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Comparison of direct and indirect skeletal anchorage systems combined with 2 canine retraction techniques
ORIGINAL ARTICLE
Comparison of direct and indirect skeletal anchorage systems combined with 2 canine retraction techniques Serkan Ozkana and Mehmet Bayramb Ordu and Trabzon, Turkey
Introduction: We compared the effectiveness of 2 canine retraction springs and anchorage systems (direct and indirect skeletal anchorage) in patients requiring first premolar extractions and maximum anchorage in the maxilla. Methods: Thirty-six patients were included (17 male, 19 female; mean age, 16.8 6 2.4 years). A mini-implant–supported Nance appliance with indirect skeletal anchorage system was used in 18 patients and a mini-implant–supported direct skeletal anchorage system in the remaining patients. In each patient, a segmental retraction arch with a reverse closing loop was applied to a maxillary canine, and a Ladanyi spring (Dentaurum, Ispringen, Germany) was applied to the other canine randomly after extraction of the maxillary first premolars. The retraction process was continued until a Class I canine relationship was obtained. Lateral cephalometric films and orthodontic casts taken before and after retraction in the distalization process were used to evaluate changes during canine distalization. The measurements were statistically evaluated using paired and independent t tests with 95% confidence intervals. Results: The reverse closing loop and the Ladanyi spring were found to be effective in canine distalization (P #0.001). There were no statistically significant differences between the reverse closing loop and the Ladanyi spring with regard to canine distalization rates (P $0.05). Both systems were effective in providing maximum anchorage (P $0.05); no statistically significant differences were detected in molar anchorage loss rates between the 2 methods (P $0.05). Conclusions: These 2 systems can be used during segmental distalization of canines requiring maximum anchorage with no significant anchorage loss. (Am J Orthod Dentofacial Orthop 2016;150:763-70)
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oss of anchorage is a common problem faced during maximum anchorage orthodontic treatment, and prevention of this plays a significant role in the design of the orthodontic appliances.1 For a patient with a Class II occlusion, if the treatment plan is the extraction of the premolars, the maxillary canines must be distalized completely to the extraction sites. Therefore, anchorage reinforcement of the posterior teeth must be ensured using intraoral or extraoral appliances.2 Previously, extraoral appliances were the only way to provide maximum anchorage, but their use was limited
a Assistant professor, Department of Orthodontics, Faculty of Dentistry, Ordu University, Ordu, Turkey. b Associate professor, Department of Orthodontics, Faculty of Dentistry, Karadeniz Technical University, Trabzon, Turkey. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. Address correspondence to: Serkan Ozkan, Department of Orthodontics, Faculty of Dentistry, Ordu University, 52100, Ordu, Turkey; e-mail, dtserkanozkan@ gmail.com. Submitted, November 2015; revised and accepted, April 2016. 0889-5406/$36.00 Ó 2016 by the American Association of Orthodontists. All rights reserved. http://dx.doi.org/10.1016/j.ajodo.2016.04.023
because they required patient cooperation. As a result, intraoral appliances became increasingly popular among clinicians for obtaining anchorage. However, popular intraoral appliances, such as palatal and lingual bars, Nance appliances, and intermaxillary elastics, have undesired side effects: protrusion, extrusion, and tipping.3 Anchorage problems encountered during the use of intraoral mechanics led to increased use of implant-borne mechanics. However, these had several limitations, such as excessive waiting times for osseointegration. These limitations were prevented by the introduction of orthodontic mini-implants as intraoral anchorage units. These implants were developed as an alternative to conventional molar and other intraoral anchorage units. Orthodontic mini-implants can be used either directly or indirectly. Direct anchorage refers to the movement of teeth using orthodontic mini-implants, whereas indirect anchorage refers to the stabilization of certain teeth in the dental arch and subsequent use of these stabilized anchors to move other teeth.4 Indirect skeletal anchorage systems are preferred by clinicians when the direct skeletal anchorage system is not applicable, such as in patients with proximity of the roots. 763
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In extraction patients, canine retraction can be carried out with fixed appliances using frictional or nonfrictional methods.5 The popularity of the nonfrictional method increased when Burstone6 reported its advantages. Thereafter, many canine retraction springs were introduced and tested for reliability and efficiency.7-10 The aim of this study was to compare the efficiency of direct and indirect skeletal anchorage systems combined with 2 canine retraction springs in patients requiring maximum anchorage for maxillary canine retraction. MATERIAL AND METHODS
After receiving ethical approval from the ethics council of Karadeniz Technical University (protocol number, B302KTU0200000/637), we conducted a pilot study to calculate the total sample size. Twenty patients were recruited for the pilot study at the beginning of the treatment, and a number from 1 to 20 was assigned to each patient. Using a randomization Web site, 4 columns of the random array of numbers were created. According to the 4 randomized number strings, patients were assigned to the following groups: (1) direct skeletal anchorage, Ladanyi spring (LS) (Dentaurum, Ispringen, Germany) on the left side; (2) direct skeletal anchorage, LS on the right side; (3) indirect skeletal anchorage, LS on the left side; and (4) indirect skeletal anchorage, LS on the right side. After randomization, the canine distalization method was applied to every patient according to the patient's assigned group. As the distalization finished, all cephalometric and cast materials were measured, and the power of the study was determined. Meanwhile, we continued to recruit new patients for the study. According to our pilot study, power calculations with G*Power (version 3.1.3; Franz Faul, Christian-Albrechts-Universitat, Kiel, Germany), group sample sizes of 16 achieved 82% power to detect a difference of 1.5mm between the null hypothesis that both group means were 1.7mm and the alternative hypothesis that the mean of group 2 was 0.1mm, with estimated group standard deviations of 1.2mm and 1.7mm at a significance level (alpha) of 0.05 using a 2-sided, 2sample t test. Thus, 16 additional patients were recruited to guarantee the exact power with a total of 36 patients (17 male, 19 female; mean age, 16.8 6 2.4 years). The aforementioned randomization method was applied to these patients as well. Patients receiving maximum anchorage with extraction of the maxillary first premolars were included in the study. The inclusion criteria were (1) permanent dentition, (2) good oral hygiene, (3) Class I or Class II skeletal pattern, (4) no anomaly in the transverse direction, (5) no maxillary canine with supraposition or excessive rotation, (6) no systemic disorder that contraindicated
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Fig 1. The palatal region after mini-implant insertion in the indirect skeletal anchorage group.
