Orthodontic implement controllable of correction force

Abstract: An orthodontic teeth alignment correction wire element made of an alloy represented by a chemical formula of Ti a Ni b-c C c , a + b = 100, b = 50-52, c = 0.25-5.0, or an alloy represented by a chemical formula of Ti a Ni b-c-d C c X d , X being at least one selected from V, Cr, Fe, Nb, Ta, W, and Al, a + b = 100, b = 50-52, c = 0.25-5.0, d = 0.25-2.0, and having a pseudoelasticity at the natural human body temperature. The wire has different recovery forces at different sections therealong by heat treating those sections at controlled temperatures of 400-600 o C and for controlled time periods of 10-150 minutes and thereafter reheat treating them . An orthodontic coil is made of each of the above-described alloy wires for correction of mismatch of the upper and the lower jaws.

Description: 1. Field of the Invention The present invention relates to orthodontic implements for use in correction of irregular and abnormal alignment of teeth and of mismatching of upper and lower jaws, and in particular, to such implements using Ti-Ni alloy wire capable of generating a controlled correction force. In order to correct the irregular alignment of teeth, an elastic wire element has usually been used as an element of the orthodontic implement. In detail, the wire element is bent in an appropriate or an ideal arch form and attached by brackets to the upper or lower teeth to move each one of the teeth due to the shape recovery force of the wire element. As the elastic wire element, although a stainless steel wire has conventionally used, a nitinol or Ti-Ni shape memory alloy wire has recently been used as disclosed in a paper entitled "Laboratory and clinical analyses of nitinol wire" by Andreasen et al, Am. J. Orthod. February 1978, pp 142-151 (reference 1) and another paper entitled "A crinical trial of alignment of teeth using a 0.019 inch thermal nitinol wire with a transition temperature range between 31 °C. and 45 °C" by Andreasen, Am. J. Orthod. November 1980, pp 528-537 Reference 2) and others. Reference 2 discloses that nitinol wire annealed at 500 °C is different from the stainless steel wire in the springback properties and is activated by mouth heat to return to its original shape. The springback property of the Nitinol is known as a pseudoelasticity. The pseudoelasticity is referred to as an unusual elastic behavior in a paper entitled "CRYSTAL STRUCTURE AND MICROSTRUCTURE OF A COLD WORKED TiNi ALLOY WITH UNUSUAL ELASTIC BEHAVIOR" by Tadaki et al, METALLURGICA, Vol. 14, 1980, Pergamon Press Ltd. pp. 911-914 (Reference 3). The cold worked TiNi alloy has the pseudoelasticity and is used for the orthodontic wire element. In correction of teeth alignment, the desired correction force is not constant for all of the teeth. In detail, providing that the desired force is one (1) for incisors of the maxillary or the upper jaw, correction forces of 1.5, 1.5, and 3.8 are desired for canines premolars and first molors of the maxillary, respectively, and 0.7, 1.1, 1.1 and 3.2 are desired for incisors canines premolars and first molors of mandible or the lower jaw, respectively. A typical desired correction force is about 80 grams for the incisors. When a single orthodontic wire element is used for correction of the upper or lower teeth, it is difficult to adjust the correction force to a desired one for each one of the teeth. Accordingly, it is difficult to reduce the patient discomfort. It is complicate to use different wires for different sections of teeth. As another aspect of the orthodontics, it is known that a coil spring is used for correction of mismatching of the maxillary and the mandible. The coil connects the maxillary with the mandible so that the coil moves them in correct occlusal relation due to the elasticity. If the coil spring is strong, it increases the patient discomfort, but if it is weak, correction is not sufficiently effected. Further, since the coil is subjected to a large number of repetitions of expansion and contraction, it is desired to have a high durability for the repetition of expansion and contraction. In the US-A-4 950 340, published on August 21, 1990, after the application date of the priority application for the present application an intermetallic compound type alloy suitable for making metallic molds for forming a depolarizing mixture for dry cells, dies for drawing optical fibers and the likes and miscellaneous wear resistant metallic articles, such as valve parts and pump parts is described. The intermetallic compound alloy consists essentially of Ni or Co or both 45 - 60%, Si 0,01 - 1%, Re 0 - 2%, Hf 0 - 2%, C 0 - 2%, and one or more elements selected from a group consisting of Zr, Fe, V, Nb, Ta, Cr, Mo, W, and Mn from 0 - 5%, one or more elements selected from a group consisting of P, Cu, Zn, Ga, Ge, Cd, In, Sn, Sb, Pb, and Bi from 0 - 2%, and the balance Ti and incidental impurities. The % being atomic %. Therefore, it is an object of the present invention to provide an orthodontic implement which can reduce the patients discomfort without effect against correction, in particular, as a wire element which makes it possible to generate correction forces desired for teeth located at different positions. It is also an object of the present invention to provide a process for manufacturing such an orthodontic wire element. This object is solved by an orthodontic wire element as is specified in claim 1. The object is also solved by