U.S. patent application number 13/282130 was filed with the patent office on 2012-02-16 for torque overcorrection model.
Invention is credited to Craig A. Andreiko.
Application Number | 20120036719 13/282130 |
Document ID | / |
Family ID | 39713987 |
Filed Date | 2012-02-16 |
United States Patent
Application |
20120036719 |
Kind Code |
A1 |
Andreiko; Craig A. |
February 16, 2012 |
Torque Overcorrection Model
Abstract
A custom orthodontic appliance comprised of brackets to be
positioned on a patient's teeth, and an archwire, is customized to
provide a desired torque to a tooth by selecting an angle for the
slot of at least one bracket so as to provide a torque interaction
between that bracket slot and the archwire. The torque interaction
is computed to compensate for tooth tilt resulting from
misalignment of the force vector applied by the archwire with the
tooth center of resistance. The torque interaction is computed at
the desired final position of the teeth, and may be computed to
provide for an applied torque even where the tooth is positioned in
the desired final tooth position to compensate for force
diminution. Material properties of the archwire and the relative
archwire slot geometry are evaluated to determine an archwire/slot
angular offset in which torque is applied to the bracket.
Inventors: |
Andreiko; Craig A.; (Alta
Loma, CA) |
Family ID: |
39713987 |
Appl. No.: |
13/282130 |
Filed: |
October 26, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12114968 |
May 5, 2008 |
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13282130 |
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60916189 |
May 4, 2007 |
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Current U.S.
Class: |
29/896.11 |
Current CPC
Class: |
A61C 7/12 20130101; A61C
7/146 20130101; Y10T 29/49568 20150115; A61C 7/002 20130101 |
Class at
Publication: |
29/896.11 |
International
Class: |
B21F 43/00 20060101
B21F043/00 |
Claims
1. A method of forming a custom orthodontic appliance, comprising
providing brackets to be positioned on a patient's teeth, wherein
the brackets include an archwire slot that is at an angle selected
to provide a desired torque to a tooth based upon the geometry of
the patient's teeth and masticatory system, providing an archwire
having a noncircular cross section, assembling the brackets to the
archwire to form the custom orthodontic appliance.
2. The method of claim 1 wherein the desired torque is computed to
compensate for tooth tilt resulting from misalignment of the force
vector applied by the archwire with the tooth center of
resistance.
3. The method of claim 1 wherein the desired torque is computed at
the desired final position of the tooth.
4. The method of claim 1 wherein the slot angle is computed to
provide for an applied torque when the tooth is positioned in the
desired final tooth position, whereby to compensate for force
diminution in the archwire.
5. The method of claim 1 wherein the slot angle is selected in
response to material properties of the archwire.
6. The method of claim 1 wherein the slot angle is selected in
response to relative archwire and slot geometry.
7. The method of claim 1 wherein the bracket is custom manufactured
for the appliance.
8. The method of claim 1 wherein the bracket is pre-manufactured
and selected from an inventory of premanufactured brackets having
specific configurations.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 12/114,968 filed May 5, 2008 which claims benefit of U.S.
Provisional Application 60/916,189, filed on May 4, 2007, which is
incorporated herein by reference.
[0002] This application is related to:
[0003] U.S. Pat. No. 5,431,562 issued Jul. 11, 1995 and Published
International Patent Application No. PCT/US00/35558, each of which
is incorporated in their entirety herein.
[0004] International Patent Application No. WO2004/028391, which is
assigned to the assignee hereof and hereby incorporated by
reference in its entirety.
[0005] U.S. patent application Ser. No. 10/528,036, filed Mar. 16,
2005, which is a U.S. National Stage application of International
Application No. PCT/US2003/030917, filed on Sep. 26, 2003, which
claims the benefit of U.S. Provisional Patent Application Ser. No.
60/413,712, filed Sep. 26, 2002, both hereby expressly incorporated
by reference herein.
[0006] U.S. patent application Ser. No. 09/941,591, which is a
continuation of International Patent Application No.
PCT/US00/35558, both filed Dec. 29, 2000, which is an International
Application of U.S. Provisional Patent Application Ser. No.
60/173,890, filed Dec. 29, 1999, all of which are hereby expressly
incorporated herein by reference in their entirety.
[0007] International Patent Application No. US2007/062965 filed
Feb. 28, 2007, which claims priority to U.S. Provisional
Application Ser. No. 60/777,483, filed Feb. 28, 2006, both of which
are hereby expressly incorporated herein by reference in their
entirety.
