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ORIGINAL ARTICLE  
Year : 2014  |  Volume : 17  |  Issue : 2  |  Page : 183-187
The effect of ferrule presence and type of dowel on fracture resistance of endodontically treated teeth restored with metal-ceramic crowns


1 Department of Conservative Dentistry & Endodontics, Faculty of Dentistry, Jamia Millia Islamia, New Delhi, India
2 Department of Conservative Dentistry & Endodontics, SGT Dental College, Gurgaon, India
3 Department of Prosthodontics, SGT Dental College, Gurgaon, India
4 Department of Prosthodontics, Faculty of Dentistry, Jamia Millia Islamia, New Delhi, India

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Date of Submission24-Sep-2013
Date of Decision23-Dec-2013
Date of Acceptance28-Dec-2013
Date of Web Publication1-Mar-2014
 

   Abstract 

Aim: The purpose of the present study was to comparatively evaluate the effect of presence of a 2 mm ferrule and different type of dowels on fracture resistance of mandibular premolars.
Materials and Methods: Fifty uniradicular mandibular premolars were divided into five groups (n = 10). Ten teeth received no treatment (group I). Samples in group II & III were decoronated 2 mm above cemento-enamel junction and received custom cast dowel-core and fiber dowel-composite core respectively, with 2 mm ferrule. Samples in group IV & V were decoronated at CEJ and were restored using cast dowels and fiber dowel-composite cores, without any ferrule. The restored teeth received metal ceramic crowns and were mechanically loaded. The specimens were subjected to a static load, until fracture, to determine the fracture resistance and fracture mode.
Results: The samples with 2 mm ferrule had a higher fracture resistance than non ferrule groups. Within non ferrule groups, there were no significant differences in the fracture resistance. Specimen restored with cast dowel had more incidence of non-repairable fracture.
Conclusions: Presence of ferrule increased the fracture resistance of endodontically treated teeth. In case of absence of ferrule, fiber dowels had similar fracture resistance as that of cast dowels and showed increased incidence of repairable fracture.

Keywords: Cast dowel; Fiber dowels; Fracture resistance; Fracture mode

How to cite this article:
Aggarwal V, Singla M, Yadav S, Yadav H, Sharma V, Bhasin SS. The effect of ferrule presence and type of dowel on fracture resistance of endodontically treated teeth restored with metal-ceramic crowns. J Conserv Dent 2014;17:183-7

How to cite this URL:
Aggarwal V, Singla M, Yadav S, Yadav H, Sharma V, Bhasin SS. The effect of ferrule presence and type of dowel on fracture resistance of endodontically treated teeth restored with metal-ceramic crowns. J Conserv Dent [serial online] 2014 [cited 2019 Sep 19];17:183-7. Available from: http://www.jcd.org.in/text.asp?2014/17/2/183/128053

   Introduction Top


Restoration of endodontically treated teeth may pose a restorative challenge because of damage by caries and endodontic procedures. [1],[2] To provide sufficient retention to the core, a radicular support is provided in the form of a dowel. [3],[4] Some authors believed that placement of dowel may reinforce the tooth, [1],[2],[3],[4],[5],[6] but unnecessary removal of radicular dentin during dowel space preparation may negate any reinforcement gains from dowel placement. [7],[8] Traditionally custom metallic dowels were used which involves a two stage fabrication and placement procedure. Subsequently prefabricated posts were used. These prefabricated posts were made up of metal, ceramics or glass fiber.

The teeth restored with cast posts usually show higher fracture resistance than fiber post but fail in an unrepairable manner. [9],[10] This difference in failure mode is attributed to the difference in the modulus of elasticity of cast and fiber dowels. [3] Because the fiber dowels have modulus of elasticity similar to dentin, there is an even distribution and transfer of stress to the radicular dentin. [4] Various authors have documented the usefulness of placing a '360 degree of metal collar of crown surrounding the dentin', also known as ferrule, to increase the resistance form and prevent fracture of the restored tooth. [11]

