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Rate of tooth movement under heavy and light continuous orthodontic forces
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Rate of tooth movement under heavy and light continuous orthodontic forces Jason A. Yee,a Tamer Tu¨rk,b Selma Elekdag˘-Tu¨rk,c Lam L. Cheng,d and M. Ali Darendelilere Sydney, Australia, and Samsun, Turkey Introduction: The aim of this study was to measure the rate and the amount of orthodontically induced tooth movement under heavy (300 g) and light (50 g) continuous forces with superelastic nickel-titanium closing coils over a defined time (12 weeks). Methods: Fourteen patients who required maxillary canine retraction into first premolar extraction sites as part of their orthodontic treatment completed this study. In a split-mouth design, precalibrated nickel-titanium closing coil springs delivering a force of 300 g or 50 g were used to distalize the canines after an alignment and stabilization period. Intraoral and maxillary cast measurements were made at the beginning of canine retraction (T0) and every 28 days for 84 days (T1, T2, T3) to assess total space closure, canine retraction rate, canine retraction and molar anchorage loss, and canine rotation. Results: Statistical analysis showed that the amount of initial tooth movement (T0-T1) was not related to force magnitude; however, during the T1-T2 and T2-T3 periods, increased amounts and higher rates of tooth movement were found with heavy forces. These significantly increased the rate and the amount of canine retraction, but the adverse effects of loss of canine rotation control and anchorage were concomitantly increased. Light forces provided a greater percentage of canine retraction than heavy forces, with less strain on anchorage. Conclusions: Initial tooth movement would benefit from light forces. Heavier forces tend to increase the rate and the amount of canine retraction but lose their advantage because of unwanted clinical side effects. (Am J Orthod Dentofacial Orthop 2009;136:150.e1-150.e9)
T
he general premise of orthodontic treatment is that, because of root surface differences, there is an ideal force that moves each type of tooth at an optimal rate. Storey and Smith1 reported the ‘‘optimal force’’ theory and documented that forces of 150 to 200 g applied to maxillary canines would produce the maximum rate of tooth movement for distalization. Various hypotheses have been promoted on stress magnitude and the rate of tooth movement; however, the concepts of threshold, light, heavy, and optimal forces are still unclear.1-12 There is ambiguity due to the inability to estimate stress distribution in the periodontal ligament, lack of experimental control of the type of tooth movement (tipping and translation), and individual heterogeneity of response.9
a Former postgraduate student, Discipline of Orthodontics, Faculty of Dentistry, University of Sydney, Sydney, Australia. b Professor, Department of Orthodontics, Faculty of Dentistry, University of Ondokuz Mayis, Samsun, Turkey. c Assistant professor, Department of Orthodontics, Faculty of Dentistry, University of Ondokuz Mayis, Samsun, Turkey. d Lecturer, Discipline of Orthodontics, Faculty of Dentistry, University of Sydney, Sydney, Australia. e Professor and chair, Discipline of Orthodontics, Faculty of Dentistry, University of Sydney, Sydney, Australia. The authors report no commercial, proprietary, or financial interest in the products or companies described in this article. Reprint requests to: M. Ali Darendeliler, Discipline of Orthodontics, Faculty of Dentistry, University of Sydney, Level 2, 2 Chalmers St, Surry Hills NSW 2010 Australia; e-mail, [email protected]. Submitted, January 2008; revised and accepted, June 2008. 0889-5406/$36.00 Copyright Ó 2009 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2008.06.027
According to Quinn and Yoshikawa,2 most clinical approaches to move teeth are based on the assumption of a force magnitude or range of magnitudes that, when delivered to the periodontal tissues, yields the most rapid rate of tooth movement. That is, the rate of tooth movement is sensitive to changes in force magnitude, and, for a particular tooth, there is an ideal force that will move it at the maximum rate. Ren et al10 developed a mathematic model to describe the relationship between force magnitude and rate of tooth movement. No force threshold could be defined that could switch tooth movement on, nor could an optimal force or force range be calculated to produce maximum tooth movement. The model indicated a wide range of forces, all of which led to maximum tooth movement. Since minute forces can initiate tooth movement, it was hypothesized that higher forces would actually overload the periodontium. In addition, Von Bohl et al11 found that high forces did not move teeth faster than low forces, but teeth with higher forces had more areas of hyalinization. The appearance of hyalinized tissue was related to force magnitude, but this had no influence on tooth movement. Melsen13 suggested that there is no relationship between force magnitude, type of tooth movement, and amount of displacement. She hypothesized that ‘‘indirect resorption at the pressure side is not a reaction to force but an attempt to remove ischemic bone lying adjacent to hyalinized tissue. The subsequent direct bone 150.e1
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resorption could be considered part of the remodeling process.’’ Tooth movement from this point of view was thus related to the distribution of stress and strain in the periodontal ligament and the remodeling response of the adjacent bone. Ren et al9 asked whether higher stresses and strains result in greater biologic activity and thus a faster rate of tooth movement, or whether an optimal force or range of forces would stimulate this process. Proffit14 believes that ‘‘the current concept of optimal force is based on the hypothesis that a force of a certain magnitude and temporal characteristics (continuous versus intermittent, constant versus declining) would be capable of producing a maximum rate of tooth movement without tissue damage and with maximum patient comfort.’’ Several variables—eg, force magnitude, direction, distribution and duration, initial tooth displacement, stress, strain, and biologic changes of the periodontium—thus must be considered to obtain optimal tooth movements.1,15,16 With the advent of superelastic nickel-titanium (NiTi) closing coils, their force duration, magnitude, continuity, and constancy are as close to ideal as clinically possible.17 According to Burstone,8 force constancy and continuity are vital in an optimal orthodontic force system. However, there is no consistency or agreement regarding optimal orthodontic force level. The aim of this study, therefore, was to measure the rate and the amount of orthodontically induced tooth movement under heavy (300 g) and light (50 g) continuous forces with superelastic NiTi closing coils over 12 weeks. MATERIAL AND METHODS
A total of 14 healthy teenagers (5 boys, 9 girls; mean age, 15.8 years; range, 13.0-19.5 years) who required bilateral extraction of the maxillary first premolars and retraction of the maxillary canines during their fixed orthodontic appliance treatment completed the study. All subjects and their parents or guardians consented to participation after receiving verbal and written explanations (ethics approval: SSWAHS X06-0062 and EK: 358). In selecting the patients, we used the following criteria: (1) need for bilateral maxillary first premolar extractions (necessitating moderate anchorage requirements) and fixed appliance orthodontic treatment; (2) similar minimal crowding on each side of the maxillary arch; (3) no previous orthodontic or orthopedic treatment; (4) no craniofacial anomalies; (5) no previous reported or observed dental treatment of the maxillary canines; (6) no history of trauma, bruxism, or parafunction; (7) no past or present signs and symptoms of periodontal
American Journal of Orthodontics and Dentofacial Orthopedics August 2009
disease; and (8) no significant medical history or medication that would adversely affect the development or the structure of the teeth and jaws and any subsequent tooth movement. At the start of treatment, standard orthodontic records were taken, including extraoral and intraoral photographs, alginate impressions (Dentalfarm Australia Proprietary, Sydney, Australia), and lateral cephalometric radiographs, and panoramic radiographs. All patients were fitted with maxillary and mandibular 0.022 3 0.028-in slot Speed (Strite Industries, Cambridge, Ontario, Canada) preadjusted (Roth prescription) self-ligated edgewise appliances. Self-ligation was used to allow for standardized ligation during canine retraction, thereby regulating the effects of friction. After extraction of the maxillary first premolars, initial alignment was obtained with a 0.014- or 0.016in NiTi wire (3 M Unitek, Monrovia, Calif), taking an average of 1.9 months (range, 0.5-6 months). Then, a 0.019 3 0.025-in beta-titanium-molybdenum alloy (TMA, 3 M Unitek) archwire was left in situ for 8 weeks to obtain standardized first-, second-, and third-order prescriptions for the experimental teeth. This consolidation period enabled full archwire passivity before retraction of the maxillary canines and also allowed a minimum of 3 months after removal of the maxillary first premolars for alveolar bone consolidation at the extraction sites.18 Concurrently, it permitted equal amounts of bone formation adjacent to the canines before the experimental period, ensuring that the quality of bone around the canine roots was of similar density. The distal retraction of the maxillary canines was performed on a continuous 0.020-in stainless steel archwire (Dentaurum, Ispringen, Germany) to reduce frictional effects. In the split-mouth design, each patient received a precalibrated 3-mm superelastic NiTi closing coil spring (GAC International, Bohemia, NY) elongated 200% to 500% of its activation, delivering an approximate force of 50 g (light) or 300 g (heavy), respectively.17 The coil springs were attached to the maxillary first molar tube (Speed) and the canine bracket (Speed) on each side with a 6-mm power arm constructed of 0.016 3 0.016-in stainless steel (Dentaurum) placed into the auxiliary tubes of the respective brackets. This was to deliver the force as close to the centers of resistance of the respective teeth as clinically possible, with the force vector parallel to the main archwire. No reactivation of the closing coils was needed (Fig 1). To prevent prejudice, the amount of force on each side of the mouth was randomized, and a minimum of 3 mm of canine retraction space was necessary to make the study feasible.
