| Abstract|| |
Objective: The objective of this study is to determine the best ferrule configuration of endodontically treated teeth.
Materials and Methods: A total of 40 freshly extracted maxillary central incisors were collected. Teeth were allocated into following four groups of each having 10 teeth. Group 1 teeth restored with crowns (CRN), Group 2 endodontically treated teeth restored with crowns (RCT and CRN), Group 3 endodontically treated teeth restored with cast dowel cores and crowns incorporating uniform 2 mm ferrule (2 FRL), Group 4 endodontically treated teeth restored with cast dowel cores and crowns incorporating uniform 1 mm ferrule (1 FRL). Crown preparation was performed with varying ferrule heights, crowns were cemented with resin modified glass ionomer cement and were kept in 100% humidity for 3 days, all specimens were loaded to failure in a universal testing machine at 0.5 mm/min. Results were obtained in the form of kilo Newton and Newton.
Result: The statistical analysis applied was an analysis of variance to detect the presence of group differences. Pair wise comparisons between the groups were analyzed with the Tukey Kramer significant difference test for multiple comparisons.
Conclusion: Ferrule had a significant role in the fracture resistance of custom cast post core restored teeth. Endodontically treated maxillary central incisors with a uniform 2 mm ferrule were more fracture resistant than those with a uniform 1 mm ferrule.
Keywords: Ferrule; fracture resistance; post and core
|How to cite this article:|
Singh S, Thareja P. Fracture resistance of endodontically treated maxillary central incisors with varying ferrule heights and configurations: In vitro study. J Conserv Dent 2014;17:115-8
|How to cite this URL:|
Singh S, Thareja P. Fracture resistance of endodontically treated maxillary central incisors with varying ferrule heights and configurations: In vitro study. J Conserv Dent [serial online] 2014 [cited 2019 Jul 21];17:115-8. Available from: http://www.jcd.org.in/text.asp?2014/17/2/115/128038
| Introduction|| |
The restoration of the endodontically treated tooth is an important aspect of successful endodontic therapy. There are wide ranges of treatment options of varying complexity. The clinician must be able to predict the probability of restoring such teeth successfully. In general, endodontically treated teeth experience significant coronal destruction as well as loss of radicular dentin, secondary to endodontic treatment. There is evidence that these teeth have reduced levels of proprioception, which could impair normal protective reflexes.  A post and core retained crown may be indicated to fulfill these requirements  Clinical longevity of the post and core restoration can be influenced by many factors including magnitude and direction of the occlusal load, design of dowel, thickness of remaining dentin, quality of cement layer and creation of ferrule effect to enhance structural durability of the final restoration. 
A ferrule is a metal ring or cap intended for strengthening. The word probably originates from Latin, combining the words "FERRUM" or iron and "VIRIOLA" for bracelets.  A dental ferrule is an encircling band of cast metal around coronal surface tooth. Sorensen and Engelmann define the ferrule effect as a "360° metal collar of the crown surrounding the parallel walls of the dentin 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 protective or "ferrule effect" occurs owing to the ferrule resisting stresses such as functional lever forces, the wedging effect of tapered posts and the lateral forces exerted during the post-insertion  The effectiveness of the ferrule has been evaluated by a variety of methods including fracture testing, ,, impact testing,  fatigue testing , and photo elastic analysis.  Majority of studies regarding the effectiveness of a ferrule support the need for at least 1.5 mm as ferrule height. , This recommendation requires that the ferrule encompass the entire circumference of tooth. Methods of testing could include static loading to stimulate forceful clench. 
The purpose of this in vitro study is to investigate the effectiveness of uniform 2 mm and 1 mm ferrule height configurations of fracture resistance of endodontically treated teeth when subjected to static load.
| Materials and Methods|| |
A total of 40 freshly extracted maxillary central incisors were collected from the Department of Oral and Maxillofacial Surgery. The selection of teeth was carried out on the basis coronal height was limited to 9-11 mm and root length was limited to 12-14 mm were allocated into four groups of each having 10 teeth. Inclusion criterion included teeth extracted for orthodontic needs or periodontally compromised, non-carious teeth, intact teeth, teeth with single canals. Exclusion criterion included carious teeth, tooth cracks/fractures, restored teeth, primary teeth and teeth with developmental anomalies. The selected teeth were stored at room temperature in distilled water until testing.
