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 They no longer represent routine use in clinical practice, mainly due to their dark color, which impairs the aesthetics of the restoration 15 .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
Experimental design
Quartz-glass fiber posts and glass fiber posts were evaluated for light transmittance using photo light intensity test and ultraviolet visible and infrared spectrophotometer. The posts were divided into two groups: the experimental group - quartz-glass fiber posts (n=10) and the control group - glass fiber posts (n=10). The posts in the experimental group are made by optical fiber, epoxy resin and glass fiber. The exact proportion is not revealed by factory. The composition of the glass fiber posts (Exacto®, Angelus Londrina, PR, Brazil) is 80% fibers glass and 20% epoxy resin. Both have a length of 18 mm, conical with the same maximum and minimum diameters of, respectively, 1.8 and 1.0 mm.
Analysis of the light transmittance using photo light intensity test
The light intensity was made with a LED dental curing-light Radii Expert SDI and a camera Canon 70D with lens macro 100mm and exposure settings in ISO 100, aperture F18 and shutter speed 125, therefore resulting in images of 5,612x3,440 pixels in RAW format. The spectrophotometer was connected to a computer running the spectrum analyzer software (OOIBase32, Ocean Optics). The software was set in a mode to evaluate the light counts correlating with the number of photons received from the spectrometer’s CCD detector. At the test of 470 nm, to each count, 30 photons were received on the equipment’s detector.
The counts were registered in total light-darkness for ten posts from each group. The posts were vertically positioned over a bench with the curing light Radii Expert SDI leaning at the cervical region. Furthermore, the photos of the posts irradiated by the light-curing device were taken in complete darkness (Canon 70D with lens Macro 100mm). The photos were divided in thirds: coronal, middle and apical.
Analysis of the light transmittance using ultraviolet-visible spectophotometer
The same posts were sanitized with 70% alcohol, stored in Eppendorf flasks and always handled with procedure gloves. After distribution and sample identification, they were submitted to an ultraviolet visible and near infrared spectrophotometer (Cary 5000 UV-Vis-NIR Spectrophotometer®, Agilent Technologies, Santa Clara, Califórnia, EUA). The mean exposure time was set at 10s, and the application distance was set at 10mm. The analyzed wavelengths were 400 nm, 450 nm, 500 nm and 550 nm. Light transmission was measured using an optical transmission microscope coupled to spectrometer. The upward referenced light source was transmitted through the edge of the cut and measured the percentage of light intensity (compared to the 100% reference) for each post. Valúes were given as the percentage of incident light measured at the opposite length of the post.
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).
Group |
Coronal |
Mean |
Apical |
---|---|---|---|
Exacto |
97.00 (1.00) a |
68.00 (4.35) c |
27.66 (1.52) d |
Experimental |
95.33 (0.57) a |
80.66 (4.61) b |
41.33 (9.45) d |
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).
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.