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Fig 1. Speed brackets with NiTi closing coils applying a heavy distalizing force (300 g) on the right canine and a light distalizing force (50 g) on the left canine.
To ascertain the clinical viability and reliability of the constant and continuous forces from the superelastic NiTi closing coils, a sample of 10 light (50 g) and 10 heavy (300 g) coils were tested with an Autograph Calibrator AG-E (Shimadzu, Kyoto, Japan) at room temperature (25 C) and dry air conditions. The amounts of activation in the study were within the manufacturer’s recommended guidelines for clinical use. Before distal canine retraction, intraoral measurements were made with digital dental calipers (Masel, Bristol, United Kingdom), and a baseline maxillary arch impression with heavy and light polyvinyl siloxane material (Imprint II Regular Body/Light Body, 3M ESPE, St Paul, Minn) was taken. Two clinical measurements of total space closure were made. First, they were made bilaterally from the corresponding brackets of the canine to the molar, with the standardized small circular hole located on the Speed NiTi clips used for opening the clip and also with the corresponding cusp tips of the canine and the mesiobuccal cusp of the first molar. All clinical measurements were made by the same author (S.E.T.). During canine retraction, the subjects were recalled every 14 days for clinical measurements and at 28-day intervals to take maxillary impressions with the polyvinyl siloxane material. At each appointment, oral hygiene was reinforced, and the appliances were assessed for damage. As a quality-control measure, if a bracket, archwire, or spring involved in canine retraction was damaged, or if the canine attained complete retraction before the end of the study period, the patient was excluded from the study. Two patients were excluded for these reasons, making the total sample size 14. The study period extended from the beginning of canine retraction for 12 weeks. Measurements were made from the orthodontic models taken every 28 days from the beginning of canine retraction. Consequently, with the clinical measurements, 4 study models per patient were available for assessment and measurement. Stable palatal reference points and the tips of the canines and the cusp
Fig 2. Stable reference structures.
tips of the first molars (Fig 2) were marked with a 0.3mm lead pencil on all casts.19-22 The models were then scanned (Expression 1600 flatbed color image scanner, Epson Australia Proprietary, Chatswood, Australia) at 600 dpi and 16-bit gray scale from an occlusal perspective; the resulting images were printed and linear measurements made. The initial casts for each patient were used as the baseline model, with an acetate template for linear and angular measurement constructed according to the stable reference points (medial and lateral ends of the third palatal rugae) and superimposed on the subsequent cast images for measurement of canine and molar movement, and canine rotation. All measurements on study models were repeated by the same operator (J.A.Y.) to ensure intraexaminer reliability and eliminate measurement error. Measurements at each 28-day time (T0, T1, T2, T3) were total space closure via measurement along the midpoint of the labial surface of the Speed brackets approximating the spring opening hole; total space closure via measurement along the cusp tips of the canine
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Statistical analysis of space closure (mm), canine retraction (mm), canine rotation ( ), and molar anchorage loss (mm) in heavy vs light forces (independent 2-sample t test)
Table I.
Measurement
Force
n
Mean
SD
SE of the mean
Sig (2-tailed)
P value
Total space closure (measured via bracket on model)
Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy
14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14
3.5136 7.3350 3.4821 7.4329 2.79114 4.9314 2.8929 5.0893 1.6986 4.1021 1.8064 2.8143 1.29 22.36 1.8464 3.3021 1.1136 2.2807
1.29893 1.89554 1.26284 1.97015 0.89481 1.61320 0.98293 1.46792 0.58362 1.68011 0.61004 0.99696 2.673 18.587 0.85881 0.91649 0.56644 0.95267
0.34715 0.50660 0.33751 0.52654 0.23915 0.43115 0.26270 0.39232 0.15598 0.44903 0.16304 0.26645 0.714 4.968 0.22953 0.24494 0.15139 0.25461
0.000
\0.001
0.000
\0.001
0.000
\0.001
0.000
\0.001
0.000
\0.001
0.003
\0.05
0.000
\0.001
0.000
\0.001
0.001
\0.05
Total space closure (measured via bracket intraorally) Total space closure (measured via cusp tip on model) Total space closure (measured via cusp tip intra-orally) Canine retraction (measured via bracket on model) Canine retraction (measured via cusp tip on model) Canine rotation Molar anchorage loss (measured via bracket on model) Molar anchorage loss (measured via cusp tip on model) Sig, Significance.
and the mesiobuccal cusp of the first molar; amount and rate of canine retraction; and amount and rate of molar anchorage loss. Distobuccal rotation of the canines, from the angle formed between a line connecting the contact points of the mesial and distal surfaces of the canines from the superimposed T0 to T3 templates, was also measured to ascertain the effect of different force levels on rotational moments. Statistical analysis was undertaken with the Statistical Package for Social Sciences (SPSS for Windows, version 12, SPSS, Chicago, Ill). The average interindividual rates and amounts of total space closure via the bracket and cusp tips, canine retraction, and anchorage loss (in millimeters) from the light and heavy closing coils were calculated for the treatment period with Student t tests. Paired t tests were also used to compare intraindividual differences between heavy and light forces. Calculations of the total amount of space closure as measured via the cusp tips, canine retraction, anchorage loss (in millimeters), and canine rotation were made. Measurements on the study models were repeated on 3 occasions and compared with the original measurements by using paired t tests to validate the method of measurement. RESULTS
The difference in total space closure between heavy and light forces was highly significant (P \0.001) between the brackets as measured on the models. The light force group averaged 3.51 6 1.30 mm of space closure,
and the heavy force group averaged 7.34 6 1.90 mm over the 12-week period (Table I). The difference in total space closure between the 2 force groups was highly significant (P \0.001) between the brackets as measured intraorally and was similar to the measurements made on the study models. The light force group averaged 3.48 6 1.26 mm of space closure, and the heavy force group averaged 7.43 6 1.97 mm over the 12-week period (Table I). The difference in total space closure between the 2 force groups was highly significant (P \0.001) between the cusp tips of the canine and mesiobuccal cusp of the first molar as measured on the study models. The mean amounts of space closure were 2.79 6 0.89 mm in the light force group and 4.93 6 1.61 mm in the heavy group over the 12 weeks (Table I). The difference in total space closure between the 2 force groups was highly significant (P \0.001) between the cusp tips of the canine and the mesiobuccal cusp of the first molar measured intraorally. The mean amounts of space closure were 2.89 6 0.98 mm in the light force group and 5.09 6 1.47 mm in the heavy group over the 12 weeks (Table I). The correlation between measurements on study models to those made intraorally both via the bracket and cups tips was high, as measured by the Pearson correlation coefficient. They were all significant at the 0.01 level. The amount of canine retraction was significantly greater in the heavy force group than in the light force group (P \0.001) during the 12-week period. The
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Statistical analysis of heavy vs light forces for canine rotation (independent 2-sample t test)
Table II.
Percentage of Canine Retraction vs Total Space Closure
Percentage
100.00% 80.00% 60.00%
Light Heavy
40.00%
Measurement
SE of Sig P Force n Mean SD the mean (2-tailed) value
Canine rotation Light 8 0.25 0.71 Heavy 8 11.13 3.64
20.00%
0.25 1.29
0.000 0.000
\0.001
0.00% 0-28
28-56
56-84
Sig, Significance.
Days
Fig 3. Percentage of canine retraction to overall space closure. Percentage
canines retracted on average 4.10 6 1.68 mm in the heavy force group and 1.70 6 0.58 mm in the light force group (Table I). The amount of canine retraction was significantly greater in the heavy force group than the light force group as measured along the cusp tips (P \0.05) during the 12 weeks. The canines retracted on average 2.81 6 1.00 mm in the heavy force group and 1.81 6 0.61 mm in the light force group (Table I). The percentages of canine retraction to overall space closure were 55% in the heavy force group and 62% in the light force group (Fig 3). Canine rotated significantly greater in the heavy force group than in the light force group (P \0.001) during the 12-week period. The average canine rotations were 22.36 6 18.59 in the heavy force group and 1.29 6 2.67 in the light force group. The greater rotations often occurred because the heavy force overcame the retentive lock of the NiTi Speed clip, causing the archwire to ‘‘pop out’’ of the bracket (6 of 14 patients). This led to an uncontrolled distobuccal rotation moment of the affected canine and might have contributed to the greater ratio of canine retraction of the heavy force group compared with the light force group when measuring the distances between the brackets (Table I). The excessive rotation represented failure of the Speed clip, caused by the heavy forces. Consequently, the differences in canine rotation of the remaining 8 patients were measured. The canine still rotated significantly more in the heavy force group than in the light force group (P \0.001). The average rotations were 11.13 6 3.64 in the heavy force group and 0.25 6 0.71 in the light force group (Table II). The amount of anchorage loss in the heavy force group was significantly greater than in the light force group (P \0.001) during the 12 weeks. The mesial movements of the molars were 3.30 6 0.92 mm in the heavy force group and 1.85 6 0.86 mm in the light force group (Table I). The amount of anchorage loss in the heavy force group was significantly greater than in the light force
Percentage of Molar Anchorage Loss vs Total Space Closure 100.00% 80.00% 60.00%
Light Heavy
40.00% 20.00% 0.00% 0-28
28-56
56-84
Days
Fig 4. Percentage of molar anchorage loss to overall space closure.