Group 1: Teeth restored with crowns (CRN),
Group 2: Endodontically treated teeth restored with crowns (RCT and CRN),
Group 3: Endodontically treated teeth restored with cast dowel cores and crowns incorporating uniform 2 mm ferrule (2 FRL),
Group 4: Endodontically treated teeth restored with cast dowel cores and crowns incorporating uniform 1 mm ferrule (1 FRL).
The specimens were mounted in acrylic resin blocks, with the long axis of the block, midfacial extent of each tooth parallel to the long axis of the block and the mid facial extent of cemento-enamel junction located 2 mm coronal to acrylic resin.
Specimens of Groups 2, 3 and 4 were instrumented to a working length using ProTaper file system (Dentsply Maillefer) following conventional principles of the crown down technique until ProTaper file F5. At each change of file size, canals were irrigated with 3% NaOCl (Vishal Dento Care Pvt. Limited) and root canal conditioning agents ethylenediaminetetraacetic acid (Glyde-Dentsply Maillefer). After chemomechanical preparation, the canals were dried with absorbent paper points (Dentsply Maillefer, Ballaigues) and obturated by lateral condensation technique with F5 ProTaper gutta-percha cones (Dentsply Maillefer, Ballaigues) and AH Plus Sealer (Dentsply-Maillefer, Switzerland).
In Groups 3 and 4, Post-space preparation began with the removal of gutta-percha using heated plugger. Final gutta-percha removal was performed with drills (#3 Gates-Glidden; Mani, Japan) taking care to preserve 5 mm apical gutta-percha followed by Largo Peeso Reamer No. 3 (Dentsply Maillefer Ballaigues, Switzerland). The Largo Peeso Reamer was marked to a depth of 5 mm short of working length, with endoblock (Dentsply Maillefer) and stopper was fixed. Gutta-percha was removed from specimens leaving 5 mm of gutta-percha in the canal apex. Following the post-space preparation, the space was rinsed with 3% NaOCl (Vishal Dento Care Pvt. Limited, India). Final irrigation was accomplished with saline and post-space was dried using paper points (Dentsply Maillefer Ballaigues).
In Group 3 and 4, 2 mm of incisal reduction and 1.5 mm of facial and lingual reduction and Ferrule height was measured and marked as 2 mm and 1 mm uniform circumferentially on the tooth surface with digital vernier calipers (Mitutoyo, Tokyo, Japan) for standardization of ferrule height. Further crown was reduced to leave 1 mm uniform ferrule with a high-speed diamond point under abundant air-water cooling.
Post and core patterns were made from pattern wax (GC pattern wax) using the direct method to replace the coronal dentin that had been removed. Wax patterns of Groups 3 and 4 (to minimize batch variation) were invested in gypsum bonded investment material (Deguvest California USA) using a 1:1 liquid to powder ratio and allowed to set for 30 min. Then invested wax patterns were placed in pre-heated burn-out oven (Unident, India) at a temperature of 700°C and left for 45 min. The patterns were cast in non-precious gold alloy with the aid of a centrifugal induction casting machine (Unident, India). The castings were divested and cleaned. The post and cores were fitted to the respective teeth and subsequently cemented with glass ionomer cement (3M ESPE). A lingual ledge was added to create standard loading point. To summarize, a coping with the metal backing was prepared onto, which labial porcelain was veneered. Wax pattern from each group was invested and cast using the same protocol used with dowels and cores using Ni-Cr alloy.
PFM crowns were fabricated and cemented with Resin modified glass ionomer cement (3M ESPE). Cemented crown samples were stored for 3 days in an environment with 100% humidity in a humidity chamber. Prepared specimens were mounted on a holder slot, which was fixed to the lower arm of the universal testing machine to present the lingual side at an angle of 45° to the metal indenter of 4 mm diameter. The indenter was fixed to the upper arm of the universal testing machine, which was set to deliver an increasing load until failure. Failure was defined as a 25% drop in the applied load. The cross head speed was 2.5 mm/min and the load was applied to lingual ledge at a 45° angle to the long axis of the tooth. The specimens were tested under random order (samples were pooled mixed and picked without visualization) and the operator was not informed of the group designation of the specimen being tested. Variable of interest was the load at failure measured in Newton. The statistical analysis applied was one way analysis of variance (ANOVA) to detect the presence of group differences and pair wise comparisons between groups with the (Tukey Kramer multiple comparison test) significant difference test for multiple comparisons. The mode of failure for each of the specimens was noted by visual inspection. Software used for statistical analysis is SPSS 16 version.
