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ORIGINAL ARTICLE
Year : 2018  |  Volume : 10  |  Issue : 1  |  Page : 37-41

The effect of incomplete crown ferrules on fracture resistance and the failure modes of endodontically treated maxillary incisors restored with cast posts, cores, and crowns


1 Department of Dental Public Health, Sribunpot Hospital, Sribunpot, Phatthalung, Thailand
2 Department of Conservative Dentistry, Prince of Songkla University, Hat Yai, Songkhla; Dental Materials Research Unit, Faculty of Dentistry, Prince of Songkla University, Hat Yai, Songkhla, Thailand
3 Department of Conservative Dentistry, Prince of Songkla University, Hat Yai, Songkhla, Thailand
4 Department of Restorative Dentistry and Biomaterials Sciences, Harvard School of Dental Medicine, Boston, MA, USA

Date of Web Publication9-Jul-2018

Correspondence Address:
Asst. Prof. Kewalin Thammasitboon
Department of Conservative Dentistry, Faculty of Dentistry, Prince of Songkla University, Songkhla 90112
Thailand
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jofs.jofs_94_17

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  Abstract 


Introduction: The purpose of this study was to investigate fracture resistance and the fracture modes of endodontically treated teeth restored with cast posts and cores in the presence of various configurations of incomplete ferrules. Materials and Methods: Fifty maxillary anterior teeth were endodontically treated and divided into five groups (n = 10) according to ferrule design: group complete ferrule (CF) had a 2-mm circumferential ferrule; group buccal-mesial-palatal (BMP) had a 2-mm ferrule on the buccal, mesial, and palatal sides; group P had a 2-mm ferrule only on the palatal side; group B had a 2-mm ferrule only on the buccal side of the tooth; and group no ferrule (NF) had no ferrule. Each tooth was restored with a cast post, core, and a Ni–Cr crown. All specimens were lingually loaded at 135° to their long axis in a universal testing machine until fractured. Fracture patterns were recorded. Data were analyzed using one-way analysis of variance and Tukey’s honestly significant difference (Tukey�s HSD) test (α = 0.05). Results: The highest load to fracture was CF (534.33 ± 100.30 N), followed by BMP (467.71 ± 54.54 N), P (462.71 ± 54.92 N), B (330.48 ± 54.86 N), and NF (275.93 ± 28.35 N), respectively. There was no statistically significant difference in load to fracture among CF, BMP, and P and between B and NF (P > 0.05). Conclusion: A tooth with incomplete ferrule had lower fracture resistance than one with complete ferrule, but it was still higher than one with no ferrule. The presence of a palatal ferrule was more effective than a buccal ferrule in providing fracture resistance to palatal occlusal loads.

Keywords: Cast post–core, ferrule design, fracture resistance, restoration of endodontically treated teeth


How to cite this article:
Kaewtip K, Kukiattrakoon B, Sattapan B, Thammasitboon K, White RR. The effect of incomplete crown ferrules on fracture resistance and the failure modes of endodontically treated maxillary incisors restored with cast posts, cores, and crowns . J Orofac Sci 2018;10:37-41

How to cite this URL:
Kaewtip K, Kukiattrakoon B, Sattapan B, Thammasitboon K, White RR. The effect of incomplete crown ferrules on fracture resistance and the failure modes of endodontically treated maxillary incisors restored with cast posts, cores, and crowns . J Orofac Sci [serial online] 2018 [cited 2018 Dec 17];10:37-41. Available from: http://www.jofs.in/text.asp?2018/10/1/37/236212




