- Email: [email protected]
Development of a novel spike-like auxiliary skeletal anchorage device to enhance miniscrew stability
TECHNO BYTES
Development of a novel spike-like auxiliary skeletal anchorage device to enhance miniscrew stability Shouichi Miyawaki,a Hiroshi Tomonari,b Takakazu Yagi,c Takaharu Kuninori,b Yasuhiko Oga,d and Masafumi Kikuchie Kagoshima, Japan
Introduction: Miniscrews are frequently used for skeletal anchorage during edgewise treatment, and their clinical use has been verified. However, their disadvantage is an approximately 15% failure rate, which is primarily attributed to the low mechanical stability between the miniscrew and cortical bone and to the miniscrew's close proximity to the dental root. To solve these problems, we developed a novel spike-like auxiliary skeletal anchorage device for use with a miniscrew to increase its stability. Methods: The retention force was compared between miniscrews with and without the auxiliary skeletal anchorage device at each displacement of the miniscrew. The combined unit was also implanted into the bones of 2 rabbits in vivo, and implantation was visually assessed at 4 weeks postoperatively while the compression force was applied. Results: The retention force of the combined unit was significantly and approximately 3 to 5 times stronger on average than that of the miniscrew alone at each displacement. The spiked portion of the auxiliary anchorage device embedded into the cortical bone of the hind limb at approximately a 0.3-mm depth at 4 weeks postimplantation in both rabbits. Conclusions: The auxiliary skeletal anchorage device may increase miniscrew stability, allow a shortened miniscrew, and enable 3-dimensional absolute anchorage. Further evaluation of its clinical application is necessary. (Am J Orthod Dentofacial Orthop 2015;148:338-44)
B
oth miniscrews and miniplates have been used in clinical orthodontic practice to provide skeletal anchorage.1,2 Miniplates do offer a lower failure rate in comparison with other skeletal anchorage devices such as miniscrews1; however, the devices are
From the Graduate School of Medical and Dental Sciences, Kagoshima University, Kagoshima, Japan. a Professor and chair, Department of Orthodontics, Field of Developmental Medicine, Health Research Course. b Assistant professor, Department of Orthodontics, Field of Developmental Medicine, Health Research Course. c Lecturer, Department of Orthodontics, Field of Developmental Medicine, Health Research Course. d Postgraduate student, Department of Orthodontics, Field of Developmental Medicine, Health Research Course. e Professor and chair, Department of Biomaterials Science, Field of Oral and Maxillofacial Rehabilitation, Advanced Therapeutic Course. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. Partially supported by Japan Society for the Promotion of Science KAKENHI Grant Numbers 26463099, 25670879, 24593105, and 24390464. Address correspondence to: Shouichi Miyawaki, Department of Orthodontics, Field of Developmental Medicine, Health Research Course, Kagoshima University, Graduate School of Dental Sciences, 8-35-1 Sakuragaoka, Kagoshima City, Kagoshima 890-8544, Japan; e-mail, [email protected]. Submitted, August 2014; revised and accepted, February 2015. 0889-5406/$36.00 Copyright Ó 2015 by the American Association of Orthodontists. http://dx.doi.org/10.1016/j.ajodo.2015.02.030
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disadvantaged by the significantly increased patient discomfort, a highly complex implantation procedure, and greater invasiveness because of the required flap surgery. The miniscrew has recently come into use for skeletal anchorage during edgewise treatment and is the most frequently implanted of all skeletal anchorage devices. Miniscrews have enabled numerous orthodontic treatments that were difficult with conventional appliances: eg, mesial movement of a molar in a patient with oligodontia,3 distal movement of the maxillary and mandibular teeth,4 and improvement of an open bite caused by intruding posterior teeth.5 In addition, miniscrews have often been used to provide 2-dimensional absolute anchorage during edgewise treatment.6 Currently, the usefulness of miniscrews has been conclusively shown with a high level of evidence, but the devices have some disadvantages.7-11 Miniscrews require extremely precise implantation when placed into the narrow space between 2 dental roots.12 Cone-beam computed tomography is not recommended to examine the space between the roots and the cortical bone thickness because of the high radiation exposure, particularly in growing patients.13 Most critically, miniscrews have an approximately 15% failure rate on average9,14,15;
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with this high rate of failure, in a patient population implanted with 4 miniscrews each, approximately 50% of the patients will experience failure in more than 1 miniscrew, ultimately leading to reimplantation. Two primary factors are implicated in miniscrew failure. One is the low mechanical stability between the miniscrew and the cortical bone.1,16-18 This results in even higher failure rates in patients with a high mandibular plane angle1,19 and in growing patients20 than in patients with a low or an average mandibular plane angle and adult patients. Another factor is the close proximity of the miniscrew to the dental root.21-23 Prevention of miniscrew failure thus requires meeting 2 crucial conditions: increased mechanical stability, achieved by increasing the contact area between the miniscrew and the cortical bone,18 and prevention of root proximity by shortening the miniscrew.23 However, increasing the miniscrew diameter to enhance the mechanical stability also increases the risk of root proximity, whereas decreasing the miniscrew diameter decreases the risk of root proximity. As a result, no currently available miniscrews or skeletal anchorage devices meet these conditions while remaining minimally invasive. Accordingly, we developed an auxiliary skeletal anchorage device that is used in conjunction with a miniscrew to increase its stability, enabling 3-dimensional absolute anchorage and ultimately helping to avoid root proximity with a shorter miniscrew. Furthermore, we evaluated the mechanical stability force between artificial bone and the combined miniscrew and auxiliary skeletal anchorage device, and assessed the degree of embedding of the auxiliary device after the compression force was applied for 4 weeks through a silicone ring between the auxiliary device and the miniscrew head. MATERIAL AND METHODS
