Shape Memory Orthodontic Archwire Having Variable Recovery Stresses

The invention is directed to orthodontic archwires and, more particularly, to orthodontic archwires employing a shape memory alloy.

Traditional systems for the orthodontic treatment of teeth have included the use of a metal orthodontic archwire, such as a stainless steel wire, which is deformed and bent into a particular shape by an orthodontist so as to exert a particular force or forces on orthodontic brackets attached to the teeth. However, such archwires have relatively low shape recovery, and the force applied by the wire varies substantially as the teeth move, requiring frequent archwire adjustment or replacement by the orthodontist. In order to overcome these drawbacks, it has been suggested to use various shape memory alloys.

In U.S. Pat. No. 4,037,324, Andreasen teaches an orthodontic archwire formed of a Nitinol alloy, which exhibits mechanical memory. The Nitinol alloy is a near-stoichiometric alloy of nickel and titanium, preferably having a small amount of cobalt substituted for the nickel on an atom-by-atom basis. Such Nitinol alloys are quite ductile when below a critical temperature (known as transition temperature range) due to martensitic shear, where adjacent planes of atoms shift by a distance less than a full interatomic distance. However, when a Nitinol alloy is heated above its critical temperature, it displays the characteristic of mechanical memory, returning the archwire to its original or pre-deformation shape. Andreasen suggests attaching such an archwire to orthodontic brackets while the archwire is below its critical temperature, and subsequently, heating the wire above its critical temperature to activate the wire memory.

In U.S. Pat. No. 4,490,112, Tanaka et al. teach an orthodontic archwire made of an ultra-elastic material and having a transformation temperature of about 37.degree. C. The ultra-elastic material preferably is a nickel-titanium alloy containing about 50.5 to 51.0 atomic percent nickel, with the balance being titanium. Because the transformation temperature is similar to the normal temperature in the mouth, the orthodontic archwire normally maintains a light orthodontic load on the brackets. However, this load may be increased by increasing the temperature in the mouth.

In U.S. Pat. No. 4,818,226, Berendt et al. disclose an orthodontic archwire formed of a shape memory alloy substantially similar to that taught by the Andreasen '324 patent. The Berendt archwire further includes a first, second, and third curved section, with the first and second curved sections including two different radii of curvature extending distally from the mid-line of the archwire, and a third curved section including two radii of curvature extending distally from the mid-line of the archwire, so that the preset form of the archwire resembles a saddle-like configuration.

In addition, U.S. Pat. No. 5,044,947 to Sachdeva et al. discloses a shape memory orthodontic archwire formed of a nickel-titanium alloy, in which the austenitic transformation temperature (A.sub.f) of the alloy may be varied by substituting and/or adding an additional element or elements.

Although the described archwires exhibit shape memory and exert different force levels on orthodontic brackets, depending upon the temperature of the archwire relative to the austenitic transformation temperature of the alloy, the force exerted at any given temperature is substantially the same throughout all regions of the archwire. Therefore, it would also be beneficial to have a shape memory orthodontic archwire which exerts different levels of force (recovery stresses) at different regions of the archwire, when the archwire is in place in the mouth.

The present invention is directed to an orthodontic archwire and a method of making an archwire. The archwire is comprised of a shape memory alloy having a preset shape for exerting forces on teeth during orthodontic treatment. The alloy has a transformation temperature (A.sub.f) greater than about 25.degree. C., such that the orthodontic archwire produces forces at normal mouth temperature which are greater than the forces produced at normal room temperature. A preferred archwire is comprised of an alloy containing at least three elements, each of which is present in an amount greater than or equal to about 2% by weight. Furthermore, the archwires of the invention have at least one curve outside of the normal plane of the archwire. Furthermore, the forces produced by the archwire at normal mouth temperature are at least about ten percent (10%) greater than the forces produced at normal room temperature.

The orthodontic archwire may exhibit variable recovery stresses in selected sections. For example, in one contemplated embodiment, an archwire has a labial section, a plurality of buccal sections, and a plurality of posterior leg sections, with each posterior leg section having a higher modulus of elasticity (and hence greater recovery stress) relative to the buccal sections.

