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Comparison of the intrusion effects on the maxillary incisors between implant anchorage and J-hook headgear
ORIGINAL ARTICLE
Comparison of the intrusion effects on the maxillary incisors between implant anchorage and J-hook headgear Toru Deguchi,a Takashi Murakami,b Shingo Kuroda,a Toshinori Yabuuchi,b Hiroshi Kamioka,c and Teruko Takano-Yamamotod Okayama, Japan Introduction: Recently, miniscrews have been used to provide anchorage during orthodontic treatment, especially for incisor intrusion. Miniscrews during incisor intrusion are commonly used in implant orthodontics. Traditionally, effective incisor intrusion has been accomplished with J-hook headgear. In this study, we compared the effect of incisor intrusion, force vector, and amount of root resorption between implant orthodontics and J-hook headgear. Methods: Lateral cephalometric radiographs from 8 patients in the implant group and 10 patients in the J-hook headgear group were analyzed for incisor retraction. The estimated force vector was analyzed in the horizontal and vertical directions in both groups. Root resorption was also measured on periapical radiographs. Results: In the implant group, significant reductions in overjet, overbite, maxillary incisor to palatal plane, and maxillary incisor to upper lip were observed after intrusion of the incisors. In the J-hook headgear group, significant reductions in overjet, overbite, maxillary incisor to upper lip, and maxillary incisor to SN plane were observed after intrusion of the incisors. There were significantly greater reductions in overbite, maxillary incisor to palatal plane, and maxillary incisor to upper lip in the implant group than in the J-hook headgear group. Estimated force analysis resulted in significantly more force in the vertical direction and less in the horizontal direction in the implant group. Furthermore, significantly less root resorption was observed in the implant group compared with the J-hook headgear group. Conclusions: The maxillary incisors were effectively intruded by using miniscrews as orthodontic anchorage without patient cooperation. The amount of root resorption was not affected by activating the ligature wire from the miniscrew during incisor intrusion. (Am J Orthod Dentofacial Orthop 2008;133:654-60)
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n correcting a Class II deepbite, an important clinical goal is to reduce the amount of overbite. There are several methods of reducing overbite during orthodontic treatment: utility arch combined with high-pull headgear, intrusive arch, and J-hook headgear (J-HG).1-3 However, intruding the incisors with an archwire requires complex wire bending adjustments to prevent undesirable side effects such as extrusion and flaring of the posterior teeth.1,4 Extraoral From the Department of Orthodontics and Dentofacial Orthopedics, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama, Japan. a Lecturer. b Graduate student. c Associate professor. d Professor and chair. Supported by grants-in-aid for scientific research from the Japan Society for the Promotion of Science. Reprint requests to: Teruko Takano-Yamamoto, Department of Orthodontics and Dentofacial Orthopedics, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, 2-5-1 Shikata-Cho, Okayama 700-8525, Japan; e-mail, [email protected]. Submitted, January 2006; revised and accepted, April 2006. 0889-5406/$34.00 Copyright © 2008 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2006.04.047
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appliances are effective in controlling the anchorage, although patient cooperation is necessary to achieve effective intrusion of the incisors. Especially in patients with deepbite and excessive gingival display resulting in a gummy smile, absolute intrusion of the maxillary incisors is required rather than extrusion of the molars.5 Recently, several reports have described the clinical use of miniscrews in orthodontic treatment.6-9 Especially, intrusion of the maxillary posterior segments by these orthodontic implants was suggested to achieve excellent results and avoid maxillary surgery in patients with severe open bites.10,11 The use of miniscrews to intrude the maxillary incisors was also reported.12,13 These reports indicated that effective maxillary incisor intrusion was achieved with minimal side effects without patient cooperation by using miniscrews as anchorage. However, to routinely use these miniscrews as anchorage to effectively intrude the incisors, we need to quantitatively evaluate the effect of the amount of incisor movement with a larger sample size. As described above, J-HG can effectively intrude the maxillary incisors, although the intrusion effect might vary, since it depends on patient cooperation. In addition, it is
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important to consider the amount and the direction of force of these appliances, resulting in incisor movement in both vertical and horizontal directions. Furthermore, a side effect of intruding the maxillary incisors is root resorption during active orthodontic treatment.14,15 There has been no investigation of the effect of root resorption during incisor intrusion with miniscrews or J-HG. The objectives of this study were to (1) compare maxillary incisor displacement, (2) analyze and compare the vector of the force, and (3) compare the amount of root resorption between the miniscrew and J-HG. MATERIAL AND METHODS
Our subjects included 18 patients (8 in the implant group, and 10 in the J-HG group). The average ages were 20.7 ⫾ 2.5 years (1 man, 9 women) in the J-HG group and 21.5 ⫾ 3.7 years in the implant group (1 man, 7 women). All patients had their maxillary first premolars extracted. Lateral cephalometric radiographs and cast models were used for the pretreatment cephalometric analysis compared with Japanese norms,16 the peer assessment rating (PAR), and the discrepancy index (DI) in both groups. For the posttreatment analysis, cephalograms were taken just after intrusion of the maxillary incisors because, if the cephalograms were obtained after all orthodontic treatment, other factors would affect the position of the maxillary incisors. Especially during detailing, we often added additional torque in the incisors; this would obscure the real effect of these intrusion mechanics. Seven angular and 6 linear measurements were made to evaluate skeletal and dental changes before and after incisor intrusion (Figs 1 and 2). In the J-HG group, an edgewise appliance (.018-in slot) with a J-hook was adapted to .016 ⫻ .022-in stainless steel wire for 100 g per side. Force was checked once a month (every appointment) with a force gauge and adjusted to maintain a force of 100 g per side (average, 109.2 g; SD 15 g). All patients were requested to record the total time that they wore their J-HG every day. Only patients wearing their headgear more than 7 hours per day on average were included in this study. Lateral cephalometric radiographs were taken with the J-HG on at postintrusion in every patient (Fig 3, A). In the implant group, 2 miniscrews (diameter; 1.5 mm; length, 6 mm) were placed under local anesthesia at the premaxillary region by senior oral surgeons at the Department of Oral and Maxillofacial Surgery, Okayama University Hospital in Japan (Fig 3, B). The orthodontic load was applied by using a ligature wire from the implant to the soldered hook on the .016 ⫻ .022-in stainless steel wire after 1 month of healing (Fig 4). The amount of force was checked by setting the force gauge
Fig 1. Angular measurements: 1, SNA; 2, SNB; 3, ANB; 4, SN to palatal plane (PP-SN); 5, SN to occlusal plane (Occl Pl); 6, SN to mandibular plane (Mp-SN); 7, U1 to SN (SN-U1).
