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d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) e227–e237

Available online at www.sciencedirect.com

jo ur nal home p ag e: www.int l .e lsev ierhea l th .com/ journa ls /dema

he influence of ferrule effect and length of castnd FRC posts on the stresses in anterior teeth

eata Dejaka,∗, Andrzej Młotkowskib

Department of Prosthetic Dentistry, Medical University of Łódz, PolandDepartment of Strength of Materials and Structures, Technical University of Łódz, Poland

r t i c l e i n f o

rticle history:

eceived 19 December 2012

eceived in revised form 1 May 2013

ccepted 24 June 2013

eywords:

errule effect

ast and FRC post and core

ength of post

odified von Mises failure criterion

ontact stresses at the

ement–tooth adhesive interface

D finite element method

tresses in anterior teeth

a b s t r a c t

Objectives. The purpose was to assess the influence of ferrule effect and length of cast post

and cores, and FRC posts on the strength of anterior teeth.

Materials and methods. The investigations were conducted by means of finite element analysis

with the application of contact elements. Thirteen 3D models of maxillary first incisors were

generated: model 1 was an intact tooth; 2A – a tooth with all-ceramic crown with ferrule

effect, 2B–D a teeth restored with, ferrule and FRC posts of various lengths 13 mm, 8 mm,

4 mm, 3A – a tooth with ceramic crown without, ferrule effect, 3B and C teeth without ferrule

and 10 mm, 5 mm length FRC posts, 4B–D teeth with ferrule, restored by cast posts and cores

of 13 mm, 8 mm, 4 mm lengths and 5B and C teeth without ferrule, restored with cast posts

and cores of 10 mm, 5 mm lengths. Each model was subject to a load with a total force of

100 N uniformly distributed under the lingual cingulum, at an angle of 130◦. To evaluate, the

strength of tooth tissues, ceramics and composites, the modified von Mises failure criterion

was used, for FRC the Tsai-Wu criterion and for cast NiCr alloy the von Mises criterion.

Contact stresses were calculated at the luting cement–tissue interface.

Results. Ferrule effect in teeth reduces mvM stresses in dentin, in posts and in luting cement.

The contact tensile stresses around posts in teeth with ferrule effect were 1.7–3.0 times

smaller than in, teeth without ferrule. Lower mvM stresses occurred in teeth with cast posts

in comparison with FRC posts. The mvM stresses in teeth with posts of various lengths were

similar, irrespective of post material.

Significance. Ferrule effect in teeth with posts and cores has a critical influence on stress

reduction. Using posts and cores made of rigid materials leads to stress reduction in teeth.

Post length has a small, effect on stress values in tooth structures.

emy

length, good attachment of the post to tissues, presence of

© 2013 Acad

. Introduction

osts and cores are used to restore endodontically treatedeeth the supragingival structures of which have sufferedonsiderable damage and can fracture during mastication.

∗ Corresponding author at: Medical University of Łódz, Ul. Pomorska 25ax: +48 426757450; mobile: +48 601 411 480.

E-mail address: bdejak@poczta.onet.pl (B. Dejak).109-5641/$ – see front matter © 2013 Academy of Dental Materials. Puttp://dx.doi.org/10.1016/j.dental.2013.06.002

of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Strength and longevity of a tooth restored with post and coredepends on post material and length, root wall width and

1, 92-213 Łódz, Poland. Tel.: +48 60141480;

ferrule and tooth load [1].Ferrule effect – the effect of a crown encircling remaining

supragingival tooth structure – considerably improves tooth

blished by Elsevier Ltd. All rights reserved.

s 2 9

e228 d e n t a l m a t e r i a l

resistance to fracture [2–4]. Most authors demonstrate a posi-tive influence of ferrule effect on the strength of teeth restoredwith posts [5–7]. In addition, ferrule effect increases thepost/core ratio and prevents the luting cement from beingwashed away, and in turn improves post retention. The heightof the remaining tooth structure is of secondary significance[8,9], as is the location of supragingival structures [10]. How-ever, Nauman et al. [11] and Tan et al. [12] hold contraryopinions. According to Ng et al. [13], maintaining coronaldentin on the palatal aspect in upper incisors increases theirresistance to fracture 2 times, maintaining coronal dentin onthe lingual aspect increases the resistance considerably less.

