CLINICAL SECTION
Journal of Orthodontics, Vol. 41, 2014, 53–62
Orthodontic skeletal anchorage using a palatal external plate Masaru Kobayashi1 and Kenji Fushima2 1
Department of Oral and Maxillofacial Surgery, Kanagawa Dental University, Kanagawa, Japan; 2Department of Orthodontics, Kanagawa Dental University, Kanagawa, Japan
We have developed the Anchor-Lock external plate system, which is fitted on the palate for the purpose of orthodontic skeletal anchorage. The aim of this study was to introduce the Anchor-Lock and assess its success rate. The Anchor-Lock is composed of titanium screws of 2.0-mm diameter and a titanium plate of 1.0-mm thickness. The external plate is rigidly interlocked with the heads of the screws, which are implanted trans-mucosally into palatal bone. Three types of Anchor-Lock are available. These were applied to 137 orthodontic patients (104 females and 33 males) aged 10–54 years. Two types of plate were used, a straight-shaped plate applied to the hard palate and to the anterior palate and a double-Y-shaped plate applied after tooth-borne rapid maxillary expansion. Success rate of the Anchor-Lock was 92.0% overall. No significant difference in success rate was found by age or sex of patients. Type or screw length of the Anchor-Lock did not affect success rate significantly. Success rate was significantly increased by the use of the surgical stent. The Anchor-Lock was effectively applied to distalize and/or intrude the upper molars. The Anchor-Lock system appears suitable for clinical use as an alternative to conventional screw- and plate-type orthodontic implants. Key words: Skeletal anchorage, temporary anchorage, palatal implant, external plate, success rate Received 27 June 2012; accepted 13 May 2013
Introduction Modern orthodontics has adopted orthodontic implants as temporary and skeletal anchorage devices, and these include plate-type implants (Umemori et al., 1999; Sherwood et al., 2002; Erverdia and Acar, 2005; Sugawara et al., 2006), mini-screw-type implants (Kanomi, 1997; Costa et al., 1998, Deguchi et al., 2003) and palatal implants (Wehrbein et al., 1999; Keles et al., 2003). Collectively, these anchorage devices all have their own advantages and disadvantages. Although plate-type implants provide firm anchorage and are easy to apply an orthodontic force to, the associated surgical procedures can be complicated and traumatic. Further, they are expensive and subject to mucosal inflammation around the plate with poor oral hygiene during active orthodontic treatment. In contrast, surgical procedures for screw-type implants are simple and minimally invasive, and their overall cost is low. However, screw types have limitations with regard to insertion site and orthodontic force application. It has been reported that the quantity and quality of bone of the palate render it suitable for the placement of orthodontic implants (Kang et al., 2007; Baumgaertel, Address for correspondence: Department of Orthodontics, Yokohama Residency Training Center, Kanagawa Dental University, Tsuruya-cho 3-31-6, Kanagawa-ku, Yokohama, Kanagawa 221-0835, Japan Email: [email protected] # 2014 British Orthodontic Society
2009; Moon et al., 2010). The palate therefore appears to be an alternative site for skeletal anchorage devices. We have applied an external plate system to the palate for the purpose of orthodontic anchorage. The aim of this study was to introduce this new skeletal anchorage system and assess its success rate.
Materials and methods The external plate system was developed as a temporary skeletal anchorage device utilizing the cranio-maxillofacial implant Compact Lock (Synthes Co., Solothurn, Switzerland), and named the Anchor-Lock. As shown in the sectioned drawing of Figure 1a, the Anchor-Lock is composed of titanium screws of 2.0-mm diameter and a titanium bone plate of 1.0-mm thickness. Usually, two screws are implanted trans-mucosally into the palatal bone. The titanium plate is rigidly locked on to the screw heads and extends laterally across the palate supramucosally. To rigidly stabilize the plate-screw locking system, the head of the screw is uniquely designed with thread cutters to fit the thread grooves in the titanium plate opening. The Anchor-Lock system makes use of
DOI 10.1179/1465313313Y.0000000069
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Figure 1 Anchor-Lock system: (a) sectioned drawing; (b) plate types
two types of plate, straight-shaped and double-Y-shaped plates, respectively (Figure 1b). Subjects One hundred and thirty-seven consecutive patients (104 females and 33 males), who visited the Kanazawa Orthodontic Clinic from 2004 to 2012 and received the Anchor-Lock as orthodontic skeletal anchorage, participated in the present study. Median age was 20 years, ranging from 10 to 54 years. Placement sites and plate types The straight-type plate is usually placed at one or two sites, the posterior hard palate (HP) and the anterior palate (AP) vault (Figure 2a, b). Bone thickness of the hard palate is estimated using lateral cephalometric tracing, in which the bony contour of the palate is measured at the level of the first molar antero-posteriorly. In our clinical criteria for placement at HP, it is advisable that bone thickness is more than 5.0 mm, a thickness less than this carries the risk of implant failure. The AP is an alternative site, in which the screw is inserted posterior to the incisive foramen and perpendicularly to the palatal mucosa toward the anterior nasal spine. For both HP and AP, the straight-type plate is
fixed with two screws, which are implanted into the palatal bone trans-mucosally at paramedian sites about 5-mm lateral to the mid-palatal suture. The double-Y type plate (Figure 2c) is another alternative, and is applied after tooth-borne rapid maxillary expansion (RME) (Fushima and Kobayashi, 2011). To retain the maxilla in the expanded state after RME, the double-Y plate is fixed to the palatal vault using four titanium screws, two each to the hard palate and the anterior palate. In addition to ensuring bone fixation, this plate can also be used as orthodontic anchorage for further tooth movement after RME. Preparation of the surgical stent Correct placement of the Anchor-Lock requires fabrication of a surgical stent (Figure 3a). A dental cast model of the upper arch is prepared, over which a thermoformable plastic sheet of 1.5-mm thickness is pressed and the part covering the dentition and palatal vault is sectioned away. The insertion position of the screw is estimated on the lateral cephalogram beforehand, as is assessment of the palatal bone thickness (Figure 3b). On the plastic sheet covering the dental cast, a plate is bent to fit the palatal surface. In this bending process, it is very important not to deform the screw locking holes.