Fig 2. Nance appliance built for the indirect skeletal anchorage group.
orthodontic treatment, and (7) no systemic or allergic disorder that contraindicated the application of skeletal anchorage units. The maxillary first premolars were extracted to resolve the crowding in all patients. In the direct skeletal anchorage group, 2 mini-implants (Aarhus; American Orthodontics, Sheboygan, Wis; 1.5-mm diameter, 8-mm length) were inserted into the buccal aspect between the roots of the maxillary first molars and second premolars at an angle of 15 to 20 . In the indirect skeletal anchorage group, after determination of the insertion areas using cephalometric radiographs and dental casts, 2 of the same mini-implants were inserted into the paramedian section of the palatal area (Fig 1). Appropriate molar bands (3M Unitek, Monrovia, Calif) were chosen and adapted to the maxillary molars.
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Fig 3. Guide wires prepared for distinguishing the right and left sides of the maxilla on the radiographs.
Fig 4. Step bends prepared for the prevention of intrusion of the canines in the direct skeletal anchorage groups.
After the construction of banded casts of the maxilla, custom Nance appliances were built over the miniimplant sites (Fig 2). Guide wires, made from 0.021 3 0.025-in stainless steel wires (American Orthodontics), were used for assessing the movements of the teeth on cephalometric radiographs (Fig 3). In both groups, 0.022 3 0.028-in slotted Roth brackets (Gemini; 3M Unitek) were bonded to the maxillary canines before application of these wires. Guide wires with rounded ends were applied to the right canines and molars, and wires with straight ends were applied to the left canines and molars in all patients. Lateral cephalometric radiographs were taken after installation of the wires before and after the canine distalization process. Remaloy wires (Dentaurum, Ispringen, Germany), 0.016 3 0.022 in diameter, were used to prepare the reverse closing loop (RCL) retraction springs. The RCL retraction springs were given the same dimensions as the prefabricated LS to obtain standardization, and 45 antitip and 15 to 20 antirotation bends were incorporated into the springs before installation. Additional step bends were also included in the springs of
the direct skeletal anchorage groups to prevent intrusion of canines because of differences in the vertical levels of the mini-implants and the canine bracket slots (Fig 4). The application of the retraction springs was completed by activating each spring by about 2 mm to obtain 120 to 150 g of retraction force. Prefabricated LS were applied on 1 side, and RCL retractor springs were applied to the other side of the patients randomly. The retraction springs were activated at 4-week intervals, and the retraction process was continued until the canines achieved a Class I relationship. The ANS-PNS line extended from the anterior nasal spine (ANS) to the posterior nasal spine (PNS) along the maxillary plane (MaxP). Local superimpositions were carried out along this line and at the PNS point. MaxP, derived from the ANS-PNS points, was used as a horizontal reference plane, and MaxVP, derived from the vertical line extending from the pterygomaxillary fissure to the MaxP, was used as the vertical reference plane for dental changes (Fig 5, B). The mesial and distal contact points of the canines were marked on the dental casts and transferred to the computer via a scanner (Perfection V700; Epson
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Fig 5. A, Cephalometric landmarks and skeletal measurements used in this study: S, sella turcica; N, nasion; A, Point A; B, Point B; ANS, anterior nasal spine; PNS, posterior nasal spina; Me, menton; 1, SNA; 2, SNB; 3, ANB; 4, SN/ManP; 5, MaxP/ManP; 6, SN/MaxP. B, Dental measurements of the maxilla: Ptm, Pterygomaxillary fissure; 7, MaxP-U1a (angular); 8, MaxP-U1 (mm); 9, MaxVP-U1 (mm); 10, MaxP/U3a (angular); 11, MaxP-U3 (mm); 12, MaxVP-U3 (mm); 13, MaxP/U6a (angular); 14, MaxP-U6 (mm); 15, MaxVP-U6 (mm). C, Measurement used for determining the rotation of the canines on the dental casts.
America, Long Beach, Calif). Digital images were then transferred to cephalometric tracing software (Facad, trial version; Ilexis, Link€ oping, Sweden), and the amounts of predistalization to postdistalization rotation of the canines were measured (Fig 5, C).
Statistical analysis
The measurements were then analyzed using SSPS software (version 15.0; SPSS, Chicago, Ill). Kolmogorov-Smirnov tests indicated approximately normal distributions, and paired t tests and independent-sample t tests were used to compare the intragroup and intergroup changes. Three weeks after the first evaluation, the tracings and measurements were repeated by the same investigator (S.O.) on 15 preretraction and 15 postretraction lateral cephalograms as well as 15 dental models of randomly selected patients. The intraclass correlation coefficients (r) were calculated for each variable to assess the reliability of the measurements, and they ranged from 0.967 to 1.00.
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RESULTS
In the cephalometric findings, although there were no statistically significant changes in skeletal measurements, there were significant reductions in the values representing protrusion of the incisors (Table I; Fig 5, A). There were no statistically significant changes in the value of MaxVP-U6, representing the horizontal distance from maxillary molars to the maxillary vertical plane (Table II), for all skeletal anchorage and spring groups. Statistically significant reductions in the angle between the maxillary canines and the maxillary plane, which represented the canine tipping angle (MaxP/ U3a), and in the distance between the maxillary canines and the maxillary vertical plane (MaxVP-U3) were observed in all skeletal anchorage and spring groups (P #0.001). In the LS group of the direct skeletal anchorage group, a statistically significant reduction in the vertical distance between the maxillary canines and the maxillary plane (MaxP-U3; P #0.01; Table II) was observed. A significant difference in MaxP-U3 values was also observed between the direct and indirect skeletal anchorage groups (P #0.05; Table III).
