Ferrule

Review Article

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
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 is
teeth. 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, demands
The purpose of this study was to summarize the results for simpler procedures and esthetic restorations led to the development of
of research conducted on different issues related to the prefabricated posts, initially made from metal and more recently from ceramics
ferrule 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 on
using 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, it
viewed 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 be
fied 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 various
presence of a 1.5- to 2-mm ferrule has a positive effect operative procedures, such as access cavity preparation and restoration, root canal
on fracture resistance of endodontically treated teeth. If preparation, irrigation and obturation, post space preparation, and final coronal
the 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 cervical
option 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 metal
ture patters. Providing an adequate ferrule lowers the collar of the crown surrounding the parallel walls of the dentine extending coronal to
impact of the post and core system, luting agents, and the shoulder of the preparation. The result is an elevation in resistance form of the
the final restoration on tooth performance. In teeth crown from the extension of dentinal tooth structure’’ (19). More precisely, parallel
with no coronal structure, in order to provide a ferrule, walls of dentin extending coronally from the crown margin provide a ‘‘ferrule,’’ which
orthodontic extrusion should be considered rather after being encircled by a crown provides a protective effect by reducing stresses within
than 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 treated
available evidence suggests that a poor clinical outcome teeth, and, generally, the ferrule effect is considered necessary to stabilize restored
is 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 ferrule
Key Words would be desirable, it should not be provided at the expense of the remaining
Endodontically treated teeth, ferrule effect, post and coronal or root structure (25, 26). However, it is important to bear in mind
core, 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.com
0099-2399/$ - 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 Ferrule Effect 11

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 ferrule
Figure 1. A schematic drawing of endodontically treated tooth restored with survived only few fatigue cycles, and 0.5-mm and 1-mm ferrule groups
post and core system and a crown. Co, core; Cr, crown; F, dentin extending showed a significantly higher number of cycles before failure (29). Even
coronal from crown margin providing a ferrule; G, remaining gutta-percha; though between these two groups there was no statistically significant
P, 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 significantly
controversial 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). Ferrule
articles retrieved in PubMed using the following key words: ‘‘ferrule’’ heights required to significantly improve fracture resistance when
and ‘‘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 had
trials’’; English language; and publication date range from 2000 to a higher percentage of preliminary failure and lower ultimate loads
2011. This search strategy identified 59 articles. Articles that were before fracture when compared with a 2-mm high design (35). The
not 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 was
selected pertinent articles were included as well as some of the classic usually decemented, whereas for teeth with 2-mm ferrule fractures
literature. 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 permit
and 49 original articles, were found to meet the mentioned inclusion preparation of the circumferential ferrule of uniform height, the effect
criteria 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 360
categories: laboratory studies, computer simulation (finite element circumferential 2-mm ferrule was compared with a 2-mm ferrule
analysis), 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 in
Laboratory 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, teeth
static load tests to simulate single high-force application that might with a nonuniform ferrule length were still more fracture resistant

12 Juloski et al. JOE — Volume 38, Number 1, January 2012

Review Article
TABLE 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 ferrule
that 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 cores
ular 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, cast
tooth resistance (19, 31, 39). The inclusion of interproximal grooves post and core was more effective, but under cyclic fatigue testing teeth
when 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 length
a 1-mm or 2-mm ferrule preparation, there was no difference in frac- specimens, higher fracture loads were observed in teeth restored
ture 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, there
effectiveness 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). After
or fatigue loading (47–52) has been widely investigated. cycling loading, specimens were exposed to an oblique, static load
Combinations of different ferrule designs and various post and core until failure, and, likewise, all systems showed statistically equivalent
systems 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 was
resistance 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 presence
research 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 ceramic
resistance as long as sufficient dentine structure remained (13, 41, core, zirconia post and composite core, and cast gold post and
47, 49, 52), whereas other studies pointed out that post type may core system. Heydecke et al (49) found no statistically significant
have 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 recommended
a ferrule is a significant factor in improving resistance to fracture for as an esthetic alternative to cast post and cores because all specimens
different types of posts: cast post and core (13, 33, 36, 40–43, 48, remained intact during thermomechanical loading, the higher fracture
53), all ceramic post and core (40), prefabricated metallic posts (30, strengths, and the number of restorable fractures observed among
34, 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 core
additional 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 differences
was similar (40). Metal cast post-and-core-restored teeth were more in fracture pattern between teeth restored with cast post and core