Figure 2 Types of Anchor-Lock and insertion sites: (a) straight-type HP; (b) straight-type AP; (c) double-Y
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Figure 3 Surgical stent: (a) surgical stent; (b) lateral cephalogrametric assessment
The plate is placed properly and fixed with acrylic resin material on the sheet. The areas surrounding the screw holes are trimmed out from the sheet and preparation of the surgical stent is completed. Surgical procedures Fixation of the Anchor-Lock is easily carried out under local anesthesia with minimum surgical invasion. All Anchor-Locks in this series were placed by one surgeon. Figure 4 shows the surgical instruments necessary for implantation of the Anchor-Lock. First, the surgical stent is placed intra-orally (Figure 5a). Two plate openings above the paramedian to the midpalatal suture are the screw locking holes, around which local anaesthesia is given. The palatal mucosa of the insertion site is marked and removed with the diamond bur. The drill guide instrument is interlocked with one of the plate openings. The 1.1-mm drill with 29.5/17.0mm length is inserted along the guide instrument. The screw guide hole into the palate is drilled to establish the depth and direction of the screw recipient site using the contra-angle hand piece, with copious saline irrigation (Figure 5b). The drill guide instrument is then removed from the plate opening and the 1.5-mm drill with 35.0/8.0mm length is used to widen the recipient hole to a depth of
about 2-mm, corresponding to the cortical bone thickness (Figure 5c). The operating speed of the contra-angle is approximately 400–500 rev/min with the reduction ratio 20 : 1. A self-tapping screw of 2.0-mm diameter is placed at the centre of the plate opening and inserted using the manual hand piece motor into the screw guide hole drilled into the palate (Figure 5d). At the end of screw insertion, the thread cutter around the screw head is rigidly interlocked with the thread groove in the plate opening. This surgical procedure is repeated on the other side. Finally, the surgical stent is removed by splitting it into two pieces (Figure 5e) and the implantation is completed. As shown in Figure 5f, the plate is placed above the palate securely with the two screws about 2.0 mm off the palatal mucosa. The surgical procedure for the Anchor-Lock using the surgical stent and drill guide instrument was established in 2007, and has been performed since then. Before 2007, the operating procedure was carried out by visual assessment only, without the guidance of the stent. Removal of the Anchor-Lock is performed using the manual hand piece driver, usually without anesthesia. Palatal mucosa at the implant sites heals in a few days. Loading protocol of orthodontic force After a 2-week healing period, orthodontic force is loaded onto the external plate bilaterally. The loading force is from 150 to 350 g per side, and mainly utilized for distalization and/or intrusion of the molars. Typical cases with each type of Anchor-Lock are outlined below. Success rate The Anchor-Lock is usually implanted at the beginning of orthodontic treatment and remains as an orthodontic anchorage throughout the active treatment period. Success rate was clinically assessed using the following criteria:
Figure 4 Surgical instruments: (a) from left to right, diamond bur, 1.1-mm drill, 1.5-mm drill, driver and drill guide instrument; (b) manual hand piece motor
1. No loosening of the screws was found during the whole active treatment period.
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Figure 5 Surgical procedures: (a) try-in of surgical stent; (b) drilling of screw guide hole with 1.1-mm drill; (c) widening of recipient hole with 1.5-mm drill; (d) screw insertion using the manual hand piece motor; (e) removal of surgical stent; (f) completion of implantation
2. No uncontrollable inflammation was detected around the supporting screws and/or the anchorage plate. If the Anchor-Lock failed, the duration of the time of application to removal was defined as the time to failure. Statistical analysis Factors affecting the success of the Anchor-Lock were investigated. The device was applied using the surgical stent after 2007, but was applied before then without stent guidance. To examine the influence of the surgical stent, we therefore compared the success rate after 2007 with that before 2007. To examine the influence of age, patients were divided into 64 young patients aged less than 20 years (median age: 14 years; range: 10–19 years) and 73 adult patients older than 20 years (median age: 26 years; range: 20– 54 years), and the success rate between them was compared. Success rate was also investigated with regard to sex. Three types of Anchor-Lock were available and their success rates were compared. For placement of the straight-type at HP and AP the influence of the screw length on success rate was examined. Differences according to surgical technique, age and sex of patients, type of Anchor-Lock and their screw length used were evaluated using the Chi-square test for independence. Results The Anchor-Lock was widely applied to 137 patients, from growing subjects aged 10 years to older subjects aged more than 50 years. Eleven of 137 cases failed, giving a total success rate of 92.0%. In 126 successful cases, active orthodontic treatment was completed, with an average application period of 2 years 2 months,
ranging from 8 months to 5 years. During the orthodontic treatment period, 15 cases out of 126 have suffered an inflammation of the palatal mucosa around the screws and the incidence was 11.9%. The mucosal inflammation was controlled by oral hygiene. In the 11 cases with failure, the time to failure was investigated (Figure 6). In all except two cases, the device was removed soon after application, with a time to failure less than 4 months. To examine the influence of surgical technique, sex, age and plate type on success rate, the Chi-square test for independence was performed (Table 1). The success rate from 2004 to 2006 was 85.2%, while that from 2007 to 2012 was 96.4%, showing a significant increase following the improvement in surgical technique (P,0.05). Success rate in females was 90.4%, and lower than the 97.0% in males, albeit that this difference was not statistically significant. By age, success rates in young and adult subjects were 92.2% and 91.8%, respectively, again showing no statistically significant difference. Of the three types of Anchor-Lock available, the straight-type at HP was used in 46 patients, and at AP in 49; while the double-Y was placed in 42. Success rate was high with the straight-type at AP and the double-Y (95.9% and 95.2%, respectively), and lower with the straight-type at HP (84.8%, albeit without statistical significance). Typical cases of each Anchor-Lock type are shown in Figures 7–9. In a case showing anterior crowding with a class I Angle’s molar relationship, the straight-type was applied to the HP between the first molars (Figure 7). Bone thickness at the placement site assessed by the lateral cephalogram was 7.0 mm and two screws of 10.0mm length were implanted in this area (Figure 7a). A lingual arch wire was applied to the upper first molars and retracted to the Anchor-Lock (Figure 7c). As a
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Figure 6 Time to failure
result, distalization of the upper lateral teeth was successfully achieved (Figure 7b) while sufficient space to align the anterior teeth was prepared. Figure 8 shows a case of crowding with an Angle’s class II molar relationship. Since the thickness of the HP was insufficient for implantation, the straight-type was placed at AP for distalization of the molars. Screws of 12.0-mm length were implanted perpendicularly to the palatal mucosa directed towards the anterior nasal spine, and confirmed on the lateral cephalogram (Figure 8a). At the beginning of treatment, a lingual arch wire was applied to the upper second molars and brackets were bonded to the upper lateral teeth. In the occlusal view of Figure 8c, distalization of the lateral teeth is seen to have progressed, and this is confirmed by the reduction in the distance between the lingual arch wire and the Anchor-Lock plate. Finally, the anterior crowding was solved and an Angle’s class I molar relationship was established without flaring out of the anterior teeth (Figure 8b).
