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Table of Contents   
ORIGINAL ARTICLE  
Year : 2015  |  Volume : 18  |  Issue : 3  |  Page : 187-191
The effect of incomplete crown ferrules on fracture resistance and failure modes of endodontically treated maxillary incisors restored with quartz fiber post, composite core, and crowns


1 Section of Dental Public Health, Kanchanadit Hospital, Kanchanadit, Suratthani, Thailand
2 Department of Conservative Dentistry, Faculty of Dentistry, Prince of Songkla University, Hat Yai, Songkhla, Thailand
3 Department of Conservative Dentistry, Dental Materials Research Unit, Faculty of Dentistry, Prince of Songkla University, Hat Yai, Songkhla, Thailand

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Date of Submission26-Sep-2014
Date of Decision17-Feb-2015
Date of Acceptance29-Mar-2015
Date of Web Publication19-May-2015
 

   Abstract 

Aim: To investigate the fracture resistance of restored endodontically treated teeth (RETT) with fiber posts, cores, and crowns with limited ferrules.
Materials and Methods: Sixty maxillary anterior teeth were endodontically treated and decoronated 2 mm above the cemento-enamel junction, and then divided into 6 groups of 10 teeth each; Group circumferential ferrule (2FR), Group ferrule in the labial, mesial, and palatal region (2FR-LaMPa), Group ferrule in the labial, and palatal region (2FR-LaPa), Group 2FR-Pa and 2FR-La respectively, and Group 0FR (no ferrule). All 60 prepared teeth were then restored with quartz fiber posts, resin composite cores, and metal crowns. The specimens were subjected to load until failure occurred. Data were analyzed using one-way analysis of variance and Tukey's tests (α = 0.05). The mode of failure was determined under a stereoscope.
Results: A statistical significant difference was found among groups 2FR-LaMPa, 2FR-Pa, 2FR-LaPa, and 2FR from the group 2FR-La, and from the group 0FR (P < 0.01). The predominant mode of failure was an oblique palatal to labial root fracture for the groups with remaining ferrules.
Conclusion: For RETT that have incomplete crown ferrules, the location of the ferrules may affect their fracture resistance.

Keywords: Ferrule design; fracture resistance; post-core restorations; quartz fiber post; restoration of endodontically treated teeth

How to cite this article:
Muangamphan P, Sattapan B, Kukiattrakoon B, Thammasitboon K. The effect of incomplete crown ferrules on fracture resistance and failure modes of endodontically treated maxillary incisors restored with quartz fiber post, composite core, and crowns. J Conserv Dent 2015;18:187-91

How to cite this URL:
Muangamphan P, Sattapan B, Kukiattrakoon B, Thammasitboon K. The effect of incomplete crown ferrules on fracture resistance and failure modes of endodontically treated maxillary incisors restored with quartz fiber post, composite core, and crowns. J Conserv Dent [serial online] 2015 [cited 2019 Nov 12];18:187-91. Available from: http://www.jcd.org.in/text.asp?2015/18/3/187/157239

   Introduction Top


Restoration of endodontically treated teeth is a challenging issue when preserving teeth to have normal anatomy, good esthetics, and appropriate function. When a tooth has more than 50% of its coronal structure missing, the use of a post-and-core foundation is recommended prior to prosthetic restoration. [1] Prefabricated posts have an advantage in that the post space can be prepared, and the post directly bonded in a single appointment and could be repairable in certain cases when failure occurred. [2]

