the rehabilitation of severely damaged coronal hard tissue in an endodontically treated tooth is always a challenge in reconstructive dentistry .
the lack of a protective feedback mechanism after pulp removal is the main contributing factor to the failure of endodontically treated teeth .
however , besides the noncontrollable risk factors , high occurrence of fractures may be attributed to various operative procedures such as caries excavation , access cavity preparation , root canal preparation , irrigation , obturation , post space preparation , and final coronal restoration . to restore endodontically treated teeth , posts are often required to provide anchorage for the core forming material and coronoradicular stabilization . cast post and core systems have been the mainstay in dentistry for many years .
however , demand for simpler procedures and esthetic restorations has led to the development of prefabricated posts , initially made from metal and more recently from ceramics and fiber - reinforced composites .
fiber - reinforced posts have a modulus of elasticity ( 21 gpa ) similar to that of dentine ( 18 gpa ) , which allow uniform distribution of forces in the root .
they are noncorrosive and are invisible under esthetic restorations and also evert the phenomenon of  dark root show through ,  and often recommended in flared roots because of lower risk of catastrophic failures and better stress distribution .
the  ferrule effect  is a longstanding , accepted concept in dentistry that is a foundation principle for the restoration of teeth that have suffered advanced structure loss .
a ferrule is defined as a  360 metal collar of the crown surrounding the parallel walls of the dentine extending coronal to the shoulder of the preparation
. this encircled preparation on the tooth provides a protective effect against fracture of the tooth , by reducing stresses within a tooth called the 
 several studies have been performed about ferrule effects which suggested that the ferrule effect strengthens the teeth against functional , wedging , and lateral forces .
clinicians encounter situations where it is not possible to give a full 360 of circumferential ferrule of 2 mm .
different ferrule designs have been suggested according to different clinical situations . in cases where caries frequently affects some walls ( primarily the proximal ones ) , as well as in noncarious lesions such as erosion and abrasion ,
a partial ferrule of 180 either facial or lingual is beneficial rather than no ferrule .
it is important to bear in mind that a ferrule effect is just one part of the restored endodontically treated tooth that represents a complex system .
several studies have been carried out about ferrule length , showing that maintenance of about 2 mm of tooth structure above the finish line or gingival margin as ferrule is effective , but a few studied the effect of different ferrule designs on fracture resistance .
the aim of this study was to investigate the effect of different crown ferrule designs on the fracture resistance of endodontically treated mandibular premolars incorporating fiber posts , composite cores , and crowns .
the aim of this study was to investigate the effect of different crown ferrule designs on the fracture resistance of endodontically treated mandibular premolars incorporating fiber posts , composite cores , and crowns .
fifty caries - free , intact human mandibular premolars , extracted for orthodontic reasons were obtained and stored in 10% formalin until use . only those teeth whose coronal heights were 7  1 mm and root lengths 14  1 mm were selected .
surface soft tissues and debris were mechanically removed using ultrasonic scalers ( p6 newtron ; acteon satelec , merignac , france ) with attention not to damage the root surface .
endodontic access cavities were prepared using a water cooler air turbine handpiece ( unicorn denmart , india ) .
the teeth were biomechanically prepared using the  step - back  technique to a # 70 size k - file ( chlorinox , dentsply maillefer , usa ) and irrigated with 2.6% sodium hypochlorite ( dentpro ) .
each canal was obturated using the lateral condensation method with gutta - percha points ( dentsply , usa ) and ah 26 sealers ( dentsply , usa ) .
following root canal obturation , the adequacy of endodontic fillings was confirmed by radiographs exposed from various angles .
the specimens were stored in an incubator at 37c for 1 week as to allow the sealer to set .
all teeth were then prepared with a circumferential 1.5 mm shoulder meeting metal - ceramic crown requirements .
the prepared clinical crowns were decoronated leaving 2 mm above cementoenamel junction ( cej ) , providing different ferrule designs .
the samples were divided into five groups each having 10 teeth : 
 group i : circumferential ferrule 2 mm above cejgroup ii : 2 mm ferrule only on the facial aspect above cejgroup iii : 2 mm ferrule only on the lingual aspect above cejgroup iv : 2 mm ferrule on the facial and lingual aspects above cej with interproximal concavitiesgroup v : no ferrule ( used as control group ) . 
