| With the development of the dental restorative materials and adhesive technology,the restoration of teeth after root canal treatment tends to diversity.Except of the traditional restorative way like full crown or post-core crown,composite resin filling as a kind of less tooth preparation restorative way is becoming more and more popular.Clinically,the fracture of maxillary premolar is common due to its anatomical features.The tooth cusp of premolar is very sharp.When the premolar chews,the lateral force is big.Besides,the premolar had a narrow neck so the fracture resistance is weak.Secondly,the special location of premolar makes it both has the function of assisting canine to tear food and the function of assisting molars to mash food.Therefore,premolar share the greater occlusal force.After root canal treatment,the fracture probability of premolar increases.While how to restore it properly remains controversial.Especially for small tooth defect,such as MO cavity,the optimal way of restoration is still inconclusive.Clinically,dentists often use composite resin filling after root canal treatment.At present,more and more new adhesive systems have been continuously introduced in dental field.The universal adhesives are popular among clinicians for its easy operation.The universal adhesive is a kind of single component,one-bottled and light-cured adhesive.Because it contains weak acid,clinicians can choose etch-rinse technique,selective-etch technique or self-etch technique according to indication.However,there are few experimental and clinical trials of universal adhesives.The effect still need to be proved in clinical.From previous studies,after etching,the bonding strength of enamel had significant improvement.But,whether etching can improve the bonding strength of dentin is still unknown.Due to the uncontrolled factors in clinical trials,evaluation in vitro experiment is more appropriate.Various methods can be used to evaluate the effect of dental adhesive system.The dynamic fatigue cyclic experiments can simulate the situation of chewing function and in this way,the restorative method and material can be evaluated appropriately.In clinical,stress fatigue is a direct reason to tooth fracture especially the tooth after root canal treatment.In addition,after a long period of time in month,the temperature change can also cause the aging of adhesive and lead to the failure of restoration.Therefore,the aging test is also very important on the evaluation of bonding effect.[Objective]The purpose of this study was to evaluate the fracture resistance of endodontically treated maxillary premolar with MO cavity by different adhesive systems of the universal adhesive before or after thermal cycles.[Methods]Forty four intact and fresh maxillary premolars extracted from orthodontic patients.After root canal treatment,according to fixed size to prepare MO cavity.Teeth were randomly divided into four groups.TE group(total-etch group):32%phosphoric acid was used for tooth enamel for 30 seconds and used for dentin surface for 15 seconds.After washing and dry,the universal adhesive was used and light cured.Then,the cavity was filled with composite resin;SE groups(selective-etch group):32%phosphoric acid was used for tooth enamel for 30 seconds.No pretreatment was need for dentin surface.After washing and dry,the universal adhesive was used and light cured.Then,the cavity was filled with composite resin;TET group(total-etch after thermal cycle group):after total-etch,the universal adhesive was used and light cured.Then,the cavity was filled with composite resin.And then the specimens were under thermal cycles(20 000 times);SET groups(selective-etch after thermal cycle group):after selective-etch,the universal adhesive was used and light cured.Then,the cavity was filled with composite resin.And then the specimens were under thermal cycles(20 000 times).Each group randomly selected one specimen to cut along the buccolingual direction and observe the enamel and dentin bonding interface using scanning electron microscope.The rest specimens were embedded and fixed on the fatigue loading machine.The tooth length axis is 45 ° angle with the vertical direction.The loading head was on the 1/2 of palate cusp.The fatigue load cycles were 1.2 million times,50 N,2 Hz.Fracture mode of the specimen was observed by stereomicroscope.And the monitor was used to record the accurate times of tooth fracture.Survival analysis of specimens was analyzed by Log-rank test(P=0.05).Fracture mode can be divided into 4 types:type Ⅰ-not involving the composite resin and the fracture line above the CEJ;Type Ⅱ-involving the composite resin and the fracture line above the CEJ;Type Ⅲ-the fracture line between the CEJ and 2 mm below CEJ;Type Ⅳ-the fracture line below 2 mm below CEJ.[Results]Before thermal cycles,the survival analysis has no statistical difference between TE group and SE group.After thermal cycles,the survival analysis has no statistical difference between TET group and SET group as well.But there was a significant difference between SE group and SET group(p<0.05).While,no difference was found between TE and TET.After thermal cycles,the proportion of severe fracture(the fracture line below CEJ)of TET group and SET group had increased.Through scanning electron microscopy,before thermal cycles,the enamel bonding interface of TE group and SE can observe resin tags.Resin tags can be found in the dentin bonding interface of TE group while none can be seen in SE group.After thermal cycles,the enamel and dentine interface of TET group and SET groups were fractured and filler exposure can be found in universal adhesive.[Conclusions]Total-etch technique or selective-etch technique by using universal adhesive has no effect on the restorative success of maxillary premolar with MO cavity after endodontically treatment no matter before or after thermal cycles.But during thermal cycles,the descendent range of fracture resistance of maxillary premolar by using selective-etch technique is greater. |
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