To increase the mechanical stability between the miniscrew and the cortical bone, we developed an auxiliary skeletal anchorage device made of titanium alloy (raw material, Ti6Al4V; ASTM F136-96) comprising 2 portions: a washer portion that receives the force from the screw head and transfers it through the silicone at the first stage of implantation, and a spiked portion that contacts and slightly embeds into the cortical bone during the first stage: ie, as the compression force was applied through the silicone ring between the auxiliary device and the miniscrew head (Fig 1, A-C). During the first stage of implantation, a silicone ring (thickness, 1.0 mm; Durometer Shore A, 18; tensile strength, 4.14 MPa [600 psi]; tearing strength, 45; tensile elongation, 700%; elastic modulus, 0.82 MPa; Bitec Global Group, Tokyo, Japan) was placed between the screw
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head and the washer portion of the auxiliary skeletal anchorage device. A preliminary study (data not shown) showed that an approximately 1 to 2 N force is delivered to the spiked portions of the auxiliary skeletal anchorage device through the silicone ring, allowing embedding into the cortical bone over time as the compression force is applied through the silicone ring between the auxiliary device and the screw head. One month after embedding the spiked portion of the miniscrew into the bone to approximately a 0.3-mm depth, the silicone ring was removed, and the miniscrew was further tightened to allow the spiked portion to compress more securely to the cortical bone (Fig 1, D). The artificial bone blocks (Sawbones; Pacific Research Laboratories, Vashon Island, Wash) included cancellous bone constructed from 0.08 g per millileter (5 pcf) density cellular rigid polyurethane foam and 1mm-thick cortical bone constructed from 0.64 g per milliliter (40 pcf) laminated rigid polyurethane foam. The constructed blocks measured 2.54 cm (1 in) in length and width, and 4.57 cm (1.8 in) in height (Fig 2, A). For the lateral displacement test, the mechanical retention force between the skeletal anchorage and the artificial bone was determined using titanium alloy miniscrews measuring 1.6 mm in diameter and 6.0 mm in length (Dual-Top; Jeil Medical, Seoul, Korea).24-29 Two groups comprising 10 miniscrews each were implanted en masse into artificial bone blocks using a driver machine (Orthonia 111-ED-010; Jeil Medical); 1 group was implanted with an auxiliary skeletal anchorage device (auxiliary group) and 1 without the auxiliary device (nonauxiliary group). The spiked portion of the auxiliary skeletal anchorage device was implanted into the cortical region of the artificial bone to a depth of 0.3 mm. The depths of miniscrew implantation in both groups were then adjusted until they were equal between the groups. To determine the mechanical retention force, the displacement of the miniscrew head was measured after application of a compression force delivered from 2 directions (Fig 2, B) using a compression test machine (TGE-5kN; Minebea, Nagano, Japan) set at a load of 5.0 kN and a compression velocity of 0.5 mm per minute (Fig 2, C). The compression force was delivered in 2 directions parallel to the artificial bone surface until each miniscrew moved 0.01, 0.02, and 0.03 mm from its initial position. The resulting force was calculated with software (SR-06-001 version 3.400; Minebea). A miniscrew with an auxiliary skeletal anchorage device was implanted into 2 New Zealand white rabbits, 1 male and 1 female, aged 14 weeks and weighing approximately 3 kg. All surgical procedures were performed under general (1 mg/kg ketamine and 2 mg/kg xylazine) and local (2% lidocaine with 1:80,000 epinephrine)
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Fig 1. A, The auxiliary skeletal anchorage device Spike Anchor [SA]) was developed for use with a commercially available miniscrew (MS), and a silicone ring (SR) was used with the SA to add the force of SA embedding; B, immediately after implantation, the direction of force from the silicon ring to the auxiliary skeletal anchorage device is apparent (red arrows); C, 4 weeks after embedding the auxiliary skeletal anchorage device into bone to a depth of approximately 0.3 mm; D, the miniscrew was further tightened immediately after removing the silicon ring (yellow arrow).
anesthesia. Before surgery, each rabbit was placed under general anesthesia and showed no response to pain stimulation. One hind limb was shaved and aseptically cleansed with povidone-iodine and 70% ethanol. A flap was reflected on each femur using a periosteal elevator, and the miniscrews were implanted at least 10 mm apart with a driver machine. At 4 weeks postimplantation, each animal was killed by a lethal dose of pentobarbital, and the implanted limb was isolated. The specimen was visually examined to determine whether automatic embedding of the auxiliary skeletal anchorage device occurred during the intervening 4 weeks after implantation. This animal experimental protocol was approved by the institutional experimentation committee of Kagoshima University (number D13010). Statistical analysis
The retention force in each displacement was compared between the 2 study groups using the Mann-Whitney U test. Data were analyzed with statistical software (version 10.0; IBM, Armonk, NY), and statistical significance was designated at a \0.05.