Most commonly recommended at present is the use ofcustom cast metal posts and cores or standard prefabricatedfiberglass-reinforced composite (FRC) posts [14]. FRC postshave different biomechanical properties to metal posts. Gold-alloy cast and FRC posts have, respectively, 7 times (1542 MPa)and 4 times (879 MPa) the flexural strength of dentin (213 MPa)[15].

Many years of experience and strength testing haveallowed the optimum dimensions of cast metal posts andcores to be determined. The length of custom metal postsshould constitute 2/3–3/4 of root length and be at least equalto the length of the future crown [16]. In order to protect peri-apical tissues, 3 mm of apical canal filling is maintained. Deeproot preparation removes a considerable amount of its struc-ture and involves the risk of root perforation [17]. However,short posts are associated with considerable stresses in dentinaround their apex [18], which may lead to debonding or rootfracture. According to some studies, the FRC post/root lengthratio should be equal to that in metal posts. Some authors sug-gest that due to adhesive bonding and good bonding to dentinthese posts can be shorter [17,19].

The purpose was to assess the influence of ferrule effectand length of cast and FRC posts on the stresses of anteriorteeth.

2. Materials and methods

Scans of the surface of the maxillary first left incisor weremade with the laser Dental 3D Scanner D250 (3ShapeA/S,Copenhagen, Denmark). The scans were processed with3Shape Dental Designer CAD software. Files with the PTSextension, including coordinates of the points on the sur-faces of the tooth under analysis, were introduced into FEAsoftware – ANSYS 14 (ANSYS version 14, ANSYS Inc., Canons-burg, PA, USA) [20]. In addition, a CT of the tooth beinginvestigated was made with a GXCB-500/i-CAT (Gendex Den-tal Systems, Des Plaines, IL, USA) system. The points on thetooth surface (obtained from the scanner) and the points onthe enamel–dentin–pulp junction (obtained from the CT), inhorizontal layers (every 1 mm), were introduced into the codepreprocessor. The points were connected with splines and onthis basis the tooth cross-section areas were reconstructed.The connection of these crosswise cross-section areas allowed

us the construction of a solid model of the central incisor,divided into enamel, dentin and pulp. The size and shape ofthe tooth were in accordance with the data from the anatom-ical atlas [21]. The crown was 10.5 mm long, root length was

( 2 0 1 3 ) e227–e237

13 mm, bocco-lingual diameter of crown was 7 mm. A 0.2 mm-thick periodontal ligament was modeled around the tooth root(model 1). The tooth model was located in the system of coor-dinates such that the Z axis was parallel to the long axis of thetooth, the X axis was oriented mesiodistally and the Y axiswas oriented buccolingually.

The tooth was prepared for a ceramic crown in accor-dance with standard clinical recommendations [16]. 2.5 mmof the incisal edge was removed and a 0.8 mm-wide roundedshoulder was prepared at the gingival margin. The crown wasscanned with the Dental 3D Scanner D250 (3ShapeA/S, Copen-hagen, Denmark). A cloud of points was introduced into theANSYS code and on this basis a solid representing the pre-pared tooth crown was developed. An additional 0.1 mm-thicklayer was added under the prepared crown to simulate thecement layer. The solid was added to model 1, thereby con-structing an incisor with an all-ceramic crown with ferrule(model 2A) (Fig. 1a).

