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  • 1. Review ArticleFerrule 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*AbstractIntroduction: Preserving intact coronal and radiculartooth structure, especially maintaining cervical tissueto create a ferrule effect, is considered to be crucial 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-for the optimal biomechanical behavior of restored ture and the functional requirements (1). Frequently, the remaining tooth structure isteeth. The ferrule effect has been extensively studied not sufficient, and a post is indicated to provide retention for crown restoration (1–and still remains controversial from many perspectives. 3). Cast post and core systems were the standard for many years. However, demandsThe purpose of this study was to summarize the results for simpler procedures and esthetic restorations led to the development ofof research conducted on different issues related to the prefabricated posts, initially made from metal and more recently from ceramicsferrule effect and published in peer-reviewed journals and fiber-reinforced composites (FRCs) (1, 4, 5).listed in PubMed. Methods: The search was conducted A successful clinical outcome of endodontically treated teeth depends onusing the following key words: ‘‘ferrule’’ and ‘‘ferrule adequate root canal treatment as well as on adequate restorative treatment per-effect’’ alone or in combination with ‘‘literature review,’’ formed afterwards (6). Therefore, research has focused on finding the most prom-‘‘fracture resistance,’’ ‘‘fatigue,’’ ‘‘finite element anal- ising post and core system (7–9), luting agent (10–12), and crown type (13, 14).ysis,’’ and ‘‘clinical trials.’’ Results: The findings from re- Even though a strengthening effect of the post is desirable and very often needed, itviewed articles were categorized into three main was questioned in the literature (1). Despite extreme efforts to reinforce endodon-categories: laboratory studies, computer simulation, tically treated teeth, biomechanical failures still represent a critical issue (4, 15).and clinical trials. Laboratory studies were further classi- The lack of a protective feedback mechanism after pulp removal may befied into subchapters based on the main aspect investi- a contributing factor to frequent tooth fractures (16). However, beside noncontrol-gated in relation to the ferrule effect. Conclusions: The lable risk factors, the high occurrence of fractures may be attributed to variouspresence of a 1.5- to 2-mm ferrule has a positive effect operative procedures, such as access cavity preparation and restoration, root canalon fracture resistance of endodontically treated teeth. If preparation, irrigation and obturation, post space preparation, and final coronalthe clinical situation does not permit a circumferential restoration (15, 17).ferrule, an incomplete ferrule is considered a better Preserving intact coronal and radicular tooth structure and maintaining cervicaloption than a complete lack of ferrule. Including a ferrule tissue to create a ferrule effect are considered to be crucial to optimize the biomechan-in preparation design could lead to more favorable frac- ical behavior of the restored tooth (17, 18). A ferrule effect is defined as a ‘‘360 metalture patters. Providing an adequate ferrule lowers the collar of the crown surrounding the parallel walls of the dentine extending coronal toimpact of the post and core system, luting agents, and the shoulder of the preparation. The result is an elevation in resistance form of thethe final restoration on tooth performance. In teeth crown from the extension of dentinal tooth structure’’ (19). More precisely, parallelwith no coronal structure, in order to provide a ferrule, walls of dentin extending coronally from the crown margin provide a ‘‘ferrule,’’ whichorthodontic extrusion should be considered rather after being encircled by a crown provides a protective effect by reducing stresses withinthan surgical crown lengthening. If neither of the alter- a tooth called the ‘‘ferrule effect’’ (20) (Fig. 1).native methods for providing a ferrule can be performed, Growing attention has been given to the prognosis of endodontically treatedavailable evidence suggests that a poor clinical outcome teeth, and, generally, the ferrule effect is considered necessary to stabilize restoredis very likely. (J Endod 2012;38:11–19) 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 ferruleKey Words would be desirable, it should not be provided at the expense of the remainingEndodontically treated teeth, ferrule effect, post and coronal or root structure (25, 26). However, it is important to bear in mindcore, review 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: jelenajuloski@gmail.com0099-2399/$ - see front matter Copyright ª 2012 American Association of Endodontists.doi:10.1016/j.joen.2011.09.024JOE — Volume 38, Number 1, January 2012 Ferrule Effect 11
  • 2. Review Article occur with trauma or as a result of parafunctional habits. All reviewed articles used single-rooted human or bovine teeth and constant load was applied at an angle to the long axis until fracture of the specimen occurred. Maximum loads that teeth could withstand and fracture patterns 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 the clinical situation in which repetitive mechanical loading and thermal changes are characteristics. Therefore, in order to acquire more clinically relevant data with regards to the ferrule effect, in some of the reviewed studies, mechanical, thermal, or thermomechanical fatigue testing were performed, and endurance of the specimens was measured. Results of all laboratory studies that investigated the ferrule effect were further classified into following subchapters based on the main aspect that was investigated in relation to the ferrule effect: ferrule height/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 height required to provide the protective effect was frequently investigated (19, 29–35). When crowned bovine teeth were subjected to cycling loading until the crown or post was dislodged or the post or root fractured, the resistance increased