Exposure of adhesives to high temperatures resulted in a reduction of properties.
Loss of stiffness of the retainer under ordinary clinical conditions may affect treatment stability.
A BPA-free adhesive showed similar or superior results than the control and may be considered as a alternative.
To evaluate the viscoelastic properties of two experimental BPA-free and one BisGMA-based orthodontic resin composite adhesives for bonding fixed retainers.
A commercially available BisGMA-based (TXA: Transbond LR) and two bisphenol A-free experimental adhesives (EXA and EXB) were included in the study. The viscoelastic behavior of the adhesives was evaluated under static and dynamic conditions at dry and wet states and at various temperatures (21, 37, 50 °C). The parameters determined were shear modulus (G), Young’s modulus (E) under static testing and storage modulus (G 1 ), loss tangent (tan δ ) and dynamic viscosity ( n *) under dynamic testing. Statistical analysis was performed by 2-way ANOVA and Bonferroni post-hoc tests ( α = 0.05).
For static testing, a significant difference was found within material and storage condition variables and a significant interaction between the two independent variables (p < 0.001 for G and E). EXA demonstrated the highest G and E values at 21 °C/dry group. Dry specimens showed the highest G and E values, but with no significant difference from 21 °C/wet specimens, except EXA in G. Wet storage at higher temperatures (37 °C and 50 °C) adversely affected all the materials to a degree ranging from 40 to 60% (p < 0.001). For dynamic testing, a significant difference was also found in material and testing condition groups, with a significant interaction between the two independent variables (p < 0.001 for G 1 and n *, p < 0.01 for tan δ ). Reduction in G 1 , and n * values, and increase in tan δ values were encountered at increased water temperatures.
The apparent detrimental effect of high temperature on the reduction of properties of adhesives may contribute to the loss of stiffness of the fixed retainer configuration under ordinary clinical conditions with unfavorable effects on tooth position and stability of the orthodontic treatment result.
Prevention of relapse is a major issue in orthodontic therapy and often requires long-term retention with preferably fixed retainers that require minimal patient compliance . From the various types of fixed retainers described in the literature, those consisting of braided or solid metallic wires bonded to enamel with resin composite orthodontic adhesives are the most frequently used, despite the recent developments in resin impregnated polymer- and glass-fibers . The intraoral performance of such systems composed of heterogeneous materials mainly depends on the strength parameters of the weakest part, the orthodontic adhesive resin, which dominates the stress transfer characteristics of the device to the bonded teeth and also demonstrates the highest failure incidence of the components involved .
Orthodontic resin composite adhesives for lingual retainer bonding are conventional particle-filled composites of medium to high filler content. These materials have been subjected to property modifications including viscosity optimization to reduce free flow, surface tension adjustment for adequate wetting of enamel and wire surfaces, softer consistency than highly filled materials for easy wire entanglement with enamel and thixotropic behavior with high recovery rates after shear thinning to ensure precise application. Orthodontic resin composite adhesives, like the restorative resin composites, demonstrate time-dependent mechanical properties . Therefore, characterization of their viscoelastic behavior can help in understanding their performance under static and dynamic loading. Parameters such as flexural and shear modulus, loss tangent and dynamic viscosity show the ability of the polymers to withstand stresses and to recover during the unloading phase (elastic or inelastic/irreversible strain) under various testing conditions (i.e., different temperatures, presence of water) . This is more important when materials free of bisphenol-A (BPA) derivatives are designed as alternatives to the commonly used BPA derivatives (BisGMA, BisEMA, BisDMA etc) to reduce the possible exposure to BPA release and the associated biological hazards . Nevertheless, the stiff bisphenol aromatic backbone of BisGMA-type monomers, highly contributes to the rigidity of the final material and hence cannot be easily replaced in dental resin composite technology .
Despite their clinical significance, viscoelastic properties have not been thoroughly studied in orthodontic adhesives. Instead, most experimental research is focused on bracket bonding to enamel and restorative materials. In the present study, a well-established technique was used to evaluate the viscoelastic properties of two experimental BPA-free and one BisGMA-based orthodontic adhesives for bonding fixed retainers and a conventional flowable restorative liner. The null hypothesis was that there are no statistically significant differences in the viscoelastic properties among the materials selected under the experimental conditions used.
