ferrule effect: a literature review

9
Ferrule Effect: A Literature Review Jelena Juloski, DDS,* Ivana Radovic, DDS, MSc, PhD, Cecilia Goracci, DDS, MSc, PhD,* Zoran R. Vulicevic, DDS, MSc, PhD, and Marco Ferrari, MD, DDS, PhD* Abstract Introduction: Preserving intact coronal and radicular tooth structure, especially maintaining cervical tissue to create a ferrule effect, is considered to be crucial for the optimal biomechanical behavior of restored teeth. The ferrule effect has been extensively studied and still remains controversial from many perspectives. The purpose of this study was to summarize the results of research conducted on different issues related to the ferrule effect and published in peer-reviewed journals listed in PubMed. Methods: The search was conducted using the following key words: ‘‘ferrule’’ and ‘‘ferrule effect’’ alone or in combination with ‘‘literature review,’’ ‘‘fracture resistance,’’ ‘‘fatigue,’’ ‘‘finite element anal- ysis,’’ and ‘‘clinical trials.’’ Results: The findings from re- viewed articles were categorized into three main categories: laboratory studies, computer simulation, and clinical trials. Laboratory studies were further classi- fied into subchapters based on the main aspect investi- gated in relation to the ferrule effect. Conclusions: The presence of a 1.5- to 2-mm ferrule has a positive effect on fracture resistance of endodontically treated teeth. If the clinical situation does not permit a circumferential ferrule, an incomplete ferrule is considered a better option than a complete lack of ferrule. Including a ferrule in preparation design could lead to more favorable frac- ture patters. Providing an adequate ferrule lowers the impact of the post and core system, luting agents, and the final restoration on tooth performance. In teeth with no coronal structure, in order to provide a ferrule, orthodontic extrusion should be considered rather than surgical crown lengthening. If neither of the alter- native methods for providing a ferrule can be performed, available evidence suggests that a poor clinical outcome is very likely. (J Endod 2012;38:11–19) Key Words Endodontically treated teeth, ferrule effect, post and core, review T he restoration of endodontically treated teeth involves a variety of treatment options and still represents a challenging task for clinicians. Whether a tooth requires a post or not is determined by the amount of remaining coronal tooth struc- ture and the functional requirements (1). Frequently, the remaining tooth structure is not sufficient, and a post is indicated to provide retention for crown restoration (1– 3). Cast post and core systems were the standard for many years. However, demands for simpler procedures and esthetic restorations led to the development of prefabricated posts, initially made from metal and more recently from ceramics and fiber-reinforced composites (FRCs) (1, 4, 5). A successful clinical outcome of endodontically treated teeth depends on adequate root canal treatment as well as on adequate restorative treatment per- formed afterwards (6). Therefore, research has focused on finding the most prom- ising post and core system (7–9), luting agent (10–12), and crown type (13, 14). Even though a strengthening effect of the post is desirable and very often needed, it was questioned in the literature (1). Despite extreme efforts to reinforce endodon- tically treated teeth, biomechanical failures still represent a critical issue (4, 15). The lack of a protective feedback mechanism after pulp removal may be a contributing factor to frequent tooth fractures (16). However, beside noncontrol- lable risk factors, the high occurrence of fractures may be attributed to various operative procedures, such as access cavity preparation and restoration, root canal preparation, irrigation and obturation, post space preparation, and final coronal restoration (15, 17). Preserving intact coronal and radicular tooth structure and maintaining cervical tissue to create a ferrule effect are considered to be crucial to optimize the biomechan- ical behavior of the restored tooth (17, 18). A ferrule effect is defined 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’’ (19). More precisely, parallel walls of dentin extending coronally from the crown margin provide a ‘‘ferrule,’’ which after being encircled by a crown provides a protective effect by reducing stresses within a tooth called the ‘‘ferrule effect’’ (20) (Fig. 1). Growing attention has been given to the prognosis of endodontically treated teeth, and, generally, the ferrule effect is considered necessary to stabilize restored tooth (1, 3, 17, 21–24). Then again, the cost of getting this support in teeth with no coronal dentin is the additional loss of tooth tissue, and although a ferrule would be desirable, it should not be provided at the expense of the remaining coronal or root structure (25, 26). However, it is important to bear in mind that a ferrule effect is just one part of the restored endodontically treated tooth that represents a complex system. The clinical performance of the entire complex is also affected by several other factors including the post and core material, the luting agent, the overlying crown, and functional occlusal loads From the *Department of Fixed Prosthodontics and Dental Materials of Siena, Tuscan School of Dental Medicine, University of Florence and Siena, Siena, Italy; and Clinic for Pediatric and Preventive Dentistry, Faculty of Dentistry, University of Belgrade, Belgrade, Serbia. Address requests for reprints to Dr Jelena Juloski, Department of Fixed Prosthodontics and Dental Materials of Siena, Policlinico Le Scotte, Viale Bracci, 53 100 Siena, Italy. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2012 American Association of Endodontists. doi:10.1016/j.joen.2011.09.024 Review Article JOE Volume 38, Number 1, January 2012 Ferrule Effect 11

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Review Article

Ferrule Effect: A Literature ReviewJelena Juloski, DDS,* Ivana Radovic, DDS, MSc, PhD,† Cecilia Goracci, DDS, MSc, PhD,*Zoran R. Vulicevic, DDS, MSc, PhD,† and Marco Ferrari, MD, DDS, PhD*

