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Year : 2007  |  Volume : 10  |  Issue : 1  |  Page : 14-18
Metal collars - are they serving any purpose?


Department of Conservative Dentistry and Endodontics, Saveetha Dental College and Hospitals, Chennai, India

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   Abstract 

The purpose of this study was to investigate the fracture resistance of endodontically treated teeth with uniform and non-uniform ferrule configurations. Root canal treatment and post space was performed in fifty extracted intact, non carious human maxillary central incisors and were divided into 5 groups of 10 specimen each i.e. Group I -- No contra bevel (No core ferrule), Group 11 - contra bevel <2mm(core ferrule <2mm), Group III - 2mm contra bevel (2mm core ferrule), Group I V - contra bevel >2mm (core ferrule >2mm). Group V - Non­uniform contra bevel (Non uniform core ferrule). Crown preparation was done in a standardized manner in all the specimen. Based on the groups varying core ferrule height was given in all the specimen. Wax patterns were made, invested and casted. Cast posts were then luted followed by crown cementation and then tested in universal testing machine at a cross head speed of 2.5mm/min and the load was applied to the lingual edge at 45° angle to the long axis of the tooth. It was concluded that contra-bevel / core ferrule does not further enhance the fracture resistance.

How to cite this article:
Goyal S, Shyamala P V, Miglani R, Narayanan L L. Metal collars - are they serving any purpose?. J Conserv Dent 2007;10:14-8

How to cite this URL:
Goyal S, Shyamala P V, Miglani R, Narayanan L L. Metal collars - are they serving any purpose?. J Conserv Dent [serial online] 2007 [cited 2019 May 19];10:14-8. Available from: http://www.jcd.org.in/text.asp?2007/10/1/14/42275

   Introduction Top


Success of root filled teeth depends on effective coronal seal, protection of the remaining tooth, restored functions and acceptable esthetics [1] .A post retained crown may be indicated to fulfill these requirements. However, one mode of failure of the post restored tooth is root fracture. Therefore, the crown and post preparation design features that reduce the chance of root fracture would be advantageous.

There is a multitude of posts available on the dental market, and as such choosing the ideal one is difficult. A cast post or a commercial prefabricated post and core is usually selected, however the custom cast post and core is the time honored traditional recommendation for corono-radicular stabilization of endodontically treated teeth [2] .

The preparation design of pulpless teeth is a critical consideration in the restoration of endodontically treated teeth but has received only limited attention. It has been proposed that the use of a ferrule as a part of the core or artificial crown may play a significant role in the fabrication of cast post. The word ferrule probably originates from combining the Latin for iron (Ferrum) and bracelets (Viriola) (Brown 1993) [1] . A ferrule is a band of metal which totally encircles the tooth, extending 1-2mm onto sound tooth tissue to guard against longitudinal fracture [3] . The purpose of the ferrule is to improve the structural integrity of the pulpless tooth by counteracting,

1. The functional lever forces

2. The wedging effect of tapered dowels.

3. The lateral forces exerted during insertion of the dowel [4]

Although the concept of a ferrule appears logical, its benefits have not been confirmed by research nor has a superior ferrule design evolved.

Thus the aim of this in vitro study was to evaluate the fracture resistance and mode of fracture of endodontically treated maxillary anterior teeth with varying ferrule (core) configurations standardizing the crown ferrule.


   Materials and Methods Top


Fifty freshly extracted intact, non carious, human maxillary central incisors stored in saline were selected for the study. The dimensions of the teeth were standardized comparing it to the published averages as shown in [Table 1].

Root Canal Preparation

Standard access cavities were prepared and the pulp tissue was extirpated. A size 10 K file was placed into the canal until it was just visible at the apical foramen to check the patency. The working length was then established by passing a size 10 K file into the canal until the tip was visible at the apex and then subtracting 1 mm from this length. The cleaning and shaping procedure was completed following a crown down technique using Profile Rotary NiTi files in a gear reduction hand piece (W & H, TREND, WD 73, Austria) according to manufacturer's instructions. The apex was enlarged upto size 30,.06 taper. 3% of NaOCI (Prime Dental Products India) was used after each instrumentation and 17% EDTA (Glyde, Dentsply, Maillefer, Germany) was used during instrumentation. After these procedures canals were ultrasonically cleansed. The teeth were then obturated using gutta-percha and zinc oxide eugenol scalers by lateral condensation technique. Specimens were stored in 100% relative humidity at 37°C for 24 hrs.