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Table I. Descriptive statistics of the changes in skeletal structure and incisors before and after canine retraction in the
2 skeletal anchorage groups Direct skeletal anchorage group T1 Variable Skeletal SNA ( ) SNB ( ) ANB ( ) SN/ManP ( ) MaxP/ManP ( ) SN/MaxP ( ) Wits (mm) Dental MaxP-U1a ( ) MaxP-U1 (mm) MaxVP-U1 (mm)
Indirect skeletal anchorage group
T2
T1
T2
Mean
SD
Mean
SD
P
Mean
SD
Mean
SD
P
79.46 76.07 3.44 36.56 28.67 7.88 3.11
4.03 4.37 2.56 5.54 4.61 2.92 3.49
79.55 75.96 3.59 36.55 28.72 7.89 3.12
4.22 4.29 2.83 5.32 4.50 2.92 3.93
0.423 0.546 0.432 0.981 0.829 0.855 0.969
80.66 76.03 4.65 35.28 27.75 7.54 5.45
2.50 2.22 1.79 3.54 4.60 3.95 3.30
80.64 75.81 4.83 35.44 27.81 7.63 5.06
2.67 1.94 1.75 3.61 4.97 3.91 2.35
0.829 0.093 0.102 0.376 0.768 0.035* 0.466
115.90 28.02 51.32
9.59 2.36 5.68
113.60 27.97 50.51
9.80 2.36 5.14
0.001z 0.705 0.008y
111.28 28.76 51.16
8.44 3.29 5.32
109.59 28.65 50.49
7.77 3.50 5.12
0.009y 0.531 0.009y
T1, Before retraction; T2, after retraction. *P \0.05; yP \0.01; zP \0.001.
Table II. Descriptive statistics of the changes in the canines and molars before and after canine retraction in the 2
skeletal anchorage groups Direct skeletal anchorage group T1
Reverse closing loop MaxP/U3a ( ) MaxP-U3 (mm) MaxVP-U3 (mm) MaxP/U6a ( ) MaxP-U6 (mm) MaxVP-U6 (mm) Ladanyi spring MaxP/U3a ( ) MaxP-U3 (mm) MaxVP-U3 (mm) MaxP/U6a ( ) MaxP-U6 (mm) MaxVP-U6 (mm)
Indirect skeletal anchorage group
T2
T1
T2
Mean
SD
Mean
SD
P
Mean
SD
Mean
SD
P
90.19 21.12 46.30 80.06 20.85 24.18
9.33 3.28 3.67 9.06 2.59 3.18
73.37 20.40 39.79 80.64 20.51 25.38
8.39 3.06 4.38 8.73 3.05 3.61
0.000y 0.078 0.000y 0.534 0.273 0.055
82.78 23.04 47.08 77.47 21.40 21.08
8.86 3.60 5.73 6.54 2.50 4.77
67.12 23.44 40.88 77.84 21.02 22.09
9.18 3.57 5.52 6.44 2.94 3.87
0.000y 0.318 0.000y 0.145 0.089 0.162
84.28 22.50 46.61 78.14 20.84 24.29
7.75 3.14 4.97 10.91 3.06 4.17
68.04 20.96 38.84 78.54 20.27 24.32
9.00 2.51 3.77 11.63 2.61 3.23
0.000y 0.002* 0.000y 0.577 0.100 0.967
84.49 22.40 46.36 77.64 21.44 21.15
8.62 4.18 5.05 7.76 2.77 4.28
68.59 23.27 40.33 77.83 21.39 22.05
10.23 3.26 5.62 6.50 2.77 4.13
0.000y 0.103 0.000y 0.837 0.847 0.117
T1, Before retraction; T2, after retraction. *P \0.01; yP \0.001.
Comparisons of the retraction speeds and total tooth movements showed no statistically significant differences between the retraction groups and the skeletal anchorage groups (Table IV). In the dental cast findings, both types of retractors caused statistically significant distal rotations of the maxillary canine crowns in both groups (P #0.001; Table V).
DISCUSSION
Studies conducted with segmental arches have reported that 155 to 250 g of force is sufficient to induce the required canine retraction.7,9,10 According to the manufacturer of LS, the spring should be activated by 2 to 3 mm to obtain 120 to 150 g of retraction force. Although these results were observed with the LS, a 2-mm activation of the RCL resulted in approximately
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Table III. Mean differences in the changes in canines and molars in the 2 groups Reverse closing loop Variable MaxP/U3a ( ) MaxP-U3 (mm) MaxVP-U3 (mm) MaxP/U6a ( ) MaxP-U6 (mm) MaxVP-U6 (mm)
Group Direct Indirect Direct Indirect Direct Indirect Direct Indirect Direct Indirect Direct Indirect
Mean 16.82 15.66 0.72 0.41 6.51 6.20 0.59 0.37 0.34 0.38 1.21 1.01
SD 9.19 6.34 1.62 1.62 2.36 2.91 3.93 0.98 1.27 0.87 2.48 2.83
Ladanyi spring P 0.671 0.049* 0.728 0.821 0.907 0.825
Mean 16.24 15.90 1.54 0.87 7.77 6.04 0.40 0.19 0.58 0.05 0.02 0.91
SD 4.97 5.12 1.78 2.08 2.89 2.74 2.98 3.84 1.41 1.11 2.28 2.25
P 0.841 0.001y 0.077 0.860 0.231 0.257
*P \0.05; yP \0.001.
170 g of force. Therefore, to obtain similar retraction forces in both springs, some modifications were made to the amount of activation during the activation process; this gave us approximately 120 to 150 g of retraction force. Reference wires have been used on the canines and molars before and after retraction to allow distinction of the right and left regions of the maxilla.11,12 Sueri and Turk12 used wires of 2 lengths on the right and left sides, and the authors of another study achieved this by taking 1 radiograph for each side of the jaw.13 Similar to the study of C¸etins¸ahin et al,14 the tips of the reference wires of the right side were rounded, and the left side were straight in our study. Although among the skeletal measurements only the SN/MaxP value was statistically significant, this had no importance clinically. Since our treatment did not affect the skeletal pattern, all skeletal values remained unchanged. At the end of the retraction process, the incisors had repositioned palatally in both anchorage groups. Because canine retraction was done with the sectional arches, the incisors were not affected directly. Therefore, we believe that this movement of the incisors occurred because of the availability of sufficient space during canine retraction and was brought about by the transseptal ligaments. Previous studies reported tipping values of the canines ranging between 3.30 and 13.03 during canine retraction with the frictionless system. Although Sueri and Turk12 reported 11.63 of canine tipping, this could be explained by the absence of a rigid archwire. In our study, the mean tipping values of the maxillary canines were 16.82 (P 5 0.000) in the RCL group and 16.24 (P 5 0.000) in the LS group of the direct
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skeletal anchorage group. In the indirect skeletal anchorage group, the mean values were 15.66 (P 5 0.000) for the RCL group and 15.90 (P 5 0.000) for the LS group. This agreed with the results of C¸etins¸ahin et al.14 Moreover, intrusion of the canines with both canine retractors in the direct skeletal anchorage group and extrusion of the canines in the indirect skeletal anchorage groups were observed. This situation may be caused by the upper position of the anchorage region (mini-implant) in relation to the retracted teeth in the direct skeletal anchorage groups. Orthodontic dental casts are mostly used to measure the rotation of the canines.5,10,15-17 Previous studies have used submentovertical radiographs for this purpose.11,12 We used orthodontic dental casts fabricated before and after the retraction process, and our results agreed with those of Ziegler and Ingervall.10 Although statistically significant (P 5 0.000) distopalatal rotations were observed in the canines of all study groups, no statistically significant differences in canine rotations were seen. Despite application of 15 to 20 antirotation bends in both retractions springs, statistically significant rotations were observed in the canines. Therefore, we believe that it would be beneficial to use elastics lingually to balance this rotation effect. Previous studies have reported no anchorage loss in direct skeletal anchorage systems,3 whereas some losses were observed with indirect skeletal anchorage systems.14,18 In our study, no statistically significant cephalometric changes were observed in the molars before or after canine retraction. Moreover, the mean differences between these changes also did not show statistical significance in any of the anchorage or retractor groups.
