INTRODUCTION

Tooth degeneration is caused by different factors which can affect tooth's enamel, dentin and hard tissues.¹ If said degenerative process is found at an initial stage, it can be reversible, such is the case of the whitish spot; if this is not the case, an irreversible process sets in related to the cavitation presence. For these reasons, to count with ideal and longer-lasting materials is of the utmost importance when restoring original cavities caused by different carious processes in the mouth.²

Several research projects are proof that the scientific community is interested in improving mechanical properties of filling materials, remembering nevertheless that there are still some deficiencies such as low resistance to wear, micro-filtration, pigmentation and incomplete polymerization. Resistance of these materials to diverse factors is still not ideal and results in their short permanence in the mouth, nevertheless, some of these materials have proven to possess annual wear similar to that of silver amalgam.^3,4

It is important to bear in mind some current and relevant concepts of minimally invasive dentistry: when teeth require restoration, this restoration must be as conservative as possible with the dental structure when required preparations are undertaken. This has caused abandonment of certain materials requiring extensive preparations in order to acquire resistance and adhesion to the tooth. Contrarily, the use of materials not requiring extensive preparations to be used in different cases is on the rise.⁵

Certain characteristics of the material provide confidence to the clinical operator, who will play an important role when choosing materials. These characteristics, among others, are resistance to masticatory forces, acceptable esthetics, and superficial hardness.⁶

In a publication previous to this study, Taron et al, in 2015, proposed in a pilot study as experimentation model a large number of natural teeth previously extracted due to orthodontic reasons. This sample was used to develop fracture resistance and tolerance tests. The study nevertheless demanded evidence of sample increase and model refinement.⁶

Restorative materials presently used such as composite resin and glass ionomers, possess advantages and disadvantages, therefore the aim of the present study was to compare one of the multiple characteristics essential to restorative materials, that is to say compressive strength of the aforementioned two materials.

MATERIAL AND METHODS

An in vitro quasi-experimental study was conducted. In it, assessment was made of compression resistance of a reconstructive glass ionomer in contrast to a nano-hybrid composite resin, both materials were used to restore Black's class I cavities with depths of 2 and 4 mm in human premolars. The convenience-selected sample was composed of 52 premolars, extracted during orthodontic treatments, lacking extensive enamel anomalies.

The sample was divided into two groups: group A, for teeth where 2 mm deep cavities were performed, and group B where 4 mm deep cavities were established. A blunt edge, cylindrical diamond burr was used. Depth of all prepared cavities was rectified with a millimeter periodontal probe (Hu-Friedy).

A self-polymerizing acrylic support was manufactured for each tooth in the sample, so as to provide stability when positioned in the compressive strength measuring instrument.

Both groups were divided into two sub-groups. Number 1 was for teeth used as sample, restored with reconstructive glass ionomer, brand 3 M Vitremer^®. Number 2 was for teeth restored with nano-hybrid resin Tetric N-Ceram, brand Ivoclar Vivadent (Table I).

All teeth of the sample were subjected to stress tests with texturometer EZ-S SHIMADZU, series number 346-54909-33, with 50-60 Hz, with maximum capacity range of 500 Newton. Filled and restored teeth were subjected to compression in the occlusal side, with a 1 mm contact area, until achieving a 1 mm depth in one single advance (Figure 1). Strength necessary to monitor necessary strength to penetrate in the vertical aspect of the restoration existing in all teeth was monitored. It must be stressed that in all samples force application was equally performed at the central point of the restoration.

Figure 1 Superficial penetration in bi-rooted teeth restored at 2 and 4mm depth. Device EZ-SHIMADZU, series 346-54909-33. 

Ethical considerations of this project were in concordance with resolution 008430 (1983), Ministry of Health, Colombian Republic.

RESULTS

After Applying t test for independent samples analyzed two by two, it was found that cavities measuring 2 mm and filled with Vitremer^® and those filled with Tetric N-Ceram exhibited significant differences (p = 0.00000000006908). A 95% confidence interval was obtained for mean differences, supposing equal variances (-60.0973 up to -41.1631). Since confidence interval does not contain 0 there was a statistically significant difference between means of both samples, with a 95% confidence level. Tested resin exhibited greater significance, since it possessed greater mean (419.9500) as observed in Figure 2.