significantly with an increasing ferrule height (34). Similarly, the number of load cycles required to induce failure in cement layer (preliminary failure) was higher for human teeth with higher ferrule (29, 33). Teeth with no ferruleFigure 1. A schematic drawing of endodontically treated tooth restored with survived only few fatigue cycles, and 0.5-mm and 1-mm ferrule groupspost and core system and a crown. Co, core; Cr, crown; F, dentin extending showed a significantly higher number of cycles before failure (29). Evencoronal from crown margin providing a ferrule; G, remaining gutta-percha; though between these two groups there was no statistically significantP, post. difference, the authors recommended a 1-mm-high ferrule because of a large standard deviation in the 0.5-mm ferrule test group. One milli-(22). This topic has been extensively studied and yet remains meter of coronal dentin was also found to be enough to significantlycontroversial from many perspectives. Therefore, this literature increase the fracture strength when exposed to static loading (19).review aimed at summarizing research conducted on different issues On the contrary, two studies reported no statistically significant differ-related to the ferrule effect. ence in failure loads between teeth with a 1-mm ferrule and those with no remaining coronal tooth structure (30, 31). In another study, teeth Materials and Methods with a 0.5-mm and 1-mm ferrule failed at a significantly lower number A literature search was conducted based on the following criteria: of load cycles than the 1.5-mm and 2-mm ferrule groups (33). Ferrulearticles retrieved in PubMed using the following key words: ‘‘ferrule’’ heights required to significantly improve fracture resistance whenand ‘‘ferrule effect’’ alone or in combination with ‘‘literature review,’’ exposed to static loading were 2 mm (31, 32) and 3 mm (30). Addi-‘‘fracture resistance,’’ ‘‘fatigue,’’ ‘‘finite element analysis,’’ and ‘‘clinical tionally, during chewing simulation, teeth with a 1-mm ferrule hadtrials’’; English language; and publication date range from 2000 to a higher percentage of preliminary failure and lower ultimate loads2011. This search strategy identified 59 articles. Articles that were before fracture when compared with a 2-mm high design (35). Thenot found to be significant to the subject of the review based on the mode of fracture was highly dependent on the ferrule height: for resto-abstract were excluded. Conversely, noteworthy articles cited in the rations with a ferrule height of a 1-mm post/core/crown complex wasselected pertinent articles were included as well as some of the classic usually decemented, whereas for teeth with 2-mm ferrule fracturesliterature. Finally, the web sites of relevant dental journals were starting on the remote side were most common (35).explored in the search for in press papers. Sixty-two articles, 13 review Moreover, because the clinical situation does not always permitand 49 original articles, were found to meet the mentioned inclusion preparation of the circumferential ferrule of uniform height, the effectcriteria and became the object of the present review. of incomplete ferrule on tooth resistance has been studied (36–38). A ferrule of nonuniform height, varying between 0.5-mm proximal and 2- mm buccal and lingual, was less effective in preventing failure under Results static loading than a uniform 2-mm ferrule (36). Similarly, differences The findings from all articles were categorized into three main in resistance after thermomechanical loading were present when a 360categories: laboratory studies, computer simulation (finite element circumferential 2-mm ferrule was compared with a 2-mm ferruleanalysis), and clinical trials. present just on the palatal or facial aspect or with the ferrule interrupted by biproximal cavitation (38). Yet, one study reported no change inLaboratory Studies resistance between teeth with a uniform ferrule and a ferrule that incor- The majority of published articles are laboratory studies that used porated only the buccal and/or lingual wall (37). Nonetheless, teethstatic load tests to simulate single high-force application that might with a nonuniform ferrule length were still more fracture resistant12 Juloski et al. JOE — Volume 38, Number 1, January 2012
  • 3. Review ArticleTABLE 1. A List of the Reviewed Laboratory Studies According to the Main Aspects Investigated in Each Study Type of post and Crown lengthening Type Ferrule height/ core system and Luting vs orthodontic of test ferrule design final restoration Post length techniques extrusion Static load Sorensen and Zhang et al (40) Nissan et al (59) al-Hazaimeh and Gegauff (66) testing Engelman (19) da Silva et al (13) Gutteridge (55) Meng et al (70) Pereira et al (30) Pereira et al (30) Mezzomo et al (53) Meng et al (69) Cho et al (31) Pereira et al (41) Akkayan (32) Zhi-Yue and Tan et al (36) Yu-Xing (42) Dikbas (37) Sendhilnathan Kutesa-Mutebi and Nayar (43) and Osman (39) Akkayan (32) al-Hazaimeh and Massa et al (44) Gutteridge (55) Qing et al (45) Ng et al (54) Lima et al (46) Fatigue Ma et al (29) Xible et al (47) Schmitter et al (35) Dorriz et al (56) testing Isidor et al (34) Hu et al (48) Isidor et al (34) Goto et al (28) Libman and Heydecke et al (49) Buttel et al (60) Naumann et al (61) Nicholls (33) Butz et al (50) Aykent et al (63) Schmitter et al (35) Sahafi et al (51) Hsu et al (62) Naumann et al (38) Naumann et al (52) Schmitter et al (65) Dorriz et al (56) Schmitter et al (35) Junge et al (64)then teeth without a ferrule (36, 37), and, therefore, it was suggested resistant to fracture than teeth with prefabricated posts when the ferrulethat a 2-mm ferrule should be provided at least on the buccal and effect was not provided (30, 41–43) as well as in the presence of a 2-lingual wall. Irrespective of the ferrule design, the predominant mode mm ferrule (42, 43, 45). On the contrary, in the presence of a ferrule,of failure was an oblique fracture from the lingual margin to the vestib- teeth restored with prefabricated titanium posts and composite coresular tooth surface (36). had higher fracture resistance compared with cast