group (P \0.05) during the 12-week period. The mesial movement of the molars was 2.28 6 0.95 mm in the heavy force group, and the light force lost 1.11 6 0.57 mm in anchorage (Table I). This equated to 45% of total space closure due to anchorage loss in the heavy force group and 38% in the light force group (Fig 4). The use of high forces also led to a diastema between the first and second molars. The rate of space closure between the heavy and light force groups was not significant from T0 to T1, indicating equal rates. After 4 weeks of tooth movement, there were significant differences between the heavy and light forces. The statistical values were P \0.05 from T1 to T2 and P \0.05 from T2 to T3. The mean amounts of space closure were 1.00 6 0.62 mm from T0 to T1, 0.92 6 0.40 mm from T1 to T2, and 0.87 6 0.35 mm from T2 toT3 in the light force group. The mean total space closures were 1.48 6 0.84 mm from T0 to T1, 1.59 6 0.65 mm from T1 to T2, and 1.87 6 1.05 mm from T2-T3 in the heavy force group (Table III). The mean rates of canine retraction in the light force group were 0.70 6 0.36 mm from T0 to T1, 0.55 6 0.28 mm from T1 to T2, and 0.57 6 0.29 mm from T2 to T3. The mean rates of canine retraction in the heavy force group were 0.89 6 0.41 mm from T0 to T1, 0.95 6 0.41 mm from T1 to T2, and 0.98 6 0.62 mm from T2-T3. The rate of space closure between heavy and light forces from canine retraction was not significant from T0 to T1. After 4 weeks of tooth movement, there were significant differences between the heavy and light
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Table III. Statistical analysis of rate of total space closure measured at 28-day intervals
Table IV.
Time interval Force
SE of Sig P the mean (2-tailed) value
Time interval Force
n
Mean
SD
0.16521 0.22447 0.10608 0.17349 0.09448 0.28004
Canine T0-T1 Canine T1-T2 Canine T2-T3 Molar T0-T1 Molar T1-T2 Molar T2-T3
14 14 14 14 14 14 14 14 14 14 14 14
0.6957 0.8864 0.5450 0.9464 0.5693 0.9814 0.3443 0.8029 0.4700 0.6793 0.3064 0.7986
0.36287 0.40993 0.28338 0.41148 0.29306 0.61984 0.24849 0.37155 0.29887 0.41052 0.21489 0.47639
T0-T1 T1-T2 T2-T3
Light Heavy Light Heavy Light Heavy
n
Mean
SD
14 14 14 14 14 14
1.0000 1.4771 0.9229 1.5850 0.8686 1.8693
0.61817 0.83988 0.39692 0.64915 0.35353 1.04781
0.099
NS
0.003
\0.05
0.002
\0.05
Sig, Significance; NS, not significant.
force groups. The statistical values were P \0.05 from T1 to T2 and P \0.05 from T2 to T3 (Table IV). The mean rates of anchorage loss, measured as molar mesialization in the light force group, were 0.34 6 0.25 mm from T0-T1, 0.47 6 0.30 mm from T1 to T2, and 0.31 6 0.21 mm from T2-T3. The mean rates of anchorage loss in the heavy force group were 0.80 6 0.37 mm from T0 to T1, 0.68 6 0.41 mm from T1 to T2, and 0.80 6 0.48 mm from T2 to T3. The amount of space closure between the heavy and light forces from molar anchorage loss was highly significant from T0 to T1, with heavy forces losing significantly more anchorage than light forces (P \0.05). There was no significant difference from T1 to T2, but T2 to T3 showed significantly more anchorage loss in the heavy force group (P \0.05) (Table IV). The differences between heavy and light forces in each subject, measured from T0 to T3, were highly statistically significant for all values measured, with heavy forces producing significantly greater total space closure, canine retraction and molar anchorage loss, and canine rotation. The differences between heavy and light forces for total space closure as measured via the cups tips intraorally and on the study models were highly significant (P \0.001); canine retraction measured intraorally was significant (P \0.01) and was highly significant (P \0.001) on the study models. Canine rotation and molar anchorage loss were significant (P 5 0.001) (Table V). Measurements on the study models were repeated on 3 times and compared with the first measurements by using paired t tests to validate the method of measurement. The results were not significant, varying from P .0.2 to P \0.8. DISCUSSION
The results indicate a highly significant difference among all variables between heavy and light orthodontic forces measured over the 12-week period, both interindi-
Statistical analysis of rates of canine retraction and molar anchorage loss at 28-day intervals
Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy
SE of Sig P the mean (2-tailed) value 0.9698 0.10956 0.07574 0.10997 0.07832 0.16566 0.06641 0.09930 0.07988 0.10972 0.05743 0.12732
0.204
NS
0.006
\0.05
0.033
\0.05
0.001
\0.05
0.135
NS
0.002
\0.05
Sig, Significance; NS, not significant.
vidually and intraindividually. Heavy forces increased the rate and the amount of canine retraction; however, the adverse effects of loss of canine rotation control and anchorage loss were concomitantly increased. Although the actual amount of total space closure was significantly greater in the heavy force group, the percentage of space closure from canine retraction was greater in the light force group (62% vs 55%). Concomitantly, the amount of anchorage loss was proportionally less in the light force group (38% vs 45%). The results tend to support the optimal force theory, showing that heavy forces lead to loss of anchorage, and light forces are ideal for canine retraction.1 The magnitude of orthodontic forces was recently shown to influence the severity of root resorption.23,24 Reitan25,26 advocated light orthodontic forces to reduce the risk of root resorption. Gonzales et al27 tested force levels of 10, 25, 50, and 100 g to mesialize molars in rats with NiTi closing coils for 3, 14, and 28 days. The effects of the different continuous forces on tooth movement and root resorption were measured on the molars. Initially, tooth movement was not proportionally related to force magnitude, but, after 14 days, significantly more tooth movement was found in the 10-, 25-, and 50-g force groups compared with the 100-g force group. A force of 10 g produced significantly more tooth movement over 28 days than did the other forces, and heavier forces caused greater root resorption. Consequently, extrapolation of these results to humans indicates that light forces produce greater tooth movement (in terms of biologic control and mechanical advantage) and less root resorption than heavier forces. Our experiment confirmed that of Gonzales et al.27 In comparing the rate of tooth movement between heavy and light forces, measured by total space closure and
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Table V.
Statistical analysis of intraindividual differences between heavy and light forces
Measurement
Force
n
Mean
SD
SE of the mean
Sig (2-tailed)
P value
Total space closure (measured via cusp tip on model)
Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy
14 14 14 14 14 14 14 14 14 14 14 14 14 14
— 2.14000 — 2.19643 — 2.40357 — 1.00786 — 21.0714 — 1.45571 — 1.16714
1.58756
0.42429
0.000
\0.001
1.44689
0.38670
0.000
\0.001
1.53291
0.40969
0.000
\0.001
1.01737
0.27190
0.003
\0.01
17.56949
4.69564
0.001
0.001
1.27742
0.34141
0.001
0.001
1.06410
0.28439
0.001
0.001
Total space closure (measured via cusp tip intra-orally) Canine retraction (measured via cusp tip on model) Canine retraction (measured via cusp tip intra-orally) Canine rotation Molar anchorage loss (measured via cusp tip on model) Molar anchorage loss (measured via cusp tip intra-orally) Sig, Significance; NS, not significant.
Rate of Tooth Movement (Total Space Closure) 2
Distance (mm)
canine retraction, the amount of initial tooth movement was not related to force magnitude. After 28 days (T1-T2), significantly more tooth movement occurred in terms of total space closure and canine retraction with heavy forces. Molar anchorage loss was not significantly different during this period. After 56 days, heavy forces caused significant molar anchorage loss and canine retraction (Figs 5-7). In interpreting these results, we can assume that, in moderate anchorage cases, heavier forces will allow greater tooth movement with approximately equal proportions of space closure from the anterior and posterior units. In maximum anchorage cases, light forces should be used to preserve the anchorage unit. For absolute anchorage with temporary anchorage devices, heavier forces seem more appropriate to increase the rate of tooth movement. However, considerations of the biocompatibility of heavier forces, rotation control, and temporary anchorage device stability are needed. Melsen and Costa28 found that the magnitude of the force was not critical in relation to the displacement of teeth when temporary anchorage devices were used. Duration of force appeared to have a significant influence on the amount of tooth displacement. The resultant treatment regimen would require a round trip in the clinical situation, because a more flexible archwire should be placed to regain the lost control. Treatment efficiency in terms of controlling biologic damage and practice management would thus be of major consequence. According to Hanson,29 opening the spring clip requires a force of 300 6 30 g applied to the labial opening hole. A lighter closing coil force might have precluded this, or palatal power arms and closing coils in addition to the buccal coils might have
1.5 Light Heavy
1 0.5 0 0-28
28-56
56-84
Days
Fig 5. Comparison of rate of total space closure between heavy and light forces.
allowed greater 3-dimensional control. However, this is not clinically practical. If a ligature wire was used to ligate the wire to the bracket instead of (or in addition to) the NiTi clip, rotation control would have been greater, but the effects of friction would have increased. This study involved several measurement regimens to overcome the inadequacies of each measuring technique. The use of the defined labial hole for opening the Speed NiTi clip was intended to find a standardized method of measuring, both intraorally and on study models. The difficulty with this technique was that it overestimated the amount of canine retraction in the heavy force group because of the loss of rotation control and also overestimated the total amount of space closure. This concern was addressed by measuring via the cusp tips; however, there are problems inherent in this technique also. Measurement of tip errors on study models or intraorally is both difficult and inaccurate. Ideally, sequential radiographs would permit measurement of tip errors, but, for ethical reasons, this was precluded.