Abstract

Introduction: Preserving intact coronal and radiculartooth structure, especially maintaining cervical tissueto create a ferrule effect, is considered to be crucialfor the optimal biomechanical behavior of restoredteeth. The ferrule effect has been extensively studiedand still remains controversial from many perspectives.The purpose of this study was to summarize the resultsof research conducted on different issues related to theferrule effect and published in peer-reviewed journalslisted in PubMed. Methods: The search was conductedusing the following key words: ‘‘ferrule’’ and ‘‘ferruleeffect’’ alone or in combination with ‘‘literature review,’’‘‘fracture resistance,’’ ‘‘fatigue,’’ ‘‘finite element anal-ysis,’’ and ‘‘clinical trials.’’ Results: The findings from re-viewed articles were categorized into three maincategories: laboratory studies, computer simulation,and clinical trials. Laboratory studies were further classi-fied into subchapters based on the main aspect investi-gated in relation to the ferrule effect. Conclusions: Thepresence of a 1.5- to 2-mm ferrule has a positive effecton fracture resistance of endodontically treated teeth. Ifthe clinical situation does not permit a circumferentialferrule, an incomplete ferrule is considered a betteroption than a complete lack of ferrule. Including a ferrulein preparation design could lead to more favorable frac-ture patters. Providing an adequate ferrule lowers theimpact of the post and core system, luting agents, andthe final restoration on tooth performance. In teethwith no coronal structure, in order to provide a ferrule,orthodontic extrusion should be considered ratherthan surgical crown lengthening. If neither of the alter-native methods for providing a ferrule can be performed,available evidence suggests that a poor clinical outcomeis very likely. (J Endod 2012;38:11–19)

Key WordsEndodontically treated teeth, ferrule effect, post andcore, review

From the *Department of Fixed Prosthodontics and Dental Mate†Clinic for Pediatric and Preventive Dentistry, Faculty of Dentistry, U

Address requests for reprints to Dr Jelena Juloski, Department ofItaly. E-mail address: [email protected]/$ - see front matter

Copyright ª 2012 American Association of Endodontists.doi:10.1016/j.joen.2011.09.024

JOE — Volume 38, Number 1, January 2012

The restoration of endodontically treated teeth involves a variety of treatmentoptions and still represents a challenging task for clinicians. Whether a tooth

requires a post or not is determined by the amount of remaining coronal tooth struc-ture and the functional requirements (1). Frequently, the remaining tooth structure isnot sufficient, and a post is indicated to provide retention for crown restoration (1–3). Cast post and core systems were the standard for many years. However, demandsfor simpler procedures and esthetic restorations led to the development ofprefabricated posts, initially made from metal and more recently from ceramicsand fiber-reinforced composites (FRCs) (1, 4, 5).

A successful clinical outcome of endodontically treated teeth depends onadequate root canal treatment as well as on adequate restorative treatment per-formed afterwards (6). Therefore, research has focused on finding the most prom-ising post and core system (7–9), luting agent (10–12), and crown type (13, 14).Even though a strengthening effect of the post is desirable and very often needed, itwas questioned in the literature (1). Despite extreme efforts to reinforce endodon-tically treated teeth, biomechanical failures still represent a critical issue (4, 15).The lack of a protective feedback mechanism after pulp removal may bea contributing factor to frequent tooth fractures (16). However, beside noncontrol-lable risk factors, the high occurrence of fractures may be attributed to variousoperative procedures, such as access cavity preparation and restoration, root canalpreparation, irrigation and obturation, post space preparation, and final coronalrestoration (15, 17).

Preserving intact coronal and radicular tooth structure and maintaining cervicaltissue to create a ferrule effect are considered to be crucial to optimize the biomechan-ical behavior of the restored tooth (17, 18). A ferrule effect is defined as a ‘‘360� metalcollar of the crown surrounding the parallel walls of the dentine extending coronal tothe shoulder of the preparation. The result is an elevation in resistance form of thecrown from the extension of dentinal tooth structure’’ (19). More precisely, parallelwalls of dentin extending coronally from the crown margin provide a ‘‘ferrule,’’ whichafter being encircled by a crown provides a protective effect by reducing stresses withina tooth called the ‘‘ferrule effect’’ (20) (Fig. 1).

Growing attention has been given to the prognosis of endodontically treatedteeth, and, generally, the ferrule effect is considered necessary to stabilize restoredtooth (1, 3, 17, 21–24). Then again, the cost of getting this support in teeth withno coronal dentin is the additional loss of tooth tissue, and although a ferrulewould be desirable, it should not be provided at the expense of the remainingcoronal or root structure (25, 26). However, it is important to bear in mindthat a ferrule effect is just one part of the restored endodontically treated tooththat represents a complex system. The clinical performance of the entirecomplex is also affected by several other factors including the post and corematerial, the luting agent, the overlying crown, and functional occlusal loads

rials of Siena, Tuscan School of Dental Medicine, University of Florence and Siena, Siena, Italy; andniversity of Belgrade, Belgrade, Serbia.Fixed Prosthodontics and Dental Materials of Siena, Policlinico Le Scotte, Viale Bracci, 53 100 Siena,

Ferrule Effect 11

Figure 1. A schematic drawing of endodontically treated tooth restored withpost and core system and a crown. Co, core; Cr, crown; F, dentin extendingcoronal from crown margin providing a ferrule; G, remaining gutta-percha;P, post.

Review Article

(22). This topic has been extensively studied and yet remainscontroversial from many perspectives. Therefore, this literaturereview aimed at summarizing research conducted on different issuesrelated to the ferrule effect.

Materials and MethodsA literature search was conducted based on the following criteria:

articles retrieved in PubMed using the following key words: ‘‘ferrule’’and ‘‘ferrule effect’’ alone or in combination with ‘‘literature review,’’‘‘fracture resistance,’’ ‘‘fatigue,’’ ‘‘finite element analysis,’’ and ‘‘clinicaltrials’’; English language; and publication date range from 2000 to2011. This search strategy identified 59 articles. Articles that werenot found to be significant to the subject of the review based on theabstract were excluded. Conversely, noteworthy articles cited in theselected pertinent articles were included as well as some of the classicliterature. Finally, the web sites of relevant dental journals wereexplored in the search for in press papers. Sixty-two articles, 13 reviewand 49 original articles, were found to meet the mentioned inclusioncriteria and became the object of the present review.