post and core and Few studies assessed the impact of particular ferrule design on prefabricated quartz-FRC posts (44). Still, for a sudden impact, casttooth resistance (19, 31, 39). The inclusion of interproximal grooves post and core was more effective, but under cyclic fatigue testing teethwhen there is a 1-mm or more coronal dentinal extension decreased restored with carbon-FRC posts outperformed cast post and cores (48).the failure loads and therefore should be avoided. Furthermore, with Furthermore, among the 1-mm, 1.5-mm, and 2-mm ferrule lengtha 1-mm or 2-mm ferrule preparation, there was no difference in frac- specimens, higher fracture loads were observed in teeth restoredture resistance regardless of whether a butt joint or a 1-mm-wide con- with a quartz-FRC post compared with teeth restored with glass-FRC,trabevel configuration existed at the tooth-core junction (19, 39). zirconia enriched glass–FRC, and zirconia posts (32).Type of Post and Core System and Final Restoration. The After subjecting the specimens only to mechanical loading, thereeffectiveness of different procedures for restoring the function of was no difference in the survival rate (100%) among teeth with a 1.5-endodontically treated teeth subjected to static (13, 30, 32, 40–46) mm ferrule restored with zirconia, FRC, or titanium post (47). Afteror fatigue loading (47–52) has been widely investigated. cycling loading, specimens were exposed to an oblique, static loadCombinations of different ferrule designs and various post and core until failure, and, likewise, all systems showed statistically equivalentsystems and final restorations were tested in an attempt to find the fracture strength values and modes of failure. However, when speci-best treatment option that would provide the highest fracture mens were subjected to thermomechanical fatigue loading, there wasresistance and the most favorable fracture pattern of severely a lower percentage of survival (49, 50). Both studies (49, 50)compromised teeth. However, the results were controversial. Some investigated the following combinations of materials in the presenceresearch reported that changes in stiffness and design associated with of a 2-mm-high dentine ferrule and under the same fatigue condi-post and core material did not significantly influence fracture tions: titanium post and composite core, zirconia post and ceramicresistance as long as sufficient dentine structure remained (13, 41, core, zirconia post and composite core, and cast gold post and47, 49, 52), whereas other studies pointed out that post type may core system. Heydecke et al (49) found no statistically significanthave an impact (32, 42–45, 50). differences in survival rates, fracture loads, and modes of failure The vast majority of literature data suggests that the presence of among groups, but all ceramic posts and cores were recommendeda ferrule is a significant factor in improving resistance to fracture for as an esthetic alternative to cast post and cores because all specimensdifferent types of posts: cast post and core (13, 33, 36, 40–43, 48, remained intact during thermomechanical loading, the higher fracture53), all ceramic post and core (40), prefabricated metallic posts (30, strengths, and the number of restorable fractures observed among34, 41), and prefabricated fiber-reinforced composite (FRC) posts specimens from that group. Conversely, Butz et al (50) found a lower(13, 29, 46, 48, 54). Conversely, two studies reported that the survival rate and fracture strength for ceramic post/composite coreadditional use of ferrule preparation had no benefit in terms of combination and suggested that this should be avoided in clinical use.resistance to fracture of teeth restored with cast post and core With regard to fracture modes, the findings were also controver-(43) and prefabricated metallic post (55). sial. Some studies reported that the direct technique was appropriate When teeth with a 2-mm ferrule were restored with a custom- because more severe root fractures were found in cast post-and-fabricated all-ceramic or metal post and core, resistance to fracture core-restored teeth (30, 41, 56); others did not observe differenceswas similar (40). Metal cast post-and-core-restored teeth were more in fracture pattern between teeth restored with cast post and coreJOE — Volume 38, Number 1, January 2012 Ferrule Effect 13
  • 4. Review Articleor prefabricated metallic or FRC posts (13, 40, 45, 48). Also, no only fractures of teeth with reduced-length posts were considereddifference was present between ferruled teeth restored with different restorable (59). Likewise, the post length had no significant influencetypes of FRC posts (32). However, among teeth with prefabricated metal on failure during chewing stimulation or on ultimate loads when humanposts and composite cores, with no ferrule, and a 1-mm to 3-mm premolars with a 1-mm or 2-mm ferrule were restored with 2-mm andferrule, different fracture patterns were present; nonferruled specimens 7-mm long glass-FRC posts (35).failed because of core fracture and ferruled teeth because of cementa- No difference in fracture resistance occurred when bovine teethtion failure (30). When prefabricated posts were made from quartz- with no ferrule, a 1.25-mm ferrule, and a 2.5-mm ferrule were restoredFRC, teeth without a ferrule mainly failed as a result of debonding, with 5-mm, 7.5-mm, and 10-mm long prefabricated titanium posts andand in the presence of a ferrule, the most common mode of failure subjected to cyclic mechanical loading (34). Conversely, the perfor-was root fracture (54). mance of teeth without a ferrule restored with an FRC post and direct When a ferrule was provided, several studies reported that the composite crown was different when different post lengths were usedplacement of a post did not significantly improve fracture resistance (60). After thermomechanical loading, specimens restored with a 3-compared with teeth restored only with composite core and no canal mm post length yielded significantly lower fracture values than withretention (39, 41, 44, 46). However, leaving the specimens empty, a post inserted 6 mm.without post and