T1 - Biomechanics of Torque - Vijay Jayde PDF
T1 - Biomechanics of Torque - Vijay Jayde PDF
T1 - Biomechanics of Torque - Vijay Jayde PDF
ABSTRACT
Objective: To evaluate the magnitudes of initial and subsequent sequential deactivational third-
order moments generated in rectangular twisted archwires (either when the archwires are actually
twisted before insertion in the standard edgewise bracket slots or when they are indirectly twisted
in the preadjusted slots of the modern edgewise brackets) in order to judge their biologic ac-
ceptability.
Materials and Methods: A finite element study was carried out with the MSC Patran/Nastran
interface. Three-dimensional models were constructed with 170 nodes of upper 0.017- 0.025-
inch and 0.019- 0.025-inch archwire segments extending bilaterally from the maxillary central
incisors to the first premolars. Required twists were applied at the appropriate locations to derive
the applied and reactionary moments both initially and during the time needed for complete de-
activation.
Results: The results indicated that a round-tripping possibility does exist in certain clinical pro-
cedures. Furthermore, the moments produced could be quite high, thereby enhancing the pos-
sibility of root resorption.
Conclusions: Twists in rectangular archwires may be used only when reciprocal torque is needed
on adjacent teeth. In other situations, alternative torquing methods should be considered.
KEY WORDS: Torque; Archwire twist; Biomechanics
quent deactivation of the archwire, the teeth sub- Table 2. Initial moments produced by progressive incremental
torque in a straight length of wire, and the subsequent alteration in
jected to the above moments were permitted root
the moments from 2 of root displacement at either end
displacements (by altering displacement boundary
conditions) in small steps of 1.5 to 2, which is the Torquing moments on each tooth, Nmm
average root displacement per month seen in clin- Activation, Tooth 1 Tooth 2 Tooth 3 Tooth 4
ical experience. The resulting changes in the ap- Tooth 1: 0
plied and reactionary moments were derived by the Tooth 2: 10
FEM. Tooth 3: 20
c. The teeth continuing to experience sizeable mo- Tooth 4: 30 19.5 0 0 19.5
Tooth 1: 2
ments were further permitted root displacements of Tooth 2: 10
1.5 to 2, and the change in applied and reaction- Tooth 3: 20
ary moments was again computed. Tooth 4: 28 15.3 3.8 4 15.1
d. The process was continued in the same step-by-
step fashion until the moments on all teeth were
Table 3. Initial moments from progressive incremental torque in a
reduced to approximately 5 Nmm, which was con- curved segment of archwire
sidered the threshold moment needed for causing
Torquing moments on each tooth, Nmm
root displacement. At this point, the archwire was
deemed to have been effectively deactivated by the Right Right Left Left
displacement of roots of all teeth. Activation, lateral central central lateral
Right-lateral: 0
However, certain simplifications and assumptions Right-central: 10
had to be accepted as dictated by the nature of the Left-central: 20
study: Left-lateral: 30 19.46 8.97 1.68 33.1
Table 4. Initial and subsequent altered moments during progressive Table 7. Initial and altered moments in the use of MBT brackets
incremental torque applied to lateral and central incisors (cuspid bracket 7)
Torquing moments on each tooth, Nmm Torquing moments on each tooth, Nmm
Amount of Amount of
torque, Central Lateral Canine torque, Central Lateral Canine Premolar
Central: 20 Central: 17 23.42 13.4 42.56 (13.93)
Lateral: 10 30.74 6.53 13.76 Lateral: 10
Central: 18 Canine: 7
Lateral: 12a Central: 15 22.12 6.89 31.79 (10.4)
Canine: 2 22.53 0.23 8.11 Lateral: 8
Central: 16 Canine: 5
Lateral: 12
Canine: 4 17.62 0.14 0.08
Central: 14 and reached a value close to zero. Thereafter, it
Lateral: 12
Canine: 4 12.91 3.69 0.06 changed into a positive moment that gradually in-
Central: 12 creased and then reduced.