Figure 9 shows a growing female subject aged 13 years with anterior crowding and an Angle’s class II molar relationship. The upper dental arch was narrow due to the transverse maxillary deficiency. A tooth-borne type of RME was applied and the mid-palatal suture was expanded with a daily expansion of 0.4 mm. After the palatal expansion, the upper lateral teeth were overexpanded accompanying with their buccal tipping and extrusion. The RME was then removed and a double-Y plate was immediately applied to the palate (Figure 9a). Early in the following few months, the over-expanded lateral teeth had relapsed spontaneously to some extent. The DY had been utilizing as an orthodontic anchorage to distalize the upper lateral teeth, while to control their lingual cusps in an intrusive manner (Figure 9c). The anterior crowding and Angle’s class II molar relationship were improved in accompaniment with distal movement of the upper molars and mandibular growth (Figure 9b). The influence of the screw length on success rate was examined for the straight-type at HP and AP and is
Table 1 Success rate of Anchor-Lock.
Total Year Sex Age Type
2004–2012 2004–2006 2007–2012 Female Male Young (10–19) Adult (20–54) AP HP DY
*Significant difference at 5% level. n.s.: not significant difference.
Number
Success
Failure
Success rate (%)
137 54 83 104 33 64 73 49 46 42
126 46 80 94 32 59 67 47 39 40
11 8 3 10 1 5 6 2 7 2
92.0 85.2 96.4 90.4 97.0 92.2 91.8 95.9 84.8 95.2
Chi-square
*
5.56
n.s.
1.47
n.s.
0.01
n.s.
3.43
n.s.
2.61
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Figure 7 Straight-type Anchor-Lock to HP: (a) lateral cephalometric finding of HP; (b) cephalometric superimposition (cranial base) of initial (solid line) and post-treatment (dotted line); (c) intra-oral photographs show treatment progress in occlusal view, initial and post-treatment findings
summarized in Table 2. For HP, 23 cases used 8.0-mm length and 22 cases used 10.0-mm. Forty-six cases of HP were divided into two groups by the screw length, 24 cases less than 8.0-mm and 22 cases of 10.0-mm. The longer screw group showed higher success rate of 95.5% than the 75.0% in the shorter screw group, without statistically significant difference. For AP, eight cases used 10.0-mm length and 38 cases used 12.0-mm. Fortynine cases of AP was divided into two groups, which were nine cases less than 10.0-mm and 40 cases over 12.0-mm. Success rate in the longer screw group was
97.5% and higher than the 88.9% in the shorter screw group, again showing no statistically significant difference. The double-Y plate used four screws; 8.0- or 10.0mm screws were used for the posterior plate openings, while 10.0- or 12.0-mm screws were applied to the anterior plate openings.
Discussion In this study, we have introduced a new skeletal anchorage device, the Anchor-Lock and assessed its
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Figure 8 Straight-type Anchor-Lock to AP: (a) lateral cephalometric finding of AP; (b) cephalometric superimposition (cranial base) of initial (solid line) and post-treatment (dotted line); (c) intra-oral photographs show treatment progress in occlusal view, initial and post-treatment findings
success rate. Success rate of the Anchor-Lock was 92.0% as a whole, which appears good in comparison with previous studies of mini-screw (Miyawaki et al., 2003; Ohashi et al., 2006; Park et al., 2006; Chaddad et al., 2008; Moon et al., 2008; Papageorgiou et al., 2012) and palatal implants (Jung et al., 2009; Asscherickx et al., 2010). Because the external plate is fixed with the palatal
bone by more than one screw, the Anchor-Lock should exert a bridging effect. The double-Y plate is rigidly implanted with four screws, the bridging effect of which likely explains its high success rate. In a palatal anchorage system similar to the Anchor-Lock, it was reported that a hybrid combination of mini-screw and mini-plate improved stability of the skeletal anchorage
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Figure 9 Double-Y plate application following RME: (a) lateral cephalometric findings; (b) cephalometric superimposition (anterior cranial base) of initial (solid line) and post-treatment (dotted line); (c) intra-oral photographs show treatment progress in occlusal view, initial and post-treatment findings
with high success rate and effectively applied to distalize the upper molars (Wilmes et al., 2009; Wilmes and Drscher, 2010).
For the straight-type plate success rate at AP was greater than that at HP, although not statistically significant. The AP has advantages for implantation
Table 2 Success rate of the straight-type plate by screw length and anatomical position. Type
Screw length
Number
Success
Failure
Success rate (%)
Chi-square
HP
8-mm or under over 10-mm 10-mm or under Over 12-mm
24 22 9 40
18 21 8 39
6 1 1 1
75.0 95.5 88.9 97.5
n.s.
3.72
n.s.
1.39
AP
*Significant difference at 5% level. n.s.: not significant difference.
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and was reported to have great depth (Kang et al., 2007; Baumgaetel, 2009) and high density (Moon et al., 2010). Success of the anchorage devices is a key to the overall success of orthodontic treatment, because anchorage failure fundamentally upsets treatment planning. From the point of view of orthodontic implant success, AP is recommended because of the characteristics of the palatal bone around the supporting screws. However, HP has advantages in controlling the direction and/or magnitude of the orthodontic force. Screw length of the Anchor-Lock seems to be an important factor relating to success rate. Especially, the endosseous length and its ratio to the total length may affect success rate. Due to the difficulty to assess the endosseous screw length, the present study investigated the total screw length used for the Anchor-Lock. Bone and mucosal thickness of the palate vary individually and in placement sites. Since sufficient bone depth is found in the AP site, long screw of 12.0-mm length was most commonly used and a high success rate was found. For each the HP and AP, although the longer screw group showed a higher success rate than the shorter screw group, there was no statistically significant difference. A study of mini-screw implants reported that the length of the mini-screw did not significantly affect their clinical success (Park et al., 2006). Further studies to assess the influence of endosseous screw length on the success rate are required. There was no difference in success rate by age. For the failure of mini-screw implants, a systematic review reported that no evidence was found for the association of age (Papageorgiou et al., 2012). These findings indicate that the Anchor-Lock can be used not only in adult subjects but also in growing subjects. Maxillary growth is a matter of concern with application of the Anchor-Lock to growing subjects. Using landmark implants, the growth change in maxillary width from 10 years to adulthood has been reported (Bjork and Skieller, 1977). It has been summarized that the average transverse increase in the incisor region was about one-third as great as that in the zygomatic region, at 0.9 and 3.0 mm, respectively. Similar results has been reported and demonstrated that the increase in width in the palatal region was approximately 1.5 mm during the period from 8.5 to 15.5 years (Korn and Baumrind, 1990). An experimental animal study indicated that growth at the mid-palatal suture contributed to transverse growth of the maxilla (Katsaros et al., 2006). Application of the Anchor-Lock possibly suppresses the normal transverse growth at the mid-palatal suture. Although the annual increment of palatal width appears to be small, particular attention should be paid to the application of the AnchorLock to growing subjects.