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RESULTS
The retention force was significantly stronger in the auxiliary group than in the nonauxiliary group at 0.01, 0.02, and 0.03 mm miniscrew displacements (0.01 mm, P 5 0.012; 0.02 mm, P 5 0.001; and 0.03 mm, P \0.001). In addition, as the displacement of the screw head decreased, the mean difference in the applied force was greater in the auxiliary group than in the nonauxiliary group. The differences in mean forces were approximately 5.5, 4.2, and 3.4 times greater in the auxiliary group than in the nonauxiliary group at displacements of 0.01, 0.02, and 0.03 mm, respectively (Table, Fig 3). Visual assessment showed that the spiked portion of the auxiliary anchorage device was implanted into the cortical bone of the hind limb to a depth of approximately 0.3 mm after the compression force was applied for 4 weeks in both rabbits (Fig 4). DISCUSSION
The visual assessment results shown in Figure 4 suggest that a compression force of 1 to 2 N through the silicone ring between the auxiliary skeletal anchorage
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Fig 2. A, The artificial bone block and the implantation of the conventional miniscrew and the miniscrew with the auxiliary skeletal anchorage device; B, schema of the force direction and structure of the combined unit, with the compression force applied to the miniscrew in 2 directions parallel to the artificial bone surface (red arrows); C, a compression test machine was used for the lateral displacement test, with the compression force applied perpendicular to the miniscrew (red arrow).
Table. Comparison of mechanical retention forces
between the 2 groups according to miniscrew head displacement Compression force (N) Auxiliary group Miniscrew displacement (mm) 0.01 0.02 0.03
Mean 0.65 3.77 7.06
SD 0.44 1.28 1.77
Nonauxiliary group Mean 0.12 0.90 2.13
SD 0.11 0.61 1.11
Probability* 0.012 0.001 \0.001
*Mann-Whitney U test.
device and the miniscrew head enabled automatic embedding of the spiked portion of the auxiliary device into the cortical bone over time. A previous study indicated that miniscrews move only slightly into the bone30; therefore, to increase the mechanical retention force between the auxiliary skeletal anchorage device and the bone, the length of embedding between the spiked portion and the bone should be increased by
lengthening the period that the compression force is applied through the silicone ring. As shown in the Table, the auxiliary skeletal anchorage device can increase the mechanical retention forces by approximately 3 to 5 times when compared with miniscrew implantation without the auxiliary unit. This increased mechanical retention force may be due to the greater contact area between the skeletal anchorage and the bone.27 Presumably, this auxiliary device should enable the use of a shorter miniscrew instead of a conventional miniscrew because the minimal contact area is compensated for by embedding the spiked portion. This improves the safety of implantation, particularly in growing patients, in whom miniscrews are difficult to use because of the elevated failure rate compared with the rate in adults.20 Furthermore, in contrast to conventional miniscrews, the auxiliary skeletal anchorage device enables 3-dimensional absolute anchorage because an orthodontic force can be applied in various directions regardless of the loosening direction of a miniscrew. Under the limitations of this study, it is unclear whether the auxiliary skeletal anchorage device is
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Fig 3. Comparison of the mechanical retention forces between the miniscrew with (blue) and without (red) the auxiliary skeletal anchorage device we developed.
Fig 4. The auxiliary skeletal anchorage device and the miniscrew in rabbit bone in vivo after the compression force was applied for 4 weeks through the silicone ring between the washer portion of the auxiliary device and the miniscrew head. Automatic implantation of the spiked portion to a depth of approximately 0.3 mm occurred. The upper panel shows the hind limb of the female rabbit, and lower panel shows the hind limb of the male rabbit.
useful in humans because our data were obtained under experimental conditions over a 4-week period. The validity of these findings warrants confirmation, and
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the potential influence on biologic responses should be examined in both animal and clinical studies in orthodontic patients.