A ferrule, a metal band or ring that encircles the 1.5-2.0 mm height of residual tooth structure, [3],[4] has been suggested for postendodontic reconstruction of severely damaged teeth. [5],[6],[7] The majority of studies support the effectiveness of using a ferrule. [8],[9],[10],[11],[12],[13],[14],[15],[16],[17] However, a circumferential ferrule (2FR) may be difficult to achieve due to the variability of teeth damage. Therefore, they may necessitate the incorporation of different types of ferrule designs. A study by Ng et al. [1] suggested that the 360 degrees circumference around the axial wall may not be the determining feature contributing to fracture resistance. Rather, the important point may be the presence of the axial wall in a location where the opposing tooth contact generates occlusal loads. Dikbas et al. [18] also concluded that different ferrule designs did not have any influence on the fracture resistance of teeth with fiber posts. There is no significant change in the resistance of teeth with fiber posts regardless of which ferrule design is incorporated. However, a study by Dikbas et al. [18] has not included the proximal (marginal ridge) ferrule design which is the important factor of tooth strength. [19] From these points of incomplete ferrules, a question arose as to whether complete ferrules were required to be appropriate for successful restorations. Therefore, the aim of this in vitro study was to evaluate fracture resistance and failure modes of endodontically treated maxillary incisors restored with quartz fiber posts, composite cores, and crown restorations.


   Materials and Methods Top


This experiment was approved by an expedited review procedure through the Institutional Review Board Human Studies Committee for a specimen study involving previously extracted human maxillary anterior teeth.

Specimen preparations


Sixty extracted maxillary anterior teeth were obtained from private clinics and from the Oral Surgery Clinic, Department of Oral and Maxillofacial Surgery. Teeth were then examined under × 20 (Nikon SMZ model 1500, Nikon Instech, Tokyo, Japan) to ensure that they were free of caries, restorations, or cracks. Radiographs of each tooth were also made to ensure that there were no cracks, internal root resorption, or obstructions within the canal. Mesiodistal and buccolingual dimensions were recorded at the cervical margin with a digital caliper (Mitutoyo, Tokyo, Japan) to obtain a similar size distribution [Table 1]. The teeth were divided into 6 groups of 10 teeth each. Subsequently the selected teeth were stored in a 0.1% Tymol solution at room temperature.
Table 1: Mean (SD) values of root size, root length, and results of fracture load testing

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The selected teeth were subjected to endodontic treatment. Working length was set at 1 mm short of the apical foramen. Canal preparation was completed using a crown-down technique with rotary nickel-titanium instrumentation (Protaper, Dentsply Maillefer, Ballaigues, Switzerland). Teeth were instrumented to ISO size 40 apically, and canals were dried with compressed air and paper points. A sealer (AH Plus, Dentsply Maillefer) was introduced into the canal using paper points. Teeth were obturated with 0.04 taper ISO No. 40 gutta percha cones using lateral compaction technique. The endodontic access cavities were filled with a temporary filling material. The specimens were then stored in an incubator at 37°C for 1-week to ensure complete polymerization.

The clinical crowns were cut 2 mm coronal to the most incisal point of the proximal cemento-enamel junction. Tooth reduction for crown preparation was performed step-by-step, starting with a 2FR of 2 mm (group 2FR) for every tooth. The crown margin was designed to follow the simulated contours of the free gingival tissue. The margins were 1 mm wide with a rounded shoulder configuration. A diamond rotary cutting instrument (ISO #016, Shofu, Tokyo, Japan) with a 12° total occlusal convergence angle was used for the margin preparation of each tooth. Teeth in group 2FR were further reduced on the distal side, providing a 2-mm ferrule on the labial, mesial, and palatal side (group 2FR-LaMPa). Teeth in group 2FR-La had a 2-mm ferrule on the labial side, group 2FR-Pa had a 2-mm ferrule on the palatal side, group ferrule in the labial, and palatal region (2FR-LaPa) had a 2-mm ferrule on the labial and palatal side, and teeth in group 0FR were reduced to eliminate the ferrule entirely.