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Rate of Canine Retraction Distance (mm)
1.2 1 0.8 Light Heavy
0.6 0.4 0.2 0 0-28
28-56
56-84
Days
measured. The results also indicated a highly significant difference between heavy and light forces. The questions that Ren et al9 proposed—whether higher stresses and strains cause greater biologic activity and thus a faster rate of tooth movement or whether an optimal force or range of forces would stimulate this process—appear to be both valid in response to providing the maximum rate of tooth movement without tissue damage and with maximum patient comfort.
Fig 6. Comparison of rate of canine retraction between heavy and light forces. CONCLUSIONS
Distance (mm)
1
Interpretation of these results suggests 3 important findings.
Rate of Molar Movement (Anchorage Loss)
0.8 0.6
Light Heavy
0.4 0.2
1.
2.
0 0-28
28-56
56-84
Days
Fig 7. Comparison of rate of molar anchorage loss between heavy and light forces.
Any tipping of the associated teeth would overestimate tooth movements; however, an attempt was made to reduce this with power arms delivering the forces from the closing coils as close to the centers of resistance as clinically possible. Despite this, tipping of the associated teeth would have occurred. Another concern was the subjectivity in measuring these reference points clinically. This was combated by measurements made on the study models, since the cusp tips on the initial templates were superimposed on the remaining study model templates to ensure some consistency. The high correlation coefficients of these measuring regimens indicated consistent results. Although we used NiTi closing coils with specific force levels, the actual targeted forces applied to the teeth were not necessarily the actual force magnitudes delivered under clinical conditions because of variations in product manufacturing and oral temperature.30,31 In addition, the forces applied to the teeth might not have been directly translated to the periodontium, because of the effects of friction and occlusal forces. The actual magnitude of the force might be irrelevant, but the fundamental considerations are the local stresses and strains experienced by the cells in the supporting periodontium. To rule out the confounding effects on the rate and amount of tooth movement (eg, patient factors and material properties), intraindividual differences were
3.
Initial tooth movement benefits from light forces, since there was no statistically significant difference between heavy and light forces. The use of heavy forces tends to increase the rate and the amount of canine retraction, but heavy force loses its advantage because of clinical side effects, such as increased anchorage loss and loss of canine rotation control. Moderate anchorage cases could benefit from heavier forces to increase the rate of space closure (within the limits of biocompatibility), whereas maximum anchorage cases would benefit from light forces.
We thank the Australian Society of Orthodontists Foundation for Research and Education, the Australian Dental Research Foundation, and Dentaurum (Ispringen, Germany) for their sponsorship of this research and Gang Shen for his assistance with this manuscript. REFERENCES 1. Storey E, Smith RE. Force in orthodontics and its relation to tooth movement. Aust Dent J 1952;56:11-3. 2. Quinn RS, Yoshikawa DK. A reassessment of force magnitude in orthodontics. Am J Orthod 1985;88:252-60. 3. Oppenheim A. A possibility for physiologic tooth movement. Am J Orthod 1944;30:277-328. 4. Andreasen G, Johnson P. Experimental findings on tooth movements under two conditions of applied force. Angle Orthod 1967;37:9-12. 5. Hixon EH, Atikan H, Callow GE, McDonald HW, Tracy RJ. Optimal force, differential force, and anchorage. Am J Orthod 1969; 55:437-57. 6. Boester CH, Johnston LE. A clinical investigation of the concepts of differential and optimal force in canine retraction. Angle Orthod 1974;44:113-9. 7. Nikolai R. On optimum orthodontic force theory as applied to canine retraction. Am J Orthod 1975;68:290-302. 8. Burstone C. The biophysics of bone remodeling during orthodontics—optimal force considerations. In: Norton LA, Burstone CJ, editors. The biology of tooth movement. Boca Raton, Fla: CRC Press; 1986. p. 321-33.
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9. Ren Y, Maltha J, Kuijpers-Jagtman AM. Optimum force magnitude for orthodontic tooth movement: a systematic literature review. Angle Orthod 2003;73:86-92. 10. Ren Y, Maltha JC, Van’t Hof M, Kuijpers-Jagtman AM. Optimum force magnitude for orthodontic tooth movement: a mathematic model. Am J Orthod Dentofacial Orthop 2004; 125:71-7. 11. von Bohl M, Maltha J, Von Den Hoff JW, KuijpersJagtman AM. Focal hyalinization during experimental tooth movement in beagle dogs. Am J Orthod Dentofacial Orthop 2004;125:615-23. 12. von Bohl M, Maltha J, Von Den Hoff JW, Kuijpers-Jagtman AM. Changes in the periodontal ligament after experimental tooth movement using high and low continuous forces in beagle dogs. Angle Orthod 2004;74:16-25. 13. Melsen B. Biological reaction of alveolar bone to orthodontic tooth movement. Angle Orthod 1999;69:151-8. 14. Proffit W. Contemporary orthodontics. St Louis: Mosby Year Book; 1999. 15. Burstone CJ, Baldwin JJ, Lawless DT. The application of continuous forces in orthodontics. Angle Orthod 1961;31:1-14. 16. Tanne K, Shibaguchi T, Terada Y, Kato J, Sakuda M. Stress levels in the PDL and biological tooth movement. In: Carlson DS, Goldstein SA, editors. Bone biodynamics in orthodontic and orthopedic treatment. Monograph 27. Craniofacial Growth Series. Ann Arbor: Center for Human Growth and Development; University of Michigan; 1992. p. 201-9. 17. Miura F, Mogi M, Ohura Y, Karibe M. The super-elastic Japanes NiTi alloy wire for use in orthodontics. Part III. Studies on the Japanese NiTi alloy coil springs. Am J Orthod Dentofacial Orthop 1988;94:89-96. 18. Hasler R, Schmid G, Ingervall B, Gehbauer U. A clinical comparison of the rate of maxillary canine retraction into healed and recent extraction sites—a pilot study. Eur J Orthod 1997;19:711-9. 19. van der Linden F. Changes in the position of posterior teeth in relation to rugae points. Am J Orthod 1978;74:142-61.
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20. Bailey LJ, Esmailnejad A, Almeida MA. Stability of the palatal rugae as landmarks for analysis of dental casts in extraction and non extraction cases. Angle Orthod 1996;66:73-8. 21. Almeida MA, Phillips C, Kula K, Tulloch C. Stability of the palatal rugae as landmarks for analysis of dental casts. Angle Orthod 1995;65:43-8. 22. Hoggan BR, Sadowsky C. The use of palatal rugae for the assessment of anteroposterior tooth movements. Am J Orthod Dentofacial Orthop 2001;119:482-8. 23. Chan E, Darendeliler MA. Physical properties of root cementum: part 5. Volumetric analysis of root resorption craters after application of light and heavy orthodontic forces. Am J Orthod Dentofacial Orthop 2005;127:186-95. 24. Darendeliler MA, Kharbanda OP, Chan EK, Srivicharnkul P, Rex T, Swain MV, Jones AS, Petocz P. Root resorption and its association with alterations in physical properties, mineral contents and resorption craters in human premolars following application of light and heavy controlled orthodontic forces. Orthod Craniofac Res 2004;7:79-97. 25. Reitan K. Effects of force magnitude and direction of tooth movement on different alveolar bone types. Angle Orthod 1964;34: 244-55. 26. Reitan K. Initial tissue behavior during apical root resorption. Angle Orthod 1974;44:68-82. 27. Gonzales C, Hotokezaka H, Yoshimatsu M, Yozgatian JH, Darendeliler MA, Yoshida N. Force magnitude and duration effects on amount of tooth movement and root resorption in the rat molar. Angle Orthod 2008;78:502-9. 28. Melsen B, Costa A. Immediate loading of implants used for orthodontic anchorage. Clin Orthod Res 2000;3:23-8. 29. Hanson H. Superelastic nickel titanium spring clips for the SPEED appliance. J Clin Orthod 2002;36:520-3. 30. Melsen B, Topp LF, Melsen HM, Terp S. Force system developed from closed coil springs. Eur J Orthod 1994;16:531-9. 31. Tripolt H, Burstone CJ, Bantleon P, Manschiebel W. Force characteristics of nicket titanium tension coil springs. Am J Orthod Dentofacial Orthop 1999;115:498-507.