without core buildup, regardless of the presence of Luting Techniquesa 2-mm-high ferrule led to the lowest fracture resistance, suggesting Post luting. Two studies reported that the additional use of ferrulethat a ferrule is only one key element in the complex (52). A significant preparation had no benefit in terms of the resistance to fracture whenincrease in load capability was observed when only composite core was resin cement was used for post cementation (53, 55). However, whenplaced, which was contributed to an increase in retentive surface to conventional zinc-phosphate cement was used, the presence of a ferrulewithstand static loading (52). did provide improvement in metal cast post-and-core-restored teeth Furthermore, when a ferrule was not provided, fracture resistance (53). Although there was no difference in fracture strength betweento static loading of teeth restored with posts was comparable with teeth the ferrule and nonferrule groups, the modes of failure differed. Teethwith composite buildups alone (46). Then again, a positive impact of with a ferrule underwent a mostly oblique fracture, whereas in teethpost placement on fracture resistance was found (41). In terms of resis- without a ferrule vertical root fracture was the predominant failuretance to cyclic loading, no difference was found between teeth restored mode (55). Moreover, a higher resistance to the preliminary failureonly with a composite core without post and teeth restored with post and of teeth with a 1.5-mm ferrule was shown when resin cement wascore systems, irrespective of the type of post (51, 52). On the other used for luting an FRC post in comparison with cast post and core lutedhand, the shape of the post did have an impact: parallel-sided cast posts with conventional zinc-phosphate cement (28). In the same study, teethshowed a higher resistance than the ones restored with tapered posts with titanium posts luted with zinc-phosphate cement and adhesively(51). Surface treatment of prefabricated posts (ie, titanium, glass- bonded composite cores showed an intermediate level of resistance.FRC, and zirconium), which consisted in air abrasion, followed by It was concluded that a stronger union between the post and core es-silane coating significantly increased the resistance (51). However, it tablished using adhesive bonding may significantly influence crownwas anticipated that the presence of a ferrule may have resulted in cement failure (28). Bonding metal cast posts to the tooth structurehigher resistance and overshadowed the effect of post-related factors by the application of an opaque porcelain layer to post surface beforethat could have given different results. cementation with resin cement increased the survival rate and fracture Regarding the failure pattern, specimens that were left empty had strength and compensated for the lack of a ferrule (56). In terms ofmore restorable than catastrophic fractures, whereas the group with fracture patterns, more favorable failure modes were seen for teethonly core buildup had an equal number of both types of failure (52). restored with a glass-FRC post and composite cores than for teethSpecimens restored with posts and cores had more catastrophic failures restored with cast post and core (56).when a ferrule was present, irrespective of post material, but when an One study evaluated the performance of teeth with excessivelyFRC post was used, less catastrophic failures were observed compared flared root canals restored with FRC posts luted with resin cementswith titanium posts (52). (61). Results showed that, after a simulated 5-year period of service Only one study aimed to investigate the impact of different crown by thermomechanical loading, flared unferruled teeth did not exhibittypes and the ferrule effect on tooth deformation and fracture resistance load capability that could be recommended for clinical use when luted(13). The effect of alumina-reinforced ceramic and metal crowns was with self-adhesive resin cement. However, when ferrule existed andassessed, and it was reported that mechanical behavior is primarily posts were adhesively luted, irrespective of the type of resin cementdetermined by the presence of a ferrule and crown type and the effect (self-adhesive or in combination with an adhesive), clinically acceptableof post and core system was considered secondary (13). The use of mean values for the load capability were revealed. Another two studiesa ceramic crown overcame the disadvantage of ferrule absence, and used the same self-adhesive resin cement for luting FRC and metal postsa ceramic crown with glass-FRC post was recommended as the best and reported similar fracture resistance in the presence of ferrule,choice for restoring endodontically treated incisors. When a ferrule regardless of the post type (13, 52). Within the no ferrule groups,cannot be provided, authors recommended metal crowns and cast fracture resistance was unaffected by the post type (13), but also higherpost and cores (13). load values were measured for metallic post compared with FRC (52).Post Length. As stated earlier, the preservation of sound root struc- Composite cores that were bonded to a tooth structure with a 1-ture is of utmost importance. Minimizing the loss of root dentin tissue mm ferrule compared with nonbonded composite cores providedduring post space preparation can be realized by using reduced-length significantly stronger crown retention under fatigue loading (62).posts. Besides, it could ensure the preservation of 4 to 5 mm of gutta- Also, the bonding of silver amalgam cores significantly improved thepercha, which is necessary to provide a reliable apical seal (57, 58). fracture resistance of teeth previously subjected to thermal cycling When a 2-mm ferrule was designed and prefabricated metal posts only when a ferrule was not present, whereas in the presence of a 2-were luted with a titanium-reinforced composite, the resistance to frac- mm-high ferrule, there was no difference in bonded and nonbondedture under static load was unaffected by the post length (59). Besides, core groups (63).14 Juloski et al. JOE — Volume 38, Number 1, January 2012
