Three-Year Clinical Performance of Composite Restorations Placed by Undergraduate Dental Students

Despite the significant decrease in caries prevalence worldwide, dental caries is still a significant problem and clinicians spend a considerable amount of time placing restorations to substitute the dental structure lost during caries progression (1). Direct restorations provide a reliable treatment to replace this lost dental structure, with a low cost, less sound dental structure removal and also good clinical performance when compared to indirect restorations (2-4). Amalgam used to be considered the best restorative material for posterior teeth. However, its use has significantly declined due to esthetic reasons and potential toxicity of mercury from amalgam restorations (5). Three-Year Clinical Performance of Composite Restorations Placed by Undergraduate Dental Students


INTRODUCTION
Despite the significant decrease in caries prevalence worldwide, dental caries is still a significant problem and clinicians spend a considerable amount of time placing restorations to substitute the dental structure lost during caries progression (1).Direct restorations provide a reliable treatment to replace this lost dental structure, with a low cost, less sound dental structure removal and also good clinical performance when compared to indirect restorations (2)(3)(4).Amalgam used to be considered the best restorative material for posterior teeth.However, its use has significantly declined due to esthetic reasons and potential toxicity of mercury from amalgam restorations (5).
The introduction of adhesive dentistry allowed the preservation of tooth structure usually lost in cavity preparations for amalgam restorations.However, composites became reliable materials only in the 1980's, with the introduction of visible light-curing technology, adhesion to dentin and improvements in filler technology that resulted in higher filler packing.Beforehand, failure due to insufficient wear resistance, loss of anatomic form and interproximal contacts and general degradation was a common event (3).
In 1990's, composite resin was the material of choice for anterior restorations due to the fulfillment of the esthetic expectations of patients.It was also indicated for posterior restorations, yet in small occlusal or occlusoproximal cavities, preferably with little occlusal function and in premolars (6).
A general trend towards adopting composites for posterior restorations in dental schools started in the late 1990's.An increase of the time dedicated to the teaching of composite restorations was experienced from the 1990's to the years 2000 (6).Yet, the little experience of the school staff in placing composites in posterior teeth probably limited the quality of the restorations placed by the students (7).Expectations are that the clinical behavior of these restorations might have been favored by the clinical experience acquired during that decade.
The currently available universal composites, either microhybrid, nanohybrid or nanoparticulate, present volumetric shrinkage lower than 4%, mechanical strength, polishability, polish retaining and wear resistance.Such improvements associated to the stateof-the-art placement technique have led to a clinical behavior comparable to that of amalgam restorations (7).In addition, the philosophical shift towards the preservation of tooth structure resulting from the minimal intervention dentistry concept increased the indication of composites as adhesive materials (3).
This retrospective study evaluated the clinical performance and the reasons for failure of class I, II, III and IV composite restorations placed by undergraduate dental students over a 3-year period.

MATERIAL AND METHODS
The study was approved by the institutional Ethics Committee (Dental School, UFPel) and the patients gave informed writing consent to be included in the study.Information about patients was collected from their dental files at the Dental School of the Federal University of Pelotas, Brazil.Patients who had their teeth restored with composite resin (class I, II, III and IV) by third or forth year dental students were selected.
The restorations were placed by dental students under supervision of a staff member, according to a pre-determined protocol, which included shade selection under natural light prior to isolation, rubber dam isolation, removal of pre-existing restoration or excavation of carious tissue based on tissue consistency criteria associated to caries dye solution.Enamel margins were regularized with gingival margin trimmers.Bevel was performed at the buccal face of the anterior teeth with diamond bur to help masking the restorative interface (8).Proximal space between teeth was guaranteed by pre-wedging with moist wooden wedges or mechanical separators.Class II restorations were performed using transparent matrix band and reflective wedges (TDV Dental Ltda., Pomerode, SC, Brazil).A totaletch adhesive system (Prime & Bond 2.1; Dentsply Ind. e Com.Ltda, Petrópolis, RJ, Brazil) was applied according to the manufacturer's instructions, and the teeth were restored incrementally with a highly filled hybrid composite (TPH; Dentsply).The restorations were finished with a sequential protocol using fine grit diamond burs and polishing discs (Soflex; 3M ESPE, St. Paul, MN, USA) under water refrigeration (9).
After 3 years, clinical examination and assessment of the quality of the restorations was performed by 2 examiners, calibrated at 2 different moments with a 30day interval (Cohen's Kappa = 0.85).The restorations were evaluated according to the modified USPHS criteria (10), shown in Table 1.Loss of the restoration, fracture of the restoration and loss of the tooth were considered as clinical failure (Table 2).Patients with systemic diseases or periodontal disease, those who refused to participate in the study or did not sign the informed consent form, and those who did not attend to the clinic of the Dental School were excluded from the study.
The statistical software packages used were EPI INFO 6.04 (Centers for Disease Control and Prevention, Atlanta, GA, USA) and STATA 5.0 (Stata Corp LP, College Station, TX, USA), Data were submitted to descriptive analysis to show the prevalence of satisfactory and unsatisfactory restorations regarding the different aspects evaluated, using Chi-square or Fisher's exact test, with confidence level of 95%.