Preferably, the shape memory alloy has a transformation temperature in the range of about 30.degree. C. to about 36.degree. C., and more preferably, about 35.degree. C. The preferred alloy is a nickel-titanium-copper alloy, which may include additional element(s) for the purpose of adjusting and controlling the alloy's transformation temperature. One such element is chromium.

When a nickel/titanium/copper alloy is used, the alloy preferably includes about fifty (50%) percent titanium by weight, about forty-five (45%) percent nickel by weight, and about five (5%) percent copper by weight. If both copper and chromium are included, the alloy preferably includes about fifty (50%) percent titanium by weight, from about 39.5% to about 46.9% nickel by weight, from about three (3%) percent to about ten (10%) percent copper by weight, and from about 0.1% to about 0.5% chromium by weight. More preferably, the alloy having copper and chromium includes about fifty (50%) titanium, from about 42.6% to about 44.85% nickel by weight, from about five (5%) percent to about seven (7%) percent copper by weight, and from about 0.15 to 0.4% chromium by weight.

The orthodontic archwire may be formed in any of a number of preset shapes, including, for example, a reverse curve of spee, which has at least one curve outside of the normal plane of the archwire.

In the method of the invention, an orthodontic archwire comprised of a shape memory alloy is formed into a preselected arch shape, heat treated to provide the desired transformation temperature, and then thermally treated to achieve the desired modulus of elasticity so as to provide necessary stresses. The thermal treatment can be done selectively, i.e., to selected portions of the archwire, to result in an archwire with a variable modulus of elasticity.

Another aspect of the invention concerns a method of orthodontic treatment which includes adhering orthodontic brackets to the teeth and attaching a shape memory orthodontic archwire of the invention to the brackets to achieve the desired corrective tooth movement.

One particular advantage of the orthodontic archwires and methods of treatment of the invention is the ability to provide a shape memory alloy archwire which delivers different levels of force (recovery stresses) to different regions of the maxillary or mandibular arch at a given mouth temperature. Other features, benefits and advantages will become readily apparent to those skilled in the art upon review of the following figures and detailed description of the invention.

FIG. 1 is a perspective view of an orthodontic archwire having a generally parabolic shape;

FIG. 2 is a perspective view of an orthodontic archwire having a reverse-curve-of-spee configuration;

FIG. 3 is a graphical representation of the variation in critical stress for inducing martensite (.sigma..sub.MS) as a function of Ni concentration;

FIG. 4 is a graphical representation of the variation in critical stress for reverse transformation (.sigma..sub.RS) as a function of Ni concentration; and

FIG. 5 is a stress-strain curve that shows the effects of differential heat treatment.

The orthodontic archwires of the invention are made by selecting a shape memory alloy having a suitable composition to achieve the desired properties of the archwire, and forming that alloy into a preselected arch shape adapted to exert forces on teeth during orthodontic treatment. If desired, the archwire may be formed into a generally parabolic shape or a reverse-curve-of-spee shape as shown in FIGS. 1 and 2, respectively. However, the shape memory alloy may be formed into any preselected arch shape having utility in orthodontic treatment. Preferably, a nickel-titanium based alloy is used because of its beneficial shape memory characteristics and the ability to modify its physical properties by adjusting the alloy content.

Once the archwire has been formed into a preselected arch shape, such as by work hardening or the like, the archwire is treated to provide the desired austenitic transformation temperature (A.sub.f). The desired transformation temperature may be achieved by selecting the appropriate heat treatment, by adjusting the composition of the shape memory alloy, by adjusting the cold work ratio, or by a combination of the above. It is contemplated that a third and even fourth element may be added to the nickel-titanium alloy for the purpose of adjusting and controlling the transformation temperature. Specifically, it is believed that the addition of copper to the Ni--Ti alloy will increase the transformation temperature, while the addition of chromium will reduce the transformation temperature.

As shown in Table 1 below, and reflected generally in FIGS. 3 and 4, adjusting the Ni content in a Ni--Ti alloy and adjusting the anneal temperature (T.sub.a) of that alloy will change the critical stress for inducing martensite (.sigma..sub.MS) (see FIG. 3), and the critical stress for reverse transformation (.sigma..sub.RS) (see FIG. 4). All data given are for a NiTi wire of 0.018" diameter, having the noted composition and annealed at the noted temperature. Also, the stress values in Table 1 were obtained upon deformation at 25.degree. C., whereas the stress values shown in FIGS. 3 and 4 were obtained upon deformation at 37.degree. C.

