Efeito do tratamento superficial de pinos de fibra de carbono lisos na retenção da resina de preenchimento: uma avaliação in vitro
Over the past few years, improvements in dental materials have led to a decrease in the utilization of metal, enhanced aesthetics and good mechanical properties. However, researches have focused on restorative materials, with almost no concern about post and core materials. Endodontic treatment techniques have also been developed, with more predictable results. Consequently, endodontically treated teeth have their longevity increased. Despite all modern restorative materials, there are still a lot of controversies about the need for intraradicular retention and the type of coronal reconstruction2,16,19,20,26,29. SORENSEN; MARTINOFF28 (1984) stated that there is no root reinforcement related to the placement of a post; they have also pointed out the importance of coronal reconstruction in endodontically treated teeth located in the posterior region. VIRE31 (1991) evaluated the possible causes of failure in 116 endodontically treated teeth, and concluded that 59.4% of them failed for prosthetic reasons, all related to the post.
Many authors agree that the placement of posts is directly related to the need for a retention and resistance form1,2,13,17,21. The retention and resistance form depends directly on the amount of sound tooth structure, tooth position, restorative choice and occlusal relationship.
Although metal-free restorations are aimed at in many recent studies, most posts used are still metallic. Many investigations have been carried out in order to find out about the possible effects of corrosion products and also to comprehend the behavior of various post materials inside the root canal, especially their modulus of elasticity.
DURET et al.5 affirmed that the ideal restoration for endodontically treated teeth should have a shape identical to that of the root canal, the same mechanical properties as dentin and effective adhesion to the tooth. Carbon fiber posts were introduced due to their better mechanical properties in relation to metallic posts: their modulus of elasticity is closer to that of dentin5 and they have elevated resistance to fatigue7. Maybe these properties are responsible for the occurrence of more retrievable fractures related to carbon fiber posts when compared with pre-fabricated posts and cast posts in other studies8,12,13,22,27,30. Metallic posts have a modulus of elasticity that can be ten times grater than that of dentin7, resulting in greater tension in the root structure6,32. There is also a difference when comparing cast posts with posts and cores: according to YAMAN; THORSTEINSSON32 (1992), cast posts cause greater tension in the apical portion, whereas posts and cores, in the cervical region. Perhaps this fact can explain the greater number of failures with posts and cores in periodontally compromised teeth. Fatigue is a disadvantage in metallic posts when compared with carbon-fiber posts; the high percentage of fibers probably leds to incomplete fractures in these posts13.
Metal-free restorations are an aim in current dentistry, because of aesthetics and the absence of corrosion products2,9,22,23,25. HORNBROOK; HASTINGS10 (1995) pointed out that cast posts and amalgam cores can be apparent through the root surface, thus they indicate tooth-colored cores for better aesthetic results. Regardless of the presence of a luting agent, avoidance of different metal alloys for posts and crowns is recommended by DEUTSCH et al.4 (1983). The chemical stability presented by carbon fiber posts is advantageous when compared with that of metallic posts9.
Posts should fit dentin apically; the high percentage of unidirectional carbon fibers (64%) ensures good resistance to fatigue8. According to PURTON; LOVE23 (1996), the main advantages of carbon fiber posts are: high resistance to fatigue, resistance to corrosion and chemical compatibility with Bis-GMA. FREEDMAN7 (1996) states that carbon fiber posts have less transmission of stress when compared with titanium posts (about 63%) and nickel-chromium posts (about 33%). ISIDOR et al.12 (1996) applied intermittent load to teeth restored in three ways: carbon fiber posts and resin cement, Para-Posts and cast posts and zinc-phosphate cements; while 85% of the Para-Posts and 100% of the cast posts failed against tensile strength, no carbon fiber post failed. Therefore, carbon fiber posts seem to be a good alternative to metallic posts and their presumed compatibility with composite resins would avoid corrosion products. Some studies8,12,18,27 concluded that the application of force caused less damage to the structure of teeth restored with carbon fiber posts when compared with those restored with metallic posts; SIDOLI et al.27 (1997) affirmed that there were more retrievable fractures with carbon fiber posts than with other posts. KING; SETCHELL13 (1990) pointed out the ease to remove them when endodontic retreatment is necessary.
Some investigations12,24 affirmed that the main problem with carbon fiber posts is the lack of adhesion to the core material. In order to solve this problem, manufacturers have introduced a serrated version. This version gained retention to the core while lost rigidity; this mechanical property of the serrated version is now closer to that of metallic posts but smaller than that of plain carbon fiber posts15.
MATERIALS AND METHODS
Fifty carbon fiber posts (C-Post, #3, BISCO, USA) were divided into five groups. Four groups contained plain posts that received modifications in the coronal portion before the placement of the core. Surface alterations were performed as follows: Group A - sandblasting; Group B - medium grit diamond bur; Group C - diamond burs used to determine the depth for laminate veneers (depth cutter); Group D - change of the head form. The last group (Group E) consisted of posts serrated by the manufacturer.
