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Fracture resistance of endodonticallytreated molars restored withpolyethylene fiber and different postsAysun Kara Tuncera, Faruk Haznedaroğlub & Safa Tuncerc

a Faculty of Dentistry, Department of Endodontics, BezmialemVakıf University, Istanbul, Turkeyb Faculty of Dentistry, Department of Endodontics, IstanbulUniversity, Istanbul, Turkeyc Faculty of Dentistry, Department of Conservative Dentistry,Istanbul University, Istanbul, TurkeyPublished online: 25 Jul 2014.

To cite this article: Aysun Kara Tuncer, Faruk Haznedaroğlu & Safa Tuncer (2014)Fracture resistance of endodontically treated molars restored with polyethylene fiberand different posts, Journal of Adhesion Science and Technology, 28:19, 1958-1969, DOI:10.1080/01694243.2014.938453

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Fracture resistance of endodontically treated molars restored withpolyethylene fiber and different posts

Aysun Kara Tuncera*, Faruk Haznedaroğlub and Safa Tuncerc

aFaculty of Dentistry, Department of Endodontics, Bezmialem Vakıf University, Istanbul, Turkey;bFaculty of Dentistry, Department of Endodontics, Istanbul University, Istanbul, Turkey; cFaculty

of Dentistry, Department of Conservative Dentistry, Istanbul University, Istanbul, Turkey

(Received 25 April 2014; final version received 16 June 2014; accepted 21 June 2014)

The purpose of this study was to compare the fracture strength and mode of teethrestored with fiber/titanium post, polyethylene fiber, and adhesive composite. Themesial, distal, and palatal walls of human maxillary molar teeth were removed, sothat only the buccal wall remained. Group 1, with caries-free maxillary molars, wasused as a positive control group and the remaining groups were restored as follows:group 2, with only adhesive composite; group 3, with polyethylene fiber and adhe-sive composite; group 4, with fiber post and adhesive composite; group 5, with fiberpost, polyethylene fiber, and adhesive composite; group 6, with titanium post andadhesive composite; and group 7, with titanium post, polyethylene fiber, and adhe-sive composite. A universal testing machine was used for fracture tests. Compres-sive loads were applied at an angle of 90 degrees on the occlusal surface of thespecimens at crosshead speed of 1 mm/min until fracture occurred. Kruskal–Wallisand Mann–Whitney U tests were adopted for statistical analysis. The study showsthat, based on the fracture strength, the group of teeth that were restored with glassfiber post, polyethylene fiber, and adhesive composite has the most significantimprovement over all the other teeth groups. Based on the fracture mode, the teethgroups restored with only glass fiber post, adhesive composite, polyethylene fiber,and adhesive composite have relatively more restorable fractures observed.

Keywords: endodontically treated teeth; fiber post; fracture mode; fracture strength;polyethylene fiber

1. Introduction

Endodontically treated teeth with substantial hard tissue loss resulting from dental car-ies, endodontic access cavities, and root canal preparation are considered to have ahigher susceptibility to fracture. Restoration is one of the important factors in thelong-term success of endodontically treated teeth. Therefore, these teeth need specialrestoration.[1,2]

It is known that posts weaken the tooth structure rather than reinforcing it.[3,4]However, in cases of severe hard tissue loss, posts are necessary to provide the reten-tion of core materials and restoration. Indications for using posts have been determineddepending on the number of remaining axial cavity walls. A post should be inserted ifonly one cavity wall remains or no wall is remaining.[5]

*Corresponding author. Email: atuncer@bezmialem.edu.tr

© 2014 Taylor & Francis

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For many years, posts made from cast metal or prefabricated metal have been used.In the 1990s, fiber-reinforced composite posts were introduced and have been replacingmetal posts due to their superior esthetic and biomechanical properties in restoration ofendodontically treated teeth.[2] However recently, ultra high molecular weight polyeth-ylene (UHMWPE) fiber reinforcement adhesive systems have been introduced. It isalso known that the addition of fibers increases the mechanical properties of materials.These woven fibers have a modulus of elasticity similar to that of dentin and areintended to create a monoblock dentin-post-core system, allowing better distribution offorces along the root.[6] Karbhari and Wang [7] reported that the use of polyethylenefiber increases the flexural characteristic of resin composites and provides a high levelof fatigue resistance by isolating and arresting cracks. By the restoration of root-filledteeth with mesio-occluso-distal (MOD) cavities when polyethylene fiber was insertedinto the bed of flowable resin, fracture strength of teeth was increased.[8] Use of poly-ethylene fiber in combination with bonding agent and flowable composite under com-posite restoration may act as a stress absorber because of its lower elastic modulus.[9]

