Digital radiographic evaluation of optical density of porcelains used for fabrication of inlay/onlay prosthesis.

The purpose of this article was to compare the mean value of optical density of four porcelains commonly used for fabrication of inlay/onlay prostheses using direct digital radiograph. The sample consisted of 20 2-mm thick porcelain specimens (measured by digital pachymeter): Empress (Ivoclair), Simbios (Degussa), Vita Omega 900 and Vitadur Alpha (Vita Zahnfabrik). The values of optical density of the specimens were expressed in millimeters aluminum equivalent (mm eq Al). The samples were X-rayed using two charge coupled devices (CCD) - RVG (Trophy) - Visualix (Gendex) and a phosphor plate system - Digora (Soredex). The optical density reading was performed with Image Tool 1.28 in a total of 110 measurements. Statistical analysis showed that there were statistically significant differences in all materials studied (p < 0.05) regardless of the radiographic system used. The highest optical density value was found for Omega 900 (1.8988 mmeqAl - Visualix - Gendex) and the lowest for Vitadur Alpha (0.8647 - Visualix - Gendex). Thus, the material presenting the highest degree of optical density was Omega 900, Empress and Simbios presented intermediate optical density values, Vitadur Alpha presented the lowest value, and the optical density of porcelains was not influenced by the digital radiography systems.

INTRODUCTION

Since long ago, man has searched for restoration of esthetics and masticatory function in order to meet these requirements satisfactorily. The materials employed comprised metal alloys, silver amalgam and resins. Buonocore (1955) suggested the utilization of acids to alter the tooth surface and make resins more suitable.

By thinking this way, porcelain was the material that met these requirements. Porcelain has recently been introduced in dentistry, and its use as a restorative material began about 200 years ago, but it was in the 80s that a big push was given, with the appearance of the new generation of porcelains, composites and dental adhesives that contributed for the development of indirect techniques for application in posterior teeth such as inlay/onlay24.

These materials should present various characteristics. They should have good biocompatibility, adaptation to the cavity margins, good adhesion to dental tissues, resistance to wear and decay, and easy manipulation. The material must present a radiopacity different from the dental tissues to enhance definition of the presence or absence of recurrent caries and also a proper cervical adaptation and proximal contour of the restoration (Takeshita, et al,22; Tamburus23). To get to this point, the inclusion of radiopaque elements with high atomic number is necessary (barium zirconium, zinc, ytterbium and lanthanum) in organic particles (Bouschlicher, et al.5; Van Dijken, et al.24; Willans and Billington25). According to Curtis, et al.9, the manufacturers can control the optical density of materials through selection of the polymeric matrix, however the nature of elements, their different sizes and densities may remarkably influence this condition 8,9,22.

The digital image became reality in dentistry in 1987 with the Radio VisioGraphy system (Trophy Radiologie, Vincennes, France) (Nelvig, et al.18). Dynamically, new products came out into the dental market, with basically two concepts concerning image capture: the first consists of a sensor, called CCD (charge coupled device), which has a wire connected to a computer, and the other has a phosphor storage system as a photon-detector, which has the same size and thickness of the periapical film and does not have connected wires, thus depending on an appropriate scanner to yield the image after X-ray exposure 4,14,19.

According to Khademi14 the digital image allowed visualization of radiographic images, considering that with an image treatment program and a computer it is possible to alter them, improving the differentiation of density and contrast of gray tones in case of failure of human visual acuity. The professional should be aware of the different densities existing among the various porcelain types; besides, he or she can distinguish it from other materials, caries and other problems (Fenyo-Pereira10; Graziottin, et al.13). In view of this, the present paper studied porcelains commonly used for inlay/onlay and verified the degree of optical density by means of digital radiography.

MATERIAL AND METHODS

In this study, 4 commonly used porcelain brands were used for fabrication of inlay/onlay prostheses: Empress (Ivoclar, Liechtenstein, Germany), Simbios (Degussa, Hanau, Germany), Vita Omega 900 and Vitadur Alpha (Vita Zahnfabrik, Säckingen, Germany).

The porcelain samples were made in the prostheses lab at São Paulo State University (UNESP), from an aluminum matrix with an 8mm diameter orifice, 2mm thick according to specification number 57 of the American Dental Association2 which states that the radiopacity of composite resins must be superior to that of an equal thickness in aluminum. According to Van Dijiken, et al.24 restorations are up to 2mm thick, therefore it is recommended that sample tests should be 2mm thick (mmeqAL = millimeters aluminum equivalent). Fenyo-Pereira10 for example, used 2-mm thick specimens to evaluate the optical density of porcelain. The specification 57 of the American Dental Association suggests the use of a 2-mm thick specimen to evaluate the optical density. After that, using diamond stones and sandpaper discs, the finishing and polishing work was done on the specimens. As porcelain had the manufacturer's properties, a digital pachymeter (Mitutoyo – São Paulo, Brasil) was used to certify that dimensions of the specimens were correct, and readings were analyzed on the display.

After preparation of the specimens they were put in a test tube, dipped into distilled water and kept in an oven at 37±1ºC in order to simulate the oral environment 16,17.

The samples were X-rayed with the periapical machine Dentsply–Gendex 765DC® (Milan, Italy) set at 65kVp and 70mA, 40cm focal distance. The 40cm focal distance is similar to the parallelism technique and exposure time of 0.02s. According to Medici-Filho, et al.17 and Takeshita, et al.22 it is ideal to get adequate contrast and density. To capture the images we used three digital x-ray systems, being two charge-coupled device (CCD), RVG (Trophy) and Visualix (Gendex), and one phosphor storage system – Digora (Soredex) for digital imaging.

