In the field of orthodontics, it is known to adhere orthodontic appliances, such as brackets, buccal tubes and the like, directly to teeth. Typically, this is accomplished by chemically bonding an appliance to a tooth surface using an adhesive. If desired, the bonding surface area of the appliance may be roughened in order to increase the surface area of the appliance in contact with the adhesive, thereby enhancing the chemical bond. Direct bonding also may be accomplished by using a metal appliance having undercuts for forming a mechanical bond with the adhesive.
U.S. Pat. No. 5,267,854 to Schmitt teaches a metal injection molded orthodontic bracket including a plurality of raised posts extending bucco-lingually from a tooth abutting surface. Each post includes a root section having a base integrally formed with the tooth abutting surface and an apex section bucco-lingually extending from the root section. The apex section terminates in a sharp, continuous parameter edge that was originally smaller in all directions than the root section. However, in accordance with the Schmitt patent, further cold working of the parameter edges occurs such that each edge is worked at ambient temperatures into a mushroom-shaped button having a worked edge larger in all directions than its associated root section and an eave capable of mechanically bonding with an adhesive.
U.S. Pat. No. 4,661,059 to Kanno features a metal orthodontic bracket which has a base surface provided with a plurality of orthogonal grooves formed by a cutting machine having a plurality of rotatable thin circular cutter blades. The grooves so produced have small fins or flashes at their edges resulting from cutting operations of cutter blades scraping metal matrix at high speed. These small fins are pressed down into the inside of the grooves to form undercuts for the adhesive to provide mechanical bonding of the base to the tooth surface.
Although mechanical bonding provides some advantages over traditional bonding methods, the brackets taught by Schmitt and Kanno have several limitations. For example, both brackets are made of metal and therefore, lack the aesthetic qualities found in plastic or ceramic orthodontic appliances. Furthermore, the mechanical bonding surfaces of the Schmitt and Kanno brackets are formed by cold working the metal at ambient temperature. This cold working process also distorts the metal microstructure of the raised posts used for mechanical bonding.
In addition, the methods used to form the mechanical bonding surfaces are relatively complex and expensive. The Schmitt patent requires the use of a hydraulically activated metal coining die array to cold work the posts, with one die for each post, and with each die having a partially spherical concave mushroom forming cavity in the tip. Such die arrays are difficult to make, expensive to manufacture, and difficult to use. Kanno teaches the use of a cutting machine to form the posts in the bracket base, which leads to a significant amount of wasted metal. Also, the raised posts of the Schmitt bracket must be originally molded with an apex section narrower than the root section in order to remove the metallic bracket from the mold without significant risk of peg breakage. This tapering results in less metal being available at the apex to form the mushroom-shaped button and corresponding eave required for mechanical bonding. Furthermore, the extremely fine fins or flashes of the Kanno bracket formed by the cutting process must be pressed downward in order to form the mechanical bonding surfaces and are subject to stress and fracture in this process.
In the related patents and applications referred to above, the bracket bases are formed of plastic having arrays of pegs projecting from their bases, with the outer ends or tips of the pegs having mushroom shaped heads that present overhangs which facilitate mechanical bonding of the bases to the surfaces of teeth with adhesive. The heads are formed by heat softening the peg tips and, while softened, applying a force to the tips in a direction toward the base, deforming the tips into mushroom-shaped heads.
Therefore, it would be beneficial to have an improved method of deforming peg tips of projections on orthodontic appliance bases into mushroom-shaped heads without the need to use heated platens to soften the projections to facilitate their deformation or to use expensive cold working coining dies in order to make an orthodontic appliance capable of being mechanically bonded to a tooth, in which the appliance is made of an aesthetically pleasing material.