a process of manufacturing such an orthodontic wire element as is specified in claim 5. Prefered developments of the invention are given in the respective subclaims. Following is a brief description of the drawings: Fig. 1 is a graph illustrating stress-strain curves of Ti 49.5 Ni₅₀C 0.50 alloy wires annealed at different heat treatment conditions; Fig. 2 is a graph illustrating stress-strain curves of Ti 49.5 Ni₄₉C 0.5 V 1.0 alloy wires annealed at different heat treatment conditions; Fig. 3 is a graph illustrating stress-strain curves of Ti 49.3 Ni 50.7 alloy wires annealed at different heat treatment conditions; Fig. 4 is an enlarged graph illustrating stress-strain curves of Ti 49.5 Ni₅₀C 0.50 alloy wires annealed at different heat treatment conditions; Fig. 5 is an enlarged graph illustrating stress-strain curves of Ti 49.3 Ni 50.7 annealed at different heat treatment conditions; Fig. 6 is a graph illustrating a stress-strain curve of Ti 49.5 Ni₅₀C 0.50 alloy wire annealed at 500 °C for 150 minutes; Fig. 7 is a graph illustrating a stress-strain curve of Ti 49.3 Ni 50.7 annealed at the heat treatment condition similar to Fig. 6; Fig. 8 is an orthodontic wire element for the upper teeth according to an embodiment of the present invention; Fig. 9 is an orthodontic wire element for the lower teeth according to another embodiment of the present invention; and Figs. 10 and 11 are views illustrating a contraction state and an expansion state of an orthodontic coil element according to another embodiment of the present invention. For the orthodontic implement according to the present invention, a wire is used which is made of an alloy represented by a chemical formula of Ti a Ni b-c C c , a + b = 100 , b = 50-52, c = 0.25-5.0. The alloy wire has a pseudoelasticity at the natural human body temperature and the pseudoelasticity is controlled by the heat treatment condition so that the wire can be different in its shape recovery stress or force at different sections therealong. The addition of C (carbon) to the TiNi alloy lowers the austenitic and martensitic transition temperatures by producing TiC in the alloy. In addition to this, it serves to elevate the unloaded shape recovery stress within the pseudoelasticity range. To this end, the amount of C is 0.25 at% or more. When it exceeds 5 at%, the cold working of the resultant alloy is almost impossible. Therefore, the amount of C is limited 5 at% or less. Another alloy, which is obtained by replacement of a part of Ti and Ni of the above described Ti-Ni-C alloy by V, Cr, Fe, Nb, Ta, W, and Al, can be used for the orthodontic implement similar to the above-described Ti-Ni-C alloy. The resultant alloy is represented by a chemical formula of Ti a Ni b-c-d C c X d , X being at least one selected from V, Cr, Fe, Nb, Ta, W, and Al, a + b = 100 , b = 50-55, c = 0.25-5.0, d = 0.25-2.0. Now, examples of the above-described alloys will be described below as regards the pseudoelasticity in comparison with the conventional Ti-Ni shape memory alloy. Examples Alloy ingots of Ti 49.5 Ni₅₀C 0.5 (alloy A) Ti 49.5 Ni₄₉C 0.5 V 1.0 (alloy B) and Ti 49.3 Ni 50.7 (alloy C) were prepared by the melt method using a high frequency induction vacuum furnace. Those ingots were formed into wires with a diameter of 1.3 mm by hot and cold working. Then, the wires were subjected to a solution treatment at 950 °C for 10 minutes. Thereafter, the wires were cold-worked into wires with a diameter of 1.0 mm (40% cold reduction). A plurality of sample wires were cut from the resultant wires and were annealed at 500 °C for various times periods. Then, each of the sample wires was subjected to a tension test of a load-unload cycle at 37 °C (which is about the natural human body temperature) for the maximum strain ε = 5% in order to obtain a stress-strain curve. Fig. 1 shows the stress-strain curves of sample wires of alloy A, Curve 1 being a typical one of samples wires annealed for 30 minutes or less, Curve 2 a typical one of samples annealed for 100 minutes or more. Fig. 2 shows stress-strain curves of sample wires of alloy B. Curve 3 is for a sample wire annealed for 10 minutes and Curve 4 is for one annealed for 150 minutes. Fig. 3 shows stress-strain curves 5 and 6 of sample wires of alloy C annealed for 10 and 150 minutes, respectively. It is seen from Figs. 1 and 2 that wires of alloys A and B have the pseudoelasticity even at different annealing time periods within a range of 10-150 minutes. The stress level of the pseudoelasticity is high and low when the annealing time period is short and long, respectively. Therefore, the stress levels of the pseudoelasticity can be controlled by changing the annealing time period at a constant annealing temperature. In comparison with this, it is seen from Fig. 3 that the pseudoelasticity is disappeared by annealing for 150 minutes and the residual strain is caused even when the stress is perfectly removed. Wire samples (a diameter of 0.45 mm) of each of alloys A and B were also annealed at 500 °C for 130 minutes (condition I) and then subjected to a bend test of a load-unload cycle. In the bend test, the sample wire was supported at two supporting points which were separated by 50mm from each other. The load was applied to the middle point of the wire between the supporting points. A distance the wire was pushed down by the load was measured as the strain. Thereafter, the sample wires were again heat treated at 530 °C for 1 minute (condition II), and then subjected to