at the level of the hook to the ligature wire that would be approximately 80 to 120 g of intrusion force. The estimated vector analyses of the intrusion force were calculated in both groups. Intrusion force was set as 100 g from the soldered hook on the wire to implant or to the headgear hook where the elastic rubber band is attached (Fig 5). The estimated intrusion force was divided into the vertical direction and the horizontal direction. A horizontal line was determined as the line from the soldered hook that is parallel to the main archwire. The vertical line was determined as the line from the implant or the hook of the headgear that is perpendicular to the horizontal force. The calculation of the horizontal and vertical force was done as follows. Intrusion force (100 g) ⫻ cos⌰ ⫽ horizontal force Intrusion force (100 g) ⫻ sin⌰ ⫽ vertical force Measurement of root resorption was analyzed with the periapical films taken at pretreatment and postintrusion. The amount of root resorption was calculated according to the method reported previously.17 The reproducibility of the measurements on lateral cephalometric and periapical radiographs was assessed by statistically analyzing the difference between double measurements made 2 months apart in all 18 subjects.18 The method error was calculated from the equation: Sx ⫽
冑兺
D2
2N
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Fig 2. Liner measurements: 1, anterior cranial base (S-N); 2, horizontal distance between the maxillary and mandibular incisal edges (overjet); 3, vertical distance between the maxillary and mandibular incisal edges (overbite); 4, maxillary incisal edge to palatal plane (PP-U1); 5, maxillary molar cusp to palatal plane (PP-U6); 6, maxillary incisal edge to upper lip (UL-U1).
Fig 3. Lateral cephalometric radiographs of A, patient in the J-HG group and B, patient in the implant group. Arrow indicates the location of the miniscrew in the implant group.
where Sx is the error of the measurement, D is the difference between duplicated measurements, and N is the number of double measurements. The errors of measurement were 0.12 mm for the periapical radiographs and 0.28 mm for the lateral cephalometric radiographs.
and postintrusion cephalometric analyses in each group. The Wilcoxon signed rank test was used to compare the posttreatment results between the J-HG and implant groups. Probability ⬎.05 was considered insignificant. These analyses were carried out with statistical analysis software (Statview; SPSS, Chicago, III).
Statistical analysis
RESULTS
For the statistical analysis, the Mann-Whitney U test was used to examine the difference between pretreatment
There were no significant differences between the implant and the J-HG groups in any measured cepha-
American Journal of Orthodontics and Dentofacial Orthopedics Volume 133, Number 5
Fig 4. Photograph of how the orthodontic intrusion force was applied from the miniscrews.
lometric variables (Table I). There was also no significant difference in the initial PAR scores of the cast models between the groups (Table II). Furthermore, there was no significant difference in the initial DI scores between the groups (Table II). In the implant group, significant reductions in overjet (P ⬍0.01), overbite (P ⬍0.01), PP-U1 (P ⬍0.05), and UL-U1 (P ⬍0.05) were observed after intrusion and retraction of the incisors compared with pretreatment (definitions in the legend of Fig 2) (Table III). In the J-HG group, significant reductions (approximately 9°) of SN-U1 (P ⬍0.05) were observed after intrusion and retraction of the incisors compared with the pretreatment analysis (Table III). Significant reductions in the amount of overjet (P ⬍0.01), overbite (P ⬍0.01), and UL-U1 (P ⬍0.05) were observed after intrusion and retraction of the incisors. There were significantly greater reductions in overbite (P ⬍0.05), PP-U1 (P ⬍0.05), and UL-U1 (P ⬍0.05) in the implant group than in the J-HG group (Table III). There was no significant difference in the treatment period between the 2 groups (Table IV). However, there was a significant difference in the amount of root resorption after intrusion of the maxillary incisors (Table IV). Significantly more root resorption was observed in the J-HG group than in the implant group (P ⬍0.05) (Table IV). From the force vector analysis, there was a significant difference in both directions between the groups (Table IV). Significantly more force was applied in the vertical direction in the implant group than in the J-HG group (P ⬍0.0001). However, significantly more force was applied to the J-HG group in the horizontal direction (P ⬍0.0001). DISCUSSION
Correction of a deep-bite malocclusion is difficult in orthodontic treatment. Generally, there are 3 ways to correct overbite: absolute intrusion of the incisors,
Deguchi et al 657
relative intrusion of the incisors, and extrusion of the posterior teeth.5 In this study, most patients had excessive display of maxillary incisors (averages, 3.4 mm in the implant group and 3.6 mm in the J-HG group) with a high mandibular plane angle (averages, 40° in the implant group and 39° in the J-HG group). Based on cephalometric analysis, the Z-score of UL-U1 resulted in significant differences in both groups compared with the normal value. Thus, rather than correcting the deep bite by extruding the molars, absolute intrusion was necessary in these patients. Furthermore, since all were nongrowing patients, relative intrusion of the maxillary incisors was difficult. Various methods of intruding the incisors have been reported.1-3 The most common method is the utility arch. However, in patients with a high mandibular plane angle, the reciprocal forces on the posterior teeth result in additional clockwise rotation of the mandible. To cancel this unfavorable reciprocal force, either headgear or J-HG is commonly used for orthodontic treatment, but patient cooperation is necessary with headgear. Especially in adults, it is extremely difficult to get patients to constantly wear their headgear. In this study, significant decreases in the amount of overbite (approximately 5 mm in implant group and 3 mm in J-HG group) were observed. This reduction of overbite was due to either intrusion of the maxillary or the mandibular incisors, or extrusion of the molars. Since there was no change in SN to mandibular plane, we suggest that ideal absolute intrusion of the incisors was accomplished in both groups. Furthermore, the most significant advantage of the miniscrew is that the effect is not influenced by patient cooperation. In patients who require intrusion in the posterior and anterior segments, an additional miniscrew could be placed at the molars, and this could intrude the whole maxilla. In our study, a maximum of 3.0 mm and an average of 1.1 mm of intrusion in maxillary incisors were observed in the J-HG group. However, to the best of our knowledge, there is only 1 study that analyzed the effect of J-HG.19 In that study, the effect of J-HG was examined in only 1 subject, and the result was not quantitatively or statistically analyzed. The amount of incisor intrusion in the J-HG group seemed to be less than expected, but there was almost no additional clockwise rotation of the mandible, indicating almost no reciprocal effect on the posterior segments with the J-HG. The time charts recorded by each patient showed that the longer the headgear was worn, the better the effect of intrusion (data not shown). Patient cooperation is the key for successful incisor intrusion with the J-HG.
