Orthodontic coil spring

A continuous-length orthodontic coil spring is made from a shape-memory alloy wire wound into a coil. The coil defines a plurality of open coil sections defining spaces between the turns thereof, and closed coil sections formed between the open coil sections. The closed coil sections are tightly wound with insubstantial spacing between the turns thereof. The coil spring is made of a Ni-Ti alloy wire, and is heat treated to impart a predetermined range of spring force within a superelastic zone of deflection. The continuous-length coil spring is cut through the closed coil sections to form several shorter-length coil springs therefrom. The closed coil sections on either end of the shorter-length coil springs are thus adapted to engage the orthodontic brackets mounted to a patient's teeth, to accurately impart the force of the spring to the brackets.

FIELD OF THE INVENTION 
The present invention relates to coil springs and, in particular, to 
orthodontic coil springs made of alloy wires exhibiting shape-memory 
properties. 
BACKGROUND INFORMATION 
An orthodontic coil spring made of an alloy wire exhibiting shape memory 
properties, such as nickel-titanium (Ni-Ti) alloy wire, is shown in U.S. 
Pat. No. 4,849,032. The Ni-Ti alloy wires exhibiting shape-memory 
properties are often referred to as "shape-memory alloy wires." 
Shape-memory alloy wires frequently exhibit excellent superelastic and 
spring-back properties. 
Superelasticity occurs when the stress value remains substantially constant 
up to a certain point of wire deformation, and when the wire deformation 
is reduced, the stress value again remains substantially constant. 
Therefore, a coil spring made of a shape-memory alloy wire can maintain a 
substantially constant load value throughout a zone of deflection. Because 
shape-memory alloy wires possess excellent spring-back properties, they 
can also be deflected to greater degrees than other types of wires, 
without causing permanent deformation of the wire. 
A shape-memory coil spring is made by winding a shape-memory alloy wire, 
such as a Ni-Ti alloy wire, into a coil. If the coil spring is to be used 
as an open or compression coil spring, then it is wound into a coil 
defining spaces between each turn thereof. If the coil spring is to be 
used as a closed or tension coil spring, then it is tightly wound into a 
close contact shape substantially without any spacing between its turns. 
Tension coil springs are provided with hooked portions on their ends to 
connect the springs to orthodontic appliances. 
In FIG. 1, a typical open shape-memory alloy coil spring is indicated by 
the reference numeral 1. The coil spring 1 is fitted over an archwire 2 
and mounted between two orthodontic brackets 3. The orthodontic brackets 3 
are in turn mounted to adjacent teeth T1 and T2. The coil spring 1 is an 
open or compression coil spring, and is thus wound with spacing between 
its turns. As indicated by the arrow shown in FIG. 1, the coil spring 1 is 
used to shift the tooth T1 away from the tooth T2, and thus into the space 
between the teeth T1 and T3. 
One advantage of the coil spring 1 is that because it is made of a 
shape-memory alloy wire, it exerts a substantially constant spring force 
throughout a zone of deflection, often referred to as the "superelastic 
zone of deflection." Therefore, if the distance that the tooth T1 is to be 
shifted is within the superelastic zone of deflection of the spring 1, the 
spring 1 can be used to apply a substantially constant spring force 
throughout the entire movement of the tooth. 
An open shape-memory alloy coil spring, like the coil spring 1, is 
typically made by winding a shape-memory alloy wire into a 
continuous-length open coil spring. The continuous-length coil spring is 
wound to define substantially constant spacing between the turns thereof. 
The continuous-length coil spring is heat treated, and is then cut into a 
number of shorter-length coil springs. The length of each shorter coil 
spring is dimensioned so that the spring will be compressed when fitted 
over an archwire in the space provided between the orthodontic brackets 
mounted on a patient's teeth. 
One problem with open shape-memory alloy coil springs, is that because they 
are cut from larger continuous-length coil springs, the ends of each coil 
spring are not adapted to properly engage the orthodontic brackets mounted 
to a patient's teeth. Usually, the ends of each spring are cut at the 
middle of a turn or, that is, at the midpoint of the space between two 
turns of the spring. As a result, the free ends of such a coil spring are 
oriented at oblique angles relative to the longitudinal axis of the 
spring. Thus, when the spring is mounted over an archwire, only the tips 
of its free ends engage the orthodontic brackets. 
