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The “Ferrule Effect” in Fixed Prosthodontics Appropriate restoration of endodontically treated teeth must account for diminished residual coronal tooth structure, typically necessitating foundation restoration, including coronal–radicular buildup or post–core restoration. To ensure long-term structural integrity, all endodontically treated posterior teeth require cuspal coverage restoration, although anterior teeth with sufficient residual tooth structure may not. The ferrule effect indicates that a band of intact, healthy tooth structure coronal to the finish line of the crown preparation must be present in order to resist destructive forces brought to bear on the restored tooth during normal functional loading. This issue of Prosthodontics Newsletter provides a closer look at the ferrule effect and its incorporation in the management of high-quality fixed prosthodontics.

Importance of the Ferrule Effect in Restorations

B iomechanical failures of re -stored nonvital teeth can lead to leakage, recurrent caries

lesions, fissures and fractures of the root, necessitating replacement of the restoration or even extrac-tion of the tooth. Without accepted clinical standards, the practitioner must choose among a wide variety of materials and restorative tech-niques to restore endodontically treated teeth.

In 2007, Dietschi et al from the University of Geneva, Switzerland, conducted a systematic review of the literature focused on the com-position and structural alterations from endodontic treatment, in

hopes of creating recommendations for materials and techniques that would lead to successful treatment of pulpless teeth. They searched the PubMed/MEDLINE database for articles in 4 areas:

➤ dentin composition

➤ dentin or restorative material physical characteristics

➤ fracture resis-tance, tooth stiffness and other monotonic mechanical tests

➤ stress simulation using photoelastic studies and finite element analysis

Chelators used in endodontic treat-ment, such as EDTA and calcium hydroxide, tend to deplete the cal-

(continued on next page)

➤ Establishing the Proper Ferrule Length

➤ Ferrule Length and Stress Distribution

➤ Ferrules in Teeth Restored with Fiber Posts

➤ Restoring a Tooth with a Nonuniform FerruleIn

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M. Patrick Pedlar, BSc, ddS, MSd, Frcd(c) B. larry Pedlar, ddS, MSd, FacP

Restoring the Natural Appearance of Teeth310 Plains Road East • Burlington, Ontario L7T 4J2

Tel: 905-632-1882 • Fax: 905-632-1351 • www.RestorativeDentistry.ca e-mail: patrick@drpedlar.com

Certified Specialists in Prosthodontics: Crowns • Veneers • Bridges • Implants • Dentures • Aesthetic Reconstructive Dentistry

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cium in root dentin, while sodium hypochlorite creates a proteolytic reaction in the organic substrate. Dentin microhardness and elastic-ity varied within the tooth and between peritubular and intertu-bular dentin. But microhardness values remained the same in both nonvital dentin and vital dentin, suggesting that nonvital dentin is neither weaker nor more brittle than vital dentin. The loss of tis-sue in endodontic treatment had a major effect on fracture resistance and tooth stiffness.

The choice of material for posts yielded mixed results. Resin-fiber posts provided a superior resistance to fracture than did ceramic posts; while gold posts provided a superior resistance to fracture than did resin-fiber posts, more harmful fractures occurred in teeth with metal posts. However, the use of a crown in the presence of a ferrule effect reduced the influence of post material on fracture resistance and severity.

Comment

The choice of post used—even the choice of whether or not to use a post—had less effect on stress con-centration and fracture resistance than was thought. In stead, the presence of the ferrule effect had the greatest influence on a success-ful restoration.

Dietschi D, Duc O, Krejci I, Sadan A. Bio-mechanical considerations for the restora-tion of endodontically treated teeth: a sys-tematic review of the literature—part 1. Composition and micro- and macro-structure alterations. Quintessence Int 2007;38:733-743.

Establishing The Proper Ferrule Length

The ideal length of a ferrule—a metal band or ring used to fit the root or crown of a tooth—

is a matter of some controversy. One important consideration is the fatigue life—the number of load cycles required to bring about pre-liminary tooth failure. Preliminary tooth failure is the point at which a failure of the luting cement results in microleakage between the tooth and crown, which can lead to

➤ caries

➤ retention loss

➤ post fracture

➤ restoration dislodgement

Libman and Nicholls from the Uni-versity of Washington prepared the canal space of 20 extracted human maxillary central inci-sors. After insertion of a custom post and core restoration 6 mm in height, preparation finish lines were located at 0.5 mm, 1 mm, 1.5 mm and 2 mm apical to the core. The teeth were then restored with complete cast crowns. An ad -ditional 5 incisors restored with

cast crowns only (no posts and cores) served as controls.

