| Abstract|| |
Aim: The aim of this study was to evaluate the in vitro fracture resistance of teeth restored with bonded ceramic inlay and direct composite resin restoration in comparison to the normal tooth.
Materials and Methods: This study evaluated the fracture strength of the teeth restored with bonded ceramic inlay and direct composite resin restoration in comparison to the normal teeth. Thirty intact human maxillary first premolars were assigned to three groups: Group 1 - comprising sound/unprepared teeth (control). Group 2 - comprising of Class-II direct composite resin restored teeth and Group 3 - comprising Class-II ceramic inlay restored teeth. Cavities were prepared with occlusal width of 1/3 intercuspal distance and 2 mm deep pulpally. Group 2 teeth were restored with hybrid composite resin (Z350 3M ESPE, USA) and group 3 teeth were restored with Vitadur Alpha alumina (Ivoclare Vivadent, Liechtenstein, Europe). Ceramic inlay was bonded with adhesive cement (rely X resin cement of 3MESPE, USA). The specimens were subjected to a compressive load until they fractured. Data were analyzed statistically by unpaired Student's t test.
Results: The fracture resistant strength, expressed as kilonewton (KN), was group 1 - 1.51 KN, group 2 - 1.25 KN, and group 3 - 1.58 KN. Statistically, group III had highest fracture resistance followed by group I, while group II had the lowest average fracture resistance.
Conclusion: The fracture resistant strength of teeth restored with ceramic inlay was comparable to that of the normal intact teeth or slightly higher, while teeth restored with direct composite resin restoration showed less fracture resistant strength than that of the normal teeth.
Keywords: Ceramic inlay, direct composite, fracture resistance
|How to cite this article:|
Desai PD, Das UK. Comparison of fracture resistance of teeth restored with ceramic inlay and resin composite: An in vitro study. Indian J Dent Res 2011;22:877
When carious cavities occur in teeth, various materials can be used for the restoration of teeth. These include Amalgam, Gold inlay, Glass ionomer, Composite resin, and Ceramic. Amalgam is used in posterior teeth for its longevity, although there are concerns about its dark appearance and mercury toxicity. Gold inlay is used but is not very popular because of its yellow colour, high cost, and time-consuming procedure for preparation. Composite resin and Glass ionomers are good aesthetic materials, but they have their own drawbacks. Composite gets discolored with time and may lead to postoperative sensitivity due to polymerization shrinkage. Glass ionomer is a good aesthetic material, but it has low compressive strength, and so it is not used in posterior teeth restorations. To overcome these drawbacks, dental researchers have introduced high strength ceramic as restorative material for posterior teeth. Ceramic has been well used in dentistry for over 100 years but the problem with early ceramics includes porcelain weakness, microleakage, cement failure, and poor fit. Recent developments in dental material science have led to the development of all ceramic material with high strength. Ceramics is claimed to be most biocompatible material used in dentistry. It gives natural appearance and it has high patient acceptance. It is chemically indestructible in oral environment. It is the most durable aesthetic material and is impervious to oral fluids. Its colour translucency and vitality cannot be matched by other materials.
|How to cite this URL:|
Desai PD, Das UK. Comparison of fracture resistance of teeth restored with ceramic inlay and resin composite: An in vitro study. Indian J Dent Res [serial online] 2011 [cited 2013 May 23];22:877. Available from: http://www.ijdr.in/text.asp?2011/22/6/877/94663
Ceramic has been used in dentistry as inlay, onlay, veneer, crown, and bridges. Amongst these indirect adhesive restorations, a Ceramic inlay is recently most useful alternative to Class-II amalgam and gold inlay restorations. Indirect aesthetic adhesive restorations have been clinically accepted restorations for posterior teeth. Ceramic based inlay also reinforces the remaining dental hard tissue. It maintains better anatomic forms, exhibit better marginal integrity, and colour stability in oral cavity. Ceramic inlays are indicated for restoration of moderate to large defects in posterior teeth due to caries, fracture, and developmental defect or for replacement of broken or lost filling.