ResultsThe findings from all articles were categorized into three main

categories: laboratory studies, computer simulation (finite elementanalysis), and clinical trials.

Laboratory StudiesThe majority of published articles are laboratory studies that used

static load tests to simulate single high-force application that might

12 Juloski et al.

occur with trauma or as a result of parafunctional habits. All reviewedarticles used single-rooted human or bovine teeth and constant loadwas applied at an angle to the long axis until fracture of the specimenoccurred. Maximum loads that teeth could withstand and fracturepatterns were analyzed and compared. However, the clinical signifi-cance of results obtained from in vitro static testing has been ques-tioned (27–29) because a monostatic load does not represent theclinical situation in which repetitive mechanical loading and thermalchanges are characteristics. Therefore, in order to acquire moreclinically relevant data with regards to the ferrule effect, in some ofthe reviewed studies, mechanical, thermal, or thermomechanicalfatigue testing were performed, and endurance of the specimens wasmeasured.

Results of all laboratory studies that investigated the ferrule effectwere further classified into following subchapters based on the mainaspect that was investigated in relation to the ferrule effect: ferruleheight/ferrule design, type of post and core system and final restoration,post length, luting techniques, and crown lengthening versus ortho-dontic extrusion (Table 1).

Ferrule Height/Ferrule Design. The necessary ferrule heightrequired to provide the protective effect was frequently investigated(19, 29–35). When crowned bovine teeth were subjected to cyclingloading until the crown or post was dislodged or the post or rootfractured, the resistance increased significantly with an increasingferrule height (34). Similarly, the number of load cycles required toinduce failure in cement layer (preliminary failure) was higher forhuman teeth with higher ferrule (29, 33). Teeth with no ferrulesurvived only few fatigue cycles, and 0.5-mm and 1-mm ferrule groupsshowed a significantly higher number of cycles before failure (29). Eventhough between these two groups there was no statistically significantdifference, the authors recommended a 1-mm-high ferrule becauseof a large standard deviation in the 0.5-mm ferrule test group. Onemilli-meter of coronal dentin was also found to be enough to significantlyincrease the fracture strength when exposed to static loading (19).On the contrary, two studies reported no statistically significant differ-ence in failure loads between teeth with a 1-mm ferrule and those withno remaining coronal tooth structure (30, 31). In another study, teethwith a 0.5-mm and 1-mm ferrule failed at a significantly lower numberof load cycles than the 1.5-mm and 2-mm ferrule groups (33). Ferruleheights required to significantly improve fracture resistance whenexposed to static loading were 2 mm (31, 32) and 3 mm (30). Addi-tionally, during chewing simulation, teeth with a 1-mm ferrule hada higher percentage of preliminary failure and lower ultimate loadsbefore fracture when compared with a 2-mm high design (35). Themode of fracture was highly dependent on the ferrule height: for resto-rations with a ferrule height of a 1-mm post/core/crown complex wasusually decemented, whereas for teeth with 2-mm ferrule fracturesstarting on the remote side were most common (35).

Moreover, because the clinical situation does not always permitpreparation of the circumferential ferrule of uniform height, the effectof incomplete ferrule on tooth resistance has been studied (36–38). Aferrule of nonuniform height, varying between 0.5-mm proximal and 2-mm buccal and lingual, was less effective in preventing failure understatic loading than a uniform 2-mm ferrule (36). Similarly, differencesin resistance after thermomechanical loading were present when a 360�circumferential 2-mm ferrule was compared with a 2-mm ferrulepresent just on the palatal or facial aspect or with the ferrule interruptedby biproximal cavitation (38). Yet, one study reported no change inresistance between teeth with a uniform ferrule and a ferrule that incor-porated only the buccal and/or lingual wall (37). Nonetheless, teethwith a nonuniform ferrule length were still more fracture resistant

JOE — Volume 38, Number 1, January 2012

TABLE 1. A List of the Reviewed Laboratory Studies According to the Main Aspects Investigated in Each Study

Typeof test

Ferrule height/ferrule design

Type of post andcore system andfinal restoration Post length

Lutingtechniques

Crown lengtheningvs orthodontic

extrusion

Static loadtesting

Sorensen andEngelman (19)

Pereira et al (30)Cho et al (31)Akkayan (32)Tan et al (36)Dikbas (37)Kutesa-Mutebiand Osman (39)

al-Hazaimeh andGutteridge (55)

Ng et al (54)

Zhang et al (40)da Silva et al (13)Pereira et al (30)Pereira et al (41)Zhi-Yue and

Yu-Xing (42)Sendhilnathan

and Nayar (43)Akkayan (32)Massa et al (44)Qing et al (45)Lima et al (46)

Nissan et al (59) al-Hazaimeh andGutteridge (55)

Mezzomo et al (53)

Gegauff (66)Meng et al (70)Meng et al (69)

Fatiguetesting

Ma et al (29)Isidor et al (34)Libman andNicholls (33)

Schmitter et al (35)Naumann et al (38)

Xible et al (47)Hu et al (48)Heydecke et al (49)Butz et al (50)Sahafi et al (51)Naumann et al (52)Dorriz et al (56)

Schmitter et al (35)Isidor et al (34)Buttel et al (60)

Dorriz et al (56)Goto et al (28)Naumann et al (61)Aykent et al (63)Hsu et al (62)Schmitter et al (65)Schmitter et al (35)Junge et al (64)

Review Article

then teeth without a ferrule (36, 37), and, therefore, it was suggestedthat a 2-mm ferrule should be provided at least on the buccal andlingual wall. Irrespective of the ferrule design, the predominant modeof failure was an oblique fracture from the lingual margin to the vestib-ular tooth surface (36).

