Three-dimensional dental model analysis of treatment outcomes for protrusive maxillary dentition: Comparison of headgear, miniscrew, and miniplate skeletal anchorage

ORIGINAL ARTICLE

Three-dimensional dental model analysis of treatment outcomes for protrusive maxillary dentition: Comparison of headgear, miniscrew, and miniplate skeletal anchorage Eddie Hsiang-Hua Lai,a Chung-Chen Jane Yao,b Jenny Zwei-Chieng Chang,c I Chen,d and Yi-Jane Chenb Taipei, Taiwan Introduction: The aim of this retrospective study on dental models was to compare the orthodontic outcomes of maxillary dentoalveolar protrusion treated with headgear, miniscrews, or miniplates for maximum anchorage. Methods: The 40 subjects were diagnosed as having either Angle Class II malocclusion or Class I bimaxillary dentoalveolar protrusion. All patients were treated to retract the maxillary dentoalveolar process by using the extraction space of the bilateral maxillary first premolars. They were divided into 3 groups according to the type of anchorage used. Group 1 (n ⫽ 16) received traditional anchorage preparation with a transpalatal arch and headgear, group 2 (n ⫽ 15) received miniscrews, and group 3 (n ⫽ 9) received miniplates for skeletal anchorage. To investigate the movement of the maxillary teeth during dentoalveolar retraction, we used a 3-dimensional (3D) digitizer to assess the positional changes of the maxillary teeth relative to the stable palatal rugose structures on the serial dental models. The 3D coordinates representing pretreatment and posttreatment maxillary dental casts were superimposed to determine the movement of individual teeth from the positional changes of 18 landmarks of the central incisor, canine, second premolar, and first molar. Results: Three-dimensional analysis of the maxillary dental models in the buccopalatal, anteroposterior, and vertical directions showed significant differences in tooth movements between the headgear and the mini-implant (miniscrew or miniplate) groups. Both skeletal anchorage groups had greater incisor retraction (6.9 mm for the miniscrew, 7.3 mm for the miniplate) than did the headgear group (5.5 mm). Mesialization of occlusal centroid of the maxillary molar in the skeletal anchorage groups was less than that in the headgear group (1.3 mm for the miniscrew, 1.4 mm for the miniplate, 2.5 mm for the headgear). Tooth movements in the anteroposterior and buccopalatal directions did not reach a statistically significant difference between the miniscrew and miniplate groups, but the maxillary posterior teeth of the subjects receiving miniplates showed greater intrusion than those receiving miniscrews anchorage. Conclusions: This 3D analysis of serial dental models demonstrated that, compared with headgear, skeletal anchorage achieved better results in the treatment of maxillary dentoalveolar protrusion. Significant intrusion of the maxillary posterior teeth was noted in the miniplate group but not in the miniscrew and headgear groups. Greater retraction of the maxillary anterior teeth, less anchorage loss of the maxillary posterior teeth, and the possibility of maxillary molar intrusion all facilitated correction of the Class II malocclusion, especially for patients with a hyperdivergent face. (Am J Orthod Dentofacial Orthop 2008;134:636-45)

a

Attending staff, Department of Orthodontics, National Taiwan University Hospital, Taipei, Taiwan. b Assistant professor, Department of Orthodontics, School of Dentistry, College of Medicine, National Taiwan University, Taipei; attending staff, National Taiwan University Hospital, Taipei, Taiwan. c Lecturer, Department of Oral and Maxillofacial Surgery, School of Dentistry, College of Medicine, National Taiwan University, Taipei; attending staff, National Taiwan University Hospital, Taipei, Taiwan. d Graduate student, Department of Orthodontics, School of Dentistry, College of Medicine, National Taiwan University, Taipei, Taiwan. Partially supported by grant NTUH 96-S582 to Y. J. Chen and grants NTUH95-000299 and NSC95-2314-B-002-212 to C. C. Yao. Reprint requests to: Yi-Jane Chen, Department of Orthodontics, School of Dentistry, College of Medicine, National Taiwan University, No. 1, Chang-Te St, Taipei, Taiwan 100; e-mail, [email protected]. Submitted, February 2007; revised and accepted, May 2007. 0889-5406/$34.00 Copyright © 2008 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2007.05.017

