In recent years, orthodontic mini-implants have been

ONLINE ONLY Factors affecting the long-term stability of orthodontic mini-implants Mitsuru Motoyoshi, a Miwa Uemura, b Akiko Ono, c Kumiko Okazaki, d Toru Shigeeda, d and Noriyoshi Shimizu e Tokyo, Japan Introduction: The placement and removal torques of mini-implants were evaluated as an index of implant stability. We examined factors affecting the initial and long-term stability of mini-implants. Methods: We measured the placement and removal torques of 134 mini-implants placed in buccal posterior alveolar bone and assessed the relationships among placement and removal torques, placement period, age, sex, and cortical bone thickness. The mini-implants were machine-surfaced, 1.6 mm in diameter and 8 mm long. A torque screwdriver was used to measure the peak torque values. Results and Conclusions: The placement and removal torques averaged approximately 8 and 4 N cm, respectively. A torque of 4 N cm suggests sufficient anchorage capability for mini-implants. No significant correlation between placement and removal torques was found. Placement torque was significantly related to age and cortical bone thickness in the maxilla, whereas removal torque was not significantly related to placement period, age, sex, or cortical bone thickness. (Am J Orthod Dentofacial Orthop 2010;137:588.e1-588.e5) In recent years, orthodontic mini-implants have been used as orthodontic anchors. 1-7 Orthodontic treatment has been advanced by the use of miniimplants; however, mini-implants are occasionally removed because they become mobile during treatment. 3,8 Many researchers have investigated the risk factors for failure of mini-implants to improve the success rate. 9-13 They found that the primary stability of miniimplants is related to the mechanical characteristics of the interface between the mini-implant and bone in relation to factors such as bone quality and quantity, and screw diameter, length, and design. To evaluate the initial stability of mini-implants, previous studies measured implant placement torque when tightening mini-implants. 10,11 Those authors found that the recommended placement torque was 5 to 10 N cm for successful implantation, although they did not examine factors affecting the initial stability in detail. The removal torque reflects the characteristics From the Department of Orthodontics, Division of Clinical Research, Dental Research Center, School of Dentistry, Nihon University, Tokyo, Japan. a Associate professor. b Resident. c Researcher. d Postgraduate student. e Professor and chair. The authors report no commercial, proprietary, financial interest in the products or companies described in this article. Reprint requests to: Mitsuru Motoyoshi, Department of Orthodontics, Nihon University School of Dentistry, 1-8-13 Kandasurugadai, Chiyodaku, Tokyo 101-8310, Japan; e-mail, motoyoshi@dent.nihon-u.ac.jp. Submitted, March 2009; revised and accepted, May 2009. 0889-5406/$36.00 Copyright Ó 2010 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2009.05.019 of the implant-bone interface during and after longterm orthodontic treatment, and can also be used to evaluate the anchorage capability of mini-implants because removal torque is the resistance force required to remove a mini-implant after orthodontic treatment. Some researchers measured the removal torque of mini-implants in animals or bone specimens. 14-16 Kim et al 17 measured the removal torque of surface-treated mini-implants in humans. In this study, we determined the placement and removal torques of machine-surfaced mini-implants in patients in relation to placement period, age, sex, and cortical bone thickness to identify factors that affect initial and long-term stability of mini-implants. MATERIAL AND METHODS Fifty-seven orthodontic patients (148 implants) were studied. Fourteen implants that developed mobility and loosening during orthodontic treatment were excluded from the study. The final study group comprised 52 patients (10 male, with 25 implants; 42 female, with 109 implants), whose ages ranged from 13.9 to 63.5 years (average, 26.1 6 8.4 years [mean 6 SD]). Titanium mini-implants were placed in the buccal posterior alveolar bone in all subjects as anchors for orthodontic treatment at Nihon University Dental Hospital. Computerized tomography (3D Accuitomo, J. Morita, Kyoto, Japan), with 0.125-mm slices with a voxel size of 0.125 mm in super-high-resolution mode, was used for diagnostic imaging of the area around the site before implant placement. The cortical bone thickness was 588.e1

