Evaluation of the shear bond strength of various adhesives on the surface of enamel irradiated with various doses of radiotherapy

Introduction

We aimed to compare the shear bond strength (SBS) forces on the enamel surface with 2 adhesives after treatment with various radiation doses.

Methods

A total of 120 premolars were included in the study. The teeth were randomly divided into 5 main groups (n = 24): negative control (without aging), positive control (with aging), 40 Gy, 60 Gy, and 70 Gy radiation. The 40 Gy, 60 Gy, and 70 Gy groups underwent conventional radiotherapy 5 days a week with a dose of 2 Gy each day. After the radiotherapy, all samples except the negative control group were subjected to thermal cycle aging. In all 5 groups, the specimens were divided into 2 subgroups, and half were bonded using 2 adhesives. After bonding, the universal Shimadzu test device was used to analyze the SBS. After the test, the tooth surfaces were examined under a stereomicroscope to determine the adhesive remnant index.

Results

When adhesives were compared, Biofix adhesive’s bond strength value was statistically higher in the 40 Gy group than in the Transbond XT group ( P = 0.001). The SBS value was higher in all irradiated groups than in nonirradiated groups ( P = 0.001). When the adhesive remnant index score was analyzed, no significant difference was found among the groups.

Conclusions

The SBS increased in irradiated teeth compared with unirradiated teeth, and the SBS values of both adhesives were within the acceptable limits in all radiation groups.

Highlights

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The incidence of head and neck cancers has been increasing in recent years.
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Radiation affects the enamel surface in patients with a history of head and neck cancers.
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Bond strength and surface characteristics are important in these patients.
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Different adhesives and radiation doses have been observed to affect bond strength.

Head and neck cancer are important global diseases, accounting for approximately 5.7% of all cancer-related deaths worldwide. ^, Radiation therapy can be a treatment option alone or in combination with chemotherapy or surgical treatment. Today, with the increase in the success rate of cancer patients treated with radiotherapy, the number of patients undergoing fixed orthodontic treatment after radiotherapy has also increased.

In addition to acute and late side effects of radiation therapy, radiation therapy may cause changes directly in enamel and dentin. ^, Radiotherapy causes deterioration in enamel’s interprismatic matrix, damages the crystal structure of hydroxyapatite, and increases the protein-mineral ratio. ^, According to the results obtained from studies, the shear bond strength (SBS) of irradiated enamel decreased. ^, In these studies, only 60 Gy of radiation was applied. In patients undergoing head and neck cancer treatment, total dose applications of 40-70 Gy are sufficient to ensure tumor control. ^, In head and neck cancer patients in the high-risk group, total dose applications of 60-66 Gy have generally been accepted as a conventional standard of care. ^, At this point, the literature includes no research on the effect of various radiation doses on bond strength. It may not be easy to achieve sufficient SBS on the enamel surface in these patients.

One of the important factors affecting orthodontic treatment is the adequate SBS of fixed attachments. ^, ^, Considering that the duration of treatment should be kept short, especially in patients with a history of cancer, SBS is also important in selecting adhesives for these patients. Biofix, an adhesive type used in recent years, minimizes the risk of contamination by reducing the number of clinical procedures and chairside time by eliminating the need for primers. Studies have shown that Biofix’s SBS is sufficient for clinical.

This study aimed to compare the SBS of metal brackets with various adhesives after various radiation doses. The null hypothesis of this study can be expressed as there is no difference between Transbond XT and Biofix adhesives in terms of bond strength forces in teeth with various doses of radiation.

Methods

This study was initiated with the approval of the Van Yüzüncü Yıl University University Clinical Research Ethics Committee in accordance with the ethical principles of the Helsinki Declaration. G∗power software (version 3.1; University of Düsseldorf, Düsseldorf, Germany) was used to determine that the sample size should be 120 for an effect size (d) of 0.4, type I error (α = 0.05) and 94% power for 5 equal main groups. The mandible or maxillary first or second molars that were indicated for extraction for reasons other than this study, such as fixed orthodontic treatment, crowding, and periodontal reasons, were included in the study.

The extracted teeth were without caries or restorative materials, had not undergone any chemical treatment, and had no structural defects, cracks, or damage caused by trauma to the enamel. Immediately after extraction, the periodontal ligament residues remaining on the teeth were cleaned, and the teeth were washed with an air-water spray, then dried and stored in light-proof jars containing distilled water in a dark environment at room temperature. The storage fluid was changed periodically, and prevention of bacterial colonization was attempted. The flowchart in Figure 1 shows the main methodologic procedures of this study.

In preparation for the radiation phase, the teeth were embedded in chemically hardening acrylic resin (SC cold acrylic; Imicryl, Konya, Turkey) up to 2-3 mm below the enamel-cementum margin using plastic cylindrical molds with a diameter of 3 cm. Afterward, the embedded teeth were glued side by side on red wax because they had to be aligned to be subjected to the radiation. In the last stage, a bolus (a tissue-equivalent material used to increase surface doses in radiotherapy) was cut into 0.5 cm pieces and fixed with rubber bands to cover the teeth’s buccal surfaces completely without leaving an air gap to provide effective radiation absorption.