Adhesion of restoration materials to the dental structure is an important factor inimproving fracture resistance,[10] optimizing the fracture patterns in case of failures[11] and prevention of microleakage.[12] The use of a glass-fiber post inside the rootcanal combined with a UHMWPE fiber in the crown surrounding the post is a newadhesive technique for the restoration of the endodontically treated teeth with MODcavities extending towards the palatal cusps.[13] The aim of the present study was toevaluate the fracture strength and mode of endodontically treated teeth restored withpolyethylene fiber and different posts. The formulated null hypothesis was that theapplication of polyethylene fiber would increase the fracture strength of endodonticallytreated maxillary molars with one remaining cavity wall.

2. Materials and methods

Seventy human maxillary first and second molars, freshly extracted for periodontal rea-sons, were used in the present study. Teeth were obtained following an informed con-sent protocol reviewed and approved by an appropriate institutional review board at theIstanbul University, The Institute of Medical Sciences. Teeth without root resorption,caries, restoration, root-canal treatment, and uncompleted root formation were selectedand cleaned of debris and soft-tissue remnants. Then teeth were examined by usinglight microscope at magnification of X50 and X100 to detect cracks. Specimens thatdid not meet the criteria were replaced. The teeth were stored at a temperature of +4° Cin distilled water.

2.1. Access cavity preparation

The 70 teeth were randomly assigned into one control and six experimental groups(n = 10). Group 1 did not receive cavity preparation or root-canal treatment. Forgroups 2 through 7, a cavity preparation was made. Mesial, distal, and palatal wallsof teeth were removed by using a water-cooled diamond fissure bur (Komet 837/016-Brasseler, Lemgo D) in a high-speed hand piece. As a result, anatomical crowns weresimilar in dimensions, the thickness of the buccal wall of the teeth was measured2.5 mm (±0.3 mm) at the occlusal surface, 3.5 mm (±0.4 mm) at the cemento-enameljunction (CEF) bucco-lingually, and 9.1 mm (±0.5 mm) mesio-distally. The cavityfloor was perpendicular to the long axis of the tooth. To ensure uniform cavity

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dimensions, the thickness of the buccal wall and the lengths of palatal roots weremeasured by a digital caliper (Mitutoyo, 1/100, Japan) at 0.1 mm sensitivity.

2.2. Root-canal filling

A size 15-K file was introduced into each canal until it could be seen at the apical fora-men. The working length was determined by subtracting 1 mm from this length. Thecoronal portion of each canal was enlarged with a slow-speed contra-angle handpieceused with Gates Glidden burs (Mani Inc., Tochigi, Japan) at sizes of 1–3. The mesio-buccal and distobuccal canals were prepared to a master apical file size of 30 and thepalatal canals to a master apical file size of 40 at working length with a stepback tech-nique. During the cleaning and shaping process, 2.5% NaOCl solution was used.Canals were dried with paper points and filled with gutta-percha (Diadent Group Inter-national, Chongchong Buk Do, Korea) and AH 26 sealer (Dentsply De-Trey, Konstanz,Germany) using cold lateral condensation technique. Excess root-canal filling materialswere removed with hot excavators at the canal orifice and the cavity cleaned.

2.3. Post space preparation and cementation of posts

ER DentinPostX and ER TitanPost (GEBR. BRASSELER GmbH Co. KG Lemgo-Germany) with the same size and shape at 9 mm length, 0.9 mm apical diameter, 4.5 mmlength of retention head, and a taper angle of 2.1 degrees were used (Figure 1).

For groups from 4 to 7, the filling of the palatal root canal was removed at a lengthof 9 mm with a heat carrier, leaving at least 4 mm for apical seal. The post spaces wereprepared with the Erlangen Post System’s special preparation drill, according to themanufacturer’s instructions (Figure 2). Radiography was taken to control root-canal fill-ing in the apical portion and the adaptation of posts in the root canal (Figure 3).

