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Factors associated with the stability of titanium screws placed in the posterior region for orthodontic anchorage
ORIGINAL ARTICLE
Factors associated with the stability of titanium screws placed in the posterior region for orthodontic anchorage Shouichi Miyawaki, DDS, PhD,a Isao Koyama, DDS, PhD,b Masahide Inoue, DDS,a Katsuaki Mishima, DDS, PhD,c Toshio Sugahara, DDS, PhD,c and Teruko Takano-Yamamoto, DDS, PhDa Okayama and Nara, Japan Recently, implant anchors such as titanium screws have been used for absolute anchorage during edgewise treatment. However, there have been few human studies reporting on the stability of implant anchors placed in the posterior region. The purpose of this study was to examine the success rates and to find the factors associated with the stability of titanium screws placed into the buccal alveolar bone of the posterior region. Fifty-one patients with malocclusions, 134 titanium screws of 3 types, and 17 miniplates were retrospectively examined in relation to clinical characteristics. The 1-year success rate of screws with 1.0-mm diameter was significantly less than that of other screws with 1.5-mm or 2.3-mm diameter or than that of miniplates. Flap surgery was associated with the patient’s discomfort. A high mandibular plane angle and inflammation of peri-implant tissue after implantation were risk factors for mobility of screws. However, we could not detect a significant association between the success rate and the following variables: screw length, kind of placement surgery, immediate loading, location of implantation, age, gender, crowding of teeth, anteroposterior jaw base relationship, controlled periodontitis, and temporomandibular disorder symptoms. We concluded that the diameter of a screw of 1.0 mm or less, inflammation of the peri-implant tissue, and a high mandibular plane angle (ie, thin cortical bone), were associated with the mobility (ie, failure) of the titanium screw placed into the buccal alveolar bone of the posterior region for orthodontic anchorage. (Am J Orthod Dentofacial Orthop 2003;124:373-8)
I
t is no exaggeration to say that anchorage in edgewise treatment is the most important factor that affects the treatment plan and result. Until now, various techniques to reinforce anchorage have been devised and used in orthodontic practice. Recently, several kinds of implant anchors providing absolute anchorage have attracted the attention of orthodontists.1,2 Among them, titanium screws, which were originally used for intermaxillary or bone fixation, have the following advantages: minimal anatomic limitation for placement, lower medical cost, simpler placement surgery, and less discomfort after implantation when compared with dental implants for abuta
Department of Orthodontics, Okayama University Graduate School of Medicine and Dentistry, Okayama, Japan. b Koyama Orthodontic Clinic, Nara, Japan. c Department of Oral and Maxillofacial Reconstructive Surgery, Okayama University Graduate School of Medicine and Dentistry. This study was partially supported by the Japan Society for the Promotion of Science in the form of Grants-in-Aid for Scientific Research (#13672149 and #20322232). Reprint requests to: Teruko Takano-Yamamoto, Okayama University Graduate School of Medicine and Dentistry, Department of Orthodontics, 2-5-1, Shikatacho, Okayama, 700-8525 Japan; e-mail, [email protected]. Submitted, October 2002; revised and accepted, December 2002. Copyright © 2003 by the American Association of Orthodontists. 0889-5406/2003/$30.00 ⫹ 0 doi:10.1016/S0889-5406(03)00565-1
ment.3,4 Therefore, titanium screws of various sizes have gradually come to be used for orthodontic absolute anchorage.1,5-9 However, there have been few human studies in which the success rates for various kinds of implant anchors were examined. With respect to dental implants for abutment, it was reported that the success rate is approximately 97% and that the factors decreasing their stability are patient age of more than 40 years, implant design and material such as alloy basket and hydroxyapatite-coated cylinder, lower bone density, placement in posterior region in the jaw, and use of a bone tap.10 Furthermore, it is well known that inflammation of the peri-implant tissue causes peri-implant bone loss, leading to implant mobility.11,12 However, most orthodontic patients are less than 40 years old and have relatively high bone density, and titanium screws have often been used for orthodontic anchorage in the posterior region.1,7-9 Therefore, several factors affecting dental implant stability might not be associated with the stability of implant anchors. According to recent reports on implant anchors in humans,1,13 titanium screws have occasionally been removed because of their mobility before or during orthodontic force application. Thus, the orthodontist 373
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needs to understand which variables are related to this mobility. To date, however, there have been few human studies that examined factors associated with the stability of titanium screws for orthodontic anchorage. The purpose of this study was to examine the success rates of 3 kinds of titanium screws and miniplates and to find the factors associated with the stability of titanium screws placed in the buccal alveolar bone of the posterior region as orthodontic anchors. MATERIAL AND METHODS
The subjects in this study were 51 patients with malocclusions (9 males, 42 females, mean age 21.8 years, SD 7.8 years) who had surgery to place an implant anchor in the posterior region for edgewise treatment at a university dental hospital or a private orthodontic clinic from March 1997 to October 2001. Before implantation, the advantages and disadvantages were explained to each patient and his or her parents when the use of an implant anchor was considered a desirable form of orthodontic treatment.7,8 After informed consent had been obtained, the same dentist at each clinic implanted the anchors. According to a preliminary study, the success rate for implant anchors at each clinic was similar (data not shown). Each dentist had more than 10 years of clinical experience. The clinical features and treatment progress for 1 year were retrospectively examined for the 51 subjects. The variables examined were age, gender, kind of surgery (flap or flapless), swelling and pain within a week after surgery, mandibular plane to sella-nasion angle (high, average, or low), and anteroposterior jaw base relationship (skeletal Class I, II, or III) according to the normal range,14 controlled periodontitis with horizontal alveolar bone resorption,15 temporomandibular disorder (TMD) symptoms such as temporomandibular joint sounds, pain, and difficulty in jaw opening,16 and crowding with an arch-length discrepancy of less than ⫺6 mm in the maxillary and mandibular dentition. Three kinds of titanium screws with different diameters and lengths (type A: diameter, 1.0 mm; length, 6 mm; type B: diameter,1.5 mm, length, 11 mm; type C: diameter, 2.3 mm, length, 14 mm) and miniplates with 2 screws (diameter, 2.0 mm; length, 5 mm) were used as implant anchors (Fig 1). The titanium screws were placed into the buccal alveolar bone through attachment gingiva in the second premolar to second molar region of the maxilla or the mandible. The miniplates were placed into the zygomatic process of the maxilla or the buccal alveolar bone of the mandible through buccal mucosa. They were implanted after local anesthesia had been administered. For 3 days after the implantation, analgesia and antibiotics were prescribed to the pa-
Fig 1. Photographs of implant anchors used in this study. Upper left: type A titanium screw with 1.0-mm diameter and 6-mm length. Upper middle: type B titanium screw with 1.5-mm diameter and 11-mm length. Upper right: type C titanium screw with 2.3-mm diameter and 14-mm length. Lower: modified miniplate with 2 screws of 2.0 mm diameter and 5 mm length.