| Result|| |
The statistical analysis applied was one way ANOVA to detect the presence of group differences. Result showed that the maximum load required to fracture test specimens of Group 2 endodontically treated with crowns was highest, followed with Group 3 endodontically treated teeth restored with cast dowel cores and crowns incorporating uniform 2 mm ferrule (2 FRL), followed with Group 1 teeth restored with crowns (CRN), followed with Group 4 endodontically treated teeth restored with cast dowel cores and crowns incorporating uniform 1 mm ferrule (1 FRL). Where no significant difference was found between the fracture resistance of Groups 1 and 2 with P = 0.05. When Groups 3 and 4 were compared results were statistically significant difference P = 0.001. Software used for statistical analysis is SPSS 16 version.
| Discussion|| |
The test specimens can be stored in various storage media such as, distilled water, ethanol and normal saline. Studies have shown that, normal saline and ethanol decrease dentinal permeability and affect fracture resistance with time.  No significant difference was found with distilled water storage. Thus in the present study, maxillary central incisors were stored in distilled water. Maintenance of adequate obturation is critically resistance to bacterial microleakage. Although studies indicate that 4 mm of gutta-percha provides adequate seal, stopping precisely at 4 mm is difficult.  Recent studies advocates leaving 5 mm of gutta-percha apically.  In the present study, in Groups 3 and 4, 5 mm of gutta-percha was left in apical root end so that the sealed accessory canals and lateral canals remained untouched to prevent microleakage.
Gates-Glidden drills were run at 800-1000 rpm with slow-speed handpiece to prepare post-space. Generating frictional heat that softens the gutta-percha and eases its removal without disturbing apical root filling.  In the present study, Groups 3 and 4 gutta-percha was removed using Gates-Glidden drill followed by Largo Peeso Reamer No. 3 at 1000-1200 rpm and post-space was prepared according to manufacturer instructions.
A 2 mm cervical ferrule is regarded as the key to restoration longevity because of its strengthening effect. Roots resist better multiple stresses, to which they are subjected when the tooth is in use, even to loads resulting from parafunctional habits. The role of the ferrule is similar to cast ring.  In this study, 2 mm of incisal reduction, 1.5 mm of facial and lingual reduction was performed. Ferrule height was measured and marked uniformly as 2 mm and 1 mm circumferentially on the tooth surface with digital calipers for standardization of ferrule height further crown was reduced to leave 2 mm and 1 mm ferrule height for respective Groups 3 and 4 with a high-speed diamond point under abundant air-water cooling.
Both zinc phosphate and glass ionomer have similar properties and commonly used because of their ease of use, coupled with their history of clinical success. In the present study, in Groups 3 and 4, glass ionomer cement was mixed to luting consistency and inserted in the canal using lentulo spirals. This may be an effective technique for reducing voids and bubbles within the luting agent. The posts were then seated into the canal by firm finger pressure and excess cement was removed along the peripheries.
In the comparative analysis between cements, resin modified glass ionomer cement showed a pull force more than twice as that of zinc phosphate cement. In thicker layers, the better physical chemical properties of the resin modified glass ionomer cement become more evident.  In this study, PFM crowns were fabricated and cemented with resin modified glass ionomer cement (3M ESPE). Tan et al. stated after cementation of crowns with resin modified glass ionomer cement crowns should be stored in 100% humidity for 3 days,  to simulate the humidity in vivo until returned for strength testing.  Cemented crown samples were stored for 3 days in an environment with 100% humidity in a humidity chamber. In the present study, specimens were evaluated for their fracture resistance, using the universal testing machine. Static load was applied at the same angulation along the long axis of tooth in the study undertaken by Tan et al., Zhi-Yue and Yu-Xing. , This angle is found to be optimal as this is the value of interincisal angle in most patients where the forces act more.
The mode of failure for specimens was an oblique fracture extending from the lingual margin to the facial surface just below the tooth's insertion into the acrylic resin.  In this present study, Groups 3 and 4 with ferrule and cast post and core, the mode of failure of specimens was an oblique fracture in middle third of roots inside the acrylic resin block. In root canal treated tooth with crown without post and core fracture was in cervical section of the root.
| Conclusion|| |
Endodontically treated maxillary central incisors with a uniform 2 mm ferrule were more fracture resistant than those with a uniform 1 mm ferrule.
Endodontically treated teeth restored with custom cast post core were as strong as endodontically treated teeth with crowns.
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Department of Conservative and Endodontics, Darshan Dental College and Hospital, Udaipur, Rajasthan
Source of Support: None, Conflict of Interest: None