The tooth model 2A was sectioned perpendicular to thelongitudinal axis, at a distance of 3.5 mm from the cemento-enamel junction (Fig. 2a). In the ANSYS code preprocessor, a17 mm × 1.2 mm cylinder with a truncated cone at the end wasgenerated. This solid was introduced into the root canal andthen added to the model 2A (Fig. 2a). The length 13 mm ofthe cylinder was introduced into root and 4 mm into crown,measured from catting plane. A 0.1 mm-thick layer was addedaround the root part of post to simulate the cement layer. Inthis way, a tooth model 2B was created with a 13 mm-longstandard post (in relation to catting plane) and a prostheticcrown with ferrule effect (Fig. 1b). Then, the post in root wascatted from 13 mm to 8 mm and 4 mm. Similarly, tooth modelswere developed with a medium 8 mm-long (model 2C) (Fig. 1c)and short 4 mm-long FRC post (model 2D) (Fig. 1d). The lengthsof the long, medium and short posts were, respectively, about80%, 50% and 25% of root length.

Next, the coronal dentin of model 2A was sectionedperpendicular to the longitudinal axis, at the tooth cervix.Tooth models with the crowns without ferrule effect andcores beginning at the level of the gingiva were developed(model 3A) (Figs. 1g and 2b). The cylinder of the same size17 mm × 1.2 mm was introduced into the model 3A. The length10 mm was introduced into the root and 7 mm into the crown,measured from catting plane (Fig. 2b). In this way, modelswith a long FRC post – 10 mm (model 3B) (Fig. 1e) and ashort post – 5 mm (model 3C) (Fig. 1f) were generated. Inthe models without ferrule effect, the lengths of the longand short posts accounted for 77% and 38% of root length,respectively.

It was assumed that the standard posts were made offiberglass-reinforced composite FRC (models 2 and 3) (Fig. 1b–f)and that the individual posts were made of NiCr alloy (models4 and 5) (Fig. 1h–l). The cores of the FRC posts were com-posite, while in the cast posts these were made of metaland integrated with the post. The crown was modeled fromleucite ceramic. Posts and crowns were ideally bonded tothe tooth structure with Variolink II composite resin cement

(Ivoclar, Vivadent AG, Schaan, Lichtenstein). The values forYoung’s modulus, Poisson’s ratio were entered for enamel [22],dentin [23,24], periodontium [25], NiCr alloy [26], FRC posts(Young’s modulus along the long axis being 37 GPa, and in the

d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) e227–e237 e229

Fig. 1 – Tooth models of central maxillary incisor. (a) Model 2A– tooth with ceramic crown and ferrule effect. (b) Model 2B–tooth restored with FRC post 13 mm length and ceramic crown with ferrule effect. (c) Model 2C – tooth restored with FRCpost 8 mm length and ceramic crown with ferrule effect. (d) Model 2D – tooth restored with FRC post 4 mm length andceramic crown with ferrule effect. (e) Model 3B – tooth restored with FRC post 10 mm length and ceramic crown withoutferrule effect. (f) Model 3C – tooth restored with FRC post 5 mm length and ceramic crown without ferrule effect. (g) Model 3A– tooth restored with ceramic crown without ferrule effect. (h) Model 4B – tooth restored with cast post 13 mm length andceramic crown with ferrule effect. (i) Model 4C – tooth restored with cast post 8 mm length and ceramic crown with ferruleeffect. (j) Model 4D – tooth restored with cast post 4 mm length and ceramic crown with ferrule effect. (k) Model 5B – toothrestored with cast post 10 mm length and ceramic crown without ferrule effect. (l) Model 5C – tooth restored with cast post5

pcaltse2

mm length and ceramic crown without ferrule effect.

erpendicular direction 9.5 GPa) [27], luting composite resinement [28], core composite resin [29], and ceramic [30]. Datare presented in Table 1. The models were considered to beinear, elastic, homogeneous and isotropic (with the excep-ion of the FRC post), but had variable compressive and tensile

trengths. Tensile and compressive strength was assumed fornamel (11.5 MPa [31], 384 MPa [32]), dentin (105.5 MPa [33],97 MPa [32]), NiCr alloy (710 MPa) [26], FRC post (1200/35 MPa,

Table 1 – Data of materials used in models of incisors teeth.

