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DETAILED DESCRIPTION OF THE INVENTION
This invention relates to a novel orthodontic system for the orthodontic
movement of teeth.
Orthodontic systems to move malaligned teeth to desired positions generally
involved, the attachment of brackets (2) to indiviual teeth (1) by means
of bands which encircle the teeth or by adhesion, and the connection of
orthodontic wire (3) to the brackets in order to create an orthodontic
force to move the teeth toward desired position.
Conventional orthodontic systems have utilized orthodontic wire formed from
stainless steel, Co--Cr based alloy or others. In these conventional
wires, recovery range due to the elasticity of the wire is limited and the
elastic modulas has a high value.
Accordingly, complicated procedures have evolved to enlarge the moving
range of orthodontic wires. These include forming variously shaped loops
between the brackets. In spite of such treatments, the orthodontic force
still decreases gradually with the movement of the teeth. Readjustment
and/or exchange of the orthodontic wire is necessary in conjunction with
the progress of the orthodontic treatment. Furthermore, depending upon the
degree and the stage of the orthodontic procedure, wires having various
recovering forces must be adapted to the degree and stage as required.
Consequently, a lot of components, such as orthodontic wires of different
sizes with brackets fitted thereto, and others, have been necessitated.
The orthodontic procedure, therefore, became complicated. In order to
resolve these problems, a method has been developed which utilizes an
excellent shape recovery force based on the shape memory effect of Ni--Ti
base alloys. These alloys have composition of the stoichiometric
intermetallic compound, the atomic ratio of Ni and Ti on 1:1, and they
undergo thermoelastic martensitic transformation. The method utilizes the
shape memory effect which is a specific phenomena caused by said
transformation. Namely, the alloy memorized a desired shape at a high
temperature. When it is deformed at a temperature below the transformation
temperature, the wire recovers the original shape when heated to a
temperature above the transformation temperature. The method applies this
recovery force to the orthodontic procedure. This allows the number of
times necessary to adjust and set the wire to be reduced. Moreover, this
alloy has additional advantages in that the corrosion resistance and
biocompatibility are excellent. However, even in case of the shape memory
wires, various sizes wires should be prepared to obtain appropriate
orthodontic forces corresponding to the degree and the stage of the
orthodontic treatment. Accordingly, the number of components, such as
brackets, cannot be reduced, and the complication is not greatly improved.
As a result of the various investigations, the inventors have studied that
the recovery force due to the super elasticity can be adjusted by heat
treatment. Super elasticity is one of the unique phenomena caused by the
aforementioned Ni--Ti base alloy which appears at a temperature above the
martensitic transformation temperature. The inventors have developed a
orthodontic system for teeth in which a simplified procedure is made
possible by using orthodnotic wires of a single size or of only a few
kinds of sizes at almost all stages. That is, the invention involves
brackets which attach to the teeth and orthodontic wires connecting to the
brackets, and subject forces to the teeth which move the teeth toward
desired positions. The orthodontic wire of the invention comprising a
super elastic Ni--Ti base alloy in which the recovery force is adjusted by
heat treatment.
Namely, the invention utilizes wire consisting of superelastic Ni--Ti base
alloy which has excellent corrosin resistance and good biocompatibility
and provides desired recovery force adapted for the state of the
orthodontic treatment by the heat treatment. Accordingly, the invention
succeeds in the orthodontic procedure with wires of a single size or of
only a few-kinds of sizes.
Superelasticity is a property which causes the wire to recover its original
shape, as with rubber, even when deformed beyond its elastic limit. In
ordinary metal material, plastic deformation takes place when the metal is
deformed beyond the elastic limit and only a portion of the elastic
deformation, which is as much as 0.5% at most, recovers when unloaded. On
the contrary, the superelastic alloy can recover its original shape at
unloading even when strained to 10% deformation. The recovery force with
the superelasticity is decreased by the heat treatment, in which the
degree of decrement is larger when the temperature is higher and the time
is longer. Accordingly, the recovery force can be controlled arbitrarily
by selecting the temperature and the time of heat treatment.
When the superelastic Ni--Ti base alloy is utilized for this purpose, it is
required that the transformation temperature of the alloy be settled lower
than the body temperature. To achieve such a transformation temperature, a
composition ratio of Ni and Ti is selected within a range of 50.0 to 51.0%
of Ni and the remainder of Ti, or a small amount lower than 0.5% of either
one or both of Ni and Ti within a range of said composition is substituted
by either one or more than two of the elements such as Co, Cr, Fe, etc.
The transformation temperature of the super-elastic Ni--Ti base alloy is
extremely sensitive to the composition thereof, and it can be lowered with
the slight variance of the composition.
Heat treatment to adjust the recovery force comprises forming the
superelastic Ni--Ti base alloy wire in a linear shape or an arch shape to
fit the teeth, and then heat treating by an appropriate method, such as
the heating temperature of 300.degree. to 600.degree. C. and a heating
time of a few seconds to several hours in atmosphere and in inert gas. In
particular, electric heating is convenient. This heating method adjusts
the recovery force locally in the manner of varying with axial position by
position dependent heat treatment. Furthermore, the wire can be bent to a
desirable curvature at the time of heat treatment.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 and FIG. 6 illustrate an example of the orthodontic system of teeth,
wherein FIG. 1 is a side view and FIG. 6 is a ground plan.