Post space preparation began with the removal of gutta-percha using Gates Glidden burs (Brasseler, USA), taking care to preserve 4mm of apical gutta­percha. Post space was prepared using peasso reamer upto size 4. Radiographs were taken in both MD and BL direction to ensure the standardization of the remaining radicular dentin thickness around the post space preparation.

With the help of a divider 4mm height was marked on the coronal tooth structure from CEJ and the remaining incisal tooth structure was removed using a slow-speed diamond disk under constant water cooling. All the specimens had standard 4mm remaining tooth structure.

Crown Preparation

Index preparation was done for all the specimen using silicon putty material (Provil Nova, Ivoclor, Vivadent). Standardized Tooth reduction was performed as shown in figure a. No incisal reduction was performed as the incisal tooth structure was already removed. The crown margin was designed to follow the simulated contour of the free gingival tissue with the labial and lingual extents of the margin 1.5mm more apical compared to the proximal margins. The chamfer finish line was given in the crown margins using Chamfer Diamond and 12° total occlusal conversion angle was used for the margin preparation.

After crown preparation the specimens were then randomly divided into 5 groups of 10 each.

Group I - No contra bevel, Group II - contra bevel <2mm, Group III - contra bevel = 2mm, Group IV­contra bevel >2mm, Group V - Non uniform contra bevel (0.5/2mm)The contra bevel was given at an angle of 45° to the long axis of tooth using a flame shaped bur.

Core ferrule and finish line was high lightened in each specimen and was then prepared as shown in [Figure 1].

Fabrication of Cast Post

Direct impression technique was employed to fabricate the post space and core using medium inlay wax (GC Corporation Tokyo, Japan). Wax patterns were fabricated and a lingual ledge was created to standardize the loading point in all the specimen. These wax patterns were invested and cast in Ni-Cr alloy. After casting, small nodules were removed if present. The sprue was sectioned and seated to their corresponding teeth. The custom made cast posts were luted into the canals using type I GIC (Fuji I, GC Corporation, Tokyo, Japan). Metal (Ni-Cr alloy) JC were then fabricated for all the specimens and were luted with type I GIC.

The teeth were then embedded in acrylic resin poured into an acrylic resin mould. Then the specimens were mounted on a custom made steel jig of angulations of 45° and were subjected to loading in universal testing machine (Lloyds, Germany) as shown in [Figure 2]. The values at failure were recorded in Newton and the pattern of fracture was observed.


   Results Top


Inference:

There was no significant difference in the mean values between any contrasts.


   Discussion Top


The current emphasis on preservation of the natural dentition, combined with successful endodontics has resulted in various methods to restore pulpless teeth when substantial tooth structure has been lost, a post and core is made to retain the coronal restoration [5] .

Many studies have been done for evaluating the effects of ferrule heights on fracture resistance of endodontically treated teeth. According to studies by Lu Zhi Yal et al, 2mm ferrule for custom cast post had the highest value for fracture resistance'. These results are also in accordance with the result shown by Assifet al. [8]

But there is not much information available on the effects of contra bevel (core ferrule) on the fracture resistance of endodontically treated teeth. Hence this study was performed in order to evaluate the effect of varying heights of core ferrule on fracture resistance of endodontically treated teeth standardizing the crown ferrule. In the present study crown ferrule was standardized to 4mm and the core ferrule was kept at varying heights.

As seen in laboratory studies, there are many factors such as post length, diameter of the post, type of cement used, thickness of the remaining radicular dentin, which play a role in the fracture resistance, besides the ferrule effect. Most of the studies on the effect of various post lengths or designs have been performed without the use of a covering crown [7] . When the covering crowns are used, as in the present study the effect of the post length may have been overshadowed.