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Fig 5. Schema of estimated force vector in A, J-HG group and B, implant group. Intrusion force (100 g) was divided into vertical and horizontal directions. The horizontal line was determined as a parallel line to the archwire; the vertical line was determined as a line perpendicular to the archwire. Table I.
Comparison of pretreatment measurements between groups Implant group
Variable Angular (°) SNA SNB ANB PP-SN Occl Pl Mp-SN SN-U1 Linear (mm) S-N Overjet Overbite PP-U1 PP-U6 UL-U1
J-HG group
Norm
SD
Mean
SD
Z-score
Mean
SD
Z-score
Difference
80.8 77.9 2.8 9.1 16.9 37.1 105.9
3.6 4.5 2.4 3.5 4.4 4.6 8.7
83.8 78.6 5.1 10.5 16.9 39.6 107.7
3.2 2.8 2.8 2.4 4.0 6.8 8.4
0.9 0.2 0.8 0.6 0.0 0.4 0.2
83.1 77.9 5.2 9.7 16.7 39.2 103.4
2.3 4.0 2.6 2.5 3.0 4.4 8.9
1 0 0.9 0.2 ⫺0.1 0.5 ⫺0.3
NS NS NS NS NS NS NS
67.9 3.1 3.3 31.0 24.6 2.0
3.6 1.1 1.9 2.3 2.0 2.0
69.6 6.9 5.0 35.2 27.8 3.4
2.6 3.3 1.9 3.1 2.4 1.3
0.7 1.2 0.9 1.4 1.3 1.1
70.7 6.2 4.4 33.0 25.2 3.6
2.1 2.0 2.1 4.4 1.8 1.3
1.3 1.6 0.1 0.5 0.3 1.2
NS NS NS NS NS NS
NS, Not significant between groups (P ⬍0.05). Table II.
Pretreatment DI and PAR scores DI
Implant J-HG
PAR (UK)
PAR (US)
Min
Max
Mean
SD
Min
Max
Mean
SD
Min
Max
Mean
SD
Difference
6 13
38 35
22.4 21.2
10.6 7.7
30 25
47 42
36.1 34.3
6 5.6
20 18
34 33
26.8 25
5.4 5.1
NS NS
NS, Not significant between groups (P ⬍0.05); Min, minimum; Max, maximum.
A maximum of 5.0 mm and an average of 3.6 mm of incisor intrusion were observed in the implant group. One reason for the greater incisor intrusion compared with the J-HG group might be related to the amount of vertical force. From the estimated force analysis, if
100 g of force is applied to 1 side of the hook, it results in an average of 90 g of vertical force in the implant group but only 60 g in the J-HG group. In the horizontal direction, it results in 40 g of force in the implant group and 80 g in the J-HG group on average. However,
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Table III.
Comparison of pretreatment and postintrusion measurements Implant group
Variable Angular (°) SNA SNB ANB PP-SN Occl Pl Mp-SN SN-U1 Linear (mm) S-N Overjet Overbite PP-U1 PP-U6 UL-U1
Pretreatment
Postintrustion
J-HG group Difference Mean
Pretreatment
Postintrustion
Difference
SD
Mean
SD
Mean
SD
Mean
SD
Norm
SD
Mean
SD
Mean
SD
80.8 77.9 2.8 9.1 16.9 37.1 105.9
3.6 4.5 2.4 3.5 4.4 4.6 8.7
83.8 78.6 5.1 10.5 16.9 39.6 107.7
3.2 2.8 2.8 2.4 4.0 6.8 8.4
83.5 79.5 4.0 11.0 16.0 39.9 101.5
3.1 2.9 2.5 2.4 3.7 6.4 7.1
⫺0.3 0.9 ⫺1.1 0.5 ⫺0.9 0.3 ⫺6.2
0.9 0.8 1.1 1.4 3.6 1.2 8.9
83.1 77.9 5.2 9.7 16.7 39.2 103.4
2.3 4.0 2.6 2.5 3.0 4.4 8.9
82.1 78.1 4.2 10.0 17.9 39.6 94.9
3.1 4.0 2.1 2.3 4.7 5.0 7.4
⫺1.0 0.2 ⫺1.0 0.3 1.2 0.4 ⫺8.5*
1.5 1.2 2.0 0.9 2.6 1.1 9.8
67.9 3.1 3.3 31.0 24.6 2.0
3.6 1.1 1.9 2.3 2.0 2.0
69.6 6.9 5.0 34.4 28.9 3.4
2.6 3.3 1.9 3.2 2.4 1.3
70.1 3.1 0.5 30.8 28.9 –0.1
3.1 1.1 1.0 3.2 3.4 2.2
0.5 ⫺3.8† ⫺4.5†‡ ⫺3.6*‡ 0.1 ⫺3.5*‡
0.8 2.5 1.7 1.7 2.0 2.6
70.7 6.2 4.4 32.1 25.2 3.6
2.1 2.0 2.1 3.5 1.8 1.3
71.2 3.4 1.0 31.0 26.5 2.1
2.5 1.1 1.5 3.0 2.9 0.8
0.5 ⫺2.8† ⫺3.4† ⫺1.1 1.3 ⫺1.5*
0.8 1.7 1.0 1.6 2.9 1.3
Significant difference between pretreatment and postintrusion: *P ⬍0.05; †P ⬍0.01; ‡Significant difference between groups (P ⬍0.05). Table IV.
Other variables Implant
Vertical force (g) Horizontal force (g) Intrusion duration (mo) Root resorption rate (mm)
J-HG
Difference
Min
Max
Mean
SD
Min
Max
Mean
SD
P value
82.9 24.2 6 0.3
97 57.4 8 1.4
89.7 42.1 6.6 0.8
6.1 13.3 0.7 0.4
50 68.2 5 0.5
73.1 88.3 10 1.7
61 78.5 7.3 1.2
9.3 7.1 1.6 0.4
⬍0.0001 ⬍0.0001 NS ⬍0.05
NS, Not significant between groups (P ⬍0.05); Min, minimum; Max, maximum.