  • 5. Review ArticleCrown luting. It was reported by Ma et al (29) that using resin a 2-mm ferrule design was associated with an even higher fracturecement to lute crowns in their study resulted in a higher cycle count resistance (69).compared with zinc-phosphate cement used by Libman and Nicholls(33). These conclusions are consistent with findings from other Computer Simulation: Finite Element Analysisstudies (35, 64, 65). When the ferrule was present, the resin luting Finite element analysis (FEA) is the method that evaluates stressagent for bonding crowns increased the number of cycles needed to distribution within complex structures and provides results withoutcause premature failure (35, 64) and the ultimate loads that the variation (71). The accuracy of the results depends on the extent tocomplex could withstand afterwards (35, 65) in comparison with which the model approaches reality (35, 71). Because of the largea glass-ionomer cement. A higher bond strength of resin-bonded variability of the results obtained from in vitro studies, differentrestorations and the ability to transfer tensile stresses at crown/dentine concerns related to the ferrule effect have been investigated based oninterface were the possible explanations of the results (35). No signif- FEA (35, 72–74). Study designs and most important findings of eachicant differences between zinc-phosphate and resin-modified glass- study are reported in Table 2. The distribution of compressive andionomer cement were found (64). The preparation design had an tensile stress (73, 74) and stress combination, known as equivalentimportant effect on fracture resistance when crowns were cemented von Mises stresses (72), were studied on 3-dimensional models ofwith glass-ionomer cement (ie, more retentive preparation and greater monoradicular tooth under static oblique load.wall thickness led to the greater fracture resistance), whereas it did It was reported that the cervical region of the tooth is exposed tonot influence fracture strength when crowns were cemented with resin the greatest stress, irrespective of the type of coronoradicular recon-cement (65). struction and preparation design (74), and that compressive stressCrown Lengthening versus Orthodontic Extrusion. The is larger than tensile stress (73, 74). However, it was stated that therestoration of severely damaged teeth, with no clinical crown present, presence of a ferrule decreases stress levels (72, 74). Furthermore,represents a particularly important issue. In such a clinical situation, the cervical stress level seems to be lower in the presence of rootan adequate ‘‘biologic width’’ and distance between crown margin canal post than when no post is used (74). The distribution and valuesand alveolar crest should be ensured (25, 66). Biological width was of stress were different between glass-FRC and zirconium oxidedefined as ‘‘the dimension of the junctional epithelial and connective ceramic posts; an increase of the elastic modulus of the post materialtissue attachment to the root above the alveolar crest’’ (67), and it increased the dentin stress but shifted the maximum stress locationhas been recommended that at least 3 mm between the crown margin from the dentin surface adjacent to the post to the post materialand the alveolar crest should be left to avoid impingement on the (72). In the absence of ferrule, posts with a high modulus of elasticitycoronal attachment of the periodontal connective tissue (68). There- (metallic posts) generated greater cervical stress than a carbon-FRCfore, if a 1.5-mm ferrule is to be achieved, at least 4.5 mm of supra- post, which represented the most significant contributory factor inalveolar tooth structure is required (25). In a situation in which enough increasing the risk of fracture (74). Nonetheless, metallic post intooth structure does not exist, the clinician may consider two options: combination with a rigid core material (cast post and core) generatedsurgical crown lengthening or orthodontic extrusion (19, 25, 66). significantly lower tensile stress in comparison with a combination of The effects of apical placement of a ferrule on the resistance to metallic post and composite core, whereas in the presence of a ferrule,static load failure after crown lengthening (66, 69, 70) or forced core material had no impact on the stress level (74).tooth eruption (69) were investigated. Composite analogs of a premolar There are two main changes in stress distribution with an increasewithout crown lengthening and without a ferrule with the crown finish in ferrule height: a decrease in compressive stress in labial cervicalline located at the cement-enamel junction showed a significantly dentine and an increase in tensile stress in palatal cervical dentingreater load resistance than crown-lengthened specimens prepared with an increased area of palatal dentine under stress (73). Authorswith a 2-mm ferrule and the finish line moved 2 mm apically from also suggested that these tensile stress values were approaching thethe cement-enamel junction (66). The results were in accordance tensile strength of dentine and produced favorable conditions forwith another study that used human premolars (70). Both of the studies a crack to nucleate, which would run apically and obliquely alongrestored the specimens with cast post and cores. However, that differ- the palatal root dentine from the internal to external root surface.ence in fracture strength was not present between teeth with an apically Therefore, the ferrule height should be determined individually basedpositioned ferrule and teeth without a ferrule when restored with on the buccolingual cervical diameter of the tooth. Moreover, highercarbon-FRC posts and composite cores (69, 70). Moreover, most displacement values of a simulated metallic crown (labial and axialfailures for cast post and core systems were catastrophic, such as rotation) were shown in teeth without a ferrule, what makes themvertical root fractures and oblique root fractures located below the more prone to failure because of debonding, and subsequently verticalcervical third of the roots and, for prefabricated carbon-FRC posts, frac- root