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Table IV. Descriptive statistics of the retraction speeds and total tooth movement of the canines Group Speed (mm/4 weeks) RCL Direct Indirect LS Direct Indirect Total movement (mm) RCL Direct Indirect LS Direct Indirect
n
Mean
SD
P
18 18 18 18
1.57 1.45 1.80 1.42
0.53 0.69 0.67 0.63
0.560
Direct
0.102
Indirect
18 18 18 18
7.62 7.25 7.62 6.94
0.89 1.74 1.10 1.84
0.447
Direct
0.204
Indirect
n
Mean
SD
P
RCL LS RCL LS
18 18 18 18
1.57 1.80 1.45 1.42
0.53 0.67 0.69 0.63
0.284
RCL LS RCL LS
18 18 18 18
7.62 7.56 7.25 6.94
0.89 1.10 1.74 1.84
1.000
Group
0.914
0.615
RCL, Reverse closing loop; LS, Ladanyi spring.
3. Table V. Preretraction and postretraction descriptive
statistics of the angular rotations of canines measured on casts Reverse closing loop
Ladanyi spring
Time Mean SD P Mean SD P T1 39.98 9.05 0.000* 37.42 9.44 0.000* T2 6.43 12.70 3.71 13.64 Indirect ( ) T1 37.42 5.80 0.000* 35.52 12.39 0.000* T2 9.77 11.56 7.33 16.19 Direct ( )
T1, Before retraction; T2, after retraction. *P \0.001.
Therefore, it can be said that both anchorage methods and retractors yielded similar changes in the molars throughout the retraction process. Moreover, the anchorage was well retained. These findings were similar to the results of previous studies using skeletal anchorage.3,14,18 There were no statistically significant differences between the anchorage and retractor groups with respect to canine retraction speeds and total tooth movements. Most previous studies also used 4-week intervals as the unit of time.12,19,20 Our results were similar to those of Martins et al,21 who used the same time interval. CONCLUSIONS
We can summarize the results as follows. 1.
2.
4.
Both the direct and the indirect skeletal anchorage systems were effective regarding maximum anchorage. No statistically significant difference was observed between the direct and indirect skeletal anchorage methods regarding anchorage loss. Both retractors used in this study (RCL, LS) were effective for canine retraction. There was no statistically significant difference between the 2 retractors.
5.
The canine crowns were significantly tipped distally in all study groups during the distalization process. Statistically significant distopalatal rotations were observed in the maxillary canines of all groups during the distalization process, but there was no significant difference between the groups. Some intrusion was observed in the canines during the distalization in the direct skeletal anchorage group. This movement was most probably due to the gingivally positioned mini-implants in relation to the canines.
Mini-implant supported direct and indirect skeletal anchorage methods can be used in combination with canine retraction springs for retraction in patients requiring maximum anchorage. REFERENCES 1. Geron S, Shpack N, Kandos S, Davidovitch M, Vardimon AD. Anchorage loss—a multifactorial response. Angle Orthod 2003; 73:730-7. 2. Nanda R. Biomechanics and esthetic strategies in clinical orthodontics. Philadelphia: Elsevier Saunders; 2005. 3. Thiruvenkatachari B, Pavithranand A, Rajasigamani K, Kyung HM. Comparison and measurement of the amount of anchorage loss of the molars with and without the use of implant anchorage during canine retraction. Am J Orthod Dentofacial Orthop 2006;129:551-4. 4. Ren Y. Mini-implants for direct or indirect orthodontic anchorage. Evid Based Dent 2009;10:113. 5. Hayashi K, Uechi J, Murata M, Mizoguchi I. Comparison of maxillary canine retraction with sliding mechanics and a retraction spring: a three-dimensional analysis based on a midpalatal orthodontic implant. Eur J Orthod 2004;26:585-9. 6. Burstone CJ. Rationale of the segmented arch. Am J Orthod 1962; 48:805-22. 7. Burstone CJ, Koenig HA. Optimizing anterior and canine retraction. Am J Orthod 1976;70:1-19. 8. Eden JD, Waters NE. An investigation into the characteristics of the PG canine retraction spring. Am J Orthod Dentofacial Orthop 1994;105:49-60. 9. Gjessing P. Biomechanical design and clinical evaluation of a new canine-retraction spring. Am J Orthod 1985;87:353-62.