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CONCLUSIONS
The novel spike-like auxiliary skeletal anchorage device enabled automatic embedding of the spiked portion into the cortical bone over time in rabbit bone in vivo, increased miniscrew stability by approximately 3 to 5 times on average compared with implantation with a miniscrew alone, and enabled 3-dimensional absolute anchorage. The new features allow the use of a shorter miniscrew and increase the safety for patients, particularly growing patients. ACKNOWLEDGMENTS
We thank Isao Koyama at the Koyama Orthodontic Clinic and Teruko Takano-Yamamoto of the Department of Orthodontics, Tohoku University, for their helpful comments on our previous studies, and Kimura of Maruyama of Iron Works for the manufacture and architectonics of the novel skeletal anchorage device. This novel auxiliary device is under consideration for domestic and international patents, with support by the Japan Science and Technology Agency. REFERENCES 1. Miyawaki S, Koyama I, Inoue M, Mishima K, Sugahara T, TakanoYamamoto 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. 2. Sugawara J, Daimaruya T, Umemori M, Nagasaka H, Takahashi I, Kawamura H, et al. Distal movement of mandibular molars in adult patients with the skeletal anchorage system. Am J Orthod Dentofacial Orthop 2004;125:130-8. 3. Maeda A, Sakoguchi Y, Miyawaki S. Patient with oligodontia treated with a miniscrew for unilateral mesial movement of the maxillary molars and alignment of an impacted third molar. Am J Orthod Dentofacial Orthop 2013;144:430-40. 4. Kyung SH, Lee JY, Shin JW, Hong C, Dietz V, Gianelly AA. Distalization of the entire maxillary arch in an adult. Am J Orthod Dentofacial Orthop 2009;135(4 Suppl):S123-32. 5. Togawa R, Iino S, Miyawaki S. Skeletal Class III and open bite treated with bilateral sagittal split osteotomy and molar intrusion using titanium screws. Angle Orthod 2010;80:1176-84. 6. Tomonari H, Yagi T, Kuninori T, Ikemori T, Miyawaki S. The replacement of one first molar and three second molars by the mesial inclination of four impacted third molars in a Class II Division 1 adult patient. Am J Orthod Dentofacial Orthop 2015:in press. 7. Koyama I, Iino S, Abe Y, Takano-Yamamoto T, Miyawaki S. Differences between sliding mechanics with implant anchorage and straight-pull headgear and intermaxillary elastics in adults with bimaxillary protrusion. Eur J Orthod 2011;33:126-31. 8. Fudalej P, Antoszewska J. Are orthodontic distalizers reinforced with the temporary skeletal anchorage devices effective? Am J Orthod Dentofacial Orthop 2011;139:722-9. 9. Papageorgiou SN, Zogakis IP, Papadopoulos MA. Failure rates and associated risk factors of orthodontic miniscrew implants: a metaanalysis. Am J Orthod Dentofacial Orthop 2012;142:577-95.e7.
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10. Karagkiolidou A, Ludwig B, Pazera P, Gkantidis N, Pandis N, Katsaros C. Survival of palatal miniscrews used for orthodontic appliance anchorage: a retrospective cohort study. Am J Orthod Dentofacial Orthop 2013;143:767-72. 11. Grec RH, Janson G, Branco NC, Moura-Grec PG, Patel MP, Castanha Henriques JF. Intraoral distalizer effects with conventional and skeletal anchorage: a meta-analysis. Am J Orthod Dentofacial Orthop 2013;143:602-15. 12. Kravitz ND, Kusnoto B. Risks and complications of orthodontic miniscrews. Am J Orthod Dentofacial Orthop 2007;131(4 Suppl):S43-51. 13. European Commission. Radiation Protection 136. European Guidelines on Radiation Protection in Dental Radiology. Luxembourg: Office for Official Publications of the European Communities, 2004. Available at: https://ec.europa.eu/energy/ sites/ener/files/documents/136.pdf. Accessed June 26, 2015. 14. Crismani AG, Bertl MH, Celar AG, Bantleon HP, Burstone CJ. Miniscrews in orthodontic treatment: review and analysis of published clinical trials. Am J Orthod Dentofacial Orthop 2010;137:108-13. 15. Tomonari H, Yagi T, Kitashima F, Koyama I, Takano-Yamamoto T, Miyawaki S. Factors associated with the stability of miniscrews as orthodontic anchorage: analysis of published clinical trials [abstract]. Orthod Waves 2013;72:36. 16. Wilmes B, Drescher D. Impact of bone quality, implant type, and implantation site preparation on insertion torques of miniimplants used for orthodontic anchorage. Int J Oral Maxillofac Surg 2011;40:697-703. 17. Deguchi T, Yabuuchi T, Hasegawa M, Garetto LP, Roberts WE, Takano-Yamamoto T. Histomorphometric evaluation of cortical bone thickness surrounding miniscrew for orthodontic anchorage. Clin Implant Dent Relat Res 2011;13:197-205. 18. Marquezan M, Mattos CT, Sant'Anna EF, de Souza MM, Maia LC. Does cortical thickness influence the primary stability of miniscrews? A systematic review and meta-analysis. Angle Orthod 2014;84:1093-103. 19. Swasty D, Lee J, Huang JC, Maki K, Gansky SA, Hatcher D, et al. Cross-sectional human mandibular morphology as assessed in vivo by cone-beam computed tomography in patients with different vertical facial dimensions. Am J Orthod Dentofacial Orthop 2011;139(4 Suppl):e377-89. 20. Farnsworth D, Rossouw PE, Ceen RF, Buschang PH. Cortical bone thickness at common miniscrew implant placement sites. Am J Orthod Dentofacial Orthop 2011;139:495-503. 