Rate of tooth movement under heavy and light continuous orthodontic forces Jason A. Yee,a Tamer Tu¨rk,b Selma Elekdag˘-Tu¨rk,c Lam L. Cheng,d and M. Ali Darendelilere Sydney, Australia, and Samsun, Turkey Introduction: The aim of this study was to measure the rate and the amount of orthodontically induced tooth movement under heavy (300 g) and light (50 g) continuous forces with superelastic nickel-titanium closing coils over a defined time (12 weeks). Methods: Fourteen patients who required maxillary canine retraction into first premolar extraction sites as part of their orthodontic treatment completed this study. In a split-mouth design, precalibrated nickel-titanium closing coil springs delivering a force of 300 g or 50 g were used to distalize the canines after an alignment and stabilization period. Intraoral and maxillary cast measurements were made at the beginning of canine retraction (T0) and every 28 days for 84 days (T1, T2, T3) to assess total space closure, canine retraction rate, canine retraction and molar anchorage loss, and canine rotation. Results: Statistical analysis showed that the amount of initial tooth movement (T0-T1) was not related to force magnitude; however, during the T1-T2 and T2-T3 periods, increased amounts and higher rates of tooth movement were found with heavy forces. These significantly increased the rate and the amount of canine retraction, but the adverse effects of loss of canine rotation control and anchorage were concomitantly increased. Light forces provided a greater percentage of canine retraction than heavy forces, with less strain on anchorage. Conclusions: Initial tooth movement would benefit from light forces. Heavier forces tend to increase the rate and the amount of canine retraction but lose their advantage because of unwanted clinical side effects. (Am J Orthod Dentofacial Orthop 2009;136:150.e1-150.e9)
T
he general premise of orthodontic treatment is that, because of root surface differences, there is an ideal force that moves each type of tooth at an optimal rate. Storey and Smith1 reported the ‘‘optimal force’’ theory and documented that forces of 150 to 200 g applied to maxillary canines would produce the maximum rate of tooth movement for distalization. Various hypotheses have been promoted on stress magnitude and the rate of tooth movement; however, the concepts of threshold, light, heavy, and optimal forces are still unclear.1-12 There is ambiguity due to the inability to estimate stress distribution in the periodontal ligament, lack of experimental control of the type of tooth movement (tipping and translation), and individual heterogeneity of response.9
a Former postgraduate student, Discipline of Orthodontics, Faculty of Dentistry, University of Sydney, Sydney, Australia. b Professor, Department of Orthodontics, Faculty of Dentistry, University of Ondokuz Mayis, Samsun, Turkey. c Assistant professor, Department of Orthodontics, Faculty of Dentistry, University of Ondokuz Mayis, Samsun, Turkey. d Lecturer, Discipline of Orthodontics, Faculty of Dentistry, University of Sydney, Sydney, Australia. e Professor and chair, Discipline of Orthodontics, Faculty of Dentistry, University of Sydney, Sydney, Australia. The authors report no commercial, proprietary, or financial interest in the products or companies described in this article. Reprint requests to: M. Ali Darendeliler, Discipline of Orthodontics, Faculty of Dentistry, University of Sydney, Level 2, 2 Chalmers St, Surry Hills NSW 2010 Australia; e-mail, [email protected]. Submitted, January 2008; revised and accepted, June 2008. 0889-5406/$36.00 Copyright Ó 2009 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2008.06.027
According to Quinn and Yoshikawa,2 most clinical approaches to move teeth are based on the assumption of a force magnitude or range of magnitudes that, when delivered to the periodontal tissues, yields the most rapid rate of tooth movement. That is, the rate of tooth movement is sensitive to changes in force magnitude, and, for a particular tooth, there is an ideal force that will move it at the maximum rate. Ren et al10 developed a mathematic model to describe the relationship between force magnitude and rate of tooth movement. No force threshold could be defined that could switch tooth movement on, nor could an optimal force or force range be calculated to produce maximum tooth movement. The model indicated a wide range of forces, all of which led to maximum tooth movement. Since minute forces can initiate tooth movement, it was hypothesized that higher forces would actually overload the periodontium. In addition, Von Bohl et al11 found that high forces did not move teeth faster than low forces, but teeth with higher forces had more areas of hyalinization. The appearance of hyalinized tissue was related to force magnitude, but this had no influence on tooth movement. Melsen13 suggested that there is no relationship between force magnitude, type of tooth movement, and amount of displacement. She hypothesized that ‘‘indirect resorption at the pressure side is not a reaction to force but an attempt to remove ischemic bone lying adjacent to hyalinized tissue. The subsequent direct bone 150.e1
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resorption could be considered part of the remodeling process.’’ Tooth movement from this point of view was thus related to the distribution of stress and strain in the periodontal ligament and the remodeling response of the adjacent bone. Ren et al9 asked whether higher stresses and strains result in greater biologic activity and thus a faster rate of tooth movement, or whether an optimal force or range of forces would stimulate this process. Proffit14 believes that ‘‘the current concept of optimal force is based on the hypothesis that a force of a certain magnitude and temporal characteristics (continuous versus intermittent, constant versus declining) would be capable of producing a maximum rate of tooth movement without tissue damage and with maximum patient comfort.’’ Several variables—eg, force magnitude, direction, distribution and duration, initial tooth displacement, stress, strain, and biologic changes of the periodontium—thus must be considered to obtain optimal tooth movements.1,15,16 With the advent of superelastic nickel-titanium (NiTi) closing coils, their force duration, magnitude, continuity, and constancy are as close to ideal as clinically possible.17 According to Burstone,8 force constancy and continuity are vital in an optimal orthodontic force system. However, there is no consistency or agreement regarding optimal orthodontic force level. The aim of this study, therefore, was to measure the rate and the amount of orthodontically induced tooth movement under heavy (300 g) and light (50 g) continuous forces with superelastic NiTi closing coils over 12 weeks. MATERIAL AND METHODS
A total of 14 healthy teenagers (5 boys, 9 girls; mean age, 15.8 years; range, 13.0-19.5 years) who required bilateral extraction of the maxillary first premolars and retraction of the maxillary canines during their fixed orthodontic appliance treatment completed the study. All subjects and their parents or guardians consented to participation after receiving verbal and written explanations (ethics approval: SSWAHS X06-0062 and EK: 358). In selecting the patients, we used the following criteria: (1) need for bilateral maxillary first premolar extractions (necessitating moderate anchorage requirements) and fixed appliance orthodontic treatment; (2) similar minimal crowding on each side of the maxillary arch; (3) no previous orthodontic or orthopedic treatment; (4) no craniofacial anomalies; (5) no previous reported or observed dental treatment of the maxillary canines; (6) no history of trauma, bruxism, or parafunction; (7) no past or present signs and symptoms of periodontal
American Journal of Orthodontics and Dentofacial Orthopedics August 2009
disease; and (8) no significant medical history or medication that would adversely affect the development or the structure of the teeth and jaws and any subsequent tooth movement. At the start of treatment, standard orthodontic records were taken, including extraoral and intraoral photographs, alginate impressions (Dentalfarm Australia Proprietary, Sydney, Australia), and lateral cephalometric radiographs, and panoramic radiographs. All patients were fitted with maxillary and mandibular 0.022 3 0.028-in slot Speed (Strite Industries, Cambridge, Ontario, Canada) preadjusted (Roth prescription) self-ligated edgewise appliances. Self-ligation was used to allow for standardized ligation during canine retraction, thereby regulating the effects of friction. After extraction of the maxillary first premolars, initial alignment was obtained with a 0.014- or 0.016in NiTi wire (3 M Unitek, Monrovia, Calif), taking an average of 1.9 months (range, 0.5-6 months). Then, a 0.019 3 0.025-in beta-titanium-molybdenum alloy (TMA, 3 M Unitek) archwire was left in situ for 8 weeks to obtain standardized first-, second-, and third-order prescriptions for the experimental teeth. This consolidation period enabled full archwire passivity before retraction of the maxillary canines and also allowed a minimum of 3 months after removal of the maxillary first premolars for alveolar bone consolidation at the extraction sites.18 Concurrently, it permitted equal amounts of bone formation adjacent to the canines before the experimental period, ensuring that the quality of bone around the canine roots was of similar density. The distal retraction of the maxillary canines was performed on a continuous 0.020-in stainless steel archwire (Dentaurum, Ispringen, Germany) to reduce frictional effects. In the split-mouth design, each patient received a precalibrated 3-mm superelastic NiTi closing coil spring (GAC International, Bohemia, NY) elongated 200% to 500% of its activation, delivering an approximate force of 50 g (light) or 300 g (heavy), respectively.17 The coil springs were attached to the maxillary first molar tube (Speed) and the canine bracket (Speed) on each side with a 6-mm power arm constructed of 0.016 3 0.016-in stainless steel (Dentaurum) placed into the auxiliary tubes of the respective brackets. This was to deliver the force as close to the centers of resistance of the respective teeth as clinically possible, with the force vector parallel to the main archwire. No reactivation of the closing coils was needed (Fig 1). To prevent prejudice, the amount of force on each side of the mouth was randomized, and a minimum of 3 mm of canine retraction space was necessary to make the study feasible.