tients. The type A screw with the 1.0-mm diameter had seldom been used for the previous 2 years because of its lower success rate. Conversely, type B and C screws and miniplates were frequently used during the period covered in this study (Fig 2). Continuous force applied to each screw was less than 2 N. If orthodontic force could be applied to the implant anchor for 1 year (or until completion of the orthodontic treatment), we recorded the implant anchor as a success; otherwise, it was considered a failure. With respect to the method of implantation of the screw, flap or flapless surgery was performed. As for the placement of the miniplate, flap surgery was always performed. The same oral surgeon performed all flap surgeries in this study. The timing of orthodontic force application was determined in each case according to the need of treatment. Clinical features and treatment progress for 1 year were retrospectively examined for 10 screws of type A, 101 screws of type B, 23 screws of type C, and 17 miniplates. The variables examined were as follows: inflammation of peri-implant tissue after placement,17 kind of placement surgery (flap or flapless),
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Fig 2. Intraoral photographs showing implant anchors in situ. Upper left: type B titanium screw with 1.5-mm diameter and 11-mm length. Upper right: type C titanium screw with 2.3-mm diameter and 14-mm length. Lower: modified miniplate with 2 screws of 2.0-mm diameter and 5-mm length.
location of implantation (maxillary or mandibular premolar-to-molar region, or zygomatic process), and the number of days from placement to orthodontic force application. Analysis of variance and post hoc test (Fisher protected least significant difference) were used to compare the success rate of each implant anchor in 51 subjects with 151 implant anchors. The Fisher PLSD or unpaired t test was used to examine the difference between success rates according to respective classifications of each clinical variable in 41 subjects with 124 screws of types B and C. According to the data distribution, the appropriate nonparametric test was used. The chi-square or Fisher exact probability test was used to examine the correlation between the success rate and respective classification of each variable for the 124 screws of types B and C. Finally, a logistic regression analysis model was used to estimate the degree of influence of each factor on failure when the other factors were controlled. The odds ratio of each factor for failure for the types B and C screws was calculated. The odds ratio describes the proportionate risk for failure of an implant anchor. Any probability of P ⬎ .05 was considered insignificant. These analyses were carried out by statistical analysis software (Statview, SPSS, Chicago, Ill).
RESULTS
The success rate for type A screws with the 1.0-mm diameter (0.0%) was significantly lower than that of the types B and C screws with 1.5-mm and 2.3-mm diameters (83.9% and 85.0%, respectively) and miniplates (96.4%). There was no significant difference in the success rate between types B and C screws or between miniplate and type B or C screws, although the success rate of miniplates with 2 screws was approximately 10% higher than that of type B or C screws. Patients who had received flap surgery for implantation more frequently complained of both swelling and pain within a week. Conversely, patients who received noninvasive flapless surgery rarely complained of discomfort (Table I). Patients with a high mandibular plane angle showed a significantly lower success rate than those with an average or low angle. No significant correlations between the success rate and the following variables were observed: screw length, kind of placement surgery, immediate loading, age, gender, crowding of teeth, anteroposterior jaw base relationship, controlled periodontitis, and temporomandibular disorder symptoms (Table II). Titanium screws with inflammation of the periimplant tissue after implantation showed a significantly
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Success rate, size of titanium screw, number of subjects and implants, and rates of flap surgery, swelling, and pain for each type of implant anchor
Table I.
Type of screws Clinical variables Success rate (%) Size of screws (mm) Diameter Length Subjects (n) Implants (n) Flap surgery rate Swelling rate Pain rate
Screw A
Screw B
Screw C
Miniplate with 2 screws
0
83.9*
85.0*
96.4*
2.3 14 10 23 7/10 7/10 6/10
2.0 5 or 7 7 17 7/7 6/7 6/7
1.0 6 3 10 0/3 0/3 0/3
1.5 11 31 101 0/31 1/31 1/31
*P ⬍ .001 (screw A vs others).