precise measurement of the actual amount of force produced by the ligature wire in the implant group was impossible in this study. There was also a difference in SN-U1 between pretreatment and postintrusion in the groups. In the J-HG group, the incisors were retracted approximately 3° more than in the implant group. This indicates that, in the J-HG group, less flaring of the incisors was observed compared with the implant group. The reason for this might be, in the implant group, the application of force (location of miniscrew) was positioned more labially from the center of resistance compared with the J-HG group. However, because we retracted as we intruded the incisors, the actual change of the axial inclination of the incisors due to the effect of the miniscrew and the J-HG could not be evaluated. Many studies have analyzed the suspected causes of external apical root resorption (EARR). Particularly, amount of force, duration of active treatment, and amount of lingual root torque were suggested as factors in EARR.15,20-22 Intruding mechanics has also been
considered a factor related to EARR.15 In our study, root resorption averaged 0.8 mm in the implant group and 1.2 mm in the J-HG group. The amount of root resorption in both groups was similar to that of previous studies about root resorption during orthodontic treatment.21,23,24 In the implant group, the amount of root resorption tended to be less than that of previous studies. Thus, incisor intrusion does not seem to be a major factor contributing to EARR based on our results; this is consistent with some studies.14,23-25 However, there were substantive methodologic differences and various experimental designs in these studies, making it difficult to compare the results. Interestingly, the amount of EARR was significantly less in the implant group compared with the J-HG group. There were no significant differences in the amount of force (both, 100 g per side) and the duration of intrusion (both, approximately 7 months). One difference between the implant and the J-HG is the type of force application. In the J-HG, the force is applied to the teeth continuously at night and then completely released
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during the day. This cycle is constantly repeated until the next appointment. In the implant group, the force is transient (not continuous because the incisors are intruded by ligature wires instead of power chain), active only during the first several days after activation of the ligature wire, resulting in a longer quiescence period compared with the J-HG group. Thus, with the J-HG, depending on the patient’s cooperation, there might be a “jiggling” effect on the tooth; this is a known risk factor for EARR.26 We suggest that 1 reason for the lower amount of EARR observed in the implant-group might be related to the type of force during orthodontic treatment. The lower root resorption might also be related to the type of force generated by the ligature wire from the miniscrew rather than intruding the incisors with continuous force with power chain. However, since our sample size was limited and the result might not be the same with a larger sample size, further investigation is necessary to elucidate the relationship of EARR and implant-anchored orthodontics with a larger sample to confirm our suggestion. CONCLUSIONS
Miniscrews can be used as orthodontic anchorage to effectively intrude the incisors, especially in correcting the excessive gingival line without patient cooperation. Furthermore, intruding the incisors by activating the ligature wire with miniscrews does not seem to increase the rate of EARR compared with other intruding mechanics during orthodontic tooth movement. REFERENCES 1. Burstone CJ. Deep overbite correction by intrusion. Am J Orthod 1977;72:1-22. 2. Janzen EK. A balanced smile—a most important treatment objective. Am J Orthod 1977;72:359-72. 3. Ricketts RM, Bench RW, Gugino CF, Hilgers JJ, Schulhof RJ. Bioprogressive therapy. Denver: Rocky Mountain Orthodontics; 1979. 4. Shroff B, Yoon WM, Lindauer SJ, Burstone CJ. Simultaneous intrusion and retraction using a three-piece base arch. Angle Orthod 1997;67:455-61. 5. Proffit WR, Fields HW. Contemporary orthodontics. 3rd ed. St Louis: Mosby Year Book; 2000. p. 200-2. 6. Kyung HM, Park HS, Bae SM, Sung JH, Kim IB. Development of orthodontic micro-implants for intraoral anchorage. J Clin Orthod 2003;37:321-8. 7. Miyawaki S, Koyama I, Inoue M, Mishima K, Sugahara T, Takano-Yamamoto T. Factors associated with the stability of titanium screws placed in the posterior region for orthodontic anchorage. Am J Orthod Dentofacial Orthop 2003;124:373-8.
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8. Park YC, Lee SY, Kim DH, Jee SH. Intrusion of posterior teeth using mini-screw implants. Am J Orthod Dentofacial Orthop 2003;123:690-4. 9. Kuroda S, Sugawara Y, Deguchi T, Kyung HM, TakanoYamamoto T. Clinical use of miniscrew implants as orthodontic anchorage: success rates and postoperative discomfort. Am J Orthod Dentofacial Orthop 2007;131:9-15. 10. Kuroda S, Katayama A, Takano-Yamamoto T. Severe anterior open-bite case treated using titanium screw anchorage Angle Orthod 2004;74:558-67. 11. Kuroda S, Sakai Y, Tamamura N, Deguchi T, TakanoYamamoto T. Treatment of severe anterior open bite with skeletal anchorage in adults: comparison with orthognathic surgery outcomes. Am J Orthod Dentofacial Orthop 2007;132: 599-605. 12. Kanomi R. Mini-implant for orthodontic anchorage. J Clin Orthod 1997;31:763-7. 13. Ohnishi H, Yagi T, Yasuda Y, Takada K. A mini-implant for orthodontic anchorage in a deep overbite case. Angle Orthod 2005;75:393-401. 14. Dermaut LR, De Munck A. Apical root resorption of upper incisors caused by intrusive tooth movement: a radiographic study. Am J Orthod Dentofacial Orthop 1986;90:321-6. 15. Parker RJ, Harris EF. Directions of orthodontic tooth movements associated with external apical root resorption of the maxillary central incisor. Am J Orthod Dentofacial Orthop 1998;114:677-83. 16. Wada K, Matsushita K, Shimazaki S, Miwa Y, Hasuike Y, Susami R. An evaluation of a new case analysis of a lateral cephalometric roentgenogram. J Kanazawa Med Univ 1981;6: 60-70. 17. Linge L, Linge BO. Patient characteristics and treatment variables associated with apical root resorption during orthodontic treatment. Am J Orthod Dentofacial Orthop 1991;99:35-43. 18. Dahlberg G. Statistical methods for medical and biological students. London: George Allen & Unwin; 1940. p.122-32. 19. Melson B, Agerbæk N, Markenstam G. Intrusion of incisors in adult patients with marginal bone loss. Am J Orthod Dentofacial Orthop 1989;96:232-41. 20. Mullenhauer B. Towards paradigmless orthodontics. Am J Orthod Dentofacial Orthop 1987;92:437-44. 21. Mirabella AD, Årtun J. Risk factors for apical root resorption of maxillary anterior teeth in adult orthodontic patients. Am J Orthod Dentofacial Orthop 1995;108:48-55. 22. Kurol J, Owman-Moll P, Lundgren D. Time related root resorptions after application of a controlled continuous orthodontic force. Am J Orthod Dentofacial Orthop 1996;110:303-10. 23. McFadden WM, Engstrom C, Engstrom H, Anholm JM. A study of the relationship between incisor intrusion and root shortening. Am J Orthod Dentofacial Orthop 1989;96:390-6. 24. Baumrind S, Korn EL, Boyd RL. Apical root resorption in orthodontically treated adults. Am J Orthod Dentofacial Orthop 1996;110:311-20. 25. DeShields RW. A study of root resorption in treated Class II Division 1 malocclusions. Angle Orthod 1969;39:231-45. 26. Stuteville OH. Injuries caused by orthodontic forces and the ultimate results of these injuries. Am J Orthod Oral Surg 1938; 24:103-16.