There are conflicting reports regarding the effect of ferrule on fracture resistance. One study concluded that 1 mm of ferrule can significantly improve the fracture resistance while some studies recommended 2 mm of ferrule height. [12],[13],[14] Schmitter et al[15] reported that teeth with 2 mm had fractures on remote areas which pose a problem during restoration. Some studies comparing uniform and non-uniform ferrule reported a higher fracture resistance in uniform ferrule group, [16] yet one study [17] evaluating fiber posts reported no change in fracture resistance in uniform and non-uniform ferrule. Some authors have reported that ferrule placement does not improve the fracture resistance of teeth restored with cast dowels and prefabricated metal dowels. [18] However majority of the studies have utilized single static load to fracture the specimen, which is different from the actual clinical scenario. The present study evaluated the fracture resistance of teeth restored with cast and fiber dowels, with or without a 2 mm ferrule preparation, under cyclic loading. The fracture mode was classified as repairable or non-repairable. The null hypothesis was that the ferrule placement and/or type of post have no effect of fracture resistance.


   Material and Methods Top


Fifty extracted human permanent mandibular premolars were used in the study. The selected teeth had approximately similar dimensions with a mesio-distal width of 5.0-5.5mm and bucco-lingual width of 7-8 mm. Teeth with caries or restoration on the cervical third, large root canals or cracks/fissures were excluded from the study. The external debris was cleaned with a hand scaler and the samples were used within one month of their extraction. Ten teeth were kept as control and received no treatment (Group I, control group). The anatomic crowns were sectioned perpendicular to the long axis of the tooth, up to 2 mm from facial cemento-enamel junction (CEJ) in the samples with ferrule (n = 20, FR groups), and up to Facial CEJ in samples without ferrule (n = 20, NFR groups) [Figure 1]. The roots were embedded in acrylic moulds with epoxy resin liner to simulate periodontal ligament, in a manner that 2 mm of tooth height apical to facial CEJ was exposed. The root canals were then manually prepared till apical size #60 using a balanced force technique. The canals were copiously irrigated between change of each file, with sodium hypochlorite (5.25%) and EDTA solution (17%) with a 27-gauge needle and a final rinse with 10ml of distilled water, followed by a thermo-plasticized gutta-percha filling. The canal entrances were sealed with a non eugenol temporary filling material (Cavit-G 3M ESPE). The specimens were stored at 100% humidity at 37°C for 7 days. The samples were divided into four groups (n = 10), according to the ferrule and dowel method used. Group II / CD-NFR (cast dowel no ferrule): Standardized dowel spaces were prepared to a depth of 9 mm using Gates-Glidden drills followed by a commercially available dowel drill (ParaPost Fiber Lux, Coltene Whaledent). The impression of the dowel space was made with a plastic burn-out casting dowel, adapted to the canal with Duralay acrylic resin (Reliance Dental Manufacturing Company, Chicago, IL, USA). The core was build up with Duralay acrylic resin with a height of 5 mm, corresponding to the core formers of fiber dowel system. A circumferential shoulder of 1 mm was kept. The resin patterns were cast in Ni-Cr alloy (Heraus Kulzer, Hanau, Germany), sandblasted and luted in the canals with zinc phosphate cement (SS White, Rio de Janeiro, RJ, Brazil).
Figure 1: Schematic representation of samples with and without ferrule. 1- full coverage crown; 2- dowel; 3- remaining apical obturating material

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Group III / CD-FR (cast dowel 2 mm ferrule): A 2 mm high circular ferrule was prepared with a high speed flat end tapered diamond cutting instruments (Brassler, USA), with circumferential shoulder of 1 mm width. Dowel spaces were prepared to a depth of 11 mm. Cast dowel and cores were fabricated in a manner similar to group I.

Group IV / FD-NFR (fiber dowel no ferrule): Standardized dowel spaces were prepared to a depth of 9 mm. The prepared dowel spaces were dried with #60 absorbent paper points. Commercially available dual cure resin cement (ParaCore automix dual cure, Coltene Whaledent) was used to cement the dowels. ParaBond Non-Rinse Conditioner was applied to the canals with the help of #60 absorbent paper points. Mixture of Adhesive A and Adhesive B was applied and excess was removed with brush. The pastes of ParaCore automix white were mixed with the help of mixing tip and were applied into the canal with Lentulo-spiral no. 40 (Dentsply Maillefer, Ballaigues, Switzerland). Small amount of the mixture of the cement was applied to the ParaPost and the dowel was gently seated into the dowel space with the help of finger pressure. The samples were cured for 40 seconds. Core build up was done with manufacturers' supplied core formers and the core height was trimmed to 5 mm with circumferential shoulder of 1 mm width. The samples were stored at 100% humidity at 37°C for 7 days.