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ncreased upper lip procumbency is commonly associated with maxillary dentoalveolar protrusion.1,2 Patients with this feature often seek orthodontic treatment to improve their facial esthetics. With the major goal of reducing maxillary dentoalveolar protrusion, the treatment plan usually includes extraction of the bilateral maxillary premolars, followed by anterior tooth retraction with maximum anchorage.3 Maximum anchorage to prevent forward movement of the maxillary posterior teeth during anterior tooth retraction can be provided by various methods. In addition to distal tip-back and buccal root torque of the posterior teeth, orthodontic anchorage can also be prepared by using an intraoral anchoring appliance—ie, a transpalatal arch or a Nance holding arch. Moreover,

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extraoral headgear appliances are often used to enhance posterior anchorage during anterior tooth retraction.4,5 However, wearing headgear full time is too demanding for most patients, especially adults, for social reasons. Subsequently, some anchorage loss and mesial movement of the posterior teeth are usually observed if a patient’s compliance in wearing the extraoral appliance is inadequate. Patient cooperation determines the effectiveness of extraoral anchorage. There are drastic differences between the duration of forces applied with extraoral and intraoral appliances. Recently Bondemark and Karlsson,6 in a randomized study, demonstrated that an intraoral appliance incorporating an open-coil spring was more effective than cervical headgear at distalizing the maxillary first molars. Mini-implant skeletal anchorage can resolve the problems of anchorage loss while closing the extraction spaces that usually develop when traditional anchorage is used. A mini-implant as part of extraction treatment for Class II and Class I malocclusions enables maximum anterior tooth retraction against stable anchorage without undesirable movements of the posterior teeth.7-10 Orthodontic skeletal anchorage also generates desirable tooth movements more efficiently, without relying on patient cooperation in wearing an extraoral appliance, possibly resulting in a shorter treatment duration. With rapid increases in the use of mini-implants for anterior tooth retraction at our clinic in Taipei, Taiwan, in the last 5 years, we can provide scientific evidence using a series of dental models to evaluate the effectiveness of various anchorage systems.11-13 Two-dimensional analysis by superimposing pretreatment and posttreatment cephalometric tracings has commonly been used to assess orthodontic tooth movement. Cephalometric superimposition can show positional changes of the maxillary and mandibular dentition in both vertical and sagittal dimensions but not buccopalatal crown movement. Contrary to 2-dimensional analysis, 3-dimensional (3D) analysis of serial dental models can provide further information on tooth movements, especially in the buccolingual transverse direction.12,14 The projection of bilateral teeth on the midsagittal plane also causes greater tracing errors because of the difficulty in identifying the bilateral teeth.15 Subsequently, tooth movements on each side are seldom investigated with cephalometric analyses. Moreover, cephalometric assessments alone present difficulties in providing a comprehensive description of orthodontic tooth movements. The aim of this study of pretreatment and posttreatment maxillary dental models was to accurately compare the effectiveness of miniscrew and miniplate skeletal anchorage during maxillary dentoalveolar retraction in adults with Class II

Table I.

Demographic information of the 40 subjects in this study

Patients (n) Age (y) Sex (male/female) Malocclusion (Class I/II) Treatment duration (mo)

Headgear group

Miniscrew group

Miniplate group

16 21.7 ⫾ 2.5 0/16 5/11 33.6 ⫾ 7.2

15 25.1 ⫾ 4.7 1/14 3/12 27.1 ⫾ 4.2

9 24.1 ⫾ 3.2 2/7 2/7 31.4 ⫾ 4.7

or Class I malocclusions with traditional extraoral anchorage. MATERIAL AND METHODS

The subjects in this retrospective study were 40 adults (Table I). All were treated in the authors’ postgraduate clinic from 2000 to 2005. Maxillary dental casts of each patient were taken before and after active orthodontic treatment. These patients had a diagnosis, based on cephalometric evaluation and study models, of Angle Class II Division 1 malocclusion or Class I malocclusion with bimaxillary protrusion. The common feature of all patients was maxillary dentoalveolar protrusion, which requires maximum anchorage to stabilize the posterior teeth during anterior tooth retraction. According to the type of anchorage, the subjects were divided into 3 groups: headgear, miniscrew, and miniplate. The headgear group (n ⫽ 16) included 5 Class I and 11 Class II malocclusions. These subjects received traditional anchorage preparation with an extraoral headgear appliance combined with a transpalatal arch. The force level exerted by the headgear was 300 to 350 g. The directional pull of the headgear was adjusted according to the mandibular plane angle—ie, high pull for high-angle patients and straight pull for normal-angle patients. The miniscrew group (n ⫽ 15) included 3 Class I and 12 Class II malocclusions. The miniplate group (n ⫽ 9) included 2 Class I and 7 Class II malocclusions. The patients in these groups received mini-implants on the buccal aspect of the bilateral maxillary posterior teeth. The titanium L- or Y-shaped miniplates and miniscrews (Mondeal, Tuttlingen, Germany, or Leibinger, Mühlheim-Stelten, Germany) were originally designed for bone fixation in maxillofacial surgery. Another type of miniscrew with a smaller diameter (Absoanchor, Dentos, Daegu, Korea) was specifically designed for orthodontic anchorage. All mini-implants were placed by 2 oral surgeons. A mucoperiosteal flap operation was required for implanting a miniplate on the buccal side of the maxillary molars, whereas the Mondeal miniscrews (diameter, 2.0 mm; length, 9 or 11 mm) and the