Few studies assessed the impact of particular ferrule design ontooth resistance (19, 31, 39). The inclusion of interproximal grooveswhen there is a 1-mm or more coronal dentinal extension decreasedthe failure loads and therefore should be avoided. Furthermore, witha 1-mm or 2-mm ferrule preparation, there was no difference in frac-ture resistance regardless of whether a butt joint or a 1-mm-wide con-trabevel configuration existed at the tooth-core junction (19, 39).

Type of Post and Core System and Final Restoration. Theeffectiveness of different procedures for restoring the function ofendodontically treated teeth subjected to static (13, 30, 32, 40–46)or fatigue loading (47–52) has been widely investigated.Combinations of different ferrule designs and various post and coresystems and final restorations were tested in an attempt to find thebest treatment option that would provide the highest fractureresistance and the most favorable fracture pattern of severelycompromised teeth. However, the results were controversial. Someresearch reported that changes in stiffness and design associated withpost and core material did not significantly influence fractureresistance as long as sufficient dentine structure remained (13, 41,47, 49, 52), whereas other studies pointed out that post type mayhave an impact (32, 42–45, 50).

The vast majority of literature data suggests that the presence ofa ferrule is a significant factor in improving resistance to fracture fordifferent types of posts: cast post and core (13, 33, 36, 40–43, 48,53), all ceramic post and core (40), prefabricated metallic posts (30,34, 41), and prefabricated fiber-reinforced composite (FRC) posts(13, 29, 46, 48, 54). Conversely, two studies reported that theadditional use of ferrule preparation had no benefit in terms ofresistance to fracture of teeth restored with cast post and core(43) and prefabricated metallic post (55).

When teeth with a 2-mm ferrule were restored with a custom-fabricated all-ceramic or metal post and core, resistance to fracturewas similar (40). Metal cast post-and-core-restored teeth were more

JOE — Volume 38, Number 1, January 2012

resistant to fracture than teeth with prefabricated posts when the ferruleeffect was not provided (30, 41–43) as well as in the presence of a 2-mm ferrule (42, 43, 45). On the contrary, in the presence of a ferrule,teeth restored with prefabricated titanium posts and composite coreshad higher fracture resistance compared with cast post and core andprefabricated quartz-FRC posts (44). Still, for a sudden impact, castpost and core was more effective, but under cyclic fatigue testing teethrestored with carbon-FRC posts outperformed cast post and cores (48).Furthermore, among the 1-mm, 1.5-mm, and 2-mm ferrule lengthspecimens, higher fracture loads were observed in teeth restoredwith a quartz-FRC post compared with teeth restored with glass-FRC,zirconia enriched glass–FRC, and zirconia posts (32).

After subjecting the specimens only to mechanical loading, therewas no difference in the survival rate (100%) among teeth with a 1.5-mm ferrule restored with zirconia, FRC, or titanium post (47). Aftercycling loading, specimens were exposed to an oblique, static loaduntil failure, and, likewise, all systems showed statistically equivalentfracture strength values and modes of failure. However, when speci-mens were subjected to thermomechanical fatigue loading, there wasa lower percentage of survival (49, 50). Both studies (49, 50)investigated the following combinations of materials in the presenceof a 2-mm-high dentine ferrule and under the same fatigue condi-tions: titanium post and composite core, zirconia post and ceramiccore, zirconia post and composite core, and cast gold post andcore system. Heydecke et al (49) found no statistically significantdifferences in survival rates, fracture loads, and modes of failureamong groups, but all ceramic posts and cores were recommendedas an esthetic alternative to cast post and cores because all specimensremained intact during thermomechanical loading, the higher fracturestrengths, and the number of restorable fractures observed amongspecimens from that group. Conversely, Butz et al (50) found a lowersurvival rate and fracture strength for ceramic post/composite corecombination and suggested that this should be avoided in clinical use.

With regard to fracture modes, the findings were also controver-sial. Some studies reported that the direct technique was appropriatebecause more severe root fractures were found in cast post-and-core-restored teeth (30, 41, 56); others did not observe differencesin fracture pattern between teeth restored with cast post and core

Ferrule Effect 13

Review Article

or prefabricated metallic or FRC posts (13, 40, 45, 48). Also, nodifference was present between ferruled teeth restored with differenttypes of FRC posts (32). However, among teeth with prefabricated metalposts and composite cores, with no ferrule, and a 1-mm to 3-mmferrule, different fracture patterns were present; nonferruled specimensfailed because of core fracture and ferruled teeth because of cementa-tion failure (30). When prefabricated posts were made from quartz-FRC, teeth without a ferrule mainly failed as a result of debonding,and in the presence of a ferrule, the most common mode of failurewas root fracture (54).

When a ferrule was provided, several studies reported that theplacement of a post did not significantly improve fracture resistancecompared with teeth restored only with composite core and no canalretention (39, 41, 44, 46). However, leaving the specimens empty,without post and without core buildup, regardless of the presence ofa 2-mm-high ferrule led to the lowest fracture resistance, suggestingthat a ferrule is only one key element in the complex (52). A significantincrease in load capability was observed when only composite core wasplaced, which was contributed to an increase in retentive surface towithstand static loading (52).

Furthermore, when a ferrule was not provided, fracture resistanceto static loading of teeth restored with posts was comparable with teethwith composite buildups alone (46). Then again, a positive impact ofpost placement on fracture resistance was found (41). In terms of resis-tance to cyclic loading, no difference was found between teeth restoredonly with a composite core without post and teeth restored with post andcore systems, irrespective of the type of post (51, 52). On the otherhand, the shape of the post did have an impact: parallel-sided cast postsshowed a higher resistance than the ones restored with tapered posts(51). Surface treatment of prefabricated posts (ie, titanium, glass-FRC, and zirconium), which consisted in air abrasion, followed bysilane coating significantly increased the resistance (51). However, itwas anticipated that the presence of a ferrule may have resulted inhigher resistance and overshadowed the effect of post-related factorsthat could have given different results.