  Introduction Top


It has been well established that the successful outcomes of endodontically treated teeth are dependent both on the quality of the root canal treatment and the coronal restoration.[1],[2] Various systems of the post- and core-retained crowns have been used to rebuild and to increase the strength to endodontically treated teeth. Numerous studies have confirmed that the presence of ferrule increases the fracture resistance for endodontically treated teeth[3],[4] regardless of the post system.[5],[6],[7] Sorensen and Engelman[3] defined the ferrule as “a 360° metal collar of the crown surrounding the parallel walls of the dentine extending coronal to the shoulder of the preparation. The result is an elevation in resistance form of the crown from the extension of dentinal tooth structure.” A more favorable prognosis could be expected if 1.5–2 mm of healthy dentin is present coronal to the margin of the crown circumferentially.[8],[9],[10],[11],[12],[13] However, obtaining a circumferential ferrule with uniform height is very challenging in endodontically treated teeth due to substantial coronal destruction from caries, endodontic access, and changes in the chemical and physical properties of the dentin.[14],[15],[16] Cast metal posts and cores have shown predictable and successful treatment outcomes and high survival rates.[17] Restoration for the teeth with extensive coronal destruction was recommended. There have been a few studies on the effect of nonuniform ferrules on the fracture resistance of the teeth restored with cast posts and cores. Tan et al.[10] studied the effect of nonuniform ferrule height on the fracture strength of the central incisors restored with a cast post and core. The results showed a significant decrease in the fracture resistance of the teeth with a nonuniform ferrule height of 0.5 mm on the proximal and 2 mm on the buccal and lingual sides, compared to those with a uniform 2-mm ferrule. Most studies using fiber posts have shown varied results, and it has been suggested that the site of the remaining hard tissue or ferrule has an influence on fracture resistance.[18],[19],[20],[21]

The purpose of this in-vitro study was to investigate fracture resistance and the fracture modes of endodontically treated teeth restored with cast posts and cores in the presence of various configurations of incomplete ferrules.


  Materials and Methods Top


Ethical approval for this study (No. 0521.1.03/916) was provided by the ethical committee of the Faculty of Dentistry, Prince of Songkla University, on 30 September, 2010. Fifty extracted maxillary anterior teeth were collected in accordance with an approved protocol reviewed by the Research Committee of the Faculty of Dentistry, Prince of Songkla University. The teeth were examined under a microscope with X 20 magnification and by radiographs. The inclusion criteria were non-carious, restoration free, no cracks, single root, and single root canal with approximately similar root length (17–18 mm) and buccolingual/mesiodistal dimensions [Table 1].
Table 1: Root dimensions ± SD of the teeth tested

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The crowns of all teeth were sectioned perpendicularly to their long axis at 2 mm above the proximal cemental–enamel junction (CEJ) in the samples with a ferrule and at the proximal CEJ in the samples without ferrule. All specimens were subjected to endodontic treatment. The working length of each tooth was determined by subtracting 1 mm from the length of an ISO size 15 K-file placed at the apical foramen. Root canal instrumentation was performed using a crown-down technique with ProTaper NiTi rotary instruments (Dentsply Maillefer, Ballaigues, Switzerland) up to F4 file (0.40 mm tip size with 6% taper). Irrigation was conducted with 2 ml of 17% ethylenediaminetetraacetic acid followed by 3 ml of 1.0% NaOCl and then dried with paper points. Subsequently, each tooth was obturated with a No. 40 ISO gutta percha cone (Hygienic GP Points; Coltene/Whaledent Inc, Coyahoga Falls, OH) and epoxy resin-based sealer (AH Plus Sealer; Dentsply Maillefer, Ballaigues, Switzerland) using the lateral compaction technique. The access preparation was then sealed with a temporary filling material (Cavit; 3M ESPE AG, Seefeld, Germany), and the specimens were stored at 37°C and 100% humidity for 7 days. The periodontal ligament was simulated using 0.25 mm autopolymerizing silicone (Ufigel P; Voco GmbH, Cuxhaven, Germany) applied to the roots, and the specimens were embedded in an autopolymerizing acrylic resin (Meliodent; Bayer UK Ltd, Newbury, UK) block up to 2 mm below the buccal CEJ.

Crown preparation was performed by creating a circumferential 2-mm ferrule with 1.5 mm axial reduction, and a circumferential 1-mm round shoulder margin using a round-end tapered diamond bur (ISO#016; Shofu Inc, Kyoto, Japan) with a high-speed handpiece attached to a dental surveyor. The remaining dentin thickness was maintained at 1 mm. All measurements were conducted with digital vernier calipers (Mitutoyo, Tokyo, Japan). The specimens were randomly divided into five groups (n = 10) according to the ferrule design [Figure 1]. Group complete ferrule (CF) had a 2-mm circumferential ferrule; group buccal-mesial-palatal (BMP) had a 2-mm ferrule on the buccal, mesial, and palatal sides; group P had a 2-mm ferrule only on the palatal side; group B had a 2-mm ferrule only on the buccal side of the tooth; and group no ferrule (NF) had no ferrule. Incomplete ferrule was created by reducing the prepared 2-mm ferrule to the CEJ level starting from a line angle to the other line angle of the surface, using flat-end tapered diamond bur (ISO#016; Shofu Inc, Kyoto, Japan). The gutta percha was then removed using a heated carrier (System B Heat System; Sybron Endo, Orange, CA) to a depth that left 4 mm remaining at the apical portion of the root canal, which gave 12–14-mm intraradicular post length. Subsequently, a standardized post space was prepared using a post drill (D.T. Light-Post Double Taper No. 3; Bisco Inc, Schaumburg, IL).
Figure 1: Ferrule designs of specimens belonging to the experimental groups. Coronal view of the locations of the ferrule and root canal