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Absoanchor miniscrews (diameter, 1.2 mm; length, 10 or 12 mm) were implanted without flap elevation. Patients receiving headgear and skeletal anchorage were treated consecutively. However, in a transitional 2-year period, both treatment modalities were available but were separately used. The diagnoses and treatment plans for our patients were codetermined by all 3 instructors (C.C.J.Y., J.Z.C.C., and Y.J.C.) in a routine conference. Maximal anchorage preparation was incorporated into the mechanical design if retraction of the maxillary dentoalveolar protrusion was needed for overjet reduction or soft-tissue esthetics. The choice of headgear or skeletal anchorage was made by each patient after the pros and cons were fully explained. All treatments and anchorage preparations were conducted after obtaining the patients’ informed consent. The treatment plans of all patients involved extraction of the bilateral maxillary first premolars, with various treatment strategies for the mandibular dentition including nonextraction, or extraction of the bilateral first or second premolars. Although these patients were treated in a postgraduate clinic in a university, all received 0.018 ⫻ 0.025-in slot edgewise appliances. Moreover, a transpalatal arch was used routinely for transverse control of the maxillary dentition. Regardless of the anchorage preparation, the extraction space was closed by sliding mechanics with en-masse retraction after partial distalization of the canines and good alignment of the 6 anterior teeth. A 1-piece beta-titanium alloy intrusive arch was used when intrusion of the maxillary anterior teeth was indicated. The moment acting on the maxillary first molars could be counteracted by the transpalatal arch. Treatment procedures were taught and monitored by 3 clinical instructors (C.C.J.Y., J.Z.C.C., and Y.J.C.) so that consistent mechanical principles were maintained, which included the archwires and the appliance systems, the sequence of archwire changes, the mechanics of overbite control, and the minimum use of Class II elastics for Class II high-angle cases. Pretreatment and posttreatment dental models from each of the 40 patients were trimmed to keep the occlusal plane parallel to the horizontal. On each maxillary dental model, morphologic landmarks were marked with a sharp pencil on the following locations: the mesial and distal ends of the incisal edge of the bilateral central incisors, the cusp tip of the canines, the buccal and palatal cusp tips of the second premolars, and the mesiobuccal, mesiopalatal, distobuccal, and distopalatal cusps of the first molars (Fig 1). To superimpose the pretreatment and posttreatment dental models, at least 4 reliable points, mostly mesial and distal points of the third palatal rugae, were identified

American Journal of Orthodontics and Dentofacial Orthopedics November 2008

Fig 1. Desktop mechanical 3D digitizer (MicroScribe 3D-G2X). The inset shows the stylus tip of the digitizer perpendicularly touching the cusp tip of the dental cast.

as references to analyze individual tooth movements. The spatial data on each pair of dental models were recorded by a mechanical desktop 3D digitizer (Microscribe G2, Immersion, San Jose, Calif) with a stylus tip connected to a mechanical arm that allowed full range of movement. The 3D coordinate data of the landmarks on each pair of dental models were analyzed with Rhinoceros software (Robert McNeel & Associates, Seattle, Wash) (Fig 2). The positional changes of the maxillary teeth were then assessed by calculating each set of paired coordinate data. Relocation of specific cusp tips relative to the stable landmarks on the palatal rugae could be decomposed into transverse, sagittal, and vertical dimensions corresponding to the 3D coordinates. Differences in the x-coordinate at the cusp tips of the serial dental models represented movement of a tooth in the buccopalatal direction. A positive sign denoted buccal movement and a negative sign palatal movement. Differences in the y-coordinates represented movement in the anteroposterior direction. A positive sign denoted mesial movement and a negative sign distal movement. Differences in the z-coordinates represented movement in the vertical (occlusal-gingi-

