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1. INDIAN DENTAL ACADEMY
Leader in continuing dental education
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2. Optimum Force Magnitude for
Orthodontic Tooth Movement
- Review of Literature
Yijin Ren, Jaap C. Maltha.
The Angle Orthodontist
2003; 73, 86-92
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3. An optimal force system is important for
adequate biological response in the
periodontal ligament and magnitude of this
force is related to the surface area of the
root.
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4. Schwarz (1932) proposed the first classic
concept of the optimal force.
He defined optimal continuous force as „„the
force leading to a change in tissue pressure
that approximated the capillary blood
pressure, thus preventing their occlusion in
the compressed periodontal ligament.‟‟
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5. This pressure in humans is about 15-20 mm
of Hg; and this comes to be 20 - 26 g for 1
sq.cm surface.
Based on this theory, Jepsen (1963)
calculated optimal force for premolar with
mean root surface area of 2.34 cm2, which
comes to be 54g.
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6. According to Schwarz, forces well below the
optimal level cause no reaction in the
periodontal ligament.
Forces exceeding the optimal level would
lead to areas of tissue necrosis, preventing
frontal bone resorption.
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7. Schwarz‟s definition was slightly modified by
Oppenheim (1944) who advocated the use
of the lightest force capable of bringing
about tooth movement.
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8. Reitan (1957) based on his histological
findings where he demonstrated cell-free
compressed areas within the pressure site,
strongly advocated use of very light forces
to maintain health of investing tissues.
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9. Storey and smith (1952) proposed their
theory of “optimal force”.
Acc to this theory, there is optimal range of
force that produce maximum tooth
movement. When force is increased above
this range, tooth movement slows down.
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10. This theory of „optimal force‟ was critically reviewed
by Boester and Johnston (1974).
They found in their study of 10 individuals that
space closure after premolar extraction was about
same after application of 140, 225 or 310g of
force during 10 weeks.
But space closure was significantly less when about
55g force was used.
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11. Hixon et al (1969) reported a more linear
relationship between force magnitude and
tooth movement, at least up to 300g of
force.
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12. The current concept of optimal force is based on
the hypothesis that a force of a certain magnitude
and temporal characteristics (continuous vs
intermittent, constant vs declining,etc ) would be
capable of producing a maximum rate of tooth
movement without tissue damage and with
maximum patient comfort.
The optimal force for tooth movement may differ
for each individual.
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13. Relationship between stress/strain and
amount of tooth movement
Force magnitude is a popular concept in
orthodontics. However, it is an incomplete
way to describe the forces delivered by an
orthodontic appliance.
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14. The true mechanical parameter in tooth
movement is not the magnitude of the force
per se, but rather the magnitude of the
stress generated by the appliance in the
surrounding periodontium.
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15. Compressive stress patterns in the periodontal ligament
under different force systems. A, Pure force applied at the
bracket. B, Force and moment applied at the bracket
Translation
Tipping
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16. Clinical interest in characterizing the nature
of the relation between the magnitudes of
applied force (i.e stress/strain) and the rate
of orthodontic tooth movement determining
the extent of anchorage loss began in the
1950s.
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17. The assumption was that differential
movement of teeth, at first proposed by
Begg (1957), could be generally achieved
with light force without any unwanted tooth
movement.
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18. In the last two decades, one of the central
questions raised about the exact
relationship of stress/strain pattern and
biologic activity and thus rate of tooth
movement, which may affect :
- anchorage planning
- efficiency of appliance in tooth movement
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19. This question led to the classic study by
Quinn and Yoshikawa (1985) in which they
proposed four possible models for the
relation between force magnitude and the
rate of orthodontic tooth movement.
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20. HYPOTHESES OF THE STRESS-MOVEMENT
RELATIONSHIP
(Robert S. Quinn, and D. Ken Yoshikawa,
jco 1985)
The effect of periodontal stress magnitude
on the rate of tooth movement is an
important issue in plans to control the
displacement of posterior teeth.
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21. Clinicians base their strategies for
controlling anchorage on their assumptions
about tooth movement.
Quinn and Yoshikawa proposed four
possible relationships between stress
magnitude and rate of movement.
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22. Possible hypotheses of the relationship between
stress magnitude and the rate of tooth movement
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23. Hypothesis 1 shows a constant relationship
between rate of movement and stress.
The rate of tooth movement does not
increase as the stress level is increased.
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24. Acc to this hypothesis, anchorage control
does not obey concept of ‘differential
anchorage’.
Hence when elastics are used for retraction,
only one size is necessary.
Loop designs are not critical and can be
simple and uncomplicated by helices.
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25. Hypothesis 2 shows a linear increase in the rate of
tooth movement as the stress increases.
In this system, intra-arch anchorage could be
manipulated by adding teeth (second molars) to
the anchorage unit or moving the extraction site
for example, second versus first premolars.