the similar bend test. The results are shown in Figs. 4 and 5 where a solid line I and a dotted line II represent the results under the conditions I and II, respectively. It is understood from Fig. 4 and Fig. 5 that the stress level of the pseudoelasticity of alloy A can be sufficiently changed by reheat treatment in comparison with alloy B. Similar sample wires were annealed at 500 °C for 150 minutes and then subjected to the similar bend test. The resultant stress-strain curves are shown in Figs. 6 and 7. The pseudoelasticity can be obtained in the sample of alloy A but is not obtained in the sample of alloy C. It is understood from the above examples that the wires of alloys A and B can be controlled in the stress level of the pseudoelasticity by control of the annealing condition and by reheat treatment. In application of the wires onto orthodontics, the shape recovery force or stress under unload is used for correction of the teeth alignment. As will be understood from the above examples, the unloaded shape recovery stress can also be controlled by control of the annealing condition. It should be noted that the similar control of the unloaded shape recovery stress level can be realized in the alloys generally represented by the chemical formulae as described above. Wires of these alloys can also be controlled to be different in the unloaded shape recovery stress at different sections therealong by changing the annealing condition for those sections. Referring to Fig. 8, an orthodontic wire element 10 for the upper teeth according to an embodiment shown therein is made alloy A and is formed in the ideal arch form. The wire element has different correction forces at different sections, that is, an incisors section 11, a canines and premolars section 12, and a molars section 13 by changing the annealing condition and/or the reheat treatment condition for these sections. The ratio of the correction forces at the sections 11, 12, and 13 is selected 1:1.5:3.8. Referring to Fig. 9, another orthodontic wire element 20 for the lower teeth shown therein comprises the incisors section 21, the canines and premolars section 22, and the molars section 23. The ratio of the correction forces at the sections 11, 12, and 13 is selected 0.7:1.1:3.2, in relation to the correction force of the incisors section 11 for the upper teeth. Referring to Fig. 10, a coil spring 30 was formed of a wire (with a diameter of 0.25 mm) of alloy A for correction of mismatch of the upper and the lower jaws. The wire was annealed at 500°C for 30 minutes to have an appropriate unloaded shape recovery stress level so as to reduce the patient's discomfort. The coil spring 30 has an outer diameter 0.75 mm at the contraction state. The coil spring 30 was subjected to a durability test for expansion (Fig. 11) and contraction at about 37 °C and a number of expansion and contraction is counted until the coil spring is damaged. The test was repeated by changing the expansion to contraction ratio (L2 to L1). In comparison, a similar coil spring was formed of a wire of alloy C and was subjected to the similar test. The results of the tests are shown in the following table. L2/L1 COIL OF ALLOY A COIL OF ALLOY C 2 100,000 100,000 3 13,000 8,000 4 5,000 2,000 5 500 50 6 30 10 It will be seen from the table the coil spring according to the present invention is superior to, in the durability, the coil spring made of the conventional TiNi alloy.

Claims: An orthodontic wire element for use in correction of an irregular and abnormal alignment of teeth which is made of an alloy represented by a chemical formula of Ti a Ni b-c C c , a+b=100 at% , b=50-52 at%, c=0.25-5.0 at%, said wire element having a pseudoelasticity at the natural human body temperature, said pseudoelasticity is controlled in its shape recovery stress by a heat treating at a controlled temperature, of 400-600 °C and for a controlled time period of 10-150 minutes. An orthodontic wire element as claimed in Claim 1 wherein a part of Ni in said alloy is substituted by at least one metallic element X selected from V, Cr, Fe, Nb, Ta, W, and Al, so that the resultant alloy is represented by a chemical formula of Ti a Ni b-c-d C c X d , a+b=100 at% , b=50-52 at%, c=0.25-5.0 at%, d=0.25-2.0at%. An orthodontic wire element as claimed in Claim 1 or 2, said wire element having said pseudoelasticity at the natural human body temperature different in its shape recovery stress at different sections therealong. An orthodontic implement comprising a coil element made of said orthodontic wire element as claimed in Claim 1 or 2, said orthodontic implement being for use in correction of mismatch between the upper and lower jaws, said wire element having a high durability. Process of manufacturing said orthodontic wire element as Claimed in Claim 3, characterized by preparing a wire element of said alloy, and heat treating different parts of said wire element at different controlled temperatures within the range up to 600 °C and for different controlled time periods of up to 150 minutes. Process according to Claim 5, characterized in that the heat treatment is carried out at a temperature of 400-600°C for a time period of 10-150 minutes.

Inventors: YAMAMOTO NAOHARU C O TOKIN COR, YAMAUCHI KIYOSHI C O TOKIN COR, YAMAMOTO, NAOHARU, C/O TOKIN CORPORATION, YAMAUCHI, KIYOSHI, C/O TOKIN CORPORATION
Applicants: TOKIN CORP, TOMY INC, TOKIN CORPORATION, TOMY INC.
Application year: 1991
Main classification: A61K6
IPC codes: A61C7, A61K6