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Fig 2. Three-dimensional analysis of positional changes of the maxillary teeth by using the 3D digitizer and Rhinoceros software: A, 18 landmarks on the central incisors, canines, second premolars, and first molars; B, top view of the superimposed coordinate input from the pretreatment and posttreatment maxillary dental models; C, lateral view; D, frontal view.

val) direction. A positive sign denoted extrusion and a negative sign intrusion. To provide more concise insight to the positional change of the first molar, the measurements at the 4 molar cusp tips were averaged to denote the mean movement of occlusal centroid.

not significant. One-way ANOVA and the post-hoc Scheffé test were used to check for differences among the headgear and the 2 mini-implant groups. Statistical significance was set at P ⬍0.05.

Statistical analysis

Our subjects included those with Angle Class I and Class II malocclusions with the common characteristic of maxillary dentoalveolar protrusion. Their underlying skeletal patterns ranged from skeletal Class I to Class II. The mean measurement of the ANB angle, an indicator of jaw-base discrepancy, in the miniplate group (6.4° ⫾ 3.8°) was higher than that of the headgear (4.8° ⫾ 2.5°) and miniscrew (4.4° ⫾ 2.9°) groups. However, differences in the ANB angle among the 3 groups did not reach statistical significance (P ⫽ 0.116). The mean treatment duration of the miniscrew group (27.1 months) was shorter than for the headgear and miniplate groups (33.6 and 31.4 months, P ⬍0.001). However, differences between the headgear and the miniplate groups did not reach statistical significance

To study the measurement errors, 10 pairs of randomly selected pretreatment and posttreatment maxillary dental models were measured again 2 weeks later. The method error was calculated according to Dahlberg’s formula.16 The data analyses and statistical significance at the level of P ⬍0.05 were checked by using SPSS software (version 10.0, SPSS, Chicago, Ill). The means and standard deviations of the treatmentinduced positional changes at specific landmarks of the maxillary teeth were calculated for each of the headgear, miniscrew, and miniplate groups. The data of a specific tooth were pooled with those from the contralateral side because differences between the corresponding teeth in the first and second quadrants were

RESULTS

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Table II.

Tooth Incisor Canine Premolar First molar

American Journal of Orthodontics and Dentofacial Orthopedics November 2008

Buccopalatal movement (mm) at the various cusp tips of the maxillary teeth Headgear group

Miniscrew group

Miniplate group

P value

Landmark

Mean

SD

Mean

SD

Mean

SD

(1-way ANOVA)

Incisal edge Cusp tip Buccal cusp Palatal cusp Mesiobuccal cusp Mesiopalatal cusp Distobuccal cusp Distopalatal cusp Occlusal centroid

0.2 0.9 ⫺0.9 ⫺0.7 ⫺0.3 ⫺0.3 0.0 0.0 ⫺0.1

0.9 1.1 1.2 1.1 0.7 0.7 0.5 0.8 0.6

0.0 0.6 ⫺1.0 ⫺1.0 ⫺0.7 ⫺0.7 ⫺0.6* ⫺0.7* ⫺0.7*

0.9 1.2 1.4 1.3 1.0 1.1 1.0 1.2 1.0

⫺0.1 1.3 ⫺1.1 ⫺0.8 ⫺0.3 ⫺0.4 0.0† ⫺0.3 ⫺0.2

0.8 1.3 1.3 1.2 1.2 1.0 0.8 0.9 0.9

0.625 0.130 0.824 0.648 0.169 0.107 0.005 0.026 0.028

Positive value indicates buccal expansion; negative value indicates palatal constriction. *Significant difference between headgear and miniscrew groups (based on the Scheffé test, P ⬍0.05). † Significant difference between miniscrew and miniplate groups (based on the Scheffé test, P ⬍0.05).