The teeth were randomly divided into 5 main groups (n = 24): negative control (without aging), positive control (with aging), 40 Gy, 60 Gy, and 70 Gy. All groups were exposed to 2-Gy fractions 5 days a week until the required radiation dose was reached. The radiation was administered in a hospital environment using a linear accelerator (Artiste Linear Accelerator; Siemens, Munich, Germany). No radiation dose was administered to the control groups. Teeth with and without radiotherapy were kept in distilled water, and the water was changed daily. Figure 2 presents the schematic of the adhesives and groups used.

After the aging process, the 5 main groups were divided into 2 subgroups. All teeth’s buccal surfaces were roughened with 37% orthophosphoric acid. Subsequently, premolar brackets (UnitekTM GeminiTM Series, 018 MBT brackets; 3M Unitek, Monrovia, Calif) were positioned on the buccal surfaces of the premolars in the subgroup, separated by Transbond XT adhesive (3M Unitek) and Transbond XT primer (3M Unitek, Monrovia, Calif) and Biofix (Biodinamica, Ibipora, Brazil) adhesive. The brackets placed in the appropriate position on the teeth were illuminated with a light device (D-Light; Woodpecker, Guilin, China) that emits blue light at a 420-480-nm wavelength for a total of 20 seconds, 10 seconds mesially and 10 seconds distally.

The specimens were kept in distilled water in an oven at 37°C (UN 110; Memmert, Schwabach, Germany) for 24 hours. The specimens were placed in a universal testing machine (Shimadzu AGS-X, Shimadzu Corporation, Kyoto, Japan) for the SBS test. A blade-shaped indenter with a speed of 0.5 mm/min was placed at the bracket base, directly at the interface with the tooth surface, and subjected to the test. Based on the information obtained from the manufacturer, the fracture load in Newtons was divided by the surface area in mm ² (9.096 mm ² ) and converted into strength in megapascals.

After the test, the surfaces were examined under a stereomicroscope (SZX-ILLB100; Olympus Optical Co Ltd, Tokyo, Japan) at 10× magnification, and the adhesive remnant index (ARI) was determined (0, no adhesive remained on the enamel surface; 1, less than half of the adhesive remained on the enamel surface; 2, more than half of the adhesive remained on the enamel surface; 3, the projection of the bracket mesh could be traced and all the adhesive remained on the enamel surface). The residual adhesives on all tooth surfaces were removed with a 12-blade tungsten carbide bur (H281K314012, Komet; Gebr Brasseler, Lemgo, Germany), and then all tooth surfaces were cleaned with fluorine-free pumice (Imıpomza, Imıcryl, Turkey) using a rubber disc attached to a low-speed micromotor for 10 seconds, and then washed with water.

One sample was randomly selected from each group of samples in the Transbond group, and 5 samples were analyzed. The sample rupture surfaces were cleaned with compressed air and coated with 5-nm-thick gold in a gold-plating device (Quorum SC7620 Sputter Coater, Quorum Technologies Ltd, East Sussex, UK). The samples were then placed in a scanning electron microscope (SEM) (Sigma 300 Field Emission Scanning Electron Microscope, ZEISS Germany, Oberkochen) and microscopically examined at 200× and 500× magnification.

Statistical analysis

Descriptive statistics for continuous variables are expressed as mean, standard deviation, and minimum and maximum values, whereas categorical variables are expressed as numbers and percentages. Two-way (factorial) variance analysis was used to compare groups and treatments for SBS. After the analysis of variance, the Duncan multiple comparison tests was conducted to determine the various groups. The Kruskal-Wallis test was conducted to compare the ARIs. In addition, the chi-square test was conducted to determine the relationship among groups and treatments and ARI scores. Statistical significance was set at 5%, and SPSS software (version 21; IBM, Armonk, NY) was used for calculations.

Results

Table I shows the number of samples in the groups according to various adhesives, SBS values, standard deviation, and minimum and maximum values. The lowest bond strength values were 9.83 ± 3.96 MPa, and the highest values were 18.07 ± 8.32 MPa in all samples containing both adhesives.

Table I

The number of samples, bond strength values, standard deviation, and minimum and maximum values in the groups according to different adhesives are shown

Groups	n	Biofix			n	Transbond XT
Groups	n	Mean ± SD	Minimum	Maximum	n	Mean ± SD	Minimum	Maximum
NC	12	9.86 ± 3.60	5.87	16.13	12	10.19 ± 2.79	6.38	13.78
PC	12	9.83 ± 3.96	5.47	18.13	12	10.40 ± 2.81	7.01	14.51
40 Gy	12	17.61 ± 7.60	7.44	28.97	12	11.78 ± 6.14	4.68	24.11
60 Gy	12	19.50 ± 6.25	4.40	26.35	12	18.07 ± 8.32	6.63	29.96
70 Gy	12	15.97 ± 4.97	5.00	25.09	12	17.81 ± 4.76	6.37	24.32

SD , standard deviation; NC , negative control; PC , positive control.

Table II presents intragroup and intergroup comparative statistics for various adhesives. In the 40 Gy, 60 Gy, and 70 Gy treated groups, SBS values were higher than in both control groups ( P = 0.001). However, no statistical difference was observed among the 40 Gy, 60 Gy, and 70 Gy groups in the Biofix group. No statistically significant difference was observed among the negative and positive controls and 40 Gy applied groups in the samples in which Transbond XT adhesive was used. In the 60 Gy and 70 Gy groups, SBS values were higher than in both control groups, and this difference was statistically significant ( P = 0.001).

Table II

Intragroup and intergroup comparative statistics are presented according to different adhesives

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