In this study, the samples were divided into three groups: flowable composite (Nano-Hybrid, Tetric N-Flow, Ivoclar Vivadent, Liechtenstein), packable composite (Nano-Hybrid, Tetric N-Ceram, Ivoclar Vivadent, Liechtenstein), and the combined use of flowable and packable composite. These groups were compared for SBS, ARI, and wire pull-out resistance. The study was approved by the Medical Ethics Committee of the Shiraz University of Medical Science (IR.SUMS.DENTAL.REC.1401.063) and is reported in accordance with ARRIVE guidelines.

Specimens characteristics

Seventy-two extracted sound bovine incisor teeth were used for SBS and ARI evaluations. Based on a study by Reicheneder et al. [29], twelve pairs were allocated to each of the three study groups, resulting in a total of 72 samples. The minimum sample size was calculated to be nine pairs in each group using G*Power software with B = 0.2, α = 0.05, and a study power of 90%. However, to ensure higher accuracy, the minimum sample size was increased to 12 pairs in each group. Previous studies have validated the use of bovine teeth as a substrate for SBS testing [30]. These teeth were sourced from animals euthanized in a slaughterhouse for reasons unrelated to this study. After extraction, the teeth were rinsed with water and cleaned of any debris using a scaler. They were then stored in distilled water at 24 °C to maintain hydration [31]. Any teeth showing hypoplastic or anomalous enamel areas were excluded from the sample groups. The teeth were paired and embedded in chemically cured acrylic resin molds to simulate dental arch positioning and interdental contacts. The surfaces were oriented to allow for parallel cutting of the retainer in relation to the crown.

To conduct the wire pull-out test, a total of 192 cylindrical acrylic blocks were fabricated, each measuring 25 mm in width and 10 mm in depth. Custom molds compatible with the testing machine were utilized for this purpose. The minimum sample size for each of the three groups was determined to be 64, resulting in a total of 192 blocks, based on parameters including effect size (0.5), study power (90%), significance level (α = 0.05), and a non-centrality parameter (B = 0.2). These calculations were performed using G*power software. Consequently, each of the three test groups was allocated 64 blocks.

Shear bond strength testing

The samples were divided into three test groups, each comprising 24 teeth. A 37% phosphoric acid gel (3 M, USA, fluoride-free) was applied to etch the teeth’ lingual surface for 30 s, a standard etching time in orthodontic bonding [32]. After rinsing with water and air drying for 10 s, bonding resin (Tetric-N-bond, Ivoclar Vivadent, Liechtenstein) was applied and light-cured for 10 s in each test group. Subsequently, 15 mm lengths of passive retainer wire (American Orthodontics, three-strand, 17.5 twists) were bonded to the lingual surface of the teeth parallel to the acrylic base. Flowable composite was used for Group 1, packable composite for Group 2, and a combination of both composites for Group 3. The amount of composite used was standardized using a minidome-shaped mold (Fig. 1).

Fig. 1

3D designed molds used for administration of composite resin

The composite resins were placed into a custom-made mini mold featuring an internal diameter of 4 mm and a height of 3 mm (Fig. 2). Within the mold, a groove facilitated the positioning of the composite to align with the wire at the center of the composite connection. The excess composite material was meticulously removed using a dental explorer, followed by curing with an LED curing light for 30 s—notably, the transparency of the mold allowed for effective light curing. In the case of utilizing both composites in Group 3, a flowable composite was initially packed and light-cured within a mini mold featuring an internal diameter of 2 mm and a height of 2.5 mm. Subsequently, the flowable composite was overlaid with a packable composite using a mini mold measuring 4 mm in diameter and 3 mm in height (Fig. 3). Post-application, the samples were de-molded and subjected to a second curing cycle lasting 20 s.

Fig. 2

Mini molds used for the administration of composite resin

Fig. 3

Preparing samples in the combined group

To maintain consistency in the bonding process, all retainers were standardized to a length of 12 mm. A flexible custom mold was employed to ensure adherence to standardized testing protocols, resulting in adhesive surfaces with a diameter of 4 mm and positioned 4 mm apart from each other. Each sample underwent a storage period of seven days in distilled water prior to testing.

