**3.4. Friction**

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

**Figure 2** pictures a tie-wing fracture of the lower second premolar bracket. Most likely this

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 radio-

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

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).

extrapolate in vitro results to the clinical setting, i.e., the oral environment.

tie-wing was damaged with pliers during arch wire insertion into the molar tube.

*3.3.1. Tie-wing fracture*

8 Current Approaches in Orthodontics

graphs [15].

polycrystalline microstructure.

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 close to metal brackets [4].

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