lower success rate than those without inflammation. No significant correlation between the success rate and flap surgery was detected. Neither the location of the implant (maxilla or mandible) nor the period from implantation to orthodontic force application was significantly associated with the success rate (Table III). Three screws of types B and C were removed before force application because of severe mobility after placement surgery. The odds ratio (ie, relative risk) for failure of titanium screws for orthodontic anchorage was 4.6 in subjects with inflamed peri-implant tissue after implantation and 4.1 in those with a high mandibular plane angle. Other factors were not significant according to logistic regression analysis (Table IV). DISCUSSION
With respect to the factors associated with the stability of titanium screws for orthodontic anchorage, our results showed for the first time that the diameter of the implant, inflammation of peri-implant tissue, and mandibular plane angle are associated with the stability of the titanium screws. As for the inflammation of peri-implant tissue, our results coincided with previous findings on dental implants for abutment.11,12 Therefore, we believe that the prevention of inflammation is important to prevent the mobility of the implant anchor. On the other hand, the number of days from implantation to force application was not associated with stability. The results suggest that immediate loading of a screw-type implant anchor is possible if the applied force is less than 2 N. In fact, recent reports recommended immediate loading to the implant anchor.1,2 Such immediate loading is probably possible because of successful mechanical interdigitation between the implant anchor and the alveolar bone in the posterior region.
As for the size of titanium screws as orthodontic anchors, the length of the screw was not associated with its stability if the screw was longer than 5 mm, because almost all miniplates were successfully fixed with 2 screws of 2.0-mm diameter and 5-mm length. On the other hand, the diameter of the screw was significantly associated with its stability. Furthermore, we found a high mandibular plane angle to be a risk factor for failure of screw-type implant anchors. According to recent studies,18,19 the thickness of buccal cortical bone in subjects with a high mandibular plane angle (1.5-2.7 mm) was thinner than that in subjects with a low angle (2.3-3.7 mm) in the mandibular first molar region. Therefore, we suggest that sufficient mechanical interdigitation between the screw and the cortical bone is an important factor that affects the stability of the screwtype implant anchor. Furthermore, we think that dentists should, if possible, examine the thickness of cortical bone by computed tomography before implantation, particularly in patients with a high mandibular plane angle. With respect to the comparison between the type B screw with a 1.5-mm diameter and the type C screw with a 2.3-mm diameter, the flap surgery rate or surgical invasion for the type B screw was lower than that for the type C screw. Less surgical invasion leads to less swelling and less pain. Furthermore, the anatomic limitation for implantation of the type B screw is less than that of the type C screw because the former is smaller. Therefore, we think that, in patients with an average-to-low mandibular plane angle, the type B screw is more desirable, but, in patients with a high mandibular plane angle, which often exists with thin cortical bone, the type C screw might be more desirable. It is well known that the buccal cortical bone of the maxilla is slightly thinner than that of mandible. There-
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Success rate and number of subjects according to clinical variables of 41 subjects treated with type B or C titanium screw(s) with 1.5-mm or 2.3-mm diameter, respectively
Table II.
Clinical variables
Success rate (%)
Success rate and number of screws according to inflammation of peri-implant tissue, flap surgery, location of implant (lower or upper), and period from implantation to force application for 124 screws of type B and C
Table III.
No. of subjects Clinical variables
Mandibular plane angle High angle Average angle Low angle Skeletal pattern Class I Class II Class III Age (y) ⬍20 ⱖ20-⬍30 ⱖ30 yrs old Gender Male Female Crowding of teeth† Absence Presence Periodontitis Absence Presence TMD symptoms Absence Presence
72.7 96.2* 100.0*
22 13 6
80.4 83.7 100.0
14 23 4
80.3 88.2 85.0
19 17 5
80.0 84.7
5 36
87.5 75.0
30 11
84.9 75.0
38 3
87.5 78.3
26 15
*P ⬍.05 vs high angle. † Moderate to severe crowding of teeth with upper and lower arch length discrepancies of less than ⫺6 mm
fore, it can be assumed that the success rate in the mandible is slightly higher than that in the maxilla. However, there was no significant difference between the success rates when titanium screws with 1.5- to 2.3-mm diameter were implanted. Therefore, these results suggest that the 1.5-mm diameter is enough to stabilize the titanium screw placed in the maxilla and the mandible if sufficient cortical bone exists to stabilize the screw. However, if the diameter is between 1.0 and 1.5 mm, there might be a significant difference between the success rates in the maxilla and the mandible. With respect to the comparison between the titanium screw with a 1.5-mm or 2.3-mm diameter and the miniplate with 2 screws having a 2.0-mm diameter, there was no significant difference between their success rates. The result might perhaps be due to the small sample size, because the success rate of the miniplate was nearly 100% (96.4%) in contrast to that of screws (83.9% and 85.0%, respectively). In fact, it was reported that a miniplate without mobility was effective during edgewise treatment of patients with open bite,
Success rate (%)
No. of screws
86.7 54.5*
113 11
85.2 75.0
108 16
84.1 83.6
63 61
85.0 82.8 87.5
20 29 72
Inflammation Absence Presence Flap surgery Flapless Flap Location of implant Upper Lower Non-force period (mo)† ⬍1 ⱖ1-⬍3 ⱖ3 months
*P ⬍.05 vs absence. † Data of 3 screws that showed early mobility were removed.
Odds ratios for failure of 124 titanium screws of type B and C*
Table IV.