fracture through the lever action of the loose post or fracture of thetures were less severe and located at or above the cervical third of the post in the cervical portion (73). However, displacement potential isroot (70). On the other hand, the presence or absence of a ferrule did reduced with an increasing ferrule height up to 1.5 mm, over whichnot influence the root fracture pattern, irrespective of the post and core there was no difference (73). Additionally, this study did not assesssystem used (70). the influence of the cement material. Because the simulated intact state Disadvantages of surgical crown lengthening treatment are an of the dentin-crown interface when cemented with glass-ionomerincrease in crown/root ratio caused by the reduced effective root cement was considered analogous to the debonding state when thelength and the increased effective crown length and reduced volume crown was adhesively luted with a resin cement, it was proposed byof root dentine that remains (66, 69, 70). An alternative approach another study that crowns with small ferrule heights should be resinto treatment could be an orthodontic extrusion. From the bio- bonded (35).mechanical point of view when orthodontic extrusion is performed, FEA does not support the notion that crown lengthening may resultbone support reduces, but the coronal lever arm does not change. in a tooth with less ability to withstand load because patterns and stressThe preparation of a 1-mm ferrule after simulated forced tooth extrac- values of tensile and compressive stress did not differ between standardtion significantly improved the fracture strength of the tooth, and and crown-lengthened models (73). With regards to post length, theJOE — Volume 38, Number 1, January 2012 Ferrule Effect 15
  • 6. Review ArticleTABLE 2. List of FEA Studies Investigating the Ferrule Effect, the Study Design, and the Most Important Findings Post (cement for post)/core/ Amount of coronal final restoration (cement for Author, year Tooth model dentin (ferrule) final restoration) Most important findings Pierrisnard et al, Monoradicular No ferrule 1. NiCr cast post-and-core/NiCr Regardless of the model, the 2002 (74) tooth with 2-mm ferrule crown greatest stress in the cervical simulated 2. NiCr post/composite/NiCr region. bone base crown Post placement reduced cervical 3. Carbon-FRC/composite/NiCr stress. crown Absence of ferrule resulted in 4. No post/composite/NiCr increased stress levels. crown Ferrule eliminated effect of core Cement layers considered material on cervical stress. insignificant Ichim et al, Maxillary No ferrule Metal cast post-and-core/metal Crown lengthening did not 2006 (73) central 0.5-mm ferrule crown decrease tooth resistance. incisor with 1-mm ferrule Displacement potential of simulated 1.5-mm ferrule a crown is reduced with periodontal 2-mm ferrule increasing ferrule height. ligament Thickness 1 mm Restoration without ferrule failed With and without primarily because of crown debonding and subsequently lengthening root fracture. Eraslan et al, Maxillary No ferrule 1. Glass-FRC/composite/ Stress values were lower for both 2009 (72) central 1-mm ferrule ceramic crown post systems when ferrule was incisor with 2-mm ferrule 2. Zirconium oxide ceramic present. simulated post/composite/ceramic Stress values were lower for glass- periodontal crown FRC post. ligament and Increase of the elastic modulus of alveolar bone the post material increased the dentin stress. Schmitter et al, Premolar 1-mm ferrule 1. Glass-FRC: 2-mm and 7-mm Post length had a minor effect on 2010 (35) 2-mm ferrule long (resin)/composite/ force distribution when ferrule metal crown (resin) was present. 2. Glass-FRC: 2-mm and 7-mm FRC posts do not contribute to long (resin)/composite/ load transfer as long as the metal crown (glass-ionomer) bonding between the tooth and composite core is intact.FRC, fiber-reinforced composite; NiCr, nickel-chromium.results of FEA suggested that it has a minor effect on force distribution for the prognosis of restored endodontically treated molars (75). Itwhen a 1-mm or 2-mm-high ferrule is present (35), which is in agree- was commented that judgment on a preexisting ferrule is difficult toment with the results from in vitro testing (34, 35, 59). obtain retrospectively and should be evaluated by a model study or One study aimed to combine the advantages of in vitro tests and cone-beam tomography to get a 3-dimensional rather than two-FEA (35). The results obtained from both methods were compared with dimensional picture of the prerestorative condition. However, theagreement. In order to evaluate distribution of the external forces finding was in line with the results of a prospective clinical trial (77)among different internal substructures, virtual models of intact teeth that investigated 1-year survival of teeth restored with FRC posts and tita-and models of teeth with different damage levels were investigated. nium screw posts. The short-term clinical performance of FRC postsThe results indicated that for intact restoration forces were exclusively was superior to that of titanium screw posts, but the ferrule heighttransferred at the dentine-crown interface, and the post did not did not have an influence on the survival of neither post type. However,contribute to load transfer until the bond between the composite one should bear in mind that the mean ferrule height in the study wascore and the dentine failed (35). more than 3 mm in both groups, and survival rates after 1 year were 93.5% and 75.6% for FRC and titanium post, respectively. Furthermore, no impact of the post material on the clinicalClinical Trials outcome after 2 years was found in the presence of a ferrule (78). Teeth One retrospective (75) and few prospective (76–80) clinical were restored with either a titanium or glass-FRC post; all the teethstudies have so far specifically addressed whether and to what extent received a 2-mm ferrule preparation; and, if necessary, surgical crownthe presence of ferrule has an influence on the clinical behavior of lengthening was performed. It was concluded that both posts can beendodontically treated and restored teeth. All