Materials and methods
The materials tested along with their composition are summarized in Table 1 . The mechanical properties of the orthodontic adhesives were investigated under static and dynamic testing. The specimens were prepared by inserting the adhesive into glass capillary tubes ( Ø = 1 mm, L = 18 mm, n = 4 per product and testing condition) and thoroughly light curing by two 20-s sequential overlapping light exposures employing a LED curing unit (Bluephase G2, Ivoclar Vivadent, Schaan, Liechtenstein) emitting 1200 mW/cm 2 light intensity at the wavelength region 500–400 nm.
|Resin: BisGMA, TEGDMA, catalysts: dimethylbenzocaine, diphenyliodonium hexafluorophosphate, filler: silanated quartz, silnated silica (75–85 wt%)||3 M ESPE, St. Paul, MN, USA|
|Experimental adhesive I
|Resin: PGDMA, TEGDMA, UEDMA, catalysts: CQ, DEAEMA filler: silanated glass (70 wt%)||–|
|Experimental adhesive II
|Resin: TEGDMA, UEDMA, catalysts: CQ, DEAEMA filler: silanated glass (60 wt%)||–|
The method used in the present study has been successfully employed for determination of the viscoelastic behavior of resin composite samples under creep, constant load rate, resonant and subresonant dynamic experiments in both torsion and flexure . The apparatus utilized is described in Fig. 1 . Each specimen was mounted between a 0.5 mm thick plexiglas disc and a rod by using a centering jig. A high intensity permanent Sm–Co magnet (M = 1.12 × 10 −2 Nm/A) with a thin mirror ( Ø = 1.55 mm) bonded to the magnet, was attached at the end of each specimen, and the assembly was placed at the center of a Helmhotz coil. The weight of the magnet caused only a minor constant axial tensile stress with no constraints on specimen torsion or extension. The torque on the specimen was controlled by the current in the coil. The spot of a He–Ne laser beam reflected by the mirror was traced onto a calibrated chart placed at a distance D = 944 cm and the rotation angle of the mirror ( φ ) was calculated from the displacement of the laser beam on the chart (X) by Eq. (1) :
The materials were tested after 24 h storage under the following conditions: (i) Dry at 21 °C, (ii) immersed in water at 21 °C, (iii) immersed in water at 37 °C and (iv) immersed in water at 50 °C. The conditions were controlled by placing a thin plastic tube ( Ø = 16 mm, L = 18 mm) over the specimen which was attached to the disk, creating thus a water containing chamber capable of temperature control (±0.5 °C) via a heating element and a thermocouple.
Under static testing a constant torque was applied to each specimen for 10 s and then instantly released with the angular displacement being recorded. Depending on the alignment of the coil the specimen was tested either under torsion or bending. In the former case, shear modulus is the ratio of shear stress to shear strain ( G = σ / γ ) and was calculated from Eq. (2) :
where r is the specimen radius, L the length and M the magnet torque. In the case of bending, the coil was rotated for 90°. Young’s modulus E was calculated by the ratio of flexural stress to flexural strain ( E = σ / ε ), which for cylindrical specimens is given by Eq. (3) :
where d is the diameter of the specimen.
For dynamic testing, frequencies ranging from 1 to 150 Hz were applied to the specimens. A function generator connected to the Helmholtz coil created a sinusoidal torque. The displacement or amplitude was measured on the chart for each frequency. The viscoelastic properties were calculated from the resonance frequency ν 0 , corresponding to the peak amplitude and also from the resonance full width Δ ν , which is the difference between the two frequencies at which the amplitude is half of the maximum.
In the confines of linear viscoelasticity, stress and strain vary sinusoidally. Storage modulus (G 1 ) is in-phase with strain, while loss modulus G 2 (related to the dissipation of energy) is 90° out-of-phase with strain. In stiff solids the complex modulus G * is almost equal in magnitude to the storage modulus G 1 , because G 2 is small when compared to G 1 . Storage modulus is given by Eq. (4):
ν 0 = ( 1 2 π ) G 1 π r 2 L I
with r and L being the radius and length of the specimen and I the moment of inertia of the magnet.
The ratio of the imaginary part to the real part (G 2 /G 1 ) of the complex modulus G* is the loss tangent (tan δ ) that expresses the phase angle between stress and strain sinusoids. Loss tangent is proportional to the energy loss per cycle and is given by Eq. (5):
tan δ = ( 1 3 ) Δ ν ν 0
Dynamic viscosity was calculated from Eq. (6) :
n * = ( 1 2 π ν 0 ) ( G 1 2 + G 2 2 )