To prepare a post space, the root canal filling was removed to the depth that leaves 4-mm gutta percha at the apical portion of the root using heated carrier (System B heat system, Sybron Endo, Orange, CA, USA). Subsequently, a post drill (No. 3 D.T. Light-Post Double Taper, Bisco, Schaumburg, IL, USA) featuring a double-taper design that provides proper post adaptation to the prepared canal was used to prepare the post spaces, ensuring at least a 4-mm gutta percha seal apically (approximately 14 mm intraradicular post length). Root canal walls were etched with 32% phosphoric acid for 15 s, rinsed with a compressed air water spray, and dried with paper points and compressed air. A bonding agent (All-Bond 2, Bisco) was applied to moisten dentine, and a dual polymerized resin luting agent (Duo-Link, Bisco) was introduced into the canal. A prefabricated quartz fiber post (1.2 mm in diameter, No. 3 D.T. Light-Post, Bisco) was introduced into the prepared canal space, using a pumping motion to release hydraulic pressure created during the cementation process. The excess luting agent was removed, and a quartz-tungsten-halogen unit (600 mW/cm2 light output) was placed directly over and in contact with the post for 20 s to ensure complete polymerization of the resin luting agent. Exposed coronal tooth structures and posts were etched for 15 s, then a bonding agent (All-Bond 2, Bisco) was placed and light polymerized.

Core build-up was performed over the posts using prefabricated plastic core formers (Paraform, Coltene/Whaledent, Coyahoga Falls, OH, USA) and resin composite (Light-Core, Bisco). The core was then light polymerized for 40 s. The core foundation was refined with a tapered flat-end diamond rotary cutting instrument (FG ISO #025, Shofu) with water irrigation. Full metallic crowns, 10 mm in height, were fabricated using a Ni-Cr alloy (Wiron 99, Bego, Germany). The crowns were cemented using zinc phosphate cement (Hy-Bond, Shofu) under a static 20-N load.

All roots were blocked out with wax 2 mm below the finish line to imitate the biologic width. The roots were covered with a 0.25 mm thick layer of autopolymerizing silicone (Ufigel P, Voco GmbH, Cuxhaven, Germany) to simulate a human periodontium. The teeth were then embedded in an autopolymerizing acrylic resin orienting their long axes to a 135° angle with the loading force. Subsequently, all specimens were stored in an incubator at 37°C for 24 h before testing.

Fracture resistance testing

Specimens were tested with a universal testing machine (LRX-Plus, Ametek Lloyd instrument, Hampshire, UK) set to deliver an increasing load until failure. The crosshead speed was 1.0 mm/min, and the load was applied to the lingual ledge at an angle of 135 degrees to the long axis of the teeth to simulate Angle's class I occlusion. [1],[17],[20] The variable of interest was the load at failure, measured in Newtons. Subsequently, specimens were examined under ×20 (Nikon SMZ model 1500, Nikon Instech, Co. Ltd.) for the type and location of failure, as well as the direction of failure if a fracture occurred.

Statistical analysis

Because the samples follow normal distribution, the statistical analyses used included one-way analysis of variance to detect the presence of group differences, and multiple comparisons between groups with Tukey's Honestly significant difference (HSD) adjustment (α = 0.05).


   Results Top


Fracture loads of each group are presented in [Table 1]. The highest fracture loads were found for the buccal, palatal, and mesial ferrule design (group 2FR-LaMPa; 778.14 (224.81 N)). The lowest fracture loads were found for the no ferrule design (group 0FR; 425.42 (141.07 N)). According to Tukey's HSD comparisons, there was significant difference among group 2FR-LaMPa, 2FR-Pa, 2FR-LaPa, and 2FR from the group 2FR-La, and from the group 0FR (P < 0.01 for all comparisons).

The fracture modes are presented in [Table 2]. The most frequent type of failure was an oblique fracture from the palatal to the border of the second and third quarters of the root on the labial surface, which were found in groups 2FR-LaMPa, 2FR-La, 2FR-LaPa, and 2FR-Pa.
Table 2: Mode of failures and frequencies of all groups tested

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   Discussion Top


In this present study, maxillary incisor teeth were selected due to their high susceptibility to trauma whereby they may eventually require the placement of a crown restoration. Various types of hard tissue damage may occur in the tooth structure requiring variations in ferrule design appropriate for individual fracture scenarios. Therefore, different possibilities of hard tissue loss were tested, simulating various clinical conditions. To examine the effect of different designs more precisely, a group with tooth preparation (group 2FR) and a group with no ferrule above the cemento-enamel junction (group 0FR) were included in the study. The reason for this approach was to evaluate the effect of the ferrule design factor more accurately.