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Fig 1. Speed brackets with NiTi closing coils applying a heavy distalizing force (300 g) on the right canine and a light distalizing force (50 g) on the left canine.
To ascertain the clinical viability and reliability of the constant and continuous forces from the superelastic NiTi closing coils, a sample of 10 light (50 g) and 10 heavy (300 g) coils were tested with an Autograph Calibrator AG-E (Shimadzu, Kyoto, Japan) at room temperature (25 C) and dry air conditions. The amounts of activation in the study were within the manufacturer’s recommended guidelines for clinical use. Before distal canine retraction, intraoral measurements were made with digital dental calipers (Masel, Bristol, United Kingdom), and a baseline maxillary arch impression with heavy and light polyvinyl siloxane material (Imprint II Regular Body/Light Body, 3M ESPE, St Paul, Minn) was taken. Two clinical measurements of total space closure were made. First, they were made bilaterally from the corresponding brackets of the canine to the molar, with the standardized small circular hole located on the Speed NiTi clips used for opening the clip and also with the corresponding cusp tips of the canine and the mesiobuccal cusp of the first molar. All clinical measurements were made by the same author (S.E.T.). During canine retraction, the subjects were recalled every 14 days for clinical measurements and at 28-day intervals to take maxillary impressions with the polyvinyl siloxane material. At each appointment, oral hygiene was reinforced, and the appliances were assessed for damage. As a quality-control measure, if a bracket, archwire, or spring involved in canine retraction was damaged, or if the canine attained complete retraction before the end of the study period, the patient was excluded from the study. Two patients were excluded for these reasons, making the total sample size 14. The study period extended from the beginning of canine retraction for 12 weeks. Measurements were made from the orthodontic models taken every 28 days from the beginning of canine retraction. Consequently, with the clinical measurements, 4 study models per patient were available for assessment and measurement. Stable palatal reference points and the tips of the canines and the cusp
Fig 2. Stable reference structures.
tips of the first molars (Fig 2) were marked with a 0.3mm lead pencil on all casts.19-22 The models were then scanned (Expression 1600 flatbed color image scanner, Epson Australia Proprietary, Chatswood, Australia) at 600 dpi and 16-bit gray scale from an occlusal perspective; the resulting images were printed and linear measurements made. The initial casts for each patient were used as the baseline model, with an acetate template for linear and angular measurement constructed according to the stable reference points (medial and lateral ends of the third palatal rugae) and superimposed on the subsequent cast images for measurement of canine and molar movement, and canine rotation. All measurements on study models were repeated by the same operator (J.A.Y.) to ensure intraexaminer reliability and eliminate measurement error. Measurements at each 28-day time (T0, T1, T2, T3) were total space closure via measurement along the midpoint of the labial surface of the Speed brackets approximating the spring opening hole; total space closure via measurement along the cusp tips of the canine
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Statistical analysis of space closure (mm), canine retraction (mm), canine rotation ( ), and molar anchorage loss (mm) in heavy vs light forces (independent 2-sample t test)
Table I.
Measurement
Force
n
Mean
SD
SE of the mean
Sig (2-tailed)
P value
Total space closure (measured via bracket on model)
Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy
14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14
3.5136 7.3350 3.4821 7.4329 2.79114 4.9314 2.8929 5.0893 1.6986 4.1021 1.8064 2.8143 1.29 22.36 1.8464 3.3021 1.1136 2.2807
1.29893 1.89554 1.26284 1.97015 0.89481 1.61320 0.98293 1.46792 0.58362 1.68011 0.61004 0.99696 2.673 18.587 0.85881 0.91649 0.56644 0.95267
0.34715 0.50660 0.33751 0.52654 0.23915 0.43115 0.26270 0.39232 0.15598 0.44903 0.16304 0.26645 0.714 4.968 0.22953 0.24494 0.15139 0.25461
0.000
\0.001
0.000
\0.001
0.000
\0.001
0.000
\0.001
0.000
\0.001
0.003
\0.05
0.000
\0.001
0.000
\0.001
0.001
\0.05
Total space closure (measured via bracket intraorally) Total space closure (measured via cusp tip on model) Total space closure (measured via cusp tip intra-orally) Canine retraction (measured via bracket on model) Canine retraction (measured via cusp tip on model) Canine rotation Molar anchorage loss (measured via bracket on model) Molar anchorage loss (measured via cusp tip on model) Sig, Significance.
and the mesiobuccal cusp of the first molar; amount and rate of canine retraction; and amount and rate of molar anchorage loss. Distobuccal rotation of the canines, from the angle formed between a line connecting the contact points of the mesial and distal surfaces of the canines from the superimposed T0 to T3 templates, was also measured to ascertain the effect of different force levels on rotational moments. Statistical analysis was undertaken with the Statistical Package for Social Sciences (SPSS for Windows, version 12, SPSS, Chicago, Ill). The average interindividual rates and amounts of total space closure via the bracket and cusp tips, canine retraction, and anchorage loss (in millimeters) from the light and heavy closing coils were calculated for the treatment period with Student t tests. Paired t tests were also used to compare intraindividual differences between heavy and light forces. Calculations of the total amount of space closure as measured via the cusp tips, canine retraction, anchorage loss (in millimeters), and canine rotation were made. Measurements on the study models were repeated on 3 occasions and compared with the original measurements by using paired t tests to validate the method of measurement. RESULTS
The difference in total space closure between heavy and light forces was highly significant (P \0.001) between the brackets as measured on the models. The light force group averaged 3.51 6 1.30 mm of space closure,
and the heavy force group averaged 7.34 6 1.90 mm over the 12-week period (Table I). The difference in total space closure between the 2 force groups was highly significant (P \0.001) between the brackets as measured intraorally and was similar to the measurements made on the study models. The light force group averaged 3.48 6 1.26 mm of space closure, and the heavy force group averaged 7.43 6 1.97 mm over the 12-week period (Table I). The difference in total space closure between the 2 force groups was highly significant (P \0.001) between the cusp tips of the canine and mesiobuccal cusp of the first molar as measured on the study models. The mean amounts of space closure were 2.79 6 0.89 mm in the light force group and 4.93 6 1.61 mm in the heavy group over the 12 weeks (Table I). The difference in total space closure between the 2 force groups was highly significant (P \0.001) between the cusp tips of the canine and the mesiobuccal cusp of the first molar measured intraorally. The mean amounts of space closure were 2.89 6 0.98 mm in the light force group and 5.09 6 1.47 mm in the heavy group over the 12 weeks (Table I). The correlation between measurements on study models to those made intraorally both via the bracket and cups tips was high, as measured by the Pearson correlation coefficient. They were all significant at the 0.01 level. The amount of canine retraction was significantly greater in the heavy force group than in the light force group (P \0.001) during the 12-week period. The
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Statistical analysis of heavy vs light forces for canine rotation (independent 2-sample t test)
Table II.
Percentage of Canine Retraction vs Total Space Closure
Percentage
100.00% 80.00% 60.00%
Light Heavy
40.00%
Measurement
SE of Sig P Force n Mean SD the mean (2-tailed) value
Canine rotation Light 8 0.25 0.71 Heavy 8 11.13 3.64
20.00%
0.25 1.29
0.000 0.000
\0.001
0.00% 0-28
28-56
56-84
Sig, Significance.
Days
Fig 3. Percentage of canine retraction to overall space closure. Percentage
canines retracted on average 4.10 6 1.68 mm in the heavy force group and 1.70 6 0.58 mm in the light force group (Table I). The amount of canine retraction was significantly greater in the heavy force group than the light force group as measured along the cusp tips (P \0.05) during the 12 weeks. The canines retracted on average 2.81 6 1.00 mm in the heavy force group and 1.81 6 0.61 mm in the light force group (Table I). The percentages of canine retraction to overall space closure were 55% in the heavy force group and 62% in the light force group (Fig 3). Canine rotated significantly greater in the heavy force group than in the light force group (P \0.001) during the 12-week period. The average canine rotations were 22.36 6 18.59 in the heavy force group and 1.29 6 2.67 in the light force group. The greater rotations often occurred because the heavy force overcame the retentive lock of the NiTi Speed clip, causing the archwire to ‘‘pop out’’ of the bracket (6 of 14 patients). This led to an uncontrolled distobuccal rotation moment of the affected canine and might have contributed to the greater ratio of canine retraction of the heavy force group compared with the light force group when measuring the distances between the brackets (Table I). The excessive rotation represented failure of the Speed clip, caused by the heavy forces. Consequently, the differences in canine rotation of the remaining 8 patients were measured. The canine still rotated significantly more in the heavy force group than in the light force group (P \0.001). The average rotations were 11.13 6 3.64 in the heavy force group and 0.25 6 0.71 in the light force group (Table II). The amount of anchorage loss in the heavy force group was significantly greater than in the light force group (P \0.001) during the 12 weeks. The mesial movements of the molars were 3.30 6 0.92 mm in the heavy force group and 1.85 6 0.86 mm in the light force group (Table I). The amount of anchorage loss in the heavy force group was significantly greater than in the light force
Percentage of Molar Anchorage Loss vs Total Space Closure 100.00% 80.00% 60.00%
Light Heavy
40.00% 20.00% 0.00% 0-28
28-56
56-84
Days
Fig 4. Percentage of molar anchorage loss to overall space closure.