 
group i : circumferential ferrule 2 mm above cej group ii : 2 mm ferrule only on the facial aspect above cej group iii : 2 mm ferrule only on the lingual aspect above cej group iv : 2 mm ferrule on the facial and lingual aspects above cej with interproximal concavities group v : no ferrule ( used as control group ) . the post spaces of length 8 mm were prepared using 1 , 2 and 3 no .
peeso reamers ( mani inc . , japan ) in sequential manner , leaving 45 mm of gutta - percha inside the canal in the apical third of the root .
glassix fiber posts ( harald nordin sa , chailly / montreux , switzerland ) was cemented in the dried prepared post space using resin luting cement relyx unicem ( 3 m espe , st .
paul , mn , usa ) , and using core build - up composite material ( luxacore , germany ) , core was built to a height of 6 mm .
metal , ceramic crowns of 7 mm in height were fabricated and were placed using resin cement , using constant finger pressure applied for 40 s then the excess cement was removed . for building artificial periodontal ligament just like the natural periodontium , root surfaces of all teeth were planned and were marked by a copying pencil 1 - 2 mm under cej .
then , an aluminum foil with 0.2 mm thickness was cut in root form and was adapted to the root dimensions from the marked line to the apex .
after the complete polymerization , samples were taken out of the acrylic in the straight route and the foils were taken off from the root surfaces .
addition of silicon impression material ( 3 m espe ) in appropriate consistency was injected into the acrylic space to stimulate the natural periodontium .
the fracture resistance test was performed by using a universal testing machine ( instron universal testing machine model 8872 , instron co. , canton , massachusetts , usa ) .
a unidirectional compressive load was applied to the buccal cusp of crowns , at an angle of 1600 from the long axis of the tooth ( to stimulate the functional working side buccal cusp loading ) , using a cylindrical ni - cr alloy rod in universal load testing machine at a crosshead speed of 1 mm / minute until fracture occurred .
data were analyzed by statistical software ( spss statistics version 20.0 , spss inc . ,
fifty caries - free , intact human mandibular premolars , extracted for orthodontic reasons were obtained and stored in 10% formalin until use . only those teeth whose coronal heights were 7  1 mm and root lengths 14  1 mm were selected .
surface soft tissues and debris were mechanically removed using ultrasonic scalers ( p6 newtron ; acteon satelec , merignac , france ) with attention not to damage the root surface .
endodontic access cavities were prepared using a water cooler air turbine handpiece ( unicorn denmart , india ) .
the teeth were biomechanically prepared using the  step - back  technique to a # 70 size k - file ( chlorinox , dentsply maillefer , usa ) and irrigated with 2.6% sodium hypochlorite ( dentpro ) .
each canal was obturated using the lateral condensation method with gutta - percha points ( dentsply , usa ) and ah 26 sealers ( dentsply , usa ) . following root canal obturation ,
the specimens were stored in an incubator at 37c for 1 week as to allow the sealer to set .
all teeth were then prepared with a circumferential 1.5 mm shoulder meeting metal - ceramic crown requirements .
the prepared clinical crowns were decoronated leaving 2 mm above cementoenamel junction ( cej ) , providing different ferrule designs .
the samples were divided into five groups each having 10 teeth : 
 group i : circumferential ferrule 2 mm above cejgroup ii : 2 mm ferrule only on the facial aspect above cejgroup iii : 2 mm ferrule only on the lingual aspect above cejgroup iv : 2 mm ferrule on the facial and lingual aspects above cej with interproximal concavitiesgroup v : no ferrule ( used as control group ) . 
