The distal movement of mandibular molars is

ORIGINAL ARTICLE Distal movement of mandibular molars in adult patients with the skeletal anchorage system Junji Sugawara, DDS, PhD, a Takayoshi Daimaruya, DDS, PhD, b Mikako Umemori, DDS, PhD, b Hiroshi Nagasaka, DDS, PhD, c Ichiro Takahashi, DDS, PhD, b Hiroshi Kawamura, DDS, PhD, d and Hideo Mitani, DDS, MS, PhD e Sendai, Japan The skeletal anchorage system (SAS) consists of titanium anchor plates and monocortical screws that are temporarily placed in either the maxilla or the mandible, or in both, as absolute orthodontic anchorage units. Distalization of the molars has been one of the most difficult biomechanical problems in traditional orthodontics, particularly in adults and in the mandible. However, it has now become possible to move molars distally with the SAS to correct anterior crossbites, maxillary dental protrusion, crowding, and dental asymmetries without having to extract premolars. This study evaluated the treatment and posttreatment changes during and after distalization of the mandibular molars. In 15 adult patients (12 women and 3 men), a total of 29 mandibular molars were successfully distalized with SAS. The amount of distalization and relapse and the type of tooth movement were analyzed with cephalometric radiographs and dental casts. The average amount of distalization of the mandibular first molars was 3.5 mm at the crown level and 1.8 mm at the root level. The average amount of relapse was 0.3 mm at both the crown and root apex levels. Of 29 mandibular molars, 9 were tipped back, and the others were translated distally in accordance with the established treatment goals. SAS is a viable modality to move mandibular molars for distally correcting anterior crossbites, malocclusions characterized by mandibular anterior crowding, and dental asymmetries. (Am J Orthod Dentofacial Orthop 2004;125:130-38) The distal movement of mandibular molars is recognized as one of the most difficult-toachieve treatment objectives in clinical orthodontics; it is much more difficult than the distalization of maxillary molars. 1 Until now, several biomechanical strategies have been proposed to move the mandibular molars distally, eg, mandibular headgear, 2 lip bumper, 3,4 distal extension lingual arch, 5 Jones jig, 6 Franzulum appliance, 7 and multiloop edgewise archwire. 8 However, most of these appliances have not been widely used, especially in adult treatment, because the amount of molar distalization achieved depends on patient cooperation. Although a distal extension lingual From the Graduate School of Dentistry, Tohoku University, Sendai, Japan. a Associate professor, Division of Orthodontics and Dentofacial Orthopedics. b Research associate, Division of Orthodontics and Dentofacial Orthopedics. c Research associate, Division of Maxillofacial Surgery. d Associate professor, Division of Maxillofacial Surgery. e Professor and chairman, Division of Orthodontics and Dentofacial Orthopedics. This research was supported by grant-in-aid #12671985, Ministry of Education, Culture, Sports, Science and Technology, Japan. Reprint requests to: Dr Junji Sugawara, Tohoku University Graduate School of Dentistry, Division of Orthodontics and Dentofacial Orthopedics, 4-1, Seiryomachi, Aoba-ku, Sendai 980-8575, Japan; e-mail, sugahara@mail.cc. tohoku.ac.jp. Submitted, September 2002; revised and accepted, February 2003. 0889-5406/$30.00 Copyright 2004 by the American Association of Orthodontists. doi:10.1016/s0889-5406(03)00682-6 arch does not require patient cooperation, the type of tooth movement is mostly that of tipping. In distalization with the Jones jig or Franzulum appliance, reciprocal forces cause anchorage loss and protrusion of the anterior teeth. A multiloop edgewise archwire technique has also been used to distalize the mandibular molars, but again by tipping rather than bodily movement. In addition, the patient must use short Class III elastics so that the mandibular incisors do not become flared. Recently, a skeletal anchorage system (SAS) has been developed that uses pure titanium anchor plates and screws as absolute orthodontic anchorage units. 9-12 The anchor plates are monocortically placed at the piriform opening rim, the zygomatic buttresses, and any regions of the mandibular cortical bone. Because the anchor plates work as the onplant and the screws function as the implant, SAS enables the rigid anchorage that results from the osseointegration effects in both the anchor plates and screws. In addition, because all portions of the anchor plates and screws are placed outside the maxillary and mandibular dentition, the SAS does not interfere with tooth movement. Therefore, it is possible to distalize the mandibular molars with anchor plates placed at the anterior border of the mandibular ramus or mandibular body. Distalization of 130