RESULTS
One hundred and nine patients were eligible for evaluation and 102 were examined with 256 restorations being evaluated.Seven patients refused to participate of the study and were excluded.The mean age of the patients was 36.5 (±12.1)years.After 3 years of follow up, 85.5% of the restorations were considered as satisfactory (Table 3).
Class II restorations presented the highest prevalence of failure (72%), including restorations classified as unsatisfactory, loss of the restoration and loss of the tooth (Table 3).The second highest prevalence of failure was observed in class IV restorations.Although no class IV restoration was classified as unsatisfactory, 16.7% of them were lost.More than 90% of class I and class III restorations were classified as satisfactory (Table 3).Within the 3-year period, no restoration failed due to secondary caries (Table 4).

DISCUSSION
The present study evaluated the 3-year survival rate and the reasons for failure of composite restorations placed by dental students.One hundred seventy restorations were placed in anterior teeth (134 class III restorations and 36 class IV restorations) and 86 composite restorations were placed in posterior teeth (25 class I restorations and 61 class II restorations).Anterior composite restorations were regularly taught in our Dental School since the 1980's.However, the shift towards using composite in posterior teeth intensified only in the late 1990's.The restorations were placed with bevel at the cavosurface margins, since this procedure could increase the marginal adaptation and sealing ability (8).We have also used a immediate finishing and polishing of restorations in the present study; however, these immediate procedures seem not to affect the quality of composite restorations, when compared to mediate polishing and finishing (9).
The survival rate of anterior restorations was considerably higher for class III than for class IV restorations (91.8 and 77.8%, respectively) and confirms the influence of the restoration size on survival probability (4).

Criteria
Score Description

Alfa
The restoration appears to match the shade and translucency of adjacent tooth tissues.

Color match Bravo
The restoration does not match the shade and translucency of adjacent tooth tissues, but the mismatch is within the normal range of tooth shades.

Charlie
The restoration does not match the shade and translucency of adjacent tooth structure, and the mismatch is outside the normal range of tooth shades and translucency.

Alfa
There is no visual evidence of marginal discoloration different from the color of the restorative material and from the color of the adjacent tooth structure.

Marginal staining Bravo
There is visual evidence of marginal discoloration at the junction of the tooth structure and the restorations, but the discoloration has not penetrated along the restoration in a pulpal direction.

Charlie
There is visual evidence of marginal discoloration at the junction of the tooth structure and the restorations that has penetrated along the restoration in a pulpal direction.

Alfa
Surface texture is similar to enamel.

Surface roughness Bravo
Surface texture is similar to conventional composite resin or to white mounted stone.

Charlie
Surface has porosities, impairing the continuous movement of the explorer.

Alfa
The restoration appeared to adapt closely to the tooth.There are no catch or crevice at any point on the margin.There is no retention of the explorer on the margin.

Marginal adaptation Bravo
The explorer catches and there is visible evidence of a crevice, which the explorer penetrates, indicating that the edge of the restoration does not adapt closely to the tooth structure.The dentin and/or the base are not exposed, and the restoration is not mobile.

Charlie
The explorer penetrates crevice defect extended to the dento-enamel junction.

Alfa
The restorations were continuous with existing anatomic form.

Anatomic form Bravo
The restoration was discontinuous with the existing anatomic form, but the missing material was insufficient to expose base or dentin.

Charlie
Sufficient material has been lost to expose dentin or base.