Regarding the failure pattern, specimens that were left empty hadmore restorable than catastrophic fractures, whereas the group withonly core buildup had an equal number of both types of failure (52).Specimens restored with posts and cores hadmore catastrophic failureswhen a ferrule was present, irrespective of post material, but when anFRC post was used, less catastrophic failures were observed comparedwith titanium posts (52).

Only one study aimed to investigate the impact of different crowntypes and the ferrule effect on tooth deformation and fracture resistance(13). The effect of alumina-reinforced ceramic and metal crowns wasassessed, and it was reported that mechanical behavior is primarilydetermined by the presence of a ferrule and crown type and the effectof post and core system was considered secondary (13). The use ofa ceramic crown overcame the disadvantage of ferrule absence, anda ceramic crown with glass-FRC post was recommended as the bestchoice for restoring endodontically treated incisors. When a ferrulecannot be provided, authors recommended metal crowns and castpost and cores (13).

Post Length. As stated earlier, the preservation of sound root struc-ture is of utmost importance. Minimizing the loss of root dentin tissueduring post space preparation can be realized by using reduced-lengthposts. Besides, it could ensure the preservation of 4 to 5 mm of gutta-percha, which is necessary to provide a reliable apical seal (57, 58).

When a 2-mm ferrule was designed and prefabricated metal postswere luted with a titanium-reinforced composite, the resistance to frac-ture under static load was unaffected by the post length (59). Besides,

14 Juloski et al.

only fractures of teeth with reduced-length posts were consideredrestorable (59). Likewise, the post length had no significant influenceon failure during chewing stimulation or on ultimate loads when humanpremolars with a 1-mm or 2-mm ferrule were restored with 2-mm and7-mm long glass-FRC posts (35).

No difference in fracture resistance occurred when bovine teethwith no ferrule, a 1.25-mm ferrule, and a 2.5-mm ferrule were restoredwith 5-mm, 7.5-mm, and 10-mm long prefabricated titanium posts andsubjected to cyclic mechanical loading (34). Conversely, the perfor-mance of teeth without a ferrule restored with an FRC post and directcomposite crown was different when different post lengths were used(60). After thermomechanical loading, specimens restored with a 3-mm post length yielded significantly lower fracture values than witha post inserted 6 mm.

Luting TechniquesPost luting. Two studies reported that the additional use of ferrulepreparation had no benefit in terms of the resistance to fracture whenresin cement was used for post cementation (53, 55). However, whenconventional zinc-phosphate cement was used, the presence of a ferruledid provide improvement in metal cast post-and-core-restored teeth(53). Although there was no difference in fracture strength betweenthe ferrule and nonferrule groups, the modes of failure differed. Teethwith a ferrule underwent a mostly oblique fracture, whereas in teethwithout a ferrule vertical root fracture was the predominant failuremode (55). Moreover, a higher resistance to the preliminary failureof teeth with a 1.5-mm ferrule was shown when resin cement wasused for luting an FRC post in comparison with cast post and core lutedwith conventional zinc-phosphate cement (28). In the same study, teethwith titanium posts luted with zinc-phosphate cement and adhesivelybonded composite cores showed an intermediate level of resistance.It was concluded that a stronger union between the post and core es-tablished using adhesive bonding may significantly influence crowncement failure (28). Bonding metal cast posts to the tooth structureby the application of an opaque porcelain layer to post surface beforecementation with resin cement increased the survival rate and fracturestrength and compensated for the lack of a ferrule (56). In terms offracture patterns, more favorable failure modes were seen for teethrestored with a glass-FRC post and composite cores than for teethrestored with cast post and core (56).

One study evaluated the performance of teeth with excessivelyflared root canals restored with FRC posts luted with resin cements(61). Results showed that, after a simulated 5-year period of serviceby thermomechanical loading, flared unferruled teeth did not exhibitload capability that could be recommended for clinical use when lutedwith self-adhesive resin cement. However, when ferrule existed andposts were adhesively luted, irrespective of the type of resin cement(self-adhesive or in combination with an adhesive), clinically acceptablemean values for the load capability were revealed. Another two studiesused the same self-adhesive resin cement for luting FRC and metal postsand reported similar fracture resistance in the presence of ferrule,regardless of the post type (13, 52). Within the no ferrule groups,fracture resistance was unaffected by the post type (13), but also higherload values were measured for metallic post compared with FRC (52).

Composite cores that were bonded to a tooth structure with a 1-mm ferrule compared with nonbonded composite cores providedsignificantly stronger crown retention under fatigue loading (62).Also, the bonding of silver amalgam cores significantly improved thefracture resistance of teeth previously subjected to thermal cyclingonly when a ferrule was not present, whereas in the presence of a 2-mm-high ferrule, there was no difference in bonded and nonbondedcore groups (63).

JOE — Volume 38, Number 1, January 2012

Review Article

Crown luting. It was reported by Ma et al (29) that using resincement to lute crowns in their study resulted in a higher cycle countcompared with zinc-phosphate cement used by Libman and Nicholls(33). These conclusions are consistent with findings from otherstudies (35, 64, 65). When the ferrule was present, the resin lutingagent for bonding crowns increased the number of cycles needed tocause premature failure (35, 64) and the ultimate loads that thecomplex could withstand afterwards (35, 65) in comparison witha glass-ionomer cement. A higher bond strength of resin-bondedrestorations and the ability to transfer tensile stresses at crown/dentineinterface were the possible explanations of the results (35). No signif-icant differences between zinc-phosphate and resin-modified glass-ionomer cement were found (64). The preparation design had animportant effect on fracture resistance when crowns were cementedwith glass-ionomer cement (ie, more retentive preparation and greaterwall thickness led to the greater fracture resistance), whereas it didnot influence fracture strength when crowns were cemented with resincement (65).