 
group i : circumferential ferrule 2 mm above cej group ii : 2 mm ferrule only on the facial aspect above cej group iii : 2 mm ferrule only on the lingual aspect above cej group iv : 2 mm ferrule on the facial and lingual aspects above cej with interproximal concavities group v : no ferrule ( used as control group ) .
the post spaces of length 8 mm were prepared using 1 , 2 and 3 no .
peeso reamers ( mani inc . , japan ) in sequential manner , leaving 45 mm of gutta - percha inside the canal in the apical third of the root .
glassix fiber posts ( harald nordin sa , chailly / montreux , switzerland ) was cemented in the dried prepared post space using resin luting cement relyx unicem ( 3 m espe , st .
paul , mn , usa ) , and using core build - up composite material ( luxacore , germany ) , core was built to a height of 6 mm .
metal , ceramic crowns of 7 mm in height were fabricated and were placed using resin cement , using constant finger pressure applied for 40 s then the excess cement was removed .
for building artificial periodontal ligament just like the natural periodontium , root surfaces of all teeth were planned and were marked by a copying pencil 1 - 2 mm under cej .
then , an aluminum foil with 0.2 mm thickness was cut in root form and was adapted to the root dimensions from the marked line to the apex .
after the complete polymerization , samples were taken out of the acrylic in the straight route and the foils were taken off from the root surfaces .
addition of silicon impression material ( 3 m espe ) in appropriate consistency was injected into the acrylic space to stimulate the natural periodontium .
the fracture resistance test was performed by using a universal testing machine ( instron universal testing machine model 8872 , instron co. , canton , massachusetts , usa ) .
a unidirectional compressive load was applied to the buccal cusp of crowns , at an angle of 1600 from the long axis of the tooth ( to stimulate the functional working side buccal cusp loading ) , using a cylindrical ni - cr alloy rod in universal load testing machine at a crosshead speed of 1 mm / minute until fracture occurred .
data were analyzed by statistical software ( spss statistics version 20.0 , spss inc . ,
the obtained data showed that maximum amount of loading was sustained by group i [ table 1 ] .
group ii showed moderate amount of force sustainability [ table 2 ] as compared to groups iii and iv .
initial and final failure readings for circumferential ferrule ( group i ) initial and final failure readings for facial ferrule ( group ii ) initial and final failure readings for lingual ferrule ( group iii ) initial and final failure readings for facial and lingual ferrule with interproximal concavities ( group iv ) initial and final failure readings for no ferrule ( group v ) spss 20.0 software was used to calculate mean and standard deviation [ figure 1 ] .
there was high to moderate significant difference ( critical value f 9.24 with p < 0.001 ) as well as there was positive correlation between the groups [ tables 6 and 7 ] . mean and standard deviation values of the groups statistical data showing the mean and the standard deviations for group i , group ii , group iii , group iv and group v and anova test
it is believed that the presence of ferrule protects the restored teeth because it reinforces the tooth / prosthesis assembly .
this portion of dental tissues adjacent to the core increases the fracture resistance , providing a positive effect by reducing the stress concentration on the teeth .
several studies have been carried out about ferrule heights showing that maintenance of about 2 mm of the tooth structure above the finish line or gingival margin is beneficial .
in contrast , others found no benefits of adding a ferrule to the preparation . only a few studied the effect of different ferrule designs on the fracture resistance . a common observation , after deliberate search in endodontic literature , led to a general perception that incorporation of ferrule whether complete or partial is always beneficial in strengthening the teeth against functional , wedging and lateral forces .
juloski stated that to achieve the full benefit of the ferrule effect , it should be a minimum of 1 - 2 mm in height , have parallel dentine walls , totally encircle the tooth , end on sound tooth structure , and avoid invasion of the attachment apparatus of the tooth .
the consensus is that a properly constructed ferrule significantly reduces the incidence of fracture in nonvital teeth by reinforcing the tooth at its external surface and redistributing applied forces which concentrate at the narrowest point around the circumference of the tooth .
in addition , it helps to maintain the integrity of the cement seal of the crown .
considering the results obtained , different ferrule designs improve the fracture resistance or the failure pattern of the tested specimens . on the other hand , the use of fiber post with modulus of elasticity close to that of dentin changed the catastrophic failure type to almost complete favorable fracture in all the four groups except the group without ferrule .