FIELD OF THE INVENTION
[0008] The present invention relates to computer-aided orthodontia,
and in particular the computerized generation of orthodontic
appliances that are individualized for a particular patient.
BACKGROUND OF THE INVENTION
[0009] In orthodontics it is difficult to create a pure
translational movement of a tooth, or to control rotation of the
tooth in what is called the Y-Z plane. This problem is most acute
when the archwire has a round cross-section, but is also observed
with archwires of square and rectangular sections.
[0010] FIG. 1 provides a simplified illustration of an archwire,
bracket and tooth, and their relative landmarks, and FIG. 2
provides a more detailed view of landmark points within the tooth.
The tooth's Center of Resistance (CR.pt) and the archwire axis
(which is herein identified as AW.pt), are typically not
coincident. As a consequence, pressure from the archwire other than
along the Crown Long Axis (CLA.ax) causes some amount of angular
change in the position of the tooth. The amount of rotation force
is related to the alignment of the force vector of the deflected
wire to the tooth's CR.pt. FIG. 1 illustrates the resulting
rotational effects on a tooth caused by archwire force in the
gingival, occlusal, facial and lingual directions.
[0011] Rotation of the tooth under applied archwire force increases
the complexity of orthodontic correction and avoiding such
rotations is an important part of orthodontic correction.
[0012] Clinicians have traditionally attempted to solve torque
correction issues by wire bending. From the clinician's point of
view when the case is submitted the clinician would intuit those
areas that should be corrected, based on experience drawn from a
menu of typical scenarios. However, this solution is not
necessarily systematic or consistent.
SUMMARY OF THE INVENTION
[0013] In accordance with principles of the present invention, a
custom orthodontic appliance comprised of brackets to be positioned
on a patient's teeth, and an archwire, is customized to provide a
desired torque to a tooth by selecting an angle for the slot of at
least one bracket so as to provide a torque interaction between
that bracket slot and the archwire.
[0014] In preferred embodiments, the torque interaction is computed
to compensate for tooth tilt resulting from misalignment of the
force vector applied by the archwire with the tooth center of
resistance. Furthermore, the torque interaction is computed at the
desired final position of the teeth. In further specific
embodiments, the slot angle is computed to provide for an applied
torque even where the tooth is positioned in the desired final
tooth position to compensate for force diminution in the
archwire.
[0015] In the disclosed particular embodiment the material
properties of the archwire and the relative archwire slot geometry
are evaluated to determine an archwire/slot angular offset in which
torque is applied to the bracket.
[0016] The above and other objects and advantages of the present
invention shall be made apparent from the accompanying drawings and
the description thereof.
BRIEF DESCRIPTION OF THE DRAWING
[0017] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention and, together with a general description of the
invention given above, and the detailed description of the
embodiments given below, serve to explain the principles of the
invention.
[0018] FIG. 1 is a simplified illustration of archwire force
causing tooth rotation about the tooth Center of Resistance.
[0019] FIG. 2 is an illustration defining points and planes on a
tooth, used to define movement of tooth objects.
[0020] FIG. 3 is a simplified version of FIG. 2 showing only data
required for calculation.
[0021] FIG. 4 is an illustration of a typical desired translational
tooth movement, shown in schematic.
[0022] FIG. 5 is an illustration of the movements of a tooth when
seeking to accomplish the translational tooth movement illustrated
in FIG. 4, in the case that the archwire force vector from AW.pt
does not pass through CR.pt.
[0023] FIG. 6 is an illustration of the results of steps 1 through
11 in FIG. 4, illustrating the path followed by the center of
resistance CR.pt, and illustrating the undesired rotation of line
Tran.li which extends from the archwire axis AW.pt, and the tooth
center of resistance CR.pt.
[0024] FIG. 7 illustrates the torque needed to prevent or undo the
undesired rotation of Tran.li in the example of FIG. 6 to create a
purely translatory tooth movement as originally desired.
[0025] FIG. 8 illustrates the stress-strain relationships of a
number of archwire materials including stainless steel, TMA and
NiTi.
[0026] FIGS. 9A and 9B illustrate the use of round and rectangular
archwires and the uncontrolled movements and rotations that are not
controlled by the archwire.