Crown Lengthening versus Orthodontic Extrusion. Therestoration of severely damaged teeth, with no clinical crown present,represents a particularly important issue. In such a clinical situation,an adequate ‘‘biologic width’’ and distance between crown marginand alveolar crest should be ensured (25, 66). Biological width wasdefined as ‘‘the dimension of the junctional epithelial and connectivetissue attachment to the root above the alveolar crest’’ (67), and ithas been recommended that at least 3 mm between the crown marginand the alveolar crest should be left to avoid impingement on thecoronal attachment of the periodontal connective tissue (68). There-fore, if a 1.5-mm ferrule is to be achieved, at least 4.5 mm of supra-alveolar tooth structure is required (25). In a situation in which enoughtooth structure does not exist, the clinician may consider two options:surgical crown lengthening or orthodontic extrusion (19, 25, 66).

The effects of apical placement of a ferrule on the resistance tostatic load failure after crown lengthening (66, 69, 70) or forcedtooth eruption (69) were investigated. Composite analogs of a premolarwithout crown lengthening and without a ferrule with the crown finishline located at the cement-enamel junction showed a significantlygreater load resistance than crown-lengthened specimens preparedwith a 2-mm ferrule and the finish line moved 2 mm apically fromthe cement-enamel junction (66). The results were in accordancewith another study that used human premolars (70). Both of the studiesrestored the specimens with cast post and cores. However, that differ-ence in fracture strength was not present between teeth with an apicallypositioned ferrule and teeth without a ferrule when restored withcarbon-FRC posts and composite cores (69, 70). Moreover, mostfailures for cast post and core systems were catastrophic, such asvertical root fractures and oblique root fractures located below thecervical third of the roots and, for prefabricated carbon-FRC posts, frac-tures were less severe and located at or above the cervical third of theroot (70). On the other hand, the presence or absence of a ferrule didnot influence the root fracture pattern, irrespective of the post and coresystem used (70).

Disadvantages of surgical crown lengthening treatment are anincrease in crown/root ratio caused by the reduced effective rootlength and the increased effective crown length and reduced volumeof root dentine that remains (66, 69, 70). An alternative approachto treatment could be an orthodontic extrusion. From the bio-mechanical point of view when orthodontic extrusion is performed,bone support reduces, but the coronal lever arm does not change.The preparation of a 1-mm ferrule after simulated forced tooth extrac-tion significantly improved the fracture strength of the tooth, and

JOE — Volume 38, Number 1, January 2012

a 2-mm ferrule design was associated with an even higher fractureresistance (69).

Computer Simulation: Finite Element AnalysisFinite element analysis (FEA) is the method that evaluates stress

distribution within complex structures and provides results withoutvariation (71). The accuracy of the results depends on the extent towhich the model approaches reality (35, 71). Because of the largevariability of the results obtained from in vitro studies, differentconcerns related to the ferrule effect have been investigated based onFEA (35, 72–74). Study designs and most important findings of eachstudy are reported in Table 2. The distribution of compressive andtensile stress (73, 74) and stress combination, known as equivalentvon Mises stresses (72), were studied on 3-dimensional models ofmonoradicular tooth under static oblique load.

It was reported that the cervical region of the tooth is exposed tothe greatest stress, irrespective of the type of coronoradicular recon-struction and preparation design (74), and that compressive stressis larger than tensile stress (73, 74). However, it was stated that thepresence of a ferrule decreases stress levels (72, 74). Furthermore,the cervical stress level seems to be lower in the presence of rootcanal post than when no post is used (74). The distribution and valuesof stress were different between glass-FRC and zirconium oxideceramic posts; an increase of the elastic modulus of the post materialincreased the dentin stress but shifted the maximum stress locationfrom the dentin surface adjacent to the post to the post material(72). In the absence of ferrule, posts with a high modulus of elasticity(metallic posts) generated greater cervical stress than a carbon-FRCpost, which represented the most significant contributory factor inincreasing the risk of fracture (74). Nonetheless, metallic post incombination with a rigid core material (cast post and core) generatedsignificantly lower tensile stress in comparison with a combination ofmetallic post and composite core, whereas in the presence of a ferrule,core material had no impact on the stress level (74).

There are two main changes in stress distribution with an increasein ferrule height: a decrease in compressive stress in labial cervicaldentine and an increase in tensile stress in palatal cervical dentinwith an increased area of palatal dentine under stress (73). Authorsalso suggested that these tensile stress values were approaching thetensile strength of dentine and produced favorable conditions fora crack to nucleate, which would run apically and obliquely alongthe palatal root dentine from the internal to external root surface.Therefore, the ferrule height should be determined individually basedon the buccolingual cervical diameter of the tooth. Moreover, higherdisplacement values of a simulated metallic crown (labial and axialrotation) were shown in teeth without a ferrule, what makes themmore prone to failure because of debonding, and subsequently verticalroot fracture through the lever action of the loose post or fracture of thepost in the cervical portion (73). However, displacement potential isreduced with an increasing ferrule height up to 1.5 mm, over whichthere was no difference (73). Additionally, this study did not assessthe influence of the cement material. Because the simulated intact stateof the dentin-crown interface when cemented with glass-ionomercement was considered analogous to the debonding state when thecrown was adhesively luted with a resin cement, it was proposed byanother study that crowns with small ferrule heights should be resinbonded (35).