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26. This would distribute the stress over a larger
root surface, lowering the local stresses and
slowing the rate of tooth movement.
On the other hand, appliance that deliver
higher stresses would close extraction sites
most rapidly.
(since large periodontal stressed will lead to
faster tooth movement of both anterior and
posterior teeth)
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27. Hypothesis 3 depicts a relationship in which
increasing stress causes the rate of
movement to increase to a maximum.
Once this optimal level is reached,
additional stress causes the rate of
movement to decline.
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28. This hypothesis was originally proposed by
Smith and Storey (1952) and following
clinical strategies have evolved to take
advantage of its implications :
- Use light forces to retract canines which
will prevent anchorage loss, but using
heavy forces to protract posterior teeth and
"anchor" the canines. (differential
anchorage)
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29. Hypothesis 4 is a composite of some of the
foregoing concepts.
Here the relationship of rate of movement and
stress magnitude is linear up to a point; after this
point, an increase in stress causes no appreciable
increase or decrease in tooth movement.
Main difference between 3rd and 4th hypothesis is
effect of force beyond optimal range.
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30. EVALUATION OF HYPOTHESES
None of the studies in literature support
hypothesis 1 (constant relationship between
stress and rate of movement).
Hypothesis 2 (continuing linear relationship
between stress level and amount of tooth
movement) is also not substantiated well in
literature.
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31. Hypothesis 3, the original Smith and Storey
proposal (1952), can no longer be
considered viable in light of subsequent
clinical experience.
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32. During canine retraction, canine moves
more than molar at both the high and low
force levels and more importantly, there is
no evidence for the rate of movement to
suddenly reverse as the stress levels
increase past a certain "optimum" value.
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33. Evidence for 4th Hypothesis is more
compelling
Burstone and Groves, (1961), Hixon et al.
(1969) and Boester and Johnston (1974)
provide evidence that beyond a certain
stress level, increasing stress no longer
alters the rate of tooth movement. (no
increase or decrease in rate of movement)
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34. This study conclude that beyond an optimal
range, there is no effect of increased force
level on rate of tooth movement.
But “what is the upper limit of this optimal
range”?
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35. Both type (intermittent or continuous) and
level (magnitude) of force are important
factors in determining optimal force which
will preserve health of tissue.
Any abnormal force will lead to adverse
tissue reaction in form root resorption.
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36. Effect of force level (magnitude) on
rate of tooth movement and root
resorption
Many studies have been performed to
investigate the relationship between
magnitude of applied force and amount of
tooth movement and root resorption.
(Storey and Smith, 1952;Reitan, 1960; Burstone and
Groves, 1961; Andreasen and Johnson, 1967; Hixon et
aI., 1969, 1970; Boester and Johnston, 1974;
Andreasen and Zwanziger, 1980; Maltha et al., 1993)
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37. A clinical inter-individual study was recently
carried out to investigate tooth movements
and adverse reactions of the toothsupporting tissues when the applied
continuous force was doubled from 50g to
100g for tipping movement of premolar
tooth (Owman-Moll et al., 1995).
Note – force of 50-55g is optimal for premolar tipping (acc. to
Schwarz)
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38. The results demonstrated that the rate of
tooth movement increased but
severity of root resorption (surface
extension and depth of root resorption)
showed no significant difference when
force of 100g was applied and compared
with 50g.
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39. An additional investigation was undertaken
by same authors to determine whether a
further substantial increase of force
magnitude would result in faster movement
of the teeth without deleterious side effects.
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40. The effects of a four-fold increased
orthodontic force magnitude on tooth
movement and root resorption.
- intra-individual study in adolescents
Owman-Moll, Juri Kurol and Dan Lundgren
EJO 1996;18, 287-294
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41. This clinical and histological study was
designed as an intra-individual study to
investigate the effect of continuous force of
50g and 200g :
- on tooth movements and
- adverse tissue reactions
(root resorption)
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42. Subjects and methods
Experimental design and orthodontic
appliance
The maxillary first premolars bilaterally in
eight individuals, six boys and two girls
aged 12.1-13.6 years (mean age 13.0
years), formed test teeth.
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43. A fixed orthodontic appliance was inserted
the day the experimental period started and
consisted of molar bands on the first
maxillary molars joined with a half round
transpalatal bar for reinforcement of the
anchorage.
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44. A lingual arch with an
anterior acrylic bite
block was soldered to
the molar bands to
reduce the occlusal
forces on the test
teeth.
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45. The buccally directed tooth movement was
performed with a sectional arch (Sentalloy
0.018” when 50g force was applied and
Australian wire 0.018” when 200g force was
applied).
These wires were attached to the molar
band and ligated to a bonded 0.018”
bracket on the test teeth.
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46. This continuous orthodontic force was
applied for 7 weeks (total duration of
study).