The inner walls of the root canals were roughened by using a diamond-surfacedinstrument with same diameter as the post diameter. The post surfaces were cleaned

Figure 1. Post types used in this study.

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with alcohol and dried. Dentin adhesive system Adper Single Bond 2 was applied tothe surface of ER DentinPostX and cured for 10 s, whereas it was not applied to thesurface of ER TitanPost. Post spaces were etched with 35% phosphoric acid, washedcarefully with water spray, and then dried with air flow and endodontic paper points.One coat of the adhesive system was applied in the canals by using a micro-brush. Theadhesive was light polymerized for 20 s following the removal of the excessive adhe-sive with a paper point. All posts were cemented with dual-polymerizing adhesiveresin-luting agent (Rely X ARC; 3M/ESPE). The luting cement mix was preparedaccording to the manufacturer’s instructions and introduced into each root canal by

Figure 2. Preparation of post spaces.

Figure 3. Radiographic control.

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using a lentulospiral drill (Dentsply Maillefer) on a low-speed handpiece. Lutingcement was also applied to the surface of the post and posts were gently seated by fin-ger pressure into the prepared post spaces. Excessive luting cement was removed byusing a micro-brush and then the post was light-cured for 40 s.

2.4. Experimental groups

2.4.1. Group 1

This group did not receive cavity preparation or root-canal treatment and was used as acontrol group.

2.4.2. Group 2 (direct composite resin restoration)

First, the enamel and then the dentin were etched with 35% phosphoric acid Scotch-bond (3 M Co., MN, USA) for 15 s and rinsed with water spray for 15 s. Dentin adhe-sive system Adper Single Bond 2 (3 M Co., MN, USA) was applied to enamel anddentin according to the manufacturer’s instructions and light-cured for 20 s with aquartz tungsten halogen polymerizing light (Astralis 3; Ivoclar Vivadent, Schaan, Liech-tenstein) with an output of 600 mW/cm2. The pulp chamber floor was filled with flow-able composite resin Filtek Supreme XT Flowable (3 M ESPE, St. Paul, USA) andthen cured for 20 s. A matrix retainer (Tofflemire; KerrHawe SA, Bioggio, Switzerland)and a metal band (Tofflemire; KerrHawe SA) was applied to restore the teeth in a waythat the anatomical structure of a maxillary molar was simulated. This applicationallowed the palatal cusp to be restored 0.5 mm below the buccal cusp in the buccopala-tal direction. The cavities were then subjected to a centripetal restoration technique byusing Filtek Supreme XT (3M Co., MN, USA) and cured for 20 s. The matrix retainerwas removed and the restoration was then completed by using an incremental restora-tion technique. Each increment in reference to occlusal surface was cured for 20 s.

2.4.3. Group 3 (direct composite restoration + polyethylene fiber insertion)

The same approach explained above was also adopted for Group 3 with a difference inthe fiber inclusion process. Flowable composite resin was applied on the buccal walland palatal dentin surface. Ribbond THM (Ribbond Inc., Seattle, USA) fiber of 2 mmwide was cut by using scissors (Ribbond Shears, Ribbond, Seattle, USA). The fiberwas first saturated with adhesive resin. Before curing, a piece of polyethylene fiber wasplaced like a U-shaped profile into the bed of uncured flowable composite resin fromthe buccal wall to the palatal dentin surface. After curing for 20 s, the cavities wererestored with composite as described above.

2.4.4. Group 4 (glass fiber post + direct composite restoration)

The teeth in this group were restored with glass fiber post (ER DentinPostX) andcomposite, as described previously.

2.4.5. Group 5 (glass fiber post + polyethylene fiber + composite restoration)

Glass fiber posts were cemented as described above. Flowable composite was appliedon the pulp chamber floor and on the buccal wall. Polyethylene fiber was embedded

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inside the flowable composite resin in the shape of a U profile starting from buccalwall, turning around the post retention head and adapting again to other side of thebuccal wall (Figure 4). After curing for 20 s, composite restorations were completed asdescribed previously.

2.4.6. Group 6 (titanium post + direct composite restoration)

ER TitanPosts (ER TitanPost; GEBR. BRASSELER GmbH Co. KG) were cementedand teeth were restored with composite resin, as described previously.