Alfa
Absence of secondary caries.

Bravo
Presence of secondary caries.F.R.R. Moura et al.
No anterior restoration was scored Charlie for color match and marginal staining, properties that usually call for replacement or repair of the restoration, due to the esthetic impact (11).About 15-17% of the class III and class IV restorations were scored Bravo for color match and about 30% for marginal staining.The higher percentage of Bravo rating for marginal staining after 3 years might be explained by the little experience of students with the total-etch adhesive technique (12).
Class II and class IV restorations presented the highest prevalence of failures, confirming again the trend of higher failure probability in larger restorations.Indeed, the involvement of proximal surfaces in the restoration has been shown to reduce drastically the survival probability up to 15 years.The same result has been observed when multiple surfaces are involved in the restoration (4).
Taking into consideration all types of restoration, the main cause of failure was the loss of the restoration due to limited adhesiveness (8.6%), contrasting with previous studies that point out caries or fracture of the restoration as the main causes of failure of composite restorations (3,4).Debonding of the restorations in the study also might have been caused by the lack of experience of the operators with the adhesive technique.Inexperienced operators have shown less ability to produce restorations with higher bond strength, when compared to experienced operators (12).In class IV restorations the involvement of the incisal angle also generates stresses not observed in other cavity configurations, challenging the interface tooth/ restoration.Also, it is important to highlight that the present study used a simplified acetone-based adhesive system, which has demonstrated lower bond strength than similar ethanol or water based adhesive systems (13), exhibiting worse clinical performance as well (14).In addition, the adhesive system used in this clinical trial has a high rate of volatilization than other adhesive systems (15), and the restorations were performed in a Dental School clinic environment where the bottles could remain more time opened, favoring increased volatilization and impairing the bonding mechanism.All these reasons could contribute to the adhesive failures observed in the present study.Proximal restorations should be able to reestablish the proximal contacts properly and seal the cervical margins, which have been recognized as the main site of secondary caries lesions.The transparent matrix band and the reflective wedges used in class II restorations were first recommended under the assumption that the composite's directional shrinkage occurred towards the light and could be controlled by application of the light through reflective structures, including the tooth itself.This matrix system is no longer used due difficulties of handling and definition of a proper proximal contact area when compared with the metal matrix/wooden wedge system.Even so, similar clinical performance of posterior restorations has been observed in a randomized clinical trial comparing both matrix systems (16).The present results revealed that no restoration failed due to secondary caries, despite extensive evidence of secondary caries being the main reason for restorative failure and replacement of restorations (3).Such results could be explained by the current concept about secondary caries, which was been recognized as carious lesions located in the margins of existing restorations and dependent upon the sealing, antimicrobial and roughness characteristics of the restorative materials used (17,18).Noteworthy, a systematic review on clinical evaluations of posterior composite restorations showed that the fracture of the restorations was the main reason for restoration failure up to 5 years, while in evaluation periods over 5 years, the main reason for failure was secondary caries (2).
Limitations of marginal seal resulting from the composite's polymerization shrinkage and the longterm degradation of the adhesive system have been pointed out as possible reasons for the development of secondary caries in composite restorations.However, in a 17-year clinical evaluation of light-curable composite restorations bonded with a third generation adhesive system no secondary caries was found (4).Also, a 10-year micromorphological evaluation of composite restorations detected surface deterioration patterns and marginal integrity imperfections from the first years of the restoration in function, features not associated to secondary caries (19).
Recently, in situ evaluations have been undertaken to determine the influence of material's characteristics, namely marginal leakage and surface roughness, biofilm control and fluoride intake on the formation of secondary caries (17,18).Only biofilm control and fluoride intake significantly influenced the formation of caries adjacent to restorations (17,18), indicating a minimum role of the restoration itself and reinforcing the importance of a health promoting approach in which caries prevention  (20).
In conclusion, in the studied population attending a Dental School clinic with a health promotion approach, most restorations placed by dental students could be considered satisfactory after 3 years of evaluation.Failure was more prevalent in larger restorations and was mainly caused by poor adhesiveness, not being associated with secondary caries.

Table 2 .
Classification of the restorations.
*Considered as clinical success.**Considered as clinical failure.

Table 3 .
Type of restorations evaluated in the study.

Table 4 .
Classification of the restorations according to the modified USPHS criteria.