Log odds Clinical variables Inflammation High mandibular plane angle
Estimate
Standard error
Odds ratio
95% confidence interval
1.52 1.41
0.74 0.68
4.6† 4.1†
1.1-19.4 1.1-15.4
*Nonsignificant factors were removed from the equation for logistic regression analysis. † P ⬍.05.
and a relatively strong orthodontic force could be applied,20 with a success rate of nearly 100% (97.2%).17 In addition, it was also reported that titanium screws have occasionally been removed because of their mobility before or during orthodontic force application.1,13 Therefore, the success rate of the miniplate might be slightly greater than that of the titanium screw without attention to the patient’s particular circumstances. However, because we identified factors associated with the stability of the screw-type implant anchor, we think that the success rate of the titanium screw would be nearly 100% if attention is paid to preventing inflammation of the peri-implant tissue, to the diameter of the screw, and to the mandibular plane angle. Furthermore, as was predicted, we showed that the miniplate had a disadvantage, in that almost all subjects
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complained of swelling and pain21 within a week after the surgery because flap surgery had to be performed. Accordingly, we think that if an implant anchor is to be placed into the buccal alveolar bone of the posterior region, titanium screws with a diameter of more than 1.0 mm (eg, 1.5 mm) should be used in patients with an average-to-low mandibular plane angle, and titanium screws with a diameter of more than 2.3 mm, or miniplates if the use of a screw is difficult, should be used in patients with a high mandibular plane angle (ie, with thin cortical bone). Inflammation of the periimplant tissue should be prevented as much as possible.
orthodontics change the conventional orthodontic treatment? Dental Diamond 2002;27:26-47. Roberts WE, Marshall KJ, Mosary PG. Rigid endosseous implant utilized as anchorage to protract molars and close an atrophic extraction site. Angle Orthod 1990;60:135-52. Roberts WE, Nelson CL, Goodacre CJ. Rigid implant anchorage to close a mandibular first molar extraction site. J Clin Orthod 1994;28:693-704. Wehrbein H, Glatzmaier J, Mundwiller U, Diedrich P. The Orthosystem—a new implant system for orthodontic anchorage in the palate. J Orofac Orthop 1996;57:142-53. Kanomi R. Mini-implant for orthodontic anchorage. J Clin Orthod 1997;31:763-7. Koyama I, Miyawaki S, Takano-Yamamoto T. Application of titanium screws in orthodontic treatment. Orthod Waves 2001; 60:313-8. Miyawaki S, Takano-Yamamoto T, Koyama I, Inoue M, Mishima K, Sugahara T. Application of caps with brackets to titanium screws as stationary anchorage in edgewise treatment. Orthod Waves 2001;60:409-13. Lee JS, Park HS, Kyung HM. Micro-implant anchorage for lingual treatment of a skeletal Class II malocclusion. J Clin Orthod 2001;35:643-7. Orenstein IH, Tarnow DP, Morris HF, Ochi S. Factors affecting implant mobility at placement and integration of mobile implants at uncovering. J Periodontol 1998;69:1404-12. Artzi Z, Tal H, Moses O, Kozlovsky A. Mucosal considerations for osseointegrated implants. J Prosthet Dent 1993;70:427-32. Forna N, Burlui V, Luca IC, Indrei A. Peri-implantitis. Rev Med Chir Soc Med Nat Iasi 1998;102:74-9. Sawa Y, Goto N, Suzuki K, Kamo N, Kamo K. The new method for the maxillary retraction of the anterior teeth using a titanium microscrew as anchorage. Orthod Waves 2001;60:328-31. Susami R. A cephalometric study of dentofacial growth in mandibular prognathism. J Jpn Orthod Soc 1967;26:278-311. Lindhe J. Textbook of clinical periodontology. Copenhagen: Munksgaard; 1984. Okeson JP. Management of temporomandibular disorders and occlusion. 4th ed. St Louis: Mosby; 1998. Nagasaka H, Sugawara J, Kawamura H, Kasahara T, Umemori M, Mitani H. A clinical evaluation on the efficacy of titanium miniplates as orthodontic anchorage. Orthod Waves 1999;58: 136-47. Tsunori M, Mashita M, Kasai K. Relationship between facial types and tooth and bone characteristics of the mandible obtained by CT scanning. Angle Orthod 1998;68:557-62. Masumoto T, Hayashi I, Kawamura A, Tanaka K, Kasai K. Relationships among facial type, buccolingual molar inclination, and cortical bone thickness of the mandible. Eur J Orthod 2001;23:15-23. Umemori M, Sugawara J, Mitani H, Nagasaka H, Kawamura H. Skeletal anchorage system for open-bite correction. Am J Orthod Dentofacial Orthop 1999;115:166-74. Miyawaki S, Yasuhara M, Koh Y. Discomfort caused by bonded lingual orthodontic appliances in adult patients as examined by retrospective questionnaire. Am J Orthod Dentofacial Orthop 1999;115:83-8.
CONCLUSIONS
A diameter of 1.0 mm or less, inflammation of the peri-implant tissue, and a high mandibular plane angle, which often exists with thin cortical bone, were associated with the mobility (ie, failure) of titanium screws placed into the buccal alveolar bone of posterior region as orthodontic anchors. Therefore, the clinical implications are as follows. If an implant anchor is to be placed into the buccal alveolar bone of the posterior region, then: 1. The use of titanium screws with a diameter of more than 1.0 mm (eg, 1.5 mm) is desirable in patients with an average-to-low mandibular plane angle, and the smaller the better so that there is less surgical invasion and less anatomic limitation. 2. The use of titanium screws with a diameter of more than 2.3 mm, or of miniplates if the use of a screw is difficult, is desirable in patients with a high mandibular plane angle (ie, with thin cortical bone). 3. Prevention of inflammation of peri-implant tissue is important to prevent mobility of the implant anchor. 4. Flapless surgery is desirable to minimize patient discomfort. 5. Immediate loading is possible if the applied force is less than 2 N.
3.
4.
5.
6. 7.
8.
9.
10.
11. 12. 13.
14. 15. 16. 17.
18.
19.
The authors thank Dr Fujii for preparing the materials. 20. REFERENCES 1. Park HS. The orthodontic treatment using micro-implant: the clinical application of MIA (micro-implant anchorage). Seoul: Narae Publishing; 2001. 2. Takano-Yamamoto T, Miyawaki S, Koyama I. Can implant
21.