studies with study highly successful in postendodontic restorations when used withdesign and most important findings are reported in Table 3. a self-adhesive resin cement and composite core. Two studies evaluated the influence of various preoperative The effect of the remaining tooth structure on the survival of teethfactors, including the ferrule height, on the clinical performance of restored with a cast post and core system, a direct metal post, or no postendodontically treated and restored teeth (75, 77). The amount of was also studied (76). The expected dentin height remaining after toothtooth structure available for a ferrule calculated retrospectively from preparation was predicted preoperatively and categorized as substantialbitewing radiographs was not the preoperative parameter significant (a 1-mm or 2-mm ferrule could be achieved) or minimal (a ferrule16 Juloski et al. JOE — Volume 38, Number 1, January 2012
  • 7. Review ArticleTABLE 3. The List of Clinical Trials Investigating the Ferrule Effect, the Study Design, and the Most Important Findings Type of study; Tooth type; amount Post (cement for post)/core/final follow-up of coronal dentin restoration (cement for final Author, year period (ferrule) restoration) Most important findings Setzer et al, Retrospective Molars Endodontically treated molars Amount of tooth structure 2011 (75) study Ferrule with subsequent crown available for ferrule was not 4 years Could not be observed restoration the preoperative parameter 1.5 mm significant for the 1-1.5 mm prognosis. 1 mm Schmitter et al, Prospective Anterior (20%), 1. Glass-FRC (resin)/composite/ Ferrule did not have an 2007 (77) randomized posterior (80%) crown, FPD or crown integrated influence on the survival of controlled Mean ferrule 3 mm in RPD (no information) neither post type. trial 2. Ti post (zinc phosphate)/ Performance of FRC posts was 1 year composite/crown, FPD or crown superior to that of metal integrated in RPD (no posts. information) Ferrari et al, Prospective Premolars 1. Quartz-FRC (resin)/composite/ Post placement reduced the 2007 (79) clinical trial All coronal walls PFM crown (no information) risk of failure. 2 years 3 coronal walls 2. No root canal retention/ Preservation of at least one 2 coronal walls composite/PFM crown (no coronal wall reduced the 1 coronal wall information) failure risk. 2-mm ferrule Teeth missing all coronal walls No ferrule had similar failure risks regardless of the presence or absence of ferrule. Cagidiaco et al, Prospective Premolars 1. Prefabricated quartz-FRC Post placement, irrespective of 2008 (80) clinical trial All coronal walls (resin)/composite/PFM crown post type, provided 3 years 3 coronal walls (no information) a significant contribution to 2 coronal walls 2. Customized fiber post (resin)/ teeth survival. 1 coronal wall composite/PFM crown (no The contribution was more 2-mm ferrule information) effective for prefabricated No ferrule 3. No root canal retention/ FRC post than for composite/PFM crown (no customized fiber post. information) Creugers et al, Prospective Incisors, canines, 1. Cast post and core (zinc- Teeth with substantial dentin 2005 (76) clinical trial premolars, molars phosphate or glass-ionomer)/ height performed better 5 years Substantial or minimal PFM crown (zinc-phosphate or than teeth with less dentin height glass-ionomer) remaining tooth structure. 2. Prefabricated metal post (zinc- Type of restoration did not phosphate or glass-ionomer)/ influence. composite/PFM crown (zinc- phosphate or glass-ionomer) 3. No root canal retention/ composite/PFM crown (zinc- phosphate or glass-ionomer) Naumann et al, Randomized Incisors, canines, 1. Titanium post (self-adhesive Both posts resulted in 2007 (78) controlled premolars, molars resin)/composite/crown, FPD or successful treatment clinical pilot 2-mm ferrule crown integrated in RPD (no outcome after 2 years. trial Surgical crown information) 2–3 years lengthening 2. Glass-FRC post (self-adhesive performed if resin)/composite/crown, FPD or necessary crown integrated in RPD (no information)FRC, fiber-reinforced composite; FPD, fixed partial denture; NiCr, nickel-chromium; PFM, porcelain fused to metal; RPD, removable partial denture; Ti, titanium.could not be achieved). Over a 5-year follow-up period, teeth with crown preparation, which varied from full chamfer with a bevela substantial dentin height performed significantly better than teeth interproximally and lingually, to a feather finish, depending on thewith less remaining tooth structure, whereas the type of restoration height and thickness of the remaining dentine. All teeth weredid not have an influence. restored with a single-unit porcelain fused to metal crown. Over Two studies evaluated the clinical behavior of endodontically a 2- (79) or 3-year (80) observation period, the post placementtreated premolars with varying degrees of coronal tissue loss that provided a significant contribution to the survival of restored teeth.were restored with FRC post and composite cores or without any Nevertheless, irrespective of the restorative procedure, the preserva-root canal retention (79, 80). In both studies, posts were luted tion of at least one coronal wall significantly reduced the failure risk.inside the root canal with composite cement in combination with When all coronal walls were missing, similar failure risks existedan appropriate adhesive system. The amount of dentin left at the regardless of the presence or absence of 2-mm-high ferrule retentioncoronal level was assessed before composite abutment buildup and (79, 80).JOE — Volume 38, Number 1, January 2012 Ferrule Effect 17
