in the united states , 166 million dental restorations were placed in 2005,1 and clinical studies suggest that more than half were replacements for 
 failed restorations.2 it is anticipated that the 
 emphasis on replacement therapy will increase with the phasing out of dental amalgam .
global 
 concerns regarding mercury in the environment are the primary driver for the discontinuation of 
 dental amalgam . identified as one of the top five mercury - added products ,
dental amalgam is ranked 
 fifth behind batteries , measuring devices , electrical switches and relays , and mercury - containing 
 light bulbs.3 resin composite is the most common alternative to dental amalgam,4 but numerous studies report that composite restorations have more 
 recurrent caries , higher failure rates , and increased frequency of replacement.2,410 simecek et al reviewed the dental records of more 
 than 3000 patients and concluded that there was a significantly higher risk of replacement for 
 posterior composite restorations as compared to amalgam.4 in a study of posterior restorations placed by 243 norwegian dentists , failed amalgam 
 restorations had a mean age of about 11 years , while the mean age of failed composite restorations 
 was statistically significantly lower at 6 years.8 
 a study of composite and amalgam restorations in the pediatric population indicated that the need 
 for additional treatment was 50% greater in children receiving composite restorations.11 depending on factors , including the size of the 
 restoration , tooth location , and patient type , the lifetime of large to moderate posterior composite 
 restorations is approximately one - half that of dental amalgam.12 the use of composite to restore form and function for posterior teeth damaged by disease , age , or 
 trauma is gaining wide acceptance by the dental community .
a myriad of factors can influence the 
 clinical success of class ii composite restorations .
clinical parameters , including patient 
 characteristics , tooth preparation , matrix utilization , and composite composition 
the popularity and demand for resin - based posterior restorations has been increasing steadily 
 since the introduction of these materials in the mid-1950s . the societal focus on aesthetics
as well 
 as the worldwide move toward eliminating amalgam restorative materials has contributed to this 
 phenomenon.3 unfortunately the success and/or 
 failure of resin - based composite restorations is dependent upon variables that may be difficult for 
 the operator to control .
for example , restorations placed in patients with high caries risk have 
 restoration failure rates two times those of patients with low caries risk.13 these findings have been documented in the adult as well as the 
 pediatric dental patient population.14 clinical 
 data indicate that regardless of which preparation design is adopted or the type of posterior 
 resin - based restoration that is utilized , the practitioner must give careful consideration to the 
 caries status of the patient and adjust recommendations for restorative materials accordingly .
posterior resin restorations have been indicated for various types of tooth preparations . in 
 particular ,
resins are utilized to maximize aesthetics and minimize the loss of tooth structure 
 during preparation . due to the location of the caries and
thus the need to restore proximal surfaces 
 in class ii restorations , a number of tooth preparation designs have been advocated .
the underlying 
 goal of all of these tooth preparation designs is a reduction in the loss of sound tooth 
 structure .
the  tunnel  technique , as reported by hunt15 and knight,16 has 
 been used to remove proximal caries while leaving the marginal ridge intact .
although potentially 
 promising , the lack of long - term clinical studies limits wide adoption of this technique.17 the ability to access and restore a proximal carious 
 lesion directly represents the most conservative proximal restorative technique available.17 this technique is relatively successful in 
 preserving intact tooth structure ( figures 1 and 
 2 ) .
minibox or 
  slot  preparations for the restoration of proximal lesions in posterior teeth have 
 also been recommended by clinicians and researchers .
these preparation designs have been described 
 as minimally invasive and relatively successful with a reported 70% success rate over an 
 average of 7 years.18 the aforementioned tooth preparation designs successfully limit the removal of sound tooth 
 structure and take advantage of appropriate etching techniques in bonding to intact enamel and 
 dentin . however , depending upon the location and extent of the caries , traditional preparation 
 designs , which involve access through the carious marginal ridge and the removal of infected 
 occlusal enamel and dentin , may be required .
