Manufacturing of Orthodontic Brackets: A Review of Metallurgical Perspectives and Applications
Theodore Eliades, Spiros Zinelis, Christoph Bourauel and George Eliades
Affiliation: 57 Agnoston Hiroon, Nea Ionia 14231, Greece.
Keywords: Stainless steel, titanium, metal injection molding, brazing, laser
Orthodontic brackets comprise the basic medium of transmission of force to teeth in orthodontics; this is achieved by the development of loads from activated archwire into the bracket slot. As a standard manufacturing process, brazing alloys to join the base and wing components of brackets are adopted by the industry. Some of these alloys also contain traces of the cytotoxic cadmium, which is added to lower the melting temperature and improve wetting. Moreover, silver-based brazing alloys form a galvanic couple that can lead to ionic release, mainly copper and zinc. Corrosion, which has been substantially minimized in current materials, is the main reason for the progressive dissolution of brazing filler metal, leading to detachment of the wing from the bracket base during orthodontic therapy or at the debonding stage. To overcome this problem, several manufacturers have introduced gold-based brazing materials that might lead to the dissolution of stainless steel, because of the formation of the galvanic couple. Thus, although brazing alloys can facilitate the manufacturing of brackets with alloys of certain properties, e.g., a stiffer alloy for the wing to withstand the loads from activated wires and a softer alloy for the base to facilitate a peel-off effect during debonding-they have several problems. Laser welding was relatively recently introduced in bracket manufacturing as an alternative to alloy soldering. With this method, welding of the wing to the base does not extend to the bulk material, and thus a “surface seal” is formed that is confined to the periphery of the joint. This technique eliminates the intermediate phases such as soldering alloys and shows acceptable mechanical performance with a low risk of joint failure. The metal injection molding (MIM) process, which has significantly expanded during the past few years, involves mixing metal powders with particle sizes of a few microns with organic binders, lubricants, and dispersants to obtain a homogeneous mixture. Injection of the feedstock is performed by using an injection-molding machine, similar to that used in the plastics industry. MIM-manufactured products are one-piece appliances with tolerances of the desired dimensions of approximately 0.3% and density values more than 97% of the theoretical density of the material. Porosity is a known defect of MIM parts, with adverse effects on the mechanical and corrosion resistance of most MIM-manufactured products. The hardness of the MIM-made brackets tested varied from 154 to 287 HV, a value much lower than the hardness of wing components of conventional stainless steel brackets, introducing the problems associated with soft and compliant wing components, as noted previously. This paper reviews the available evidence recent patent on bracket manufacturing with specific reference to limitations of each method from metallurgical and application perspectives.
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