Evaluation of resistance to fracture of temporary implant-supported prosthesis with extension in cantilever enhanced with glass fibre

This study aims to evaluate the fracture resistance of acrylic resin used in temporary prosthesis with an extension cantilever, using glass fibers treated with silane as reinforcement, varying the distribution and positioning within the matrix. Fifty specimens were produced and divided equally into five groups: Group I, without reinforcement; Group II, reinforced with continuous, concentrated and aligned fibers; Group III, enhanced with simple fiber laminate; Group IV, a doubly reinforced fiber laminate; Group V, reinforced with fibers surrounding the implants and parallel to the occlusal surface. There was statistical variation between Groups I and III, I and IV, I and V, II and III, II and IV, II and V, and III and IV. The results demonstrated that temporary prosthesis reinforced with glass fibers treated with silane exhibited an increased resistance to fracture.


INTRODUCTION
The oral prosthetic rehabilitation using osteointegrated implants is becoming a daily clinic routine (Rosa et al., 2008;Cooper, 2009). An alternative for this treatment, being the provisional cantilever with distal extension is important (Zurdo et al., 2009). This prosthesis can provide comfort and facilitate adaptation for the patient. However, the masticatory forces are concentrated in these extensions. Often we can see fractures in the union between the cantilever and the last implant (Van Zyl, 1995).
The material choices for making temporary prosthesis is polymethylmethacrylate (PMMA), that presents favorable aesthetic property, easy handling and low cost. However, the mechanical properties of PMMA present low resistance under occlusal force (Berrong et al., 1990). Thus, several authors have proposed the inclusion of ribs in these polymers which are: nylon fibers (John et al., 2001), silica fibers , aluminum *Corresponding author. E-mail: rodrigopoluha@gmail.com. Tel:+554299459674. Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License fibers (Grant and Greener, 2009), polyethylene fibers (Bae et al., 2001;Dixon and Breeding, 1992;Samadzadeh et al., 1997), steel wires (Carroll and von Fraunhofer, 1985;Hazelton et al., 1995;Powell et al., 1994), polyaramid fibers (Bae et al., 2001), carbon fibers (Ekstrand et al., 1987;Larson et al., 1991) and glass fibers (Vallittu, 1993;Keyf et al., 2003;Nohrström et al., 2000) in order to improve their properties, among them, the flexural strength and modulus of elasticity (Haselton et al., 2002). The purpose of these reinforcements is to improve its strength properties and flexural modulus.
According to Fonseca et al. (2011), among the types of observed reinforcements, the glass fiber treated with silane stood out by virtue of better incorporation of the fibers and the matrix, increased fracture resistance, easy handling and low cost. The development of resistant temporary prosthesis to be used in cantilever can bring greater functional and aesthetic comfort for edentulous patients.
This study aims to evaluate the fracture resistance of acrylic resins (PMMA) with the use of glass fibers treated with silane, depending on their distribution and positioning within the PMMA matrix used in temporary prosthesis with extension cantilevers.

MATERIALS AND METHODS
A stainless steel base I was used for the specimens and the other tests. Three sets of abutments (Neodent, Curitiba, Brazil) constitute a component of type titanium UCLA (4.1 mm diameter, 10.0 mm height) set on an analog of implant external hexagon platform (HE) titanium (same dimensions) with the same diameter Neotorque screw by means of lateral screws (Figure 1; 1).
A second metallic matrix II is constructed in two parts (upper and lower) to produce standardized samples simulating teeth 5, a canine, three pre-molars and molars, with interproximal areas of all teeth 5.0 ± 0.1 mm high and 5.5 ± 0.1 mm wide, with the area of cantilever of 21 mm. All dimensions were measured with a digital caliper (Figure 1; 2).
The preparation of specimens occurred positioning the bottom of the array II on the matrix I. A precision scale was used to weigh the portion of acrylic resin Dencrilay (Dencril, São Paulo, SP, Brazil). 3.8 g powder and 4.0 ml of monomer was used. The resin was mixed according to the manufacturer's specifications. When the resin reached the sandy phase, it was applied into the matrix II (Figure 1; 3). The upper part II to the matrix was coupled in the matrix I during the plastic phase of the resin and excesses were removed (Figure 1; 4). At that moment, an elastic was applied to about two matrices, and immediately taken to the orthodontic pan (VH Essence Dental Equipment, Araraquara, SP, Brazil) with water and under pressure of 20 psi for 10 min to complete the polymerization, facilitating subsequent polishing and avoiding the formation of bubbles inside, thereby eliminating this interference.
A single evaluator produced fifty specimens. The specimens were equally divided into 5 groups. The Group I was the control group. This group did not receive fiber reinforcement. In the other groups, samples were produced with glass fiber. All groups received 0.1 g of glass fibers (Maxi Rubber, Diadema, SP, Brazil). These fibers are treated with silane (Prosil, FGM, Joinville, SC, Brasil).
In Group II, the glass fibers were shredded and regrouped. These fibers were arranged in parallel bundles (Figures 2; 5).
The specimens were produced in two stages. At first, the resin was inserted into the individual matrix in the sandy phase (Figure 2; 6). The glass fibers were treated with a silane and inserted into this matrix, making a previous strengthening infrastructure. This structure was taken to orthodontic pan. In the second step, these infrastructures were placed on the components for it to be located at the same level at the top of the pillars. Then, the specimens' preparation process continued being equal to the control group.
The specimens of Group III were constructed with a laminated glass fiber weft (Figure 2; 7). These fibers were positioned in the center of the specimens (Figure 2; 7.1). The glass fibers of the Group IV has been divided into two parts (Figure 2, 8). One of the parts was positioned at the level of the pillars. The other part was positioned in the upper portion of the matrix II (Figure 2, 8.1). In Group V, the specimens were produced with fibers arranged in two ways: encircling the three abutments and parallel to the occlusal surface. The beams which surrounded the implants were placed in the bottom of the matrix and measured 2 cm each, with a quantity of three filaments for each abutments (Figure 2; 9). The beam parallel to the occlusal surface was positioned on top of the matrix which contained ten glass fiber filaments with 3.5 cm in length (Figure 2; 9.1) ( Table 1).
The specimens were evaluated for the presence of defects and bubbles, and were discarded when necessary. The specimens were polished and stored for 72 h in deionized water at 37°. The samples were tested in a universal testing machine (EMIC, São José dos Pinhais, PR, Brazil). An independent evaluator, conducted compression tests on the central groove of the molar. The end point of the tests was to fracture the specimen (Figure 3). Fracture strength of each specimen was recorded, and data were analyzed statistically by analysis of variance (ANOVA) and Tukey's test at 99%. Statistical analysis was performed using the statistical software BioEstat 5.3 (Mamirauá Civil Society/CNPq). Table 2 shows the fracture resistance values for each specimen. The mean and standard deviation can also be observed in this table. ANOVA and Tukey test showed statistically significant difference among groups. Groups III, IV and V were significantly more resistant to fracture as compared to Groups I and II. Assessing the means obtained by groups, it is observed that Group IV was the best. The other groups (Groups V, III, II and I respectively) showed lower performance. Comparing the group with each other, this study observe a statistically significant difference between Groups I and III (4.076); I and IV (5.826); I and V (4.484); II and III (3.188); II and IV (4.939); II and V (3.596). However, there were no statistically significant difference between Groups I and II (0.888); III and IV (1.75); III and V (0.408), IV and V (1.342), respectively.