Lateral: 12 The moments generated during torquing of two central
Canine: 4 8.11 7.58 0.08 incisors for lingual root torque (with twists placed be-
Central: 10 tween the central and lateral incisors on either side) are
Lateral: 10
Canine: 4 6.8 4.97 2.64
given in Table 5. Although the moments generated on
the central incisors were close to the optimum recom-
a
It is assumed that the root of the lateral incisor will move labially
mended value (23.94 Nmm) and in the lingual direction
owing to the moment generated. This will increase the archwire-
bracket angle, and hence the resultant torque will be 12. as intended, the lateral incisors experienced a labial root
torquing effect of slightly lesser magnitude (19.46 Nmm)
that may or may not be desirable. During deactivation,
Table 5. Initial and subsequent altered moments during lingual root the labial root torque on the lateral incisors reduced
torquing of two central incisors
sharply and then changed to lingual root torque. The
Torquing moments on each tooth, Nmm cuspids, which were resisting the displacement of the
Amount of
torque, Central Lateral Canine lateral incisors, experienced a labial root torque that
10 23.94 19.46 gradually increased and then reduced slightly.
8 15.8 9.15 2.76 The moments generated during torquing of a single
6.5 9.49 1.22 4.8 tooth (Table 6) by both 0.019- 0.025-inch and 0.017-
5 3.4 6.43 6.9 0.025-inch SS archwires on the right central incisor
Central: 5 5.8 1.71 4.84
were very high initially (39.19 Nmm and 30.21/Nmm).
Lateral: 3.5
Although the moments on the left central and right lat-
eral incisors were not very high to begin with, these
dropped sharply when some relaxation in the wire was
Table 4. Although the initial moment on the central in-
permitted to simulate small amount of root movement.
cisor was quite high (30.74 Nmm) and that the cuspid
experienced a negative moment of much lesser mag-
Moments Generated in a PAE Setup
nitude (6.53 Nmm), the lateral incisor experienced a
(MBT Prescription and 0.019- 0.025-inch
negative moment (13.76 Nmm) and not a zero mo-
SS Archwires)
ment. With deactivation, the moment on the central
incisor gradually reduced whereas the moment on the The moments differed considerably in the lateral in-
cuspid dropped sharply and continued to be close to cisor and cuspid area, depending on whether the up-
zero. The interesting observation was that when the per cuspid brackets had a positive torque, negative
root of lateral incisor moved labially because of the torque, or zero torque (Tables 7 and 8). With 7 of
initial negative moment, the moment soon reduced torque in the cuspid brackets, the negative moments
Table 6. Initial and subsequent altered moments during single-root torquing (in this case, the right central incisor)
Torquing moments on each tooth, Nmm
SS archwire
size, inches Activation Right canine Right lateral Right central Left central Left lateral
0.019 0.025 10 19.29 39.19 15.37
0.019 0.025 8 2.82 9.14 24.68 4.1 3.81
0.017 0.025 10 14.96 30.21 11.97
0.017 0.025 8 2.2 7.11 19 3.24 2.96
Table 8. Initial moments in the use of MBT brackets (cuspid bracket DISCUSSION
0 or 7)
Effects of Curvature in Modifying the Archwire
Torquing moments on each tooth, Nmm
Amount of Behavior
torque, Central Lateral Canine Premolar
In recent years, it has been realized that the me-
Central: 17
Lateral: 10 chanics enunciated for two-dimensional models of
Canine: 0 23.42 0.58 13.76 straight length of wires need to be cautiously applied
Central: 17 to the archwires, for the latter are 3D entities and their
Lateral: 10 curvature is likely to affect the bending and torsional
Canine: 7 23.42 10.22 15.05 (13.93)
behavior. Isaacson et al9 reported this in a subsequent
article in connection with the V bend mechanics. Our
study also noted this while assessing the veracity of
Table 9. Initial moments generated with the use of inverted upper Isaacsons hypothesis. It was validated in relation to a
lateral incisor bracket and cuspid bracket having 7, 0, or 7 torque straight length of wire. However, when applied to the
curved segment of the archwire, though the end teeth
Torquing moments on each tooth, Nmm
Amount of experienced moments in opposite directions their
torque, Central Lateral Canine Premolar magnitudes were not the same. Furthermore, one of
Central: 17 the two middle teeth experienced a moment close to
Lateral: 10 zero, but the other middle tooth experienced a sizable
Canine: 7 57.67 79.35 42.56 (13.93)
amount of moment.