Review Article
or prefabricated metallic or FRC posts (13, 40, 45, 48). Also, no only fractures of teeth with reduced-length posts were considered
difference was present between ferruled teeth restored with different restorable (59). Likewise, the post length had no significant influence
types of FRC posts (32). However, among teeth with prefabricated metal on failure during chewing stimulation or on ultimate loads when human
posts 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 and
ferrule, 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 teeth
tion failure (30). When prefabricated posts were made from quartz- with no ferrule, a 1.25-mm ferrule, and a 2.5-mm ferrule were restored
FRC, teeth without a ferrule mainly failed as a result of debonding, with 5-mm, 7.5-mm, and 10-mm long prefabricated titanium posts and
and 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 used
placement 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 with
retention (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 Techniques
a 2-mm-high ferrule led to the lowest fracture resistance, suggesting Post luting. Two studies reported that the additional use of ferrule
that a ferrule is only one key element in the complex (52). A significant preparation had no benefit in terms of the resistance to fracture when
increase in load capability was observed when only composite core was resin cement was used for post cementation (53, 55). However, when
placed, which was contributed to an increase in retentive surface to conventional zinc-phosphate cement was used, the presence of a ferrule
withstand 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 between
to static loading of teeth restored with posts was comparable with teeth the ferrule and nonferrule groups, the modes of failure differed. Teeth
with composite buildups alone (46). Then again, a positive impact of with a ferrule underwent a mostly oblique fracture, whereas in teeth
post placement on fracture resistance was found (41). In terms of resis- without a ferrule vertical root fracture was the predominant failure
tance to cyclic loading, no difference was found between teeth restored mode (55). Moreover, a higher resistance to the preliminary failure
only 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 was
core 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 luted
hand, the shape of the post did have an impact: parallel-sided cast posts with conventional zinc-phosphate cement (28). In the same study, teeth
showed 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 crown
was anticipated that the presence of a ferrule may have resulted in cement failure (28). Bonding metal cast posts to the tooth structure
higher resistance and overshadowed the effect of post-related factors by the application of an opaque porcelain layer to post surface before
that 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 of
more restorable than catastrophic fractures, whereas the group with fracture patterns, more favorable failure modes were seen for teeth
only core buildup had an equal number of both types of failure (52). restored with a glass-FRC post and composite cores than for teeth
Specimens 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 excessively
FRC post was used, less catastrophic failures were observed compared flared root canals restored with FRC posts luted with resin cements
with 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 exhibit
types 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 and
assessed, and it was reported that mechanical behavior is primarily posts were adhesively luted, irrespective of the type of resin cement
determined by the presence of a ferrule and crown type and the effect (self-adhesive or in combination with an adhesive), clinically acceptable
of post and core system was considered secondary (13). The use of mean values for the load capability were revealed. Another two studies
a ceramic crown overcame the disadvantage of ferrule absence, and used the same self-adhesive resin cement for luting FRC and metal posts
a 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 higher
post 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 provided
during 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 the
percha, 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 nonbonded
ture under static load was unaffected by the post length (59). Besides, core groups (63).