The results of this present study demonstrated that incomplete ferrules were appropriate for successful restorations. This corresponded with results of previous studies. [1],[17] Group 2FR-LaMPa had the highest fracture load, which was not a significant difference from groups 2FR-Pa, 2FRLaPa, and 2FR. These results support the findings of previous studies [1],[18] in which similar methodology was used but with different ferrule designs. Complete ferrule may not be contributing to the highest fracture resistance in endodontically treated tooth restored with fiber post. Higher fracture resistance was found when palatal ferrule is present, which the location where the load is applied. Consequently, the arc of displacement of the complete crown places the root/remaining tooth structure under tension, buttressed against the post/core. Thus, the strength of the root/remaining tooth structure and not the bond between the post/core and the root/remaining tooth structure, is primarily challenged. On the contrary, if the remaining axial wall is on the labial aspect, the arc of displacement of the complete crown challenges the bond between the post/core and the root first, and as the resistance of the post/core bond to the root and the strength of the remaining tooth structure is overcome, root fracture results at a lower force level than the former force level. With no coronal tooth structure remaining, the resistance to displacement is primarily a function of the bond between the post/core and the root. In this present study, in 4 groups (2FR-LaMPa, 2FR-Pa, 2FR-LaPa, and 2FR), the remaining axial tooth structure was located in sites that seemed to directly oppose labially directed dislodging forces. Specimens in these groups appeared to more effectively resist fracture compared to specimens in the other 2 groups. In the other 2 groups, tooth preparations completely lacked axial tooth structure (group 0FR) or axial tooth structure was located labially (group 2FR-La), where it seemed labially directed dislodging forces would not be directly opposed. In the absence of an axial tooth structure in a location that could oppose dislodging forces, the prognosis for a restored endodontically treated teeth (RETT) may be unfavorable. However, this study also demonstrated that specimens with labially located axial tooth structures (group 2FR-La) incisal to the preparation finish line provided some degree of fracture resistance to RETT. The fracture resistance of all specimens having tooth preparations with axial walls was significantly greater than specimens without axial walls incisal to the finish line (0FR group). In addition, inferior fracture resistance was found in a tooth with complete ferrule. This may be caused by the lower adhesive area for the core built-up when compared with a tooth with interrupted ferrule that possess higher bonging surface area.

The primary mode of failure for 2FR-LaMPa, 2FR-Pa, 2FR-LaPa, 2FR-La, and 2FR groups was root fracture, which is a function of the modulus of elastic of fiber post similar to dentine. [21] Even though root fracture is a catastrophic event, it occurred at a much higher load compared to the Group 0FR in which debonding of the post was the initial failure. The present study used similar materials and methods as Dikbas et al's. [18] However, LaMPa ferrule design was added into the study in order to confirm and the findings both of previous studies [1],[18] and to provide more information regarding the site of ferrule which it has been shown that palatal ferrule is the most significant feature.

The limitations of this study should be noted and mentioned. The type of static loading test is used, which does not represent the intraoral condition. Teeth, intraorally, are subject to cyclic loading through mastication. This study design examined angle and force from a single direction, and this design is not necessarily representative of clinical conditions. Moreover, the oral cavity presents a different testing environment. For example, the presence of water, temperature changes and pH levels in the oral cavity may also considerably affect the results. The study also simulated maxillary central incisors, and therefore, the results can be applied only to that group of teeth. Therefore, further studies are required to simulate clinical conditions in vivo.


   Conclusion Top


Within the limitations of this study, it was concluded that for restoration of endodontically treated anterior teeth that have incomplete crown ferrules, location of the remaining coronal tooth structure may affect fracture resistance.

 
   References Top

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Correspondence Address:
Assoc. Prof. Boonrat Sattapan
Department of Conservative Dentistry, Dental Materials Research Unit, Faculty of Dentistry, Prince of Songkla University, Hat Yai, Songkhla
Thailand
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Source of Support: Faculty of Dentistry research fund, Prince of Songkla University, Conflict of Interest: None


DOI: 10.4103/0972-0707.157239

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