Orthodontics is a branch of dentistry that embraces correcting tooth position by delivering force to the malaligned teeth. This is achieved with orthodontic brackets bonded to the tooth. Force is applied with the help of an archwire engaged in the slots of the bracket.

Stainless steel, which has been used promisingly in the field of orthodontics for decades, was introduced by Lucien De Coster.^[1] The initial brackets made from stainless steel had a mesh base morphology. This was based on the mechanism that increased surface area increases bond strength. Maijer and Smith demonstrated the release of corrosive products from American Iron and Steel Institute type 316 stainless steel brackets.^[2] A newer stainless steel alloy was proposed by Oshida called 2205 stainless steel alloy, which has better corrosion resistance and improved microhardness compared to 316 L stainless steel. As stainless steel is less biocompatible and allergic due to the presence of nickel, other metal brackets have been launched. This includes titanium brackets and cobalt chromium brackets. In the 1980s, there was an increasing surge in the number of adult patients in the field of orthodontics, which paved the way for research into esthetic orthodontic brackets. In 1960, the first transparent bracket was introduced by Newman et al. In 1980, the first alumina-based ceramic brackets emerged. It suffered from the limitations of being bulky, an increased incidence of tie wing fracture, higher reports of enamel damage during debonding, and increased friction. This led to further research, leading to the discovery of zirconia-based ceramic brackets. The first plastic bracket was an unfilled polycarbonate bracket launched in the early 1970s. These plastic brackets, which were fabricated earlier, had the disadvantages of having a low elastic modulus and increased absorption of colorants.^[3] To overcome these shortcomings, reinforced polycarbonate brackets were launched. Other polymers that have been utilized in the fabrication of orthodontic brackets are polyurethane, polyoxymethylene, and many more. Studies are being conducted worldwide to minimize their drawbacks and provide brackets for improved properties.

With the ongoing research, the polymer brackets might be considered an effective alternative to the conventional bracket system, but there is no clear evidence available in the literature comparing the mechanical properties of the polymer brackets with the conventional brackets.

This systematic review was prepared according to the Cochrane Handbook for Systematic Reviews of Interventions and Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA).^[4-6] This systematic review could not be registered in the International Prospective Register of Systematic Reviews (PROSPERO) as it is only dedicated to reviews of studies in humans or animals, and this is a review of in vitro studies.

Stainless steel brackets are the conventional bracket system being used clinically. With aesthetics becoming an important requisite, it has become indispensable to incorporate a ceramic bracket system into the treatment plan. Scott discussed the mechanical properties of ceramic brackets with metal brackets, especially fracture toughness.^[19] He concluded that ceramics are more likely to fracture than metals under the same conditions. Even though ceramic has transparency attributes, high hardness, and high strength, it has the limitation of having low fracture toughness. Furthermore, the efficiency of tooth movement was reduced with the ceramic bracket due to increased friction.

Torque is an activation generated by the torsion of an archwire in a bracket slot, and the archwire moves the root in a palatal direction through the torsional tension. Some authors have specified that a minimum torque of 0.5 Ncm is necessary.^[20] As the deformability of the bracket during torsional force has an impact on the torquing moment, it is dispensable to know the torquing capacity of the bracket. Harzer et al. and Morina et al. concluded that polycarbonate brackets have the highest torque loss in comparison with conventional brackets.^[13,14] This is due to the elastic deformation of the bracket slot. The results obtained are in accordance with the results of Feldner et al. and Alkire et al.^[9,10] The transmitted torquing moment is less predicted in polycarbonate, as the shock absorber effect claimed by the manufacturer is not to be expected. Only polycarbonate brackets with metal slots displayed adequate stability.

Creep is a permanent deformation that occurs when a material is subjected to a constant load over an extended period of time. Creep is important for thermoplastic materials such as polycarbonate resins. Dobrin et al. found that polycarbonate bracket slots distorted with time under physiological stress of 2000 g/mm².^[3] Henner et al. reported that the torquing capacity of the filler-reinforced polycarbonate bracket showed better torquing capacity than the nonreinforced polymer brackets, even though they were lower than the conventional metal brackets.^[11] Feldner et al. have concluded that ceramic-reinforced polymer brackets lacked clinically acceptable strength to withstand torquing forces and would lead to distortion.^[9] Moreover, this was confirmed by Möller et al. and Sadat-Khonsari et al.^[12,16] The addition of ceramic or fiberglass particles may influence the water absorption properties of plastics. This absorbed water reduced the adhesion between the ceramic fillers and the polycarbonate resins. If these phenomena occur, increased creep may result. If the filler particles added are silanized and their size and proportion are varied, it might show improved mechanical properties. Other factors that influence their mechanical properties are their manufacturing processes and the design of the apparatus. Within each bracket type, the bracket width was always greater than the displacement of the bracket wings, suggesting that the deformation occurred not only at the bracket wings but also at the bracket base when torquing force was applied.

Hence, reinforcing the polycarbonate material with ceramic filler did not significantly affect the resulting creep. Polymer brackets with a metal slot were more effective in reducing creep. Based on the data of this study and Feldner et al.’s, clinicians who use polycarbonate brackets and fill the bracket slot with a full-size wire to apply significant torque to teeth should consider using brackets with metal slot reinforcement.^[9] The polycarbonate bracket experiences approximately 12% to 15% torque loss as a result of creep. Hence, an additional torque to the wire to obtain the intended torque has been suggested in the clinical use of polycarbonate brackets.

In terms of ceramic brackets, Nishio et al. reported that the ceramic brackets showed significantly higher deformation resistance than plastic brackets, and in particular, ceramic brackets reinforced with a stainless steel slot showed the highest resistance.^[15] However, the plastic brackets are reversibly or plastically deformed on loading.

The clinical torque was simulated using the orthodontic measuring and simulation system (OMSS). Although this integrated system had the advantage of analyzing the issues faced in the field of orthodontics, it had some downsides. It failed to take into effect the long-term effect of the torque on the brackets, the influence of saliva on the bracket material, and its effects on the adjacent teeth. In the clinical dental model in OMSS, the neighboring brackets permitted additional play. The actual torque loss was thus well above the values registered in the in vitro experiments.

Another reason for the low torques was that the torque generated by the outer edges of the wire resulted in the shortening of the archwire, causing deformation of the continuous archwire. This leads to auxiliary forces that generate counter torque in the anterior segment and at the incisors. The simulated tooth then starts the torque movement and reacts rapidly to these forces. Then, the force as well as the torque disappear.

In terms of hardness, Iwasaki et al. measured by the microindentation method ceramic and polycarbonate brackets. Alumina has a higher hardness and is superior to that of CO made of zirconia, and monocrystalline alumina is harder than polycrystalline alumina. The hardness depends on the orientation of the crystals.^[18] A decrease in grain size increases the strength of polycrystalline alumina. The ceramic brackets have low plastic deformation compared with the polymer bracket materials. Glass fiber-reinforced polycarbonate brackets showed increased hardness whereas glass fiber acts as a reinforcing material. This study lacked the influence of water absorption, hence the varied results from the previous studies.

The following conclusions were obtained: Polymer brackets have low mechanical properties in terms of torque loss, fracture resistance, hardness, and torsional creep compared to metal brackets. Among the polymers listed in the studies, it was found that polyamide exhibited low hardness and polyoxymethylene exhibited the highest torque loss. Torque deformation was maximum with a ceramic-reinforced polymer bracket, followed by pure polymer. Torque deformation was minimal with metal slot- and ceramic-reinforced polymers, followed by metal slot-reinforced polymers. Future studies must be carried out to check for the reliability of the obtained results.