Material Modulus ofelasticity (GPa)

Poisson ratio

Enamel 84.1 0.33

Dentin 18.6 0.31

Periodontium 0.05 0.45Cast NiCr post 188 0.33

Glass fiber post EX = 37EY = 9.5EZ = 9.5

�X = 0.34�Y = 0.27�Z = 0.27

Crowns leuciteceramic

65.0 0.19

Composite core 14.1 0.24

Luting resin cement 8.3 0.35

910/110 MPa) [34], core composite resin (41 MPa, 293 MPa) [35],ceramic (48.8 MPa, 162.9 MPa) [36] and luting composite resincement (45.1 MPa, 178 MPa) [37].

For calculation purposes, each tooth model was dividedinto 10-node structural solid elements (Solid 187). Used in

each of the thirteen tooth models were approximately 120,000elements joined at 150,000 nodes. Pairs of bonded contact ele-ments, Targe 170 and Conta 174, were used around the posts,

Tensilestrength (MPa)

Compressivestrength (MPa)

11.5 384105.5 297

710 710RtX = 1200RtY = 35RtZ = 35

RcX = 910RcY = 110RcZ = 110

48.8 162.9

41 29345.1 178

e230 d e n t a l m a t e r i a l s 2 9

Fig. 2 – Longitudinal cross-sections of central maxillaryincisor model restored with FRC post of different length andceramic crowns (a) with ferrule effect (b) without ferruleeffect.

( 2 0 1 3 ) e227–e237

at the interface of the luting cement–dentin bond and underthe crown.

The models were fixed in nodes on the outer surface of theperiodontium and subjected to loads simulating forces actingon incisors during clenching in central occlusion. Their totalvalue was 100 N [38] and they were applied evenly to the nodesunder the lingual cingulum, at 130◦ to the longitudinal axis[39].

The contact simulation conducted with finite elementanalysis is a nonlinear analysis and therefore requires that theload be divided into steps. The components of stresses (nor-mal, shear and principal stresses) in the models during obliqueloading were calculated. The use of von Mises’ theory for mate-rial failure requires that the compressive and tensile strengthsbe equal, whereas for ceramic, tooth tissues, composite resinthe compressive strength values are several times larger thanthe tensile strength values. One criterion used to assess thefailure of such brittle materials under complex stress condi-tions is the modified von Mises failure criterion (mvM) [40].This considers the ratio between compressive and tensilestrength (enamel 33.4; dentin 2.8; leucite ceramic 3.3; com-posite resin cement 3.9). The mvM stresses were calculatedfrom principal stresses in models, using the original comput-ers programme in APDL (Ansys Parametric Design Language).According to the criterion, the material will fail when the val-ues of the equivalent stresses exceed its tensile strength. Themaximum values of the equivalent stresses which appearedin the model materials were compared to one another and totheir respective tensile strengths. To evaluate effort for FRCposts, which have strong anisotropic properties, the Tsai-Wucriterion was applied [41]. The inverse Tsai-Wu ratio index(STWSR) was estimated and the index values above 1 indicatematerial damage. The results of the calculation are presentedas maps of these stresses in the dentin, post and cores, lutingcement and crowns of the incisor models. Compressive, ten-sile, and shear contact stresses at the luting cement–dentininterface around posts and under the crown were also cal-culated. These were presented graphically as maps on thecontact surfaces of the restoration and dentin.

3. Results

The values of the maximum mvM stresses occurring in themodel materials under an oblique load are presented inTable 2, and maximum contact stresses in Table 3.