The force magnitude was controlled weekly
and reactivated to 50g and 200g, and was
measured to the nearest 1g with a strain
gauge (Haldex'E Halmstad, Sweden).
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47. The orthodontic force magnitude declined on
average from 50g to 41g (18 per cent) and
on average from 200 to 145g (28 per cent)
during each week of appliance reactivation.
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48. Tooth movement registration
Alginate impressions were taken just before
start and at the end of the experiment.
With a sharp pencil, a point on each of the
buccal and palatal cusps of the test and
control teeth was marked on the cast.
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49. The horizontal (buccal) tooth movement was
measured with a coordinate measuring
machine (Validator lOO@, TESA SA, Renens,
Switzerland) to the nearest 0.01 mm.
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50. Radiographic registrations
Periapical radiographs using a long cone
parallel technique were taken within a week
before the start of the tooth movement and
immediately before extraction of the teeth.
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51. Histological procedures
At the end of the experiment, the teeth
were extracted.
With the microtome set to 4 µm, the teeth
were serially sectioned parallel to the long
axis in a bucco--palatal direction from the
mesial surface to the middle of the root
(with total 3 levels).
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52. The sections were stained with haematoxylin
and eosin.
A light microscope with a micrometer fitted
into the eye-piece was used to measure
surface extension and depth of root
resorption.
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53. The surface extension
of resorption was
measured parallel to
the root surface.
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54. The depth of each
resorption lacuna was
measured at the
deepest point by using
the distance from the
bottom of the cavity
perpendicular to the
tangent passing
through the borders of
the resorption lacuna
on the root surface.
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55. The mean value of root contour and root area were
calculated in order to describe the:
1 Resorbed root contour (%).
The sum of the extension of the resorption along
the root surface in the three longitudinal and
bucco-palatally directed histological sections of
each tooth was registered and a mean was
calculated and related to a registered mean root
contour.
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56. 2 Resorbed root area (%).
The sum of the resorbed root area
(extension x depth of the resorption lacuna)
in the three longitudinal and bucco-palatally
directed histological sections of each tooth
was registered and a mean was calculated
and related to a registered mean root area.
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57. Results
Amount of tooth movement
After application of a continuous force of
50g for 7 weeks, the tooth displacements
varied between 1.5 and 5.9 mm (mean 3.5
+/- 1.2 mm).
When a continuous force of 200g was
applied, the movements varied between 1.9
and 7.9 mm (mean 5.1 +/- 1.9 mm).
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58. This difference in horizontal tooth movement
was significant (P=0.02) with a 95 per cent
confidence interval.
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59. Root resorption
Root resorption was registered in all test
teeth and there were no significant
difference in number (n) or severity of root
resorption (i.e. resorbed root contour (%)
and resorbed root area (%)) after
application of a 50g compared with a 200g
force.
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61. Frequency and severity of root resorption showed
great individual variation, whether 50g or 200g force
was used.
200g force
50g force
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62. Discussion
A force of 50g has often been used and
recommended when buccal tipping of
premolars is desired.
The results of this study showed that when
applied continuous force increased four fold,
(200g) tooth movement increased 50 %
without any significant increase in root
resorption.
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63. One of the main findings in this
investigation was that the individual
variations in tooth movement as well as in
frequency and severity of root resorption
were large, irrespective of amount of force.
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64. This finding indicate that the major source of
variation is probably not the magnitude of
force, but variation in metabolic response.
It is well known that prostaglandins play a
major role in resorption processes.
(Klein and Raisz, 1970; Somjen et a/., 1980; Ngan et al., 1988;
Brudvik and Rygh, 1991).
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65. In yet another hypothesis, it may be
speculated that a force magnitude of 200g
may be too large to express cellular
reactions close to the root surface and that
tooth movement may take place mainly by
undermining resorption of the alveolar
bone. (as proposed by Reitan, 1985).
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66. Where adverse tissue reactions are concerned,
heavy force application may primarily prevent
cellular reactions on the root surface.
But still, in a longer perspective, when the force is
reduced due to tooth movement, extensive root
resorption may take place.
(as in this study also, higher forces have influenced
the root surface)
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67. But this short term study of early tissue
reactions in adolescents can not answer this
assumption.
The experimental model used in this study
does not allow long-term investigations due
to the limited bucco-lingual extension of the
alveolar process.
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68. It is therefore, necessary to utilize another
type of model, permitting tooth movement
along the alveolar ridge, if long-term results
are to be studied.
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69. Effect of type of force on rate of
tooth movement and root
resorption
Reitan (1957, 1970, 1985) advocated use
of intermittent forces to prevent the
development of root resorption by
enabling reparative processes to occur
during periods with little or no force.
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70. Maltha and Dijkman (1996) reported more
resorption in dogs when using continuous rather
than intermittent forces.
