**3.2. Tensile strength**

The ultimate tensile strength, often shortened as tensile strength, is defined as the maximum stress that a material can withstand while being stretched or pulled before failing or breaking [9]. When stress is placed on a ceramic material, its unyielding atomic structure makes the redistribution and the relief of stress close to impossible [2]. Ductile materials, such as metals and polymers, experience plastic deformation before failure [10]. In other words, the elongation of ceramics at failure (brittle fracture) is less than 1%, yet the elongation of stainless steel at failure (ductile fracture) is approximately 20% [11]. Hence, ceramic brackets do not flex. This implies that ceramic brackets are much more likely to fracture than metal brackets under identical conditions [11].

#### **3.3. Fracture toughness**

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

6 Current Approaches in Orthodontics

brackets are available in polycrystalline and monocrystalline forms [2].

cise items with smooth surfaces in large quantities at fast rates [3].

as sapphire brackets, is completely different. Here, the Al<sup>2</sup>

to the difficulty of milling, i.e., the cutting process [2].

**3. Properties of ceramic brackets**

performed to prevent enamel damage.

**3.1. Hardness**

**3.2. Tensile strength**

cal conditions [11].

Most ceramic brackets are produced from aluminum oxide (alumina) particles, and these

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>

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 pre-

The production process for monocrystalline (single crystal) ceramic brackets, also referred to

tant 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

Ceramic brackets are known for their hardness. They are notably harder than enamel [4–7]. Thus, contact between enamel and ceramic brackets has to be avoided by all means. This type of contact can lead to severe enamel damage [8]. Particular care has to be exercised with deep bite and/or class II canine relationship patients. If required, bite opening applications must be

The ultimate tensile strength, often shortened as tensile strength, is defined as the maximum stress that a material can withstand while being stretched or pulled before failing or breaking [9]. When stress is placed on a ceramic material, its unyielding atomic structure makes the redistribution and the relief of stress close to impossible [2]. Ductile materials, such as metals and polymers, experience plastic deformation before failure [10]. In other words, the elongation of ceramics at failure (brittle fracture) is less than 1%, yet the elongation of stainless steel at failure (ductile fracture) is approximately 20% [11]. Hence, ceramic brackets do not flex. This implies that ceramic brackets are much more likely to fracture than metal brackets under identi-

O3

particles are melted. The resul-

O3 ) 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 material to crack propagation as well as the environment of the material [6].

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

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

Finally, it should be noted that the exposure of alumina to water or saliva decreases fracture toughness [10]. This characteristic is important to remember when the clinician attempts to extrapolate in vitro results to the clinical setting, i.e., the oral environment.

#### *3.3.1. Tie-wing fracture*

**Figure 2** pictures a tie-wing fracture of the lower second premolar bracket. Most likely this tie-wing was damaged with pliers during arch wire insertion into the molar tube.

Complete fragmentation of a damaged bracket might occur during arch wire ligation or during the course of treatment. Thus, the removal of an impaired bracket and its replacement with a new bracket is a prudent risk management strategy. The risk of ceramic fragment penetration into the patient's oral soft tissues, inhalation or swallowing by the patient does exist. Ceramics are radiolucent, i.e., ceramic bracket fragments are not visible on radiographs [15].

An interesting in vitro study [16] tested tie-wing fracture strength of polycrystalline and monocrystalline brackets after being exposed to fluoride prophylactic agents (Prevident 5000 and Phos-flur gel; Colgate Pharmaceuticals, Canton, Mass, USA). The researchers stated that the fluoride-alumina surface interaction most likely caused strain in the surface bonds of both types of brackets. Yet, this presumed bond strain only affected the fracture strength of the monocrystalline alumina brackets. The results of this study imply that the use of topical fluoride agents may increase the susceptibility of tie-wing fractures of monocrystalline brackets under clinical conditions and that polycrystalline brackets might be the appropriate choice for poor oral hygiene patients that require fluoride prophylactic agents. The authors [16] pointed out that this outcome was most likely related to the inhibition of cracks at the GBs of the polycrystalline microstructure.

**3.4. Friction**

**3.5. Optics**

brackets basically clear [2, 10, 21].

close to metal brackets [4].

**Figure 3.** The semitwin tie-wing complex.

When polycrystalline ceramics were compared with monocrystalline ceramics, it was concluded that polycrystalline ceramics have a higher coefficient of friction. In fact, more than a decade ago, it was pointed out that monocrystalline brackets have frictional characteristics

Ceramic Brackets Revisited

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http://dx.doi.org/10.5772/intechopen.79638

To overcome the problem of frictional resistance of polycrystalline brackets, manufacturers carried out numerous modifications. Polycrystalline ceramic brackets with metal inserts in the arch wire slot (metal slots) were developed [18]. Nevertheless, it was reported that the sharp edges of the metal insert may "dig into" the softer arch wire material, thus increasing resistance to sliding and thereby reducing the efficiency of tooth movement [7, 19]. Another modification was the addition of bumps along the floor of the polycrystalline ceramic bracket slot. Nevertheless, these bumps were not effective in reducing frictional resistance [20].

A recent study, including ceramic and metal brackets that were manufactured by different production methods, including CIM and metal injection molding (MIM), concluded that the manufacturing technologies do not present a critical difference regarding friction [3]. It was reiterated that the complex phenomenon of friction depends on a multitude of factors, such as the bracket/ligature/arch wire combinations, the surface quality of the arch wire/bracket slot,

The optical properties of ceramic brackets provide an attractive option for a great number of patients. As previously mentioned, polycrystalline ceramic brackets possess a microstructure of crystal GBs. This microstructure reflects light, resulting in some degree of opacity. In contrast, single-crystal brackets lack GBs, thus permitting the passage of light, making these

the bracket design, and the force exerted by the ligature on the arch wire [3].

The tie-wing complex of polycrystalline ceramic brackets can be manufactured as either semitwin or true twin. Semitwin differs from true twin by having an isthmus of ceramic joining the mesial and distal tie-wings, i.e., the mesial and distal tie-wings are not four independent projections from the bracket base as with the true twin configuration (**Figure 3**). This semitwin configuration has been stated to possess a better tie-wing fracture strength. It has been proposed that such a ceramic connector produces a cross-stabilizing effect [13, 17].

**Figure 2.** Distogingival tie-wing fracture (the red elastic ligature was used to accentuate this fracture).

**Figure 3.** The semitwin tie-wing complex.