Group V / FD-FR (fiber dowel 2 mm ferrule): As in group II, a circumferential ferrule of 2 mm height was prepared with 1 mm shoulder width. Fiber dowels were luted and core build up was done as in group III.

Coronal impressions were made using a polyvinylsiloxane impression material and metal ceramic crowns were fabricated using nickel-chromium alloy and a veneering of ceramic. The crowns were cemented with glass-ionomer cement (GC Fuji I, Tokyo, Japan). The specimens were subjected to cyclic loading of 150,000 cycles at 60N (simulating 6 months of oral masticatory stresses) at a frequency of 50Hz. The fracture resistance was measured with Universal Instron testing machine (Zwick GmbH and Co., postf. 4350, D-7900u/m, Germany). A compressive load was applied on the lingual ridge of buccal cusp. The acrylic block was placed at an angulation to provide a 45° angle between the long axis of tooth and 5 mm wide spherical loading tip. The samples were stressed to failure at a crosshead speed of 0.5mm/minute until a first major load drop. The load was recorded in newtons. The specimens were analyzed to ascertain the mode of failure and were divided into two groups: Repairable (fractures above cemento-enamel junction (CEJ), horizontal cervical fracture, core-tooth fracture); and non-repairable (oblique fracture entirely below CEJ, fracture in the middle or apical third of the root, vertical root fracture) [Figure 2]. The data was recorded and was statistically analyzed using one way analysis of variance for the fracture resistance and Fisher's exact test for the mode of fracture. Pairwise multiple comparison procedures for the fracture resistance of different groups were carried out using Holm-Sidak method using the program BioEstat (version 4.0; Mamiraua Institute, Belem, Brazil).
Figure 2: Classification of fracture modes; repairable fracture includes fractures above CEJ or core-tooth fracture, non-repairable fracture includes fractures below CEJ, apical third fractures or vertical root fractures

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   Results Top


The fracture resistance and fracture mode of all the groups is presented [Table 1]. The control group had the highest fracture resistance (883 ± 47 N) which was significantly higher than all the groups (one way ANOVA, [Table 2]). The next higher fracture resistance was demonstrated by ferrule groups (CD-FR and FD-FR) with fracture resistance of CD-FR (816 ± 61 N) greater than FD-FR (708 ± 41 N). There was no significant difference between the CD-NFR (629 ± 50 N) and FD-NFR (596 ± 52 N) groups. The FD-NFR group showed repairable fracture in 90% of samples [Table 2], which was significantly higher that all other groups.
Table 1: Mode of failure in different experimental groups

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Table 2: One way ANOVA (P < 0.001)


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   Discussion Top


The endodontically treated are usually more prone to fracture because of loss of the structural integrity. [1],[2],[3],[4],[5],[6],[7],[8],[9],[10] Other factors that could affect the fracture resistance of root canal treated teeth are stress during obtuaration; post space preparation, inappropriate use of the tooth as abutment, [19] age of the patient [20] and reduced proprioception. [21] The restoration of a root canal treated tooth should prevent the tooth from fracture under masticatory stresses. In cases of extensive coronal damage, dowel placement will provide support to the core. [3]

The null hypothesis that ferrule placement and/or type of post have no effect of fracture resistance was rejected on the basis of data obtained. However, the fracture resistance difference between the non ferrule groups was not statistically significant. The decision to place a dowel depends upon the extent of structural loss and amount of functional forces. [3] It has been well reported in the literature that fractures resistance of endodontically treated teeth with loss of one or more walls, is improved after placement of dowel, core and a full coverage restoration. [1],[2],[3],[4],[5] However, there several studies reporting that placement of dowel (irrespective of its type) does not positively affect the fracture resistance or fracture mode, and even considered dowel placement as a potential risk towards root fracture. [18] The higher incidence of 'catastrophic' root fracture was reported with the use of metallic dowels. [9],[10]