Comparison of the intrusion effects on the maxillary incisors between implant anchorage and J-hook headgear Toru Deguchi,a Takashi Murakami,b Shingo Kuroda,a Toshinori Yabuuchi,b Hiroshi Kamioka,c and Teruko Takano-Yamamotod Okayama, Japan Introduction: Recently, miniscrews have been used to provide anchorage during orthodontic treatment, especially for incisor intrusion. Miniscrews during incisor intrusion are commonly used in implant orthodontics. Traditionally, effective incisor intrusion has been accomplished with J-hook headgear. In this study, we compared the effect of incisor intrusion, force vector, and amount of root resorption between implant orthodontics and J-hook headgear. Methods: Lateral cephalometric radiographs from 8 patients in the implant group and 10 patients in the J-hook headgear group were analyzed for incisor retraction. The estimated force vector was analyzed in the horizontal and vertical directions in both groups. Root resorption was also measured on periapical radiographs. Results: In the implant group, significant reductions in overjet, overbite, maxillary incisor to palatal plane, and maxillary incisor to upper lip were observed after intrusion of the incisors. In the J-hook headgear group, significant reductions in overjet, overbite, maxillary incisor to upper lip, and maxillary incisor to SN plane were observed after intrusion of the incisors. There were significantly greater reductions in overbite, maxillary incisor to palatal plane, and maxillary incisor to upper lip in the implant group than in the J-hook headgear group. Estimated force analysis resulted in significantly more force in the vertical direction and less in the horizontal direction in the implant group. Furthermore, significantly less root resorption was observed in the implant group compared with the J-hook headgear group. Conclusions: The maxillary incisors were effectively intruded by using miniscrews as orthodontic anchorage without patient cooperation. The amount of root resorption was not affected by activating the ligature wire from the miniscrew during incisor intrusion. (Am J Orthod Dentofacial Orthop 2008;133:654-60)
I
n correcting a Class II deepbite, an important clinical goal is to reduce the amount of overbite. There are several methods of reducing overbite during orthodontic treatment: utility arch combined with high-pull headgear, intrusive arch, and J-hook headgear (J-HG).1-3 However, intruding the incisors with an archwire requires complex wire bending adjustments to prevent undesirable side effects such as extrusion and flaring of the posterior teeth.1,4 Extraoral From the Department of Orthodontics and Dentofacial Orthopedics, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama, Japan. a Lecturer. b Graduate student. c Associate professor. d Professor and chair. Supported by grants-in-aid for scientific research from the Japan Society for the Promotion of Science. Reprint requests to: Teruko Takano-Yamamoto, Department of Orthodontics and Dentofacial Orthopedics, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, 2-5-1 Shikata-Cho, Okayama 700-8525, Japan; e-mail, [email protected]. Submitted, January 2006; revised and accepted, April 2006. 0889-5406/$34.00 Copyright © 2008 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2006.04.047
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appliances are effective in controlling the anchorage, although patient cooperation is necessary to achieve effective intrusion of the incisors. Especially in patients with deepbite and excessive gingival display resulting in a gummy smile, absolute intrusion of the maxillary incisors is required rather than extrusion of the molars.5 Recently, several reports have described the clinical use of miniscrews in orthodontic treatment.6-9 Especially, intrusion of the maxillary posterior segments by these orthodontic implants was suggested to achieve excellent results and avoid maxillary surgery in patients with severe open bites.10,11 The use of miniscrews to intrude the maxillary incisors was also reported.12,13 These reports indicated that effective maxillary incisor intrusion was achieved with minimal side effects without patient cooperation by using miniscrews as anchorage. However, to routinely use these miniscrews as anchorage to effectively intrude the incisors, we need to quantitatively evaluate the effect of the amount of incisor movement with a larger sample size. As described above, J-HG can effectively intrude the maxillary incisors, although the intrusion effect might vary, since it depends on patient cooperation. In addition, it is
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important to consider the amount and the direction of force of these appliances, resulting in incisor movement in both vertical and horizontal directions. Furthermore, a side effect of intruding the maxillary incisors is root resorption during active orthodontic treatment.14,15 There has been no investigation of the effect of root resorption during incisor intrusion with miniscrews or J-HG. The objectives of this study were to (1) compare maxillary incisor displacement, (2) analyze and compare the vector of the force, and (3) compare the amount of root resorption between the miniscrew and J-HG. MATERIAL AND METHODS
Our subjects included 18 patients (8 in the implant group, and 10 in the J-HG group). The average ages were 20.7 ⫾ 2.5 years (1 man, 9 women) in the J-HG group and 21.5 ⫾ 3.7 years in the implant group (1 man, 7 women). All patients had their maxillary first premolars extracted. Lateral cephalometric radiographs and cast models were used for the pretreatment cephalometric analysis compared with Japanese norms,16 the peer assessment rating (PAR), and the discrepancy index (DI) in both groups. For the posttreatment analysis, cephalograms were taken just after intrusion of the maxillary incisors because, if the cephalograms were obtained after all orthodontic treatment, other factors would affect the position of the maxillary incisors. Especially during detailing, we often added additional torque in the incisors; this would obscure the real effect of these intrusion mechanics. Seven angular and 6 linear measurements were made to evaluate skeletal and dental changes before and after incisor intrusion (Figs 1 and 2). In the J-HG group, an edgewise appliance (.018-in slot) with a J-hook was adapted to .016 ⫻ .022-in stainless steel wire for 100 g per side. Force was checked once a month (every appointment) with a force gauge and adjusted to maintain a force of 100 g per side (average, 109.2 g; SD 15 g). All patients were requested to record the total time that they wore their J-HG every day. Only patients wearing their headgear more than 7 hours per day on average were included in this study. Lateral cephalometric radiographs were taken with the J-HG on at postintrusion in every patient (Fig 3, A). In the implant group, 2 miniscrews (diameter; 1.5 mm; length, 6 mm) were placed under local anesthesia at the premaxillary region by senior oral surgeons at the Department of Oral and Maxillofacial Surgery, Okayama University Hospital in Japan (Fig 3, B). The orthodontic load was applied by using a ligature wire from the implant to the soldered hook on the .016 ⫻ .022-in stainless steel wire after 1 month of healing (Fig 4). The amount of force was checked by setting the force gauge
Fig 1. Angular measurements: 1, SNA; 2, SNB; 3, ANB; 4, SN to palatal plane (PP-SN); 5, SN to occlusal plane (Occl Pl); 6, SN to mandibular plane (Mp-SN); 7, U1 to SN (SN-U1).