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Success rate of the Anchor-Lock did not differ by sex significantly. This finding is consistent with the previous studies of the success rate of mini-screw implants, which also reported no significant difference by sex (Papageorgiou et al., 2012). In the present study, nine out of 11 failure cases showed the time to failure of less than 4 months. In the other two cases, the Anchor-Lock functioned successfully as an anchorage for more than 14 months before failing in the course of treatment. Similar results were reported in a study of mini-screw orthodontic implants, with dislodgement of the mini-screw occurring frequently in the first 1– 2 months, and most failures occurring within the first 4 months (Moon et al., 2008). In a study of palatal orthodontic implant, three of 34 implants were lost within 3 months after placement (Asscherickx et al., 2010). We therefore consider that if the Anchor-Lock remains stable without loosening at more than 4 months after surgery, it will likely continue to function without interruption as an orthodontic anchorage in the following treatment period. Although most patients accept the Anchor-Lock, there are some disadvantages. For an oral sensation, one patient complained of some difficulty in pronunciation of the k sound. The speech problem may be due to the device positioning, in which the HP was placed too posteriorly, near the border on the soft palate. Mucosal inflammation around the Anchor-Lock was found temporarily in 11.9% of 126 successful cases and controlled by oral hygiene. Since 2007, a surgical stent and drill guide instrument have been available and success rate of the Anchor-Lock increased significantly. The surgical stent facilitates the systematic implantation of the AnchorLock and ensures the stable orthodontic anchorage. By using the stent, interlocking between the plate openings and the screw heads is accomplished passively without any deformation of their titanium structures. This condition should suppress the loosening of screw-plate locking as well as the development of screw mobility in the palatal bone. Accurate placement of the AnchorLock is important not only for orthodontic force application, but also for soft tissue management of the palatal mucosa. The surgical stent controls the distance between the plate and the palatal mucosa. A larger plate– mucosa distance might induce disorders in oral function, such as in swallowing or pronunciation, while a smaller distance may give rise to mucosal inflammation due to plaque accumulation around the implanted screws and the underside of the plate facing the mucosa. Once inflammation occurs, tissue swelling of the palatal mucosa will occupy the space between it and the plate, adversely affecting oral hygiene.
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An endosseous implant applied to the palate, the Orthosystem, has been reported and successfully used as an orthodontic implant (Wehrbein et al., 1999). This palatal implant system is usually applied to a midpalatal site posterior to the incisive foramen. Since the positioning of this palatal implant is determined by the need to minimize perforation risk to adjacent structures (nasal cavity, nasopalatine canal and incisor roots) (Cousley, 2005), its length is limited to 4.0 or 6.0 mm and its diameter to 3.3 or 4.1 mm. The AP type is also implanted posterior to the incisive foramen, but to a paramedian site about 5.0 mm lateral to the mid-palatal suture, and therefore, carries little risk of injuring the nasopalatine canal. The Anchor-Lock can be easily emplaced and removed under local anaesthesia, and surgical procedures are not complicated and can be carried out systematically within 15 min. The screw diameter of the Anchor-Lock is 2.0 mm and thus sufficiently small to obviate problematic tissue damage on insertion, or tissue defect on removal. The Anchor-Lock appears to be a minimally invasive system which is suitable for clinical settings. Further, it is able to stabilize orthodontic loading in all directions. Orthodontic force is then loaded to the external plate for intrusion and/or distalization of the molars. The Anchor-Lock appears to be an alternative to conventional screw- and plate-type anchorage devices. Conclusions Here, we introduced a new skeletal anchorage device named Anchor-Lock. In this system, an external titanium plate is fixed with titanium screws, which are implanted into the palatal bone trans-mucosally. Three types of Anchor-Lock are available; two utilize a straight-shaped plate which is applied to the hard palate and to the anterior palate. The third type utilizes a double-y-shaped plate which is applied after toothborne rapid maxillary expansion. Eleven of 137 cases failed, giving an overall success rate of 92.0%. No significant difference in success rate was seen by age or sex of patients. Type or screw length of the AnchorLock did not affect success rate significantly. Success rate was significantly increased by the use of the surgical stent. The Anchor-Lock was effectively applied to distalize and/or intrude the upper molars. References Asscherickx K, Vannet BV, Bottenberg P, Wehrbein H, Sabzevar MM. Clinical observations and success rates of palatal implants. Am J Orthod Dentofacial Orthop 2010; 137: 114–122.
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Baumgaertel S. Quantitative investigation of palatal bone depth and cortical bone thickness for mini-implant placement in adults. Am J Orthod Dentofacial Orthop 2009; 136: 104–108. Bjork A, Skieller V. Growth of the maxilla in three dimensions as revealed radiographically by the implant method. Br J Orthod 1977; 4: 53–64. Chaddad K, Ferreira AF, Geurs N, Reddy MS. Influence of surface characteristics on survival rates of mini-implants. Angle Orthod 2008; 78: 107–113. Costa A, Raffainl M, Melsen B. Miniscrews as orthodontic anchorage: a preliminary report. Int J Adult Orthodon Orthognath Surg 1998; 13: 201– 209. Cousley R. Critical aspects in the use of orthodontic palatal implants. Am J Orthod Dentofacial Orthop 2005; 127: 723–729. Deguchi T, Takano-Yamamoto T, Kanomi R, Hartsfield JK, Jr, Roberts WE, Garetto LP. The use of small titanium screws for orthodontic anchorage. J Dent Res 2003; 82: 377–381. Erverdia N, Acar A. Zygomatic anchorage for en masse retraction in the treatment of