Effects of ionizing radiation and different resin composites on shear strength of ceramic brackets: an in vitro study

ABSTRACT Introduction: Head and neck cancer is considered one of the most common types, and its treatment includes radiotherapy, which can trigger side effects and undesirable sequelae in the oral cavity and dental tissues. Objective: This study aimed to make an in vitro evaluation of the shear strength and failure mode of ceramic orthodontic brackets bonded with two different composites in enamel submitted to ionizing radiation. Methods: After the study was approved by the Research Ethics Committee, 60 healthy human premolars were selected and divided into two groups, based on the absence or presence of ionizing irradiation of the enamel. The fragments were thermocycled and then randomly subdivided into two subgroups, according to the composite used for bonding the ceramic brackets (Inspire Ice - Ormco) to the enamel (n = 15): Transbond XT composite (3M), and Light Bond composite (Reliance). After 24 hours, the specimens were submitted to the shear strength test, and the failure mode was analyzed using a stereomicroscope and confocal microscopy. The shear strength data were submitted to two-way ANOVA, considering a significance level of 5%. Results: The groups submitted to radiation presented lower shear strength values (4.48MPa) than those not irradiated (9.23MPa) (p< 0.001), and the tested composites were not statistically different (p= 0.078). Regarding the fracture mode, all the groups presented mostly adhesive fractures. Conclusion: It was concluded that ionizing radiation negatively affects the adhesion of ceramic brackets, regardless of the composite used for bonding.


INTRODUCTION
Cancer treatment is administered with ionizing radiation, which destroys neoplastic tissues, interacts with tissues, and forms electrons that ionize the environment, create chemical effects by water hydrolysis, and disrupt DNA strands. 1 However, treatment with radiotherapy may have undesirable side effects and sequelae in the oral cavity. 2,3 In addition to these side effects, changes may occur in the physical and adhesive properties of dental enamel following ionizing radiation. 4,5 These changes should be considered by the dental surgeon when performing restorative procedures, and bonding orthodontic brackets. 5 The advancement of cancer treatment methods, and the early diagnosis of the disease 6-10 have substantially improved the survival and cure rates in recent decades. Therefore, more and more dentists are subject to treat patients who have undergone cancer treatment, such as radiotherapy. This increase in cancer treatments demands greater attention on the changes that may be occurring in the oral cavity, both in soft and hard tissues. 11 In cases where radiotherapy is applied to the head and neck region, enamel and dentin may present structural changes, and modifications in their physicochemical properties. 4,[12][13][14][15][16][17] Dental Press J Orthod. 2022;27(2):e2219330 Tomasin Neto A, Amaral F, Romano F -Effects of ionizing radiation and different resin composites on shear strength of ceramic brackets: an in vitro study 4 Despite the increasing use of ceramic brackets for orthodontic treatments, few studies in the literature have evaluated how ionizing radiation interferes with the bonding of these brackets to dental enamel 5,18 . Thus, this study aimed to make an in vitro evaluation of the shear strength and failure mode of ceramic orthodontic brackets bonded with two different enamel composites submitted to ionizing radiation. The null hypothesis tested is that there would be no difference in the shear strength of different composites used for bonding ceramic brackets, in either irradiated or non-irradiated enamel.

MATERIAL AND METHODS
After the study was approval by the Research Ethics Committee  Subsequently, the premolar roots were introduced into and centered in PVC tubes approximately 1.5-cm high, filled with self-curing acrylic resin, and the excess resin was removed with a Lecron spatula (Duflex, Juiz de Fora/MG, Brazil). This device not only enabled the tooth roots to be introduced into the PVC tubes, but also helped maintain the tooth perpendicular to the base of the tube. This positioning is essential to provide parallelism between the exposed enamel face and the shear chisel during the mechanical test, given that any change in this angle may alter the test result.  The specimens were subjected to a fractioned dose of 2 Gy over five consecutive days, until reaching the total 60 Gy dose for all 30 fractions after six weeks (Table 2)  triple syringe free of oil and moisture for the same period. Each rubber cup was used on only five teeth, thus preventing rubber wear from impairing the prophylaxis efficiency. Immediately afterwards, the enamel was conditioned with 37% phosphoric acid for 15 seconds, followed by vigorous washing with air/water jet for 10 seconds, and drying for the same period.
Enamel fragments with or without irradiation were divided into two subgroups, according to the bonding composite: Transbond XT composite (3M Unitek, Monrovia, CA, USA), and Light Bond composite (Reliance Orthodontic Products, Itasca, IL, USA). Table 3 describes the composition of the materials.

Enamel/adhesive cohesive mixed fracture (CE/Ad) -the
dental structure is partially removed on the same specimen, and the remaining structure is fractured between the composite and the enamel.

Resin cohesive/Enamel cohesive mixed fracture (CR/CE) -part
of the composite remains on the dental surface of the same specimen, and the dental structure is partially removed.

Tomasin Neto A, Amaral F, Romano F -Effects of ionizing radiation and different resin composites on shear strength of ceramic brackets: an in vitro study
The flowchart presented in the Figure 1 shows the main methodological procedures of this study.

SHEAR STRENGTH TEST
The two-way analysis of variance showed that the ionizing radiation significantly affected the shear bond strength of ceramic brackets to enamel (p < 0.001). Table 4   were no statistical differences between Transbond XT and Lightbond composites (p = 0.078, Table 4 and Fig 2).

FRACTURE PATTERN ANALYSIS
Analysis of fracture patterns showed that the adhesive fracture was predominant in all groups. However, there was a high percentage of cohesive fracture in resin (CR) and resin/adhesive cohesive mixed fracture (CR/Ad) in the non-irradiated group, in which brackets were bonded with Light Bond composite (Fig 3).

MORPHOLOGICAL ANALYSIS
Representative images of each failure mode can be seen in Figure 4.

DISCUSSION
Structural changes in enamel and dentin from ionizing radiation present as decreased or increased microhardness, increased solubility, reduced stability of the dentin-enamel junction, altered enamel prism structure, increased calcium concentration, and reduced oxygen in the enamel. [12][13][14][15]24 Thus, dental surfaces altered by radiotherapy become susceptible to adhesive failures, such as failures related to bracket bonding used in corrective orthodontic treatment. 4,17,20,25,26 Adequate adhesion of brackets is essential, whether metallic or ceramic, since frequent rebonding prolongs the time of corrective orthodontic treatment and hinders orthodontic mechanics. 25  Tomasin Neto A, Amaral F, Romano F -Effects of ionizing radiation and different resin composites on shear strength of ceramic brackets: an in vitro study

CONCLUSION
The bond strength of ceramic composites and brackets to enamel subjected to ionizing radiation was reduced.
There were no significant differences between Transbond XT and Light Bond composites, regardless of the presence of ionizing irradiation to enamel or not.
There was a predominance of adhesive fractures for all the groups evaluated, indicating that the fracture occurred between the enamel and the composite.

Fábio Romano (FR)
Conception or design of the study:

ATN, FA, FR
Data acquisition, analysis or interpretation:

ATN, FR
Writing the article:

ATN
Critical revision of the article:

FA, FR
Final approval of the article:

FA
Overall responsibility:

FA
The authors report no commercial, proprietary or financial interest in the products or companies described in this article.