Figure 2 Comparison of superficial hardness of different restoration materials at 2mm depth. 

Figure 3 shows results of the comparison of both used restorative materials, after conducting an analysis of the 4 mm cavities filled with Vitremer^® and Tetric N-Ceram resin. They exhibited significant difference (p = 0.000) and a confidence interval comprised between values of -94.8257 up to -76.604. Due to the fact that confidence interval does not contain 0, there was a statistically significant difference with confidence interval of 95%. Resin was more significant since it possessed greater mean (438.9784 N).

Figure 3 Comparison of superficial hardness of different restoration materials at 4mm depth. 

When comparing 2 and 4 mm cavities filled with Vitremer^®, results showed no statistically significant difference (p = 0.00257443), confidence interval 6.07823 up to 25.3742, since interval contains 0 no statistically significant difference was found between both samples, with a 95% confidence level. Comparison with 2 and 4 mm cavities filled with Tetric N-Ceram resin, results revealed statistically significant difference (p = 0.000155887), alternatively, appealing to the confidence interval (-28.2774 up to -10.4401); since interval did not contain 0 there was a statistically significant difference between means of both samples, with confidence interval of 95%. Significance of resin at 4 mm was greater since it possessed greater media (438.9784 N) (Tables II and III).

DISCUSSION

In a previous publication of this research group, the cavity model was primed in natural teeth, in order to try to establish the importance of research in an environment much more similar to the reality of the oral cavity in human beings.⁶ Irrespectively of cavity depth, greater superficial hardness could be observed in all teeth restored with nano-hybrid resin. Nevertheless, in all study groups resin placed in 4 mm deep cavities exhibited higher hardness. Group B1 exhibited lowest results upon penetration.

Carrillo (2008)⁷ reported similar results to those obtained in the present study with respect to resistance of some filling materials used in dentistry. This study reported comparison of composite resin, reconstructive glass ionomer and fluid resin; in it, hardness values of composite resin were widely greater than those of the two remaining materials.^8,9

In 2014, Suarez and Lozano¹⁰ studied hardness of different types of resins, but, differing from the present study, they conducted their study examining the material in the shape of pre-formed elements, built with the studied materials, and not in a tooth by filling directly a prepared cavity, mimicking thus clinical reality. It is considered that the model proposed in the present study, far more resembles a real scenario of resistance measurement and compressive forces.

Sun Ae Song et al, in 2014¹¹ conducted research on resin hardness at different polymerization stages. Nevertheless, they conducted that research with Vickers’ microdurometer, which differs from the texturometer using in the present study, since its measurement is not directed to assess what force is needed by the machine in order to achieve penetration.¹²

To conduct studies on assessment of superficial hardness of two dental filling materials at two different thicknesses or depths is very important for the industry of dental materials, and for modern dentistry since contributions achieved with these research projects help to refine clinical indications and guide dental materials manufacturers in the search for further benefits for dental patients. This point was taken by Shanthala (2013)¹² and Erazo (2010)¹³ since they considered this a series of factors which allowed to achieve longer and more effective dental treatments for patients in cases when glass ionomer or resins were used as filling materials.^14,15

CONCLUSION

Bearing in mind limitations inherent to an in vitro study, it could be concluded that teeth with 4 mm deep cavities restored with Tetric N-Ceram exhibited greater hardness than those restored with Vitremer^® at 2 and 4 mm and 2 mm deep cavities with the same resin, nevertheless it must be accepted that evolution of present research might alter these results.

Resistance to compressive strength showed that to restore posterior teeth, studied resin possessed significantly higher hardness when compared to reconstructive glass ionomer.

Range of obtained forces by no means compare to range of forces recorded in human teeth bite. This points out to the need to improve presently used dental materials.

Bearing in mind diverse applications of used methods and materials, it would be possible to create new research projects targeting changes in hardness of restorative materials.