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Post and core patterns were made from acrylic resin (Duralay; Reliance Dental Manufacturing Company, Worth, IL, USA) using a direct method. The core was built up to a height of 4 mm from the proximal CEJ. The resin patterns were then invested and cast in Ni–Cr alloy (Dentsply-Sankin KK, Tokyo, Japan). All posts were cleaned and checked for passive retention and uniform adaptation using pressure spot indicator (Coltène/Whaledent AG, Altstätten, Switzerland). The post was cemented with zinc phosphate cement (HY-bond Zinc Phosphate Cement, Shofu Inc, Kyoto, Japan), and a load of 3 kg was placed on the seated post. After setting, the excess cement was removed.

Wax patterns were fabricated on the specimens using a silicone putty index (ExaFlex Putty; GC America, Alsip, IL, USA). A lingual ledge was made to create a standard loading point for fracture resistance testing. Wax patterns were invested and cast in Ni–Cr alloys using the same protocol used with posts and cores. The final crowns were cemented on their respective teeth using zinc phosphate cement (HY-bond Zinc Phosphate Cement, Shofu Inc, Kyoto, Japan) mixed according to the manufacturer’s instructions. A load of 3 kg was placed on the cemented crown until the cement set. Excess cement was removed, and the specimens were stored in 100% humidity at 37°C for 24 h before testing.

Fracture resistance testing

The fracture resistance was measured with a Universal Instron testing machine (model LRX-plus; Ametek Lloyd Instrument, Hampshire, UK). The specimen was placed at an angulation to provide a 135° angle between the long axis of the tooth and the loading tip. The specimens were subjected to static loading with 1-mm/min crosshead speed until failure. Failure was defined as a 10% drop in the applied load. Load to failure was recorded in Newtons (N).

The mode of fracture was determined by visual inspection under a stereo microscope with X 20 magnification (Nikon SMZ model 1500; Nikon Instech Co Ltd).

Statistical analysis

The mean loads to failure were compared and analyzed by one-way analysis of variance, and the Tukey’s HSD test was used for pairwise multiple comparisons (α = 0.05).


  Results Top


The means load required to fracture and the distribution of the fracture modes in each group are presented in [Table 2]. The highest load to fracture was group CF, followed by groups BMP, P, B, and NF, respectively. Tukey’s HSD comparisons revealed no statistically significant difference in load to fracture among groups CF, BMP, and P and between groups B and NF. Fracture modes observed in all specimens could be categorized into four patterns [Figure 2] including oblique fracture extending from the palatal crown margin to the labial side below the acrylic level, horizontal root fracture at 3–5 mm below the palatal crown margin, horizontal root fracture at the level of post tip, and vertical root fracture with post and core dislodgement.
Table 2: Mean load to fracture (Newton) (SD) and the fracture mode of each group

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Figure 2: Fracture modes: (A) Oblique fracture. (B) Horizontal root fracture 3–5 mm below the palatal crown margin. (C) Horizontal root fracture at the level of post tip. (D) Vertical root fracture with post and core dislodgement

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  Discussion Top


The results of this investigation confirm the findings of many laboratory studies and finite element analysis (FEA),[22],[23],[24],[25],[26] which have shown the reinforcement effect of a circumferential ferrule to endodontically treated tooth restored with a cast post, core, and a complete crown. In this study, the highest fracture resistance was found on a tooth with a complete 2-mm ferrule, while a tooth with no ferrule had the lowest resistance. The results also demonstrated that a tooth with an incomplete ferrule has lower fracture resistance than a tooth with a complete ferrule, but it still has higher fracture resistance than one without a ferrule. These findings correspond to a study by Tan et al.,[10] which has shown that a central incisor, restored with a cast post and core in the presence of a 0.5-mm proximal and 2-mm labial and palatal ferrule, was less resistant to fracture under static loading than a tooth with a complete 2-mm ferrule, and the tooth that lacked a ferrule had the lowest fracture resistance.