The radiographic images are measured by means of a program named Image Tool for Windows version 1.28 (San Antonio, US) as for their digital optical density (gray tones). The gray tones measured by the program go from 0 to 255, then 256 different tones are recognized by the program 17.

Twenty specimens were made i.e., 5 for each porcelain brand. Considering that 5 different areas were measured for each specimen, 100 measurements in porcelain were obtained. Five different areas in each sample were analyzed in order to minimize the error, so the mean of the 5 areas was used in this study (Fenyo-Pereira10 and Graziottin, et al.13)

RESULTS

The mean optical density values were subjected to ANOVA (2-way) and Tukey tests (p < 0.05). The porcelain presenting the highest optical density was Omega 900 and the lowest was Vitadur Alpha on the three digital systems (Figures 1, 2 and 3).

FIGURE 1

Mean values of optical density of porcelains. Means followed by same letters are not statistically different to each other at a significance level of 5%. Digora (Soredex)

FIGURE 2

Mean values of optical density of porcelains. Means followed by same letters are not statistically different to each other at a significance level of 5%. RVG (Trophy)

FIGURE 3

Mean values of optical density of porcelains. Means followed by same letters are not statistically different to each other at a significance level of 5%.. Visualix (Gendex)

Table 1 represents the mean values of optical density for the different porcelain brands measured in gray tones.

TABLE 1

Mean optical density values in millimeters equivalent aluminum (mmeqAl) of the porcelains evaluated by the three different radiographic systems

RADIOGRAPHIC SYSTEM	PORCELAINS
	Empress(mmeqal)	Omega 900(mmeqal)	Simbios(mmeqal)	Vitadur Alpha(mmeqal)
Digora (Soredex)	1.1815a	1.6056a	0.7643b	0.8771b
RVG (Trophy)	1.2788a	1.8412a	0.8724b	0.8724b
Visualix (Gendex)	1.3458a	1.8988a	1.0605b	0.8647b

Groups with the same letter have no statistically significant differences to each other.

DISCUSSION

Porcelain appeared in China at nearly 1000 years BC but it was first used to for fabrication of inlay/onlay as an esthetic means for caries restoration only 200 years ago; however, due to the high percentage of failures, those prostheses were substituted by gold during decades (Chain, et al.7). The inlays/onlays produced in gold had the disadvantage of not being esthetically favorable, so in the 60s the porcelain was brought back to the market.

Today, the porcelain is the first material to be used for esthetic and functional restorations: it is chemically inert, has low thermal and electrical conductibility, is resistant to compression and has an excellent potential to simulate the appearance of natural teeth7,11.

The porcelain used in this research is classified as: feldspathic i.e., it has a great quantity of feldspar in its composition besides quartz and kaolin. It can be used for fabrication of inlay/onlay, ceramometal and porcelain veneers restorations. It can also be applied as a coating, since it presents excellent translucence characteristics and similar color as natural teeth. The Empress is reinforced with leucite, which provides good mechanical and visual properties and enables the porcelain to be used for jacket crowns, inlays and onlays20,24.

The composite resins used for class I and II restorations are accepted to be radiopaque2.

The radiopacity is a prerequisite for dental materials. In 1979 the Standard Association of Australia required the restorative materials of composite resin to have a high radiopacity. The American Dental Association2 included the radiopacity in the specifications for dental materials in 1981. Thus, it was established that dental materials should have same or greater radiopacity compared to radiolucent areas that suggest caries 1,12,13,15.

In order to evaluate the radiopacity, authors like Abou-Tabl, et al.1 and Taira, et al.21, suggested teeth cutting similar to the specimen for comparison, while Cook8 stated that aluminium is the material of choice because it presents the same linear coefficient of absorption as enamel.

The porcelain Omega 900 presented 1.6056mmeqAl (Digora), 1.8412 (RVG) and 1.8988 (Visualix). Empress presented 1.1815mmeqAl (Digora), 1.2788mmeqAl (RVG) and 1.3458mmeqAl (Visualix). The values of Simbios were 0.7643mmeqAl (Digora), 0.8724mmeqAl (RVG) and 1.0605mmeqAl (Visualix) and the Vitadur Alpha porcelain presented 0.8771mmeqAl (Digora), 0.8724 mmeqAl (RVG) and 0.8647mmeqAl (Visualix). Vitadur Alpha was the one that showed the lowest value, thus converging with Fenyo-Pereira's10 studies. The Empress and Simbios had intermediate values and the Omega 900 presented the highest optical density.

Although the Omega 900 has shown statistically higher radiopacity values than enamel, Curtis, et al.9, consider that extremely high radiopacity values as that of metallic restorations e.g. silver amalgam can cover suspect areas and interfere with caries evaluation, as well as in cervical adaptation of class II restorations. This fact is due to an optical illusion described by Berry3 as mach bands, which are radiolucent areas along the cementoenamel junction. In his article he still points out that they are influenced by the density of the object, the type of film or even the projections on the radiography.

The objective of the restorations is to re-establish the occlusion. Therefore, they must offer proper characteristics for normal functioning of the stomatognathic system11. Radiopacity is essential to evaluate the restoration quality, Graziottin, et al.13, Fenyo-Pereira10, Loguercio, et al.15, Medici-Filho, et al.17, used the digital image to measure the optical density. Therefore, digital radiography represents an alternative to measure the optical density, besides contributing for dental diagnosis.

The material that presented the highest degree of optical density was Omega 900;

Empress and Simbios presented intermediate optical density values;

Vitadur Alpha presented the lowest optical density value;

The optical density of porcelains was not influenced by the digital radiography systems.