In the present study load was applied at an angle of 45° to the long axis. This loading angle was selected because it simulates the average angle of contact between maxillary and mandibular incisors in class I occlusion and has been used by many investigator previously (Barkhordar et al, 1989, Saupe et al, 1996). Loading was continued until failure occurred as a result of root fracture at a cross head speed of 2.5mm/min. Generally the cross head speed varies from 0.5mm/min to 5cm/min. A slow speed was selected in this study since it enables to record the exact amount of load required to cause failure.

The results of this study showed that specimen in group III & IV showed higher values for fracture resistance but the difference was not statistically significant. Thus showing the core ferrule has no / minimal influence on fracture resistance of an endodontically treated teeth.

The ferruled group specimen demonstrated a horizontal fracture in midroot at a level corresponding to the end of the post as observed in [Figure 3]. In the non-ferruled group the fracture line was vertical and extended to involve the root face as observed in [Figure 4]. This pattern is very similar to that described by Barkhorder et al (1989). Whereas specimens with a metal collar had horizontal fractures and specimen without collar had vertical or oblique fracture.

As mentioned in previous studies the fracture resistance is increased with increase in ferrule height [1] .Thus in this study keeping the crown ferrule height constant, the varying core ferrule height did not further enhance the fracture resistance, but the pattern of root failure was changed. This may be attributed to the hugging action of the ferrule which prevents shattering of the root [1] .

Horizontal root fractures have better prognosis than vertical root fractures. Moreover, the vertical root fracture leaves no other choice of treatment except extraction of the teeth. Hence, by providing a core ferrule along with standardized crown ferrule the pattern of root fracture is influenced.


   Conclusion Top


Within the limits of this study it can be concluded that although contra-bevel / core ferrule does not further enhance the fracture resistance, it is an important feature in fabrication of cast post, as it affects the pattern of root fracture.



 
   References Top

1.Stankiewiez N.R. & P.R.Wilson The ferrule effect : The literature review. Int. Endodontic Journal 2002: 35,575-581.  Back to cited text no. 1    
2.Steven M.Morgano &Patrice Milot. Clinical success of cast metal posts and cores. Journal of Prosthetic Dentistry. 1993,60:70, 11-16.  Back to cited text no. 2    
3.Whitworth J.M. A.W.G.Walls and R.W.Wassell. Crowns and extra-coronal restorations: endodontic considerations: the pulp, the root-treated tooth and the crown: British Dental Journal 1992:6,315-327.  Back to cited text no. 3    
4.Sorensen. A. and Michel J.Engelman. Ferrule design and fracture resistance of endodontically treated teeth: Journal of Prosthetic Dentistry. 1990:63,529-536.  Back to cited text no. 4    
5.Rahmat A.Barkhordar, Ryle Radke and Jan Abbasi. Effect of metal collars on resistance of endodontically treated teeth to root fracture: Journal of Prosthetic Dentistry. 1986:61, 676-678.  Back to cited text no. 5    
6.Lu Zhi-Yue and Zhang Yu-Xing. Effects of post-core design and ferrule on fracture resistance of endodontically treated maxillary central incisors. Journal of Prosthetic Dentistry. 2003:89,368-373.  Back to cited text no. 6    
7.Flemming Isidor, Knud Brondum and Gert Ravnholt. The influence of post length and crown ferrule length on the resistance to cyclic loading of bovine teeth with prefabricated titanium posts. Int Journal of Prosthodont. 1999:12,78-82.  Back to cited text no. 7    
8.Assif D, Bitenski A, Pilo R, Oren E. Effect of post design on resisitance to fracture of endodontically treated teeth with complete crowns. Journal of Prosthetic Dentistry 1993:69,36-40.  Back to cited text no. 8    

Top
Correspondence Address:
Sameer Goyal
Department of Conservative Dentistry and Endodontics, Saveetha Dental College and Hospitals, Chennai
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0707.42275

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

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    Abstract
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    Materials and Me...
    Results
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