INTRODUCTION

Fiber-reinforced prefabricated intraarticular posts have gained popularity due to several favorable characteristics for clinical use compared to metallic intraradicular posts. ^{1, 2} Notable qualities of the material include dentin-like mechanical properties, the ability to adhere to the root structure, and good esthetics ^{2, 3} . Scientific evidence demonstrates that prefabricated fiber-reinforced posts have an elastic modulus around 20 Gpa ^4-6 , which is very similar to dentin (18 GPa) ⁷ . The convergence of properties standardizes the stress distribution and reduces the risk of fracture ^8-10 . Another relevant aspect of fiber-reinforced prefabricated intraradicular posts is their ability to transmit light, enhanced by the association of carbon, glass, quartz, and zirconia fibers ¹¹ . Translucent posts can allow lighter to pass through, improving the depth and quality of polymarization of the material chosen for cementation ^12-14 .

Carbon fibers represent one of the first prefabricated posts available on the market. They are composed of unidirectional carbon fibers inserted in an epoxy resin matrix. They no longer represent routine use in clinical practice, mainly due to their dark color, which impairs the aesthetics of the restoration ¹⁵ . Glass fiber posts can be made of two types: S-glass and E-glass. They have different properties, but generally, are amorphous, and they are formed of a three-dimensional network of silica, with oxygen and other atoms arranged randomly ¹⁶ . Furthermore, Glass fiber posts can be associated with quartz fibers, which are pure silica in their crystallized form. Posts reinforced with glass and quartz fiber lead to better stress distribution when compared to rigid metal posts or zirconium oxide ceramic posts ¹⁴ . Fiber-reinforced posts also have advantageous optical properties over metallic post systems, reinforced by carbon fibers or metal oxide post systems ¹⁶ .

Glass fibers posts have a refractive index like resin; therefore, they allow efficient light transmission ^{16, 17} . Consequently, the addition of glass fibers to the dental composite will improve its mechanical properties without affecting the degree of conversion of the resin matrix, unlike opaque-colored fibers, as carbon, or zirconia ^{12, 16} . The clinician must realize that there are recognizably substantial differences in the mechanical load capacity of different fiber-reinforced posts and must be aware of such differences to select an appropriate post system to use ¹⁸ . Therefore, the aim of this study was to evaluate the luminous transmittance of quartz-glass fiber posts and conventional fiberglass posts.

MATERIAL AND METHOD

RESULTS

In the transmission of light in the posts of the experimental group was 97% on the coronal third, 68% in the middle third, and 27.66% in the apical third. In the posts of the control group, the light transmission was 95.33% in the coronal third, 80.66% in the middle third, and 41.33% in the apical third (Fig. 1). Figure 2 represents the difference in light transmittance in relation to the experimental and control fiber posts, respectively. Light transmission was significantly higher in the middle third of the posts of the experimental group when compared to the control group (p=0.0155) (Table 1).

Fig. 1 Luminous transmittance between the thirds of the posts of the experimental and control groups. 

Fig. 2 Illustrated photo of the luminous transmittance between the thirds of the posts of the experimental and control groups, respectively. 

The luminous transmittance of the posts of the experimental group was 97.4% with om wavelengths of 400 nm, 97% at 450 and 500 nm, and 96.9% at 550 nm. In the posts of the control group, the luminous transmittance was 72.3% with wavelengths of 400 nm, 68.6% at 450nm; 64.6% at 500nm and 61.5% at 550 nm (Fig. 1). The values of light transmittance measured by spectrometer (mean and standard deviations) and the differences in the groups are shown in Table 2. The post of the experimental group demonstrated significantly higher light transmittance than the control group (p<0.001).

Figure 3 is related to the linear representation of the data obtained, in which the experimental group showed greater homogeneity and stability in light transmission throughout the analyzed spectrum (close to 0.97). The control group showed the greatest change throughout the analysis, and at the wavelength of 550 nm, there was the lowest mean transmittance (0.615); while the highest transmittance occurred at 400 nm, (0.723).

Fig. 3 Values of linear representation of the data obtained from the light transmittance of the groups as a function of the wavelength. 

DISCUSSION

In this study, the luminous transmittance through glass fiber posts and quartz-glass fiber posts was investigated. According to the results of this in vitro study, quartz-glass fiber posts showed significantly better luminous transmittance compared to glass fiber posts. The nuil hypothesis tested in this study that the luminous transmittance of different glass fiber posts would not differ was therefore rejected. From a clinical point of view, this may indicate better polymerization of the cementing agent used in the root canal ¹⁸ .

Resin-based cement is widely used today for the cementation of various indirect dental restorations that have received intraradical support ¹⁹ . Dual-curing resin-based types of cement are clinically preferred over purely light-curing types of cement because the former can better tolerate light exposure in locations that do not allow optimal access to curing light due to indirect-direction morphology ^{19, 20} . Compared to photopolymerizable resin-based materials, free radical-mediated polymerization of dual-polymerized materials is more complex, as two initiation reactions occur simultaneously and interact with each other ²¹ . Recognition of this complexity led to numerous studies on dual-polymerization resin types of cement ²⁰ and, among other specificities, also the availability of the intraradical retainer in relation to luminous transmittance ^11-14 .

Scientific evidence shows that there is no difficulty for light to reach a more superficial region of the intraradicular post ^{14, 22-25} . In this study, the evaluate of luminous transmittance was performed in thirds; the depth was defined in millimeters at six pre-established points ²² . Our results showed significantly higher luminous transmittance in the middle third of the quartz-glass fiber post when compared to the glass fiber post. This fact becomes important since favorable degrees of cure of the cementing agent have been described in depths above 8mm ^{23, 26} . Silva et al. studied the degree of conversion after the cementation of two types of posts at three levels of depth and demonstrated statistically favorable results between the middle thirds of the two types of posts also related to translucency ²³ .

The luminous transmittance of glass fiber posts and zirconia, quartz, and silica have already been previously tested ²⁷ . However, methodological differences were found in the digital camera, in the variety of posts designs such as, non-standard diameter and lengths, and a more robust sample. Thus, replication of this study is suggested, considering the results of the apical third. In the apical third, the p-value was very close to the significance level. The null hypothesis could be rejected if the sample was larger.

Regarding luminous transmittance analyzed by the UV/Vis spectrophotometer, better luminous transmittance was also demonstrated by the quartz-glass fiber posts, which may be related to the type and arrangement of the fibers, thus increasing the luminous transmission valúes. The result of this method can be considered extremely efficient in the present study, as the posts have the same design both in length and thickness. When irradiation occurs, the light beams are distributed along the posts by total internal reflection. The critical angle is the product of the difference in refractive indices between the core and surface material. Rays that exceed the surface boundary of the material at an angle greater than the critical angles are reflected ²⁷ . Thus, differences in optical properties are explained by variability in fiber diameter, orientation pattern and variable matrix composition. All these factors contribute to refraction divergences and make it difficult to compare posts from different commercial brands ^{8,11-14,27, 28} .

The two luminous transmittance methods used in this study should be carefully used to estímate clinical performance. However, they can be correlated with the bond strength tests of the posts to the resin cement in the root canal, thus explaining possible favorable results for the quartz-glass fiber posts due to the greater luminous transmittance.

CONCLUSION

Quartz-glass fiber posts show significantly better light transmittance compared to glass fiber posts.