(P ⫽ 0.432). The high correlation between the repeated digitization of the same morphologic point was confirmed in a method error study. The overall point-torepeated-point distance was decomposed into x-, y-, z-coordinates, which imply the error in the buccopalatal, mesiodistal, and occlusal-gingival directions. The method errors of the x-coordinate for the morphologic landmarks on the incisor, canine, premolar, and molar were 0.04, 0.04, 0.19, and 0.06 mm, respectively. The method error of y-coordinate for those on the incisor, canine, premolar, and molar were 0.10, 0.004, 0.09, and 0.07 mm, respectively. The y-coordinate for those of the incisor, canine, premolar, and molar were 0.11, 0.16, 0.15, and 0.11 mm, respectively. Among the 3D coordinate measurements, the highest method error was in the z-coordinate. It might be associated with the variation in the vertical contact of the stylus tip of the 3D digitizer to the morphologic landmarks on the dental models. The means and standard deviations of buccopalatal movement at the various cusp tips of the maxillary teeth measured from the paired pretreatment and posttreatment dental models were determined and are shown in Table II. A positive value denotes buccal movement; a negative value indicates palatal movement. Generally, the amount of tooth movement in the transverse dimension was within 1 mm. Although 1.3 mm of buccal movement of a canine occurred in the miniplate group, no significant difference was noted among the 3 groups by 1-way ANOVA. There was also no significant group difference for the incisors and the premolars. The buccopalatal movements of the incisors could be interpreted as rotation and midline corrections. As to the first molar, statistical analysis showed a significant difference among the 3 groups for the distobuccal and distopalatal cusps but not for the mesiobuccal or

mesiopalatal cusps. The post-hoc Scheffé test showed that the distal cusps of the maxillary first molar in the miniscrew group had moved palatally more than those in the headgear and miniplate groups. Table III shows the means and standard deviations of the anteroposterior movement of the maxillary teeth. A positive value denotes mesial movement (anchorage loss) in cases of anterior protrusion. A negative value denotes distal movement (retraction) of the anterior teeth. One-way ANOVA showed significant group differences for the positional changes of each dental landmark. The post-hoc Scheffé test detected significant differences between the miniscrew and the headgear groups, and between the miniplate and the headgear groups, but not between the miniscrew and the miniplate groups. Both groups with skeletal anchorage had greater anterior tooth retraction than the headgear group. Less anterior teeth retraction and greater mesial movement of the posterior teeth were noted in the headgear group compared with the mini-implant groups (Fig 3). Tooth movements in the vertical direction were investigated by the occlusal-gingival positional changes of the maxillary teeth. The means and standard deviations calculated for each group are shown in Table IV. A positive value denotes extrusion, and a negative value denotes intrusion. In both the headgear and the miniscrew groups, the standard deviations were generally larger than the absolute values of the mean measurements, especially the measurements associated with the incisors and the canines. This was also found in the miniplate group. The wide range of variations in the amount of anterior tooth intrusion might have been due to various amounts of anterior overbite in our subjects according to the inclusion criterion of maxillary dentoalveolar protrusion. Positional changes of each cusp tip were compared among the 3 groups. One-way

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Table III.

Mesiodistal movement (mm) at the various cusp tips of the maxillary teeth Headgear group

Tooth Incisor Canine Premolar First molar

Miniscrew group

Miniplate group †

P value

Landmark

Mean

SD

Mean*

SD

Mean

SD

(1-way ANOVA)

Incisal edge Cusp tip Buccal cusp Palatal cusp Mesiobuccal cusp Mesiopalatal cusp Distobuccal cusp Distopalatal cusp Occlusal centroid

⫺5.5 ⫺4.9 2.8 2.0 2.6 2.3 2.7 2.3 2.5

2.0 1.2 1.0 1.1 1.2 0.8 1.2 0.8 0.9

⫺6.9 ⫺5.9 1.7 1.1 1.5 1.1 1.4 1.2 1.3

1.8 1.3 0.9 1.0 1.0 1.0 1.3 1.0 1.0

⫺7.3 ⫺5.9 1.5 0.8 1.7 1.3 1.6 1.3 1.4

2.0 1.6 1.5 1.5 1.4 1.3 1.3 1.3 1.3

0.003 0.004 0.000 0.001 0.001 0.000 0.000 0.000 0.000

Positive value indicates mesial movement; negative value indicates distal retraction. *Significant difference between headgear and miniscrew groups (based on the Scheffé test, P ⬍0.05). † Significant difference between headgear and miniplate groups (based on the Scheffé test, P ⬍0.05).

Fig 3. Scatterplot of incisor retraction and mesial movement in the 3 groups with different types of anchorage preparation.