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10. Ziegler P, Ingervall B. A clinical study of maxillary canine retraction with a retraction spring and with sliding mechanics. Am J Orthod Dentofacial Orthop 1989;95:99-106. 11. Darendeliler MA, Darendeliler H, Uner O. The drum spring (DS) retractor: constant and continuous force for canine retraction. Eur J Orthod 1997;19:115-30. 12. Sueri MY, Turk T. Effectiveness of laceback ligatures on maxillary canine retraction. Angle Orthod 2006;76:1010-4. 13. Dincer M, Iscan HN. The effects of different sectional arches in canine retraction. Eur J Orthod 1994;16:31723. 14. Cetinsahin A, Dincer M, Arman-Ozcirpici A, Uckan S. Effects of the zygoma anchorage system on canine retraction. Eur J Orthod 2010;32:505-13. 15. Iwasaki LR, Haack JE, Nickel JC, Morton J. Human tooth movement in response to continuous stress of low magnitude. Am J Orthod Dentofacial Orthop 2000;117:175-83. 16. Rajcich MM, Sadowsky C. Efficacy of intraarch mechanics using differential moments for achieving anchorage control in
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extraction cases. Am J Orthod Dentofacial Orthop 1997;112: 441-8. Shpack N, Davidovitch M, Sarne O, Panayi N, Vardimon AD. Duration and anchorage management of canine retraction with bodily versus tipping mechanics. Angle Orthod 2008;78: 95-100. Wehrbein H, Feifel H, Diedrich P. Palatal implant anchorage reinforcement of posterior teeth: a prospective study. Am J Orthod Dentofacial Orthop 1999;116:678-86. Deguchi T, Takano-Yamamoto T, Kanomi R, Hartsfield JK Jr, Roberts WE, Garetto LP. The use of small titanium screws for orthodontic anchorage. J Dent Res 2003;82:377-81. Dixon V, Read MJ, O'Brien KD, Worthington HV, Mandall NA. A randomized clinical trial to compare three methods of orthodontic space closure. J Orthod 2002;29:31-6. Martins RP, Buschang PH, Gandini LG Jr, Rossouw PE. Changes over time in canine retraction: an implant study. Am J Orthod Dentofacial Orthop 2009;136:87-93.
American Journal of Orthodontics and Dentofacial Orthopedics
Comparison of direct and indirect skeletal anchorage systems combined with 2 canine retraction techniques Serkan Ozkana and Mehmet Bayramb Ordu and Trabzon, Turkey
Introduction: We compared the effectiveness of 2 canine retraction springs and anchorage systems (direct and indirect skeletal anchorage) in patients requiring first premolar extractions and maximum anchorage in the maxilla. Methods: Thirty-six patients were included (17 male, 19 female; mean age, 16.8 6 2.4 years). A mini-implant–supported Nance appliance with indirect skeletal anchorage system was used in 18 patients and a mini-implant–supported direct skeletal anchorage system in the remaining patients. In each patient, a segmental retraction arch with a reverse closing loop was applied to a maxillary canine, and a Ladanyi spring (Dentaurum, Ispringen, Germany) was applied to the other canine randomly after extraction of the maxillary first premolars. The retraction process was continued until a Class I canine relationship was obtained. Lateral cephalometric films and orthodontic casts taken before and after retraction in the distalization process were used to evaluate changes during canine distalization. The measurements were statistically evaluated using paired and independent t tests with 95% confidence intervals. Results: The reverse closing loop and the Ladanyi spring were found to be effective in canine distalization (P #0.001). There were no statistically significant differences between the reverse closing loop and the Ladanyi spring with regard to canine distalization rates (P $0.05). Both systems were effective in providing maximum anchorage (P $0.05); no statistically significant differences were detected in molar anchorage loss rates between the 2 methods (P $0.05). Conclusions: These 2 systems can be used during segmental distalization of canines requiring maximum anchorage with no significant anchorage loss. (Am J Orthod Dentofacial Orthop 2016;150:763-70)
L
oss of anchorage is a common problem faced during maximum anchorage orthodontic treatment, and prevention of this plays a significant role in the design of the orthodontic appliances.1 For a patient with a Class II occlusion, if the treatment plan is the extraction of the premolars, the maxillary canines must be distalized completely to the extraction sites. Therefore, anchorage reinforcement of the posterior teeth must be ensured using intraoral or extraoral appliances.2 Previously, extraoral appliances were the only way to provide maximum anchorage, but their use was limited
a Assistant professor, Department of Orthodontics, Faculty of Dentistry, Ordu University, Ordu, Turkey. b Associate professor, Department of Orthodontics, Faculty of Dentistry, Karadeniz Technical University, Trabzon, Turkey. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. Address correspondence to: Serkan Ozkan, Department of Orthodontics, Faculty of Dentistry, Ordu University, 52100, Ordu, Turkey; e-mail, dtserkanozkan@ gmail.com. Submitted, November 2015; revised and accepted, April 2016. 0889-5406/$36.00 Ó 2016 by the American Association of Orthodontists. All rights reserved. http://dx.doi.org/10.1016/j.ajodo.2016.04.023
because they required patient cooperation. As a result, intraoral appliances became increasingly popular among clinicians for obtaining anchorage. However, popular intraoral appliances, such as palatal and lingual bars, Nance appliances, and intermaxillary elastics, have undesired side effects: protrusion, extrusion, and tipping.3 Anchorage problems encountered during the use of intraoral mechanics led to increased use of implant-borne mechanics. However, these had several limitations, such as excessive waiting times for osseointegration. These limitations were prevented by the introduction of orthodontic mini-implants as intraoral anchorage units. These implants were developed as an alternative to conventional molar and other intraoral anchorage units. Orthodontic mini-implants can be used either directly or indirectly. Direct anchorage refers to the movement of teeth using orthodontic mini-implants, whereas indirect anchorage refers to the stabilization of certain teeth in the dental arch and subsequent use of these stabilized anchors to move other teeth.4 Indirect skeletal anchorage systems are preferred by clinicians when the direct skeletal anchorage system is not applicable, such as in patients with proximity of the roots. 763
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In extraction patients, canine retraction can be carried out with fixed appliances using frictional or nonfrictional methods.5 The popularity of the nonfrictional method increased when Burstone6 reported its advantages. Thereafter, many canine retraction springs were introduced and tested for reliability and efficiency.7-10 The aim of this study was to compare the efficiency of direct and indirect skeletal anchorage systems combined with 2 canine retraction springs in patients requiring maximum anchorage for maxillary canine retraction. MATERIAL AND METHODS
After receiving ethical approval from the ethics council of Karadeniz Technical University (protocol number, B302KTU0200000/637), we conducted a pilot study to calculate the total sample size. Twenty patients were recruited for the pilot study at the beginning of the treatment, and a number from 1 to 20 was assigned to each patient. Using a randomization Web site, 4 columns of the random array of numbers were created. According to the 4 randomized number strings, patients were assigned to the following groups: (1) direct skeletal anchorage, Ladanyi spring (LS) (Dentaurum, Ispringen, Germany) on the left side; (2) direct skeletal anchorage, LS on the right side; (3) indirect skeletal anchorage, LS on the left side; and (4) indirect skeletal anchorage, LS on the right side. After randomization, the canine distalization method was applied to every patient according to the patient's assigned group. As the distalization finished, all cephalometric and cast materials were measured, and the power of the study was determined. Meanwhile, we continued to recruit new patients for the study. According to our pilot study, power calculations with G*Power (version 3.1.3; Franz Faul, Christian-Albrechts-Universitat, Kiel, Germany), group sample sizes of 16 achieved 82% power to detect a difference of 1.5mm between the null hypothesis that both group means were 1.7mm and the alternative hypothesis that the mean of group 2 was 0.1mm, with estimated group standard deviations of 1.2mm and 1.7mm at a significance level (alpha) of 0.05 using a 2-sided, 2sample t test. Thus, 16 additional patients were recruited to guarantee the exact power with a total of 36 patients (17 male, 19 female; mean age, 16.8 6 2.4 years). The aforementioned randomization method was applied to these patients as well. Patients receiving maximum anchorage with extraction of the maxillary first premolars were included in the study. The inclusion criteria were (1) permanent dentition, (2) good oral hygiene, (3) Class I or Class II skeletal pattern, (4) no anomaly in the transverse direction, (5) no maxillary canine with supraposition or excessive rotation, (6) no systemic disorder that contraindicated
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Fig 1. The palatal region after mini-implant insertion in the indirect skeletal anchorage group.