Oblique loading of the intact tooth (model A) producedmvM stresses in dentin of 14 MPa, concentrated in the palatalwall of the root (Table 2). In models with ferrule (models 2 and4) stresses in dentin were reduced to 11 MPa (Fig. 3a), while inteeth structures without ferrule (models 3 and 5) stresses indentin increased to 17.6 MPa (Fig. 4a) (Table 2). Ferrule effectin teeth reduces mvM stresses in dentin by 42–60% and inposts by 20–33%, depending of materials of posts (Table 2 andGraph 1). In posts and cores, mvM stresses were concentratedaround the core-post junction, in the tooth cervix (Fig. 3b). In

FRC posts in teeth with ferrule, the maximum inverse Tsai-Wu ratio was 0.06, while in teeth without ferrule it increasedto 0.08 (Table 2) (Fig. 4b). This poses no risk of post fracture asonly values above 1.0 are dangerous. The highest von Mises

d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) e227–e237 e231

Fig. 3 – Distribution of mvM stresses in materials of the tooth restored with cast NiCr post 13 mm length and ceramic crownwith ferrule effect (model 4B). (a) Distribution of mvM stresses in dentin. (b) Distribution of mvM stresses in cast post andcore. (c) Distribution of mvM stresses in resincement around cast post and core. (d) Distribution of mvM stresses in ceramiccrown covered cast post and core. (e) Compressive and tensile contact stresses distribution in adhesive cement–dentininterface around the cast post (contact tensile stresses are marked in blue color and their values are negative; MN denotesthe maximum value of tensile stresses; contact compressive stresses are marked in red and yellow color and their valuesare positive, MX denotes the maximum value of compressive stresses). (f) Shear contact stresses distribution in adhesivecement–dentin interface around the cast post (MX and red color indicates maximal shear stresses). (For interpretation of thereferences to color in text, the reader is referred to the web version of this article.)

e232 d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) e227–e237

Fig. 4 – Distribution of stresses in different materials of the tooth model restored with FRC post 5 mm length and ceramiccrown without ferrule effect (model 3C). (a) Distribution of mvM stresses in dentin. (b) Distribution of inverse Tsai-Wu ratioindex (STWSR) in FRC post. (c) Distribution of mvM stresses in luting resin cement around FRC post. (d) Distribution of mvMstresses in ceramic crown based on FRC post and core. (e) Compressive and tensile contact stresses distribution in adhesivecement–dentin interface around FRC post (contact tensile stresses are marked in blue color and their values are negative;MN denotes the maximum value of tensile stresses; contact compressive stresses are marked in red and yellow color andtheir values are positive, MX denotes the maximum value of compressive stresses). (f) Shear contact stresses distribution inadhesive cement–dentin interface around FRC post (MX and red color indicates maximal shear stresses). (For interpretationof the references to color in text, the reader is referred to the web version of this article.)

d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) e227–e237 e233

Table 2 – Maximal mvM stresses in the materials in the models of teeth restored with different posts and cores (MPa).

Material Ferruleeffect

Symbol ofmodel

Model oftooth with

Maximal stresses mvM (MPa)

Ceramiccrown

Cement ofcrown

Dentin Post Cement ofpost

FRC

Model 1 Tooth 26.7 14.0 – –

With

Model 2A Crown 33.5 14.3 11.3 – –Model 2B Long post 13 mm 31.8 10.3 11.0 0.064 Tsai-Wu ratio 19.9Model 2C Middle post 8 mm 31.8 10.5 11.0 0.063 20.4Model 2D Short post 4 mm 31.8 10.4 11.0 0.062 21.5

Without

Model 3A Crown 31.3 16.8 16.5 – –Model 3B Long post 10mm 28.5 14.5 17.6 0.08 26.4Model 3C Short post 5 mm 28.5 14.5 17.5 0.08 27.4

MetalWith

Model 4B Long post 13 mm 21.7 10.3 11.0 83.3 14.8Model 4C Middle post 8 mm 22.5 10.4 11.0 81.5 15.9Model 4D Short post 4 mm 22.5 9.9 11.0 75.3 16.6

WithoutModel 5B Long post 10 mm 15.6 10.6 15.6 99.7 17.7Model 5C Short post 5 mm 15.6 10.4 15.4 90.4 19.1

Table 3 – Maximal contact compressive and tensile stresses in the models of teeth restored with different posts and cores(MPa).