Moreover, when the ferrule was incomplete, the location of the ferrule appeared to be a critical factor to the resistance of the tooth. This study showed that the means fracture load of a tooth with a 2-mm ferrule on the palatal, and buccal–mesial–palatal aspects were not significantly different from those of a tooth with a complete ferrule. However, when there was only a buccal ferrule present, the mean load to fracture was significantly reduced and not different from a tooth without a ferrule. These results are in accordance with a study by Ng et al.,[19] which reported that for a tooth restored with a quartz fiber post, a composite resin core, and a metal crown, the presence of a palatal ferrule was more effective than a labial ferrule in providing fracture resistance to occlusal loads. The authors provided an explanation for the findings that when the ferrule is on the palatal side, where the load is applied, the arc of displacement of the crown causes tension on the tooth structure, buttressed against the post and core. Thus, the strength of the root and remaining tooth structure is primarily challenged. On the contrary, if the ferrule is on the labial side, the arc of displacement of the crown challenges the bond between the post, core, and the tooth structure first; once the bond has failed, the root is subsequently fractured, which happened at lower load levels than that when a palatal ferrule is present. Naumann et al.[18] reported different results. They found a great variation in tooth resistance when the ferrule was incomplete. The highest median fracture load value was found in the group with a facial ferrule, followed by a palatal and a complete 2-mm ferrule. The lowest median was found in the group with no proximal ferrule. However, there were no statistically significant differences between the groups with no proximal ferrule and a complete ferrule or a palatal ferrule. The author stated that high resistance in the group with a facial or a palatal ferrule might be attributed to the greater remaining dentin thickness and adhesive area between the tooth substance and the composite core material. It should be noted that the tooth specimens were restored with glass fiber posts, composite cores, and all-ceramic crowns, which were subjected to thermal cycling and mechanical loading prior to a fracture resistance test. There has been only one study[25] in which teeth restored with cast posts and cores with metal crowns were assessed for the impact of the different designs of incomplete ferrules on tooth strength. The results showed that the site of the incomplete ferrule did not have an effect on the fracture resistance of the tooth. However, mandibular premolars, not central incisors, were tested in this study.

The most frequent mode of fracture for all five groups was an oblique fracture extending from the palatal margin to the labial root surface below the acrylic resin margin except for the teeth with complete ferrules, in which horizontal root fracture at 3–5 mm below the crown margin was the most frequent mode. Interestingly, post dislodgement and vertical root fractures were observed in three teeth with no ferrule. These data supported the results based on FEA,[24] which have shown that maximal tensile stress was found at two locations including the palatal wall of the root at the junction between the ferrule and the cervical margin of the preparation and the periphery of the root dentine approximately 3–5 mm from the palatal margin of the crown. When a 2-mm complete ferrule was present, compressive stress in the labial cervical dentine decreased, while tensile stress in the palatal cervical dentin increased. Moreover, the area of tensile stress expanded toward the cervical margin leading to a more constant tension area, compared with a preparation with no ferrule. The study also reported that in the absence of a ferrule, rotational movement followed by a dislodgement of the post, core, and crown was created by palatal occlusal force. Subsequent lever action could lead to a fracture of the post in the coronal portion or the development of a vertical root fracture. These findings may explain the horizontal and vertical root fractures found in a tooth with a complete ferrule or with no ferrule, respectively.

Even though many studies have been conducted using the combinations of various ferrule designs and different post and core systems, inconsistent data still exist. Difference in experimental designs have a significant impact and could lead to different results. More importantly, there are several limitations to in-vitro studies, including in this study. Static loading may not accurately replicate the real situations in the oral cavity. Cyclic loading and thermocycling might be recommended. Generally, only one post and core system and one tooth type, mostly maxillary central incisors, were tested in the studies. Therefore, data interpretation should be limited to the studied system and the group of teeth.


  Conclusion Top


Under the conditions of this in-vitro study, it was concluded that for the endodontically treated anterior teeth restored with a cast post, core, and a metal crown, a complete ferrule provided the highest fracture resistance. A tooth with incomplete ferrule had lower fracture resistance, but it was still higher than one with no ferrule. The presence of a palatal ferrule was more effective than a labial ferrule in providing fracture resistance to palatal occlusal loads.

Financial support and sponsorship

This study was supported by Prince of Songkla University.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2]



 

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