Fig 2. Nance appliance built for the indirect skeletal anchorage group.
orthodontic treatment, and (7) no systemic or allergic disorder that contraindicated the application of skeletal anchorage units. The maxillary first premolars were extracted to resolve the crowding in all patients. In the direct skeletal anchorage group, 2 mini-implants (Aarhus; American Orthodontics, Sheboygan, Wis; 1.5-mm diameter, 8-mm length) were inserted into the buccal aspect between the roots of the maxillary first molars and second premolars at an angle of 15 to 20 . In the indirect skeletal anchorage group, after determination of the insertion areas using cephalometric radiographs and dental casts, 2 of the same mini-implants were inserted into the paramedian section of the palatal area (Fig 1). Appropriate molar bands (3M Unitek, Monrovia, Calif) were chosen and adapted to the maxillary molars.
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Fig 3. Guide wires prepared for distinguishing the right and left sides of the maxilla on the radiographs.
Fig 4. Step bends prepared for the prevention of intrusion of the canines in the direct skeletal anchorage groups.
After the construction of banded casts of the maxilla, custom Nance appliances were built over the miniimplant sites (Fig 2). Guide wires, made from 0.021 3 0.025-in stainless steel wires (American Orthodontics), were used for assessing the movements of the teeth on cephalometric radiographs (Fig 3). In both groups, 0.022 3 0.028-in slotted Roth brackets (Gemini; 3M Unitek) were bonded to the maxillary canines before application of these wires. Guide wires with rounded ends were applied to the right canines and molars, and wires with straight ends were applied to the left canines and molars in all patients. Lateral cephalometric radiographs were taken after installation of the wires before and after the canine distalization process. Remaloy wires (Dentaurum, Ispringen, Germany), 0.016 3 0.022 in diameter, were used to prepare the reverse closing loop (RCL) retraction springs. The RCL retraction springs were given the same dimensions as the prefabricated LS to obtain standardization, and 45 antitip and 15 to 20 antirotation bends were incorporated into the springs before installation. Additional step bends were also included in the springs of
the direct skeletal anchorage groups to prevent intrusion of canines because of differences in the vertical levels of the mini-implants and the canine bracket slots (Fig 4). The application of the retraction springs was completed by activating each spring by about 2 mm to obtain 120 to 150 g of retraction force. Prefabricated LS were applied on 1 side, and RCL retractor springs were applied to the other side of the patients randomly. The retraction springs were activated at 4-week intervals, and the retraction process was continued until the canines achieved a Class I relationship. The ANS-PNS line extended from the anterior nasal spine (ANS) to the posterior nasal spine (PNS) along the maxillary plane (MaxP). Local superimpositions were carried out along this line and at the PNS point. MaxP, derived from the ANS-PNS points, was used as a horizontal reference plane, and MaxVP, derived from the vertical line extending from the pterygomaxillary fissure to the MaxP, was used as the vertical reference plane for dental changes (Fig 5, B). The mesial and distal contact points of the canines were marked on the dental casts and transferred to the computer via a scanner (Perfection V700; Epson
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Fig 5. A, Cephalometric landmarks and skeletal measurements used in this study: S, sella turcica; N, nasion; A, Point A; B, Point B; ANS, anterior nasal spine; PNS, posterior nasal spina; Me, menton; 1, SNA; 2, SNB; 3, ANB; 4, SN/ManP; 5, MaxP/ManP; 6, SN/MaxP. B, Dental measurements of the maxilla: Ptm, Pterygomaxillary fissure; 7, MaxP-U1a (angular); 8, MaxP-U1 (mm); 9, MaxVP-U1 (mm); 10, MaxP/U3a (angular); 11, MaxP-U3 (mm); 12, MaxVP-U3 (mm); 13, MaxP/U6a (angular); 14, MaxP-U6 (mm); 15, MaxVP-U6 (mm). C, Measurement used for determining the rotation of the canines on the dental casts.
America, Long Beach, Calif). Digital images were then transferred to cephalometric tracing software (Facad, trial version; Ilexis, Link€ oping, Sweden), and the amounts of predistalization to postdistalization rotation of the canines were measured (Fig 5, C).
Statistical analysis
The measurements were then analyzed using SSPS software (version 15.0; SPSS, Chicago, Ill). Kolmogorov-Smirnov tests indicated approximately normal distributions, and paired t tests and independent-sample t tests were used to compare the intragroup and intergroup changes. Three weeks after the first evaluation, the tracings and measurements were repeated by the same investigator (S.O.) on 15 preretraction and 15 postretraction lateral cephalograms as well as 15 dental models of randomly selected patients. The intraclass correlation coefficients (r) were calculated for each variable to assess the reliability of the measurements, and they ranged from 0.967 to 1.00.
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RESULTS
In the cephalometric findings, although there were no statistically significant changes in skeletal measurements, there were significant reductions in the values representing protrusion of the incisors (Table I; Fig 5, A). There were no statistically significant changes in the value of MaxVP-U6, representing the horizontal distance from maxillary molars to the maxillary vertical plane (Table II), for all skeletal anchorage and spring groups. Statistically significant reductions in the angle between the maxillary canines and the maxillary plane, which represented the canine tipping angle (MaxP/ U3a), and in the distance between the maxillary canines and the maxillary vertical plane (MaxVP-U3) were observed in all skeletal anchorage and spring groups (P #0.001). In the LS group of the direct skeletal anchorage group, a statistically significant reduction in the vertical distance between the maxillary canines and the maxillary plane (MaxP-U3; P #0.01; Table II) was observed. A significant difference in MaxP-U3 values was also observed between the direct and indirect skeletal anchorage groups (P #0.05; Table III).