Factors associated with the stability of titanium screws placed in the posterior region for orthodontic anchorage Shouichi Miyawaki, DDS, PhD,a Isao Koyama, DDS, PhD,b Masahide Inoue, DDS,a Katsuaki Mishima, DDS, PhD,c Toshio Sugahara, DDS, PhD,c and Teruko Takano-Yamamoto, DDS, PhDa Okayama and Nara, Japan Recently, implant anchors such as titanium screws have been used for absolute anchorage during edgewise treatment. However, there have been few human studies reporting on the stability of implant anchors placed in the posterior region. The purpose of this study was to examine the success rates and to find the factors associated with the stability of titanium screws placed into the buccal alveolar bone of the posterior region. Fifty-one patients with malocclusions, 134 titanium screws of 3 types, and 17 miniplates were retrospectively examined in relation to clinical characteristics. The 1-year success rate of screws with 1.0-mm diameter was significantly less than that of other screws with 1.5-mm or 2.3-mm diameter or than that of miniplates. Flap surgery was associated with the patient’s discomfort. A high mandibular plane angle and inflammation of peri-implant tissue after implantation were risk factors for mobility of screws. However, we could not detect a significant association between the success rate and the following variables: screw length, kind of placement surgery, immediate loading, location of implantation, age, gender, crowding of teeth, anteroposterior jaw base relationship, controlled periodontitis, and temporomandibular disorder symptoms. We concluded that the diameter of a screw of 1.0 mm or less, inflammation of the peri-implant tissue, and a high mandibular plane angle (ie, thin cortical bone), were associated with the mobility (ie, failure) of the titanium screw placed into the buccal alveolar bone of the posterior region for orthodontic anchorage. (Am J Orthod Dentofacial Orthop 2003;124:373-8)
I
t is no exaggeration to say that anchorage in edgewise treatment is the most important factor that affects the treatment plan and result. Until now, various techniques to reinforce anchorage have been devised and used in orthodontic practice. Recently, several kinds of implant anchors providing absolute anchorage have attracted the attention of orthodontists.1,2 Among them, titanium screws, which were originally used for intermaxillary or bone fixation, have the following advantages: minimal anatomic limitation for placement, lower medical cost, simpler placement surgery, and less discomfort after implantation when compared with dental implants for abuta
Department of Orthodontics, Okayama University Graduate School of Medicine and Dentistry, Okayama, Japan. b Koyama Orthodontic Clinic, Nara, Japan. c Department of Oral and Maxillofacial Reconstructive Surgery, Okayama University Graduate School of Medicine and Dentistry. This study was partially supported by the Japan Society for the Promotion of Science in the form of Grants-in-Aid for Scientific Research (#13672149 and #20322232). Reprint requests to: Teruko Takano-Yamamoto, Okayama University Graduate School of Medicine and Dentistry, Department of Orthodontics, 2-5-1, Shikatacho, Okayama, 700-8525 Japan; e-mail, [email protected]. Submitted, October 2002; revised and accepted, December 2002. Copyright © 2003 by the American Association of Orthodontists. 0889-5406/2003/$30.00 ⫹ 0 doi:10.1016/S0889-5406(03)00565-1
ment.3,4 Therefore, titanium screws of various sizes have gradually come to be used for orthodontic absolute anchorage.1,5-9 However, there have been few human studies in which the success rates for various kinds of implant anchors were examined. With respect to dental implants for abutment, it was reported that the success rate is approximately 97% and that the factors decreasing their stability are patient age of more than 40 years, implant design and material such as alloy basket and hydroxyapatite-coated cylinder, lower bone density, placement in posterior region in the jaw, and use of a bone tap.10 Furthermore, it is well known that inflammation of the peri-implant tissue causes peri-implant bone loss, leading to implant mobility.11,12 However, most orthodontic patients are less than 40 years old and have relatively high bone density, and titanium screws have often been used for orthodontic anchorage in the posterior region.1,7-9 Therefore, several factors affecting dental implant stability might not be associated with the stability of implant anchors. According to recent reports on implant anchors in humans,1,13 titanium screws have occasionally been removed because of their mobility before or during orthodontic force application. Thus, the orthodontist 373
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needs to understand which variables are related to this mobility. To date, however, there have been few human studies that examined factors associated with the stability of titanium screws for orthodontic anchorage. The purpose of this study was to examine the success rates of 3 kinds of titanium screws and miniplates and to find the factors associated with the stability of titanium screws placed in the buccal alveolar bone of the posterior region as orthodontic anchors. MATERIAL AND METHODS
The subjects in this study were 51 patients with malocclusions (9 males, 42 females, mean age 21.8 years, SD 7.8 years) who had surgery to place an implant anchor in the posterior region for edgewise treatment at a university dental hospital or a private orthodontic clinic from March 1997 to October 2001. Before implantation, the advantages and disadvantages were explained to each patient and his or her parents when the use of an implant anchor was considered a desirable form of orthodontic treatment.7,8 After informed consent had been obtained, the same dentist at each clinic implanted the anchors. According to a preliminary study, the success rate for implant anchors at each clinic was similar (data not shown). Each dentist had more than 10 years of clinical experience. The clinical features and treatment progress for 1 year were retrospectively examined for the 51 subjects. The variables examined were age, gender, kind of surgery (flap or flapless), swelling and pain within a week after surgery, mandibular plane to sella-nasion angle (high, average, or low), and anteroposterior jaw base relationship (skeletal Class I, II, or III) according to the normal range,14 controlled periodontitis with horizontal alveolar bone resorption,15 temporomandibular disorder (TMD) symptoms such as temporomandibular joint sounds, pain, and difficulty in jaw opening,16 and crowding with an arch-length discrepancy of less than ⫺6 mm in the maxillary and mandibular dentition. Three kinds of titanium screws with different diameters and lengths (type A: diameter, 1.0 mm; length, 6 mm; type B: diameter,1.5 mm, length, 11 mm; type C: diameter, 2.3 mm, length, 14 mm) and miniplates with 2 screws (diameter, 2.0 mm; length, 5 mm) were used as implant anchors (Fig 1). The titanium screws were placed into the buccal alveolar bone through attachment gingiva in the second premolar to second molar region of the maxilla or the mandible. The miniplates were placed into the zygomatic process of the maxilla or the buccal alveolar bone of the mandible through buccal mucosa. They were implanted after local anesthesia had been administered. For 3 days after the implantation, analgesia and antibiotics were prescribed to the pa-
Fig 1. Photographs of implant anchors used in this study. Upper left: type A titanium screw with 1.0-mm diameter and 6-mm length. Upper middle: type B titanium screw with 1.5-mm diameter and 11-mm length. Upper right: type C titanium screw with 2.3-mm diameter and 14-mm length. Lower: modified miniplate with 2 screws of 2.0 mm diameter and 5 mm length.