FEA does not support the notion that crown lengthening may resultin a tooth with less ability to withstand load because patterns and stressvalues of tensile and compressive stress did not differ between standardand crown-lengthened models (73). With regards to post length, the

Ferrule Effect 15

TABLE 2. List of FEA Studies Investigating the Ferrule Effect, the Study Design, and the Most Important Findings

Author, year Tooth modelAmount of coronaldentin (ferrule)

Post (cement for post)/core/final restoration (cement for

final restoration) Most important findings

Pierrisnard et al,2002 (74)

Monoradiculartooth withsimulatedbone base

No ferrule2-mm ferrule

1. NiCr cast post-and-core/NiCrcrown

2. NiCr post/composite/NiCrcrown

3. Carbon-FRC/composite/NiCrcrown

4. No post/composite/NiCrcrown

Cement layers consideredinsignificant

Regardless of the model, thegreatest stress in the cervicalregion.

Post placement reduced cervicalstress.

Absence of ferrule resulted inincreased stress levels.

Ferrule eliminated effect of corematerial on cervical stress.

Ichim et al,2006 (73)

Maxillarycentralincisor withsimulatedperiodontalligament

No ferrule0.5-mm ferrule1-mm ferrule1.5-mm ferrule2-mm ferruleThickness 1 mmWith and withoutcrownlengthening

Metal cast post-and-core/metalcrown

Crown lengthening did notdecrease tooth resistance.

Displacement potential ofa crown is reduced withincreasing ferrule height.

Restorationwithout ferrule failedprimarily because ofdebonding and subsequentlyroot fracture.

Eraslan et al,2009 (72)

Maxillarycentralincisor withsimulatedperiodontalligament andalveolar bone

No ferrule1-mm ferrule2-mm ferrule

1. Glass-FRC/composite/ceramic crown

2. Zirconium oxide ceramicpost/composite/ceramiccrown

Stress values were lower for bothpost systems when ferrule waspresent.

Stress values were lower for glass-FRC post.

Increase of the elastic modulus ofthe post material increased thedentin stress.

Schmitter et al,2010 (35)

Premolar 1-mm ferrule2-mm ferrule

1. Glass-FRC: 2-mm and 7-mmlong (resin)/composite/metal crown (resin)

2. Glass-FRC: 2-mm and 7-mmlong (resin)/composite/metal crown (glass-ionomer)

Post length had a minor effect onforce distribution when ferrulewas present.

FRC posts do not contribute toload transfer as long as thebonding between the toothand composite core is intact.

FRC, fiber-reinforced composite; NiCr, nickel-chromium.

Review Article

results of FEA suggested that it has a minor effect on force distributionwhen a 1-mm or 2-mm-high ferrule is present (35), which is in agree-ment with the results from in vitro testing (34, 35, 59).

One study aimed to combine the advantages of in vitro tests andFEA (35). The results obtained from both methods were compared withagreement. In order to evaluate distribution of the external forcesamong different internal substructures, virtual models of intact teethand models of teeth with different damage levels were investigated.The results indicated that for intact restoration forces were exclusivelytransferred at the dentine-crown interface, and the post did notcontribute to load transfer until the bond between the compositecore and the dentine failed (35).

Clinical TrialsOne retrospective (75) and few prospective (76–80) clinical

studies have so far specifically addressed whether and to what extentthe presence of ferrule has an influence on the clinical behavior ofendodontically treated and restored teeth. All studies with studydesign and most important findings are reported in Table 3.

Two studies evaluated the influence of various preoperativefactors, including the ferrule height, on the clinical performance ofendodontically treated and restored teeth (75, 77). The amount oftooth structure available for a ferrule calculated retrospectively frombitewing radiographs was not the preoperative parameter significant

16 Juloski et al.

for the prognosis of restored endodontically treated molars (75). Itwas commented that judgment on a preexisting ferrule is difficult toobtain retrospectively and should be evaluated by a model study orcone-beam tomography to get a 3-dimensional rather than two-dimensional picture of the prerestorative condition. However, thefinding was in line with the results of a prospective clinical trial (77)that investigated 1-year survival of teeth restored with FRC posts and tita-nium screw posts. The short-term clinical performance of FRC postswas superior to that of titanium screw posts, but the ferrule heightdid not have an influence on the survival of neither post type. However,one should bear in mind that the mean ferrule height in the study wasmore than 3 mm in both groups, and survival rates after 1 year were93.5% and 75.6% for FRC and titanium post, respectively.

Furthermore, no impact of the post material on the clinicaloutcome after 2 years was found in the presence of a ferrule (78). Teethwere restored with either a titanium or glass-FRC post; all the teethreceived a 2-mm ferrule preparation; and, if necessary, surgical crownlengthening was performed. It was concluded that both posts can behighly successful in postendodontic restorations when used witha self-adhesive resin cement and composite core.

The effect of the remaining tooth structure on the survival of teethrestored with a cast post and core system, a direct metal post, or no postwas also studied (76). The expected dentin height remaining after toothpreparation was predicted preoperatively and categorized as substantial(a 1-mm or 2-mm ferrule could be achieved) or minimal (a ferrule

JOE — Volume 38, Number 1, January 2012

TABLE 3. The List of Clinical Trials Investigating the Ferrule Effect, the Study Design, and the Most Important Findings

Author, year

Type of study;follow-upperiod

Tooth type; amountof coronal dentin

(ferrule)

Post (cement for post)/core/finalrestoration (cement for final

restoration) Most important findings

Setzer et al,2011 (75)

Retrospectivestudy

4 years

MolarsFerruleCould not be observed

>1.5 mm1-1.5 mm<1 mm

Endodontically treated molarswith subsequent crownrestoration

Amount of tooth structureavailable for ferrulewas notthe preoperative parametersignificant for theprognosis.

Schmitter et al,2007 (77)

Prospectiverandomizedcontrolledtrial

1 year

Anterior (20%),posterior (80%)

Mean ferrule >3 mm

1. Glass-FRC (resin)/composite/crown, FPD or crown integratedin RPD (no information)

2. Ti post (zinc phosphate)/composite/crown, FPD or crownintegrated in RPD (noinformation)

Ferrule did not have aninfluence on the survival ofneither post type.