American Journal of Orthodontics and Dentofacial Orthopedics Volume 125, Number 2 Sugawara et al 131 Table I. Sample characteristics Patient no. Sex Age Treatment period (mo) Anteroposterior jaw relationship Extraction of lower third molars Chief complaint Procedure for retention 1 F 38y7mo 30 Class III Rs/Ls Crowding LBR 2 F 21y7mo 35 Class III Rs/Ls Crowding LBR 3 F 28 y 11 mo 36 Class I Rs Crowding LBR 4 F 36y5mo 25 Class II CM Crowding LBR 5 M 20y1mo 39 Class II Rs/Ls Crowding LA LBR 6 M 24y6mo 24 Class III Rs Asymmetry LBR 7 F 43y6mo 33 Class III Rs/Ls Crowding LA LBR 8 F 27y9mo 19 Class III CM Crossbite LBR 9 F 30y5mo 27 Class III CM Crossbite 10 F 16 y 10 mo 31 Class III Rs/Ls Crowding LBR 11 M 15 y 11 mo 30 Class III Rs/Ls Crowding LBR 12 F 22 y 10 mo 29 Class III Rs/Ls Asymmetry LBR 13 F 23y8mo 27 Class III Rs/Ls Crowding LBR 14 F 16y1mo 21 Class III Rs Crossbite LBR 15 F 32y0mo 27 Class I Rs/Ls Crowding LBR F, Female; M, male; Rs, right side; Ls, left side; CM, congenital missing; LBR, lingual bonded retainer; LA, lingual arch. the mandibular molars enables the clinician to correct anterior crossbites, mandibular incisor crowding, and mandibular dental asymmetry without extracting premolars. Until now, because there have been only a few clinical studies involving the distalization of mandibular molars, 2-8 little information has been available regarding the type of tooth movement that occurs, the limitations of distal movement, and posttreatment stability. Joondeph and Riedel 13 believed that posttreatment stability is not a separate problem in orthodontics but one to be considered in diagnosis and treatment planning. Thus, it is as important to investigate the posttreatment stability of distalized mandibular molars as it is to demonstrate the overall effectiveness of this procedure. The aims of the present study were (1) to measure the average amount of distalization of the mandibular molars, (2) to evaluate the type of tooth movement that occurs, and (3) to determine the stability of the distalized molars 1 year posttreatment. MATERIAL AND METHODS Fifteen adult patients (12 women and 3 men) who had undergone orthodontic treatment in Tohoku University Dental Hospital, Sendai, Japan, were selected as subjects in this study. All of them satisfied the following criteria for case selection: (1) diagnosed as having no severe skeletal disharmonies, (2) sufficient space behind the second molar for the application of mandibular molar distalization, (3) treated by distalization of 1 mandibular first molar, and (4) followed for at least 1 year posttreatment. The sample characteristics are shown in Table I. The most common chief complaint of these patients was mandibular incisor crowding. The average age of the patients at the beginning of treatment was 26.9 years and ranged from 16.1 to 43.5 years. The average treatment period was 28.9 months, ranging from 21 to 39 months. The anchor plates (Leibinger, Muhlheim-Stelten, Germany) (Fig 1, A), made of pure titanium, were placed behind the second molars at the anterior border of the mandibular ramus, as shown in Figure 1, B-D. Implantation was performed under local anesthesia, and the titanium plates were secured with pure titanium screws 10 (Leibinger) (Fig 1, B). The diameter and the length of the monocortical screws were 2.0 and 5.0 mm, respectively (Fig 1, A). The 2 fundamental methods of applying distalizing forces to the subjects in this study are shown in Figure 2. One is for single molar distalization (Fig 2, A). Extraction of the third molars is needed to create the space for the molar distalization. After the buccal segments are leveled and aligned, stiff archwires (.018.025-in or.019.026-in stainless steel) are engaged, and the L-shaped anchor plates are placed at the anterior border of the mandibular ramus. Then the bands or brackets of the first molars are taken off, and a retractive force is applied to the second molars with an open coil spring. To avoid the side effects of the reciprocal coil spring, the first premolars must be firmly ligated with anchor plates. After the distalization of the second molars, distalization of the first molars is done with the same procedure. The other method used is en masse distalization of the entire buccal segments (Fig 2, B). The procedures