Orthodontic archwires according to the present invention are formed with a transformation temperature which allows the archwire to produce corrective forces at normal mouth temperature which are greater than the forces produced at normal room temperature. These different levels of force are due to the fact that the shape memory alloy is relatively ductile when below its transformation temperature due to martensitic shear, wherein adjacent planes of atoms shift by a distance less than a full interatomic distance. However, when the archwire is heated above its transformation temperature, the alloy displays its characteristic mechanical memory and attempts to return toward its original preset shape.

As used herein, "normal mouth temperature" refers to the ambient temperature of the mouth without cold or hot foods or liquids present, i.e., a temperature of from about 35.degree. C. to about 38.degree. C. "Normal room temperature" refers to a temperature of from about 18.degree. C. to about 22.degree. C.

The shape memory property of the archwire enables engaging the archwire into the bracket slot by the clinician in a relatively easy manner. Because the transformation temperature of the alloy preferably is at or above normal room temperature, the archwire will readily bend at room temperature and may be connected to the brackets with relative ease. However, as the archwire warms to mouth temperature, the alloy approaches and, depending on the transformation temperature selected, surpasses its transformation temperature, whereby the alloy displays its shape memory characteristic and exerts force (unloading force) on the bracket as the archwire returns to its preselected shape.

Because of the ability to adjust the transformation temperature, it is possible to provide an orthodontic archwire which exerts a relatively mild force, a moderate force, a strong force, or even a pulsing force. For example, if only a mild force is desired, the archwire may be heat treated to provide a transformation temperature relatively close to normal mouth temperature. In this manner, the archwire may be installed relatively easily at room temperature. Then when the archwire reaches normal mouth temperature, it will be in a martensitic/austenitic phase and will exert only a mild shape memory effect. Furthermore, the forces exerted by such an archwire may be pulsed by drinking hot liquids, for example. In a preferred form, the archwire has a transformation temperature of 35.degree. C., in which case a slightly stronger continuous force may be exerted by the archwire on the brackets and teeth when the archwire achieves normal mouth temperature.

Orthodontic archwires of the present invention may also be thermally treated with a controlled heat treatment to provide the desired modulus of elasticity to the archwire. It is contemplated that different sections of the archwire may be subjected to different heat treatments so as to provide a variable modulus of elasticity (and hence variable recovery stresses) along the length of the archwire, which provides a significant advantage. While the force exerted by standard shape memory alloy archwires may be controlled by selecting a particular transformation temperature, the force is relatively uniform along its length. And although such a force may be modified somewhat by various loops and bends in the archwire, these loops or bends may be somewhat unpredictable. However, orthodontic archwires of the present invention are thermally treated, typically by the manufacturer, to provide various moduli of elasticity in different sections of the archwire. Therefore, such archwires may deliver a relatively higher level of force to those portions of the arch requiring more movement, while delivering a lower force to those teeth which require less adjustment.

Referring to FIGS. 1 and 2, preferred embodiments of the orthodontic archwire 10, 110 are shown including a labial section 12, 112, a plurality of buccal sections 14, 114, and a plurality of posterior leg sections 16, 116. These archwires can be selectively treated to provide a modulus of elasticity which is higher in the posterior leg sections 16, 116 than in the buccal sections 14, 114. With reference to FIG. 5, there is represented, in a graphic manner, the effect of selective heat treatment. The FIG. 5 data is for a Ni--Ti wire (50.6% Ni) of 0.018" diameter wherein a first section was heat treated (annealed) at 450.degree. C., and a second section was heat treated at 350.degree. C. The variation in stress plateaus for inducing martensite and for reverse transformation are apparent at the noted anneal temperatures. Thus it will be appreciated by persons skilled in the art that variable heat treatments of the archwires 10, 110 of the present invention can be advantageously utilized to achieve the desired properties.

While several particular embodiments of the invention have been discussed above, it will be apparent to one of ordinary skill in the art that various other changes and modifications may be made without departing from the scope of the invention, which is determined by the following claims.