ANOVA showed significant differences at the palatal cusp of the second premolar and all cusps of the first molars. Intrusive movements of the second premolar and the first molar in the miniplate group were significantly greater than those in the headgear and miniscrew groups (Fig 4). DISCUSSION

This 3D dental model study provided a comprehensive description of individual tooth movements of protrusive maxillary dentition during orthodontic treatment by

using 3 types of anchorage. Our results demonstrated that orthodontic treatment with skeletal anchorage resulted in greater retraction of the maxillary anterior teeth and less anchorage loss of the posterior teeth than traditional headgear anchorage. Treatment changes shown in the 3D dental model analysis fulfilled our treatment goals for correcting maxillary dentoalveolar protrusion. In addition to anterior retraction, miniplates can facilitate intrusion of the maxillary posterior teeth; this is favorable for closing an anterior open bite and providing vertical control during orthodontic treatment for

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Table IV.

Tooth Incisor Canine Premolar First molar

American Journal of Orthodontics and Dentofacial Orthopedics November 2008

Occlusal-gingival movement (mm) at the various cusp tips of the maxillary teeth Headgear group

Miniscrew group

Miniplate group

P value

Landmark

Mean

SD

Mean

SD

Mean

SD

(1-way ANOVA)

Incisal edge Cusp tip Buccal cusp Palatal cusp Mesiobuccal cusp Mesiopalatal cusp Distobuccal cusp Distopalatal cusp Occlusal centroid

0.9 0.3 ⫺0.8 ⫺0.6 ⫺0.8 ⫺0.3 ⫺0.5 ⫺0.2 ⫺0.5

2.4 2.2 1.0 0.8 0.8 0.8 0.9 0.7 0.7

1.8 0.8 ⫺0.6 ⫺0.6 ⫺0.6 ⫺0.5 ⫺0.4 ⫺0.6 ⫺0.6

1.6 1.1 1.0 0.8 1.0 0.9 1.3 0.9 0.9

1.9 ⫺0.3 ⫺1.1 ⫺1.1*,† ⫺1.4† ⫺1.3*,† ⫺1.3*,† ⫺1.2* ⫺1.3*,†

1.5 0.8 0.9 0.6 1.1 0.9 1.2 1.0 0.9

0.181 0.064 0.209 0.012 0.018 0.000 0.014 0.001 0.002

Positive value indicates extrusive movement; negative value indicates intrusive movement. *Significant difference between headgear and miniplate groups (P ⬍0.05). † Significant difference between miniscrew and miniplate groups (P ⬍0.05).

Fig 4. Scatterplot of vertical movements of the mesial and distal surfaces of the maxillary first molar.

Class II high-angle cases. Since mini-implant skeletal anchorage is stable and reliable without patient compliance, this increases its acceptability to many adults. Compared with our previous case profile, adult patient numbers have increased since mini-implants were adopted as routine anchorage procedures for comprehensive orthodontic treatment.17 Mini-implants are increasingly used for orthodontic anchorage. Many types of mini-implants— endosseous implants,18 miniplates,19,20 miniscrews,21 microscrews,7,22,23 and palatal orthodontic implants from

Straumann (Orthosystem)24— can provide skeletal anchorage when simultaneously used with various orthodontic techniques, such as sliding mechanics,7 frictionless loop mechanics,2 and lingual orthodontic appliances.25 In our previous cephalometric study, we investigated the skeletal and dentoalveolar effects of mini-implant anchorage in treating maxillary dentoalveolar protrusion.26 In this study, pretreatment and posttreatment positional changes of the morphologic landmarks of each maxillary tooth were analyzed and decomposed as x-, y-, and z-coordinates, which indi-