In literature search, we found that there is not enough strong evidence to compare ceramic inlays with other types of restorations, but there are limited researches on ceramic inlay as restorative material. , Hence this study was undertaken to compare the fracture resistance of teeth restored with high strength ceramic inlay and hybrid composite resin restoration.  The null hypothesis tested was that there is no significant difference in fracture strength between sound teeth and teeth restored with ceramic inlay, while teeth restored with composite resin would show less fracture strength. 
| Materials and Methods|| |
0Thirty recently extracted sound human maxillary permanent premolars were selected. Teeth were free of caries and cracks and restorations. Any calculi and soft tissue deposits were removed with hand scalar. Numbers of teeth were divided into three groups. There were ten teeth in each group. Group 1 - comprising sound/unprepared teeth (control). Group 2 - comprising of Class-II direct composite resin restored teeth, and group 3 - comprising Class-II ceramic inlay restored teeth.
The selected teeth were mounted with their roots embedded in autopolymerized acrylic resin in plastic ring (which was of one inch in diameter) so that the cemento-enamel junction of tooth remained 1 mm above the acrylic resin surface. The teeth were positioned in the center of the ring.
In group I, sound/unprepared teeth were kept intact for comparison with study group.
In group II, Class-II cavity was prepared in each tooth with 2 mm depth pulpally, 4 mm deep in proximal boxes, width of cavity was 1/3 rd of intercuspal distance, and cavo surface angle is beveled. Total etching was done with 37% phosphoric acid for 15 seconds. Then the teeth were rinsed for 30 seconds. After air drying, two coats of single bond bonding agent (single bond, 3M ESPE, USA) were applied in each cavity and light cured for 20 seconds and then teeth were restored with high-strength hybrid composite resin (Z 350, 3M ESPE, USA) with incremental technique and light curing each increment for 30 seconds according to the instructions given by the manufacturer.
In group III teeth, Class-II inlay cavity was prepared with 2 mm depth pulpally, 4 mm deep in proximal boxes, width of cavity was 1/3 rd of intercuspal distance, and cavo surface angle is not beveled.  Impression of prepared teeth was taken with an addition silicon impression material in an individual plastic tray and working dies were prepared in investment material in vacuum mixer. Vitadur Alpha alumina (Ivoclare Vivadent, Liechtenstein, Europe) ceramic restorations were prepared on these dies according to manufacturer's instructions. The internal fits of restorations were evaluated. Restorations were measured with thickness gauge and an occlusal thickness of 1.5-2 mm was considered as adequate. Internal or adhesive surface of ceramic inlay was treated with 10% hydrofluoric acid solution for 3 minutes, and then rinsed with water and then air dried. These etched porcelain surfaces were treated with a silane coupling agent (scotch bond ceramic primer 3M ESPE USA) for 5 minutes. Enamel surfaces of cavities were air dried and then acid etched for 15 seconds with 37% phosphoric acid gel and rinsed with water for 30 seconds. The rinsed surfaces were air dried lightly. Two coats of adhesive (single bond 3M ESPE USA) were applied on cut tooth surface and light cured for 30 seconds. Ceramic inlays were cemented with adhesive cement (relyx resin cement of 3M ESPE USA). Cement was hand mixed and applied to both prepared teeth and ceramic inlay restoration. After seating of restoration on the prepared teeth once cement sets, excess cement was removed with scaler. Finishing burs were used to smooth the surface.
All the restored teeth were stored in distilled water at room temperature for 2 weeks. Teeth were subjected to a thermal cycling regime of 700 cycles between 5 and 55° C with dwell time of 30 seconds at each temperature. Before the fracture strength test, again these teeth were stored in water for 2 weeks. Thereafter, the specimen were individually tested in a universal testing machine at cross head speed of 0.5 mm/ min, for this purpose a 2.8 mm diameter rod was mounted in the moving arm which was in contact with the center of occlusal surface of the restored tooth during fracture test. All specimens were loaded by compression until they fractured. The ultimate fracture load was recorded. 