132 Sugawara et al American Journal of Orthodontics and Dentofacial Orthopedics February 2004 before distalization are the same as those for the single molar distalization, but the mechanics are less complex. Direct retractive force is applied from the anchor plates to the first premolars to perform en masse distalization of the buccal segments. Elastic modules or nickeltitanium closed coil springs usually provide the retractive orthodontic force. After debonding, retention consisted of a lingual bonded retainer with.0175-in multistranded flexible wire on lingual surfaces of the mandibular anterior teeth. The method for retention used in each subject is indicated in Table I. Lateral cephalometric radiographs, panoramic radiographs, photographs, and dental casts were taken immediately before treatment, at debonding, and 1 year after debonding. Cephalometric radiographs were traced, and then mandibular tracings were carefully superimposed on the detailed anatomic structures (ie, inferior alveolar canals and fine structures in the symphysis). 14,15 The left and right molars are distinguished on the cephalograms by referring to the panoramic radiographs. The occlusal surfaces of the mandibular dental casts were photocopied perpendicularly to the mandibular occlusal plane, and occlusograms 16 were produced by tracing the outlines of all teeth. The occlusograms were magnified by 1.06 to adjust to the magnification of the cephalometric tracings. In aligning the edge of the incisors and the midline of the mandibular occlusograms, the superimposed mandibular tracings and the occlusograms were combined to analyze the type of molar movement 3-dimensionally. Then the posterior displacement of the medial surfaces of the first molars was measured with calipers at a precision of 0.1 mm, as shown in Figure 3. The amount of posterior displacement at the crown and root levels was measured on the occlusograms and the cephalometric tracings, respectively. The type of tooth movement was evaluated by the crown and root movement ratio. When the percentage ratio of the root movement to the crown movement (the tipping ratio) was less than 25%, the type of tooth movement was determined to be tipping. All cephalometric tracings and measurements were performed by a single researcher (T.D.), and the intraindividual method error did not exceed 0.2 mm. The correlation between the amount of posterior displacement at the crown level, the tipping ratio, and Fig 1. Basic components of SAS and required surgical procedures. A, Titanium anchor plates and monocortical bone screws; B, surgical procedure; C, intraoral photograph of implanted anchor plate; D, panoramic radiograph of implanted anchor plate.

American Journal of Orthodontics and Dentofacial Orthopedics Volume 125, Number 2 Sugawara et al 133 Fig 2. Two fundamental mechanical modalities for mandibular molar distalization. A, Distalizing force is applied to mandibular molars by ligating between stiff archwires (.019.026-in stainless steel) at premolar regions and first hook of miniplate, and open coil spring is placed between molars and premolars; B, elastic modules are tied to first hook of miniplates and brackets to load distalizing force on buccal segment with passively ligated stiff archwire (.019.026-in stainless steel). the amount of relapse was statistically analyzed. A paired t test was applied for the statistical analysis for comparison between the position of the mandibular first molars at debonding and at 1 year posttreatment. RESULTS The results of this study are shown in Table II. The average amount ( standard deviation) of distal movement was 3.5 1.4 mm at the crown level. The maximum amount of distalization at the crown level was 7.1 mm, and the minimum was 1.0 mm at the first molar. The amount of root movement was 1.8 mm, on average. In 2 of the 29 first molars, the roots moved forward. The average tipping ratio was 46.3%. Although most of the first molars showed bodily movement, 9 of 29 molars showed tipping movement, in which the tipping ratios were less than 25% (Table II). The calculated correlation coefficient between the tipping ratio and the amount of distalization was 0.33, which was not statistically significant (P.01). The amount of relapse in the mandibular first molars is shown in Table II. The average amount and

134 Sugawara et al American Journal of Orthodontics and Dentofacial Orthopedics February 2004 was to maintain the anteroposterior position of the mandibular incisors by distalization of the molars and lateral expansion of the buccal segments. To achieve this treatment goal, 2 titanium anchor plates were placed at the anterior border of the bilateral mandibular ramus. A multibracket system was applied, and the molars were uprighted and distalized with SAS mechanics, as shown in Figure 4, A. After 36 months of active treatment, the appliances and the anchor plates were removed. The patient was then given a wraparound retainer for the maxillary dental arch and a lingual bonded retainer for the mandibular anterior teeth. The amount of distal movement of the mandibular first molars was 7.1 mm on the left and 