588.e2 Motoyoshi et al American Journal of Orthodontics and Dentofacial Orthopedics May 2010 Fig 1. The commercial titanium mini-implant (ISA orthodontic implants, Biodent, Tokyo, Japan) with a 1.6-mm diameter and an 8-mm length used in this study. measured at the prepared sites in the maxilla and mandible. The site of implant placement was identified by measuring the height from the archwire to the position of the implant on the tomogram at the prepared site in the interroot gap between the second premolar and the first molar, or the first molar and the second molar. 11 Titanium mini-implants measuring 1.6 mm in diameter and 8 mm in length (ISA orthodontic implants, Biodent, Tokyo, Japan) were used (Fig 1). After local anesthesia was administered, a pilot hole was drilled with a bone drill into the buccal alveolar bone posterior to the second premolar or the second molar of the maxilla or mandible without raising a flap. To improve the success rate, we used bone drills with diameters of 1.0 mm in the maxilla and 1.3 or 1.4 mm in the mandible to control the placement torque, based on published results. 10-12 For the mandible, a 1.3-mm or 1.4-mm diameter bone drill was chosen depending on bone stiffness; when the placement torque exceeded 10 N cm during placement of a mini-implant in a pilot hole 1.3 mm in diameter, the pilot hole was enlarged with the 1.4-mm bone drill. The peak value of the placement torque when tightening the mini-implant was measured by using a torque screwdriver (N2DPSK, Nakamura, Tokyo, Japan) (Fig 2). The torque screwdriver has a round dial gauge with a pointer indicating the peak value, and the manufacturer stated that its accuracy is 6 3%. The peak placement torque was recorded at the final turn when the mini-implant was tightened into the implant hole. Immediately after placement, an orthodontic force of less than 2 N was applied to the mini-implant. Each patient was prescribed an antibiotic for 3 days after placement to control infection. After clinical use of the mini-implant, the peak removal torque was Fig 2. Torque screwdriver (N2DPSK, Nakamura, Tokyo, Japan) used to record peak placement and removal torque values. measured by using the torque screwdriver when the mini-implant was removed. To verify the hypotheses that removal torque increases with greater placement torque, longer placement period, and thicker cortical bone, we investigated the relationships among the placement and removal torques, placement period, cortical bone thickness, sex, and age. The Pearson correlation coefficient was calculated to evaluate the relationships among placement torque, removal torque, placement period, age, and cortical bone thickness. The paired t test was used to examine the difference between placement and removal torques. The unpaired t test was used to compare torque with sex and age. These analyses were performed with the SPSS statistical program (SPSS Japan, Tokyo, Japan); P \0.05 was considered significant. This study was approved by the ethics committee of Nihon University School of Dentistry, and all patients consented to participate in this study. RESULTS The rate of mobility and loosening of the miniimplants was less than 9.5%. There were no significant

American Journal of Orthodontics and Dentofacial Orthopedics Motoyoshi et al 588.e3 Volume 137, Number 5 Table I. Changes of torque values after clinical usage (N cm) Placement torque Removal torque Mean SD Mean SD n Maxilla 7.67 2.62 4.37* 2.20 67 Mandible 8.40 2.51 4.09* 1.90 67 Maxilla and mandible 8.03 2.58 4.23* 2.05 134 *P \0.05 (placement vs removal). Table II. Changes of torque values in the 3 placement torque groups Placement torque Removal torque Group Mean SD Mean SD n 0-5 N cm 3.67 1.03 4.31 2.03 18 5-10 N cm 7.89 1.47 4.33* 2.00 89 10-15 N cm 11.41 1.03 3.83* 2.26 27 *P \0.05 (placement vs removal). Table III. Differences by sex (N cm) Placement torque Removal torque Difference Mean SD Mean SD Mean SD n Female 8.09 2.68 4.07 2.00 4.02 3.50 109 Male 7.80 2.16 4.93 2.17 2.87 2.97 25 No significant difference between the sexes. Table IV. Correlation coefficients among placement period, age, and torque (n 5 134) Placement period Placement torque Removal torque Difference Age (y) 0.299* 0.287* 0.087 0.165 Mean, 26.1 SD, 8.4 Placement period (mo) 0.195* 0.101 0.086 Mean, 23.1 SD, 6.7 *P \0.05. differences in the placement and removal torques between the right and left sides, or in cortical bone thickness. No significant correlation between the placement and removal torques was found in the maxilla (r 5 0.063, P 5 0.611) or the mandible (r 5 0.083, P 5 0.503). The removal torque was significantly lower than the placement torque in both the maxilla and the mandible (Table I). When the subjects were divided into 3 groups according to placement torque (low, 0-5; intermediate, 5-10; and high, 10-15 N cm) the removal torque did not change in the low-torque group, but it decreased significantly in the intermediate- and hightorque groups (Table II). No sex differences in the torques were found (Table III). Significant correlations were found between age and placement period, age and placement torque, and placement period and placement torque, whereas no parameters were correlated with the removal torque (Table IV). Placement torque was significantly related to maxillary cortical bone thickness, but removal torque was not related to cortical bone thickness (Table V). No significant correlation was found between torque and cortical bone thickness in the mandible (Table VI). DISCUSSION Some researchers investigated the removal torque of orthodontic mini-implants to evaluate the implant-bone interface. 