Prefabricated glass fiber posts
Ten cylindrical prefabricated glass fiber posts were acquired from Reforpost® (Angelus, Paraná, Brazil), with 1.5 mm in diameter and 15 mm in length, which were used as a control group.
Manufacture of biological posts
This study was approved by the Human Research Ethics Committee under protocol number 065/09 and by the Animal Experimentation Committee of the Federal University of Minas Gerais. Twenty bovine teeth from slaughterhouses and 50 human extracted teeth, duly donated, were acquired. These teeth were sterilized for 7 days in a 10% formaldehyde solution [16] and maintained in distilled water during all stages of the study.
The biological posts were created from the roots of bovine incisors and human canines (Fig. 1) using a drill specially designed and made for this study (Fig. 1), which standardized the specimens in a cylindrical form.
All of the teeth had their coronal portion cut by a cylindrically-shaped diamond tip burr at high rotation under cooling, with this tooth portion being subsequently discarded (Fig. 1). The root canal portion was sectioned along the long axis, in four parts (Fig. 1), with a carborundum disc at low rotation under cooling.
Bovine posts
Although each bovine root provided four posts, only one post was made from each root. Twenty cylindrical posts of 13 mm length were configured with a cylindrical intraradicular portion of 10 mm length (1.5 mm in diameter), and a cylindrical coronal portion of 3 mm length (2 mm in diameter) (Fig. 2).
Human posts
Ten human maxillary canines were cut out, acquiring one post per root, totaling 10 biological posts made of human root dentin. They presented the same proportions as the bovine posts (Fig. 2).
The dimensions of both bovine and human posts were in accordance with the size of the prefabricated glass fiber post in order to standardize the posts.
All procedures performed in studies involving extracted human teeth were in accordance with the ethical standards of the institutional and/or national research committee (Research Ethics Committee of Federal University of Minas Gerais, protocol 065/09) and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. The subjects signed an informed consent form prior to the study beginning.
Specimen preparation
Forty human canines were selected and placed in four experimental groups (n = 10/group) for the fracture test. The teeth had their crowns removed, and endodontic treatment was performed. Briefly, the root canals of all groups were prepared chemomechanically. The working length of the roots was determined by the visual method by introducing a file #15 (Maillefer, Dentsply, Rio de Janeiro, Brazil), inside the root canal until it reaches the foramen, then backing up one millimeter. It was adopted the modified classical technique until the file #80 (Maillefer, Dentsply, Rio de Janeiro, Brazil). After intermittent rinsing with 2.5% sodium hypochloride, the roots were dried with paper points (Maillefer, Dentsply, Rio de Janeiro, Brazil), and the roots were obturated with laterally condensed gutta-percha (Maillefer, Dentsply, Rio de Janeiro, Brazil) and an endodontic cement (Sealer 26®, Dentsply, Rio de Janeiro, Brazil).
The teeth presented a mean root length of 15 mm, with a cervical diameter of 5–5.5 mm in the mesiodistal direction and 7–7.5 mm in the vestibular-palatal direction. These were sterilized by immersion in a formaldehyde solution for 7 days and maintained in distilled water until the beginning of the experimental procedures.
To simulate the periodontal ligament, each tooth was marked with a pen at a distance of 2.0 mm below the cementoenamel limit, covering 13 mm of the root. This area was covered by a #7 wax (Wilson, São Paulo, Brazil), liquefied in a water bath until reaching its demarcation line, at approximately 0.3 mm in thickness.
On a #7 wax plate (Wilson, São Paulo, Brazil), eight punctures were made at equal distances from each other. In each puncture, a root was set in the area marked by the liquefied wax (Fig. 3). Polyvinyl chloride (PVC) cylinders (Tigre, São Paulo, Brazil) of 25 mm in diameter were also stuck to the wax and placed over the exposed roots, with one of its borders having been previously heated (Fig. 3).
Next, the chemically activated acrylic resin was applied in the inner portion of the PVC cylinder and the elements were maintained in distilled water. Two hours later, they were removed from the wax plate. The teeth were removed from the artificial alveoli in acrylic resin and cleaned for posterior simulation of the periodontal ligament. The preparation of the roots and the insertion of the PVC tube was performed 24 h after the biological posts were made, time in which the posts were conserved in distilled water.