Faltin et al. (2001) confirmed that a reduction of
continuous force magnitude should be considered
to preserve the integrity of the tissues.
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71. Constant versus dissipating forces in
orthodontics: the effect on initial tooth
movement and root resorption.
F. Weiland
EJO 2003; 25, 335-342
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72. The aim of this clinical and laser scanning
microscopic study was to compare the
effects of two frequently used arch wires on
tooth movement and root resorption.
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73. In a inter-individual comparison study
design, a total of 90 premolars in 27
individuals (10 boys, 17 girls, with a mean
age of 12.5 years) were used in study.
Out of these 90 teeth, 6 teeth served as
control.
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74. Therefore, 84 teeth (maxilla/mandible) were
moved buccally with an experimental fixed
orthodontic appliance.
Impressions using alginate were taken
immediately before insertion of the
experimental appliance.
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75. Appliance design
A fixed orthodontic appliance was cemented
at the start of the experiment.
Appliance consisted of an acrylic splint
covering all but the experimental teeth.
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76. A fixed orthodontic appliance was cemented at the start
of the experiment. It consisted of an acrylic splint
covering all but the experimental teeth in one arch.
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77. Brackets (0.018 inch slot) were bonded to
the experimental teeth.
The brackets were incorporated in the
splints in such a way that a normal interbracket distance (5mm) existed between
the brackets on the splint and the bracket
on the experimental tooth
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78. Base of the slot of the „splint brackets‟ was
4.5 mm more buccally positioned than that
of the bracket on the experimental tooth in
the middle.
The premolar on one side was activated with
a stainless steel wire (0.016 inch) with a
buccal offset of 1 mm, which was
reactivated every four weeks.
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79. The contra lateral premolar was moved with
a super elastic wire (0.016 inch) with a
force plateau of 0.8–1 N.
(This wire had an initial activation of 4.5
mm and was not reactivated during the 12
week experimental period).
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81. At the end of the experimental period, tooth
displacement was studied threedimensionally on dental casts with a coordinate measuring machine.
Teeth were then extracted. Six premolars
were used as control teeth and were
extracted before the experiment started.
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82. The depth, area, and volume of the
resorption lacunae were measured using
three-dimensional digital images made with
a confocal laser scanning microscope
(CLSM).
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83. Result
Tooth movement
Teeth with the superelastic wire moved
significantly more (3.50 versus 2.30 mm)
and tipped buccally to a larger degree
(9.26° versus 7.81°) during the 12-week
experimental period than those moved with
the stainless steel wire.
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84. Resorption
The number of resorptions on the roots of
the teeth moved with a super elastic wire
was significantly greater than those moved
with a stainless steel wire (22 versus 16, P
< 0.001).
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86. The amount of resorptive damage,
defined as the largest depth, perimeter,
area, and volume were compared between
these two groups.
The teeth moved with the super elastic wire
showed significantly more resorptive
damage regarding all these parameters.
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88. Conclusion
This study confirms earlier studies regarding
potential damage to tissues with use of
continuous force.
(Gibson et al. 1992; Owman-Moll et al. 1995;
Daskalogiannakis and McLachlan. 1996; Darendeliler et
al., 1997 and Faltin et al. 2001)
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89. It could not be confirmed that maxillary
teeth are at higher resorptive risk than
mandibular teeth, as has been stated in the
literature
(Ketcham, 1927, 1929;Massler and Malone, 1954; Massler and
Perreault, 1954;Phillips, 1955; McFadden et al., 1989).
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90. The differing sensitivity is mostly explained
by the differing mechanical load of the
upper and lower teeth during treatment or
differing amounts of tooth movement during
orthodontic therapy.
In this investigation, the force system used
in the maxilla and mandible was the same.
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91. The amount of tooth movement and the amount of
resorptive activity were correlated. However, this
correlation was weak, the correlation coefficient
(r) being 0.35
This confirms data from the literature (Stuteville, 1937, 1938; Morse,
1971; Von der Ahe, 1973; Hollender et al., 1980; Sharpe et al., 1987;
Kelley et al., 1993; Beck and Harris, 1994; Baumrind et al., 1996;
Costopoulos and Nanda, 1996).
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92. Individual variations have been reported to
be an important factor for both tooth
movement in both force systems.
(Hixon et al., 1970; Maltha et al., 1993; Lundgren et al., 1996) and
root resorption (Henry and Weinman, 1951; Massler and Malone,
1954; Kvam, 1972; Reitan, 1974; Zachrisson, 1976; Linge and Linge,
1983). This clinical study confirms these findings.
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93. Rygh and Brudvik (1993) stated :
„New wire qualities pose challenges for the
orthodontist who must try to avoid
continuous forces that are heavy enough to
lead to necrosis of the periodontal ligament,
and last long enough to prevent the root
from recovering from damage inflicted on its
surface‟.
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94. Thank you
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