caries , fractures , attrition , and endodontic access cavities are the most common reasons of tooth structure loss rendering them susceptible to fracture . according to cavity preparation principles , it is recommended to remove all the undermined enamel to prevent functional fracture . when restoring teeth with large cavities , choosing the most ideal method and material to restore strength and esthetics is a real challenge .
often it is necessary to use full or partial coverage instead of a simple intracoronal restoration .
this fact has been based on clinical studies and indicates the need for special restorative considerations for teeth susceptible to fracture .
effects of amalgam , gold and a variety of composite systems have been evaluated on the resistance of teeth to fracture [ 46 ] .
wendt et al   have shown that cast gold restorations providing cuspal protection could result in a significant increase in the resistance of teeth to fracture , compared to unaltered teeth .
salis et al   showed that gold on - lays provided good support for the remaining tooth structure .
in addition , it has been revealed that in case of high stress , such teeth suffered only superficial cuspal fractures .
some researchers have claimed that if the teeth are over loaded , composite restorations make the tooth fracture more promising at the tooth - resin interface [ 57 ] . in recent years , there has been an increased acceptance and use of bonding techniques in strengthening tooth structure [ 810 ] . due to the success of bonding techniques
, there is an increasing interest in determining how a restoration bonded to the tooth by different methods can affect its conservation and longevity .
it has been shown that the acid - etch technique might influence the deformation of a cusp under occlusal forces [ 1113 ] .
latino et al   found no difference between restorative materials ( amalgam , bonded amalgam , and composite ) regarding the ability to reinforce unsupported enamel ; however , sound dentin was the most reinforcing factor .
enamel has been proved to be the most appropriate tissue against the opposing tooth enamel , both esthetically and mechanically . as the major role of the enamel is to preserve function and the natural contact points of the teeth , reinforcement of the unsupported enamel
apparently , two major factors may provide the dentinal support of the enamel ; namely , strong connection at the dentino - enamel junction ( dej ) and appropriate dentin elasticity .
inherent dentin elasticity enables it to absorb the mechanical energy applied to the tooth ; consequently , leading to resistance against masticatory forces , whereas the enamel does not possess such characteristics .
needless to say that the dentino - enamel junction may serve the tooth strength by its unique characterizations .
the purpose of the present study was to investigate the effect of various restorative materials and bonding agents on the unsupported enamel reinforcement and to compare it with sound dentin .
in this in vitro study , forty - five extracted human molar teeth were selected , washed thoroughly , and stored in chloramine t for one week . after cleansing with rubber cup and pumice ,
the teeth were randomly divided into five groups of nine . high - speed hand piece and a diamond fissure bur ( no=837 l 0.012 , teeskavan )
was used to remove the lingual cusps of each tooth . as proposed by probhakar et al ,
two cuts were used in each tooth ; the first cut was vertical , parallel to the long axis and 1 mm facial to the central groove extending mesially and distally through the marginal ridges .
the second cut was horizontal , perpendicular to the long axis of the tooth on the lingual surface and approximately 2 mm coronal to the cemento - enamel junction ( cej ) , extending to join the first cut .
the group with sound dentin was considered as the positive control . in groups 2 to 5
, the dentin supporting the buccal cusps was removed at approximately 3 mm gingival to the occlusal enamel of the buccal cusps using a no .
all steps were carried out under sufficient light , taking care to reduce the risk of enamel crack or wall crazing , while undermining the enamel .
after this step , these groups received different treatments as mentioned below : 
 -pc , the dentin of facial cusps was left intact ( positive control group).-nc , the prepared cavities were not restored ( negative control group).-a , the prepared cavities were filled using two layers of varnish ( kimia ) and spherical amalgam ( sina ) inserted in 0.51 mm thick increments and thoroughly condensed . preparation and manipulation of the amalgam was performed according to manufacturer s instructions .-
ba , the teeth were restored with an amalgam bonded system , ( panavia , kuraray company ) and spherical amalgam ( sina ) .
c , the cavities were filled using resin composite and a fifth generation dentin - bonding agent , single bond ( 3 m ) .
the composite resin ( tetric ceram viva dent ) was inserted incrementally , each layer light cured for 10 seconds using colten light cure unit ( coltolux 2.5 ) with light intensity of 300 mw / cm .
-pc , the dentin of facial cusps was left intact ( positive control group ) .
-a , the prepared cavities were filled using two layers of varnish ( kimia ) and spherical amalgam ( sina ) inserted in 0.51 mm thick increments and thoroughly condensed . preparation and manipulation of the amalgam
-ba , the teeth were restored with an amalgam bonded system , ( panavia , kuraray company ) and spherical amalgam ( sina ) .
