Dental Biomaterials and Dentistry

Dear Colleagues,

New biomaterials are constantly being developed to respond to the need for better mechanical properties and biocompatibility in medicine and dentistry. Extensive research published in journals of materials science and orthodontics has allowed the properties of materials to be appreciated and used in an appropriate clinical manner. Furthermore, acquiring comprehensive knowledge of the biological, chemical, physical, and mechanical proprieties of titanium and titanium alloys in relation to biomedical applications requires a multidisciplinary approach.

Titanium alloys such as TMA (Ti–Mo–Sn–Zr), Ti–Nb, Ni–Ti, and Cu–Ni–Ti are generally used as biomaterials, mainly as orthodontic wires, because of their interesting elasticity and shape memory capacity, which allow these materials to return to their original configurations after being submitted to a stress. These properties, that are not observed in the traditionally used stainless steel wires, permit the orthodontist to apply continuous force on the teeth without causing discomfort to the patient. This is a positive aspect and has stimulated the use of titanium alloys wires in place of stainless steel ones, mainly during the beginning of treatment, when teeth are more misaligned.

However, Ni–Ti-based alloys comprise a large Ni content (about 48%–55% mass), and a negative aspect that hinders their wide use in prolonged orthodontic treatment takes into account the Ni corrosion in the oral cavity. This point is a constant concern for many dentistry professionals, since the presence of the Ni²⁺ ion in the oral medium can cause several allergies to patients. It implies that corrosion resistance must be another important item to be estimated for the prolonged use of these materials in orthodontic treatments.

For metallic biomaterials, the chemical composition is the important factor determining the behaviour of the metal toward a passivity state, while other factors such as potential values, strain rate, surface roughness, degree of oxidation, acidity (pH), temperature of media, and the presence of corrosion inhibitors determine the degree of passivating. The oral cavity is a complex dynamic environment that often undergoes rapid and substantial change in response to temperature and pH. This has implications for biological interactions contributed to by a continuous flow of saliva and microbiological activity. Corrosions of Ni–Ti prostheses in the mouth are expected to release Ni ions, which then come into contact with cells and tissues in the immediate environment and are subsequently distributed throughout the entire body.

In clinical orthodontics, the importance of friction due to mechanical and biological factors has received much attention. Mechanical factors include surface roughness due to material type, in particular, archwire material, which is related to the coefficient of friction and type of ligatures. Biological factors include saliva, corrosion, and debris. The surface properties of archwires and their possible intraoral degradation might affect the friction force and, hence, the effectiveness of orthodontic treatment.

Until recently, studies on friction have been performed on as-received archwires and brackets, that have been aged as a result of either in vitro or in vivo means. Some investigations reported that stainless steel (SS) rectangular wires, when exposed to the intraoral environment, showed a significant increase in the amount of debris accumulation and, also, a significant correlation was observed between the amount of debris and friction. However, the assessment of the amount of debris accumulation was performed mainly using semiquantitative methods on short-term in vivo exposed material. Brackets and archwires are usually exposed intraorally to corrosion and debris accumulation for several months or even years, which might influence their surface characteristics and affect friction. Therefore, it is important to assess friction forces and the influence of wear and debris accumulation after long-term in vivo exposure.

Therefore, this editorial would provide an overview of research from various backgrounds in order to provide insights into titanium and titanium-based materials in the field of dentistry and orthodontics as divided into the four distinctive areas of research studies on dental archwires: corrosion of dental archwire surface; corrosion and wear properties of new and in vivo exposed nickel titanium (Ni–Ti) and stainless steel (SS) orthodontic archwires; biocompatibility and nickel release, especially of Ni–Ti archwires; and versatile surface treatments to tailor properties, such as oxidation procedures, and different coatings on dental archwires.

Prof. Dr. Maja Ovsenik
Guest Editor

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