The present in-vitro study was designed and conducted in the Department of Orthodontics and Dentofacial Orthopaedics, Best Dental Science College, Madurai, Tamil Nadu, India, between July 2022 and December 2023. Ethical clearance was obtained from the Institutional Ethical Committee (BDSC-IEC2022-STU-BrV-AQD-22). Informed consent was obtained from the patients who required upper premolar extraction for orthodontic treatment.

The inclusion and exclusion criteria for the study are as follows:

Inclusion criteria:

Exclusion criteria:

Sample size calculation: The required sample size was estimated from a previous study [5] using G*Power software version 3.1 (power=0.80), resulting in a total of 60 teeth used in the study [Table/Fig-1]. Based on the inclusion and exclusion criteria, a sample of 60 non carious, non fluorosed extracted maxillary first premolar teeth was collected from the study Institute.

[Table/Fig-1]:

Sample size distribution.

All the teeth were cleaned with pumice for five seconds, rinsed in water for ten seconds, and then air-dried. The buccal surface of the teeth was etched with 37% orthophosphoric acid gel for thirty seconds, rinsed with water for ten seconds, and air-dried until a white, chalky surface appeared. The primer (Transbond ^TM XT, 3M Unitek, Monrovia, California) was applied to the etched surface in a gingival-occlusal direction and cured for fifteen seconds. MBT stainless steel brackets with a slot size of 0.022×0.028 inches were used, and each tooth was bonded with the composite and cured for 40 seconds [Table/Fig-2,3,4 and 5].

[Table/Fig-2]:

Sample size distribution.

[Table/Fig-3]:

Application of 37% orthophosphoric acid on the buccal surface of the tooth.

[Table/Fig-4]:

Application of primer on the buccal surface of the tooth.

[Table/Fig-5]:

Light curing of the bracket for 40 seconds.

Based on the type of adhesive used, the samples were divided into two groups:

Preparation of nanocomposite: To create a mixture of 1% TiO₂ nanocomposite, the amount of composite required for each tooth was estimated based on clinical experience. A conventional composite (Transbond ^TM XT; 3M Unitek, Monrovia, California, USA) weighing 245 mg (approximately 2.5 mg of composite required per tooth×10 teeth) was manually mixed with 5 mg of 1% TiO₂ nanoparticle powder (Ultrananotech TiO₂ nanoparticles, Purity-99.9%, APS-30-50 nm, Bengaluru, India) on a glass slab in a semi-dark room, away from direct light, to achieve a homogeneous mixture of 250 mg (245 mg of Transbond ^TM XT+5 mg of TiO₂ powder=250 mg TiO₂ nanocomposite) for a sample of 10 teeth. This amount was then multiplied by 3 to obtain 750 mg of nanocomposite for a total sample of 30 teeth [Table/Fig-6a,b] [11,12].

[Table/Fig-6]:

a,b) Weighing of TiO₂ powder using electronic balance machine and mixed with the conventional composite resin.

Debonding procedure: The Instron 3000 universal testing machine was used to analyse the debonding force of the bracket, in which the crosshead of the machine was moved at a uniform speed of 1 mm/minute. An acrylic block was prepared, and the tooth was mounted in a vertical direction with the help of a jig. The acrylic was covered up to the cementoenamel junction of the tooth to ensure equal distribution of force. The tooth was positioned in the lower crosshead of the Instron machine with the tooth crown facing upwards. The necessary force to debond the bracket was applied in an occlusal-gingival direction parallel to the bracket base. The shear bond strength was evaluated using the unit of Megapascals (MPa) [Table/Fig-7,8].

[Table/Fig-7]:

Mounting of tooth in acrylic blocks.

[Table/Fig-8]:

Debonding of the brackets using Instron 3000 universal testing machine.

After bracket debonding, the ARI score [7] was calculated under a 10× microscope according to the following criteria:

According to Artun J and Bergland S (1984), a score of 3 was considered indicative of better bond strength because the fracture occurs at the bracket-adhesive interface. However, there is a difficulty in removing the adhesive, which might result in some enamel being removed from the surface, potentially mistaken for composite due to its transparent nature [13]. Therefore, a score of either 1 or 2 is considered safe for removal without damaging the enamel surface.

The obtained data values were entered into Microsoft Excel 2013 and statistically analysed using Statistical Package for Social Sciences (SPSS) software version 22.0, Chicago, Illinois. A Student’s t-test was conducted to estimate the difference in the mean compressive stress at the maximum debonding force required for the two groups, and a Chi-square test was performed to evaluate the ARI scores between the two groups. The significance level was set at a p-value of less than 0.05.

The highest bond strength was observed in the TiO₂ nanocomposite (Mean±SD: 14.3±1.21 MPa) compared to the conventional composite (Mean±SD: 12.9±1.05 MPa), with a mean difference of 1.4 MPa that was statistically significant (p-value=0.045) [Table/Fig-9].

The results from the Chi-square test indicated that ARI score 1 (<50% of adhesive remnant on the tooth) was more prevalent in the conventional composite group, suggesting a higher occurrence of adhesive bond failure with less adhesive remaining on the enamel. In contrast, score 2 (>50% of the adhesive remnant on the tooth) was greater for the nanocomposite group, indicating a cohesive bond failure that left adhesive remnants on both the enamel surface and the bracket. The difference in the ARI scores between the two groups was statistically not significant (p-value=0.682) [Table/Fig-10].

