loss of fracture resistance of the tooth structure following root canal treatment and mod cavity preparation is one of the most complicated dental problems . when tooth fracture occurs below cej , it may need more complex treatments for restoration or even lead to tooth loss in future .
when tooth is weakened and cusps are undermined probability of cuspal fracture is much higher , especially in posterior areas which are more frequently exposed to robust occlusal forces .
there are many factors that can affect incidence of tooth fracture among which are extension of cavity preparation , amount of tooth structure left , and type of restorative material .
one of the most critical and important principles of restorative dentistry is long - term prognosis of tooth and restoration which is directly affected by the treatment plan , skill of the operator and type of restorative material used for restoration .
direct composite restorations have been a subject to attention because of their superior properties such as low cost and esthetics . with the advent of low - shrinkage composites with lower rate of microleakage and enhancement of mechanical properties and esthetic characteristics , using such composites for posterior restorations
recently it has been showed that reinforcing composite with fibers can increase fracture strength of restoration .
there are a wide variety of commercial fibers available in market , among them is  leno - woven polyethylene ribbon 
fibers , using this fiber along with composites increases tensile strength , elastic modulus and fracture toughness of restorations .
kanie et al . have shown that tensile and compressive strength of composite restorations used with woven polyethylene fibers increased significantly compared to non - fiber restorations .
another investigation done by garoushi et al . in 2007 revealed that the use of fiber in the deepest part of composite restorations that contained particular fillers can increase fracture strength of restoration .
some other studies have indicated that if longitudinal axis of fibers would be subjected to compressive forces in perpendicular direction , fibers can increase strength of restoration , but , if direction is parallel to longitudinal axis of fibers , no enhancement will occur in restoration due to matrix failure . in most of the studies ,
fibers were located in central part of composite restoration and it seems that fiber position can affect mechanical properties of the restoration .
the aim of this study was to determine the effect of fiber orientation on fracture strength of root - canal treated and restored teeth .
in this experimental - lab investigation , 50 intact human maxillary premolar teeth that had been extracted due to periodontal disease or orthodontic treatments were chosen , samples were evaluated for any cracks or carries on crown and root surface utilizing a stereomicroscope at 25 times magnification .
prior to cavity preparation impressions were taken from all samples using a sillicon - based impression material ( colten speedex putty , feldwiesenstrasse , aistatten , switzerland ) .
this impression was cut occlusoapically and used as an index for full cuspal reconstruction in the proceeding stages .
specimens were then root - canal treated and the cavities were prepared with the high - speed handpiece ( w&h ) with the air - cooling system and diamond bur ( d&z germany).in the mod preparation , the width of the cavity in isthmus area was kept 2/3 of the intercuspal distance
. then cusps of teeth in four groups were reduced 1.5 mm using a fissure bur .
after cavity preparation teeth were restored as follow : group 1 : no preparation or restoration .
group 2 : after cavity preparation , teeth were etched with 37% phosphoric acid for 20 s , washed for 10 s , air dried , then single bond ( 3 m espe , usa)bonding agent was applied to the etched surfaces . following a 10 s delay , excess bonding
was removed with medium power air syringe and then cured using the light - cure apparatus ( vsi , arialux , ariadent ) for 10 s. then composite restorative material was applied by layering technique in the cavity .
the thickness of each layer was less than 2 mm and curing time for z-250 ( 3 m espe , usa ) was according to company 's instructions . for forming the occlusal surface ,
a proper amount of composite was put on the tooth surface and teeth were put in pre - prepared moulds , after removing the excess composite , the last layer was also cured .
group 3 : all stages were repeated like the aforementioned group except that instead of z250 composite , spectrum composite ( cauk - dentsply , milfmd , usa ) was used .
group 4 : cavity preparation , etching , irrigation and bonding stages were done like group 2 , for restoring the tooth , primarily composite with less than 2 mm thickness was placed at the bottom of pulp chamber and cured , then a thin layer of flowable composite ( cauk - dentsply , milfmd , usa ) was applied and fibers ( angelus , rua - goias , brazil)were placed in a u - shaped pattern at the bottom of pulp chamber embedded in flow composite with 1 mm distance from the occlusal level and both were cured for 20 s , then additional composite layers were added to complete the restoration as it was done for group 2 .