group (P \0.05) during the 12-week period. The mesial movement of the molars was 2.28 6 0.95 mm in the heavy force group, and the light force lost 1.11 6 0.57 mm in anchorage (Table I). This equated to 45% of total space closure due to anchorage loss in the heavy force group and 38% in the light force group (Fig 4). The use of high forces also led to a diastema between the first and second molars. The rate of space closure between the heavy and light force groups was not significant from T0 to T1, indicating equal rates. After 4 weeks of tooth movement, there were significant differences between the heavy and light forces. The statistical values were P \0.05 from T1 to T2 and P \0.05 from T2 to T3. The mean amounts of space closure were 1.00 6 0.62 mm from T0 to T1, 0.92 6 0.40 mm from T1 to T2, and 0.87 6 0.35 mm from T2 toT3 in the light force group. The mean total space closures were 1.48 6 0.84 mm from T0 to T1, 1.59 6 0.65 mm from T1 to T2, and 1.87 6 1.05 mm from T2-T3 in the heavy force group (Table III). The mean rates of canine retraction in the light force group were 0.70 6 0.36 mm from T0 to T1, 0.55 6 0.28 mm from T1 to T2, and 0.57 6 0.29 mm from T2 to T3. The mean rates of canine retraction in the heavy force group were 0.89 6 0.41 mm from T0 to T1, 0.95 6 0.41 mm from T1 to T2, and 0.98 6 0.62 mm from T2-T3. The rate of space closure between heavy and light forces from canine retraction was not significant from T0 to T1. After 4 weeks of tooth movement, there were significant differences between the heavy and light
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Table III. Statistical analysis of rate of total space closure measured at 28-day intervals
Table IV.
Time interval Force
SE of Sig P the mean (2-tailed) value
Time interval Force
n
Mean
SD
0.16521 0.22447 0.10608 0.17349 0.09448 0.28004
Canine T0-T1 Canine T1-T2 Canine T2-T3 Molar T0-T1 Molar T1-T2 Molar T2-T3
14 14 14 14 14 14 14 14 14 14 14 14
0.6957 0.8864 0.5450 0.9464 0.5693 0.9814 0.3443 0.8029 0.4700 0.6793 0.3064 0.7986
0.36287 0.40993 0.28338 0.41148 0.29306 0.61984 0.24849 0.37155 0.29887 0.41052 0.21489 0.47639
T0-T1 T1-T2 T2-T3
Light Heavy Light Heavy Light Heavy
n
Mean
SD
14 14 14 14 14 14
1.0000 1.4771 0.9229 1.5850 0.8686 1.8693
0.61817 0.83988 0.39692 0.64915 0.35353 1.04781
0.099
NS
0.003
\0.05
0.002
\0.05
Sig, Significance; NS, not significant.
force groups. The statistical values were P \0.05 from T1 to T2 and P \0.05 from T2 to T3 (Table IV). The mean rates of anchorage loss, measured as molar mesialization in the light force group, were 0.34 6 0.25 mm from T0-T1, 0.47 6 0.30 mm from T1 to T2, and 0.31 6 0.21 mm from T2-T3. The mean rates of anchorage loss in the heavy force group were 0.80 6 0.37 mm from T0 to T1, 0.68 6 0.41 mm from T1 to T2, and 0.80 6 0.48 mm from T2 to T3. The amount of space closure between the heavy and light forces from molar anchorage loss was highly significant from T0 to T1, with heavy forces losing significantly more anchorage than light forces (P \0.05). There was no significant difference from T1 to T2, but T2 to T3 showed significantly more anchorage loss in the heavy force group (P \0.05) (Table IV). The differences between heavy and light forces in each subject, measured from T0 to T3, were highly statistically significant for all values measured, with heavy forces producing significantly greater total space closure, canine retraction and molar anchorage loss, and canine rotation. The differences between heavy and light forces for total space closure as measured via the cups tips intraorally and on the study models were highly significant (P \0.001); canine retraction measured intraorally was significant (P \0.01) and was highly significant (P \0.001) on the study models. Canine rotation and molar anchorage loss were significant (P 5 0.001) (Table V). Measurements on the study models were repeated on 3 times and compared with the first measurements by using paired t tests to validate the method of measurement. The results were not significant, varying from P .0.2 to P \0.8. DISCUSSION
The results indicate a highly significant difference among all variables between heavy and light orthodontic forces measured over the 12-week period, both interindi-
Statistical analysis of rates of canine retraction and molar anchorage loss at 28-day intervals
Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy
SE of Sig P the mean (2-tailed) value 0.9698 0.10956 0.07574 0.10997 0.07832 0.16566 0.06641 0.09930 0.07988 0.10972 0.05743 0.12732
0.204
NS
0.006
\0.05
0.033
\0.05
0.001
\0.05
0.135
NS
0.002
\0.05
Sig, Significance; NS, not significant.
vidually and intraindividually. Heavy forces increased the rate and the amount of canine retraction; however, the adverse effects of loss of canine rotation control and anchorage loss were concomitantly increased. Although the actual amount of total space closure was significantly greater in the heavy force group, the percentage of space closure from canine retraction was greater in the light force group (62% vs 55%). Concomitantly, the amount of anchorage loss was proportionally less in the light force group (38% vs 45%). The results tend to support the optimal force theory, showing that heavy forces lead to loss of anchorage, and light forces are ideal for canine retraction.1 The magnitude of orthodontic forces was recently shown to influence the severity of root resorption.23,24 Reitan25,26 advocated light orthodontic forces to reduce the risk of root resorption. Gonzales et al27 tested force levels of 10, 25, 50, and 100 g to mesialize molars in rats with NiTi closing coils for 3, 14, and 28 days. The effects of the different continuous forces on tooth movement and root resorption were measured on the molars. Initially, tooth movement was not proportionally related to force magnitude, but, after 14 days, significantly more tooth movement was found in the 10-, 25-, and 50-g force groups compared with the 100-g force group. A force of 10 g produced significantly more tooth movement over 28 days than did the other forces, and heavier forces caused greater root resorption. Consequently, extrapolation of these results to humans indicates that light forces produce greater tooth movement (in terms of biologic control and mechanical advantage) and less root resorption than heavier forces. Our experiment confirmed that of Gonzales et al.27 In comparing the rate of tooth movement between heavy and light forces, measured by total space closure and
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Table V.
Statistical analysis of intraindividual differences between heavy and light forces
Measurement
Force
n
Mean
SD
SE of the mean
Sig (2-tailed)
P value
Total space closure (measured via cusp tip on model)
Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy Light Heavy
14 14 14 14 14 14 14 14 14 14 14 14 14 14
— 2.14000 — 2.19643 — 2.40357 — 1.00786 — 21.0714 — 1.45571 — 1.16714
1.58756
0.42429
0.000
\0.001
1.44689
0.38670
0.000
\0.001
1.53291
0.40969
0.000
\0.001
1.01737
0.27190
0.003
\0.01
17.56949
4.69564
0.001
0.001
1.27742
0.34141
0.001
0.001
1.06410
0.28439
0.001
0.001
Total space closure (measured via cusp tip intra-orally) Canine retraction (measured via cusp tip on model) Canine retraction (measured via cusp tip intra-orally) Canine rotation Molar anchorage loss (measured via cusp tip on model) Molar anchorage loss (measured via cusp tip intra-orally) Sig, Significance; NS, not significant.
Rate of Tooth Movement (Total Space Closure) 2
Distance (mm)
canine retraction, the amount of initial tooth movement was not related to force magnitude. After 28 days (T1-T2), significantly more tooth movement occurred in terms of total space closure and canine retraction with heavy forces. Molar anchorage loss was not significantly different during this period. After 56 days, heavy forces caused significant molar anchorage loss and canine retraction (Figs 5-7). In interpreting these results, we can assume that, in moderate anchorage cases, heavier forces will allow greater tooth movement with approximately equal proportions of space closure from the anterior and posterior units. In maximum anchorage cases, light forces should be used to preserve the anchorage unit. For absolute anchorage with temporary anchorage devices, heavier forces seem more appropriate to increase the rate of tooth movement. However, considerations of the biocompatibility of heavier forces, rotation control, and temporary anchorage device stability are needed. Melsen and Costa28 found that the magnitude of the force was not critical in relation to the displacement of teeth when temporary anchorage devices were used. Duration of force appeared to have a significant influence on the amount of tooth displacement. The resultant treatment regimen would require a round trip in the clinical situation, because a more flexible archwire should be placed to regain the lost control. Treatment efficiency in terms of controlling biologic damage and practice management would thus be of major consequence. According to Hanson,29 opening the spring clip requires a force of 300 6 30 g applied to the labial opening hole. A lighter closing coil force might have precluded this, or palatal power arms and closing coils in addition to the buccal coils might have
1.5 Light Heavy
1 0.5 0 0-28
28-56
56-84
Days
Fig 5. Comparison of rate of total space closure between heavy and light forces.
allowed greater 3-dimensional control. However, this is not clinically practical. If a ligature wire was used to ligate the wire to the bracket instead of (or in addition to) the NiTi clip, rotation control would have been greater, but the effects of friction would have increased. This study involved several measurement regimens to overcome the inadequacies of each measuring technique. The use of the defined labial hole for opening the Speed NiTi clip was intended to find a standardized method of measuring, both intraorally and on study models. The difficulty with this technique was that it overestimated the amount of canine retraction in the heavy force group because of the loss of rotation control and also overestimated the total amount of space closure. This concern was addressed by measuring via the cusp tips; however, there are problems inherent in this technique also. Measurement of tip errors on study models or intraorally is both difficult and inaccurate. Ideally, sequential radiographs would permit measurement of tip errors, but, for ethical reasons, this was precluded.