Performance of FRC posts wassuperior to that of metalposts.

Ferrari et al,2007 (79)

Prospectiveclinical trial

2 years

PremolarsAll coronal walls3 coronal walls2 coronal walls1 coronal wall2-mm ferruleNo ferrule

1. Quartz-FRC (resin)/composite/PFM crown (no information)

2. No root canal retention/composite/PFM crown (noinformation)

Post placement reduced therisk of failure.

Preservation of at least onecoronal wall reduced thefailure risk.

Teethmissing all coronal wallshad similar failure risksregardless of the presenceor absence of ferrule.

Cagidiaco et al,2008 (80)

Prospectiveclinical trial

3 years

PremolarsAll coronal walls3 coronal walls2 coronal walls1 coronal wall2-mm ferruleNo ferrule

1. Prefabricated quartz-FRC(resin)/composite/PFM crown(no information)

2. Customized fiber post (resin)/composite/PFM crown (noinformation)

3. No root canal retention/composite/PFM crown (noinformation)

Post placement, irrespective ofpost type, provideda significant contribution toteeth survival.

The contribution was moreeffective for prefabricatedFRC post than forcustomized fiber post.

Creugers et al,2005 (76)

Prospectiveclinical trial

5 years

Incisors, canines,premolars, molars

Substantial or minimaldentin height

1. Cast post and core (zinc-phosphate or glass-ionomer)/PFM crown (zinc-phosphate orglass-ionomer)

2. Prefabricated metal post (zinc-phosphate or glass-ionomer)/composite/PFM crown (zinc-phosphate or glass-ionomer)

3. No root canal retention/composite/PFM crown (zinc-phosphate or glass-ionomer)

Teeth with substantial dentinheight performed betterthan teeth with lessremaining tooth structure.

Type of restoration did notinfluence.

Naumann et al,2007 (78)

Randomizedcontrolledclinical pilottrial

2–3 years

Incisors, canines,premolars, molars

2-mm ferruleSurgical crown

lengtheningperformed ifnecessary

1. Titanium post (self-adhesiveresin)/composite/crown, FPD orcrown integrated in RPD (noinformation)

2. Glass-FRC post (self-adhesiveresin)/composite/crown, FPD orcrown integrated in RPD (noinformation)

Both posts resulted insuccessful treatmentoutcome after 2 years.

FRC, fiber-reinforced composite; FPD, fixed partial denture; NiCr, nickel-chromium; PFM, porcelain fused to metal; RPD, removable partial denture; Ti, titanium.

Review Article

could not be achieved). Over a 5-year follow-up period, teeth witha substantial dentin height performed significantly better than teethwith less remaining tooth structure, whereas the type of restorationdid not have an influence.

Two studies evaluated the clinical behavior of endodonticallytreated premolars with varying degrees of coronal tissue loss thatwere restored with FRC post and composite cores or without anyroot canal retention (79, 80). In both studies, posts were lutedinside the root canal with composite cement in combination withan appropriate adhesive system. The amount of dentin left at thecoronal level was assessed before composite abutment buildup and

JOE — Volume 38, Number 1, January 2012

crown preparation, which varied from full chamfer with a bevelinterproximally and lingually, to a feather finish, depending on theheight and thickness of the remaining dentine. All teeth wererestored with a single-unit porcelain fused to metal crown. Overa 2- (79) or 3-year (80) observation period, the post placementprovided a significant contribution to the survival of restored teeth.Nevertheless, irrespective of the restorative procedure, the preserva-tion of at least one coronal wall significantly reduced the failure risk.When all coronal walls were missing, similar failure risks existedregardless of the presence or absence of 2-mm-high ferrule retention(79, 80).

Ferrule Effect 17

Review Article

ConclusionsBased on the evidence from in vitro and in vivo studies reviewed,

the presence of ferrule has a positive effect on fracture resistance ofendodontically treated teeth. More successful prognosis could be ex-pected if healthy dentin extending 1.5 to 2 mm coronal to the marginof the crown is provided circumferentially. If the clinical situationdoes not permit a 360� circumferential ferrule because of extensivecaries lesions, previous restorations, or fractures, an incomplete ferruleis still considered a better option than complete absence of a ferrule.Moreover, including a ferrule in the preparation design could possiblylead to more favorable fracture patters. With regard to the effect ofdifferent restorative procedures on tooth resistance, literature dataare often controversial, probably because of the different methodolo-gies and study designs used. However, it could be generally concludedthat providing an adequate ferrule lowers the impact of the post andcore system, luting agents, and the final restoration on the performanceof endodontically restored teeth.

When it comes to severely damaged teeth with no coronal struc-ture, in order to provide space for a ferrule, orthodontic extrusionshould be considered rather than surgical crown lengthening. Thisapproach preserves more tooth structure and ensures more favorablebiomechanical behavior. If neither of the alternative methods forproviding a ferrule can be performed, currently available evidencesuggests that poor treatment outcome is very likely. Therefore, clini-cians may take in consideration tooth extraction followed by appro-priate surgical and/or prosthetic rehabilitation.

However, in all laboratory and FEA studies, single-rooted teethwere investigated and, therefore, the influence of the ferrule effect onmultirooted teeth remains an area for further research. Also, currentliterature lacks the information on optimal ferrule thickness. It isdirectly related to the tooth structure remaining after endodontic treat-ment and the amount of dentin that needs to be removed during crownpreparation, which depends on the design of the finish line and crowntype. However, this topic has not been investigated so far. More impor-tantly, long-term prospective clinical trials with precise assessment ofremaining tooth structure after crown preparation and uniform studydesign are needed in order to provide more reliable conclusions.

AcknowledgmentsThe authors deny any conflicts of interest related to this study.

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