group 5 : all stages were done like group 4 except for fiber position and orientation . in this
fibers were orientated mesiodistally on the surface of uncured composite , adjusted and cured together , then a little bit of flowable composite was applied on it and fibers were positioned buccopallataly and cured after adjustment .
then , restoration was completed in such a way that approximately 1 mm of composite would be present around the fiber .
after restoration of teeth , they were placed in distilled water and kept in incubator for 7 days ; then samples were subjected to compressive force using the mechanical testing machine ( ulm , zwick gamj & co. kg germany ) at cross head speed of 1mm / min perpendicular to occlusal plane of teeth .
force loading phases were recorded in a stress ( n)/strain ( m ) graph utilizing test xpert software in the mechanical testing machine .
maximal amount of forces applied until tooth failures were determined . for evaluation of the failure pattern , all specimens were analyzed visually and assigned to four groups based on the fracture position in relation with cej .
the four groups were as follows : up to the cej , below cej , on the restoration and on the tooth .
the data were analyzed by one - way anova analysis under the significant level of   0.05 .
the amounts of fracture strength of specimens in different groups were as follows : group 1 : 1271.16722.04 n group 2 : 952.05330.16 n group 3 : 918.52256.6 n group 4 : 857.13184.58 n group 5:1138.81406.64 n the results of fracture strength of different groups are shown in graph 1 .
average of fracture resistance of different groups data assessed with unilateral variance analysis and our results showed that use of fiber can increase fracture resistance of tooth but no significant differences were detected between groups.(p = 0.21 ) in other words , using or not using fibers did not show any significant differences in increasing fracture strength of specimens but had an important effect in the fracture pattern . in evaluation of the fracture type , most of fractures above the cej occurred in group 5 and the least occurred in group 4 . in evaluation of fracture 's occurrence on restoration or tooth , most of fractures on restorations occurred in group 5 and the least occurred in group 3 .
fiber - reinforced composite ( frc ) restoration has been considered as a new alternative for posterior composite restoration . in this study
, we investigate the effect of fiber - orientation and composite type in four groups of cases using two kinds of orientation of fibers plus two composite types and compared them to a control group .
our data showed that reinforcement of composite with fiber does not increase fracture resistance of root - filled premolars significantly .
fibers act as crack stoppers and enhance the property of composite . the resin matrix acts to protect the fiber and fix their geometrical orientation .
boron oxide , a glass - forming agent is present at 69 wt% in e - fibers and < 1 wt% in s - fibers .
e- and s - fibers are the ones most commonly used in dentistry . the fibers can be arranged in one direction ( unidirectional ) , with the fibers running from one end to other in a parallel fashion .
alternatively , the fibers can be arranged in different directions to one another , resulting either in a weave- or mesh - type architecture .
when the directional orientation of the fiber long axis is perpendicular to the applied forces , it will result in strength reinforcement .
forces that are parallel to the fiber orientation will produce matrix - dominated failures and consequently yield little reinforcement .
multidirectional reinforcement is accompanied by a decrease in strength in any one direction when compared with unidirectional fiber .
chemically , polyethylene fibers switch from a hydrophobic phase into a hydrophilic phase after exposure to cold gas plasma .
treatment of polyethylene fibers with plasma not only increases wet ability , but also creates chemical bond between polyethylene fiber lwuhmw and resin.[1517 ] rudoand karbhari showed that successful function of pe fiber lwuhmw in enhancing the fracture strength of restoration is related to intrinsic characteristics of fibers , degree / level of chemical bond between resin and fiber , effect of leno tissue on fracture strength and tension of sample and also resistance against dislocation in the resin matrix .
frc provides clinicians with esthetic , superior handling characteristics and one - visit treatment , which are expected characteristics of composite restoration .
moreover , fiber treatment with plasma causes better bonding to resin matrix , so it might be possible that increased fracture strength in these kinds of restorations may be because of cuspal splinting together after using fiber . despite the fact that using fiber can improve mechanical characteristics of composite and increase fracture strength of specimens , this method is just effective in certain directions in a way that using fiber in mesiodistal and buccopalatal can cause teeth surface splinting and play an effective role in increasing fracture strength of specimens . despite the knowledge about fiber characteristics , a little is known about their effect on composite restorations .
eakel showed that , fiber composition , form , direction , volume percentage to resin and the strength of bond to resin affects fiber function in composite restorations .
showed that mechanical characteristics of fiber - reinforced materials are affected by the aforementioned issue above .