-c , the cavities were filled using resin composite and a fifth generation dentin - bonding agent , single bond ( 3 m ) .
the composite resin ( tetric ceram viva dent ) was inserted incrementally , each layer light cured for 10 seconds using colten light cure unit ( coltolux 2.5 ) with light intensity of 300 mw / cm .
all specimens were thermocycled for 1500 cycles at 3c and 45c with a dwell time of 20 seconds , and then kept at room temperature in distilled water before mounting them in self - cure acrylic resin .
then a flat horizontal surface of approximately 1.52.5 mm was cut at the tips of the remaining enamel cusps using a diamond disk .
all the specimens were then loaded compressively to fracture using an instron testing machine ( model , 1195 ) .
the end of the rod was placed against the flattened area of the enamel ( fig 1 ) .
the data were subjected to one way analysis of variance ( anova ) and duncan post hoc tests .
the results showed that the negative control group with not restored undermined enamel was significantly weaker than all the other groups , except the composite group .
the resin composite group was not significantly different from either the negative or the amalgam restored groups ( p=0.056 , p=0.642 , respectively ) .
bonded amalgam restorations were not significantly different from the positive group ( p=0.762 ) . both amalgam and
in addition , the amalgam group revealed a significantly less reinforcing effect than the amalgam bonded restorations and the positive group ( p=0.00 ) .
nowadays , bonding technology is believed to possess the capability to strengthen tooth structure [ 810 ] . in the present study
, there was no significant difference in the amount of support provided by bonded amalgam ( panavia f ) and sound dentin ( p=0.762 ) , showing the ability of this method of restoration in maintenance and reinforcement of the undermined enamel .
latino et al   showed no significant difference among different restorative materials in their ability to support undermined enamel .
grisanti et al   showed no significant difference in enamel support between tooth colored restorative materials ( composite resin , resin - modified glass ionomer and conventional glass - ionomer ) . in both the above - mentioned studies
mc cullock et al   revealed that adhesive materials increased tooth resistance to fracture as much as two to six times , depending on the applied technique .
pilo et al   proved that adhesives enhanced the reinforcement effects of bonded amalgam up to 39%61% , resulting in a recovery in amalgam stiffness .
eakle   also attributed fruitful results to the application of dentinal adhesive in the promotion of tooth resistance to fracture .
these results support our study , which shows that amalgam bond restoration causes the same strength as sound dentin and significantly more than unrestored cusp .
it should be mentioned that dias de souza et al   reported no significant differences in fracture resistance of non - bonded amalgam combined with varnish restorations as compared with those restored with bonded amalgam plus panavia f , which is not supported by the results of the present study .
molinaro et al   showed that both packable and conventional composite resins could not provide much cuspal stiffness compared to bonded amalgam .
this confirms the results of the present study , which surprisingly showed no statistically significant difference between resin composite and undermined enamel groups ( p=0.056 ) , meaning that resin composite is not able to provide satisfactory support for the undermined enamel .
this does not agree with mckenzie   who showed that composite restored teeth were found to be significantly stronger than those unrestored ; however , in their study , resin composite was not compared with bonded and non - bonded amalgam .
other than the type of material , the method of insertion with care to prevent voids is important .
bonded amalgam ( panavia f ) could support undermined enamel as well as sound dentin .
 objective : preservation of unsupported occlusal enamel after removal of underlying carious dentin may result in maintenance of aesthetics as well as wear resistance against the opposing enamel . 
 this study investigates the influence of different restorative materials and bonding agents on reinforcement of unsupported enamel in molars and compares it with sound dentin.materials and methods : in this in vitro study , forty- five extracted human molars were selected and randomly divided into five groups of nine . 
 all lingual cusps were cut off . 
 the dentin underlying the buccal cusps was removed in all groups except the positive control . 
 the negative control group received no restorations . 
 after application of varnish and panavia f , spherical amalgam ( sina ) and after application of single - bond ( 3 m ) , composite resin ( tetric ceram ) was used to replace missing dentin.all specimens were thermocycled , then mounted in acrylic resin using a surveyor . 
 lingual inclination of facial cusps was positioned horizontally . 
 load was applied by an instron machine at a crosshead speed of 10 mm / min until fracture.data were subjected to anova ( one way ) and post hoc test ( duncan).results : statistically significant differences were found between the five groups ( p<0.001 ) ; however , no significant difference was revealed between bonded amalgam and the positive control groups ( p=0.762 ) . 
 composite and amalgam had the same effect ( p=0.642 ) , while the composite and negative group had no significant difference ( p=0.056).conclusion : bonded amalgam systems ( panavia f ) could reinforce the undermined occlusal enamel effectively . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSION