**3.5. Optics**

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 brackets basically clear [2, 10, 21].

As mentioned above, monocrystalline brackets have more optical clarity than polycrystalline brackets (**Figure 4**). Whether this difference is of essential importance from an esthetic point of view is a decision to be made by the orthodontist as well as the patient [2].

**3.8. Biocompatibility**

is proven, alternative materials have to be used.

**3.9. Magnetic resonance imaging (MRI) compatibility**

**4. Characteristics of ceramic bracket bases**

oral tissues [5].

required [10, 27].

Biocompatibility is the ability of a material to provide successful service in a host while causing minimal response [9]. It has been stated that conventional ceramic brackets are chemically stable (inert) in the oral environment and that they exhibit excellent biocompatibility with

Ceramic Brackets Revisited

11

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

In 2012, Retamoso et al. [26] carried out an in vitro cytotoxicity study evaluating various orthodontic brackets. These researchers reported that monocrystalline ceramic brackets had good biocompatibility. On the other hand, polycrystalline ceramic brackets with metal slots demonstrated some toxic effects. It was pointed out that the metallic slot was the essential factor responsible for a decrease of cell viability due to nickel ion release. They [26] concluded that it is essential to continue with studies evaluating cytotoxicity. If toxicity of any material

Orthodontists are often asked to remove fixed orthodontic appliances prior to an MRI scan—a diagnostic tool that does not expose the patient to radiation—particularly when looking for pathology in the head and neck region or when information regarding the articular disc is

Beau et al. [27] provided a detailed flowchart concerning the indications for the removal of fixed orthodontic appliances prior to MRI scans of the head and neck region. According to this flowchart, ceramic brackets do not have to be removed prior to an MRI scan. They are MRI-safe. However, ceramic brackets with any metal components, such as stainless steel slots, have to be removed if the region under examination is adjacent to these brackets. Stainless steel causes extensive artifacts, which may degrade image quality beyond clinical acceptability. The authors [27] pointed out that they did not include arch wires or removable appliances

Several retention mechanisms were developed for the attachment of ceramic bracket bases to the adhesive. These are chemical retention, mechanical retention, and a combination of both methods [21, 28]. The first developed method was the chemical retention method. This method, now obsolete, used a coating of glass on the flat ceramic bracket base and then a silane coupler to achieve a chemical bond between the glass-coated bracket base and the adhesive. The silane molecule is a bifunctional molecule; that is, one end reacted with the glass coating on the bracket base, while the other end reacted with the adhesive [11, 29]. It was pointed out that the chemical retention mechanism produced very strong bonds that harmed the tooth surface in the form of cracks and enamel tear-outs during debracketing [4, 7, 11, 29–32].

in their research, since these can be easily removed prior to an MRI scan.

Apart from esthetics, the optical properties of ceramic brackets have been shown to affect the amount of light transmitted through these brackets during photocuring. The amount of light transmitted through ceramic brackets affects the curing efficiency of the light-cured adhesive. Polycrystalline brackets and polycrystalline brackets with a polymer mesh base were found to block direct light transmittance to a greater extent than monocrystalline brackets. It was pointed out that the color-coded holders designed for identification and handling of ceramic brackets also hinder light transmittance. The use of clear holders with colored edges has been suggested [22].
