INTRODUCTION

Flowable resin composites have been indicated as bioprotective materials on orthodontic mini-implant heads¹ to prevent the implants from traumatizing soft tissues. However, the leachable components from the resins, such as bisphenol A-diglycidyl dimethacrylate (Bis-GMA), triethylene glycol dimethacrylate, and urethane dimethacrylate (UDMA) found in a wide range of resins, have been shown to have a definite cytotoxic effect.^2,3 Because of the proximity of mini-implants to the gingiva and other oral tissues, a similar effect could occur in these tissues.^1,2

The behavior of resins is directly linked to their chemical formulation, and the release of 25% to 45% of the monomers, which are not converted into polymers after polymerization,^4,5 occurs when using the conventional irradiation method.⁵ Therefore, residual monomers⁶ may trigger discrete to moderate—or even severe—inflammatory reactions,⁷ in addition to having a direct influence on the physical, mechanical, and biologic properties of the material.^8,9

Studies have been conducted with adhesives and resin materials,^7,10 but there is a lack of studies that directly relate the degree of monomer conversion, polymerization, and the in vivo inflammatory events of flowable resins. Thus, the authors’ aim was to test the hypothesis that there is no difference between the biocompatibility and degree of monomer conversion of flowable resins used as bioprotective materials of orthodontic mini-implants at different time intervals.

MATERIALS AND METHODS

Animal Model and Experimental Groups

For this study, 48 adult male Wistar rats with a mean weight of 250 g were used. The animals were divided into four experimental groups (12 rats per group): Group Control (polyethylene), Group Filtek, Group Top Comfort, and Group Wave (Table 1). The animal experiment was approved by the Ethics Committee on Animal Research, UACB\UFCG\0102012.

Table 1. Composition of the Tested Flowable Resins

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The rats were anesthetized with an intraperitoneal injection of sodium thiopental (50 mg/kg) (Cristália, Campinas, SP, Brazil). Then trichotomy was performed in the dorsal region, using 4% chlorhexidine gluconate for antisepsis of the operative field.¹⁰ On the midline, equidistant from the base of the tail to the head of the animal, two incisions approximately 8 mm long were made using an No.15 scalpel blade (Embramac, Itapira, SP, Brazil).

The subcutaneous tissue was laterally parted to create a tunnel in the lateral direction, forming two surgical recesses, each approximately 18 mm deep. Each rat was fitted with two tube implants (1.5 mm inner diameter × 5 mm long) made of polyethylene (nontoxic Scalp Vein 19G, Embramac, Itapira, Brazil), which were washed with deionized water and autoclaved at a temperature of 110°C for 20 minutes and then used as inoculation vehicles for the tested materials.

The resins were introduced into the openings at the extremities of the tubes, using a syringe (Centrix, Shelton, Conn) supported on a glass slide at one extremity and a small glass slide at the other to flatten the material. Afterward, they were light polymerized for 40 seconds, using a LED appliance (Radii/SDI, Baywater, Victoria, Australia) fixed on a rod to ensure that the distance between the specimens remained constant and using a light intensity of 1000 mw/cm².

After the materials were implanted, the surgical recesses were sutured with a 4.0 suture needle with thread, then the animals were given a 0.2-mL intramuscular dose of veterinary pentabiotic (Wyeth Laboratory, Collegeville, Pa) and an injection of sodium dipyrone (0.3 mL/100 g; Novalgina, São Paulo, Brazil). All the procedures of this study were performed in accordance with the Canadian Council on Animal Care (1981). The animals were kept in individual cages under adequate conditions with appropriate rations and water ad libitum.

After time intervals of 7, 15, and 30 days, the animals were anesthetized to obtain excisional biopsies of the implant area, including sufficient normal surrounding tissue. Afterward, the rats were sacrificed by the cervical dislocation technique after having been sedated with sodium thiopental (50 mg/kg).

RESULTS

Biocompatibility

The flowable resins evaluated presented an intense inflammatory infiltrate, observed after 7 days, with Groups Top Comfort and Filtek (Figure 1A and B) differing statistically from Group Control (P  =  .016). All the groups presented circulatory alterations (edema) and granulation tissue with greater expressiveness after 7 days, showing gradual reduction in the subsequent periods, without statistical difference among them (P > .05). The resins demonstrated the presence of tissue degeneration (necrosis) around or within the cavity, as a result of resin implantation, in a more expressive manner after 7 days. Multinucleated giant cells were rarely found in response to the nonpersistence of severe inflammatory infiltrate, and there was good tolerance of the body to the materials (Table 2).

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Table 2. Mean of Scores Attributed to Resins After Time Intervals of 7, 15, and 30 Days, for the Six Conditions Evaluated^a

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Regarding the presence of collagen fibers during the repair process, Group Top Comfort demonstrated slower repair than did Group Wave (Figure 1C), differing statistically from the control group (P  =  .037) after 15 days (Figure 1D), which persisted after 30 days, with Group Top Comfort (Figure 1E and F) differing statistically from Groups Filtek and Control (P  =  .008) (Figure 1G; Table 2).

Degree of Conversion

Monomer conversion of the resins increased progressively up to the 30th day, with lower conversion of 62.3% in Group Top Comfort at 7 days and higher conversion of 79.1% in Group Filtek at 30 days. There were no statistically significant differences between the resins at 7 and 15 days (P > .05). The flowable resin Top Comfort showed the lowest conversion values during the experiment, with a statistically significant difference from Filtek (P  =  .006) after 30 days (Table 3). When the time intervals for the same resin were compared, there was significant difference between the times evaluated (P < .01) for all groups.