[0027] FIGS. 10A and 10B illustrate two cases of bracket-archwire
interaction; in FIG. 10A the archwire torque tilts the tooth
lingually and in FIG. 10B the archwire force tilts the tooth
labially.
[0028] FIG. 10C illustrates a case in which the archwire force
moves the tooth from a labial tilt to an upright position, and the
archwire force complements the natural effect of this motion by
applying a lingual torque.
[0029] FIG. 11A illustrates the use of traction forces to a bracket
on a molar, to tilt an incisor.
[0030] FIG. 11B illustrates a high cuspid, which when brought in
line with surrounding teeth would typically tilt lingually.
[0031] FIG. 11C illustrates lingually erupting laterals creating a
crossbite, which when the crossbite is resolved typically tilt
labially.
[0032] FIG. 11D illustrates buccal expansion, which typically leads
to labial tilt.
[0033] FIG. 12 illustrates the use of tilt correction in appliance
design in accordance with principles of the present invention.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0034] In accordance with the present invention, a computer aided
orthodontia design system utilizes tooth shape data to approximate
the actual translational and angular effects. This process is
performed after appliance design and prior to final
fabrication.
[0035] Referring now to FIG. 2, the tooth landmarks used in
accordance with principles of the present invention may be
identified:
[0036] MDL.pl (Mid Developmental Lobe Plane)
[0037] CC.pt (Cervical Center Point)
[0038] CL.ax (Crown Long Axis)
[0039] FEGJ.pt (facial Enamel Gingival Junction Point)
[0040] IE.pt (Incisal Edge Point): Found by projecting a line
normal to MDL.pl through BC.pt
[0041] Crown Height (CH.li): The distance from IE.pt to FEGJ.pt.
This is used to find CR.pt by using data from a lookup table
proportionate to CH.li along CL.ax from CC.pt extending toward the
root tip.
[0042] CR.pt (Center of Resistance Point): Found by extending CL.ax
gingivally by an amount proportional to the length of the line
segment CH.li (approximate crown height) specific to each tooth.
Typically, the crowns and roots are reasonably proportional to each
other and the literature references the location of the Centers of
Resistance.
[0043] AW.pt (Archwire Axis Point): Found by locating the
intersection of the center of the wire and MDL.pl after appliance
design is completed.
[0044] Translation Line (Tran.li): Line through and containing
AW.pt and Cr.pt.
[0045] Referring now to FIG. 3, a simplified view of the tooth
geometry and landmarks, showing only data required for calculation,
is useful for further discussion. Each of the points illustrated in
FIG. 3 is on the Mid-Developmental Lobe Plan for the tooth
MDL.pl.
[0046] FIG. 4 illustrates a typical desired translational tooth
movement, shown in schematic. This tooth movement is
"translational" because AW.pt and CR.pt are translated such that
the angle of Tran.li is unchanged by the movement. A number of
typical cases of this type of movement are illustrated below.
[0047] FIG. 5 illustrates a model for the movement of AW.pt and
CR.pt on the tooth that results from the use of a round orthodontic
archwire positioned within a bracket and generating a force that is
not aligned with the center of resistance CR.pt. FIG. 5 illustrates
11 steps in the resulting movement identified by the numbers 1-11,
each of which is described below.
[0048] Step 1: Tran.li is drawn in the initial position, the force
vector from the archwire from AW.pt is also illustrated. Notably,
the force vector does not pass through CR.pt.
[0049] Step 2: For a small motion, AW.pt moves along the force
vector and CR.pt moves identically.
[0050] Step 3: Because the force vector is not parallel to Tran.li,
ultimately Tran.li is rotated (in this case counterclockwise) some
amount about the new location of AW.pt, until CR.pt falls on
Tran.li in its original position.
[0051] Steps 4-5, 6-7, 8-9, 10-11: Steps 2-3 repeat, until CR.pt,
AW.pt and Tran.li arrive at the final positions shown in Step 11.
Note that Tran.li has, at this point, substantially rotated.
[0052] FIG. 6 provides a composite view showing the rotation of
Tran.li and movement of CR.pt through the steps illustrated in FIG.
5. Clearly, the rotation of Tran.li is undesired as the original
objective was translation only of Tran.li. The observed rotation is
caused by the fact that the force vector applied at AW.pt does not
pass through CR.pt
[0053] FIG. 7 illustrates the rotation that needs to be applied to
Tran.li (rotated clockwise) to restore Tran.li to its original
angle to net a translatory move only.