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cate changes in the buccopalatal, mesiodistal, and occlusal-gingival directions. In correcting a dentoalveolar protrusion, extraction of the bilateral premolars is a common approach to provide space for anterior retraction. When the extraction of maxillary premolars is indicated for Class II or Class I malocclusion, the mechanics must be designed to maximize anterior tooth retraction,4 whereas extrusion and mesial movements of the maxillary molars should be minimized until dental crowding and dentoalveolar protrusion are corrected.27 Well-controlled mechanotherapy can result in a satisfactory soft-tissue response with reductions in lip eversion and protrusion. A previous study found that anterior tooth retraction and the corresponding lip response varied among premolar extraction patterns with wide individual variations.1 Differences in extraction patterns produce changes in relative root surface area of the opposing posterior unit to the anterior segment; this then affects the amount of anterior retraction. Factors other than simply the choice of a premolar extraction pattern— eg, the amount of original crowding—also influence the amounts of forward molar movement and incisor retraction.28 Although skeletal anchorage from a miniimplant expands the envelope of traditional orthodontic tooth movement, biomechanical considerations and special concerns for facial esthetics should be addressed for an efficient and realistic treatment for patients with Class II malocclusion and maxillary dentoalveolar protrusion. With cephalometric analyses, blurred images resulting from superimposition of bilateral structures make it difficult to identify individual teeth. The errors associated with measuring small linear distances on cephalometric tracings further compromise the quantitative assessment of orthodontic movements of each tooth. The linear measurement errors of 3D dental model data were fewer compared with our previous data using cephalometric analyses (median values, 0.09 vs 0.15 mm). Another limitation inherent in cephalometric assessments is the inability to evaluate tooth movement in the transverse direction. Three-dimensional dental model imaging can be analyzed by using laser scanners, which provide a hologram of dental models with more expensive equipment.29 The use of a 3D digitizer with dental models has made possible a more comprehensive assessment of tooth position changes during treatment.12 Cone-beam computed tomography is another tool to generate 3D dentofacial reconstruction images, so that alveolar bone changes after orthodontic tooth movements can be studied. In this study, the 3D digitizer and computer-aided analysis made possible the 3D assessment of tooth

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movements. As noted in Table III, movements of the maxillary central incisor were 5.5 mm in the headgear group, 6.9 mm in the miniscrew group, and 7.3 mm in the miniplate group. The mesial movement of the maxillary first molar was more than 2 mm in the headgear groups, but less than 1.5 mm in both the miniscrew and miniplate groups. Our previous cephalometric data showed that the central incisors were retracted by 6.7 mm in the headgear group and 8.2 mm in the skeletal anchorage group, including both miniscrews and miniplates.26 Mesial movements of the maxillary first molar were 2.1 mm in headgear group and 0.9 mm in the skeletal anchorage group. Comparing measurements on virtual dental models, the cephalometric data were affected by the magnification of lateral cephalometric images, on which the actual tooth movements were projected on the midsagittal plane. Three-dimensional analysis of serial dental models facilitates the investigation of individual orthodontic tooth movements. To compare the positional changes between the pretreatment and posttreatment dental models, an important factor is the selection of a stable reference for superimposition. Abdel-Aziz and Sabet30 studied the stability of the palatal rugose area before and after orthodontic treatment in adults and found that the lateral points of the third rugae were the most reliable and could be used as reference points for cast superimposition. Another study also reported that the medial and lateral end of the third palatal rugae can be as reliable as cephalometric superimpositions for assessing anteroposterior molar movements.31 Retraction of a maxillary dentoalveolar protrusion can be facilitated by various types of mini-implant anchorage. Understanding the effects and applying the proper mini-implant can help achieve efficient orthodontic treatment. This study on dental models demonstrates that mini-implant anchorage, with either miniscrews or miniplates, performed better than conventional headgear anchorage. These results are compatible with our previous research findings on cephalometric analyses for a greater amount of incisor retraction with skeletal anchorage.26 Although this study on 3D dental models showed subtle differences between miniscrews and miniplates in the correction of maxillary dentoalveolar protrusion, the performance of the miniscrew was similar to that of the miniplate in both anteroposterior and transverse dimensions but not in the vertical dimension. In addition to anterior tooth retraction, miniplate anchorage also facilitates maxillary molar intrusion; this is favorable for correcting high-angle Class II malocclusions. In this study, shorter treatment times in the miniscrew group were noted, whereas treatment duration in the miniplate group did not