DISCUSSION
A composite material is formed by a combination of two or more materials. These materials differ in form and chemical composition being insoluble in each other. In the case of polymer matrix composites (PMC), materials are a polymer resin matrix and glass fibers (Smith, 2008;Callister, 2007;Narva et al., 2004). The resistance of the composites is influenced significantly by factors such as arrangement, orientation and distribution of fibers. Composites with uniform distribution of the fibers should have better properties (Callister, 2007;Colán et al., 2008;Uzun et al., 1999;Hazelton er al., 1995). Another factor is expected that the use of the silane promotes an effective bond between the fiber and resin (Ekstrand et al., 1987;Vallittu, 1993). The results of this study showed that Group III (single laminated glass fibers), IV (double laminated glass fibers) and V (glass fibers surrounding the implant and parallel to the occlusal surface) significantly increased fracture toughness when compared with the Groups I (control group) and II (aligned glass fibers).
Group II (aligned glass fibers) was constructed with the addition of glass fibers. However, the specimens showed no resistance to fracture which was significantly higher, than that in the control group. This finding can be explained by the manufacturing process in two steps of these specimens. In this group, the fibers were applied to the element constructed in different time and after separate manufacture, these reinforcements were added to specimens, assuming a central position within the resin matrix. This device does not seem to interact with the provisional acrylic resin. The fiber reinforcement behaves independently with the acrylic matrix. So, the interim did not benefit from the addition of fiberglass. A union between these two similar materials built and polymerized at different times does not form a composite. Some manufacturers provide glass fibers impregnated    with resin composite. Groups III (single laminated glass fibers) and IV (double laminated glass fibers) made use of laminated fibers, interwoven and oriented in strategic positions over the specimens. The fibers were similar in weight and interlacing. However, they were distributed at different positions in the acrylic resin matrix. The oriented positioning is beneficial because it is stable and reduces flexing of the specimen. However, there was no statistically significant difference between these two groups despite heterogeneity of distribution of the fibers position. Possibly, this happened because the vertical length of the specimens does not have enough greatness to be influenced by the fiber positioning. In addition, the reduced size of the samples probably also helped that there was no significant divergence.
Glass fibers (Group V) were separated by removing the interlacing. With this change, it was possible to arrange the fibers in parallel. Thus, the fibers were distributed with greater uniformity in the positioning. In this group, the fibers also was arranged to involve the implants at the level of the pillar. Thus, it was possible to increase the resistance in this region. The results of fracture toughness of these specimens were higher than those of Groups I and II. These results came from the fact that all reinforced species has the same amount of fibers. Once groups were positioned in two different ways between them, they seem to be acted with individual entity and so with the half of quality of the fibers of the other groups. However, there was no statistically significant difference when compared with Groups III and IV. These results seem to indicate that the disposition of the fibers is less important than the weight used. This study did not aim to evaluate the variation of the weight of glass fibers.
The results obtained from the Group III (single laminated glass fibers), IV (double laminated glass fibers) and V (glass fibers surrounding the implant and parallel to the occlusal surface) are similar to those reported by John et al. (2001), Keyf et al. (2003), Kim and Watts (2004), Vallittu (1993Vallittu ( , 1998, Vallittu and Lassila (1992), Vallittu et al. (1994), and Narva et al. (2004) who found a significant increase of the flexural strength of specimens PMMA resin reinforced by glass fibers compared to nonreinforced specimens. New research should to be developed with the aim of greater fracture resistance and the assessment of this prosthesis in testing fatigue and thermal cycling.

Conclusion
According to the methodology used and the results obtained in this study, it could be concluded that the provisional prosthesis with an extension cantilever is reinforced with glass fibers, which can be a viable alternative to oral rehabilitation treatment because Group III (single laminated glass fibers), IV (double laminated glass fibers) and V (glass fibers surrounding the implant and parallel to the occlusal surface) increased significantly with the fracture resistance compared with provisional built without reinforcement.