The tips of the spring, however, are not shaped or oriented to conformably 
engage the surfaces of the brackets. This problem is enhanced with open 
shape-memory alloy coil springs, because they are typically deflected or 
opened to a greater degree than other types of orthodontic open coil 
springs. As a result, the force of an open shape-memory alloy coil spring 
is usually not applied to the orthodontic brackets in a smooth and 
reliable manner. 
It is an object of the present invention, therefore, to provide an 
orthodontic coil spring that overcomes the problems of known shape-memory 
alloy coil springs. 
SUMMARY OF THE INVENTION 
The present invention is directed to an open orthodontic coil spring for 
imparting forces to orthodontic appliances mounted to a patient's teeth. 
The orthodontic coil spring comprises a shape-memory alloy wire exhibiting 
superelastic properties wound into a coil. The coil includes at least one 
open coil section between the free ends thereof, which defines 
predetermined spaces between its turns. The free ends of the coil are 
tightly wound substantially without any spacing between the turns thereof. 
The free ends of the coil are thus adapted to substantially engage 
orthodontic appliances to impart the spring forces thereto. 
In one coil spring of the present invention, the shape-memory alloy wire is 
a Ni-Ti alloy wire, and the coil spring is heat treated to impart a 
substantially predetermined range of spring force within a superelastic 
zone of deflection. Preferably, each of the free ends of the coil includes 
at least 1-1/2 turns. The coil spring thus defines an engaging surface on 
either end thereof. The turns defining the engaging surfaces are 
preferably oriented substantially perpendicular to the longitudinal axis 
of the coil spring. Each of the engaging surfaces is equal in length to 
about one-half of a turn of the coil spring. 
The present invention is also directed to a continuous-length coil spring 
for forming several shorter-length open orthodontic coil springs 
therefrom. The continuous-length coil spring comprises a shape-memory 
alloy wire exhibiting superelastic properties wound into a coiled shape. 
The coiled shape includes a plurality of open coil sections defining 
predetermined spaces between the turns thereof, and closed coil sections 
formed between the open coil sections. The closed coil sections are 
tightly wound with insubstantial spacing between the turns thereof. The 
continuous-length coil spring is separable into more than one coil spring 
by cutting through the shape-memory alloy wire in at least one of the 
closed coil sections. 
In one coil spring of the present invention, each of the open coil sections 
includes the same number of turns and each of the closed coil sections 
includes the same number of turns. Preferably, each open coil section 
includes about five turns and each closed coil section includes about 
three turns. The shape-memory alloy wire is preferably made of a Ni-Ti 
alloy. 
The present invention is also directed to a method of making orthodontic 
coil springs, comprising the following steps: 
winding a shape-memory alloy wire exhibiting superelastic properties into a 
coil defined by a plurality of open coil sections and closed coil sections 
formed between the open coil sections, wherein the open coil sections 
define predetermined spaces between the turns thereof, and the closed coil 
sections are tightly wound with insubstantial spacing between the turns 
thereof; and 
cutting through the shape-memory alloy wire in at least one of the closed 
coil sections to form at least two coil springs therefrom. 
In one method of the present invention, each of the open coil sections are 
formed with the same number of turns and each of the closed coil sections 
are formed with the same number of turns. The shape-memory alloy wire is 
preferably made of a Ni-Ti alloy. 
One advantage of the present invention, is that because the free ends of 
the coiled wire are tightly wound with insubstantial spacing between the 
turns thereof, the coil spring has relatively large, smooth surfaces on 
either end thereof for engaging orthodontic appliances. As a result, the 
substantially constant force of the coil spring can be accurately and 
smoothly imparted to orthodontic appliances. Thus, the problems of known 
open, shape-memory alloy coil springs, wherein only the tips of the free 
ends of the coil springs engage the appliances, are overcome by the open 
coil spring of the present invention. 
Other advantages of the apparatus and method of the present invention will 
become apparent in view of the following detailed description and drawings 
taken in connection therewith.

DETAILED DESCRIPTION 
In FIG. 2, a continuous-length coil spring embodying the present invention 
is indicated generally by the reference numeral 10. The continuous-length 
coil spring 10 is made of a shape-memory alloy wire, such as a Ni-Ti alloy 
wire, which is wound into a coil. When the coil spring 10 is in a relaxed 
state (neither being stretched nor compressed), it defines a plurality of 
open coil sections 12 and closed coil sections 14 located therebetween. 