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American Journal of Orthodontics and Dentofacial Orthopedics August 2009
Rate of Canine Retraction Distance (mm)
1.2 1 0.8 Light Heavy
0.6 0.4 0.2 0 0-28
28-56
56-84
Days
measured. The results also indicated a highly significant difference between heavy and light forces. The questions that Ren et al9 proposed—whether higher stresses and strains cause greater biologic activity and thus a faster rate of tooth movement or whether an optimal force or range of forces would stimulate this process—appear to be both valid in response to providing the maximum rate of tooth movement without tissue damage and with maximum patient comfort.
Fig 6. Comparison of rate of canine retraction between heavy and light forces. CONCLUSIONS
Distance (mm)
1
Interpretation of these results suggests 3 important findings.
Rate of Molar Movement (Anchorage Loss)
0.8 0.6
Light Heavy
0.4 0.2
1.
2.
0 0-28
28-56
56-84
Days
Fig 7. Comparison of rate of molar anchorage loss between heavy and light forces.
Any tipping of the associated teeth would overestimate tooth movements; however, an attempt was made to reduce this with power arms delivering the forces from the closing coils as close to the centers of resistance as clinically possible. Despite this, tipping of the associated teeth would have occurred. Another concern was the subjectivity in measuring these reference points clinically. This was combated by measurements made on the study models, since the cusp tips on the initial templates were superimposed on the remaining study model templates to ensure some consistency. The high correlation coefficients of these measuring regimens indicated consistent results. Although we used NiTi closing coils with specific force levels, the actual targeted forces applied to the teeth were not necessarily the actual force magnitudes delivered under clinical conditions because of variations in product manufacturing and oral temperature.30,31 In addition, the forces applied to the teeth might not have been directly translated to the periodontium, because of the effects of friction and occlusal forces. The actual magnitude of the force might be irrelevant, but the fundamental considerations are the local stresses and strains experienced by the cells in the supporting periodontium. To rule out the confounding effects on the rate and amount of tooth movement (eg, patient factors and material properties), intraindividual differences were
3.
Initial tooth movement benefits from light forces, since there was no statistically significant difference between heavy and light forces. The use of heavy forces tends to increase the rate and the amount of canine retraction, but heavy force loses its advantage because of clinical side effects, such as increased anchorage loss and loss of canine rotation control. Moderate anchorage cases could benefit from heavier forces to increase the rate of space closure (within the limits of biocompatibility), whereas maximum anchorage cases would benefit from light forces.
We thank the Australian Society of Orthodontists Foundation for Research and Education, the Australian Dental Research Foundation, and Dentaurum (Ispringen, Germany) for their sponsorship of this research and Gang Shen for his assistance with this manuscript. REFERENCES 1. Storey E, Smith RE. Force in orthodontics and its relation to tooth movement. Aust Dent J 1952;56:11-3. 2. Quinn RS, Yoshikawa DK. A reassessment of force magnitude in orthodontics. Am J Orthod 1985;88:252-60. 3. Oppenheim A. A possibility for physiologic tooth movement. Am J Orthod 1944;30:277-328. 4. Andreasen G, Johnson P. Experimental findings on tooth movements under two conditions of applied force. Angle Orthod 1967;37:9-12. 5. Hixon EH, Atikan H, Callow GE, McDonald HW, Tracy RJ. Optimal force, differential force, and anchorage. Am J Orthod 1969; 55:437-57. 6. Boester CH, Johnston LE. A clinical investigation of the concepts of differential and optimal force in canine retraction. Angle Orthod 1974;44:113-9. 7. Nikolai R. On optimum orthodontic force theory as applied to canine retraction. Am J Orthod 1975;68:290-302. 8. Burstone C. The biophysics of bone remodeling during orthodontics—optimal force considerations. In: Norton LA, Burstone CJ, editors. The biology of tooth movement. Boca Raton, Fla: CRC Press; 1986. p. 321-33.
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9. Ren Y, Maltha J, Kuijpers-Jagtman AM. Optimum force magnitude for orthodontic tooth movement: a systematic literature review. Angle Orthod 2003;73:86-92. 10. Ren Y, Maltha JC, Van’t Hof M, Kuijpers-Jagtman AM. Optimum force magnitude for orthodontic tooth movement: a mathematic model. Am J Orthod Dentofacial Orthop 2004; 125:71-7. 11. von Bohl M, Maltha J, Von Den Hoff JW, KuijpersJagtman AM. Focal hyalinization during experimental tooth movement in beagle dogs. Am J Orthod Dentofacial Orthop 2004;125:615-23. 12. von Bohl M, Maltha J, Von Den Hoff JW, Kuijpers-Jagtman AM. Changes in the periodontal ligament after experimental tooth movement using high and low continuous forces in beagle dogs. Angle Orthod 2004;74:16-25. 13. Melsen B. Biological reaction of alveolar bone to orthodontic tooth movement. Angle Orthod 1999;69:151-8. 14. Proffit W. Contemporary orthodontics. St Louis: Mosby Year Book; 1999. 15. Burstone CJ, Baldwin JJ, Lawless DT. The application of continuous forces in orthodontics. Angle Orthod 1961;31:1-14. 16. Tanne K, Shibaguchi T, Terada Y, Kato J, Sakuda M. Stress levels in the PDL and biological tooth movement. In: Carlson DS, Goldstein SA, editors. Bone biodynamics in orthodontic and orthopedic treatment. Monograph 27. Craniofacial Growth Series. Ann Arbor: Center for Human Growth and Development; University of Michigan; 1992. p. 201-9. 17. Miura F, Mogi M, Ohura Y, Karibe M. The super-elastic Japanes NiTi alloy wire for use in orthodontics. Part III. Studies on the Japanese NiTi alloy coil springs. Am J Orthod Dentofacial Orthop 1988;94:89-96. 18. Hasler R, Schmid G, Ingervall B, Gehbauer U. A clinical comparison of the rate of maxillary canine retraction into healed and recent extraction sites—a pilot study. Eur J Orthod 1997;19:711-9. 19. van der Linden F. Changes in the position of posterior teeth in relation to rugae points. Am J Orthod 1978;74:142-61.
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20. Bailey LJ, Esmailnejad A, Almeida MA. Stability of the palatal rugae as landmarks for analysis of dental casts in extraction and non extraction cases. Angle Orthod 1996;66:73-8. 21. Almeida MA, Phillips C, Kula K, Tulloch C. Stability of the palatal rugae as landmarks for analysis of dental casts. Angle Orthod 1995;65:43-8. 22. Hoggan BR, Sadowsky C. The use of palatal rugae for the assessment of anteroposterior tooth movements. Am J Orthod Dentofacial Orthop 2001;119:482-8. 23. Chan E, Darendeliler MA. Physical properties of root cementum: part 5. Volumetric analysis of root resorption craters after application of light and heavy orthodontic forces. Am J Orthod Dentofacial Orthop 2005;127:186-95. 24. Darendeliler MA, Kharbanda OP, Chan EK, Srivicharnkul P, Rex T, Swain MV, Jones AS, Petocz P. Root resorption and its association with alterations in physical properties, mineral contents and resorption craters in human premolars following application of light and heavy controlled orthodontic forces. Orthod Craniofac Res 2004;7:79-97. 25. Reitan K. Effects of force magnitude and direction of tooth movement on different alveolar bone types. Angle Orthod 1964;34: 244-55. 26. Reitan K. Initial tissue behavior during apical root resorption. Angle Orthod 1974;44:68-82. 27. Gonzales C, Hotokezaka H, Yoshimatsu M, Yozgatian JH, Darendeliler MA, Yoshida N. Force magnitude and duration effects on amount of tooth movement and root resorption in the rat molar. Angle Orthod 2008;78:502-9. 28. Melsen B, Costa A. Immediate loading of implants used for orthodontic anchorage. Clin Orthod Res 2000;3:23-8. 29. Hanson H. Superelastic nickel titanium spring clips for the SPEED appliance. J Clin Orthod 2002;36:520-3. 30. Melsen B, Topp LF, Melsen HM, Terp S. Force system developed from closed coil springs. Eur J Orthod 1994;16:531-9. 31. Tripolt H, Burstone CJ, Bantleon P, Manschiebel W. Force characteristics of nicket titanium tension coil springs. Am J Orthod Dentofacial Orthop 1999;115:498-507.