In the present study, the teeth restored with cast dowel and 2 mm ferrule demonstrated highest fracture resistance among all the restored groups. However, it gave maximum incidence of non-repairable root fracture. The metallic posts have higher modulus of elasticity (MOE) than radicular dentin and cause stress concentrations at the weaker sections of root. [3] The fiber dowel with 2 mm ferrule had significantly less incidence of non-repairable fractures but had less fracture resistance than cast dowel group with ferrule. The fiber dowels have a low MOE and dissipate stress evenly along the root surface. [4] Moreover, the low MOE acts as a protective mechanism fracturing the fiber dowel before tooth fracture. This was evident in the teeth restored with fiber dowels in the present study where majority of samples of FD-NFR group fractured at core tooth junction.

Placement of ferrule enhanced the fracture resistance of restored teeth, both in cast and fiber dowel groups. Ferrule effect is provided by the encircling of parallel walls of dentin crown, thus decreasing the splitting stresses within the tooth. [11] It has been shown that teeth with ferrule have high fracture resistance and could bear more load cycles than teeth restored with ferrule. [22] Akkayan [14] reported that 2 and 3 mm ferrule significantly improved the fracture resistance as compared with teeth with less/no ferrule. Some authors have even reported that if ferrule is provided, the placement of dowel does not enhance the fracture resistance and the ferrule is the only 'key element'. [23],[24],[25],[26],[27] Regarding the effect of ferrule on fracture mode, it has been reported that teeth with 2 mm ferrule failed in a 'catastrophic' manner. [15] If the ferrule height is 1 mm or less, the fracture mode was generally decementation or fracture at core-tooth junction. In the present study, the non ferrule cast dowel group had a majority of non-repairable fracture while majority of the teeth restored with fiber dowels fractured at the core-tooth junction.

During clinical function, the teeth are unavoidably subjected to dynamic masticatory stresses. These sub-critical stresses are not constant in nature and provide a fatigue loading. To simulate these stresses 150000 cycles of cyclic loading was applied. After cyclic loading, the teeth were subjected to a static load until fracture to determine the fracture resistance. The functional loads play an important role in stress generation in endodontically treated teeth restored with dowels. It has been reported that the effect of cyclic load was more significant than type of post design in generation of stress in the tooth. [27] Xible et al[28] reported that there was difference between the survival rate and fracture resistance of teeth restored with a 1.5 mm ferrule and different prefabricated posts after mechanical loading and an oblique, static load. A clinical trial with 2 years of follow up reported that there was no difference in the clinical outcome of teeth restored with titanium or glass FRC posts with 2 mm ferrule. [29]

There are several limitations to the present study. Only a single type of force (60N) was applied during cyclic loading, which does not exactly replicate the type of dynamic masticatory forces on the tooth during function. Also the thermal cycling was not applied which has been shown to affect the luting agent and affect the outcome. There are certain avenues for further research. These include thermo-mechanical loading of samples with variable mechanical loading, using the same research protocol. The effect of different ferrule heights, ferrule configuration and different post methods could be evaluated in a clinical research.


   Conclusion Top


Under the limitations of this in vitro study, it can be concluded that presence of ferrule increased the fracture resistance of endodontically treated teeth restored with cast metal or fiber dowel. The cast dowel with 2 mm ferrule had a high fracture resistance but led to non-repairable fracture. In specimen without ferrule, fiber post had a high incidence of repairable fracture.

 
   References Top

1.Baraban DJ. The restoration of endodontically treated teeth: An update. J Prosthet Dent 1988;59:553-8.  Back to cited text no. 1
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29.Naumann M, Sterzenbac G, Alexandra F, Dietrich T. Randomized controlled clinical pilot trial of titanium vs. glass fiber prefabricated posts: Preliminary results after up to 3 years. Int J Prosthodont 2007;20:499-503.  Back to cited text no. 29
    

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Correspondence Address:
Vivek Aggarwal
Department of Conservative Dentistry & Endodontics, Faculty of Dentistry, Jamia Millia Islamia, New Delhi - 110 024
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0707.128053

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