FIG. 2 illustrates an example of the measuring methods of the recovery
force of orthodontic wire.
FIG. 3 through FIG. 5 are diagrams of the load-deflection curves,
respectively, which show the relationship between the heat treatment and
the recovery force of orthodontic wire of the invention.
1.--Tooth
2.--Bracket
3.--Orthodontic wire
4, 4' 5--bar
The invention will be presented in detail using examples.
An alloy comprising 51% of Ni and the remainder of Ti was melted and casted
in vacuum. An ingot was forged and hot rolled and then cold drawn.
Intermediate annealing was repeated during the drawing process. The alloy
was finally finished to a wire of 0.4 mm diameter.
After the scales on the surface were removed, the wire material was fixed
in a linear shape and subjected to heat treatment, the recovery force of
the wire was evaluated as follows: As shown in FIG. 2, a pair of bars (4)
(4') with a diameter of 7 mm were placed oppositely so as to the distance
between centers being 14 mm, the brackets (2) were settled on said bars
(4) (4'), and the orthodontic wire (3) was fitted. Load was applied to the
middle point of the wire through a round bar (5) with a diameter of 5.0
mm, and the recovery force was estimated by the flexure (.delta.). Results
are shown in FIG. 3 through FIG. 5. This measurement simulates an
orthodontic procedure of a portion from canine to premolar.
The figures show the load-deflection curves. A curve of the alloy wire
which was subjected to the straightening treatment in intermediate
temperature after cold drawing is cited as (6) in FIG. 3. Curves of the
wires which were heat treated at 500.degree. C. for 5, 10, 60 and 120
minutes, are cited as (7), (8), (9) and (10) respectively. Curves cited as
(11), (12), (13) and (14) in FIG. 4 are those of wires subjected to the
heat treatment at 400.degree. C. for 5, 10, 60 and 120 minutes,
respectively. Curves of (15), (16), (17), and (18) in FIG. 5 are of wires
subjected to the heat treatment at 600.degree. C. for 5, 10, 60 and 120
minutes respectively.
The figures show clearly that the superelastic Ni--Ti base alloy wires can
recover its very large displacement with a constant force and stress at
loading (arrow indicating upward) and unloading (arrow indicating
downward) are. For the orthodontic treatment of the teeth, the stress at
unloading, that is, the recovery force is important, and this recovery
force varys with both heat treatment temperature and treatment time.
Therefore, desired recovery force can be obtained by selecting these
conditions appropriately. The recovery force of the flat portion at
unloading is changed from 270 g of No. 6 to 220 g by the heat treatment at
500.degree. C. for 5 minutes (No. 7). The recovery force further varys to
200 g, 100 g and 65 g with the treatment for 10 (No. 8), 60 (No. 9) and
120 (No. 10) minutes, respectively. Recovery forces of the wires treated
at 400.degree. C. for 5 (No. 11), 10 (No. 12), 60 (No. 13) and 120 (No.
14) minutes are 250 g, 230 g, 200 g and 190 g, respectively. In the case
of the wires treated at 600.degree. C. for 5 (No. 15), 10 (No. 16), 60
(No. 17) and 120 (No. 18), the flat portions are not observed because of a
certain amount of residual deformation, although the recovery behaviors
corresponding to the treatment times is indicated.
In above, the description was made with regard to the linear shape
orthodontic wire, but the invention is not confined to this. For example,
the recovery force can also be adjusted by the heat treatment after or at
a time of forming an arch shape. The recovery force can also be adjusted
by the heat treatment. The wire is attached to the brackets attached to
the teeth to connect them, and, at this time, the curvature of the arch
can be adjusted after heat treatment to disired recovery force. Also, the
recovery force can be adjusted through the formation of a bend between
respective brackets. These operations can be conducted separately from the
heat treatment or at the same time as the heat treatment. Particularly,
when using electric heating, both ends or appropriate intermediate
portions of the wire are grasped with two or more pliers while providing
electric current between the pliers and at this time the wire can be
deformed to desired shape.
As described above, according to the invention, the recovery force of the
orthodontic wire can be adjusted to the desired strength arbitrarily.
Accordingly, not only the wire of the invention provides orthodontic
procedure with the recovery force fit for the state of the treatment, but
also a constant recovery force which does not decrease with the progress
in the orthodontic treatment. Moreover, the number of times to exchange or
reattach the wire is reduced markedly and, the size of the bracket
depending upon the size of the wire is also reduced. As a result,
complication in orthodontic procedure is relieved significantly.
Furthermore, the recovery force can be adjusted locally, that is, can be
strengthened or weakened against individual tooth, such special treatment
are exerted as the teeth intended for the treatment can be treated
selectively, and so on.
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