these more invasive preparations are indicated in this 
 clinical situation ( figure 3 ) and are well 
 documented in the literature.19 whenever 
 possible , conservative structure - sparing preparation techniques should be used . when restoring 
 proximal surfaces with resin - based composite .
considerable attention has been devoted to the relationship between cavity type , cavity size , 
 number of surfaces restored , and the risk of restoration failure .
as the number of restored surfaces 
 increases , the risk of restoration failure also increases.2022 for 
 example , as reported in the 2012 review by demarco et al,23 single - surface and class i restorations are less likely to fail as compared to 
 multisurface restorations , and class ii restorations . to minimize restoration failure and mitigate 
 the effects of bonding multiple tooth surfaces , most clinical strategies have focused on methods to 
 decrease the ratio between the bonded surface area to the nonbonded surface area , also described as 
 the cavity configuration or c - factor . the higher the c - factor the less chance for relaxation of 
 polymerization shrinkage .
some studies have indicated that the increase in c - factor is also 
 associated with decreased bond strength.24,25 however , recent investigations have suggested that 
 this finding may not be valid for the newer low - shrink resin - based composites.26 along with preparation design and extent of tissue removal , the position of the tooth in the 
 mouth directly influences the overall clinical performance and longevity of the restoration .
studies 
 suggest that restorations placed in premolars fail less often than similar restorations placed in 
 molars.20,21 intuitively this finding makes sense in that the masticatory forces 
 and stresses placed on restorations in molar teeth are higher than those placed in premolars . 

nonetheless , the findings in terms of tooth position and number of restored surfaces indicate that 
 clinicians should utilize posterior resin composites in areas where aesthetics is deemed essential 
 and should maintain as much tooth structure as possible .
figures 4 and 5 illustrate the aesthetic 
 results obtained when replacing a proximal amalgam restoration with a resin - based composite 
 restoration .
the techniques used to fill and cure resin - based composites , particularly in areas of high 
 masticatory stresses , have received considerable attention .
the debate among researchers as well as 
 practitioners regarding bulk cure versus incremental cure continues .
incremental filling techniques 
 ( figure 6 ) have long been recommended due to the 
 polymerization shrinkage associated with dental composites . reducing the volume of composite
that is 
 polymerized at each stage of the restorative procedure minimizes shrinkage and maximizes the 
 conversion of monomers to polymer .
this is achieved , in part , by decreasing the attenuation of the 
 curing light.27 while incremental filling 
 techniques have been taught and utilized for decades , some studies indicate that incremental filling 
 of resin - based composites produces higher shrinkage stress.27,28 in direct 
 contrast , more recent studies report that incremental filling produces lower shrinkage stress when 
 compared to bulk filling techniques.29,30 these diverse and contradictory conclusions are 
 likely due to different testing methods.31 
 currently , manufacturers are striving to produce resin - based composite systems that have less 
 polymerization shrinkage ( < 2% ) and , more importantly , reduced polymerization 
 shrinkage stress .
strategies to improve shrinkage include utilizing new low - shrinking monomers or 
 those with an increased molecular weight.26 as 
 the low - shrinking composite resins improve , incremental filling and curing of posterior composites 
 may no longer be recommended .
however , until the long - term clinical success of the lower shrinking 
 composite resin systems is confirmed , using an incremental filling technique in deep cavity 
 preparations is recommended.26 the influence of matrix type on the quality of the proximal contact and the ease of placement of 
 class ii resin restorations has also been evaluated .
the ability to reproduce an appropriate , 
 functional , proximal contact with a class ii resin restoration is important to minimize food 
 impaction and thus maintain healthy periodontal tissues .
open margin  through which oral fluids , eg , 
 saliva , enzymes , water , and cariogenic bacteria , may penetrate .