  • 8. Review Article Conclusions 10. Monticelli F, Osorio R, Albaladejo A, et al. Effects of adhesive systems and luting agents on bonding of fiber posts to root canal dentin. J Biomed Mater Res B Based on the evidence from in vitro and in vivo studies reviewed, Appl Biomater 2006;77:195–200.the presence of ferrule has a positive effect on fracture resistance of 11. Soares CJ, Raposo LH, Soares PV, et al. Effect of different cements on the biomechan-endodontically treated teeth. More successful prognosis could be ex- ical behavior of teeth restored with cast dowel-and-cores-in vitro and FEA analysis.pected if healthy dentin extending 1.5 to 2 mm coronal to the margin J Prosthodont 2010;19:130–7. 12. Radovic I, Mazzitelli C, Chieffi N, et al. Evaluation of the adhesion of fiber posts ce-of the crown is provided circumferentially. If the clinical situation mented using different adhesive approaches. Eur J Oral Sci 2008;116:557–63.does not permit a 360 circumferential ferrule because of extensive 13. da Silva NR, Raposo LH, Versluis A, et al. The effect of post, core, crown type, andcaries lesions, previous restorations, or fractures, an incomplete ferrule ferrule presence on the biomechanical behavior of endodontically treated bovineis still considered a better option than complete absence of a ferrule. anterior teeth. J Prosthet Dent 2010;104:306–17. 14. Salameh Z, Sorrentino R, Ounsi HF, et al. The effect of different full-coverage crownMoreover, including a ferrule in the preparation design could possibly systems on fracture resistance and failure pattern of endodontically treated maxillarylead to more favorable fracture patters. With regard to the effect of incisors restored with and without glass fiber posts. J Endod 2008;34:842–6.different restorative procedures on tooth resistance, literature data 15. Tang W, Wu Y, Smales RJ. Identifying and reducing risks for potential fractures inare often controversial, probably because of the different methodolo- endodontically treated teeth. J Endod 2010;36:609–17.gies and study designs used. However, it could be generally concluded 16. Randow K, Glantz PO. On cantilever loading of vital and non-vital teeth. An exper- imental clinical study. Acta Odontol Scand 1986;44:271–7.that providing an adequate ferrule lowers the impact of the post and 17. Dietschi D, Duc O, Krejci I, et al. Biomechanical considerations for the restoration ofcore system, luting agents, and the final restoration on the performance endodontically treated teeth: a systematic review of the literature—part 1. Composi-of endodontically restored teeth. tion and micro- and macrostructure alterations. Quintessence Int 2007;38:733–43. When it comes to severely damaged teeth with no coronal struc- 18. Dietschi D, Duc O, Krejci I, et al. Biomechanical considerations for the restoration of endodontically treated teeth: a systematic review of the literature, part II (evaluationture, in order to provide space for a ferrule, orthodontic extrusion of fatigue behavior, interfaces, and in vivo studies). Quintessence Int 2008;39:should be considered rather than surgical crown lengthening. This 117–29.approach preserves more tooth structure and ensures more favorable 19. Sorensen JA, Engelman MJ. Ferrule design and fracture resistance of endodonticallybiomechanical behavior. If neither of the alternative methods for treated teeth. J Prosthet Dent 1990;63:529–36.providing a ferrule can be performed, currently available evidence 20. Stankiewicz N, Wilson P. The ferrule effect. Dent Update 2008;35:222–4. 21. Slutzky-Goldberg I, Slutzky H, Gorfil C, et al. Restoration of endodontically treatedsuggests that poor treatment outcome is very likely. Therefore, clini- teeth review and treatment recommendations. Int J Dent 2009;2009:150251.cians may take in consideration tooth extraction followed by appro- 22. Al-Omiri MK, Mahmoud AA, Rayyan MR, et al. Fracture resistance of teeth restoredpriate surgical and/or prosthetic rehabilitation. with post-retained restorations: an overview. J Endod 2010;36:1439–49. However, in all laboratory and FEA studies, single-rooted teeth 23. McLean A. Predictably restoring endodontically treated teeth. J Can Dent Assoc 1998;64:782–7.were investigated and, therefore, the influence of the ferrule effect on 24. Morgano SM. Restoration of pulpless teeth: application of traditional principles inmultirooted teeth remains an area for further research. Also, current present and future contexts. J Prosthet Dent 1996;75:375–80.literature lacks the information on optimal ferrule thickness. It is 25. Stankiewicz NR, Wilson PR. The ferrule effect: a literature review. Int Endod J 2002;directly related to the tooth structure remaining after endodontic treat- 35:575–81.ment and the amount of dentin that needs to be removed during crown 26. Jotkowitz A, Samet N. Rethinking ferrule—a new approach to an old dilemma. Br Dent J 2010;209:25–33.preparation, which depends on the design of the finish line and crown 27. Kelly JR. Clinically relevant approach to failure testing of all-ceramic restorations.type. However, this topic has not been investigated so far. More impor- J Prosthet Dent 1999;81:652–61.tantly, long-term prospective clinical trials with precise assessment of 28. Goto Y, Nicholls JI, Phillips KM, et al. Fatigue resistance of endodontically treatedremaining tooth structure after crown preparation and uniform study teeth restored with three dowel-and-core systems. J Prosthet Dent 2005;93:45–50. 29. Ma PS, Nicholls JI, Junge T, et al. Load fatigue of teeth with different ferrule lengths,design are needed in order to provide more reliable conclusions. restored with fiber posts, composite resin cores, and all-ceramic crowns. J Prosthet Dent 2009;102:229–34. 30. Pereira JR, de Ornelas F, Conti PC, et al. Effect of a crown ferrule on the fracture Acknowledgments resistance of endodontically treated teeth restored with prefabricated posts. J Prosthet Dent 2006;95:50–4. The authors deny any conflicts of interest related to this study. 31. Cho H, Michalakis KX, Kim Y, et al. Impact of interproximal groove placement and remaining coronal tooth structure on the fracture resistance of endodontically treated maxillary anterior teeth. J Prosthodont 2009;18:43–8. 32. Akkayan B. An in vitro study evaluating the effect of ferrule length on fracture resis- References tance of endodontically treated teeth restored with fiber-reinforced and zirconia 1. Schwartz RS, Robbins JW. 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J Prosthodont 1995;4: custom made posts affects the 3-year survival of endodontically treated premolars. 51–3. Am J Dent 2008;21:179–84.JOE — Volume 38, Number 1, January 2012 Ferrule Effect 19