| Results|| |
According to [Table 1] the fracture strength means (KN) and standard deviation for three groups were as follows-
Sound/unprepared group - 1.51 ± 0.19
Composite resin restoration group - 1.25 ± 0.15
Ceramic inlay group - 1.58 ± 0.17
Group III had highest average KN followed by group I. Group II had the lowest average KN value as shown in the table. Statistical analysis was done using Student's t test. We found that group III had highest KN value compared to group II (t=4.50, d.f.=18, P<0.001). However, group I and group III did not differ significantly from each other (t=1.17, d.f.=18, P>0.05). The average KN value for group I was significantly higher than that of group II (t=3.29, d.f.=18, P<0.01).
| Discussion|| |
0Increasing demand of nonmetallic aesthetic restorations has led dental researchers to develop many materials such as Glass ionomer cement, Hybrid ionomer cement, Composite resins, and Ceramic. Among them composite and ceramics are used for restoration of posterior teeth. Composite resins used as a direct or indirect adhesive restorations exhibit similar flexure strength, flexure modulus, and hardness. However, primary reason for failure of Composite resin restoration was fracture of tooth or restoration, wear off, loss of bond between tooth, and restoration or loss of marginal adaptation.  Recently many types of all Ceramic materials with high value of compressive strength have been developed and improved adhesive techniques have enabled their use in fabricating all ceramic restorations. Ceramics may be sintered, milled infiltrated, pressed, or cast. , Ceramic inlay restorations perform well in terms of long term retention, colour match, and anatomic contour stability. There is limited marginal deterioration; therefore, they are not predisposed to marginal discoloration or secondary caries. Patients rarely suffer from postoperative sensitivity after placement of the ceramic inlay. 
Teeth also become more brittle with age and are therefore more susceptible for cracking and fracture especially if the tooth has been weakened by restorative procedure.  The effect of masticatory stresses on teeth either restored or unrestored is variable. Sound teeth rarely fracture from masticatory stresses, but cusp fracture may occur in teeth with cavity preparation and restorations.  Cavity preparations significantly weaken the remaining tooth structure. Direct and indirect intracoronal adhesive restorations can partly restore fracture resistance of teeth weakened by wide cavity preparation.  Therefore, adhesive restorations are useful to reinforce the tooth weaken by cavity preparation. Considering the effect of cavity design, the onlay design is more efficacious in protecting the tooth structures than the inlay design.  Resin cement used for cementation of adhesive restoration is elastic and tends to deform under stress making them capable of absorbing more stress. Therefore, success of ceramic inlay is absolutely dependent on the creation of an uncompromised adhesive tooth ceramic interface. 
Dalpino et al. have checked fracture resistance of teeth restored with direct and indirect composite resin and indirect ceramic restorations. They found that bonded indirect ceramic restoration requires higher load to fracture than direct and indirect composite resin restorations. A bonded indirect restoration with ceramic is ideal option for restoring teeth weakened by wide cavity preparation.  But the advantage of posterior composites is that it can be placed in one appointment, while ceramic inlays usually require two appointments with the dentist, because of the time needed for its fabrication in the laboratory.
Yamanel et al. had done an evaluation on the effects of restorative materials and cavity design on stress distribution on the tooth structures and restorative materials by means of three-dimensional (3-D) finite-element analysis. Two different nano filled composites and two different all ceramic materials were used in their study. A permanent molar tooth was modeled with enamel and dentin structures. 3-D inlay and onlay cavity designs were created; the results of this study showed that in the case of materials with low elastic moduli, more stress was transferred to the tooth structures. Therefore, compared to the nanofilled composites, the all ceramic inlay and onlay materials tested transferred less stress to the tooth structures. 