this marginal leakage can lead to 
 recurrent caries , which is the most often cited reason for composite restoration failure.2,410 manufacturers have introduced various types of matrices into the dental market with the goal of 
 affecting or influencing the direction of composite shrinkage during polymerization.32 the literature no longer supports the concept of 
  directional polymerization,33 but these 
 matrices still exist .
although there are a myriad of different shapes and sizes , the majority of 
 matrices fall into one of two basic types : ( 1 ) metal matrices , which are straight or 
 circumferential / precontoured and ( 2 ) transparent matrices which are either straight or 
 circumferential / precontoured .
despite the theory that transparent matrices will enhance 
 polymerization at the gingival margin , the recent literature suggests that the choice of matrix does 
 not influence the clinical success of class ii posterior resins.32 in addition to matrix type , there are numerous tooth separation ( wedging ) products and 
 techniques .
the literature suggests that the type 
 of matrix material / wedge does not influence the clinical performance of class ii composite 
 restorations.34 however , the literature does 
 indicate that no matrix / wedge combination can accurately reproduce an intact proximal surface 
 contact at the precise location of the natural intact tooth.35
researchers and industry continue their efforts to modify composite resin restorative materials 
 in order to improve their handling characteristics , mechanical and physical properties , and clinical 
 performance .
the majority of the current resin composites have mechanical properties that make them 
 suitable for use in all areas of the mouth .
the functionality of these restorations , however , in 
 areas of high masticatory stress is still a concern .
resin restorations that are placed in areas of 
 high function are more prone to exhibit excessive wear and/or marginal fracture despite the advances 
 in the current materials .
clinicians must exercise caution when placing large resin - based composite 
 restorations in areas of high function .
the longevity of posterior resin restorations placed in 
 patients who have a history of clenching or grinding may be particularly limited.35 while resin composition , tooth preparation design , and matrix systems may influence the lifetime 
 of posterior composite restorations , the primary factor in the clinical failure of moderate to large 
 composite restorations is secondary caries at the margins of the restorations.8 as an example , in a study of radiographs from 459 adults , age 
 1819 years , the investigators reported that , among interproximal restorations , the failure 
 rate as a result of secondary or recurrent caries was 43% for composite as compared to 
 8% for amalgam.7 in a separate study of 
 amalgam and composite restorations placed in 812-year - old children , the primary reason for 
 failure of both materials was secondary caries , but secondary caries was 3.5 times higher in 
 composite restorations.5 an increase in secondary caries at the margins of composite restorations suggests that the seal 
 at the composite  tooth interface is not adequate to resist the physical , chemical , and 
 mechanical stresses that are present in the mouth .
the failure of moderate to large composite 
 restorations has been linked to the degradation of the bond at the tooth surface  composite 
 material interface12,3641 and an 
 increase in the concentration of the cariogenic bacterium streptococcus mutans at 
 the perimeter of these materials.4246 degradation of the bond at the interface between the 
 tooth and composite has been associated with the failure of adhesives to form an impervious seal 
 with the dentin.2,4150 
 failure of the adhesive / dentin ( a / d ) bond leads to open pores at the composite 
tooth 
 interface and bacterial enzymes , oral fluids , and even bacteria can penetrate these open pores.51 data from in vivo and in vitro studies indicate that 
 the infiltration of these agents into the voids between the tooth and composite will lead to 
 recurrent caries , hypersensitivity , and pulpal inflammation.41,47,52,53 results from clinical studies indicate loss of retention , poor marginal adaptation , and 
 marginal discoloration when the a / d interface is exposed to the oral cavity.54 effective mechanical bonding between the composite restoration and 
 treated enamel has been achieved using appropriate acid - etching protocols , but failure of the bond 
 at the a / d interface threatens the long - term clinical survival of moderate to large posterior 
 composite restorations.39,41,43,52,5557 bonding failures have been commonly tracked to the gingival margin of class ii composite 
 restorations.58 a separation between the 
 composite material and tooth surface has been noted at the gingival margin.55 in class ii composite restorations , there is generally little enamel 
 available for bonding at the gingival margin ; therefore , the bond at this site depends on the 
 integrity of the seal formed with dentin .