15,16 The force required for removing a miniimplant is related to the torsion resistance of the implant-bone interface and thus can be used to evaluate anchorage capability indirectly. 18 To improve the initial stability and anchorage capability of a mini-implant, it is necessary to identify factors that affect placement and removal torques. We investigated the relationships between the placement and removal torques of miniimplants and the parameters of placement period, age, sex, and cortical bone thickness. Removal torque was not significantly related to placement torque. Therefore, removal torque is not affected by the degree of firmness when tightening the mini-implants. Okazaki et al 16 investigated removal torque after mini-implants measuring 1.2 mm in diameter were placed in 1.0- and 1.2-mm cavities prepared in dog femurs. They found that the measurements for the 1.2- mm cavities 6 weeks after placement were similar to those for the 1.0-mm cavities, whereas the immediately postinsertion removal torques for the 1.2-mm cavities were significantly lower than those for the 1.0-mm cavities. Our study supports their results, because we demonstrated that removal torque does not depend on placement torque in humans. The torque decreased from approximately 8 to 4 N cm after clinical use. When the subjects were divided into 3 torque groups, the torque in the low-torque group had a constant value of approximately 4 N cm whereas the torque values in the intermediate- and high-torque groups decreased significantly to approximately 4 N cm. Immediately after placement, the implant-bone interface is affected by bone stiffness at the prepared site, screw design, and size of the pilot hole relative to the diameter of the mini-implant. 9-11 Several months after

588.e4 Motoyoshi et al American Journal of Orthodontics and Dentofacial Orthopedics May 2010 Table V. Correlation coefficients of torque vs cortical bone thickness in the maxilla (n 5 60) Placement torque Removal torque Cortical bone thickness (N cm) (N cm) (mm) Mean 7.76 4.39 1.13 SD 2.71 2.14 0.33 r 0.392* 0.156 r, Cortical bone thickness vs torque value. *P \0.05. Table VI. Correlation coefficients of torque vs cortical bone thickness in the mandible (n 5 59) Placement torque Removal torque Cortical bone thickness (N cm) (N cm) (mm) Mean 8.21 4.16 1.73 SD 2.45 1.86 0.56 r 0.019 0.043 r, Cortical bone thickness vs torque value. placement, the increased compressive stress on the bone surrounding the mini-implant might disappear with accompanying bone metabolism, thus reducing the torque. All mini-implants in this study were subjected to sustained orthodontic force of less than 4 N cm regardless of the torque. Therefore, 4 N cm might be sufficient torque for orthodontic anchorage. For successful implantation, the pilot drill could prevent overtorquing to improve the success rate when tightening the miniimplant into stiff bone, according to Motoyoshi et al. 10,11 When the prepared site is fragile and the placement torque is less than 4 N cm even with selfdrilling without a pilot drill, a healing period might improve the anchorage capability of machine-surfaced mini-implants. To identify other factors that affect torque, we investigated the placement and removal torques of the miniimplants according to placement period, age, sex, and cortical bone thickness. No sex differences were found in either placement or removal torque. No significant correlations were found between removal torque and age or placement period, but placement torque was significantly related to both age and placement period. The correlation between placement period and placement torque is related to the significant correlations between placement period and age, and between age and placement torque. The significant correlation between placement period and age is related to the fact that limited orthodontic treatment was carried out in older subjects (these 5 patients were 45 years or older), and the treatment period was shorter than 12 months. A negative correlation was found between age and placement torque, with lower placement torque in older subjects. This might be related to the lower bone density observed in older people. 19 The results showed a significant correlation between cortical bone thickness and placement torque in the maxilla; this agrees with a previous study. 10 In contrast, no correlation was found between cortical bone thickness and placement torque in the mandible. This was most likely caused by our placement procedure; we used a bone drill with a diameter of 1.3 or 1.4 mm in the mandible to control the placement torque to improve the success rate based on published results. 