21. Kuroda S, Yamada K, Deguchi T, Hashimoto T, Kyung HM, Takano-Yamamoto T. Root proximity is a major factor for screw failure in orthodontic anchorage. Am J Orthod Dentofacial Orthop 2007;131(4 Suppl):S68-73. 22. Min KI, Kim SC, Kang KH, Cho JH, Lee EH, Chang NY, et al. Root proximity and cortical bone thickness effects on the success rate of orthodontic micro-implants using cone beam computed tomography. Angle Orthod 2012;82:1014-21. 23. Watanabe H, Deguchi T, Hasegawa M, Ito M, Kim S, TakanoYamamoto T. Orthodontic miniscrew failure rate and root proximity, insertion angle, bone contact length, and bone density. Orthod Craniofac Res 2013;16:44-55. 24. Pickard MB, Dechow P, Rossouw PE, Buschang PH. Effects of miniscrew orientation on implant stability and resistance to failure. Am J Orthod Dentofacial Orthop 2010;137:91-9. 25. Petrey JS, Saunders MM, Kluemper GT, Cunningham LL, Beeman CS. Temporary anchorage device insertion variables: effects on retention. Angle Orthod 2010;80:446-53. 26. Brettin BT, Grosland NM, Qian F, Southard KA, Stuntz TD, Morgan TA, et al. Bicortical vs monocortical orthodontic skeletal anchorage. Am J Orthod Dentofacial Orthop 2008;134:625-35.
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27. Morarend C, Qian F, Marshall SD, Southard KA, Grosland NM, Morgan TA, et al. Effect of screw diameter on orthodontic skeletal anchorage. Am J Orthod Dentofacial Orthop 2009;136:224-9. 28. Hong C, Lee H, Webster R, Kwak J, Wu BM, Moon W. Stability comparison between commercially available mini-implants and a novel design: part 1. Angle Orthod 2011;81:692-9.
29. Song HN, Hong C, Banh R, Ohebsion T, Asatrian G, Leung HY, et al. Mechanical stability and clinical applicability assessment of novel orthodontic mini-implant design. Angle Orthod 2013;83:832-41. 30. Wang YC, Liou EJ. Comparison of the loading behavior of selfdrilling and predrilled miniscrews throughout orthodontic loading. Am J Orthod Dentofacial Orthop 2008;133:38-43.
Nostalgia Advertisement from a 1983 issue of the Journal
August 2015 Vol 148 Issue 2
American Journal of Orthodontics and Dentofacial Orthopedics
Development of a novel spike-like auxiliary skeletal anchorage device to enhance miniscrew stability Shouichi Miyawaki,a Hiroshi Tomonari,b Takakazu Yagi,c Takaharu Kuninori,b Yasuhiko Oga,d and Masafumi Kikuchie Kagoshima, Japan
Introduction: Miniscrews are frequently used for skeletal anchorage during edgewise treatment, and their clinical use has been verified. However, their disadvantage is an approximately 15% failure rate, which is primarily attributed to the low mechanical stability between the miniscrew and cortical bone and to the miniscrew's close proximity to the dental root. To solve these problems, we developed a novel spike-like auxiliary skeletal anchorage device for use with a miniscrew to increase its stability. Methods: The retention force was compared between miniscrews with and without the auxiliary skeletal anchorage device at each displacement of the miniscrew. The combined unit was also implanted into the bones of 2 rabbits in vivo, and implantation was visually assessed at 4 weeks postoperatively while the compression force was applied. Results: The retention force of the combined unit was significantly and approximately 3 to 5 times stronger on average than that of the miniscrew alone at each displacement. The spiked portion of the auxiliary anchorage device embedded into the cortical bone of the hind limb at approximately a 0.3-mm depth at 4 weeks postimplantation in both rabbits. Conclusions: The auxiliary skeletal anchorage device may increase miniscrew stability, allow a shortened miniscrew, and enable 3-dimensional absolute anchorage. Further evaluation of its clinical application is necessary. (Am J Orthod Dentofacial Orthop 2015;148:338-44)
B
oth miniscrews and miniplates have been used in clinical orthodontic practice to provide skeletal anchorage.1,2 Miniplates do offer a lower failure rate in comparison with other skeletal anchorage devices such as miniscrews1; however, the devices are
From the Graduate School of Medical and Dental Sciences, Kagoshima University, Kagoshima, Japan. a Professor and chair, Department of Orthodontics, Field of Developmental Medicine, Health Research Course. b Assistant professor, Department of Orthodontics, Field of Developmental Medicine, Health Research Course. c Lecturer, Department of Orthodontics, Field of Developmental Medicine, Health Research Course. d Postgraduate student, Department of Orthodontics, Field of Developmental Medicine, Health Research Course. e Professor and chair, Department of Biomaterials Science, Field of Oral and Maxillofacial Rehabilitation, Advanced Therapeutic Course. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. Partially supported by Japan Society for the Promotion of Science KAKENHI Grant Numbers 26463099, 25670879, 24593105, and 24390464. Address correspondence to: Shouichi Miyawaki, Department of Orthodontics, Field of Developmental Medicine, Health Research Course, Kagoshima University, Graduate School of Dental Sciences, 8-35-1 Sakuragaoka, Kagoshima City, Kagoshima 890-8544, Japan; e-mail, [email protected]. Submitted, August 2014; revised and accepted, February 2015. 0889-5406/$36.00 Copyright Ó 2015 by the American Association of Orthodontists. http://dx.doi.org/10.1016/j.ajodo.2015.02.030