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significantly differ from that in the headgear group. This might have been due to our anchorage strategy of using miniscrews for Class II patients with normal facial divergency because there is usually less difficulty in vertical control during treatment when compared with those with hyperdivergent facial patterns. Conventional orthodontic appliances for molar distalization often extrude the molars. Miniplate skeletal anchorage, implanted buccally and apically to the maxillary first molar crown, can simultaneously distalize and intrude the maxillary posterior teeth. The incorporation of auxiliary attachments— eg, power chain elastic, coiled spring, or sliding jig—facilitates adjustments in the relative amounts of horizontal and vertical forces. In this study, miniscrews were primarily used for anterior tooth retraction but not for maxillary molar intrusion. To prepare skeletal anchorage for patients with normal or hypodivergent facial patterns, miniscrews are preferred over miniplates, since the implantation procedure of miniscrews is simpler and has fewer complications. All subjects in this study were treated with maxillary premolar extraction and maximal anchorage preparation. Although the standard deviations were larger than the mean measurements, the mean values of retraction of the maxillary incisors were higher in the miniplate and miniscrew groups than in the headgear group. Contrary to the intermittent force application from the headgear, the skeletal anchorage was consistently stable throughout treatment. For Class II patients, overretraction of the maxillary incisors seldom occurs with headgear anchorage but is possible with skeletal anchorage. For proper retraction of the maxillary incisors and to maintain adequate lip support, it is necessary to monitor the patient’s facial esthetics and the incisor-to-lip relationship if anterior tooth retraction is conducted with skeletal anchorage. In the finishing stages, overall chain elastic is often required for residual space closure and to establish stable posterior occlusion. This can partially explain why more than 1 mm of mesial movement of the maxillary molar was still noted in the miniscrew and miniplate groups. Torque control of the maxillary incisors is important to facial esthetics when treating Class II malocclusions. In our system of sliding mechanics, the retraction force connecting the maxillary skeletal anchorage to the maxillary incisors tends to produce a clockwise moment and cause incisor uprighting. A 1-piece betatitanium alloy intrusion arch provided a counterclockwise moment and controlled the palatal root torque of the maxillary incisors. The side effect of an intrusive arch on the maxillary first molar is controlled with a transpalatal arch, which helps maintain intermolar width and prevents crown buccal tipping when the

American Journal of Orthodontics and Dentofacial Orthopedics November 2008

retraction force applied by mini-implants is buccal to the dentoalveolar process. Thus, from the mechanical point of view, the palatal orthodontic implant (Orthosystem) provides excellent anchorage control in both anteroposterior and transverse dimensions when compared with mini-implants at the buccal sides. However, the high cost, longer waiting period for osseointegration, and the difficulty of removal made this palatal implant less attractive. Our data show that buccopalatal movement of individual maxillary teeth was small in each group of the 3 anchorage systems. This implies that the transverse dimension of the maxillary dentition can be well controlled to establish good occlusion, regardless of whether traditional headgear or miniimplant anchorage is used. Class II elastic is commonly used to retract maxillary anterior teeth during the treatment of Class II malocclusions. However, the side effects of maxillary incisor elongation and mandibular molar extrusion can hinder the correction of Class II malocclusions, especially in patients with high mandibular plane angles. For high-angle patients, stable skeletal anchorage can control anterior facial height and posterior vertical dimension, followed by improvements in vertical facial balance.32 Traditional orthodontic mechanics are unpredictable for many orthodontists when treating Class II malocclusions in adults with a skeletal open bite. Skeletal anchorage systems with titanium miniplates have been reported to predictably distalize and intrude the posterior teeth.19 Orthodontic treatment of severe anterior open bites by using titanium skeletal anchorage has consistently been shown to achieve genuine intrusion of the maxillary molars.3,33 Miniplate anchorage is also recommended for Class II high-angle patients because intrusion of the maxillary molars can be followed by counterclockwise rotation of the mandible, subsequently improving the Class II malocclusion.25 In this study, we compared treatment outcomes of miniscrew and miniplate anchorage to a traditional headgear appliance, and the results showed that the maxillary posterior teeth were significantly intruded only in the miniplate group. This might have resulted from the different anchorage strategies to meet the treatment goals, because we chose the type of mini-implant— miniscrew or miniplate—to fit each patient’s needs. These results substantiate the advantages of skeletal anchorage by mini-implants to retract a protrusive maxillary dentition, especially with miniplates for Class II high-angle patients. CONCLUSIONS

For the treatment of maxillary dentoalveolar protrusions, both miniscrew and miniplate skeletal anchorage

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American Journal of Orthodontics and Dentofacial Orthopedics Volume 134, Number 5

achieved better control in the anteroposterior direction than did the traditional headgear appliance. Significantly more intrusion of the maxillary posterior teeth was noted in the miniplate group, but not in the miniscrew or headgear group. Skeletal anchorage favored the correction of Class II malocclusions with the biomechanical advantages of greater retraction of the maxillary anterior teeth and less anchorage loss of the posterior teeth. Moreover, miniplate-facilitated maxillary molar intrusion is a reliable strategy for treating Class II malocclusions in patients with a hyperdivergent face. This 3D dental model analysis provides an evidence-based rationale for applying orthodontic mini-implants.

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