The open coil sections 12 are wound with a pitch "P", so as to define 
substantially equal predetermined spaces "A" between the turns thereof. 
The closed coil sections 14, on the other hand, are tightly wound 
substantially without any spacing between the turns thereof. 
As shown in FIG. 2, each individual turn of the open coil sections 12 is 
oriented at an oblique angle "B" relative to the longitudinal or helical 
axis "X" of the coil spring. The turns of the closed coil sections 14, on 
the other hand, are substantially perpendicular to the X axis. The coil 
spring 10 is wound so that each open coil section 12 includes about five 
turns and each closed coil section 14 includes about three turns. As can 
be seen, the same winding pattern is repeated throughout the length of the 
coil spring 10. After the coil spring 10 is wound, it is then heat treated 
to impart a predetermined range of spring force within a superelastic zone 
of deflection, as described in U.S. Pat. No. 4,849,032, which is hereby 
incorporated by reference as part of the present disclosure. 
The continuous-length coil spring 10 is then cut into several 
shorter-length open coil springs 16, shown typically in FIG. 3. The inner 
diameter "D" of the continuous-length coil spring 10, which is the same as 
the inner diameter "D" of each coil spring 16, is dimensioned to fit over 
an archwire (not shown). The free ends of the coil spring 16 are each cut 
at about the middle of two adjacent closed coil sections 14. Therefore, 
because each closed coil section 14 includes about three turns, about 
1-1/2 turns on either end of the coil spring 16 are tightly wound 
substantially without any spacing between the turns thereof. 
As a result, each free end of the coil spring 16 defines an engaging 
surface C, indicated by crosshatch in FIG. 4. Each engaging surface C is 
equal in length to about one-half of a turn, and is oriented substantially 
perpendicular to the longitudinal axis X of the coil spring 16. When the 
coil spring 16 is mounted over an archwire, the engaging surfaces C engage 
the orthodontic brackets mounted to a patient's teeth (not shown) to 
impart the compressive force of the coil spring 16 to the brackets. 
One advantage of the open coil spring of the present invention, is that 
because the engaging surfaces C are each equal in length to about 1/2 of a 
turn, and are oriented substantially perpendicular to the helical axis X 
of the spring, the compressive force of the coil spring 16 is accurately 
imparted to the orthodontic brackets. Known open shape-memory alloy coil 
springs, on the other hand, are not formed with the closed coil sections, 
but are typically trimmed at about the midpoints between the turns of the 
coil springs. Therefore, the free ends of such known coil springs are 
usually oriented at oblique angles relative to the longitudinal axes of 
the coil springs, like the angle B shown in FIG. 2. As a result, usually 
only the tips of the free ends of such springs engage the orthodontic 
brackets. Accordingly, the force of such a spring is typically 
inaccurately or unreliably applied to the brackets. Thus, the problems 
normally encountered with known open shape-memory alloy coil springs, are 
overcome by forming the continuous-length coil spring 10 with the closed 
coil sections 14 of the present invention. 
As will be recognized by those skilled in the art, the number of turns in 
each open coil section 12 can be varied, as compared to the embodiment 
shown in FIG. 2. Likewise, the open coil sections 12 do not have to 
include the same number of turns, but each can be wound with a different 
number of turns. Moreover, if needed for an individual patient, a coil 
spring 16 may comprise two or more open coil sections 12 with a closed 
coil section 14 formed between each successive open coil section. For 
example, the length of an open coil section 12 may not be long enough to 
fit within the space provided between adjacent orthodontic brackets, 
whereas the length of two or three open coil sections 12 may be 
appropriate. Therefore, a single spring 16 can be trimmed from a 
continuous-length coil spring 10 that has more than one open coil section 
12 and/or closed coil section 14. 
FIG. 5 illustrates another continuous-length coil spring embodying the 
present invention which is substantially the same as the continuous-length 
coil spring 10 of FIG. 2. Therefore, like reference numerals are used to 
indicate like elements. The continuous-length coil spring 10 of FIG. 5 
differs from the coil spring described above in that each of the closed 
coil sections 14 includes about four turns, and each of the open coil 
sections 12 includes about five turns. Therefore, when the shorter-length 
open coil springs are cut therefrom, each one has about two turns tightly 
wound substantially without any spacing on either end thereof. The number 
of windings in either the open coil sections 12 or closed coil sections 14 
can thus be varied to meet the needs of each particular application.