[0054] To model the desired torque, to factors must be known:
[0055] 1). The translational and rotational movements of the
individual teeth in reference to MDL.pl from the original position
to the final treated position within each arch. This is provided by
modeling that movement as described above.
[0056] 2). What movement has occurred between the two arches in an
anterior/posterior direction. This may involve doctor input when
the case is submitted.
[0057] To apply corrective torque, it is necessary to elaborate
upon the interactions between orthodontic archwires and
brackets.
[0058] In orthodontic appliances there is an interplay between the
wire and the slot of the bracket. The slot is typically rectangular
and the wires can be round or rectangular and of various
dimensions, alloys and sections. Alloys are chosen for different
force and elastic capabilities. Typical alloys are 300 series
stainless steel, a beta titanium/molybdenum alloy (TMA), and
various nickel titanium (superelastic) alloys (NiTi).
[0059] FIG. 8 is a graph showing some relative characteristics of
these typical alloys. Per unit of deflection, stainless is the
stiffest, and the NiTi is the softest alloy. In orthodontics, the
principle of "force diminution" states that as the deflection is
reduced, at some point the force output by the wire becomes
insufficient to move the teeth. As illustrated in FIG. 8, force
diminution is most rapid with NiTi wires, and least rapid with
stainless. In each case, however, it is believed that the wire
never relaxes completely, and so, the tooth is never taken to the
exact endpoint that would be computed for a completely relaxed
archwire. The stainless wire will bring a tooth closer to the
desired end point than will the NiTi, all other things being equal.
(Youngs modulus for stainless is 26 million psi, TMA 14 million psi
and NiTi about 6 million psi.)
[0060] The following table details, for various wire sizes (width
and height) the relative torsional stiffness of the wire for the
three alloys noted above, where the reference stiffness is that of
a 0.016.times.0.016 NiTi wire.
TABLE-US-00001 Wire Height Wire Width 18 Slot 22 Slot 0.016 0.016
17.1 NA 0.016 0.022 9.5 29.0 0.014 0.025 14.5 30.3 0.016 0.025 7.8
22.5 0.017 0.025 4.6 18.7 0.018 0.025 1.5 15.0 0.019 0.025 NA 11.4
0.021 0.025 NA 4.7
[0061] The interplay between the round and rectangular wires and
the slot is illustrated in FIG. 9A. Three sources of error between
the bracket and archwire are discussed herein, as others are
negligible.
[0062] Type A error results from variation in the location of the
wire in the slot in the direction along the height of the tooth.
This results in errors that are generally small since, for an
0.018'' slot, the wires used to finish cases are typically 0.016''
which results in an error too small to detect.
[0063] Type B error results from variation in the archwire angle
relative to the slot in the bracket. This source of error affects
the tip angle of tooth, and has somewhat greater effect than Type A
error, but is also generally insignificant because the bracket
width is approximately 0.103'' and the "play" is only +/-0.002''
resulting in an angular error of 0.88 degrees.
[0064] Type C errors, which are errors in torque and/or tooth tilt
angle, are quite significant relative to Type A and Type B errors,
and are the topic explored below. A typical wire for an 0.018''
slot would be 0.016''.times.0.022'' and for an 0.022'' slot would
be 0.019.times.0.025. This can generate and error of +/-10 degrees
in tooth tilt angle. This is especially true if one considers that
the corner radius of the wire is typically 0.003''. Undersize wires
are and are likely to remain the most popular in orthodontic
treatment for a variety of reasons.
[0065] The following table illustrates, for a variety of
rectangular or square cross section wire sizes (designated by
height and width), the play in an 18 and 22 bracket slot.
TABLE-US-00002 Wire Height Wire Width 18 Slot 22 Slot 0.016 0.016
17.1 NA 0.016 0.022 9.5 29.0 0.014 0.025 14.5 30.3 0.016 0.025 7.8
22.5 0.017 0.025 4.6 18.7 0.018 0.025 1.5 15.0 0.019 0.025 NA 11.4
0.021 0.025 NA 4.7
[0066] As can be seen in this table, interplay between the wire and
the slot coupled with the force output of different sections and
alloys can create a loss of control or a divergence from where the
appliance was programmed to position the teeth. (As noted in the
background, the loss of control is complete in the case of a round
wire.)