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Table I. Descriptive statistics of the changes in skeletal structure and incisors before and after canine retraction in the
2 skeletal anchorage groups Direct skeletal anchorage group T1 Variable Skeletal SNA ( ) SNB ( ) ANB ( ) SN/ManP ( ) MaxP/ManP ( ) SN/MaxP ( ) Wits (mm) Dental MaxP-U1a ( ) MaxP-U1 (mm) MaxVP-U1 (mm)
Indirect skeletal anchorage group
T2
T1
T2
Mean
SD
Mean
SD
P
Mean
SD
Mean
SD
P
79.46 76.07 3.44 36.56 28.67 7.88 3.11
4.03 4.37 2.56 5.54 4.61 2.92 3.49
79.55 75.96 3.59 36.55 28.72 7.89 3.12
4.22 4.29 2.83 5.32 4.50 2.92 3.93
0.423 0.546 0.432 0.981 0.829 0.855 0.969
80.66 76.03 4.65 35.28 27.75 7.54 5.45
2.50 2.22 1.79 3.54 4.60 3.95 3.30
80.64 75.81 4.83 35.44 27.81 7.63 5.06
2.67 1.94 1.75 3.61 4.97 3.91 2.35
0.829 0.093 0.102 0.376 0.768 0.035* 0.466
115.90 28.02 51.32
9.59 2.36 5.68
113.60 27.97 50.51
9.80 2.36 5.14
0.001z 0.705 0.008y
111.28 28.76 51.16
8.44 3.29 5.32
109.59 28.65 50.49
7.77 3.50 5.12
0.009y 0.531 0.009y
T1, Before retraction; T2, after retraction. *P \0.05; yP \0.01; zP \0.001.
Table II. Descriptive statistics of the changes in the canines and molars before and after canine retraction in the 2
skeletal anchorage groups Direct skeletal anchorage group T1
Reverse closing loop MaxP/U3a ( ) MaxP-U3 (mm) MaxVP-U3 (mm) MaxP/U6a ( ) MaxP-U6 (mm) MaxVP-U6 (mm) Ladanyi spring MaxP/U3a ( ) MaxP-U3 (mm) MaxVP-U3 (mm) MaxP/U6a ( ) MaxP-U6 (mm) MaxVP-U6 (mm)
Indirect skeletal anchorage group
T2
T1
T2
Mean
SD
Mean
SD
P
Mean
SD
Mean
SD
P
90.19 21.12 46.30 80.06 20.85 24.18
9.33 3.28 3.67 9.06 2.59 3.18
73.37 20.40 39.79 80.64 20.51 25.38
8.39 3.06 4.38 8.73 3.05 3.61
0.000y 0.078 0.000y 0.534 0.273 0.055
82.78 23.04 47.08 77.47 21.40 21.08
8.86 3.60 5.73 6.54 2.50 4.77
67.12 23.44 40.88 77.84 21.02 22.09
9.18 3.57 5.52 6.44 2.94 3.87
0.000y 0.318 0.000y 0.145 0.089 0.162
84.28 22.50 46.61 78.14 20.84 24.29
7.75 3.14 4.97 10.91 3.06 4.17
68.04 20.96 38.84 78.54 20.27 24.32
9.00 2.51 3.77 11.63 2.61 3.23
0.000y 0.002* 0.000y 0.577 0.100 0.967
84.49 22.40 46.36 77.64 21.44 21.15
8.62 4.18 5.05 7.76 2.77 4.28
68.59 23.27 40.33 77.83 21.39 22.05
10.23 3.26 5.62 6.50 2.77 4.13
0.000y 0.103 0.000y 0.837 0.847 0.117
T1, Before retraction; T2, after retraction. *P \0.01; yP \0.001.
Comparisons of the retraction speeds and total tooth movements showed no statistically significant differences between the retraction groups and the skeletal anchorage groups (Table IV). In the dental cast findings, both types of retractors caused statistically significant distal rotations of the maxillary canine crowns in both groups (P #0.001; Table V).
DISCUSSION
Studies conducted with segmental arches have reported that 155 to 250 g of force is sufficient to induce the required canine retraction.7,9,10 According to the manufacturer of LS, the spring should be activated by 2 to 3 mm to obtain 120 to 150 g of retraction force. Although these results were observed with the LS, a 2-mm activation of the RCL resulted in approximately
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Table III. Mean differences in the changes in canines and molars in the 2 groups Reverse closing loop Variable MaxP/U3a ( ) MaxP-U3 (mm) MaxVP-U3 (mm) MaxP/U6a ( ) MaxP-U6 (mm) MaxVP-U6 (mm)
Group Direct Indirect Direct Indirect Direct Indirect Direct Indirect Direct Indirect Direct Indirect
Mean 16.82 15.66 0.72 0.41 6.51 6.20 0.59 0.37 0.34 0.38 1.21 1.01
SD 9.19 6.34 1.62 1.62 2.36 2.91 3.93 0.98 1.27 0.87 2.48 2.83
Ladanyi spring P 0.671 0.049* 0.728 0.821 0.907 0.825
Mean 16.24 15.90 1.54 0.87 7.77 6.04 0.40 0.19 0.58 0.05 0.02 0.91
SD 4.97 5.12 1.78 2.08 2.89 2.74 2.98 3.84 1.41 1.11 2.28 2.25
P 0.841 0.001y 0.077 0.860 0.231 0.257
*P \0.05; yP \0.001.