The detachment procedure was conducted utilizing a Zwick Roell Universal Testing Machine-Z020, operating at a crosshead speed of 1 mm/min (Fig. 4). To simulate preliminary bite stress, the applied strain was directed along the occlusal-apical axis of the incisors. Consistent with established methodologies [28,29,30, 33, 34], the edge of the shear bar was positioned at the midpoint of the interdental segment, owing to its heightened sensitivity. Stress was incrementally applied to the wire until separation ensued, with the resulting SBS recorded in Newtons (N).

Fig. 4

Measurement of shear bond strength on debonding using universal testing machine

Adhesive remnant index (ARI)

Following debonding, all teeth and retainers were examined using an optical microscope (Bestscope 300, BestScope Technology Co., Ltd., China) to assess the residual adhesive on the enamel surfaces. The quantification of adhesive remnants adhering to the teeth surfaces was carried out according to established guidelines for assessing the ARI [35]. The criteria were as follows:

Score 0 = No adhesive left on the tooth.

Score 1 = Less than half of the adhesive left on the tooth.

Score 2 = More than half of the adhesive left on the tooth.

Score 3 = All adhesive left on the tooth, with a distinct impression of the retainer.

Wire pull-out

A small rectangle was crafted on the top of each block using a wire measuring 2.5 mm wide, 5 mm long, and 4 mm deep, representing the quantity of wire typically inserted in a bonded dental retainer. Additionally, a small groove measuring 1 mm in width was carved into the top surface of the block, extending across its entire diameter, with the intention of securing the wire in place. This groove was made approximately 2 mm deep in each test group to demonstrate the depth of penetration of both the wire and the composite material into the tooth surface.

The fabrication of the rectangle and groove was accomplished using a stamp designed in 3D and printed with resin (Fig. 5). Subsequently, the upper surface of the block, formed by the stamp, was polished and prepared for bonding. Initially, any debris within the slot and center hole was manually removed. Then, the slot and hole were dried using compressed air. An 8.5-cm long three-strand rope was passively positioned at the bottom of the groove, followed by the passive placement of an 8.5-cm length three-strand wire at the base of the groove. A 3.5-cm length was marked on both sides of the wire, and a node was tied by two Matthew knots (Fig. 6). Subsequently, acrylic was poured onto the node to secure it in place during the test.

Fig. 5

3D-designed stamp for wire pull-out test

Fig. 6

The sample used for the wire pull-out test

Following this, the wire was inserted into the specific composite material designated for testing. In Group 1, the wire was embedded in the packable composite, while in Group 2, it was embedded in the flowable composite. Group 3 involved a combination of both composite types. The void in the middle of the slot was filled with the test material. Special attention was paid to ensuring intimate contact between the plastic and the wall of the center hole of the slot without any obstruction from air bubbles. Any excess material was removed by the sculptor. Finally, the composite was treated with light for 30 s. To prepare Group 3, a flexible silicone mold with dimensions of 2.5 mm in width, 1 mm in length, and 2 mm in depth was used as a barrier against the spread of flowable composite throughout the cavity.

The molds were positioned on both sides of the wire, after which the wire was embedded in flowable composite up to half of the mold’s depth, equivalent to 1 mm. Following the curing of the flowable composite, the molds were removed, and the remaining space, encompassing the former mold placement and extending 1 mm above the flowable composite, was filled with packable composite (Fig. 7). Subsequently, the ends of the wires were pulled and connected to enable fixation using the tension sensor fixing lever of the universal tester (Zwick Z020). This setup allowed for the application of force perpendicular to the dip cord’s length to initiate the movement of the rope. Testing for damage was conducted by moving the crosshead at a speed of 10 mm per minute [36]. The force required to extract the wire from the device was measured in Newtons (Fig. 7).

Fig. 7Statistical analysis

The average and variation values for each study group were calculated using the collected data from the experimental groups. ANOVA was employed to assess significant differences among the groups, followed by the application of the Tukey HSD range test to confirm any observed disparities with a 95% confidence level. Furthermore, the Chi-square test was used to investigate variations in ARI scores across different groups, and pairwise comparisons were performed using the Kruskal-Wallis test. Data analysis was performed using SPSS software (IBM Corp. Released 2013. IBM SPSS Statistics for Windows, Version 22.0. Armonk, NY: IBM Corp.)