2.4.7. Group 7 (titanium post + polyethylene fiber + composite restoration)

After cementation of ER TitanPosts, polyethylene fiber was used to reinforce thecomposite resin restoration as described in group 5.

After restorations of all teeth groups were completed, teeth were subject to thermo-cycling with 1000 cycles at 5–55 °C (±2 °C) and a dwell time of 30 s. To simulatehuman periodontium, root surfaces were dipped into melted wax up to 2.0 mm belowthe CEJ, resulting in a 0.2–0.3 mm-thick wax layer. The teeth were embedded in au-topolymerizing acrylic resin (Paladent 20, Heraeus Kulzer GmbH., Hanau, Germany) inorder that the CEF was located approximately 2 mm above the simulated bone level.After resin polymerization, the roots were removed from the resin blocks; the wax waseliminated from the root surface creating a space in the resin blocks. The impressionmaterial (Zhermack Oranwash L; Zhermack Indurent Gel) was mixed and placed in thespace created in the resin blocks. The teeth were re-inserted into the resin block andthe excess material was removed with a scalpel blade. Therefore, the impression mate-rial filled the space previously occupied by wax, thus providing a simulated ‘PDL’ witha thickness of 0.2–0.3 mm. Following that process, the specimens were stored at 100%humidity and 37 °C for 24 h prior to mechanical loading test.

A 90-degree vertical compressive load was applied to the specimens using a univer-sal testing machine (Autograph AG-IS, Shimadzu Co., Kyoto, Japan) with a stainlesssteel cylindrical bar (3 mm). The load was applied to the center of the occlusal surfaceof restored teeth until fracture occurred with crosshead speed of 1 mm/min. Force dataapplied over time were recorded using the universal testing machine’s computer soft-ware. The failure of the specimen was determined when the force-versus-time graphshowed an abrupt change in load, indicating a sudden decrease in the specimen’s resis-tance to compressive loading.[14] The fracture loads were determined in Newtons (N).

Figure 4. Application of polyethylene fiber around the post.

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The specimens were examined using a stereomicroscope under 10x magnification todetermine the fracture modes. The fracture modes were classified as fracture in toothstructure (adhesive failure), fracture in restoration (cohesive failure), and fracture bothin tooth structure and restoration (mix failure). The main groups were then divided intosubgroups as restorable and non-restorable.

2.5. Statistical analyses

Number Cruncher Statistical System 2007 and PASS 2008 Statistical Software (Utah,USA) were used for the statistical analysis. Fracture strength data were analyzed fornormality of distribution (Kolmogorov-Smirnov test) and homogeneity of variancesamong groups (Levene’s test), followed by Kruskal–Wallis test. When a significant dif-ference was indicated, multiple comparisons were performed using the Mann–WhitneyU test to determine which group differed from the others. Differences were consideredto be significant at p < 0.05.

3. Results

The statistical analysis by using the Kruskal–Wallis test revealed significant differencesbetween the test groups (p < 0.01). The results of the mean failure load values andstandard deviation for the seven groups are shown in Table 1.

The highest fracture strength was observed in the sound teeth group. Followingthat, Group 5 showed the second highest values for F max, which were statistically dif-ferent than the other groups. Among the other groups, statistically significant differencewas not observed.

Distribution of the fracture modes among groups after failure is shown in Table 2.All fractures observed in restorations (cohesive failures) were restorable (Figure 5(a)and (b)) however, fractures in tooth structure (adhesive failure) were non-restorable(Figure 5(c)). The most restorable and non-restorable fractures were observed in group5 and 3, respectively. All fractures observed both in tooth structure and restoration(mix failure) (Figure 5(d)) were non-restorable.

4. Discussion

Numerous studies have been conducted to determine the effect of different post typesand restoration methods regarding the fracture resistance and fracture mode of

Table 1. Mean and median fracture resistance (N) and the standard deviation (SD) for each ofthe seven groups (CR, composite resin; GFP, glass fiber post; TP, titanium post).

n Restoration type Mean ± SD (N) Median (N)

Group 1 10 intact teeth 2667.07 ± 730.24 2833.26Group 2 10 CR 1665.20 ± 532.52 1570.16Group 3 10 polyethylene fiber+CR 1735.21 ± 498.73 1609.29Group 4 10 GFP+CR 1731.05 ± 515.99 1640.31Group 5 10 GFP+polyethylene fiber+CR 2184.87 ± 454.34 2062.11Group 6 10 TP+CR 1749.27 ± 382.83 1732.89Group 7 10 TP+polyethylene fiber+CR 1640.25 ± 441.38 1570.70