5.5 mm on the right. The treatment goal was almost achieved, as shown in Figure 4 B, D, and E. Both mandibular molars were uprighted to the appropriate inclination, and a desirable occlusion was obtained (Fig 4, D). Although the mandibular right first molar relapsed 1.0 mm by the 1-year follow-up, the left molar, which showed the maximum amount of distalization, was very stable, showing no relapse. Fig 3. Method to measure amount of mandibular molar distalization. Superimposition of mandible on occlusogram. C, crown movement; R, root movement. rate of relapse 1 year posttreatment were 0.3 mm and 9%, respectively. No statistically significant differences were observed in the position of the first molars between the time of debonding and 1 year posttreatment. Maximum relapse was 0.8 mm, and the maximum relapse rate was 40%. No statistically significant correlation between the relapse rate and the amount of posterior displacement or tipping ratio at the first molars was observed. Figure 4 shows the intraoral photographs and composite superimpositions of the occlusograms and tracings in patient 3, who had the maximum amount of molar distalization among all subjects in this study. This patient was a 36-year-old Japanese woman who complained of a high maxillary left canine (Fig 4, A) and bilateral linguoversion of the mandibular second premolars (Fig 4, C). She had no skeletal but many dental problems (crowding, a Class II molar relationship, a large overjet, a missing maxillary right canine, a mesial rotation, and tipping of the mandibular first molars). The treatment goal in the mandibular dentition DISCUSSION Numerous extraoral and intraoral mechanical modalities have been proposed for distalizing maxillary molars, 17-30 but only a few have been reported for mandibular molars. 2-8 Each previously reported mechanism has a disadvantage the need for patient cooperation, tipping movement, anchorage loss, and flaring of the incisors. In addition, it was more difficult to distalize mandibular than maxillary molars. 1 Therefore, clinical attention has been focused on the use of endosseous implants to provide rigid, intraoral anchorage units for distalizing mandibular molars. However, neither the design of the implant itself nor the position for implantation has been practical for distalizing the mandibular molars, because the implant disturbed tooth movement or became loose because of the heavy force necessary for molar distalization in the alveolar bone. Only Jenner and Fitzpatrick 31 reported a patient in whom skeletal anchorage was applied with surgical bone plates to move a mandibular molar distally. Accordingly, we recently proposed the SAS, a safe and useful system for skeletal anchorage that uses titanium anchor plates and monocortical titanium screws, which provide rigid anchorage units for distalizing the mandibular molars. 9,12 The monocortical bone screws in the SAS are fixed on the anterior border of the mandibular ramus or the mandibular body beyond the root apices or outside the alveolar region and never

American Journal of Orthodontics and Dentofacial Orthopedics Volume 125, Number 2 Sugawara et al 135 Table II. Tooth position at debonding and 1 year posttreatment and amount of relapse and mode of tooth movement Patient no. Side Tooth position at debonding (mm) Position of tooth at1y posttreatment (mm) Tipping Relapse (mm) Crown Root ratio (%) Mode of tooth movement Crown Relapse ratio (%) 1 R 3.0 3.0 3.0 100.0 Translated 0.0 0.0 L 2.0 0.0 1.5 0.0 Tipping 0.5 25.0 2 R 4.5 3.8 4.2 84.4 Translated 0.3 6.7 L 3.5 1.8 3.5 51.4 Translated 0.0 0.0 3 R 5.5 4.0 4.5 72.7 Translated 1.0 18.2 L 7.1 3.2 7.2 45.1 Translated 0.1 1.4 4 R 2.7 2.1 2.7 77.8 Translated 0.0 0.0 L 1.0 0.0 0.6 0.0 Tipping 0.4 40.0 5 R 3.0 2.2 3.0 73.3 Translated 0.0 0.0 L 5.0 1.0 4.2 20.0 Tipping 0.8 16.0 6 R 4.3 2.0 3.7 46.5 Translated 0.6 14.0 L 1.7 0.0 1.5 0.0 Tipping 0.2 11.8 7 R 3.0 3.0 3.0 100.0 Translated 0.0 0.0 L 3.0 0.0 3.0 0.0 Tipping 0.0 0.0 8 R 2.0 0.0 2.0 0.0 Tipping 0.0 0.0 L 4.2 0.0 4.2 0.0 Tipping 0.0 0.0 9 R 3.3 2.8 2.6 84.8 Translated 0.7 21.2 10 R 4.0 3.5 3.5 87.5 Translated 0.5 12.5 L 4.8 4.2 4.8 87.5 Translated 0.0 0.0 11 R 4.6 3.6 4.1 78.3 Translated 0.5 10.9 L 2.0 1.6 1.7 80.0 Translated 0.3 15.0 12 R 5.7 1.6 5.7 28.1 Translated 0.0 0.0 L 2.6 0.7 2.0 26.9 Tipping 0.6 23.1 13 R 3.3 1.8 3.3 54.5 Translated 0.0 0.0 L 3.4 1.5 3.2 44.1 Translated 0.2 5.9 14 R 3.4 2.4 2.4 70.6 Translated 1.0 29.4 L 1.6 0.4 1.6 25.0 Tipping 0.0 0.0 15 R 2.0 1.0 2.0 50.0 Translated 0.0 0.0 L 4.0 2.3 3.5 57.5 Translated 0.5 12.5 Average 3.5 1.8 3.2 46.3 0.3 9.0 SD 1.4 1.4 1.4 38.7 0.3 11.0 R, Right; L, left; SD, standard deviation. Patient 9 showed asymmetric dentition; left molar was in exact position, so only right molar was