Review Article
Crown luting. It was reported by Ma et al (29) that using resin a 2-mm ferrule design was associated with an even higher fracture
cement 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 Analysis
studies (35, 64, 65). When the ferrule was present, the resin luting Finite element analysis (FEA) is the method that evaluates stress
agent for bonding crowns increased the number of cycles needed to distribution within complex structures and provides results without
cause premature failure (35, 64) and the ultimate loads that the variation (71). The accuracy of the results depends on the extent to
complex could withstand afterwards (35, 65) in comparison with which the model approaches reality (35, 71). Because of the large
a glass-ionomer cement. A higher bond strength of resin-bonded variability of the results obtained from in vitro studies, different
restorations and the ability to transfer tensile stresses at crown/dentine concerns related to the ferrule effect have been investigated based on
interface were the possible explanations of the results (35). No signif- FEA (35, 72–74). Study designs and most important findings of each
icant differences between zinc-phosphate and resin-modified glass- study are reported in Table 2. The distribution of compressive and
ionomer cement were found (64). The preparation design had an tensile stress (73, 74) and stress combination, known as equivalent
important effect on fracture resistance when crowns were cemented von Mises stresses (72), were studied on 3-dimensional models of
with 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 to
not 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 stress
Crown Lengthening versus Orthodontic Extrusion. The is larger than tensile stress (73, 74). However, it was stated that the
restoration 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 root
an adequate ‘‘biologic width’’ and distance between crown margin canal post than when no post is used (74). The distribution and values
and alveolar crest should be ensured (25, 66). Biological width was of stress were different between glass-FRC and zirconium oxide
defined as ‘‘the dimension of the junctional epithelial and connective ceramic posts; an increase of the elastic modulus of the post material
tissue attachment to the root above the alveolar crest’’ (67), and it increased the dentin stress but shifted the maximum stress location
has been recommended that at least 3 mm between the crown margin from the dentin surface adjacent to the post to the post material
and the alveolar crest should be left to avoid impingement on the (72). In the absence of ferrule, posts with a high modulus of elasticity
coronal attachment of the periodontal connective tissue (68). There- (metallic posts) generated greater cervical stress than a carbon-FRC
fore, 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 in
alveolar tooth structure is required (25). In a situation in which enough increasing the risk of fracture (74). Nonetheless, metallic post in
tooth structure does not exist, the clinician may consider two options: combination with a rigid core material (cast post and core) generated
surgical 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 increase
without crown lengthening and without a ferrule with the crown finish in ferrule height: a decrease in compressive stress in labial cervical
line located at the cement-enamel junction showed a significantly dentine and an increase in tensile stress in palatal cervical dentin
greater load resistance than crown-lengthened specimens prepared with an increased area of palatal dentine under stress (73). Authors
with a 2-mm ferrule and the finish line moved 2 mm apically from also suggested that these tensile stress values were approaching the
the cement-enamel junction (66). The results were in accordance tensile strength of dentine and produced favorable conditions for
with another study that used human premolars (70). Both of the studies a crack to nucleate, which would run apically and obliquely along
restored 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 based
positioned ferrule and teeth without a ferrule when restored with on the buccolingual cervical diameter of the tooth. Moreover, higher
carbon-FRC posts and composite cores (69, 70). Moreover, most displacement values of a simulated metallic crown (labial and axial
failures for cast post and core systems were catastrophic, such as rotation) were shown in teeth without a ferrule, what makes them
vertical root fractures and oblique root fractures located below the more prone to failure because of debonding, and subsequently vertical
cervical third of the roots and, for prefabricated carbon-FRC posts, frac- root fracture through the lever action of the loose post or fracture of the
tures were less severe and located at or above the cervical third of the post in the cervical portion (73). However, displacement potential is
root (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 which
not influence the root fracture pattern, irrespective of the post and core there was no difference (73). Additionally, this study did not assess
system 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-ionomer
increase in crown/root ratio caused by the reduced effective root cement was considered analogous to the debonding state when the
length and the increased effective crown length and reduced volume crown was adhesively luted with a resin cement, it was proposed by
of root dentine that remains (66, 69, 70). An alternative approach another study that crowns with small ferrule heights should be resin
to 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 result
bone support reduces, but the coronal lever arm does not change. in a tooth with less ability to withstand load because patterns and stress
The preparation of a 1-mm ferrule after simulated forced tooth extrac- values of tensile and compressive stress did not differ between standard
tion significantly improved the fracture strength of the tooth, and and crown-lengthened models (73). With regards to post length, the


Review Article
TABLE 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). It
when 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 to
ment 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, the
agreement. 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 posts
The results indicated that for intact restoration forces were exclusively was superior to that of titanium screw posts, but the ferrule height
transferred 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 was
core 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 clinical
Clinical 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 teeth
studies have so far specifically addressed whether and to what extent received a 2-mm ferrule preparation; and, if necessary, surgical crown
the presence of ferrule has an influence on the clinical behavior of lengthening was performed. It was concluded that both posts can be
endodontically treated and restored teeth. All studies with study highly successful in postendodontic restorations when used with
design 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 teeth
factors, including the ferrule height, on the clinical performance of restored with a cast post and core system, a direct metal post, or no post
endodontically treated and restored teeth (75, 77). The amount of was also studied (76). The expected dentin height remaining after tooth
tooth structure available for a ferrule calculated retrospectively from preparation was predicted preoperatively and categorized as substantial
bitewing radiographs was not the preoperative parameter significant (a 1-mm or 2-mm ferrule could be achieved) or minimal (a ferrule


Review Article
TABLE 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 bevel
a substantial dentin height performed significantly better than teeth interproximally and lingually, to a feather finish, depending on the
with less remaining tooth structure, whereas the type of restoration height and thickness of the remaining dentine. All teeth were
did 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 placement
treated 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 existed
an appropriate adhesive system. The amount of dentin left at the regardless of the presence or absence of 2-mm-high ferrule retention
coronal level was assessed before composite abutment buildup and (79, 80).


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.
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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, and
caries lesions, previous restorations, or fractures, an incomplete ferrule ferrule presence on the biomechanical behavior of endodontically treated bovine
is 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 crown
Moreover, including a ferrule in the preparation design could possibly systems on fracture resistance and failure pattern of endodontically treated maxillary
lead 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 in
are often controversial, probably because of the different methodolo- endodontically treated teeth. J Endod 2010;36:609–17.
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ture, 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.
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Acknowledgments resistance of endodontically treated teeth restored with prefabricated posts.
J Prosthet Dent 2006;95:50–4.
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Review Article
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