To standardize the coronal portion of the post (nucleus), a preparation of the total crown was performed on a healthy canine with the same dimensions of the teeth from the present study. The coronal portion presented 5 mm height and 1 mm beyond the shoulder line (Fig. 3). This procedure was then duplicated using additional silicone (EXPRESS, 3 M ESPE, USA), and a special stone plaster was applied to the molds to create the plaster models of the prepared nucleus. After the crystallization of the plaster (Vigodent, Coltene, Brazil), the polypropylene matrixes were created in a vacuum plastifying machine (FGM, Brazil).
Post cementation
Four experimental groups (n = 10) were created for testing:
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Group I: control group, Reforpost® glass fiber cylindrical; self-adhesive cement resin with dual polymerization, Rely X U-100 (3M ESPE, USA).
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Group II: biological cylindrical posts made of human dentin; self-adhesive cement resin with dual polymerization, Rely X U-100 (3M ESPE, USA) (3M ESPE, US).
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GROUP III: biological posts made of bovine dentin; self-adhesive cement resin with dual polymerization, Rely X U-100 (3M ESPE, USA).
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GROUP IV: biological posts made of bovine dentin; resin-modified glass-ionomer cements, RelyX™ Luting 2 (3M Espe, USA).
All of the root canals had their fillings partially removed (10 mm), were widened to the size of a Largo 6 drill (1.6 mm in diameter), and then cleaned with 24% EDTA (Biodinâmica Ltda., Paraná, Brazil) for 3 min, followed by abundant rinsing with distilled water for 1 min and dried with absorbent paper cones (Dentsply, USA).
For the cementation stage, each glass fiber post was cleaned with 70% alcohol and dried. The biological posts were rinsed with an air/water spray and dried with absorbent paper. Each post received a fine layer of cement along its border and was immediately placed in the root canal slowly. After excess removal, the cement was photopolymerized for 40 s, followed by a waiting period for the final chemical polymerization to be achieved.
The portion of the post that remained outside the root was maintained, together with the composite resin, to characterize the nucleus. Using the polypropylene matrixes obtained in a vacuum plastifying machine, the form and dimension of the coronal portion of the nuclei (5 mm in height and shoulder of 1 mm) were standardized for all groups (Fig. 3).
To accomplish this, after complete polymerization of the cements, the adhesive procedures were performed equally for all groups to formulate the nucleus, together with the matrix and the composite resin. Application of 37% phosphoric acid for 15 s on the post and dentin, abundant rinsing, and drying with absorbent paper; application of the Single Bond adhesive system (ESPE–USA) on the entire dentin and post, and polymerization for 20 s with a light-emitting diode (wavelength of 470 nm); placing of the Filtek™ Z250 composite resin (3 M ESPE, Sumaré, Brazil) around the cemented post; polymerization for 40 s on all sides; followed by removal of the excess material using diamond tip burrs.
Each root and artificial alveoli made of acrylic resin was cleaned, all wax removed and dried with absorbent paper. The specific adhesive of the simulator material (Polyether Adhesive, 3M ESPE, Germany) was attached to the roots and inside the artificial alveoli, waiting 15 min for it to completely dry. Next, the impression material made of polyethylene from Impregum (3M ESPE, Germany) was handled, according to manufacturer instructions, and placed in the artificial alveoli by means of a polyethylene syringe [17]. The tooth was then re-implanted upon this, removing the excesses [17].
The teeth were stored for 24 h in 100% humidity [17] to perform the 135° compression tests.
In vitro fracture resistance test
Compressive loads were applied in an EMIC universal testing machine (Instron Brasil Equipamentos Científicos Ltda, Paraná, Brazil) at a velocity of 0.5 mm/min, in the palatal region of the specimens, at a 135° angle in relation to the long axis of the tooth (Fig. 4), until fracture.
These fractures were classified according to the tooth region (cervical, middle, or apical) (Fig. 5) and whether or not they were reparable [18]. Dental feasibility was considered to characterize the repair, either by clinical crown lengthening or by apicectomy, aimed at the maintenance of appropriate bone support [18].
Statistical analysis
The data were analyzed by the Minitab® 15 statistics software. Descriptive analyses were performed to provide mean, standard deviation, and frequencies. The data were then submitted to the normality test. The differences in the fracture resistance values were verified by means of Analysis of Variance (One-Way ANOVA). The distribution of the fracture patterns among the groups were verified by Chi-Square test. A significance level of 5% was adopted.