The occlusion research (study
of the non-orthodontic normals) had been launched to get a better understanding
of occlusion and how it was related to the buccolabial surfaces of the crowns.
There was no intention originally to produce a new appliance, but the need for
one became evident.
The arising
concept was to Pre-program suitable treatment into the appliance and therefore
the SWA was developed.
The built-in features of the
SWA guide the teeth along direct vector lines, eliminating jiggling,
round-tripping and other excessive movements.
Andrews’s philosophy of treatment
A bracket type for each tooth type:
Since within any dentition, teeth
differ from each other, therefore, each tooth would require its own bracket.
Measurements of the non-orthodontic normal
models had provided data about the angular positions of crowns in excellent
occlusion. Such data are useless unless known to be taken from specified,
unchanged crown sites.
If an appliance is not located
properly, all that has been programmed into it will be altered.
Where the bracket should
be located on the tooth?
The LACC
Crown morphology includes many
developmental landmarks. Among these are the ridges and grooves on the crown's
labial or buccal surfaces. These ridges and grooves can be easily recognized,
and are not significantly subject to environmental alterations such as
chipping, wear and fracture.
The long axis of the clinical
crown (LACC), (as viewed from the labial or buccal perspective), for all teeth
except molars, is located at the mid-developmental ridge that runs vertically
and is the most prominent portion in the central area of the labial or buccal
surface.
The long axis of molar crowns
is identified by the dominant vertical groove on the buccal surface.
The LA-Point
That vertical line, the LACC, is easy to
find. But how far "up" on it should the bracket be placed?
At its mid-point. The
LA-point is the mid-point on the Long Axis of the crowns. The crown has no obvious
horizontal axis but the accuracy of this procedure has been measured and found
satisfactory.
The LACC is practical for
measuring bracket position. No x-rays needed, no guessing; this axis can be
directly seen, touched, and marked with a pencil.
Keying to the long axis of
the crown makes accurate bracket placement much easier. One simply places the
tie-wings of the bracket parallel to the LACC, and the base point of the
bracket at the LA-point.
Torque in Bracket Base
"Torque" refers to buccolingual
or labiolingual inclination.
How is torquing accomplished by the SWA?
The bracket base is inclined
in relation to the stem, allowing the stem to be parallel to the Andrews plane,
and the LA-point, base point and slot point to be included in the Andrews plane.
Making the torquing function a product of the base design allows all slots (at
the completion of active treatment) to be aligned with each other and thus receive
a flat, unbent rectangular archwire.
A
separate bracket for each tooth type is necessary, with torque built into the
bracket base. Compound-Contoured Bases
Most conventional appliances have simple
horizontal curvature in the bracket base. The Straight-Wire Appliance added
vertical curvature. It is the combination of the two that we refer to as
"compound contour" or "compound curvature."
Installing a bracket with a
vertically flat base against the vertically curved surface of a tooth allows a
variety of slot-to-tooth positions. Not only is torque affected by any rolling
motion during bracket placement but also is the height of the slot in relation
to the occlusal plane or Andrews plane.
Angulated Slots
The slots are angulated for tip, but each
bracket is squarely aligned with the LACC. If any existing bracket with a
horizontally curved base is angulated on the tooth to achieve tip, this creates
a two-point contact between tooth and two diagonally-opposite corners of the
bracket base, resulting in a potential for rocking.
To obtain the full effects of the Straight-Wire
approach, tip must be built into the slot of the bracket, torque must be in the
base, and the base of each bracket must be contoured to fit firmly and
unchangingly when the bracket is installed "squarely" (not angulated)
at the chosen site.
In/Out and Molar Offset
As Teeth types differ in buccolingual
thickness, the thickness of the brackets was varied according. Elimination of first order archwire bends
has several advantages, including time-saving and the enabling of en masse
space closures without interference from wirebends.
Other Aspects
Designing an individual
bracket for each tooth type also permits innovations not involving the slot or
the base. SWA tie-wings, instead of being bilaterally symmetrical, step-out
farther on the gingival sides of posterior brackets, resulting in easier
ligation and less frequent gingival impingement.
Identification
One further special design element in the
Straight-Wire Appliance is that every bracket can be identified as to its tooth
type.
Extraction Brackets Series
Bodily tooth movement usually is necessary
in extraction cases, presenting a requirement for anti-tip and anti-rotation
forces (in addition to torque, tip and in/out). The amount required is
proportionate to the distance a tooth must be moved.