Table 3. Mean Values and Standard Deviation (SDs) of the Degree of Conversion (%) of Flowable Resins*

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DISCUSSION

Orthodontic mini-implants have been increasingly used in orthodontics; however, complications during orthodontic loading, such as aphthous ulceration, soft tissue covering the miniscrew head, soft tissue inflammation, and infection in the neighboring tissues, arising from the small head of the mini-implant may also occur.¹² To minimize these effects, flowable resin composites have been used as protective material on the head of these devices.¹

On the other hand, the monomers released by resins are cytotoxic to fibroblasts,¹³ which can induce local and systemic reactions in patients,¹⁴ causing epithelial proliferation, lichenoid reactions,¹⁵ hypersensitivity, and allergic reactions.^15,16 The main cause of the cytotoxic effects of resins is the release of unpolymerized residual monomers,^2,7,11,17 which may cause severe cellular damage^3,10,17 and favor bacterial growth around the resin.¹⁸ Studies^19,20 have demonstrated that flowable resins are more cytotoxic than traditional resin composites.

In this study, the biocompatibility of flowable resins was evaluated by means of inflammatory phenomena.^10,11 From the experiments, one may observe an intense inflammatory infiltrate for the resins evaluated, after 7 days, with a statistical difference between the resins Top Comfort and Filtek compared with the control group. This suggests a similar release of monomers between them, as observed in the degree of DC, but of a more cytotoxic nature when compared with the resin Wave in this initial period. In this line of research, these findings support the idea that resin components show cytotoxic effects that are dependent on time and their concentration.²¹

One can observe the low level of expressiveness for the occurence of edema, tissue necrosis, and multinucleated giant cells, which demonstrates a low level of concomitant aggressiveness of these resins. There was expressive presence of granulation tissue after 7 and 15 days, but there was no statistical difference between the materials. This, associated with the reduced inflammatory reaction in subsequent time intervals, demonstrated that the body tolerates the resins well.

The gradual reduction in inflammatory events and the ascendance of the polymerization process at 30 days corroborate the findings of other studies^10,11 and relate to the pattern of monomer conversion into polymers and consequent release of residual monomers²² in the first 4 weeks.^10,23 Regarding the tissue repair process (presence of collagen fibers), the resins Wave and Filtek presented no statistical difference from the control group in any of the time intervals evaluated. The resin Top Comfort demonstrated statistical difference from the control group (P  =  .037) after 15 days, persisting after 30 days, and differing statistically from the resin Filtek and the control group (P  =  .008). The tissue exposed to the resins of Groups Wave and Filtek showed good healing capacity, with this process being slower and showing a lower quantity of collagen fibers for the resin Top Comfort.

As for the DC, other authors have shown immediate conversion values ranging from 60% to 68%.²⁴ In the present study, the values ranged from 62.3% for immediate conversion to 79.1% for conversion in 30 days. The DC depends on the light source used. Other investigators²⁵ have suggested that in order to obtain a similar DC among tested resins, the values could be achieved in 10 to 15 seconds by fast halogen (850 mW/cm²) polymerization. In this study, a LED system with a light intensity of 1000 mW/cm² was used for 40 seconds. The higher DC observed with this light source might be attributed to the greater light energy used on the material. This is in agreement with the findings of authors²⁶ who demonstrated that when light energy was decreased, the DC diminished considerably, as shown with different light energy densities for the same light source.

The flowable resin Top Comfort showed the lowest conversion values during the experiment, with a statistically significant difference from Filtek Z350 (P  =  .006) after an interval of 30 days. Flowable resins contain a large quantity of a diluent monomer, commonly TEGDMA, added to a more voluminous and structurally rigid monomer base, such as Bis-GMA or UDMA, to reduce the viscosity.²⁷ This increases the number of polymerized double bonds; thus, DC increases when there is an increase in TEGDMA—up to a certain point.²⁴ Although most flowable resins have a proportion of TEGDMA that does not normally reach this critical concentration, the high proportion of monomer may be a limiting factor in the DC.²⁵

The flowable resin Top Comfort is composed of 60% BisGMA, UDMA, and TEGMA monomers. This quantity of monomers is significantly higher than the 35% found in the Filtek and Wave resins. Based on the histological findings, we affirmed that the higher level of unconverted monomers released added to the cytotoxicity of these, influencing the slower reparative capacity of the resin Top Comfort, as demonstrated by the lower number of collagen fibers after 15 and 30 days compared with the other flowable resins. The difference in DC of the resins tested may also be attributed to differences in composition: In addition to Bis-GMA and TEGDMA, the resins contain UDMA, which modifies the rheology of the material and may influence the DC of the resin.²⁴

Analysis of the inflammatory and polymerization phenomena in order to characterize and classify the experimental groups and compare them with the control allows one to affirm that the order of the best-to-worst performance was Filtek, Wave, and Top Comfort resin according to the tissue repair capacity after 30 days.

CONCLUSION

• The hypothesis was rejected. The flowable resin Top Comfort demonstrated lower tissue repair capacity with a lower number of collagen fibers compared with the Filtek and Wave resins.

• The DC ranged from 62.3% for immediate conversion to 79.1% for conversion in 30 days. The resin Top Comfort showed the lowest conversion values during the experiment.