All 25 teeth were loaded into a fa -tigue loading device where they were subjected to 72 cycles per min ute with a 4-kg load applied at a 135° angle until preliminary failure, defined as a crack in or around the luting cement initiated at the lingual margin of the crown and spreading up the lingual axial wall. Failure cracks in the cement layer were verified by placing a drop of water on the lingual mar-gin and applying intermittent pres-sure to the crown.

The results were subjected to a 1-way analysis of variance (ANOVA) test. The teeth with 0.5-mm and 1-mm ferrules failed significantly faster than did the teeth with 1.5-mm and 2-mm ferrules (Table 1).

Comment

This study suggested that a ferrule should be at least 1.5 mm in length to avoid failure of the restoration. Teeth that present with a ferrule <1.5 mm may need to undergo peri -odontal crown lengthening or ortho-dontic extrusion.

Libman WJ, Nicholls JI. Load fatigue of teeth restored with cast posts and cores and complete crowns. Int J Prosthodont 1995;8:155-161.

Importance of the Ferrule Effect in Restorations(continued from front page)

Table 1. Number of cycles to preliminary failure

Group Ferrule length (mm) Mean ± SD

1 0.5 113 ± 83

2 1.0 1140 ± 665

3 1.5 71,651 ± 53,590

4 2.0 60,045 ± 26,604

Control — 91,208 ± 49,891

Results in groups 1 and 2 were significantly different from the results in the control group and in groups 3 and 4 (p ≤ .05). SD, standard deviation.

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Ferrule Length And Stress Distribution

Most evidence points to fer-rules’ having a beneficial effect on the fracture resis-

tance of endodontically treated teeth. Unfortunately, a significant variation concerning the ideal height for a ferrule exists among studies. A ferrule reduces the stress levels of the cervical region where the tooth is exposed to the high-est stress. But evidence has failed to confirm beyond doubt that a ferrule has a significant influence on the survival of endodontically treated teeth.

Juloski et al from the University of Siena, Italy, used finite element analysis to evaluate the effect of ferrule height on stress distribu-tion in maxillary first premolars restored using adhesively luted glass fiber-reinforced resin posts and ceramic crowns. Four solid models of a maxillary first premo-lar, the periodontal ligament and the corresponding alveolar bone process simulated teeth with fer-rule heights of 0, 1, 2 and 3 mm. After being validated by compari-son with an in vivo premolar, the models were subjected to a force of 200 N applied to the buccal cusp at a 45° angle to the longitudinal axis, which simulated a character-istic maxillary first premolar load-ing condition.

The adhesive interfaces were the weakest places in the models. The model without a ferrule dis-played the greatest tensile stress at the abutment–root interface,

approaching the tensile strength of the abutment material. The models with a ferrule showed less tensile stress than did the tensile strength of the interface, while the stress de creased further as the length of the ferrule increased.

Comment

Based on this finite element analy -sis, the presence of a ferrule should reduce the probability of clinical failure in endodontically treated teeth with regard to tensile stress. More over, teeth restored with longer ferrules exhibit the least amount of tensile stress and should, therefore, be less likely to fail.

Juloski J, Apicella D, Ferrari M. The effect of ferrule height on stress distribution within a tooth restored with fibre posts and ceramic crown: a finite element analy-sis. Dent Mater 2014;30:1304-1315.

Ferrules in Teeth Restored With Fiber Posts

For the restoration of endodonti-cally treated teeth, fiber posts have several advantages over

cast metal posts, including better stress distribution, fewer biological side effects and a more pleasing esthetic appearance underneath the restoration. But most studies that have analyzed the ferrule effect have used cast posts and cores in the teeth. Samran et al from Christian-Albrechts University at Kiel, Germany, examined the frac-ture resistance of endodontically treated premolars restored with glass-fiber posts and various fer-rule heights.