The present study assessed the shear bond strength of an orthodontic adhesive incorporated with TiO₂ nanoparticles. The results revealed a significant increase in the bond strength of the TiO₂ nanocomposite compared to that of the conventional composite.

The introduction of the etching technique by Buonocore and Bisphenol A-glycidyl Methacrylate (Bis-GMA) by Bowen has played a major role in the successful bonding of orthodontic fixed appliances [14]. Despite the use of nanotechnology in other scientific fields, it has recently become popular in the field of orthodontics for efficient bonding. Nanoparticles have been proven to increase the strength of materials by increasing the surface area; this concept has been used to enhance the properties of adhesives [15]. TiO₂ is highly attractive for its light-scattering properties and very high refractive index, allowing low levels of the pigment to achieve a white, opaque coating with good mechanical properties [4]. Effective bonding of orthodontic brackets to the enamel surface of the teeth is necessary to save chairside time and improve operator efficiency, which requires excellent materials that provide superior bonding properties. According to the literature, the ideal bond strength of the material is considered to be between 6 Mpa and 10 to resist functional occlusal forces [14].

The present study showed that adding TiO₂ as a nanofiller enhanced the bond strength of the nanocomposite compared to the conventional composite (p-value <0.05). The results of the present study contrast with those of Poosti M et al., which indicated that there was no significant difference observed between the nanocomposite and conventional composite (p-value=0.58) [5]. Sodagar A et al., concluded in their study that a nanocomposite used with TiO₂ nanoparticles exhibited better bond strength when used at a 1% concentration [12]. However, Elsaka SE et al., concluded in their study that TiO₂ nanocomposites provided better mechanical and antibacterial properties, thus enhancing bond strength [16].

The differences observed may be due to small sample sizes, methodological variations, different bonding materials and the standardisation of particle sizes, as sizes greater than the threshold value can affect the adherence of the composite to the tooth material, potentially impacting the outcomes across various studies. Therefore, a larger sample size was used in the present study. Another reason for the differences in bond strength could arise from variations in filler content. A higher filler concentration leads to less polymerisation shrinkage and a higher degree of cure [8]. Transbond ^TM XT was used in the present study since it is the most widely used composite for comparing different bonding materials for bond strength values. Similar studies from the literature have been tabulated in [Table/Fig-11] [5-7,9,10,12,17].

Debonding the bracket carries a risk of chipping the enamel surface, which may lead to enamel loss. Therefore, it is considered clinically safe to remove the remnant composite from the enamel surface after debonding to prevent enamel loss [7]. Consequently, the ARI score is regarded as a reliable method for assessing the adhesive-tooth and adhesive-bracket interfaces. ARI scores, compared using the Chi-square test, showed no significant difference between the groups (p-value >0.05).

In the case of the conventional composite, score 1 was higher, indicating more adhesive bond failure with less adhesive on the enamel. In contrast, in the TiO₂ nanocomposite, score 2 was higher, suggesting a cohesive bond failure that left adhesive remnants on both sides, i.e., the enamel surface and the bracket. A study by Uysal T et al., showed no significant results in ARI scores between Transbond ^TM XT, Filtek Supreme Plus nanocomposite and KetacTM N100 nano-ionomer, all of which contain fluoroaminosilicate glass and nanofillers known to release fluoride for caries inhibition and improve mechanical properties [3]. According to Nagar N and Vaz AC, there were no significant differences in the ARI scores between Ceram·X^® mono and Transbond ^TM XT [18].

The incorporation of TiO₂ nanoparticles into orthodontic adhesives has substantial clinical relevance. Enhanced bond strength reduces the frequency of bracket failures, thereby minimising the need for rebonding and associated interruptions in treatment. This not only optimises clinical efficiency by reducing chairside time but also improves the patient experience by decreasing the overall treatment duration. Moreover, stronger bonding facilitates better control of orthodontic forces, contributing to more precise and reliable tooth movement. This benefit of improved bond strength enhances both the aesthetic and functional outcomes of orthodontic care, ensuring higher long-term success rates. The forces generated in the present study may not accurately reflect the forces produced by jaw movements, which typically fall within the range of 81-100 mm/sec. This range is comparatively lower than that measured by universal testing machines [19].

Though the results of the present study showed adequate bond strength between the two composites, there are certain limitations. For instance, the study was conducted in-vitro, where the bond strength of the material cannot be generalised to clinical situations. The strength of the material may differ among various brands of composites that are commercially available and routinely used in clinical cases. Additionally, the forces generated in the study may not accurately reflect the forces produced by jaw movements.

The addition of TiO₂ nanocomposites has improved bond strength compared to conventional composites. This innovation not only contributes to more durable orthodontic bonding but may also reduce clinical failures, chair time and overall treatment duration. Therefore, TiO₂ nanoparticles can be used as filler particles to increase the efficiency of bonding. Research on nanocomposite materials is quite extensive in dentistry to enhance bond strength. However, studies regarding nanocomposites are still in the initial stages and remain controversial, as they have been shown to possess antimicrobial properties alongside varying bond strengths. Therefore, clinical trials using appropriate nanoparticle concentrations are required to prove the efficacy and efficiency of the material’s strength in clinical conditions.