at the level of the hook to the ligature wire that would be approximately 80 to 120 g of intrusion force. The estimated vector analyses of the intrusion force were calculated in both groups. Intrusion force was set as 100 g from the soldered hook on the wire to implant or to the headgear hook where the elastic rubber band is attached (Fig 5). The estimated intrusion force was divided into the vertical direction and the horizontal direction. A horizontal line was determined as the line from the soldered hook that is parallel to the main archwire. The vertical line was determined as the line from the implant or the hook of the headgear that is perpendicular to the horizontal force. The calculation of the horizontal and vertical force was done as follows. Intrusion force (100 g) ⫻ cos⌰ ⫽ horizontal force Intrusion force (100 g) ⫻ sin⌰ ⫽ vertical force Measurement of root resorption was analyzed with the periapical films taken at pretreatment and postintrusion. The amount of root resorption was calculated according to the method reported previously.17 The reproducibility of the measurements on lateral cephalometric and periapical radiographs was assessed by statistically analyzing the difference between double measurements made 2 months apart in all 18 subjects.18 The method error was calculated from the equation: Sx ⫽
冑兺
D2
2N
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Fig 2. Liner measurements: 1, anterior cranial base (S-N); 2, horizontal distance between the maxillary and mandibular incisal edges (overjet); 3, vertical distance between the maxillary and mandibular incisal edges (overbite); 4, maxillary incisal edge to palatal plane (PP-U1); 5, maxillary molar cusp to palatal plane (PP-U6); 6, maxillary incisal edge to upper lip (UL-U1).
Fig 3. Lateral cephalometric radiographs of A, patient in the J-HG group and B, patient in the implant group. Arrow indicates the location of the miniscrew in the implant group.
where Sx is the error of the measurement, D is the difference between duplicated measurements, and N is the number of double measurements. The errors of measurement were 0.12 mm for the periapical radiographs and 0.28 mm for the lateral cephalometric radiographs.
and postintrusion cephalometric analyses in each group. The Wilcoxon signed rank test was used to compare the posttreatment results between the J-HG and implant groups. Probability ⬎.05 was considered insignificant. These analyses were carried out with statistical analysis software (Statview; SPSS, Chicago, III).
Statistical analysis
RESULTS
For the statistical analysis, the Mann-Whitney U test was used to examine the difference between pretreatment
There were no significant differences between the implant and the J-HG groups in any measured cepha-
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Fig 4. Photograph of how the orthodontic intrusion force was applied from the miniscrews.
lometric variables (Table I). There was also no significant difference in the initial PAR scores of the cast models between the groups (Table II). Furthermore, there was no significant difference in the initial DI scores between the groups (Table II). In the implant group, significant reductions in overjet (P ⬍0.01), overbite (P ⬍0.01), PP-U1 (P ⬍0.05), and UL-U1 (P ⬍0.05) were observed after intrusion and retraction of the incisors compared with pretreatment (definitions in the legend of Fig 2) (Table III). In the J-HG group, significant reductions (approximately 9°) of SN-U1 (P ⬍0.05) were observed after intrusion and retraction of the incisors compared with the pretreatment analysis (Table III). Significant reductions in the amount of overjet (P ⬍0.01), overbite (P ⬍0.01), and UL-U1 (P ⬍0.05) were observed after intrusion and retraction of the incisors. There were significantly greater reductions in overbite (P ⬍0.05), PP-U1 (P ⬍0.05), and UL-U1 (P ⬍0.05) in the implant group than in the J-HG group (Table III). There was no significant difference in the treatment period between the 2 groups (Table IV). However, there was a significant difference in the amount of root resorption after intrusion of the maxillary incisors (Table IV). Significantly more root resorption was observed in the J-HG group than in the implant group (P ⬍0.05) (Table IV). From the force vector analysis, there was a significant difference in both directions between the groups (Table IV). Significantly more force was applied in the vertical direction in the implant group than in the J-HG group (P ⬍0.0001). However, significantly more force was applied to the J-HG group in the horizontal direction (P ⬍0.0001). DISCUSSION
Correction of a deep-bite malocclusion is difficult in orthodontic treatment. Generally, there are 3 ways to correct overbite: absolute intrusion of the incisors,
Deguchi et al 657
relative intrusion of the incisors, and extrusion of the posterior teeth.5 In this study, most patients had excessive display of maxillary incisors (averages, 3.4 mm in the implant group and 3.6 mm in the J-HG group) with a high mandibular plane angle (averages, 40° in the implant group and 39° in the J-HG group). Based on cephalometric analysis, the Z-score of UL-U1 resulted in significant differences in both groups compared with the normal value. Thus, rather than correcting the deep bite by extruding the molars, absolute intrusion was necessary in these patients. Furthermore, since all were nongrowing patients, relative intrusion of the maxillary incisors was difficult. Various methods of intruding the incisors have been reported.1-3 The most common method is the utility arch. However, in patients with a high mandibular plane angle, the reciprocal forces on the posterior teeth result in additional clockwise rotation of the mandible. To cancel this unfavorable reciprocal force, either headgear or J-HG is commonly used for orthodontic treatment, but patient cooperation is necessary with headgear. Especially in adults, it is extremely difficult to get patients to constantly wear their headgear. In this study, significant decreases in the amount of overbite (approximately 5 mm in implant group and 3 mm in J-HG group) were observed. This reduction of overbite was due to either intrusion of the maxillary or the mandibular incisors, or extrusion of the molars. Since there was no change in SN to mandibular plane, we suggest that ideal absolute intrusion of the incisors was accomplished in both groups. Furthermore, the most significant advantage of the miniscrew is that the effect is not influenced by patient cooperation. In patients who require intrusion in the posterior and anterior segments, an additional miniscrew could be placed at the molars, and this could intrude the whole maxilla. In our study, a maximum of 3.0 mm and an average of 1.1 mm of intrusion in maxillary incisors were observed in the J-HG group. However, to the best of our knowledge, there is only 1 study that analyzed the effect of J-HG.19 In that study, the effect of J-HG was examined in only 1 subject, and the result was not quantitatively or statistically analyzed. The amount of incisor intrusion in the J-HG group seemed to be less than expected, but there was almost no additional clockwise rotation of the mandible, indicating almost no reciprocal effect on the posterior segments with the J-HG. The time charts recorded by each patient showed that the longer the headgear was worn, the better the effect of intrusion (data not shown). Patient cooperation is the key for successful incisor intrusion with the J-HG.
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Fig 5. Schema of estimated force vector in A, J-HG group and B, implant group. Intrusion force (100 g) was divided into vertical and horizontal directions. The horizontal line was determined as a parallel line to the archwire; the vertical line was determined as a line perpendicular to the archwire. Table I.