Material ofpost

Ferruleeffect

Symbol ofmodel

Model of toothwith

Maximal contact stresses (MPa)

Tensile onbearingsurface

Shear onbearingsurface

Tensilearound post

Sheararound post

FRC

With

Model 2A Crown – – – –Model 2B Long post 13 mm 4.0 2.1 6.8 1.1Model 2C Middle post 8 mm 4.1 2.1 6.7 1.0Model 2D Short post 4 mm 3.8 2.1 6.6 1.0

Without

Model 3A Crown – – – –Model 3B Long post 10 mm 11.7 3.1 11.6 1.84Model 3C Short post 5 mm 11.7 3.1 11.3 1.75

MetalWith

Model 4B Long post 13 mm 3.5 1.7 5.9 1.0Model 4C Middle post 8 mm 3.4 1.7 5.7 1.0Model 4D Short post 4 mm 3.0 1.7 5.6 1.1

WithoutModel 5B Long post 10 mm 9.3 3.0 10.6 1.5Model 5C Short post 5 mm 9.1 3.0 10.4 1.5

Graph 1 – Comparison of mvM stresses in teeth models with different designed posts and cores.

e234 d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) e227–e237

Graph 2 – Comparison of contact tensile stresses occurred in cement–dentin adhesive interface around different designed

posts.

stresses were observed in metal post and core without fer-rule 99.7 MPa (Table 2). This is less than one seventh of thetensile strength for NiCr alloy (710 MPa) [26]. Under physiolog-ical loads, ideally cemented posts in anterior teeth, regardlessof whether they are made of metal or FRC, are not prone tofractures.

MvM stresses in luting resin cement around posts of teethwith ferrule decreased by 20–32% in comparison to teeth with-out this effect (Table 2) (Figs. 3c and 4c). The maximum valuesof 27.4 MPa were achieved in the luting cement around shortFRC posts in teeth without ferrule (Fig. 4c). Analogous ten-sile contact stresses around posts in teeth with ferrule wereapproximately 2 times lower than without ferrule, while shearstresses were almost 50% lower (Fig. 3e and f, 4e and f) (Table 3and Graph 2). On the bearing area of posts and cores, ten-sile contact stresses in teeth without ferrule increased almost3 times as compared to teeth with maintained supragingivaldentin (Table 3 and Graph 2).

In ceramic crowns placed on cast metal posts and cores,maximum stresses were 32–45% lower than in crownssupported on FRC posts (Figs. 3d and 4d) (Table 3). ThemvM stresses in luting resin cement around metal postsand tensile contact stresses at the metal post-tissue inter-face were approximately 20% lower than around FRC posts(Tables 2 and 3).

The mvM stresses in dentin of teeth with posts of vari-ous lengths had similar values (Table 2 and Graph 1). Postlength did not have an influence on stresses in crowns anddentin (Table 2). Only in cement around short posts was thereobservation of a small increase in mvM stresses (Fig. 4c).

4. Discussion

The study implies that the presence of ferrule in a tooth

restored with a post reduces stresses in dentin, post and lut-ing cement around the post (Table 2). Using posts and cores inteeth with ferrule effect makes teeth stronger, but increasesstresses in dentin in teeth without supragingival structure.

Schmitter et al. [42], Eraslan et al. [43] and Pierrisnard et al.[44] obtained similar FEA results for teeth with ferrule restoredwith posts. According to Pereira [7], ferrule causes a statisti-cally significant increase in tooth resistance to fracture. Limaet al. showed that teeth with ferrule effect failed at a loadof 573.3 N, and at less than half of this value, 275.3 N, with-out ferrule [5]. According to a three-year clinical follow-up byMancebo [45], only 6.67% of incisors restored with posts witha 2 mm ferrule were damaged, while in teeth without ferrulethis was as high as 26.20%.