severe class II division 1. Angle Orthod 2005; 75: 483–490. Fushima K, Kobayashi M. Bone fixation with palatal external plate following tooth-borne rapid maxillary expansion. Bull Kanagawa Dent Col 2011; 39: 3–9. Jung BA, Kunkel M, Go¨llner P, Liechti T, Wehrbein H. Success rate of secondgeneration palatal implants. Angle Orthod 2009; 79: 85–90. Kang S, Lee SJ, Ahn SJ, Heo MS, Kim TW. Bone thickness of the palate for orthodontic mini-implant anchorage in adults. Am J Orthod Dentofacial Orthop 2007; 131: S74–S81. Kanomi R. Mini-implant for orthodontic anchorage. J Clin Orthod 1997; 31: 763–767. Katsaros C, Zissis A, Bresin A, Kiliaridis S. Functional influence on sutural bone apposition in the growing rat. Am J Orthod Dentofacial Orthop 2006; 129: 352–357. Keles A, Erverdi N, Sezen S. Bodily distalization of molars with absolute anchorage. Angle Orthod 2003; 73: 471–482. Korn EL, Baumrind S. Transverse development of the human jaws between the ages of 8.5 and 15.5 years, studied longitudinally with use of implants. J Dent Res 1990; 69: 1298–1306. 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: 373–378. Moon CH, Lee DG, Lee HS, Im JS, Baek SH. Factors associated with the success rate of orthodontic miniscrews placed in the upper and lower posterior buccal region. Angle Orthod 2008; 78: 101–106. Moon SH, Park SH, Lim WH, Chun YS. Palatal bone density in adult subjects: implications for mini-implant placement. Angle Orthod 2010; 80: 137–144. Ohashi E, Pecho OE, Moron M, Lagravere MO. Implant vs screw loading protocols in orthodontics. Angle Orthod 2006; 76: 721–727. Papageorgiou SN, Zogakis IP, Papadopoulos MA. Failure rates and associated risk factors of orthodontic miniscrew implants: a meta-analysis. Am J Orthod Dentofacial Orthop 2012; 142: 577–595. Park HS, Jeong SH, Kwon OW. Factors affecting the clinical success of screw implants used as orthodontic anchorage. Am J Orthod Dentofacial Orthop 2006; 130: 18–25. Sherwood KH, Burch JG, Thompson WJ. Closing anterior open bites by intruding molars with titanium miniplate anchorage. Am J Orthod Dentofacial Orthop 2002; 122: 593–600. Sugawara J, Kanzaki R, Takahashi I, Nagasaka H, Nanda R. Distal movement of maxillary molars in nongrowing patients with the skeletal anchorage system. Am J Orthod Dentofacial Orthop 2006; 129: 723–733. Umemori M, Sugawara J, Mitani H, Nagasaka H, Kawamura H. Skeletal anchorage system for open-bite correction. Am J Orthod Dentofacial Orthop 1999; 115: 166–174. Wehrbein H, Feifel H, Diedrich P. Palatal implant anchorage reinforcement of posterior teeth: a prospective study. Am J Orthod Dentofacial Orthop 1999; 116: 678–686. Wilmes B, Drescher D. Application and effectiveness of the Beneslider: a device to move molars distally. World J Orthod 2010; 11: 331–340. Wilmes B, Drescher D, Nienkemper M. A miniplate system for improved stability of skeletal anchorage. J Clin Orthod 2009; 43: 494–501.
Journal of Orthodontics, Vol. 41, 2014, 53–62
Orthodontic skeletal anchorage using a palatal external plate Masaru Kobayashi1 and Kenji Fushima2 1
Department of Oral and Maxillofacial Surgery, Kanagawa Dental University, Kanagawa, Japan; 2Department of Orthodontics, Kanagawa Dental University, Kanagawa, Japan
We have developed the Anchor-Lock external plate system, which is fitted on the palate for the purpose of orthodontic skeletal anchorage. The aim of this study was to introduce the Anchor-Lock and assess its success rate. The Anchor-Lock is composed of titanium screws of 2.0-mm diameter and a titanium plate of 1.0-mm thickness. The external plate is rigidly interlocked with the heads of the screws, which are implanted trans-mucosally into palatal bone. Three types of Anchor-Lock are available. These were applied to 137 orthodontic patients (104 females and 33 males) aged 10–54 years. Two types of plate were used, a straight-shaped plate applied to the hard palate and to the anterior palate and a double-Y-shaped plate applied after tooth-borne rapid maxillary expansion. Success rate of the Anchor-Lock was 92.0% overall. No significant difference in success rate was found by age or sex of patients. Type or screw length of the Anchor-Lock did not affect success rate significantly. Success rate was significantly increased by the use of the surgical stent. The Anchor-Lock was effectively applied to distalize and/or intrude the upper molars. The Anchor-Lock system appears suitable for clinical use as an alternative to conventional screw- and plate-type orthodontic implants. Key words: Skeletal anchorage, temporary anchorage, palatal implant, external plate, success rate Received 27 June 2012; accepted 13 May 2013
Introduction Modern orthodontics has adopted orthodontic implants as temporary and skeletal anchorage devices, and these include plate-type implants (Umemori et al., 1999; Sherwood et al., 2002; Erverdia and Acar, 2005; Sugawara et al., 2006), mini-screw-type implants (Kanomi, 1997; Costa et al., 1998, Deguchi et al., 2003) and palatal implants (Wehrbein et al., 1999; Keles et al., 2003). Collectively, these anchorage devices all have their own advantages and disadvantages. Although plate-type implants provide firm anchorage and are easy to apply an orthodontic force to, the associated surgical procedures can be complicated and traumatic. Further, they are expensive and subject to mucosal inflammation around the plate with poor oral hygiene during active orthodontic treatment. In contrast, surgical procedures for screw-type implants are simple and minimally invasive, and their overall cost is low. However, screw types have limitations with regard to insertion site and orthodontic force application. It has been reported that the quantity and quality of bone of the palate render it suitable for the placement of orthodontic implants (Kang et al., 2007; Baumgaertel, Address for correspondence: Department of Orthodontics, Yokohama Residency Training Center, Kanagawa Dental University, Tsuruya-cho 3-31-6, Kanagawa-ku, Yokohama, Kanagawa 221-0835, Japan Email: [email protected] # 2014 British Orthodontic Society
2009; Moon et al., 2010). The palate therefore appears to be an alternative site for skeletal anchorage devices. We have applied an external plate system to the palate for the purpose of orthodontic anchorage. The aim of this study was to introduce this new skeletal anchorage system and assess its success rate.