gaps at the gingival margin have been attributed to 
 unreliable dentin bonding.55,57 in a study comparing the microtensile a / d bond 
 strength of gingival and proximal walls of class ii composite restorations , the adhesive bond to the 
 gingival wall was significantly weaker.59 a 
 complementary spectroscopic study reported a twofold difference in the extent of dentin 
 demineralization at the proximal and gingival margins.50 the difference in demineralization suggests less mineralized dentin at the gingival 
 margin .
the cumulative effect of less mineral , increased density , and size of the tubules60 would mean faster and deeper etching at the gingival 
 margin as compared to the proximal wall .
although the etch was deeper at the gingival margin , there 
 was considerably less adhesive infiltration of the demineralized dentin matrix at the gingival 
 margin.50 the discrepancy between etching depth 
 and adhesive infiltration led to a large area of exposed collagen at the gingival margin .
yoshiyama et al suggested that the increased number of tubules per unit area at the gingival 
 margin would promote efficient adhesive infiltration at this margin.61 however , other variables , including water content , interfere with 
 efficient adhesive infiltration at the gingival margin .
water content is higher in dentin at the 
 gingival margin as compared to the proximal wall .
water content is increased because of the water 
 present within the demineralized dentin matrix and patent tubules that contain a great deal of 
 dentinal fluid .
the presence of this fluid contributes to the contamination of the prepared 
 surface.62 the increased water leads to reduced 
 adhesive infiltration and lower monomer / polymer conversion of the adhesive at the gingival margin as 
 compared to the proximal wall.50 the impact of 
 water upon the effectiveness of bonding is further supported by in vitro investigations that 
 indicate that adhesive monomers or oligomers and unprotected collagen at the gingival margin of 
 class ii composite restorations undergo hydrolytic degradation after 90 days of aqueous 
 storage.49 wet bonding techniques were introduced in the early 1990s to counteract the problems noted with 
 collagen collapse following drying of the demineralized dentin matrix.6366 wet 
 bonding means that the demineralized dentin matrix is fully hydrated throughout the bonding 
 protocol . using this procedure ,
the channels between the demineralized dentin collagen fibrils are 
 filled with water , solvent , conditioner , and/or oral fluids.67,68 adhesive must 
 diffuse into the fluid - filled spaces of the substrate and along the collagen fibrils . ideally , the 
 solvent in combination with hydrophilic monomers , eg , hydroxyethyl methacrylate ( hema ) conditions 
 the collagen to remain expanded during adhesive infiltration . however
, hema , a primary component in 
 many single - bottle , commercial , dentin adhesives , can dramatically reduce the evaporation of 
 water.69 hydrophobic monomers , such as 
 2,2-bis[4(2-hydroxy-3-methacryloyloxy - propyloxy)-phenyl ] propane ( bisgma ) , would 
 resist diffusing into these sites where there is residual water.7072 in the in vivo situation , there may be little control over the amount of water left on the tooth . 
 thus , it is possible to leave the dentin surface so wet that the adhesive physically separates into 
 hydrophobic- and hydrophilic - rich phases.71,73,74 indeed , results from laboratory investigations indicate that excess moisture prohibited 
 the formation of an impervious structurally integrated a / d bond at the gingival margin of class ii 
 composite restorations.49,50 under clinical conditions , dentists must routinely attempt to bond to naturally wet substrates , 
 eg , caries - affected dentin75 or deep dentin.7679 the water content of caries - affected dentin is 2.7 times greater than that of normal 
 dentin.75 exposed tubules account for 22% 
 of the surface area in deep dentin . in contrast ,
exposed tubules account for 1% of the 
 surface area of dentin close to the dentino - enamel junction.80 the large increase in exposed tubules in deep dentin means that 
 pulpal fluid will contribute additional moisture to that already present within the demineralized 
 dentin matrix . with the sensitivity of our current adhesives to excess moisture ,
it is obvious that 
 bonding to these clinically relevant substrates is a formidable challenge.79,8183 this difficulty highlights the potential limitations 
 in utilizing resin - based composites to restore large , deep , carious lesions .