The root canals of 80 mandibular premolars were prepared for end-odontic treatment, then assigned to 1 of 5 groups: no ferrule, 0.5-mm ferrule, 1-mm ferrule, 1.5-mm fer-rule or 2-mm ferrule. Each group was then divided into 2 subgroups: 1 residual facial wall or 2 residual facial and lingual walls. The teeth were restored with glass-fiber posts luted with adhesive composite-resin cement and cast crowns. After undergoing masticatory loading simulation with a nominal load of 5 kg for 1.2 million cycles, and thermocycling at 5°C to 55°C for 6499 cycles, the teeth were quasi-statically loaded until fracture.

Fracture resistance increased with increasing ferrule size and with the existence of a second residual wall. Two-way analysis of variance (ANOVA) showed that both ferrule height (p ≤≤ .001) and number of residual walls (p = .006) individu-ally had a significant influence on fracture resistance, although there was no statistically signifi cant in -teraction between the 2 var iables. Only 5 teeth, and ≤1 tooth in each group, fractured in a catastrophic mode (in the middle or apical third); the remaining 75 teeth frac -tured in a favorable mode (in the cervical third).

Comment

The study results indicated that the use of a ferrule in teeth restored with glass-fiber posts provides a benefit similar to that of teeth re -stored with cast posts.

Samran A, El Bahra S, Kern M. The influ-ence of substance loss and ferrule height on the fracture resistance of endodonti-cally treated premolars. An in vitro study. Dent Mater 2013;29:1280-1286.

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Restoring a Tooth with a Nonuniform Ferrule

More endodontically treated teeth fail due to prosthetic reasons than to biologic

reasons. Restoration with crowns has proven to increase the success of restorations in teeth with extensive work. Tan et al from the University of Iowa investigated whether unifor-mity of a ferrule was necessary, or if a nonuniform ferrule configuration would provide equivalent results. They divided 50 intact maxillary central incisors into 5 groups:

➤ CRN group: teeth restored with crowns

➤ RCT/CRN group: endodonti-cally treated teeth restored with crowns

➤ 2 FRL group: endodontically treated teeth restored with cast dowels and cores, and crowns incorporating a 2-mm ferrule

➤ 0.5/2 FRL group: endodon-tically treated teeth restored with cast dowels and cores, and crowns incorporating a ferrule of nonuniform height ranging from 0.5 mm proximal to 2 mm mid-facial and midlingual

➤ 0 FRL group: endodontically treated teeth restored with cast dowels and cores, and crowns without a ferrule

Teeth in the RCT/CRN, 2 FRL, 0.5/2 FRL and 0 FRL groups under-went standard endodontic treat-ment and cast crowns. After being

mounted on a universal testing machine, the teeth received an increasing load until they failed, with failure defined as a 25% drop in the applied load. Results were compared using a 1-way analysis of variance (ANOVA) to detect group differences and pairwise comparisons among groups with the Tukey adjustment.

Teeth in the 2 FRL group fractured at 587 ± 110 N, while teeth in the 0.5/2 FRL group fractured at 427 ± 88 N; both values are >2 standard deviations beyond the maximal occluding force exerted on these teeth. Teeth in the 0 FRL group failed at 265 ± 78 N, which sug-gests that they would be vulnerable under maximal stress levels.

The 1-way ANOVA comparison found significant differences among the groups (p < .0001). The Tukey analysis found that the groups sorted into 3 clusters: Groups CRN, RCT/CRN and 2 FRL demonstrated more fracture resis-tance than did group 0.5/2 FRL (p < .05), which in turn demon-strated more fracture resistance than did group 0 FRL (p < .01; Figure 1).

Comment

Teeth with a uniform ferrule out-performed those with a nonuniform ferrule. Both significantly outper-formed teeth restored with crowns and cast dowels and cores without a ferrule. The results of this study suggested that a ferrule should be employed in these restorations, even if a perfectly uniform one can-not be achieved.

Tan PLB, Aquilino SA, Gratton DG, et al. In vitro fracture resistance of endodontically treated central incisors with varying ferrule heights and configurations. J Prosthet Dent 2005;93:331-336.

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Figure 1. Load at failure and Tukey groupings for experimental groups

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In the Next Issue

Our next report features a discussion of these issues and the studies that analyze them, as well as other articles exploring topics of vital interest to you as a practitioner.