There are three brackets
designed for each posterior tooth type. The three are designated Minimum,
Medium and Maximum.
SWA extraction brackets, like Standard SWA
brackets, have built-in tip, torque and in/out. In addition, they provide two
functions not needed in the Standard brackets: anti-tip and anti-rotation.
Incisor Bracket Sets
As shown by Steiner, the inclination
(torque) requirement of incisors is directly related to the ANB angle. SWA
design accommodates these requirements with three sets of incisor brackets,
designated A, S and C, with S representing Standard.
Classification of
Treatment
Angle's classification of malocclusion does
not indicate the degree of difficulty of a case. Another approach proposed by Andrews
can be beneficial in selection of brackets sets.
This approach divides cases
into three categories: Class I, Class II and Class II-Classic. Each of these is
then subdivided into more types.
Class I cases are those that
have an Angle Class I molar relationship. Class II cases are those with an
Angle Class II molar relationship and lower arch length problems. Class
II-Classic also deals with Angle Class II molars, but only those cases with no
lower arch length problems and the lower anteriors in good relationship to the
face.
Brackets types are determined
by the amount of crowding and/or protrusion found in the lower dental arch.
The brackets are organized
into sets and series. These are referred to with numerals and letters that
match each such combination of brackets with the Classification for which it is
designed. This permits immediate selection of the brackets most suitable for
resolving a particular problem.
Roth Modifications
The extraction brackets had
counter-tip and counter-rotation built in to offset relapse, and to keep teeth
from rotating and tipping into the extraction sites as they were being moved
along a rectangular wire. In other words, the brackets were constructed to
allow translation of teeth as much as possible and to offset any relapse
tendency by overcorrection.
If teeth tend to relapse back
whence they started, and if counter-tip, counter-rotation, counter-torque, and
leveling of the curve of Spee were applied to the SWA in every possible
direction, then it should be possible to use primarily one prescription for
most cases, and to finish to an "end of appliance therapy" goal in
which all tooth positions are slightly overcorrected and from which the teeth
will most likely settle into non-orthodontic normal positions.
Also, Roth introduced changes
in the configuration of the appliance. Power arms were cast as integral parts
of the brackets designed by Andrews to place the force near the CR of the
teeth.
The Advantages of the
Straight-Wire Appliance
1. Ease of arch wire
construction, since there is no need for bends of the arch wire.
2. No need for inter-bracket
span, since there are no bends in the arch wire.
3. Ease of arch wire
placement. When the teeth are properly aligned into ideal positions, all the
bracket slots lie parallel to the occlusal plane.
4. Less
"round-tripping." Since the teeth move in more direct vector lines
from their maloccluded positions to their individualized positions.
5. Better control of tooth positions. Since all requirements for idealized
tooth positions are built into the brackets rather than the arch wires.
6. Better and more consistent
results with shorter treatment time. The movement of teeth in direct vector
lines to their final positions, limited by what is engineered into the
appliance, results in shorter treatment time for the patient and less chair
time for the orthodontist .
7. Patient comfort. Placing
of large rectangular arch wires is easier for both the patient and the
orthodontist with less jiggling of the teeth and therefore much less
discomfort.
8. Complete space closure in
both extraction and non-extraction cases can be accomplished with one set of
arch wires since there are no bends in the arch wires to interfere with the
brackets themselves during space closure.
9. Ease of Ligation. The
gingival wings of the brackets are constructed so that they stand away from the
gingiva labially and buccally.
10. Bracket Identification.
Easy bracket identification due to the identifying markings that are cast into
the brackets.
11. Easier, more accurate
bracket placement. Specific bracket base compound curvature insures consistent,
reliable fit.
12. The SWA also allows one
to take full advantage of the new nickel titanium wires. These wires are
difficult to bend, and bends do not stay in nickel titanium wires.
At last
The
SWA has proven itself under the test of time. It has been upgraded with
technical advances, and perhaps its more recent "facelifts" have made
it look like a different appliance. The tooth positioning ingredients, however,
are the same as ever.
The SWA key is to get the
brackets properly placed and if they are not properly placed, they must be
reset.
The SWA will not diagnose cases, it will
not set up the treatment plan, and it will not figure out the mechanics needed
to correct the malocclusion but it will allow us to reach our treatment goals….which
are the six key of normal occlusion.
Thank you dr.lily for this great article 🙏
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