In Group A, sandblasting (with 50 mm aluminum oxide) was carried out at a distance of 1 mm, over a length of 3 mm, on the coronal portion. Posts from Group B also had a length of 3 mm prepared with a medium grit diamond bur (Diamond Burs, number 315, Moyco, USA) in a direction parallel to their long axis; each bur was discarded after being used in one specimen. Group C received the same preparation with a depth cutter (Diamond Series, number S4, Moyco, USA); each bur was discarded after a single use. Group D had their head machined as shown in Figure 1.
All posts received a double coat of Primer B (All-Bond, BISCO, USA) and were dried. A machined acrylic resin mold was developed and divided in two halves, where a #3 C-Post fitted exactly (Figure 2); at the coronal end, a 3 mm space was created in order to receive a composite resin core. A fine brush with a thin layer of solid Vaseline lubricated the coronal end before the placement of the core.
Then, a composite resin especially developed for core build-up (Core-Flo, BISCO, USA) was mixed as recommended by the manufacturer and then inserted in the 3-millimeter space. The post was then placed and both halves of the mold were screwed and kept under pressure; according to DURET et al.6 (1990) it is essential to push the composite material with some pressure into the microscopic retentions of the post, in order to link it with the matrix of the carbon fiber post. The composite resin that exceeded the capacity of the mold was removed with a spatula before setting. Any remaining excess of composite was trimmed with abrasive paper discs (Mooreplastics, garnet fine, Moore) placed in a handpiece mandrel.
The samples were thermocycled (600 cycles, 30 seconds, 5ºC and 55ºC) and then kept in distilled water for one week.
The specimens were submitted to tension test in an Instron machine (model 4,301). A mandrel was adapted to the Instron machine and a metallic frame was developed in order to transmit the force exactly in the long axis of the samples (Figure 3), at a crosshead speed of 0.5 mm/min. The test was stopped when there was lack of adhesion or fracture of the core.
The data were submitted to a test of analysis of variance (ANOVA), with a confidence interval of 5%. There was statistical difference between the groups at a level of 5%. The Tukey comparison test demonstrated statistically significant difference between Group B and the other groups. There was no statistically significant difference related to the retention of the core between Groups A, C, D (treated plain posts) and E (serrated posts). Graphic 1 presents the mean values for all experimental conditions.
Regarding the type of failure, there was also a visual difference between the groups: the samples from Group B presented only dislodgment of the core, which was a completely different result from those of the other Groups. In 80% of the specimens from Group A, there was dislodgment of the core with partial or total fracture of the composite resin. In Groups C, D and E there was core fracture in all specimens (Figure 4).
As stated by LOVE; PURTON15(1996), plain carbon posts are mechanically superior to serrated ones, because of their greater rigidity. In spite of that, smaller adhesion to the core compromises the retention and resistance form that retains the crown. The results of this study concluded that the macroscopic retention encountered in Groups C, D and E is favorable to the retention of the core, which corroborates the results of other studies3,15,16,24. The surface treatments performed in Groups C and D transformed plain carbon posts into good clinical alternatives to serrated posts because of their mechanical properties. Comparing the results obtained by LOVE; PURTON15 (1996) with the results of this study, it is possible to observe that the same kind of failure that occurred in Groups C, D and E was found, i.e., fracture of the core in all samples, which reflects the tension induced in the core material, depending on the shape of the head. The alteration produced in Group D is inconvenient for clinical purposes, but it is important to suggest that the manufacturers keep the posts plain in the root portion and alter the coronal end for greater retention.
From the clinical standpoint, non-catastrophic failures (100% in Group B and 20% in Group A) are not only failures of the restoration – they may also cause great damage to the tooth because of the time interval between the failure and its detection, allowing the development of microleakage and root caries. The catastrophic failures in Groups C, D and E11 are clinical failures of the restorations and demand their immediate substitution.
The use of composite resins as core materials is more popular nowadays because they are easy to handle and can be immediately prepared2. However, many factors can contribute to the integrity of the interface between the post and the core. The possible effect of thermal stress in composite resins is otherwise minimized because there is less transmission of heat under a crown than it would be expected in a direct composite restoration, but further studies are necessary. CHANG; MILLSTEIN3 (1993) considered posts and cores less reliable when compared with cast posts because of the greater number of interfaces. DURET et al.6 (1990) stated that the different moduli of elasticity of the post material and the core material can affect the bonding resistance. The chemical compatibility between the composite resin of the core and the epoxy matrix of carbon posts would lead to a better interface; however, the heat treatment performed during the fabrication of carbon posts can play an important role in this adhesion, resulting in a worse interface.
The mechanical tension between posts and radicular dentin is also related to the type of core material15,16,32. Tension is greater for cast posts when compared with posts and composite cores, according to YAMAN; THORSTEINSSON32 (1992). Failures related to the use of composite resins as core materials are mainly related to their low modulus of elasticity29. KOVARIK et al.14 (1992) pointed out that failures with composite resin cores occur at the interface, stressing the importance of a good interface; a good quality interface should be expected with chemical and mechanical adhesion.
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Recebido para publicação em 06/01/00
* Ph.D Student, UNESP, São José dos Campos;
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