tients. The type A screw with the 1.0-mm diameter had seldom been used for the previous 2 years because of its lower success rate. Conversely, type B and C screws and miniplates were frequently used during the period covered in this study (Fig 2). Continuous force applied to each screw was less than 2 N. If orthodontic force could be applied to the implant anchor for 1 year (or until completion of the orthodontic treatment), we recorded the implant anchor as a success; otherwise, it was considered a failure. With respect to the method of implantation of the screw, flap or flapless surgery was performed. As for the placement of the miniplate, flap surgery was always performed. The same oral surgeon performed all flap surgeries in this study. The timing of orthodontic force application was determined in each case according to the need of treatment. Clinical features and treatment progress for 1 year were retrospectively examined for 10 screws of type A, 101 screws of type B, 23 screws of type C, and 17 miniplates. The variables examined were as follows: inflammation of peri-implant tissue after placement,17 kind of placement surgery (flap or flapless),
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Fig 2. Intraoral photographs showing implant anchors in situ. Upper left: type B titanium screw with 1.5-mm diameter and 11-mm length. Upper right: type C titanium screw with 2.3-mm diameter and 14-mm length. Lower: modified miniplate with 2 screws of 2.0-mm diameter and 5-mm length.
location of implantation (maxillary or mandibular premolar-to-molar region, or zygomatic process), and the number of days from placement to orthodontic force application. Analysis of variance and post hoc test (Fisher protected least significant difference) were used to compare the success rate of each implant anchor in 51 subjects with 151 implant anchors. The Fisher PLSD or unpaired t test was used to examine the difference between success rates according to respective classifications of each clinical variable in 41 subjects with 124 screws of types B and C. According to the data distribution, the appropriate nonparametric test was used. The chi-square or Fisher exact probability test was used to examine the correlation between the success rate and respective classification of each variable for the 124 screws of types B and C. Finally, a logistic regression analysis model was used to estimate the degree of influence of each factor on failure when the other factors were controlled. The odds ratio of each factor for failure for the types B and C screws was calculated. The odds ratio describes the proportionate risk for failure of an implant anchor. Any probability of P ⬎ .05 was considered insignificant. These analyses were carried out by statistical analysis software (Statview, SPSS, Chicago, Ill).
RESULTS
The success rate for type A screws with the 1.0-mm diameter (0.0%) was significantly lower than that of the types B and C screws with 1.5-mm and 2.3-mm diameters (83.9% and 85.0%, respectively) and miniplates (96.4%). There was no significant difference in the success rate between types B and C screws or between miniplate and type B or C screws, although the success rate of miniplates with 2 screws was approximately 10% higher than that of type B or C screws. Patients who had received flap surgery for implantation more frequently complained of both swelling and pain within a week. Conversely, patients who received noninvasive flapless surgery rarely complained of discomfort (Table I). Patients with a high mandibular plane angle showed a significantly lower success rate than those with an average or low angle. No significant correlations between the success rate and the following variables were observed: screw length, kind of placement surgery, immediate loading, age, gender, crowding of teeth, anteroposterior jaw base relationship, controlled periodontitis, and temporomandibular disorder symptoms (Table II). Titanium screws with inflammation of the periimplant tissue after implantation showed a significantly
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Success rate, size of titanium screw, number of subjects and implants, and rates of flap surgery, swelling, and pain for each type of implant anchor
Table I.
Type of screws Clinical variables Success rate (%) Size of screws (mm) Diameter Length Subjects (n) Implants (n) Flap surgery rate Swelling rate Pain rate
Screw A
Screw B
Screw C
Miniplate with 2 screws
0
83.9*
85.0*
96.4*
2.3 14 10 23 7/10 7/10 6/10
2.0 5 or 7 7 17 7/7 6/7 6/7
1.0 6 3 10 0/3 0/3 0/3
1.5 11 31 101 0/31 1/31 1/31
*P ⬍ .001 (screw A vs others).
lower success rate than those without inflammation. No significant correlation between the success rate and flap surgery was detected. Neither the location of the implant (maxilla or mandible) nor the period from implantation to orthodontic force application was significantly associated with the success rate (Table III). Three screws of types B and C were removed before force application because of severe mobility after placement surgery. The odds ratio (ie, relative risk) for failure of titanium screws for orthodontic anchorage was 4.6 in subjects with inflamed peri-implant tissue after implantation and 4.1 in those with a high mandibular plane angle. Other factors were not significant according to logistic regression analysis (Table IV). DISCUSSION
With respect to the factors associated with the stability of titanium screws for orthodontic anchorage, our results showed for the first time that the diameter of the implant, inflammation of peri-implant tissue, and mandibular plane angle are associated with the stability of the titanium screws. As for the inflammation of peri-implant tissue, our results coincided with previous findings on dental implants for abutment.11,12 Therefore, we believe that the prevention of inflammation is important to prevent the mobility of the implant anchor. On the other hand, the number of days from implantation to force application was not associated with stability. The results suggest that immediate loading of a screw-type implant anchor is possible if the applied force is less than 2 N. In fact, recent reports recommended immediate loading to the implant anchor.1,2 Such immediate loading is probably possible because of successful mechanical interdigitation between the implant anchor and the alveolar bone in the posterior region.