11,12 For the mandible, the 1.3- or 1.4-mm diameter bone drill was chosen depending on the bone stiffness. Consequently, the placement torque in 49 of 67 implants in the mandible was less than 10 N cm so that any correlation between placement torque and cortical bone thickness in the mandible disappeared, although the remaining 18 implants slightly exceeded torque values of 10 N cm regardless of pilot drilling with the 1.4-mm diameter drill. No significant correlation was found between removal torque and cortical bone thickness. Obviously, a thicker layer of compact bone is not likely to cause an increase in removal torque, whereas thick cortical bone would increase placement resistance when tightening mini-implants in the maxilla. Therefore, a thicker layer of compact bone does not always ensure long-term stability and anchorage capability of the mini-implant. This might be explained by the fact that mini-implants in a mandible with thick cortical bone are not always stable compared with those in a maxilla with thinner cortical bone. 20 Kim et al 17 assessed the removal torque of surfacetreated mini-implants and reported an average removal torque of 16.4 N cm. This value was 4 times greater than our result based on machine-surfaced miniimplants. Obviously, removal torque is affected by the properties of the surface of the mini-implant. Therefore, the removal torque might be related to implant design, such as shape of the screw thread and the taper of the angle of the screw, although more detailed studies should be conducted to identify other factors that affect removal torque. CONCLUSIONS Placement and removal torques averaged approximately 8 and 4 N cm respectively, when machinesurfaced mini-implants 1.6 mm in diameter and 8 mm long were placed in the buccal posterior alveolar

American Journal of Orthodontics and Dentofacial Orthopedics Motoyoshi et al 588.e5 Volume 137, Number 5 bone. A torque of 4 N cm should have sufficient anchorage capability for machine-surfaced mini-implants. Placement torque was significantly lower among older patients and with thinner cortical bone in the maxilla, whereas removal torque was not significantly related to placement torque, placement period, age, sex, or cortical bone thickness. REFERENCES 1. Kanomi R. Mini-implant for orthodontic anchorage. J Clin Orthod 1997;31:763-7. 2. Freudenthaler JW, Bantleon HP, Haas R. Bicortical titanium screws for critical orthodontic anchorage in the mandible: a preliminary report on clinical applications. Clin Oral Implants Res 2001;12:358-63. 3. Sawa Y, Goto K, Suzuki N, 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. 4. Bac SM, Park HS, Kyung HM, Kwon OW, Sung JH. Clinical application of micro-implant anchorage. J Clin Orthod 2002;36: 298-302. 5. Park HS, Kyung HM, Sung JH. A simple method of molar uprighting with microimplant anchorage. J Clin Orthod 2002;36:592-6. 6. Herman RJ, Currier GF, Miyake A. Mini-implant anchorage for maxillary canine retraction: a pilot study. Am J Orthod Dentofacial Orthop 2006;130:228-35. 7. Kim TW, Kim H, Lee SJ. Correction of deep overbite and gummy smile by using a mini-implant with a segmented wire in a growing Class II Division 2 patient. Am J Orthod Dentofacial Orthop 2006; 130:676-85. 8. Costa A, Raffaini M, Melsen B. Miniscrews as orthodontic anchorage: a preliminary report. Int J Adult Orthod Orthognath Surg 1998;13:201-9. 9. 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. 10. Motoyoshi M, Hirabayashi M, Uemura M, Shimizu N. Recommended placement torque when tightening an orthodontic miniimplant. Clin Oral Implants Res 2006;17:109-14. 11. Motoyoshi M, Yoshida T, Ono A, Shimizu N. Effect of cortical bone thickness and implant placement torque on stability of orthodontic mini-implant. Int J Oral Maxillofac Implants 2007;22: 779-84. 12. Motoyoshi M, Matsuoka M, Shimizu N. Application of orthodontic mini-implants in adolescents. Int J Oral Maxillofac Surg 2007; 36:695-9. 13. Deguchi T, Nasu M, Murakami K, Yabuuchi T, Kamioka H, Takano-Yamamoto T. Quantitative evaluation of cortical bone thickness with computed tomographic scanning for orthodontic implants. Am J Orthod Dentofacial Orthop 2006;129:721.e7-12. 14. Wilmes B, Rademacher C, Olthoff G, Drescher D. Parameters affecting primary stability of orthodontic mini-implants. J Orofac Orthop 2006;67:162-74. 15. Song YY, Cha JY, Hwang CJ. Mechanical characteristics of various orthodontic mini-screws in relation to artificial cortical bone thickness. Angle Orthod 2007;77:979-85. 16. Okazaki J, Komasa Y, Sakai D, Kamada A, Ikeo T, Toda I, et al. A torque removal study on the primary stability of orthodontic titanium screw mini-implants in the cortical bone of dog femurs. Int J Oral Maxillofac Surg 2008;37:647-50. 17. Kim SH, Cho JH, Chung KR, Kook YA, Nelson G. Removal torque values of surface-treated mini-implants after loading. Am J Orthod Dentofacial Orthop 2008;134:36-43. 18. Favero LG, Pisoni A, Paganelli C. Removal torque of osseointegrated mini-implants: an in vivo evaluation. Eur J Orthod 2007; 29:443-8. 19. Amorim MA, Takayama L, Jorgetti V, Pereira RM. Comparative study of axial and femoral bone mineral density and parameters of mandibular bone quality in patients receiving dental implants. Osteoporos Int 2007;18:703-9. 20. Cheng SJ, Tseng IY, Lee JJ, Kok SH. A prospective study of the risk factors associated with failure of mini-implants used for orthodontic anchorage. Int J Oral Maxillofac Implants 2004;19:100-6.