Image of a polycrystalline alumina ceramic orthodontic bracket
4.1 Disadvantages of Ceramics in Orthodontics
Tooth wear is the phenomenon in the oral cavity characterized by the loss of original anatomical shape, which may be the result of physiological or pathological conditions. Excessive wear can damage the occlusal surfaces and cause functional alterations of chewing movements, which may result in the remodeling of the temporomandibular joint. Dental ceramics can cause damage to tooth enamel.
According to Oh et al. (2002), different factors influence the abrasive potential of ceramics on the enamel, such as physical factors (resistance to fracture, frictional resistance, hardness), microstructural factors (porosity, grain size), surface characteristics (polish, presence or absence of enamel) and environmental factors (salivary pH).
The wear of ceramics on the tooth structure occurs by fracture rather than by plastic deformation as occurs in metals. This type of failure through micro indentations is dependent on the arrangement of crystals in the glassy matrix.
A very important physical property of ceramic brackets is the extreme high hardness of aluminum oxide. It is generally thought that the harder a material is, the more it will wear an opposing material softer than itself. The Knoop hardness number (KHN) for ceramic brackets is in the range of 2,400–2,450, almost nine times as hard as stainless steel brackets (KHN approximately 280) or enamel (KHN 343).
Care should be taken when ceramic brackets are bonded to lower incisors once. In patients with deep overbite, they can become in touch with the lingual surface or incisal edge of the upper incisors leading to wear of the enamel. The same can occur with other teeth. In patients with deep overbite, metal brackets in the lower teeth can be a wise choice.
4.1.2 Frictional Resistance
Often, orthodontic treatments involve dental extractions to solve space issues. To close these spaces, as in other situations, the teeth are slid over the orthodontic wires. When using this technique, the surface roughness of ceramics compared to that of stainless steel, increases the frictional resistance, reducing the sliding efficiency by up to 30 % (Johnson et al. 2005). Among the ceramic brackets, those made from monocrystalline alumina seem to offer more slip resistance. The friction coefficient depends on the load and on the geometric characteristics such as surface texture, shapes and structures of the contact area.
On the market ceramic brackets can be found with metal inserts in their channels in order to minimize the effect of increased roughness of the ceramics without affecting the esthetic appeal of these devices. Authors such as Thorstenson and Kusy (2003), however, have found no greater efficiency of esthetic brackets with metal inserts.