Comparison of pretreatment measurements between groups Implant group
Variable Angular (°) SNA SNB ANB PP-SN Occl Pl Mp-SN SN-U1 Linear (mm) S-N Overjet Overbite PP-U1 PP-U6 UL-U1
J-HG group
Norm
SD
Mean
SD
Z-score
Mean
SD
Z-score
Difference
80.8 77.9 2.8 9.1 16.9 37.1 105.9
3.6 4.5 2.4 3.5 4.4 4.6 8.7
83.8 78.6 5.1 10.5 16.9 39.6 107.7
3.2 2.8 2.8 2.4 4.0 6.8 8.4
0.9 0.2 0.8 0.6 0.0 0.4 0.2
83.1 77.9 5.2 9.7 16.7 39.2 103.4
2.3 4.0 2.6 2.5 3.0 4.4 8.9
1 0 0.9 0.2 ⫺0.1 0.5 ⫺0.3
NS NS NS NS NS NS NS
67.9 3.1 3.3 31.0 24.6 2.0
3.6 1.1 1.9 2.3 2.0 2.0
69.6 6.9 5.0 35.2 27.8 3.4
2.6 3.3 1.9 3.1 2.4 1.3
0.7 1.2 0.9 1.4 1.3 1.1
70.7 6.2 4.4 33.0 25.2 3.6
2.1 2.0 2.1 4.4 1.8 1.3
1.3 1.6 0.1 0.5 0.3 1.2
NS NS NS NS NS NS
NS, Not significant between groups (P ⬍0.05). Table II.
Pretreatment DI and PAR scores DI
Implant J-HG
PAR (UK)
PAR (US)
Min
Max
Mean
SD
Min
Max
Mean
SD
Min
Max
Mean
SD
Difference
6 13
38 35
22.4 21.2
10.6 7.7
30 25
47 42
36.1 34.3
6 5.6
20 18
34 33
26.8 25
5.4 5.1
NS NS
NS, Not significant between groups (P ⬍0.05); Min, minimum; Max, maximum.
A maximum of 5.0 mm and an average of 3.6 mm of incisor intrusion were observed in the implant group. One reason for the greater incisor intrusion compared with the J-HG group might be related to the amount of vertical force. From the estimated force analysis, if
100 g of force is applied to 1 side of the hook, it results in an average of 90 g of vertical force in the implant group but only 60 g in the J-HG group. In the horizontal direction, it results in 40 g of force in the implant group and 80 g in the J-HG group on average. However,
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Table III.
Comparison of pretreatment and postintrusion measurements Implant group
Variable Angular (°) SNA SNB ANB PP-SN Occl Pl Mp-SN SN-U1 Linear (mm) S-N Overjet Overbite PP-U1 PP-U6 UL-U1
Pretreatment
Postintrustion
J-HG group Difference Mean
Pretreatment
Postintrustion
Difference
SD
Mean
SD
Mean
SD
Mean
SD
Norm
SD
Mean
SD
Mean
SD
80.8 77.9 2.8 9.1 16.9 37.1 105.9
3.6 4.5 2.4 3.5 4.4 4.6 8.7
83.8 78.6 5.1 10.5 16.9 39.6 107.7
3.2 2.8 2.8 2.4 4.0 6.8 8.4
83.5 79.5 4.0 11.0 16.0 39.9 101.5
3.1 2.9 2.5 2.4 3.7 6.4 7.1
⫺0.3 0.9 ⫺1.1 0.5 ⫺0.9 0.3 ⫺6.2
0.9 0.8 1.1 1.4 3.6 1.2 8.9
83.1 77.9 5.2 9.7 16.7 39.2 103.4
2.3 4.0 2.6 2.5 3.0 4.4 8.9
82.1 78.1 4.2 10.0 17.9 39.6 94.9
3.1 4.0 2.1 2.3 4.7 5.0 7.4
⫺1.0 0.2 ⫺1.0 0.3 1.2 0.4 ⫺8.5*
1.5 1.2 2.0 0.9 2.6 1.1 9.8
67.9 3.1 3.3 31.0 24.6 2.0
3.6 1.1 1.9 2.3 2.0 2.0
69.6 6.9 5.0 34.4 28.9 3.4
2.6 3.3 1.9 3.2 2.4 1.3
70.1 3.1 0.5 30.8 28.9 –0.1
3.1 1.1 1.0 3.2 3.4 2.2
0.5 ⫺3.8† ⫺4.5†‡ ⫺3.6*‡ 0.1 ⫺3.5*‡
0.8 2.5 1.7 1.7 2.0 2.6
70.7 6.2 4.4 32.1 25.2 3.6
2.1 2.0 2.1 3.5 1.8 1.3
71.2 3.4 1.0 31.0 26.5 2.1
2.5 1.1 1.5 3.0 2.9 0.8
0.5 ⫺2.8† ⫺3.4† ⫺1.1 1.3 ⫺1.5*
0.8 1.7 1.0 1.6 2.9 1.3
Significant difference between pretreatment and postintrusion: *P ⬍0.05; †P ⬍0.01; ‡Significant difference between groups (P ⬍0.05). Table IV.
Other variables Implant
Vertical force (g) Horizontal force (g) Intrusion duration (mo) Root resorption rate (mm)
J-HG
Difference
Min
Max
Mean
SD
Min
Max
Mean
SD
P value
82.9 24.2 6 0.3
97 57.4 8 1.4
89.7 42.1 6.6 0.8
6.1 13.3 0.7 0.4
50 68.2 5 0.5
73.1 88.3 10 1.7
61 78.5 7.3 1.2
9.3 7.1 1.6 0.4
⬍0.0001 ⬍0.0001 NS ⬍0.05
NS, Not significant between groups (P ⬍0.05); Min, minimum; Max, maximum.