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disadvantaged by the significantly increased patient discomfort, a highly complex implantation procedure, and greater invasiveness because of the required flap surgery. The miniscrew has recently come into use for skeletal anchorage during edgewise treatment and is the most frequently implanted of all skeletal anchorage devices. Miniscrews have enabled numerous orthodontic treatments that were difficult with conventional appliances: eg, mesial movement of a molar in a patient with oligodontia,3 distal movement of the maxillary and mandibular teeth,4 and improvement of an open bite caused by intruding posterior teeth.5 In addition, miniscrews have often been used to provide 2-dimensional absolute anchorage during edgewise treatment.6 Currently, the usefulness of miniscrews has been conclusively shown with a high level of evidence, but the devices have some disadvantages.7-11 Miniscrews require extremely precise implantation when placed into the narrow space between 2 dental roots.12 Cone-beam computed tomography is not recommended to examine the space between the roots and the cortical bone thickness because of the high radiation exposure, particularly in growing patients.13 Most critically, miniscrews have an approximately 15% failure rate on average9,14,15;
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with this high rate of failure, in a patient population implanted with 4 miniscrews each, approximately 50% of the patients will experience failure in more than 1 miniscrew, ultimately leading to reimplantation. Two primary factors are implicated in miniscrew failure. One is the low mechanical stability between the miniscrew and the cortical bone.1,16-18 This results in even higher failure rates in patients with a high mandibular plane angle1,19 and in growing patients20 than in patients with a low or an average mandibular plane angle and adult patients. Another factor is the close proximity of the miniscrew to the dental root.21-23 Prevention of miniscrew failure thus requires meeting 2 crucial conditions: increased mechanical stability, achieved by increasing the contact area between the miniscrew and the cortical bone,18 and prevention of root proximity by shortening the miniscrew.23 However, increasing the miniscrew diameter to enhance the mechanical stability also increases the risk of root proximity, whereas decreasing the miniscrew diameter decreases the risk of root proximity. As a result, no currently available miniscrews or skeletal anchorage devices meet these conditions while remaining minimally invasive. Accordingly, we developed an auxiliary skeletal anchorage device that is used in conjunction with a miniscrew to increase its stability, enabling 3-dimensional absolute anchorage and ultimately helping to avoid root proximity with a shorter miniscrew. Furthermore, we evaluated the mechanical stability force between artificial bone and the combined miniscrew and auxiliary skeletal anchorage device, and assessed the degree of embedding of the auxiliary device after the compression force was applied for 4 weeks through a silicone ring between the auxiliary device and the miniscrew head. MATERIAL AND METHODS
To increase the mechanical stability between the miniscrew and the cortical bone, we developed an auxiliary skeletal anchorage device made of titanium alloy (raw material, Ti6Al4V; ASTM F136-96) comprising 2 portions: a washer portion that receives the force from the screw head and transfers it through the silicone at the first stage of implantation, and a spiked portion that contacts and slightly embeds into the cortical bone during the first stage: ie, as the compression force was applied through the silicone ring between the auxiliary device and the miniscrew head (Fig 1, A-C). During the first stage of implantation, a silicone ring (thickness, 1.0 mm; Durometer Shore A, 18; tensile strength, 4.14 MPa [600 psi]; tearing strength, 45; tensile elongation, 700%; elastic modulus, 0.82 MPa; Bitec Global Group, Tokyo, Japan) was placed between the screw
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head and the washer portion of the auxiliary skeletal anchorage device. A preliminary study (data not shown) showed that an approximately 1 to 2 N force is delivered to the spiked portions of the auxiliary skeletal anchorage device through the silicone ring, allowing embedding into the cortical bone over time as the compression force is applied through the silicone ring between the auxiliary device and the screw head. One month after embedding the spiked portion of the miniscrew into the bone to approximately a 0.3-mm depth, the silicone ring was removed, and the miniscrew was further tightened to allow the spiked portion to compress more securely to the cortical bone (Fig 1, D). The artificial bone blocks (Sawbones; Pacific Research Laboratories, Vashon Island, Wash) included cancellous bone constructed from 0.08 g per millileter (5 pcf) density cellular rigid polyurethane foam and 1mm-thick