Materials and methods The external plate system was developed as a temporary skeletal anchorage device utilizing the cranio-maxillofacial implant Compact Lock (Synthes Co., Solothurn, Switzerland), and named the Anchor-Lock. As shown in the sectioned drawing of Figure 1a, the Anchor-Lock is composed of titanium screws of 2.0-mm diameter and a titanium bone plate of 1.0-mm thickness. Usually, two screws are implanted trans-mucosally into the palatal bone. The titanium plate is rigidly locked on to the screw heads and extends laterally across the palate supramucosally. To rigidly stabilize the plate-screw locking system, the head of the screw is uniquely designed with thread cutters to fit the thread grooves in the titanium plate opening. The Anchor-Lock system makes use of
DOI 10.1179/1465313313Y.0000000069
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Figure 1 Anchor-Lock system: (a) sectioned drawing; (b) plate types
two types of plate, straight-shaped and double-Y-shaped plates, respectively (Figure 1b). Subjects One hundred and thirty-seven consecutive patients (104 females and 33 males), who visited the Kanazawa Orthodontic Clinic from 2004 to 2012 and received the Anchor-Lock as orthodontic skeletal anchorage, participated in the present study. Median age was 20 years, ranging from 10 to 54 years. Placement sites and plate types The straight-type plate is usually placed at one or two sites, the posterior hard palate (HP) and the anterior palate (AP) vault (Figure 2a, b). Bone thickness of the hard palate is estimated using lateral cephalometric tracing, in which the bony contour of the palate is measured at the level of the first molar antero-posteriorly. In our clinical criteria for placement at HP, it is advisable that bone thickness is more than 5.0 mm, a thickness less than this carries the risk of implant failure. The AP is an alternative site, in which the screw is inserted posterior to the incisive foramen and perpendicularly to the palatal mucosa toward the anterior nasal spine. For both HP and AP, the straight-type plate is
fixed with two screws, which are implanted into the palatal bone trans-mucosally at paramedian sites about 5-mm lateral to the mid-palatal suture. The double-Y type plate (Figure 2c) is another alternative, and is applied after tooth-borne rapid maxillary expansion (RME) (Fushima and Kobayashi, 2011). To retain the maxilla in the expanded state after RME, the double-Y plate is fixed to the palatal vault using four titanium screws, two each to the hard palate and the anterior palate. In addition to ensuring bone fixation, this plate can also be used as orthodontic anchorage for further tooth movement after RME. Preparation of the surgical stent Correct placement of the Anchor-Lock requires fabrication of a surgical stent (Figure 3a). A dental cast model of the upper arch is prepared, over which a thermoformable plastic sheet of 1.5-mm thickness is pressed and the part covering the dentition and palatal vault is sectioned away. The insertion position of the screw is estimated on the lateral cephalogram beforehand, as is assessment of the palatal bone thickness (Figure 3b). On the plastic sheet covering the dental cast, a plate is bent to fit the palatal surface. In this bending process, it is very important not to deform the screw locking holes.
Figure 2 Types of Anchor-Lock and insertion sites: (a) straight-type HP; (b) straight-type AP; (c) double-Y
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Figure 3 Surgical stent: (a) surgical stent; (b) lateral cephalogrametric assessment
The plate is placed properly and fixed with acrylic resin material on the sheet. The areas surrounding the screw holes are trimmed out from the sheet and preparation of the surgical stent is completed. Surgical procedures Fixation of the Anchor-Lock is easily carried out under local anesthesia with minimum surgical invasion. All Anchor-Locks in this series were placed by one surgeon. Figure 4 shows the surgical instruments necessary for implantation of the Anchor-Lock. First, the surgical stent is placed intra-orally (Figure 5a). Two plate openings above the paramedian to the midpalatal suture are the screw locking holes, around which local anaesthesia is given. The palatal mucosa of the insertion site is marked and removed with the diamond bur. The drill guide instrument is interlocked with one of the plate openings. The 1.1-mm drill with 29.5/17.0mm length is inserted along the guide instrument. The screw guide hole into the palate is drilled to establish the depth and direction of the screw recipient site using the contra-angle hand piece, with copious saline irrigation (Figure 5b). The drill guide instrument is then removed from the plate opening and the 1.5-mm drill with 35.0/8.0mm length is used to widen the recipient hole to a depth of
about 2-mm, corresponding to the cortical bone thickness (Figure 5c). The operating speed of the contra-angle is approximately 400–500 rev/min with the reduction ratio 20 : 1. A self-tapping screw of 2.0-mm diameter is placed at the centre of the plate opening and inserted using the manual hand piece motor into the screw guide hole drilled into the palate (Figure 5d). At the end of screw insertion, the thread cutter around the screw head is rigidly interlocked with the thread groove in the plate opening. This surgical procedure is repeated on the other side. Finally, the surgical stent is removed by splitting it into two pieces (Figure 5e) and the implantation is completed. As shown in Figure 5f, the plate is placed above the palate securely with the two screws about 2.0 mm off the palatal mucosa. The surgical procedure for the Anchor-Lock using the surgical stent and drill guide instrument was established in 2007, and has been performed since then. Before 2007, the operating procedure was carried out by visual assessment only, without the guidance of the stent. Removal of the Anchor-Lock is performed using the manual hand piece driver, usually without anesthesia. Palatal mucosa at the implant sites heals in a few days. Loading protocol of orthodontic force After a 2-week healing period, orthodontic force is loaded onto the external plate bilaterally. The loading force is from 150 to 350 g per side, and mainly utilized for distalization and/or intrusion of the molars. Typical cases with each type of Anchor-Lock are outlined below. Success rate The Anchor-Lock is usually implanted at the beginning of orthodontic treatment and remains as an orthodontic anchorage throughout the active treatment period. Success rate was clinically assessed using the following criteria:
Figure 4 Surgical instruments: (a) from left to right, diamond bur, 1.1-mm drill, 1.5-mm drill, driver and drill guide instrument; (b) manual hand piece motor
