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 .