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teeth.[15–18] Human incisors, canines, and premolars were used in most of these stud-ies. Bader et al. reported that tooth fracture occurred mostly in maxillary premolar andmolar.[19] Additionally, Wayman et al. [20] indicated that maxillary first molars werethe second most frequently endodontically treated teeth group after mandibular firstmolars. Therefore, maxillary molar teeth were used in the present study.

The posts used in the present study were chosen to present frequently used end-odontic post systems in daily practice. To eliminate the differences raised from the formof the posts between the groups, prefabricated titanium and glass fiber posts with at 9mm length, 0.9 mm apical diameter, 4.5 mm length of retention head, and a taper angleof 2.1 degrees were used. Cavity preparations for post-and-core foundations were per-formed approximately 2 mm deep into the coronal root portion to allow the core to berecessed. Posts with a retention head also can be placed into such a cavity to stabilizethe post-and-core foundation. However, the coronal heads provide no protection fromrotation, as they can be prepared for the cast post-and-core foundations. With theirretention head, they are fixed on the floor of the auxiliary cavity; this avoids the trans-fer of stress to the root by preventing the post from being pressed deeper into the postpreparation during insertion or function.[21]

In the present study, polyethylene fiber was only used for reinforcement under thecomposite resin restoration. Due to the difficulty of standardization, polyethylene fiberwas not preferred as root-canal post.

In the present study, group 1 (sound teeth) specimens exhibited the highest mean ofresistance to fracture; following that, group 5 (restored with glass fiber post, polyethyl-ene fiber, and composite resin) had the second highest mean of resistance to fracture.

Table 2. Distribution of the fracture mode among groups.

Fracture in tooth structure Fracture in restorationFracture both in toothstructure and restoration

% Restorable Non-restorable Restorable Non-restorable Restorable Non-restorable

Group 2 – – 70 – – 30Group 3 – 20 40 – – 40Group 4 – – 80 – – 20Group 5 – – 90 – – 10Group 6 – 20 50 – – 30Group 7 – 20 60 – – 20

Figure 5. Representative failure modes: (a), (b) fracture in restoration (cohesive failure), (c)fracture in tooth structure (adhesive failure), and (d) fracture both in tooth structure and restora-tion (mix failure).

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Thus, the null hypothesis is accepted in part. The fracture strength of group 5 might beexplained with the integrity of these substances forming a composite mono-block thathas higher restoration strength. Our results agree with those by Costa et al. [13] whoreported an improvement in fracture strength of endodontically treated premolarsrestored with the combination of the fiber post, polyethylene fiber, and composite. Thehigher fracture strength could explained by the physical properties and the additionalanchorage from the palatal cusp of tooth provided by polyethylene fiber. On the otherhand, in the present study a statistically significant difference was not observed amongthe other groups. Adapting a piece of the fiber from buccal to palatal direction did notsignificantly increase fracture strength of endodontically treated molar teeth restoredwith titanium post, polyethylene fiber, and composite resin in comparison to the resto-ration with glass fiber post, polyethylene fiber, and composite resin. The explanation ofthis result might be the inadequate bonding ability of fiber in combination with a tita-nium post retention head and composite resin. The other possible explanation of theresults might be the rigid structure of titanium post that does not allow the post toabsorb stress and to prevent root fractures.[22]

In the present study, the mean failure loads between the groups restored with fiberpost and titanium post were not significantly different. These results were in agreementwith the other studies.[23] However, contrasting results were reported showing signifi-cantly higher mean failure loads for titanium posts [16,24,25] or significantly highermean failure loads for fiber posts.[15]

Similar to our results, restoration of the endodontically treated premolars with thecombination of the fiber post, polyethylene fiber, and composite resin showed higherfracture strength than the other restorative techniques. In a study by Belli et al.[26]embedding a Leno Wave Ultra High Modulus polyethylene fiber into the bed of flow-able resin under an extensive composite, restoration increases the fracture strength inroot-filled molars with MOD cavities. They suggested that the presence of fibers createsa change in stress dynamics at the enamel-composite adhesive interface. This disagreedwith the other studies.[27,28] In the study by Fennis et al. [28] the fracture resistanceof fiber reinforced cusp-replacing composite restorations in premolars was evaluatedand it was indicated that the application of fibers does not increase the static load-bear-ing capacity. In addition to this, Cobankara et al. [27] found no statistically significantdifference by the fracture resistance between a resin composite restoration and a fiber-reinforced resin composite restoration.

Oskee et al. [29] evaluated the effect of the fiber position under the composite res-toration and showed that placing glass fibers in the occlusal third of MOD cavitiesbetween composite layers significantly increase fracture resistance. It was explainedwith the levers principle. According to the levers principle, fracture resistance increaseswhen fibers are placed close to the point where force is applied because it leads to ashorter working arm.

According to the mode of fracture in the present study, adhesive and mix failuresshowed non-restorable fractures, however cohesive failures from the composite resinwere restorable fractures. Direct adhesive restorative materials bond to tooth structure,and thus increase fracture resistance.[17–21] Composite resin restorative materials havean elastic modulus similar to that of dentin.[22,23] This mechanical property is impor-tant with respect to both the stress–strain ratio of the tooth-restoration complex duringocclusal load application, and behavioral differences between tooth structure and restor-ative materials.[30] In addition to this, Rosentritt et al. [31] concluded that the postsstabilized the entire system and thereby partially compensated for the stress on the

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adhesive composite. In the present study, although most of the specimens in Groups 4and 5 restored with glass fiber post and composite resin showed cohesive failures(restorable fracture) from the composite resin, the percentage of restorable and non-restorable fracture pattern in Group 6 restored with titanium post and composite resinwas equal. The reason for this might be the dentin-like modulus of the glass fiber postthat was noted by a number of publications and, in correlation to that, the more advan-tageous stress distribution on the residual tooth structure.[15,17,32,33]

In all specimens with polyethylene fiber groups except the group restored withpolyethylene fiber and composite, restorable fractures were observed. This was inagreement with Fennis et al. [28] who investigated the effect of FRC on the fractureresistance of cusp-replacing composite restorations, whereby it was concluded that glassFRCs had a beneficial effect on the fracture mode and thereby on re-restorability in theevent of a fracture.

In a study by Belli et al. [8] cusp fracture was observed in the specimens restoredonly with composite resin, which was also observed in the present study. In the com-posite resin restorations reinforced with polyethylene fiber, cohesive fracture wasobserved between the flowable composite resin and polyethylene fiber. These resultswere in agreement with the present study but, in addition to cohesive fracture, non-restorable fracture in tooth structure also occurred. It was thought that there were tworeasons for the occurrence of non-restorable teeth fractures in the group restored withpolyethylene fiber/composite resin compared to the specimens restored only with com-posite resin. First, polyethylene fiber was thoroughly adapted on the vestibular teethstructure but, on the other hand, there was not enough area to adapt the fiber on thepalatal teeth surface. Secondly, not thoroughly adapted fiber might have caused the lackof integrity of composite resin restoration and, eventually, decreases in the fracturestrength of the teeth.

Despite efforts to achieve standardization, the use of extracted human teeth for peri-odontal reasons of different age introduces significant variation. In in vitro studies,there are many factors such as the thickness of enamel, inherent weakness, variationamongst the size of teeth and variations in the level of contact of the metal rods withthe cuspal inclines during the fracture test that may have contributed to the large stan-dard deviations.[34] The standard deviations may have been reduced if a standardizedtooth model had been available.[35]

Other limitations of the present study were the lack of chewing simulation, fatigueloading, and different loading angles. Test designs of laboratory studies can only par-tially simulate the clinical situation. Clinical loading of teeth is a dynamic process; inwhich loading force, frequency, and direction vary greatly.[36]

5. Conclusions

Within the limitations of this study, the combined use of glass fiber posts, polyethylenefibers, and composite resin improved the fracture resistance of maxillary molar teethwith only buccal wall remaining. In addition, it has a beneficial effect on the fracturemode and thereby on the re-restorability in case of fracture. Further, in vitro and clini-cal studies are required to compare the results obtained by use of alternative applicationmethods of polyethylene fiber and post systems before recommending such treatmenton a regular basis.

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