fennis et al . compared fracture resistance of maxillary premolars but did not find any significant difference between composite restorations reinforced with unilateral single buccopalatal fiber and restorations with two fibers in the buccopalatal and mesiodistal direction .
belli et al . evaluated the effect of fibers in fracture resistance of root canal treated teeth and showed that use of polyethylene fibers at the bottom or upper part of mod cavities had a significant effect in increasing the fracture strength of specimens .
dyer et al . also studied the effect of fiber position and orientation in initial and final fracture load values of fiber - reinforced composite .
they reported that position and fiber orientation influenced the load to initial and final failure and specimen deflection .
in many cases , a simple change in fiber orientation statistically affected the load to failure and also led to changes in flexural strength of composite restorations . in a way that in three - point loading test , positioning fibers in the tension side can result in an increase flexural strength of restorations .
composites are isotropic materials and have similar characteristics in every directions , thus with adding fibers they show different behaviors in relation to the force direction which at the bottom of cavity in flowable composite , tooth fracture is applied to them in such a way that their mechanical characteristics switches from isotropic to nonisotropic characteristics . this structural complexity ,
belli et al . showed that if fibers are positioned resistance may increase that can be a result of less modulus of elasticity and stress absorption in fiber complex with flowable composite .
in addition , fiber - reinforced inclusion on composite allows better stress distribution and dissipation in the structure in which it had been incorporated , thus decreasing and homogenizing stress transmission to the supporting teeth . in theory , the reinforcing effect of fiber fillers is not only based on stress transfer from the polymer matrix to fibers , but also on the behavior of individual fibers as crack stoppers . the ability of fiber to slow or arrest crack propagation could result in an increase on the fracture strength of composite fiber - reinforced .
it is important to stand out that system success depends on the cohesiveness between the fibers and the surrounding resin matrix , which should ensure uniformity of stress transfer from the matrix to the fibers .
it has been shown that formation of voids between matrix and fibers also lowers the fracture toughness of frc . in the case of void formation at the interface of matrix and fiber a huge surface of oxygen - inhibited layer
the presence of void and structural defect in fiber - composite complex can be considered a probable reason in less increase of fracture resistance in frcs .
this study showed that there is no significant difference in fracture strength of teeth restored with spectrum and z-250 composites .
composite contains zirconium and silica filler with 60% volume percent and the average particle size is 0.6 m .
modulus of elasticity of z-250 composite is equal to 11 and in spectrum composite is 10.572 .
similarity of mechanical properties may be responsible for similar fracture strength of these two composites .
during restoration of teeth with composite restorative materials different factors such as cavity dimension and restorative system may influence resistance of teeth against fracture . considering these issues ,
further research is needed to evaluate the effect of ageing in water or saliva on fiber reinforcement .
the results of this study showed that use of fibers , fiber orientation and type of composite have no significant difference in fracture resistance of the restored , root canal- treated maxillary premolars .
however , the fiber positioning in a buccopalatal and mesiodistal orientation in the occlusal area showed higher fracture resistance ; moreover , fiber utilization could change fracture type in restored maxillary premolars and switched it to more restorable fractures .
 aim : to determine the effect of fiber and its orientation on the fracture resistance of composite restorations in root - filled premolars.materials and methods:50 sound human premolars were collected and assigned to five groups of ten specimens each . in group 1 ( control ) , specimens were not root - filled . in the other four groups , mod cavities were prepared and the cusps were reduced to 1.5 mm . in groups 2 and 3 , the samples were filled with z-250 and spectrum composites respectively without fiber reinforcement . 
 groups 4 and 5 were reinforced with fiber - ribbon-(angelus ) and restored with spectrum composite . 
 fibers were placed in the u - shaped design in group 4 while they were placed in the cross - shape in occlusal region in group 5 . 
 the fracture resistance was tested using a mechanical testing machine at crosshead speed of 1mm / min.results : the fracture strength of control group was 1271.16722.04 n while it was 952.05330.16 n for group 2 , 918.52256.6 n for group 3 , 857.13184.58 n for group 4 and 1138.81406.64 n for group 5 . 
 fracture strength of group 5 was more than other groups except for group 1 , there was no statistically significant differences.conclusions:the reinforcement of composite with fiber does not increase fracture resistance of root - filled premolars significantly . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSIONS