As is well known, orthodontic appliances are used to move or manipulate certain teeth to correct irregularities and/or abnormalities in their relationships with surrounding members. Orthodontic appliances include systems comprising wires and brackets as well as systems comprising removable aligners. This manipulation is achieved by the application of designed force systems to selected teeth. The forces for these systems are provided by a force delivery component such as an arch wire or spring. The wire is elastically deformed, or activated, to absorb energy. The wire slowly releases this energy as it deactivates and returns to the relaxed condition. The released energy is applied to selected teeth, for instance by interaction of the loaded wire with brackets attached to the teeth, to provide the desired tooth movement.
Tooth movement can best be achieved by producing an optimal force system capable of delivering relatively light but continuous corrective forces. Some desirable biomechanical characteristics of the orthodontic force system include low to moderate force magnitude, constancy of force magnitude during deactivation and accurate location of the force application point. The use of a low to moderate force magnitude will allow the teeth to move rapidly and relatively painlessly with minimum tissue damage. A constant force magnitude over time will provide maximum tissue response. Additionally, if the orthodontic appliance releases force too rapidly, it becomes more difficult to accurately produce the desired effect, requiring more frequent adjustments to maintain the force at some minimum desired level.
There are several additional criteria that are important for orthodontic appliances in general. For example, the orthodontic material must be non-toxic, resistant to the corrosive environment within a patient's mouth and available in desired shapes and dimensions. Some other important parameters, especially for orthodontic force delivery components, include strength, elastic deformation, yield strength, stiffness, formability and joinability. More recently, the aesthetic appearance of orthodontic components has become very important, with many patients expressing a strong preference for orthodontic components and appliances that are less visually apparent against the patient's teeth.
Elastic deformation or “spring back” is a measure of the amount of deflection or activation that the wire or other component can sustain and still be totally elastic, that is, to recover to its original shape and position. The elastic deformation of an orthodontic component is fundamentally proportional to its ratio of flexure strength to flexure modulus; or similarly its ratio of tensile yield strength to modulus of elasticity. The higher the ratio of yield strength to modulus, the greater the elastic deformation. Design factors also affect elastic deformation, for example, the elastic deformation of round wire varies inversely as the first power of the diameter and the second power of the length of the wire. Elastic deformation is important because it determines the distance over which an appliance can provide an effective force system before readjustment is necessary. Appliances that can sustain larger elastic deformation (deflection) can more readily engage teeth that are severely malposed.
Yield strength must be high enough to assure achieving desired force levels for tooth movement and preventing appliance failure associated with permanent deformation. The lack of adequate yield strength can not be corrected by design changes such as increase in size or bulk because of size limitations in the oral cavity. Metals have traditionally been used in orthodontics because in the necessary cross-sections they provide desirable force levels that other categories of structural materials, such as engineering plastics, have not been able to provide.
Stiffness is the ratio of force/unit activation. The stiffness or rigidity of an appliance varies significantly with appliance design, for example, stiffness varies as the fourth power of the diameter for round wire. For rectangular wire, stiffness varies as bd3, where b is the base or cross-sectional dimension perpendicular to the force and and d is the depth or cross-sectional dimension parallel to the force. Wires of unique cross-sections, such as polygonal, offer different stiffnesses, and hence different forces, in different planes. Although not available with metal appliances, it is desirable for an appliance to have unique cross-sectional shapes that give greater control over tooth movement by varying force as required in the three dimensional planes.
The stiffness of an appliance component, when stiffness is linear in the range of use, is a primary determinant of the force that can be applied to teeth during manipulation. Greater stiffness results in more force for each unit of activation. Generally, low stiffness orthodontic components are required for active tooth movement and high stiffness components for passive holding components. High stiffness may be required for small deflection applications. For example, if a tooth were 4 mm out of alignment and 100 g of force is needed, 25 g/mm would be a low stiffness and 1,000 g/mm would be a high stiffness.
Some orthodontic components, such as a wire, require sufficient ductility to be formed to a desired customized shape for a particular patient. Additionally, the wire has to be joinable to other wires or components, while retaining its strength and elasticity characteristics. Naturally, the wire must be available in a variety of desired cross-sectional shapes and dimensions as variability in cross-sectional shape can allow greater potential control of orthodontic force systems. All orthodontic wires have conventionally had either rectangular or circular cross sections.
Some orthodontic components, such as attachments, that translate the force from the wire directly to the tooth have additional criteria that have to be considered. For example, the design, geometry and overall dimensions of an attachment such as a bracket are important for both its ease of manipulation as well as its ability to help contribute to effective application of the orthodontic force system. Attachments may be bonded directly to the tooth surface or mechanically fastened using a band that typically circumscribes the entire tooth. An attachment that is bonded may have certain functional shapes and contours on the surface contacting the tooth in order to aid adhesion. Attachments should be easy to fabricate or manufacture. Attachments must have sufficient strength to transfer force to the joined tooth without attachment deformation or fracture. Additionally, it is desirable for the bracket to be comprised of a material that provides a low level of friction to wires within the slot. Aesthetics of attachments are again very important to some patients.
There have been attempts to use material selection in conjunction with appliance design to control orthodontic force systems. Over the years dental practitioners have used orthodontic force delivery components made from gold alloys, stainless steel alloys, nickel-titanium memory type alloys of the type described in U.S. Pat. No. 4,037,324 and beta titanium alloys of the type described in U.S. Pat. No. 4,197,643 in an effort to design orthodontic components that can impart a desired force system. While the above materials have been successfully used for orthodontic applications, some deficiencies remain.
Orthodontic component aesthetics is an increasingly important consideration, particularly for labial appliances and components. Metal components have a characteristic gray or silver color that is quite obvious against the color background of the tooth structure and aesthetically objectionable to many patients. The use of clear or tooth-colored components and appliances would be considerably more aesthetically pleasing to many patients. Attempts have been made to overcome the aesthetic deficiencies of metal orthodontic components. Tooth colored plastic coatings have been applied to the metal components. Such coatings can lose adhesion to the underlying metal surface and peel off; exhibit an undesirably high amount of friction when used with metal or ceramic brackets and are relatively soft and can be scraped or gouged by contact with harder surfaces.
Metallic orthodontic components have also specifically been identified as a problem area for the nuclear magnetic resonance diagnostic procedure, since metals do not exhibit the requisite radiolucency and interfere with the resulting images.
There have also been attempts to use material selection to improve characteristics of other orthodontic components. Brackets have been fabricated from ceramic materials in an attempt to provide a more aesthetically pleasing appliance. However, ceramic brackets, while available, are expensive; are not available in more complex shapes and sizes; are brittle; and are very hard and can wear contacting teeth. Ceramic brackets may also be difficult to debond, leading to tooth enamel fracture.
Another approach to orthodontic tooth movement is the use of a removable appliance, such as an aligner, in place of wires and brackets. Such aligners can be very aesthetic and “patient friendly” since they are removable by the patient and require no bonding of attachments. One limitation of current materials with respect to aligners is the occurrence of permanent deformation adjacent to the imprint of the final crown position, which does not allow exact tooth movement because the shape of the aligner has been altered and no longer applies the required force. This permanent deformation is related to inadequate mechanical properties of available materials used in removable appliances, for example yield strength and modulus.
It is generally believed that thermoplastic polymers such as polymethylmethacrylate (PMMA) or polycarbonate and even high strength polymers such as polyetheretherketone (PEEK) do not possess the requisite flexural strength, modulus and elastic deformation desirable, or in some cases necessary, for use as a force delivery component. Table 1 lists the mechanical properties of some known high strength engineering polymers as well as properties for some metals useful in orthodontic use.
TABLE 1TensileFlexureTensileStrength,FlexureStrength,Modu-MPaModulus,MPalus,Ulti-MaterialGPaYieldGPaYieldmatePolybenzimidazole (PBI)6.62215.8160Polyamide-imide (PAI)5.2185Polyphenylene sulfide3.8963.865(PPS)Polyetheretherketone4.11703.597120(PEEK)Polyether-imide (PEI)3.31183.3103Polymethylmethacrylate2.3912.55153(PMMA)Polycarbonate (PC)2.8882.36270Acrylonitrile-butadiene-2.5832.350styrene (ABS)Polyamides (nylon)1.8801.96075Thermoplastic0.51.237PolyurethaneNickel-Titanium41.41489Beta Titanium71.71276Stainless Steel179.02117
Brackets have been fabricated from polycarbonate materials in an attempt to provide a more aesthetically pleasing appliance. However, polycarbonate brackets cannot resist the high stress magnitudes frequently encountered in orthodontics so that the bracket slot distorts or spreads apart under torque loading well below the levels desirable for clinical use. In addition, polycarbonate brackets have tying wings that have been known to fail. Polycarbonate as an orthodontic material can also stain from contact with food.
More recently, highly fiber reinforced composite materials such as those described in U.S. Pat. No. 4,717,341 have been proposed for use in orthodontics. Such highly fiber reinforced composite materials show promise in this application, however, these materials presently are anisotropic, are somewhat difficult to form into complex shapes, require effective coupling of the high strength reinforcing phase into the polymer matrix and have low ductility.
Some Definitions Used in the Specification
The following terms will have the given definitions unless otherwise explicitly defined.
Elastic deformation or spring back—the amount of deflection or activation that the wire or other component can sustain and still be totally elastic, that is, to recover to its original shape and position.
Filler material—Particles, powder or other materials having having approximately equal dimensions in all directions. Filler material is added to a polymer primarily to enhance polymer properties such as wear resistance, mechanical properties or color.
Neat—Without admixture or dilution, that is substantially free of materials such as additives, filler materials, other polymers, plasticizers and reinforcing agents.
Non-Thermoplastic polymer—Any polymer which does not fall within the definition of a thermoplastic polymer.
Orthodontic appliance—A device used for tooth alignment, occlusal correction and non-surgical jaw alteration. Appliances can be fixed or removable. Removable appliances, such as aligners, are inserted and removed by the patient.
Orthodontic attachment—Brackets, tubes or other shapes bonded to a tooth or to a band that joins an orthodontic wire with the tooth.
Orthodontic auxiliary—Items added to supplement an appliance, including springs separate from the arch wire and hooks and buttons joined to a wire or tooth.
Orthodontic component—Any part of a fixed or removable appliance, for example attachments, wires, ligating mechanisms and auxiliaries.
Orthodontic force delivery component.—Any part of an orthodontic appliance that is capable of storing energy for tooth movement.
Orthodontic ligating mechanism—Mechanism such as metal ligature wires, elastomeric rings or caps for joining wires to an attachment.
Orthodontic wire—A force delivery component of the appliance.
Polymer—A long chain of covalently bonded, repeating, organic structural units. Generally includes, for example, homopolymers, copolymers, such as for example, block, graft, random and alternating copolymers, terpolymers, etc, and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term “polymer” includes all possible geometric configurations. These configurations include, for example, isotactic, syndiotactic and random symmetries.
Reinforcing agent—a filament, fiber, whisker, insert, etc. having a length much greater than its cross sectional dimensions. Reinforcing agents are primarily used to increase the mechanical properties of a polymer.
Stiffness—The ratio of a steady force acting on a deformable elastic material to the resulting displacement of that material.
Thermoplastic polymer—A polymer that is fusible, softening when exposed to heat and returning generally to its unsoftened state when cooled to room temperature. Thermoplastic materials include, for example, polyvinyl chlorides, some polyesters, polyamides, polyfluorocarbons, polyolefins, some polyurethanes, polystyrenes, polyvinyl alcohol, copolymers of ethylene and at least one vinyl monomer (e.g., poly (ethylene vinyl acetates), cellulose esters and acrylic resins.
Unreinforced—A material with substantially no reinforcing agent.