[0067] Most of the effect of the above discussion might be
acceptable orthodontically if it weren't for patient related
effects. Specifically, depending on the tooth's original starting
position (which way the tooth was torqued as the beginning), the
torque applied by the archwire can be substantial. As seen in FIGS.
10A and 10B, a labially tilted tooth (FIG. 10A) receives a torque
that is opposite to that of a lingually tilted tooth (FIG.
10B).
[0068] In accordance with principles of the present invention, a
torque correction is applied to the bracket design in a custom
orthodontic appliance, to correct for tilt error in an orthodontic
correction. In the particular described embodiment, the direction
of the correction is determined based on the direction of movement
of that particular tooth during the treatment, rather than initial
torque of the tooth, to ensure that the wire seeks to rotate the
tooth-bracket-pad assembly in the desired direction. For example,
in the case of FIG. 10C, the wire is attempting to push the tooth
in the lingual direction.
[0069] It will be noted that wire torque is defined based on tooth
surface, and in some cases the torque direction in initial
locations can be misleading if the curvature of the surface is
opposite due to any region's particular variance. Furthermore,
there are effects from treatment mechanics. One of several
scenarios is shown below. FIG. 11 illustrates the starting position
and final position created by pulling the archwire the molar tooth
as if to close spaces. The resulting tooth tilt repositions the
wire in the bracket in a manner that produces or relieves
wire-bracket torque depending upon the relative location of the
bracket and the incisor.
[0070] Other treatment cases are illustrated in FIGS. 11B and 11C.
In FIG. 11B, an insufficiently erupted tooth must be brought level
with its neighboring teeth, a process that often introduces tilt.
In FIG. 11C, maxillary teeth erupted on the labial side of the
mandibular teeth must be moved labially, often leading to tilt. In
FIG. 11D, buccal expansion leads to labial tilt of bicuspids.
[0071] Using an orthodontic appliance design system, such as is
described in detail in the above-referenced patent applications,
the torque applied by the archwire can be computed. For this
calculation the finishing (final) wire size and alloy must be
known. Although these may take any configuration, typically the
size for an 0.018'' slot would be 0.016''.times.0.022'' and for a
0.022'' slot the equivalent would be 0.019''.times.0.025''. In the
Insignia software the operator would then have to input the alloy,
size and circumstance is to be used.
[0072] In accordance with principles of the invention, a lookup
table is generated to characterize the torque and positioning of
the wire for the various combinations of alloy, size and
circumstance. The brackets, which are custom manufactured, or
premanufactured and selected from inventory, are selected to have
slots with adjusted angles as illustrated in FIG. 12. As seen in
FIG. 12, the system computes the interplay between the wire and
bracket to determine the angular offset between the wire plane and
bracket slot. The lookup table compensates for the rounding of wire
corners, and the relative wire and slot sizes. In some wire
sections like 19.times.25 in an 0.022 slot the effect of the radius
is minimal but in some like 16.times.16 in an 0.018 slot the effect
is large. Both the section and radius are taken into consideration.
(Radii are almost always 0.003''.)
[0073] Once the backlash or interplay between the bracket and wire
are computed, the shift of the wire plane relative to the bracket
slot is determined, and the bracket slot angle is determined to
generate a torque tending to prevent undesired tilt during
orthodontic correction.
[0074] In view of the foregoing, it will be appreciated that a
computer assisted orthodontic appliance design system may
mathematically calculate the required bracket torques to overcome
the side effects of archwire mechanics regardless of the wire size
or alloy chosen for finishing. These calculations may incorporate
Class II or III mechanics, expansion, ectopic eruption, and just
about any orthodontic scenario possible. In those embodiments in
which the bracket slots are individually created based on the case
data, it is possible to correct for almost any situation that can
be imagined. For example, correction can alleviate balancing
interferences from buccally erupting upper second molars, dangling
lingual cusps in expansion cases, flaring of lower incisors in
crowded or Class II situations, and lingual tipping of upper
incisors in extraction cases.
[0075] While the present invention has been illustrated by a
description of various embodiments and while these embodiments have
been described in considerable detail, it is not the intention of
the applicants to restrict or in any way limit the scope of the
appended claims to such detail. Additional advantages and
modifications will readily appear to those skilled in the art. The
invention in its broader aspects is therefore not limited to the
specific details, representative apparatus and method, and
illustrative example shown and described. Accordingly, departures
may be made from such details without departing from the spirit or
scope of applicant's general inventive concept.
* * * * *