As for the size of titanium screws as orthodontic anchors, the length of the screw was not associated with its stability if the screw was longer than 5 mm, because almost all miniplates were successfully fixed with 2 screws of 2.0-mm diameter and 5-mm length. On the other hand, the diameter of the screw was significantly associated with its stability. Furthermore, we found a high mandibular plane angle to be a risk factor for failure of screw-type implant anchors. According to recent studies,18,19 the thickness of buccal cortical bone in subjects with a high mandibular plane angle (1.5-2.7 mm) was thinner than that in subjects with a low angle (2.3-3.7 mm) in the mandibular first molar region. Therefore, we suggest that sufficient mechanical interdigitation between the screw and the cortical bone is an important factor that affects the stability of the screwtype implant anchor. Furthermore, we think that dentists should, if possible, examine the thickness of cortical bone by computed tomography before implantation, particularly in patients with a high mandibular plane angle. With respect to the comparison between the type B screw with a 1.5-mm diameter and the type C screw with a 2.3-mm diameter, the flap surgery rate or surgical invasion for the type B screw was lower than that for the type C screw. Less surgical invasion leads to less swelling and less pain. Furthermore, the anatomic limitation for implantation of the type B screw is less than that of the type C screw because the former is smaller. Therefore, we think that, in patients with an average-to-low mandibular plane angle, the type B screw is more desirable, but, in patients with a high mandibular plane angle, which often exists with thin cortical bone, the type C screw might be more desirable. It is well known that the buccal cortical bone of the maxilla is slightly thinner than that of mandible. There-
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American Journal of Orthodontics and Dentofacial Orthopedics Volume 124, Number 4
Success rate and number of subjects according to clinical variables of 41 subjects treated with type B or C titanium screw(s) with 1.5-mm or 2.3-mm diameter, respectively
Table II.
Clinical variables
Success rate (%)
Success rate and number of screws according to inflammation of peri-implant tissue, flap surgery, location of implant (lower or upper), and period from implantation to force application for 124 screws of type B and C
Table III.
No. of subjects Clinical variables
Mandibular plane angle High angle Average angle Low angle Skeletal pattern Class I Class II Class III Age (y) ⬍20 ⱖ20-⬍30 ⱖ30 yrs old Gender Male Female Crowding of teeth† Absence Presence Periodontitis Absence Presence TMD symptoms Absence Presence
72.7 96.2* 100.0*
22 13 6
80.4 83.7 100.0
14 23 4
80.3 88.2 85.0
19 17 5
80.0 84.7
5 36
87.5 75.0
30 11
84.9 75.0
38 3
87.5 78.3
26 15
*P ⬍.05 vs high angle. † Moderate to severe crowding of teeth with upper and lower arch length discrepancies of less than ⫺6 mm
fore, it can be assumed that the success rate in the mandible is slightly higher than that in the maxilla. However, there was no significant difference between the success rates when titanium screws with 1.5- to 2.3-mm diameter were implanted. Therefore, these results suggest that the 1.5-mm diameter is enough to stabilize the titanium screw placed in the maxilla and the mandible if sufficient cortical bone exists to stabilize the screw. However, if the diameter is between 1.0 and 1.5 mm, there might be a significant difference between the success rates in the maxilla and the mandible. With respect to the comparison between the titanium screw with a 1.5-mm or 2.3-mm diameter and the miniplate with 2 screws having a 2.0-mm diameter, there was no significant difference between their success rates. The result might perhaps be due to the small sample size, because the success rate of the miniplate was nearly 100% (96.4%) in contrast to that of screws (83.9% and 85.0%, respectively). In fact, it was reported that a miniplate without mobility was effective during edgewise treatment of patients with open bite,
Success rate (%)
No. of screws
86.7 54.5*
113 11
85.2 75.0
108 16
84.1 83.6
63 61
85.0 82.8 87.5
20 29 72
Inflammation Absence Presence Flap surgery Flapless Flap Location of implant Upper Lower Non-force period (mo)† ⬍1 ⱖ1-⬍3 ⱖ3 months
*P ⬍.05 vs absence. † Data of 3 screws that showed early mobility were removed.
Odds ratios for failure of 124 titanium screws of type B and C*
Table IV.