1. No loosening of the screws was found during the whole active treatment period.
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Figure 5 Surgical procedures: (a) try-in of surgical stent; (b) drilling of screw guide hole with 1.1-mm drill; (c) widening of recipient hole with 1.5-mm drill; (d) screw insertion using the manual hand piece motor; (e) removal of surgical stent; (f) completion of implantation
2. No uncontrollable inflammation was detected around the supporting screws and/or the anchorage plate. If the Anchor-Lock failed, the duration of the time of application to removal was defined as the time to failure. Statistical analysis Factors affecting the success of the Anchor-Lock were investigated. The device was applied using the surgical stent after 2007, but was applied before then without stent guidance. To examine the influence of the surgical stent, we therefore compared the success rate after 2007 with that before 2007. To examine the influence of age, patients were divided into 64 young patients aged less than 20 years (median age: 14 years; range: 10–19 years) and 73 adult patients older than 20 years (median age: 26 years; range: 20– 54 years), and the success rate between them was compared. Success rate was also investigated with regard to sex. Three types of Anchor-Lock were available and their success rates were compared. For placement of the straight-type at HP and AP the influence of the screw length on success rate was examined. Differences according to surgical technique, age and sex of patients, type of Anchor-Lock and their screw length used were evaluated using the Chi-square test for independence. Results The Anchor-Lock was widely applied to 137 patients, from growing subjects aged 10 years to older subjects aged more than 50 years. Eleven of 137 cases failed, giving a total success rate of 92.0%. In 126 successful cases, active orthodontic treatment was completed, with an average application period of 2 years 2 months,
ranging from 8 months to 5 years. During the orthodontic treatment period, 15 cases out of 126 have suffered an inflammation of the palatal mucosa around the screws and the incidence was 11.9%. The mucosal inflammation was controlled by oral hygiene. In the 11 cases with failure, the time to failure was investigated (Figure 6). In all except two cases, the device was removed soon after application, with a time to failure less than 4 months. To examine the influence of surgical technique, sex, age and plate type on success rate, the Chi-square test for independence was performed (Table 1). The success rate from 2004 to 2006 was 85.2%, while that from 2007 to 2012 was 96.4%, showing a significant increase following the improvement in surgical technique (P,0.05). Success rate in females was 90.4%, and lower than the 97.0% in males, albeit that this difference was not statistically significant. By age, success rates in young and adult subjects were 92.2% and 91.8%, respectively, again showing no statistically significant difference. Of the three types of Anchor-Lock available, the straight-type at HP was used in 46 patients, and at AP in 49; while the double-Y was placed in 42. Success rate was high with the straight-type at AP and the double-Y (95.9% and 95.2%, respectively), and lower with the straight-type at HP (84.8%, albeit without statistical significance). Typical cases of each Anchor-Lock type are shown in Figures 7–9. In a case showing anterior crowding with a class I Angle’s molar relationship, the straight-type was applied to the HP between the first molars (Figure 7). Bone thickness at the placement site assessed by the lateral cephalogram was 7.0 mm and two screws of 10.0mm length were implanted in this area (Figure 7a). A lingual arch wire was applied to the upper first molars and retracted to the Anchor-Lock (Figure 7c). As a
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Figure 6 Time to failure
result, distalization of the upper lateral teeth was successfully achieved (Figure 7b) while sufficient space to align the anterior teeth was prepared. Figure 8 shows a case of crowding with an Angle’s class II molar relationship. Since the thickness of the HP was insufficient for implantation, the straight-type was placed at AP for distalization of the molars. Screws of 12.0-mm length were implanted perpendicularly to the palatal mucosa directed towards the anterior nasal spine, and confirmed on the lateral cephalogram (Figure 8a). At the beginning of treatment, a lingual arch wire was applied to the upper second molars and brackets were bonded to the upper lateral teeth. In the occlusal view of Figure 8c, distalization of the lateral teeth is seen to have progressed, and this is confirmed by the reduction in the distance between the lingual arch wire and the Anchor-Lock plate. Finally, the anterior crowding was solved and an Angle’s class I molar relationship was established without flaring out of the anterior teeth (Figure 8b).
Figure 9 shows a growing female subject aged 13 years with anterior crowding and an Angle’s class II molar relationship. The upper dental arch was narrow due to the transverse maxillary deficiency. A tooth-borne type of RME was applied and the mid-palatal suture was expanded with a daily expansion of 0.4 mm. After the palatal expansion, the upper lateral teeth were overexpanded accompanying with their buccal tipping and extrusion. The RME was then removed and a double-Y plate was immediately applied to the palate (Figure 9a). Early in the following few months, the over-expanded lateral teeth had relapsed spontaneously to some extent. The DY had been utilizing as an orthodontic anchorage to distalize the upper lateral teeth, while to control their lingual cusps in an intrusive manner (Figure 9c). The anterior crowding and Angle’s class II molar relationship were improved in accompaniment with distal movement of the upper molars and mandibular growth (Figure 9b). The influence of the screw length on success rate was examined for the straight-type at HP and AP and is
Table 1 Success rate of Anchor-Lock.
Total Year Sex Age Type
2004–2012 2004–2006 2007–2012 Female Male Young (10–19) Adult (20–54) AP HP DY
*Significant difference at 5% level. n.s.: not significant difference.
Number
Success
Failure
Success rate (%)
137 54 83 104 33 64 73 49 46 42
126 46 80 94 32 59 67 47 39 40
11 8 3 10 1 5 6 2 7 2
92.0 85.2 96.4 90.4 97.0 92.2 91.8 95.9 84.8 95.2
Chi-square
*
5.56
n.s.
1.47
n.s.
0.01
n.s.
3.43
n.s.
2.61
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Figure 7 Straight-type Anchor-Lock to HP: (a) lateral cephalometric finding of HP; (b) cephalometric superimposition (cranial base) of initial (solid line) and post-treatment (dotted line); (c) intra-oral photographs show treatment progress in occlusal view, initial and post-treatment findings
summarized in Table 2. For HP, 23 cases used 8.0-mm length and 22 cases used 10.0-mm. Forty-six cases of HP were divided into two groups by the screw length, 24 cases less than 8.0-mm and 22 cases of 10.0-mm. The longer screw group showed higher success rate of 95.5% than the 75.0% in the shorter screw group, without statistically significant difference. For AP, eight cases used 10.0-mm length and 38 cases used 12.0-mm. Fortynine cases of AP was divided into two groups, which were nine cases less than 10.0-mm and 40 cases over 12.0-mm. Success rate in the longer screw group was
97.5% and higher than the 88.9% in the shorter screw group, again showing no statistically significant difference. The double-Y plate used four screws; 8.0- or 10.0mm screws were used for the posterior plate openings, while 10.0- or 12.0-mm screws were applied to the anterior plate openings.
Discussion In this study, we have introduced a new skeletal anchorage device, the Anchor-Lock and assessed its
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Figure 8 Straight-type Anchor-Lock to AP: (a) lateral cephalometric finding of AP; (b) cephalometric superimposition (cranial base) of initial (solid line) and post-treatment (dotted line); (c) intra-oral photographs show treatment progress in occlusal view, initial and post-treatment findings
success rate. Success rate of the Anchor-Lock was 92.0% as a whole, which appears good in comparison with previous studies of mini-screw (Miyawaki et al., 2003; Ohashi et al., 2006; Park et al., 2006; Chaddad et al., 2008; Moon et al., 2008; Papageorgiou et al., 2012) and palatal implants (Jung et al., 2009; Asscherickx et al., 2010). Because the external plate is fixed with the palatal
bone by more than one screw, the Anchor-Lock should exert a bridging effect. The double-Y plate is rigidly implanted with four screws, the bridging effect of which likely explains its high success rate. In a palatal anchorage system similar to the Anchor-Lock, it was reported that a hybrid combination of mini-screw and mini-plate improved stability of the skeletal anchorage
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Figure 9 Double-Y plate application following RME: (a) lateral cephalometric findings; (b) cephalometric superimposition (anterior cranial base) of initial (solid line) and post-treatment (dotted line); (c) intra-oral photographs show treatment progress in occlusal view, initial and post-treatment findings
with high success rate and effectively applied to distalize the upper molars (Wilmes et al., 2009; Wilmes and Drscher, 2010).
For the straight-type plate success rate at AP was greater than that at HP, although not statistically significant. The AP has advantages for implantation
Table 2 Success rate of the straight-type plate by screw length and anatomical position. Type
Screw length
Number
Success
Failure
Success rate (%)
Chi-square
HP
8-mm or under over 10-mm 10-mm or under Over 12-mm
24 22 9 40
18 21 8 39
6 1 1 1
75.0 95.5 88.9 97.5
n.s.