170 g of force. Therefore, to obtain similar retraction forces in both springs, some modifications were made to the amount of activation during the activation process; this gave us approximately 120 to 150 g of retraction force. Reference wires have been used on the canines and molars before and after retraction to allow distinction of the right and left regions of the maxilla.11,12 Sueri and Turk12 used wires of 2 lengths on the right and left sides, and the authors of another study achieved this by taking 1 radiograph for each side of the jaw.13 Similar to the study of C¸etins¸ahin et al,14 the tips of the reference wires of the right side were rounded, and the left side were straight in our study. Although among the skeletal measurements only the SN/MaxP value was statistically significant, this had no importance clinically. Since our treatment did not affect the skeletal pattern, all skeletal values remained unchanged. At the end of the retraction process, the incisors had repositioned palatally in both anchorage groups. Because canine retraction was done with the sectional arches, the incisors were not affected directly. Therefore, we believe that this movement of the incisors occurred because of the availability of sufficient space during canine retraction and was brought about by the transseptal ligaments. Previous studies reported tipping values of the canines ranging between 3.30 and 13.03 during canine retraction with the frictionless system. Although Sueri and Turk12 reported 11.63 of canine tipping, this could be explained by the absence of a rigid archwire. In our study, the mean tipping values of the maxillary canines were 16.82 (P 5 0.000) in the RCL group and 16.24 (P 5 0.000) in the LS group of the direct
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skeletal anchorage group. In the indirect skeletal anchorage group, the mean values were 15.66 (P 5 0.000) for the RCL group and 15.90 (P 5 0.000) for the LS group. This agreed with the results of C¸etins¸ahin et al.14 Moreover, intrusion of the canines with both canine retractors in the direct skeletal anchorage group and extrusion of the canines in the indirect skeletal anchorage groups were observed. This situation may be caused by the upper position of the anchorage region (mini-implant) in relation to the retracted teeth in the direct skeletal anchorage groups. Orthodontic dental casts are mostly used to measure the rotation of the canines.5,10,15-17 Previous studies have used submentovertical radiographs for this purpose.11,12 We used orthodontic dental casts fabricated before and after the retraction process, and our results agreed with those of Ziegler and Ingervall.10 Although statistically significant (P 5 0.000) distopalatal rotations were observed in the canines of all study groups, no statistically significant differences in canine rotations were seen. Despite application of 15 to 20 antirotation bends in both retractions springs, statistically significant rotations were observed in the canines. Therefore, we believe that it would be beneficial to use elastics lingually to balance this rotation effect. Previous studies have reported no anchorage loss in direct skeletal anchorage systems,3 whereas some losses were observed with indirect skeletal anchorage systems.14,18 In our study, no statistically significant cephalometric changes were observed in the molars before or after canine retraction. Moreover, the mean differences between these changes also did not show statistical significance in any of the anchorage or retractor groups.
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Table IV. Descriptive statistics of the retraction speeds and total tooth movement of the canines Group Speed (mm/4 weeks) RCL Direct Indirect LS Direct Indirect Total movement (mm) RCL Direct Indirect LS Direct Indirect
n
Mean
SD
P
18 18 18 18
1.57 1.45 1.80 1.42
0.53 0.69 0.67 0.63
0.560
Direct
0.102
Indirect
18 18 18 18
7.62 7.25 7.62 6.94
0.89 1.74 1.10 1.84
0.447
Direct
0.204
Indirect
n
Mean
SD
P
RCL LS RCL LS
18 18 18 18
1.57 1.80 1.45 1.42
0.53 0.67 0.69 0.63
0.284
RCL LS RCL LS
18 18 18 18
7.62 7.56 7.25 6.94
0.89 1.10 1.74 1.84
1.000
Group
0.914
0.615
RCL, Reverse closing loop; LS, Ladanyi spring.
3. Table V. Preretraction and postretraction descriptive
statistics of the angular rotations of canines measured on casts Reverse closing loop
Ladanyi spring
Time Mean SD P Mean SD P T1 39.98 9.05 0.000* 37.42 9.44 0.000* T2 6.43 12.70 3.71 13.64 Indirect ( ) T1 37.42 5.80 0.000* 35.52 12.39 0.000* T2 9.77 11.56 7.33 16.19 Direct ( )
T1, Before retraction; T2, after retraction. *P \0.001.
Therefore, it can be said that both anchorage methods and retractors yielded similar changes in the molars throughout the retraction process. Moreover, the anchorage was well retained. These findings were similar to the results of previous studies using skeletal anchorage.3,14,18 There were no statistically significant differences between the anchorage and retractor groups with respect to canine retraction speeds and total tooth movements. Most previous studies also used 4-week intervals as the unit of time.12,19,20 Our results were similar to those of Martins et al,21 who used the same time interval. CONCLUSIONS
We can summarize the results as follows. 1.
2.
4.
Both the direct and the indirect skeletal anchorage systems were effective regarding maximum anchorage. No statistically significant difference was observed between the direct and indirect skeletal anchorage methods regarding anchorage loss. Both retractors used in this study (RCL, LS) were effective for canine retraction. There was no statistically significant difference between the 2 retractors.
5.
The canine crowns were significantly tipped distally in all study groups during the distalization process. Statistically significant distopalatal rotations were observed in the maxillary canines of all groups during the distalization process, but there was no significant difference between the groups. Some intrusion was observed in the canines during the distalization in the direct skeletal anchorage group. This movement was most probably due to the gingivally positioned mini-implants in relation to the canines.
Mini-implant supported direct and indirect skeletal anchorage methods can be used in combination with canine retraction springs for retraction in patients requiring maximum anchorage. REFERENCES 1. Geron S, Shpack N, Kandos S, Davidovitch M, Vardimon AD. Anchorage loss—a multifactorial response. Angle Orthod 2003; 73:730-7. 2. Nanda R. Biomechanics and esthetic strategies in clinical orthodontics. Philadelphia: Elsevier Saunders; 2005. 3. Thiruvenkatachari B, Pavithranand A, Rajasigamani K, Kyung HM. Comparison and measurement of the amount of anchorage loss of the molars with and without the use of implant anchorage during canine retraction. Am J Orthod Dentofacial Orthop 2006;129:551-4. 4. Ren Y. Mini-implants for direct or indirect orthodontic anchorage. Evid Based Dent 2009;10:113. 5. Hayashi K, Uechi J, Murata M, Mizoguchi I. Comparison of maxillary canine retraction with sliding mechanics and a retraction spring: a three-dimensional analysis based on a midpalatal orthodontic implant. Eur J Orthod 2004;26:585-9. 6. Burstone CJ. Rationale of the segmented arch. Am J Orthod 1962; 48:805-22. 7. Burstone CJ, Koenig HA. Optimizing anterior and canine retraction. Am J Orthod 1976;70:1-19. 8. Eden JD, Waters NE. An investigation into the characteristics of the PG canine retraction spring. Am J Orthod Dentofacial Orthop 1994;105:49-60. 9. Gjessing P. Biomechanical design and clinical evaluation of a new canine-retraction spring. Am J Orthod 1985;87:353-62.
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