**2. Ceramic bracket production: a short overview**

Most ceramic brackets are produced from aluminum oxide (alumina) particles, and these brackets are available in polycrystalline and monocrystalline forms [2].

**3.3. Fracture toughness**

Fracture toughness is a property which describes the ability of a material containing a crack to resist fracture [6, 12]. This is an important material property since the presence of imperfections, such as microscopic scratches, cracks, voids, and pores are not completely avoidable during the fabrication of materials. These microscopic imperfections may or may not be harmful to the material, depending on a number of factors such as the fracture toughness of the material examined, the stress on the material, length of the crack, and resistance of the

Ceramic Brackets Revisited

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The higher the fracture toughness, the more difficult it is to propagate a crack in that material [12]. The fracture toughness of polycrystalline alumina brackets is higher than the fracture toughness of monocrystalline alumina brackets. This implies that crack propagation is relatively easier in single-crystal alumina brackets when compared with polycrystalline alumina brackets [12]. Polycrystalline brackets have a higher resistance to crack propagation due to crack interaction with grain boundaries (GBs). A GB is the interface between two "grains" (crystals) in a polycrystalline (multiple crystals) material (**Figure 1**). Cracks are impeded at these GBs [10]. Clinical applications that may scratch the surfaces of ceramic brackets may greatly reduce the fracture toughness, thereby predisposing ceramic brackets to eventual fracture [12]. Thus, utmost care has to be taken not to scratch ceramic bracket surfaces with instruments and stainless steel ligature wires during treatment. Also, the clinician should not overstress when ligating with steel ligature wires. This might initiate crack growth and propagation, leading to the eventual fracture of the bracket. Careful ligation is mandatory, and elastomeric modules (ligatures) or coated ligatures are advised to prevent ceramic bracket fractures, particularly tie-wing fractures [6, 13, 14]. Arch wire sequencing also has to be performed carefully. The use of resilient full-size arch wires before the placement of full-size stainless steel arch wires is recommended [7]. Furthermore, the patient has to be advised to restrain from chewing and/or biting on any hard substances [6] as well as from intraoral/lip piercings. A prudent choice is to avoid ceramic brackets with

material to crack propagation as well as the environment of the material [6].

orthognathic surgery patients as well as with patients involved in contact sports.

**Figure 1.** Schematic presentation of "grains" and GBs.

Nowadays, the majority of polycrystalline (multiple crystals) brackets are produced by ceramic injection molding (CIM). An outline of CIM is as follows: the aluminum oxide (Al<sup>2</sup> O3 ) particles are mixed with a binder. This mixture is rendered flowable through heat and pressure application and injected into a bracket mold. The binder is removed, i.e., burned out. Subsequently, sintering—the production of a coherent mass by heating without melting—is carried out. The advantage of CIM is that this technology can manufacture complex and precise items with smooth surfaces in large quantities at fast rates [3].

The production process for monocrystalline (single crystal) ceramic brackets, also referred to as sapphire brackets, is completely different. Here, the Al<sup>2</sup> O3 particles are melted. The resultant mass is slowly cooled to permit a controlled crystallization, leading to the production of a large, single crystal. This large, single crystal in rod or bar form is then milled into brackets with ultrasonic cutting techniques and/or diamond cutting tools. After milling, the monocrystalline brackets are heat-treated to eliminate surface imperfections and to relieve the stress caused by the milling procedure. The production of these brackets is more expensive when compared to the production of polycrystalline brackets. This increased expense is mainly due to the difficulty of milling, i.e., the cutting process [2].