these results are in agreement with a study conducted by dikbas to assess the impact of the different types of ferrule on fracture resistance of the upper incisors . those teeth without post which received endodontic treatment and had the crown showed the highest mean fracture resistance and the group lacking ferrule had the lowest mean among other groups .
although it is clear that a full 360 circumferential ferrule is desirable , there are clinical circumstances where adopting a partial ferrule is still a better option than giving a no ferrule .
facial ferrule of 2 mm above cej showed higher fracture resistance when compared with other groups but showed less fracture resistance when compared to circumferential ferrule .
facial and lingual ferrule of 2 mm with interproximal concavities showed almost equal results as that of lingual ferrule .
posterior teeth with deep proximal boxes are a common outcome of interproximal caries which commonly results in a compromised ferrule in these areas .
tjan and whang in 1986 did a study confirming that a ferrule preparation increases the mechanical resistance by opposing displacement .
tan and aquilino did a similar study concluding that a tooth with a nonuniform ferrule is more effective at resisting fracture than a tooth with no ferrule but not as effective as a tooth with a uniform 2-mm ferrule .
most of the specimens in the study ( 90% ) showed a failure pattern wherein crown displacement preceded tooth fracture .
the highest concentration of stress appeared at the cej as sharp angles cause high concentration of stresses during force application .
favorable fractures were observed in all the four groups except the group with no ferrule .
fractures were in oblique direction from lingual to buccal surface involving the middle or apical third of the roots and vertically on buccal surface or below the acrylic resin .
as per the results obtained , high significant difference was visible among all the groups which in turn can indicate the fact that the different locations of ferrule will have an impact on the mean fracture resistance that is different from the results which have been achieved in the above - mentioned studies .
a facial loading on mandibular premolars teeth may cause stress in the form of tension in the facial margin and may also cause stress in the form of compression in the lingual margin . as these studies were in vitro , it is difficult to achieve standardization with regard to functional age of teeth , morphologic variations of the pulp , and abnormalities in dentin composition before tooth extraction .
differences in dentin composition may affect the resilience of the dentin and thus , change the fracture pattern during compression .
these variations were not controlled in this study which probably constitutes a limitation of the present analysis .
hence , it is suggested to carry out similar researches in more simulating conditions to obtain more precise results .
under data obtained in the sample size and limitations of the study , we can conclude that there was a high statistically significant difference among all the groups ( p < 0.001 ) .
there was maximum fracture resistance in group i ( circumferential ferrule design ) and significantly moderate difference was found in group ii ( facial ferrule design ) when compared with group iii ( lingual ferrule ) and group iv ( facial and lingual ferrule with interproximal concavities ) .
 introduction : in cases of severe hard tissue loss , 2 mm circumferential ferrule is difficult to achieve which leads to incorporation of different ferrule designs.aim:to compare and evaluate the effect of different crown ferrule designs on the fracture resistance of mandibular premolars restored with fiber posts , composite cores , and crowns.materials and methods : fifty freshly extracted mandibular premolars were endodontically treated and divided into five groups : group i - 2 mm circumferential ferrule above the cementoenamel junction ( cej ) ; group ii - 2 mm ferrule on the facial aspect above cej ; group iii - 2 mm ferrule on the lingual aspect above cej ; group iv - 2 mm ferrule on the facial and lingual aspects above cej with interproximal concavities , and group v - no ferrule ( control group ) and were later restored with fiber posts , composite cores , and crowns . 
 specimens were mounted on a universal testing machine , and compressive load was applied at a crosshead speed of 1 mm / min until fracture occurred.results:the results showed that circumferential ferrule produced the highest mean fracture resistance and the least fracture resistance was found in the control group.conclusion:circumferential ferrule increases the fracture resistance of endodontically treated teeth restored with bonded post , core , and crown . 
INTRODUCTION
Aim of the study
MATERIALS AND METHODS
Preparation of the specimens
Root canal preparation
Post space preparation
Block preparation and periodontal ligament simulation
Testing fracture resistance
RESULTS
DISCUSSION
CONCLUSION
Financial support and sponsorship
Conflicts of interest