distalized. interfere with the root movement in orthodontic therapy. The SAS has 2 more outstanding advantages not provided by the other mechanisms for distalizing the mandibular molars. First, it is possible to intrude the mandibular molars with the SAS. Therefore, the extrusion of the mandibular molars after the tipping of the molar distalization can be corrected easily. Second, en masse distalization of the mandibular buccal segments or the entire dentition is also possible if the mandibular dentition is fundamentally well aligned. These advantages simplify the orthodontic procedures and significantly reduce the orthodontic treatment period. The previously reported mechanotherapies 2-8 could distalize the mandibular molars to some extent, but the amount of molar distalization was quite limited, and the mandibular molars could seldom be translated distally with those mechanisms. As the results of this study have shown, the SAS enables tooth movement to be controlled 3-dimensionally, so that treatment goals can be accomplished, even when the amount of tooth movement required is more than the mesiodistal width of the premolars. Consequently, with the SAS, it is not always necessary to extract the mandibular first or second premolars, even in patients with moderate to severe crowding. Also, the molar relationship in patients with symmetric or asymmetric Class III molar relationships can be corrected without having to extract mandibular premolars. The SAS might require dentists, especially orthodontists, to reconsider their thinking regarding arch length discrepancy, space analysis, and tooth extraction criteria as they have been described in the orthodontic literature. 32,33 Traditionally, the arch length deficiency has been calculated anterior to the

136 Sugawara et al American Journal of Orthodontics and Dentofacial Orthopedics February 2004 Fig 4. Intraoral photographs (A-D) and superimposition of mandibles and occlusograms (E) of patient treated with SAS. Intraoral photographs at initial treatment (A, C) and at debonding (B, D). Note change in molar relationship shown in C and D. Arrowheads indicate lingual bonded retainer. Solid line in E: at initial treatment in left panel and at debonding in right. Dotted line in E: at debonding in left panel and 1 year posttreatment in right. first molars because molar distalization was assumed to be nearly impossible. However, by using the space posterior to the second molars, 14 permanent teeth can be well aligned in the alveolar bone, as demonstrated by the present study. Therefore, it will now become necessary to find an indicator to determine the posterior limits of the alveolar region from the standpoints of orthodontics, anatomy, and periodontology. For example, the location of the mandibular third molars should be a very useful indicator for

American Journal of Orthodontics and Dentofacial Orthopedics Volume 125, Number 2 Sugawara et al 137 judging the posterior limit of the alveolar bone in the mandibular dentition. But even in treatable cases, the condition of oral hygiene around the distalized molars should be predicted before treatment. The posttreatment stability of tooth movement has also been an important issue in orthodontics. The short-term relapse in the distalized mandibular molars in this study was minimal and was not correlated with the amount of distalization. Previously, the tendency has been that, the larger the amount of tooth movement and the more the teeth are tipped, the greater the relapse. 13 However, no significant correlation was found between the amount of relapse and the tipping ratio and the amount of tooth movement in the present study. It was apparent that the type of tooth movement could be well controlled according to the treatment goal to achieve long-term stability. It could well be that the achieved occlusion is a contributing factor in maintaining the tooth position. Another reason for the remarkable posttreatment stability might be that the shape of the dental arch is not changed excessively and therefore does not disrupt the balance to keep equivalence between the perioral muscles and tongue function. The SAS can be used to distalize the mandibular molars; thus, it is not necessary to expand the mandibular arch excessively. Because SAS treatment is a symptomatic rather than a causal treatment, further research is needed to verify its long-term stability. CONCLUSIONS The SAS is a new and viable modality for distalizing mandibular molars. It enables en masse movement of the mandibular buccal segments and even the entire mandibular dentition with only minor surgery for placing the anchor plates at the anterior border of the mandibular ramus or the mandibular body. 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