water blisters that form in adhesives placed on overly wet surfaces8486 and 
 adhesive phase separation that leads to very limited infiltration of the critical but hydrophobic 
 dimethacry - late component71,87,88 are two 
 examples of the sensitivity of our current adhesives to excess moisture .
the optimum amount of 
 wetness varies as a function of the adhesive system.89 it is impossible to simultaneously achieve uniform wetness on all of the walls of the 
 cavity preparation.90 in short , wet bonding is a 
 very technique - sensitive procedure .
optimum bonding with our current commercial dentin adhesives 
 occurs over a very narrow range of conditions , eg , water content.78 strategies to promote bonding of the resinous materials to intrinsically wet dentin substrates 
 include the incorporation of ionic and hydrophilic monomers into the adhesive.91 these adhesives etch and prime simultaneously , thus addressing the problems of collagen collapse 
 and simplifying the bonding protocol .
the hydrophilicity of these adhesives enhances water sorption , 
 which can lead to hydrolytic breakdown in the mouth.85,90,92 with these systems , the bonded interface lacks a nonsolvated 
 hydrophobic resin coating .
the hybrid layers made with these adhesive systems behave as 
 semi - permeable membranes ; water is transferred throughout the bonded interface even after adhesive 
 polymerization.54 the increase in the 
 concentration of hydrophilic monomers in these systems has been associated with decreased structural 
 integrity at the a / d interface.54,93 deterioration of the a / d bond formed with these 
 systems was noted after 1 year of in vivo aging.94 these results suggest that hydrophilicity and hydrolytic stability of resin monomers are 
 generally antagonistic.90
when measured immediately , dentin - composite bonds are generally considered adequate to tolerate 
 conditions in the mouth , but these bonds deteriorate with time .
the two major mechanisms of 
 deterioration are fatigue and hydrolysis.95 
 fatigue has been linked to stresses transmitted to the bond by occlusal forces , thermal expansion 
 and contraction , and polymerization shrinkage of the composite .
chronic deterioration of the 
 dentin - composite bond is also related to hydrolysis and leaching of the adhesive that has 
 infiltrated the tooth structure.70,79 fatigue investigations have indicated that the overall time - dependent behavior of the 
 composite  tooth interface is a complex function of the individual material phases . for 
 example , microfinite element analyses have shown that each material phase at the a / d interface 
 experiences different stress concentrations at functional loads.96,97 the overall 
 failure behavior of the bond at the a / d interface is not determined by the weakest component but by 
 the component whose stress concentration is closest to its failure strength .
similarly , the overall 
 fatigue life of the a / d interface is governed by the material component with the shortest fatigue 
 life under a given loading condition.98 under masticatory function the material components at the composite  tooth interface are 
 subjected to both chemical and mechanical stresses . the interplay between these stresses can result 
 in a deterioration of the properties of the material over time .
the breaking of covalent bonds by 
 addition of water to ester bonds is considered one of the primary reasons for deterioration of the 
 adhesive at the interface between the composite and tooth.89,90 interestingly , 
 degradation of methacrylate ester groups produces carboxylic acids  the same functional 
 group that is the culprit in lactic acid - induced dental caries .