3.72
n.s.
1.39
AP
*Significant difference at 5% level. n.s.: not significant difference.
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and was reported to have great depth (Kang et al., 2007; Baumgaetel, 2009) and high density (Moon et al., 2010). Success of the anchorage devices is a key to the overall success of orthodontic treatment, because anchorage failure fundamentally upsets treatment planning. From the point of view of orthodontic implant success, AP is recommended because of the characteristics of the palatal bone around the supporting screws. However, HP has advantages in controlling the direction and/or magnitude of the orthodontic force. Screw length of the Anchor-Lock seems to be an important factor relating to success rate. Especially, the endosseous length and its ratio to the total length may affect success rate. Due to the difficulty to assess the endosseous screw length, the present study investigated the total screw length used for the Anchor-Lock. Bone and mucosal thickness of the palate vary individually and in placement sites. Since sufficient bone depth is found in the AP site, long screw of 12.0-mm length was most commonly used and a high success rate was found. For each the HP and AP, although the longer screw group showed a higher success rate than the shorter screw group, there was no statistically significant difference. A study of mini-screw implants reported that the length of the mini-screw did not significantly affect their clinical success (Park et al., 2006). Further studies to assess the influence of endosseous screw length on the success rate are required. There was no difference in success rate by age. For the failure of mini-screw implants, a systematic review reported that no evidence was found for the association of age (Papageorgiou et al., 2012). These findings indicate that the Anchor-Lock can be used not only in adult subjects but also in growing subjects. Maxillary growth is a matter of concern with application of the Anchor-Lock to growing subjects. Using landmark implants, the growth change in maxillary width from 10 years to adulthood has been reported (Bjork and Skieller, 1977). It has been summarized that the average transverse increase in the incisor region was about one-third as great as that in the zygomatic region, at 0.9 and 3.0 mm, respectively. Similar results has been reported and demonstrated that the increase in width in the palatal region was approximately 1.5 mm during the period from 8.5 to 15.5 years (Korn and Baumrind, 1990). An experimental animal study indicated that growth at the mid-palatal suture contributed to transverse growth of the maxilla (Katsaros et al., 2006). Application of the Anchor-Lock possibly suppresses the normal transverse growth at the mid-palatal suture. Although the annual increment of palatal width appears to be small, particular attention should be paid to the application of the AnchorLock to growing subjects.
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Success rate of the Anchor-Lock did not differ by sex significantly. This finding is consistent with the previous studies of the success rate of mini-screw implants, which also reported no significant difference by sex (Papageorgiou et al., 2012). In the present study, nine out of 11 failure cases showed the time to failure of less than 4 months. In the other two cases, the Anchor-Lock functioned successfully as an anchorage for more than 14 months before failing in the course of treatment. Similar results were reported in a study of mini-screw orthodontic implants, with dislodgement of the mini-screw occurring frequently in the first 1– 2 months, and most failures occurring within the first 4 months (Moon et al., 2008). In a study of palatal orthodontic implant, three of 34 implants were lost within 3 months after placement (Asscherickx et al., 2010). We therefore consider that if the Anchor-Lock remains stable without loosening at more than 4 months after surgery, it will likely continue to function without interruption as an orthodontic anchorage in the following treatment period. Although most patients accept the Anchor-Lock, there are some disadvantages. For an oral sensation, one patient complained of some difficulty in pronunciation of the k sound. The speech problem may be due to the device positioning, in which the HP was placed too posteriorly, near the border on the soft palate. Mucosal inflammation around the Anchor-Lock was found temporarily in 11.9% of 126 successful cases and controlled by oral hygiene. Since 2007, a surgical stent and drill guide instrument have been available and success rate of the Anchor-Lock increased significantly. The surgical stent facilitates the systematic implantation of the AnchorLock and ensures the stable orthodontic anchorage. By using the stent, interlocking between the plate openings and the screw heads is accomplished passively without any deformation of their titanium structures. This condition should suppress the loosening of screw-plate locking as well as the development of screw mobility in the palatal bone. Accurate placement of the AnchorLock is important not only for orthodontic force application, but also for soft tissue management of the palatal mucosa. The surgical stent controls the distance between the plate and the palatal mucosa. A larger plate– mucosa distance might induce disorders in oral function, such as in swallowing or pronunciation, while a smaller distance may give rise to mucosal inflammation due to plaque accumulation around the implanted screws and the underside of the plate facing the mucosa. Once inflammation occurs, tissue swelling of the palatal mucosa will occupy the space between it and the plate, adversely affecting oral hygiene.
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An endosseous implant applied to the palate, the Orthosystem, has been reported and successfully used as an orthodontic implant (Wehrbein et al., 1999). This palatal implant system is usually applied to a midpalatal site posterior to the incisive foramen. Since the positioning of this palatal implant is determined by the need to minimize perforation risk to adjacent structures (nasal cavity, nasopalatine canal and incisor roots) (Cousley, 2005), its length is limited to 4.0 or 6.0 mm and its diameter to 3.3 or 4.1 mm. The AP type is also implanted posterior to the incisive foramen, but to a paramedian site about 5.0 mm lateral to the mid-palatal suture, and therefore, carries little risk of injuring the nasopalatine canal. The Anchor-Lock can be easily emplaced and removed under local anaesthesia, and surgical procedures are not complicated and can be carried out systematically within 15 min. The screw diameter of the Anchor-Lock is 2.0 mm and thus sufficiently small to obviate problematic tissue damage on insertion, or tissue defect on removal. The Anchor-Lock appears to be a minimally invasive system which is suitable for clinical settings. Further, it is able to stabilize orthodontic loading in all directions. Orthodontic force is then loaded to the external plate for intrusion and/or distalization of the molars. The Anchor-Lock appears to be an alternative to conventional screw- and plate-type anchorage devices. Conclusions Here, we introduced a new skeletal anchorage device named Anchor-Lock. In this system, an external titanium plate is fixed with titanium screws, which are implanted into the palatal bone trans-mucosally. Three types of Anchor-Lock are available; two utilize a straight-shaped plate which is applied to the hard palate and to the anterior palate. The third type utilizes a double-y-shaped plate which is applied after toothborne rapid maxillary expansion. Eleven of 137 cases failed, giving an overall success rate of 92.0%. No significant difference in success rate was seen by age or sex of patients. Type or screw length of the AnchorLock did not affect success rate significantly. Success rate was significantly increased by the use of the surgical stent. The Anchor-Lock was effectively applied to distalize and/or intrude the upper molars. References Asscherickx K, Vannet BV, Bottenberg P, Wehrbein H, Sabzevar MM. Clinical observations and success rates of palatal implants. Am J Orthod Dentofacial Orthop 2010; 137: 114–122.
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