Tensile contact stresses around posts in teeth with ferrulewere smaller than in teeth without supragingival structure(Graph 2). Particularly high stresses in cement were observedaround short FRC posts without ferrule. Considerable tensilecontact stresses around posts and the bearing area of the rootin teeth without ferrule may predispose to leakage around therestoration and post debonding.

The calculations performed in this study have made itpossible to establish that the higher the Young’s modulus ofthe post materials, the smaller the mvM stresses exerted ondentin and cement around posts (Graph 1). There are smallermvM stresses in teeth restored with cast metal posts andcores than in teeth with FRC posts. Many FEA studies confirmthis [46–48]. Laboratory strength tests also demonstrate thatdamaging teeth with metal posts requires a force statisticallysignificantly higher than does fracturing teeth with FRC posts[49–53].

The study implies that post length had an insignificanteffect on mvM stresses in roots, irrespective of post mate-rial (Table 2). Using finite element analysis, Ferrari et al. alsofound no differences in values and distributions of stressesaround posts of various lengths [54]. Rodriguez-Cervanteset al. observed no correlation in a search for a relationshipbetween post/root length ratio and fracture force (with postlengths of 4–14 mm) [55]. Likewise, Chuang et al. discovered

that mean root fracture force in teeth restored with 5 mm- and10 mm-long FRC posts had in both cases similar values [56].When testing fracture resistance of teeth restored with 5 mm-, 7 mm- and 9 mm-long posts in vitro, Schiavetti et al. found no

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tatistically significant differences between these groups [57].owever, McLaren et al. showed a fracture resistance nearly

wo times higher in teeth with 10 mm posts as compared with mm posts [58,59]. Nissan et al. confirmed that post lengthoes not affect tooth strength where there is a 2 mm ferrule

60].The study indicates that ferrule effect and stiffness of the

ost material, not the post length, has a critical effect on levelsf mvM stresses in dentin. Teeth with ferrule and cast metalost are the most resistant to fracture. Use of such a restora-ion allows a successful reconstruction of 90.9% of heavilyamaged teeth [61]. Using FRC posts in teeth without main-aining supragingival structures is the worst solution and canead to posts debonding or failure of restoration. It was con-rmed by Al-Omiri and Al-Wahadni [62].

There was a lot of research concerning strength of teethestored with various posts and cores. In previous investiga-ions, stresses in individual materials have not been studied,nly stresses in whole models have been analyzed. To evalu-te the material strength, the authors of the above-mentionedtudies have employed the von Mises criterion, which doesot account for differences in the tensile and compressivetrength of these materials. In the present study, the modifiedon Mises failure criterion for tissues, ceramics, compos-tes and the Tsai-Wu failure criterion for fiberglass-reinforcedosts that exhibits strong orthotropic properties have beenmployed. These criterions allowed an evaluation of stressesn the materials closer to real conditions. In the formernvestigations, contact stresses at the post-dentin interfaceave not been studied, either. Here, contact elements in thebonded” option have been applied around restorations onhe cement–tissue adhesive interface, which allowed calcu-ations of contact tensile, compressive and shear stressesnd to visualize their distribution throughout the area of theement–tooth junction around posts.

. Conclusions

errule effect in damaged teeth with posts and cores has aritical effect on stress reduction in these teeth. Maintainingupragingival tooth structures embraced by crown provideshe greatest strength to restored teeth.

Using stiff materials for posts and cores leads to stresseduction in tooth tissues. Teeth restored with cast metalosts should demonstrate higher fracture resistance thaneeth with FRC posts.

Post length has a small effect on levels of stress in toothtructures.

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