the change in mechanical properties 
 of the materials can be contributed to a variety of mechanisms that include proliferation of surface 
 and subsurface flaws.95,97,99101 these flaws in combination with the chemical and 
 biochemical stresses that are present in the mouth can then lead to restoration failure . in conclusion , the a / d bond can be the first defense against substances that may penetrate and 
 ultimately undermine the gingival margin in composite restorations in vivo . it has been hypothesized 
 that the in vivo degradation of the bond at the a / d interface follows a cascade of events that 
 begins when the dentin is acid etched102,103 disruption of the tooth structure by acid etching 
 exposes and activates proteolytic enzymes , eg , matrix metalloproteinases ( mmps ) , that can degrade 
 the exposed collagen component of the hybrid layer.104,105 the following factors inhibit 
 the formation of a durable a / d bond : ( 1 ) water sorption and hydrolysis of the adhesive resin ; ( 2 ) 
 inadequate monomer / polymer conversion of the infiltrating adhesive ; ( 3 ) incomplete resin 
 infiltration of the demineralized dentin matrix ; ( 4 ) incomplete solvent evaporation ; and ( 5 ) 
 enzymatic challenges within the cavity preparation through exposure to oral fluids.49,71,83,104115 
 although durable a / d bonds are critical for maintaining a seal at the tooth  composite 
 interface , the properties of the materials are only one part of an extremely complex problem.116
restoring posterior teeth with resin - based composite materials continues to gain popularity among 
 clinicians , and the demand for such aesthetic restorations is increasing .
manufacturers are working 
 aggressively to improve resin composite materials by modifying components to decrease polymerization 
 shrinkage , to improve mechanical and physical properties , and to enhance handling characteristics . 

the two main causes of posterior composite restoration failure are secondary caries and fracture 
 ( restoration or tooth).35 a review and update of 
 posterior resin composites in terms of preparation design , matrix choice , and resin systems 
 demonstrate the limited extent to which these factors influence the overall clinical lifetime of 
 resins placed in posterior teeth .
clinical and patient factors , including caries risk , cavity size , 
 cavity type , number of restored surfaces , and position of the tooth in the mouth must be given 
 careful attention in the selection of any restorative material including composite resins .
while clinicians tend to focus on tooth form and function when evaluating the success and failure 
 of posterior resins , the emphasis must remain in advancing our understanding and knowledge of the 
 intricate and complicated characteristics of the restoration  tooth interface .
this paper 
 presents an update in existing technology and underscores the mechanisms that negatively impact the 
 durability of posterior composites in permanent teeth .
 restoring posterior teeth with resin - based composite materials continues to gain popularity among 
 clinicians , and the demand for such aesthetic restorations is increasing . indeed , the most common 
 aesthetic alternative to dental amalgam is resin composite . 
 moderate to large posterior composite 
 restorations , however , have higher failure rates , more recurrent caries , and increased frequency of 
 replacement . 
 investigators across the globe are researching new materials and techniques that will 
 improve the clinical performance , handling characteristics , and mechanical and physical properties 
 of composite resin restorative materials . despite such attention , large to moderate posterior 
 composite restorations continue to have a clinical lifetime that is approximately one - half that of 
 the dental amalgam . 
 while there are numerous recommendations regarding preparation design , 
 restoration placement , and polymerization technique , current research indicates that restoration 
 longevity depends on several variables that may be difficult for the dentist to control . 
 these 
 variables include the patient s caries risk , tooth position , patient habits , number of 
 restored surfaces , the quality of the tooth  restoration bond , and the ability of the 
 restorative material to produce a sealed tooth  restoration interface . 
 although clinicians 
 tend to focus on tooth form when evaluating the success and failure of posterior composite 
 restorations , the emphasis must remain on advancing our understanding of the clinical variables that 
 impact the formation of a durable seal at the restoration  tooth interface . 
 this paper 
 presents an update of existing technology and underscores the mechanisms that negatively impact the 
 durability of posterior composite restorations in permanent teeth . 
Clinical performance of composite versus dental amalgam restorations
Patient selection
Tooth preparation
Polymerization and matrices
Composite restoration failures
Sensitivity of adhesive to wet bonding conditions
Effects of function, fatigue, and degradation
Summary