precise measurement of the actual amount of force produced by the ligature wire in the implant group was impossible in this study. There was also a difference in SN-U1 between pretreatment and postintrusion in the groups. In the J-HG group, the incisors were retracted approximately 3° more than in the implant group. This indicates that, in the J-HG group, less flaring of the incisors was observed compared with the implant group. The reason for this might be, in the implant group, the application of force (location of miniscrew) was positioned more labially from the center of resistance compared with the J-HG group. However, because we retracted as we intruded the incisors, the actual change of the axial inclination of the incisors due to the effect of the miniscrew and the J-HG could not be evaluated. Many studies have analyzed the suspected causes of external apical root resorption (EARR). Particularly, amount of force, duration of active treatment, and amount of lingual root torque were suggested as factors in EARR.15,20-22 Intruding mechanics has also been
considered a factor related to EARR.15 In our study, root resorption averaged 0.8 mm in the implant group and 1.2 mm in the J-HG group. The amount of root resorption in both groups was similar to that of previous studies about root resorption during orthodontic treatment.21,23,24 In the implant group, the amount of root resorption tended to be less than that of previous studies. Thus, incisor intrusion does not seem to be a major factor contributing to EARR based on our results; this is consistent with some studies.14,23-25 However, there were substantive methodologic differences and various experimental designs in these studies, making it difficult to compare the results. Interestingly, the amount of EARR was significantly less in the implant group compared with the J-HG group. There were no significant differences in the amount of force (both, 100 g per side) and the duration of intrusion (both, approximately 7 months). One difference between the implant and the J-HG is the type of force application. In the J-HG, the force is applied to the teeth continuously at night and then completely released
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during the day. This cycle is constantly repeated until the next appointment. In the implant group, the force is transient (not continuous because the incisors are intruded by ligature wires instead of power chain), active only during the first several days after activation of the ligature wire, resulting in a longer quiescence period compared with the J-HG group. Thus, with the J-HG, depending on the patient’s cooperation, there might be a “jiggling” effect on the tooth; this is a known risk factor for EARR.26 We suggest that 1 reason for the lower amount of EARR observed in the implant-group might be related to the type of force during orthodontic treatment. The lower root resorption might also be related to the type of force generated by the ligature wire from the miniscrew rather than intruding the incisors with continuous force with power chain. However, since our sample size was limited and the result might not be the same with a larger sample size, further investigation is necessary to elucidate the relationship of EARR and implant-anchored orthodontics with a larger sample to confirm our suggestion. CONCLUSIONS
Miniscrews can be used as orthodontic anchorage to effectively intrude the incisors, especially in correcting the excessive gingival line without patient cooperation. Furthermore, intruding the incisors by activating the ligature wire with miniscrews does not seem to increase the rate of EARR compared with other intruding mechanics during orthodontic tooth movement. REFERENCES 1. Burstone CJ. Deep overbite correction by intrusion. Am J Orthod 1977;72:1-22. 2. Janzen EK. A balanced smile—a most important treatment objective. Am J Orthod 1977;72:359-72. 3. Ricketts RM, Bench RW, Gugino CF, Hilgers JJ, Schulhof RJ. Bioprogressive therapy. Denver: Rocky Mountain Orthodontics; 1979. 4. Shroff B, Yoon WM, Lindauer SJ, Burstone CJ. Simultaneous intrusion and retraction using a three-piece base arch. Angle Orthod 1997;67:455-61. 5. Proffit WR, Fields HW. Contemporary orthodontics. 3rd ed. St Louis: Mosby Year Book; 2000. p. 200-2. 6. Kyung HM, Park HS, Bae SM, Sung JH, Kim IB. Development of orthodontic micro-implants for intraoral anchorage. J Clin Orthod 2003;37:321-8. 7. Miyawaki S, Koyama I, Inoue M, Mishima K, Sugahara T, Takano-Yamamoto T. Factors associated with the stability of titanium screws placed in the posterior region for orthodontic anchorage. Am J Orthod Dentofacial Orthop 2003;124:373-8.
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8. Park YC, Lee SY, Kim DH, Jee SH. Intrusion of posterior teeth using mini-screw implants. Am J Orthod Dentofacial Orthop 2003;123:690-4. 9. Kuroda S, Sugawara Y, Deguchi T, Kyung HM, TakanoYamamoto T. Clinical use of miniscrew implants as orthodontic anchorage: success rates and postoperative discomfort. Am J Orthod Dentofacial Orthop 2007;131:9-15. 10. Kuroda S, Katayama A, Takano-Yamamoto T. Severe anterior open-bite case treated using titanium screw anchorage Angle Orthod 2004;74:558-67. 11. Kuroda S, Sakai Y, Tamamura N, Deguchi T, TakanoYamamoto T. Treatment of severe anterior open bite with skeletal anchorage in adults: comparison with orthognathic surgery outcomes. Am J Orthod Dentofacial Orthop 2007;132: 599-605. 12. Kanomi R. Mini-implant for orthodontic anchorage. J Clin Orthod 1997;31:763-7. 13. Ohnishi H, Yagi T, Yasuda Y, Takada K. A mini-implant for orthodontic anchorage in a deep overbite case. Angle Orthod 2005;75:393-401. 14. Dermaut LR, De Munck A. Apical root resorption of upper incisors caused by intrusive tooth movement: a radiographic study. Am J Orthod Dentofacial Orthop 1986;90:321-6. 15. Parker RJ, Harris EF. Directions of orthodontic tooth movements associated with external apical root resorption of the maxillary central incisor. Am J Orthod Dentofacial Orthop 1998;114:677-83. 16. Wada K, Matsushita K, Shimazaki S, Miwa Y, Hasuike Y, Susami R. An evaluation of a new case analysis of a lateral cephalometric roentgenogram. J Kanazawa Med Univ 1981;6: 60-70. 17. Linge L, Linge BO. Patient characteristics and treatment variables associated with apical root resorption during orthodontic treatment. Am J Orthod Dentofacial Orthop 1991;99:35-43. 18. Dahlberg G. Statistical methods for medical and biological students. London: George Allen & Unwin; 1940. p.122-32. 19. Melson B, Agerbæk N, Markenstam G. Intrusion of incisors in adult patients with marginal bone loss. Am J Orthod Dentofacial Orthop 1989;96:232-41. 20. Mullenhauer B. Towards paradigmless orthodontics. Am J Orthod Dentofacial Orthop 1987;92:437-44. 21. Mirabella AD, Årtun J. Risk factors for apical root resorption of maxillary anterior teeth in adult orthodontic patients. Am J Orthod Dentofacial Orthop 1995;108:48-55. 22. Kurol J, Owman-Moll P, Lundgren D. Time related root resorptions after application of a controlled continuous orthodontic force. Am J Orthod Dentofacial Orthop 1996;110:303-10. 23. McFadden WM, Engstrom C, Engstrom H, Anholm JM. A study of the relationship between incisor intrusion and root shortening. Am J Orthod Dentofacial Orthop 1989;96:390-6. 24. Baumrind S, Korn EL, Boyd RL. Apical root resorption in orthodontically treated adults. Am J Orthod Dentofacial Orthop 1996;110:311-20. 25. DeShields RW. A study of root resorption in treated Class II Division 1 malocclusions. Angle Orthod 1969;39:231-45. 26. Stuteville OH. Injuries caused by orthodontic forces and the ultimate results of these injuries. Am J Orthod Oral Surg 1938; 24:103-16.