procedure to obtain endodontic access, leading to dentin dehydration and, consequently, loss of elasticity, making the teeth more susceptible to fracture. [1]

In these cases, the use of intraradicular posts is recommended to promote retention of the final restoration [2],[3] since it is not the function of endodontic treatment to recover the physical structure of the tooth. [4],[5],[6] A factor capable of interfering in the retention of these posts is their length [7] which must allow adequate retention as well as maintain the 3-4 mm of gutta-percha and filling cement required for apical sealing. [8],[9]

The post length in relation to root canal is a major concern for many investigators. [10],[11] Several studies have recommend that the post must be at least as long as the clinical crown of the tooth to be restored, and the adequate depth would be 2/3 of the root length. [12],[13] Studies have reported that the increase in post length results in a more uniform distribution of stresses along the root [14],[15] and improves the tooth resistance to fracture. [16] However, Isidor et al.[17] observed that the increase in post length did not necessarily increase the tooth resistance to fracture. Giovani et al.[18] observed that there was no significant difference among the metal posts of different lengths. Santos-Filho et al., [19] comparing the fracture resistance of metal posts and glass-fiber posts, observed that longer cast metal cores presented greater fracture resistance than shorter metal posts, and glass fiber posts of different lengths showed a similar behavior.

Thus, the aim of this study was to evaluate the in vitro resistance to displacement of metal intraradicular retainers of different lengths using the shear test, to test the hypothesis that in endodontically treated teeth, longer posts show better resistance to displacement.


   Materials and Methods Top


Tooth selection and preparation

Thirty-six maxillary central incisors, obtained from the Tooth Bank of the Dentistry School - UPF, Brazil, were used in the study. The teeth were free of any type of cervical lesions and root defects, and were without previous endodontic treatment and kept in a 0.02% thymol solution [20] at a temperature of 4 ° C for 48 hours for disinfection. After this, they were cleaned with pumice and water slurry using Robinson brushes (Microdont, Socorro, Brazil). Then, the teeth were stored in distilled water at a temperature of 4 ° C for 1 week. Their crowns were removed by a cross-sectional cut through their cervical portion, using a double-faced diamond disk (911H, Brasseler Lemgo, Germany) coupled to a handpiece (Kavo, Joinvile, Brazil), in order to obtain a length of 16 mm measured from the root apex for each tested tooth.

Endodontic treatment

After cutting, the root canals were irrigated with physiological solution and when present, pulp tissue was removed with a #25 Hedströen file (Maillefer, Ballaigues, Switzerland). Working length was determined by inserting a K-file (Maillefer) into the canal until its tip was seen in the apical foramen, and then withdrawing it 1 mm from this measurement. Chemical-mechanical preparation of the root canal was performed with rotary instruments K3 (SybronEndo, Glendora, CA, USA) by the Crown-down technique, using four instruments, after determining the anatomic diameter. The irrigant solutions used were 2.5% sodium hypochlorite (FarmáciaNatupharma, Passo Fundo, Brazil), followed by final irrigation with 17% ethylenediaminetetraacetic acid (EDTA; Biodinâmica, Ibiporã, Brazil). [21]

Next, the root canals were dried with absorbent paper points (Tamari, Tamarimam Industrial Ltd., Macaçaruru, Brazil) with a diameter compatible with that of the preparation. Filling was performed with main and accessory gutta-percha cones (Dentsply-Maillefer, Petrópolis, Brazil) and Grossman cement (Endo fill, Dentsply-Maillefer) by the technique of gutta-percha lateral condensation.

After root canal filling, any excess gutta-percha and cement were removed and the coronal portion was sealed with Cavit W (Premier Dental Produtos, Rio de Janeiro, Brazil). Finally, teeth were put into an oven at 37 ° C and 40% humidity for a period of 48 hours. [22]

Test specimen preparation

After the endodontic cement setting time, the roots were removed from the oven and individually embedded in clear self-polymerizing resin (Jet Clássico, São Paulo, Brazil), in PVC cylinders (Tigre, São Paulo, Brazil), 25 mm in diameter and 20 mm high, so that the cervical surface faced the external and top surface of the cylinder.

The temporary cement was removed from the canal entrances with a 1013 round bur (KG Sorensen, Barueri, Brazil) at low speed, and canals were prepared with Largo drills (Dentsply-Maillefer, Ballaigus, Switzerland) in the following proportions of length: G1 (control) 2/3 (10.6 mm); G2: 1/2 (8.00 mm); G3: 1/3 (5.3 mm). After this, Duralay resin (Reliance Dental, Worth, IL, USA) was used to obtain molds of the intraradicular retainers by the direct technique, and prefabricated polycarbonate posts (Pin-jet, Angelus, Londrina, Brazil) were used. The coronal portion was made 5 mm high for all test specimens. Specimens were prepared to receive complete crowns with a reduction of 1.5 mm and ferrule of 2.0 mm. After casting, the intraradicular retainers were cemented in their respective roots with zinc phosphate cement (SS White, Rio de Janeiro, Brazil). On the core of each specimen, a Duralay acrylic pattern was made and cast in metal. This crown was replicated with molding material and poured in acrylic resin to make the other patterns. On the palatine face of the patterns, rectangular-shaped stops with a central concavity were made to locate and stabilize the metal tip during the fracture resistance test. Thus, standardized crowns were obtained for all teeth. All crowns were cemented with zinc phosphate cement in a ratio of 2.0 g of zinc phosphate powder to 0.5 ml of liquid. The crowns were filled with the cement, placed on the preparations, and kept under constant finger pressure for 60 seconds. After 10 minutes, the excess cement was removed with a dental explorer. The specimens were then stored in 100% relative humidity at a constant temperature of 37°C for a period of 72 hours.