Log odds Clinical variables Inflammation High mandibular plane angle
Estimate
Standard error
Odds ratio
95% confidence interval
1.52 1.41
0.74 0.68
4.6† 4.1†
1.1-19.4 1.1-15.4
*Nonsignificant factors were removed from the equation for logistic regression analysis. † P ⬍.05.
and a relatively strong orthodontic force could be applied,20 with a success rate of nearly 100% (97.2%).17 In addition, it was also reported that titanium screws have occasionally been removed because of their mobility before or during orthodontic force application.1,13 Therefore, the success rate of the miniplate might be slightly greater than that of the titanium screw without attention to the patient’s particular circumstances. However, because we identified factors associated with the stability of the screw-type implant anchor, we think that the success rate of the titanium screw would be nearly 100% if attention is paid to preventing inflammation of the peri-implant tissue, to the diameter of the screw, and to the mandibular plane angle. Furthermore, as was predicted, we showed that the miniplate had a disadvantage, in that almost all subjects
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complained of swelling and pain21 within a week after the surgery because flap surgery had to be performed. Accordingly, we think that if an implant anchor is to be placed into the buccal alveolar bone of the posterior region, titanium screws with a diameter of more than 1.0 mm (eg, 1.5 mm) should be used in patients with an average-to-low mandibular plane angle, and titanium screws with a diameter of more than 2.3 mm, or miniplates if the use of a screw is difficult, should be used in patients with a high mandibular plane angle (ie, with thin cortical bone). Inflammation of the periimplant tissue should be prevented as much as possible.
orthodontics change the conventional orthodontic treatment? Dental Diamond 2002;27:26-47. Roberts WE, Marshall KJ, Mosary PG. Rigid endosseous implant utilized as anchorage to protract molars and close an atrophic extraction site. Angle Orthod 1990;60:135-52. Roberts WE, Nelson CL, Goodacre CJ. Rigid implant anchorage to close a mandibular first molar extraction site. J Clin Orthod 1994;28:693-704. Wehrbein H, Glatzmaier J, Mundwiller U, Diedrich P. The Orthosystem—a new implant system for orthodontic anchorage in the palate. J Orofac Orthop 1996;57:142-53. Kanomi R. Mini-implant for orthodontic anchorage. J Clin Orthod 1997;31:763-7. Koyama I, Miyawaki S, Takano-Yamamoto T. Application of titanium screws in orthodontic treatment. Orthod Waves 2001; 60:313-8. Miyawaki S, Takano-Yamamoto T, Koyama I, Inoue M, Mishima K, Sugahara T. Application of caps with brackets to titanium screws as stationary anchorage in edgewise treatment. Orthod Waves 2001;60:409-13. Lee JS, Park HS, Kyung HM. Micro-implant anchorage for lingual treatment of a skeletal Class II malocclusion. J Clin Orthod 2001;35:643-7. Orenstein IH, Tarnow DP, Morris HF, Ochi S. Factors affecting implant mobility at placement and integration of mobile implants at uncovering. J Periodontol 1998;69:1404-12. Artzi Z, Tal H, Moses O, Kozlovsky A. Mucosal considerations for osseointegrated implants. J Prosthet Dent 1993;70:427-32. Forna N, Burlui V, Luca IC, Indrei A. Peri-implantitis. Rev Med Chir Soc Med Nat Iasi 1998;102:74-9. Sawa Y, Goto N, Suzuki K, Kamo N, Kamo K. The new method for the maxillary retraction of the anterior teeth using a titanium microscrew as anchorage. Orthod Waves 2001;60:328-31. Susami R. A cephalometric study of dentofacial growth in mandibular prognathism. J Jpn Orthod Soc 1967;26:278-311. Lindhe J. Textbook of clinical periodontology. Copenhagen: Munksgaard; 1984. Okeson JP. Management of temporomandibular disorders and occlusion. 4th ed. St Louis: Mosby; 1998. Nagasaka H, Sugawara J, Kawamura H, Kasahara T, Umemori M, Mitani H. A clinical evaluation on the efficacy of titanium miniplates as orthodontic anchorage. Orthod Waves 1999;58: 136-47. Tsunori M, Mashita M, Kasai K. Relationship between facial types and tooth and bone characteristics of the mandible obtained by CT scanning. Angle Orthod 1998;68:557-62. Masumoto T, Hayashi I, Kawamura A, Tanaka K, Kasai K. Relationships among facial type, buccolingual molar inclination, and cortical bone thickness of the mandible. Eur J Orthod 2001;23:15-23. Umemori M, Sugawara J, Mitani H, Nagasaka H, Kawamura H. Skeletal anchorage system for open-bite correction. Am J Orthod Dentofacial Orthop 1999;115:166-74. Miyawaki S, Yasuhara M, Koh Y. Discomfort caused by bonded lingual orthodontic appliances in adult patients as examined by retrospective questionnaire. Am J Orthod Dentofacial Orthop 1999;115:83-8.
CONCLUSIONS
A diameter of 1.0 mm or less, inflammation of the peri-implant tissue, and a high mandibular plane angle, which often exists with thin cortical bone, were associated with the mobility (ie, failure) of titanium screws placed into the buccal alveolar bone of posterior region as orthodontic anchors. Therefore, the clinical implications are as follows. If an implant anchor is to be placed into the buccal alveolar bone of the posterior region, then: 1. The use of titanium screws with a diameter of more than 1.0 mm (eg, 1.5 mm) is desirable in patients with an average-to-low mandibular plane angle, and the smaller the better so that there is less surgical invasion and less anatomic limitation. 2. The use of titanium screws with a diameter of more than 2.3 mm, or of miniplates if the use of a screw is difficult, is desirable in patients with a high mandibular plane angle (ie, with thin cortical bone). 3. Prevention of inflammation of peri-implant tissue is important to prevent mobility of the implant anchor. 4. Flapless surgery is desirable to minimize patient discomfort. 5. Immediate loading is possible if the applied force is less than 2 N.
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The authors thank Dr Fujii for preparing the materials. 20. REFERENCES 1. Park HS. The orthodontic treatment using micro-implant: the clinical application of MIA (micro-implant anchorage). Seoul: Narae Publishing; 2001. 2. Takano-Yamamoto T, Miyawaki S, Koyama I. Can implant
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