Present study evaluated the importance of ceramic as restorative material and strengthening of the teeth restored with ceramic. Cavity preparation is primarily based on the preservation of dental structure and physical properties of the restorative material in accord with the principle of adaptation, resistance and retention, occlusion, and aesthetics.  Findings of the present study [Table 1] are consistent with studies done by Morimoto et al., Dejak et al., Santos et al., Jensen et al., and Eackle et al. ,,,, Marimoto et al. reported that fracture strength of teeth restored with inlay and onlay ceramics are near to normal teeth.  Dejak et al. also reported that adhesively bonded composite resin and ceramic inlay reinforce the structure of the posterior teeth.  Santos et al. found in his research that cavity preparation significantly weakens the remaining tooth structure, and direct and indirect intracoronal adhesive restorations can partly restore fracture resistance of teeth weakened by wide cavity preparation.  Eackle et al. found in their study that fewer fractures occur in teeth if restoration is conservative.  Present study do support the other study by getting similar result and proved that ceramic inlay is better indication as posterior restoration where direct composite fail due to polymerization shrinkage. Ceramic inlay is a good aesthetic restoration because all ceramic material has good compressive and tensile strength. It is useful restoration in stress-bearing area. They considered that acid etching and adhesive technique do reinforce dental structures in a way that stiffness values of restored teeth approximate the value of sound teeth. Ceramic inlay with half or one-third of the intercuspal distance, luted with resin cements, can recover tooth strength near to that of the intact teeth, while teeth restored with composite resin had reduced or lesser strength than that of the intact teeth. ,,,
Many clinical studies are also present on ceramic inlay as restorative material for posterior teeth. Santos et al. had done clinical study for ceramic inlay and onlay for six month and found that both ceramics perform satisfactory after six months.  Manhart et al. found in their study that ceramic inlay shows 100% success rate, while composite inlay shows 90% success rate in their performance.  Thordrup et al. In their clinical study of four different tooth coloured inlay prepared by direct and indirect methods with ceramic and composite found that ceramic inlay may fail in long run. Survival rates of ceramic and composite inlay are not much different.  Thordrup et al. had done another clinical study and found that survival rates after 10 years of observation, both ceramic inlay and composite inlay, function well.  Lange and Pfeiffer had found in their clinical study that ceramic inlays performed better than direct Filtek Z250 composite restorations in marginal adaptation, colour match, and anatomic form, but with regard to survival probability, there was no significant difference.  Correlating the clinical studies, ,, in vitro study, ,, and present study we can say that ceramic inlay and onlay are better options for posterior tooth restoration because posterior tooth restoration does require material with good strength which all ceramic restoration have it.
Failures of ceramic restorations may also occur. Many reasons have been given for the failure of all ceramic restorations. The principle problem associated with the ceramic inlay appears to be related to cavity preparation, patient occlusion, cementing agents, insufficient thickness, and internal defect of ceramic.  Failure of ceramic restoration clinically appears as secondary caries, fracture of restoration or remaining tooth structure, marginal deficiency, and postoperative sensitivity. Among these bulk fracture is most common.
The method of occlusal loading during the fracture test is an important factor. Fracture resistance of teeth has been primarily tested using basic laboratory methods, where increasing load is applied on the intended surface until they fracture. These methods use the acute single axial loading failure but tooth fracture in oral cavity seems to result from the accumulation of repeated stress during oral functioning. Clinically axial forces and in addition lateral forces and fatigue loading should be considered. ,,
Limitation of the study is that the compressive test alone cannot simulate clinical conditions. Other in vitro tests such as stress distribution analysis, tension tests, and clinical studies should be developed to determine longevity of various ceramic restorations.
Ceramic inlays can offer an excellent appearance; however, their long term prognosis is uncertain, as only limited studies have reported the long term clinical performance of these restorations.
| Conclusion|| |
- Within limitation of this study following conclusions are drawn
- Fracture resistance of tooth restored with ceramic inlay is as good as intact tooth or more.
- Fracture resistance of tooth restored with hybrid composite is less than normal tooth and tooth restored with ceramic inlay.
Ceramic inlay is a better restorative alternative as compared to direct composite resin as far as the fracture resistance is concerned. More research is needed to determine whether ceramic inlays compare well over long term with amalgam, composite resin, or gold inlays.
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Priti D Desai
Department of Conservative and Endodontic Dentistry, Gurunanak Institute of Dental Science and Research, Kolkata