Shear test

The test specimens were then subjected to displacement by means of a shear test in a Universal Test Machine (EMIC DL 2000, São José dos Pinhais, Brazil) using a load cell of 2000 N at a crosshead speed of 0.5 mm/minute until the cast metal intraradicular retainers were displaced. A device was used to standardize the position of the specimens on the base of the apparatus so that the load could be applied at an angle of 135° in relation to the long axis of the roots. [22],[23] An increasing oblique compressive load was applied on the cingulum of the palatal surface (3.0 mm from the incisor region) using a cylindrical-shaped device with a round tip (2.7 mm in diameter) [Figure 1]. Fracture analysis was performed using a Binocular magnifying lens Carl Zeiss (Jena, Germany), and fractures were classified as (1) displacement of the entire prosthetic set at the post/cement interface; (2) fracture of the prosthetic set and (3) root fracture. Data were collected and subjected to analysis of variance (ANOVA) and complementary Newman-Keuls test.
Figure 1: Diagram of the fracture resistance test. (a) Metal crown; (b) acrylic resin block; (c) circular metal matrix and (d) metal device

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   Results Top


[Table 1] shows mean values in Newtons (N) and standard deviations for the studied groups.
Table 1: Means and standard deviation of tested groups

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The highest stress values were those in G1 followed by G2 (P < 0.01) which did not differ statistically between them, and were statistically higher than those of G3 (P < 0.01). In the fracture analysis, it was observed that all test specimens showed displacement of the entire prosthetic set at the post/cement interface, without fracture of the root or prosthetic set.


   Discussion Top


In present study, better results were obtained with posts of 2/3 and 1/2 the length of the root canal, suggesting that it is sufficient to prepare half of the root canal for post cementation, partially rejecting the hypothesis under study. However, posts with 1/3 of length showed the worst results among the tested groups, which indicates that very short posts showed little retention and may compromise a prosthetic rehabilitation. Similar results have been obtained in earlier studies. [15],[24],[25]

In this study, posts with 2/3 and 1/2 of the root length showed statistically similar resistance to displacement. This indicates that it is unnecessary to use longer posts. Some studies [17],[18],[26],[27],[28] corroborate the data of the present study. This also suggests that excessively long posts can diminish the resistance of the roots because of excessive wear during root canal preparation to receive the post. [23],[29] These results allow metal posts of intermediate length to be indicated for use to provide prosthetically restored roots with adequate fracture resistance for situations in which the length of 2/3 cannot be reached, for example, in situations involving curved roots. However, findings of the studies of Leary et al.[14] and Holmes et al.[15] contradict the results obtained. According to these authors, the increase in post length results in better stress distribution along the remaining dental structure.

However, Leary [14] showed that preparation for an intraradicular post considerably weakens the remaining root structure. In other words, when using a post, a large amount of dentinal structure needs to be removed during root preparation to receive the post, which may weaken the tooth rather than reinforcing it. This may suggest why 2/3 and 1/2 of the root length showed statistically similar resistance to displacement in the present study. While the length of the root canal preparation is important for retention, it is also necessary to respect the principle of biomechanical resistance directly related to the quantity of remaining dental structure by preserving it to the maximum extent. Thus, canal preparation must be both sufficient to retain the core and as minimal as possible to avoid weakening the tooth. [16]

According to endodontic principles, the quantity of endodontic filling material in the root apex must be between 3 and 4 mm in an extent to maintain the integrity of apical sealing, thus preventing leakage and contamination of the root canal system. [8],[9] The results of this study demonstrate that preparation of half of the root canal length is sufficient to retain the post as well as to preserve dental structure and maintain an adequate quantity of endodontic filling material in the apical region of the root to prevent leakage.

In this study, a metal post was used because this type has been used for intraradicular retention for a long time, [30] and zinc phosphate cement was used because it has traditionally been used for post and crown cementation [31],[32] although it does not bond to the root canal walls and is soluble in the oral medium. [33] These two characteristics may have contributed to post displacement without the occurrence of root fracture. If these posts were cemented with resinous cement, there could be an increase in the resistance to displacement of these posts and, consequently, a greater possibility of root fracture as well. Another factor to be noted is the diameter of the root portion of the post. When the post volume is large, the possibility of root fracture increases. [34] Thus, it is important to use burs of a diameter compatible with that of the root canal during canal preparation to receive the post, in order to prevent weakening of the root canal walls. In the present study, maxillary central incisors were used with thick root walls and little wear was performed in their walls, which may have contributed to the nonappearance of root fractures.

Due to the variability of results found in literature with regard to the resistance to displacement values of posts with different lengths, it is important for further investigation to be conducted to seek a consensus about the ideal post length to promote adequate retention without affecting the strength and integrity of the dental structure.


   Conclusion Top


Based on the methodology used and results obtained, it was concluded that the increase in post length did not significantly increase its resistance to displacement. Posts with 1/3 of the root canal length did not show resistance to displacement similar to that of the other groups, whereas the retention of posts with half of the length of the root canal was shown to be adequate.

 
   References Top

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33.Anusavise KJ. Phillips, Dental Materials. 10 th ed. Rio de Janeiro (BR): Guanabara Koogan; 1998.  Back to cited text no. 33
    
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Correspondence Address:
Doglas Cecchin
Department of Restorative Dentistry, Piracicaba Dental School, State University of Campinas, Piracicaba
Brazil
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9290.107351

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