cortical bone constructed from 0.64 g per milliliter (40 pcf) laminated rigid polyurethane foam. The constructed blocks measured 2.54 cm (1 in) in length and width, and 4.57 cm (1.8 in) in height (Fig 2, A). For the lateral displacement test, the mechanical retention force between the skeletal anchorage and the artificial bone was determined using titanium alloy miniscrews measuring 1.6 mm in diameter and 6.0 mm in length (Dual-Top; Jeil Medical, Seoul, Korea).24-29 Two groups comprising 10 miniscrews each were implanted en masse into artificial bone blocks using a driver machine (Orthonia 111-ED-010; Jeil Medical); 1 group was implanted with an auxiliary skeletal anchorage device (auxiliary group) and 1 without the auxiliary device (nonauxiliary group). The spiked portion of the auxiliary skeletal anchorage device was implanted into the cortical region of the artificial bone to a depth of 0.3 mm. The depths of miniscrew implantation in both groups were then adjusted until they were equal between the groups. To determine the mechanical retention force, the displacement of the miniscrew head was measured after application of a compression force delivered from 2 directions (Fig 2, B) using a compression test machine (TGE-5kN; Minebea, Nagano, Japan) set at a load of 5.0 kN and a compression velocity of 0.5 mm per minute (Fig 2, C). The compression force was delivered in 2 directions parallel to the artificial bone surface until each miniscrew moved 0.01, 0.02, and 0.03 mm from its initial position. The resulting force was calculated with software (SR-06-001 version 3.400; Minebea). A miniscrew with an auxiliary skeletal anchorage device was implanted into 2 New Zealand white rabbits, 1 male and 1 female, aged 14 weeks and weighing approximately 3 kg. All surgical procedures were performed under general (1 mg/kg ketamine and 2 mg/kg xylazine) and local (2% lidocaine with 1:80,000 epinephrine)
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Fig 1. A, The auxiliary skeletal anchorage device Spike Anchor [SA]) was developed for use with a commercially available miniscrew (MS), and a silicone ring (SR) was used with the SA to add the force of SA embedding; B, immediately after implantation, the direction of force from the silicon ring to the auxiliary skeletal anchorage device is apparent (red arrows); C, 4 weeks after embedding the auxiliary skeletal anchorage device into bone to a depth of approximately 0.3 mm; D, the miniscrew was further tightened immediately after removing the silicon ring (yellow arrow).
anesthesia. Before surgery, each rabbit was placed under general anesthesia and showed no response to pain stimulation. One hind limb was shaved and aseptically cleansed with povidone-iodine and 70% ethanol. A flap was reflected on each femur using a periosteal elevator, and the miniscrews were implanted at least 10 mm apart with a driver machine. At 4 weeks postimplantation, each animal was killed by a lethal dose of pentobarbital, and the implanted limb was isolated. The specimen was visually examined to determine whether automatic embedding of the auxiliary skeletal anchorage device occurred during the intervening 4 weeks after implantation. This animal experimental protocol was approved by the institutional experimentation committee of Kagoshima University (number D13010). Statistical analysis
The retention force in each displacement was compared between the 2 study groups using the Mann-Whitney U test. Data were analyzed with statistical software (version 10.0; IBM, Armonk, NY), and statistical significance was designated at a \0.05.
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RESULTS
The retention force was significantly stronger in the auxiliary group than in the nonauxiliary group at 0.01, 0.02, and 0.03 mm miniscrew displacements (0.01 mm, P 5 0.012; 0.02 mm, P 5 0.001; and 0.03 mm, P \0.001). In addition, as the displacement of the screw head decreased, the mean difference in the applied force was greater in the auxiliary group than in the nonauxiliary group. The differences in mean forces were approximately 5.5, 4.2, and 3.4 times greater in the auxiliary group than in the nonauxiliary group at displacements of 0.01, 0.02, and 0.03 mm, respectively (Table, Fig 3). Visual assessment showed that the spiked portion of the auxiliary anchorage device was implanted into the cortical bone of the hind limb to a depth of approximately 0.3 mm after the compression force was applied for 4 weeks in both rabbits (Fig 4). DISCUSSION
The visual assessment results shown in Figure 4 suggest that a compression force of 1 to 2 N through the silicone ring between the auxiliary skeletal anchorage
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Fig 2. A, The artificial bone block and the implantation of the conventional miniscrew and the miniscrew with the auxiliary skeletal anchorage device; B, schema of the force direction and structure of the combined unit, with the compression force applied to the miniscrew in 2 directions parallel to the artificial bone surface (red arrows); C, a compression test machine was used for the lateral displacement test, with the compression force applied perpendicular to the miniscrew (red arrow).