Central: 17
Lateral: 10
Canine: 0 57.67 65.37 13.76 Possibility of Round-trip Movements in Certain
Central: 17 Torquing Procedures
Lateral: 10
Canine: 7 57.67 51.39 15.05 (13.93) Our suggested explanation for the movement of
middle teeth consequent to displacement of end teeth
and gradual deactivation of archwires proved to be
correct, as did our suspicion that some round tripping
on the cuspids were very high (42.56 Nmm) but may occur during certain applications of torque.
dropped suddenly with small amount of relaxation in Progressive incremental torque was introduced by
the archwire. Another interesting observation was that the standard edgewise technique to overcome the re-
though the upper cuspid with 0 built-in torque was ciprocal effects on the adjacent teeth from the wire
expected to experience no torquing moment, it did ex- twists. However, as was noted in the results, the root
perience a negative moment of sufficient magnitude to of lateral incisor in the simulated setup initially expe-
be clinically effective (13.76 Nmm). rienced a labial root torquing moment, which gradually
As can be seen in Table 9, the moments generated reduced to zero and then changed to a lingual root
on the central incisor when the lateral incisor bracket torquing moment. This would mean that the lateral in-
was inverted were very high in all three situations cisor is likely to experience some round tripping and
(57.67 Nmm), and the negative moments on the lateral that its total lingual root torque is much less than the
incisor were extremely high (79.35 Nmm) when the torque undergone by the central incisor.
cuspid bracket had built-in torque of 7. Although they In the other situation simulating lingual root torque
were of lesser magnitude when the cuspid had 0 or only on the maxillary central incisors (as in the treat-
7 torque, they were still quite high (65.37 Nmm and ment of a Class II division 2 situation), the roots of
51.39 Nmm). The moment on the cuspid with its brack- maxillary lateral incisors initially experienced an almost
et having built-in torque of 7 was also very high (42.56 equal labial moment, which reduced sharply and then
Nmm). changed to a positive moment as the archwire deac-
tivated. This again indicates a possibility of round-trip
movement of the lateral incisors.
Torquing Moments from Various Wires
The moments mentioned in all the above tables are Torque Considerations in the PAE Appliances
for the SS archwires. As can be seen, they are often The setup simulated in our study was the currently
quite high. A rough estimation of the moments gen- popular MBT prescription.10 Other prescriptions have dif-
erated by the TMA and NiTi archwires of equivalent fering built-in torque values, but the trends noted here
sizes for similar situations can be made from their tor- (in the specific case of MBT with upper cuspid brackets
sional stiffness ratio, which is SS:TMA:NiTi 10:3:1. having negative torque) would apply to them also.
The high magnitudes of some of the moments ob- plications, for high moments are often produced
served in this study will not apply in clinical practice that drop suddenly when tooth movements start.
when untorqued archwires are used. This is because Also, round tripping could occur in many instances.
the largest archwire normally used is 0.019 0.025 c. The use of TMA archwires in preference to SS
inches, which has about 10 to 12 of archwire-slot archwires may reduce these adverse effects but
play that will reduce the moments substantially. How- may not eliminate them completely. Alternative
ever, additional torque (up to 20) is often needed in methods proposed by authors such as Thurow4
the 0.019- 0.025-inch SS archwire during treatment, (torquing spurs in round-base archwire), DeAngelis
as per the recommendation of the proponents of the and Davidovitch7 (use of Warren springs), and
technique.10 This added torque would overcome the Isaacson and Rebellato12 (torquing arches) deserve
archwire-slot play, and then the FEM values would be serious attention because the reciprocal reactions
close to the moments that are likely to be generated from these are spread on many (often distant) teeth
in the clinical practice, when SS archwires are used. and are controlled more easily.
Inversion of the maxillary lateral incisor bracket is d. The moments generated by the NiTi archwires,
often recommended10 in cases where the lateral inci- even those that are 0.019 0.025 inches, may be
sors are in-standing to begin with. One should keep in too low to bring about active torquing of teeth over
mind the possibility of developing extremely high mo- long periods. However, they may prove beneficial
ments in the SS archwires when following this prac- when reciprocal torque is needed on adjacent
tice. teeth, especially when using preadjusted brackets,
Another observation worth noting is that the recip- more so with some brackets inverted.
rocal effects on adjacent teeth might override the built-
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