Table. Comparison of mechanical retention forces
between the 2 groups according to miniscrew head displacement Compression force (N) Auxiliary group Miniscrew displacement (mm) 0.01 0.02 0.03
Mean 0.65 3.77 7.06
SD 0.44 1.28 1.77
Nonauxiliary group Mean 0.12 0.90 2.13
SD 0.11 0.61 1.11
Probability* 0.012 0.001 \0.001
*Mann-Whitney U test.
device and the miniscrew head enabled automatic embedding of the spiked portion of the auxiliary device into the cortical bone over time. A previous study indicated that miniscrews move only slightly into the bone30; therefore, to increase the mechanical retention force between the auxiliary skeletal anchorage device and the bone, the length of embedding between the spiked portion and the bone should be increased by
lengthening the period that the compression force is applied through the silicone ring. As shown in the Table, the auxiliary skeletal anchorage device can increase the mechanical retention forces by approximately 3 to 5 times when compared with miniscrew implantation without the auxiliary unit. This increased mechanical retention force may be due to the greater contact area between the skeletal anchorage and the bone.27 Presumably, this auxiliary device should enable the use of a shorter miniscrew instead of a conventional miniscrew because the minimal contact area is compensated for by embedding the spiked portion. This improves the safety of implantation, particularly in growing patients, in whom miniscrews are difficult to use because of the elevated failure rate compared with the rate in adults.20 Furthermore, in contrast to conventional miniscrews, the auxiliary skeletal anchorage device enables 3-dimensional absolute anchorage because an orthodontic force can be applied in various directions regardless of the loosening direction of a miniscrew. Under the limitations of this study, it is unclear whether the auxiliary skeletal anchorage device is
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Fig 3. Comparison of the mechanical retention forces between the miniscrew with (blue) and without (red) the auxiliary skeletal anchorage device we developed.
Fig 4. The auxiliary skeletal anchorage device and the miniscrew in rabbit bone in vivo after the compression force was applied for 4 weeks through the silicone ring between the washer portion of the auxiliary device and the miniscrew head. Automatic implantation of the spiked portion to a depth of approximately 0.3 mm occurred. The upper panel shows the hind limb of the female rabbit, and lower panel shows the hind limb of the male rabbit.
useful in humans because our data were obtained under experimental conditions over a 4-week period. The validity of these findings warrants confirmation, and
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the potential influence on biologic responses should be examined in both animal and clinical studies in orthodontic patients.
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CONCLUSIONS
The novel spike-like auxiliary skeletal anchorage device enabled automatic embedding of the spiked portion into the cortical bone over time in rabbit bone in vivo, increased miniscrew stability by approximately 3 to 5 times on average compared with implantation with a miniscrew alone, and enabled 3-dimensional absolute anchorage. The new features allow the use of a shorter miniscrew and increase the safety for patients, particularly growing patients. ACKNOWLEDGMENTS
We thank Isao Koyama at the Koyama Orthodontic Clinic and Teruko Takano-Yamamoto of the Department of Orthodontics, Tohoku University, for their helpful comments on our previous studies, and Kimura of Maruyama of Iron Works for the manufacture and architectonics of the novel skeletal anchorage device. This novel auxiliary device is under consideration for domestic and international patents, with support by the Japan Science and Technology Agency. REFERENCES 1. Miyawaki S, Koyama I, Inoue M, Mishima K, Sugahara T, TakanoYamamoto 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. 2. Sugawara J, Daimaruya T, Umemori M, Nagasaka H, Takahashi I, Kawamura H, et al. Distal movement of mandibular molars in adult patients with the skeletal anchorage system. Am J Orthod Dentofacial Orthop 2004;125:130-8. 3. Maeda A, Sakoguchi Y, Miyawaki S. Patient with oligodontia treated with a miniscrew for unilateral mesial movement of the maxillary molars and alignment of an impacted third molar. Am J Orthod Dentofacial Orthop 2013;144:430-40. 4. Kyung SH, Lee JY, Shin JW, Hong C, Dietz V, Gianelly AA. Distalization of the entire maxillary arch in an adult. Am J Orthod Dentofacial Orthop 2009;135(4 Suppl):S123-32. 5. Togawa R, Iino S, Miyawaki S. Skeletal Class III and open bite treated with bilateral sagittal split osteotomy and molar intrusion using titanium screws. Angle Orthod 2010;80:1176-84. 6. Tomonari H, Yagi T, Kuninori T, Ikemori T, Miyawaki S. The replacement of one first molar and three second molars by the mesial inclination of four impacted third molars in a Class II Division 1 adult patient. Am J Orthod Dentofacial Orthop 2015:in press. 7. Koyama I, Iino S, Abe Y, Takano-Yamamoto T, Miyawaki S. Differences between sliding mechanics with implant anchorage and straight-pull headgear and intermaxillary elastics in adults with bimaxillary protrusion. Eur J Orthod 2011;33:126-31. 8. Fudalej P, Antoszewska J. Are orthodontic distalizers reinforced with the temporary skeletal anchorage devices effective? Am J Orthod Dentofacial Orthop 2011;139:722-9. 9. Papageorgiou SN, Zogakis IP, Papadopoulos MA. Failure rates and associated risk factors of orthodontic miniscrew implants: a metaanalysis. Am J Orthod Dentofacial Orthop 2012;142:577-95.e7.
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