- Email: [email protected]
Treatment and posttreatment effects of a novel magnetic palatal expansion appliance with reactivation system in dogs
ORIGINAL ARTICLE
Treatment and posttreatment effects of a novel magnetic palatal expansion appliance with reactivation system in dogs Fei Tong, Zhanqi Miao, Xingchen Zhou, Chang Xiao, and Jianyong Wu Nanchang, China
Introduction: This study aimed to compare the effects of a novel magnetic palatal expansion appliance (MPEA) during the expansion and maintenance period with that of a screw expansion appliance. Methods: Based on previous research, the MPEA had a reactivation system that was modified for a broader working range and more stable expansion. Thirty-six male beagle dogs were assigned to a magnetic expansion (ME; n 5 12), screwed expansion (SE; n 5 12) or control (n 5 12) group. Half of the dogs from each group were evaluated only during 5 weeks of activation, whereas the rest were evaluated for 5 weeks of activation and 8 additional weeks of retention. Nonmagnetic metal marking implants were implanted on both sides of the midpalatal suture of all dogs. Three-dimensional assessment of treatment and posttreatment dental and skeletal effects were conducted using cone-beam computed tomography. The width of the midpalatal suture, mineralization and deposition rate of bone, and fluorescence integral optical density were calculated during the expansion and retention periods using tetracycline fluorescence labeling. Results: There were increases in the value of all cone-beam computed tomography parameters in the SE and ME groups during the expansion period, and the increase was significantly greater than that of the control group (P \0.01). However, there was no significant difference in the values of any parameters during the retention period. The width of the midline sutures, mineralization and deposition rate of bone, and integral optical density in the 2 experimental groups were significantly higher than those of the control group (P \0.01), and there was no significant difference between the SE and ME groups. After the retention period, the values of all tetracycline fluorescence evaluation parameters of the experimental groups decreased significantly. Conclusions: The novel MPEA with a reactivation system was able to expand the midpalatal suture effectively. Dental and skeletal expansion effects are similar to those of the screw expansion appliance. Wearing the appliance as a retainer can effectively maintain the expansion effect. The new bone formation rate was accelerated during the expansion process and decreased to normal levels during the retention period. (Am J Orthod Dentofacial Orthop 2020;157:194-204)
P
alatal expansion (PE) is one of the most frequently used orthodontic treatment strategies in the correction of malocclusion. It is applied for the
From the Department of Orthodontics, Affiliated Stomatological Hospital of Nanchang University, Stomatological and Biomedical Key Laboratory of Jiangxi Province, Nanchang, Jiangxi, China. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. This study was supported by funding from National Natural Science Foundation of China (No. 81360171) and Natural Science Foundation of Jiangxi province (No. 20151BAB215018). Address correspondence to: Jianyong Wu, 49# Fuzhou Road, Donghu District, Nanchang, Jiangxi 330006, China; e-mail, [email protected]. Submitted, August 2018; revised and accepted, March 2019. 0889-5406/$36.00 Ó 2019 by the American Association of Orthodontists. All rights reserved. https://doi.org/10.1016/j.ajodo.2019.03.020
194
correction of posterior crossbites occurring as a result of constricted maxilla, mainly mechanical and magnetic expansions. Mechanical expansion is widely used in clinical settings, but it requires frequent reactivation, complicated operations, and a considerable initial force, which may cause the patient to be prone to root resorption, alveolar dehiscence and fenestration, and additional side effects.1-3 On the other hand, magnetic force expansion is relatively gentle, possessing superior vector control and atraumatic periodontal remodeling.4,5 However, the magnetic energy product of early generation magnets was relatively weak, and the magnetic force generated was also inadequate. Furthermore, the force generated between the 2 magnets sharply decreased along with distance, which led
Tong et al
to an insufficient and unstable expansion force in early expanding appliances, thereby restricting their development.6,7 The new magnetic palatal expansion appliance (MPEA) developed through our previous research applies repulsion forces from third generation high-energy rareearth permanent magnetic alloy neodymium-ironboron magnets, combined with a reactivation system, in order to overcome the limitation of the magnetic force decreasing along with distance. Reactivation can be performed through limiting grooves and central release links. Thus, the operation is more convenient, and the orthodontic force is more stable than that of earlier magnets. Indeed, this method has been shown to expand the maxilla in animal experiments effectively.8 However, the appliance still suffers from numerous deficiencies, including a large size, insufficient working range (expansion amount), and the requirement of a complicated reactivation operation. In this study, we have made some improvements to the magnetic expansion appliance, based on the results of our previous study. The squared housing box was changed into a middle track-type housing box, in order to make it smaller and easier to clean, whereas the limiting grooves were replaced with a locking clasp device so that reactivation was more convenient and reliable during clinical applications. Long-term stability after maxillary expansion is one of the research hot spots of orthodontics. The stability of PE has been studied and shown that without retention, relapse immediately occurs and may reach 45%.9,10 Long-term interventions have revealed the negative influence of cumulative relapse over an extended posttreatment period. The effect of many factors, including patient's age, severity of the maxillary constriction, rate and amount of expansion, design of the device, structures surrounding the maxilla, duration of the retention period and response of the midpalatal suture, and adaptation of soft tissues to the new positions, have been detailed. Lee et al11 found that expansion of the midline suture was the most important part of PE. After deposition and mineralization of a new bone in the midpalatal suture, it can resist reconvergence of the palate plate, which is directly related to the stability of the PE. As such, evaluation of bone reconstruction at the midpalatal suture is of considerable significance in evaluating the stability of PE. In this animal experiment, we aimed to compare the treatment and posttreatment effects of the novel MPEA with that of a mechanical screw expansion device using cone-beam computed tomography (CBCT) and investigate the mineralization and deposition rate of new bone in the midpalatal suture, during the expansion
195
and retention phases, using a tetracycline fluorescence labeling method. MATERIAL AND METHODS
The novel MPEA was designed and manufactured with a reactivation system. The former MPEAs (Fig 1, A) consists of 2 magnets (2.7 mm 3 11.5 mm 3 2.3 mm) placed in a square housing box with the same pole facing each other, and reactivation is applied by zeroing the distance of the 2 magnets through the squeezing of the central release links, followed by crimping the weak points of the limiting grooves in the pushing plate (Fig 1, B).8 The novel MPEA (Fig 1, C) is composed of 4 magnets (6 mm 3 4 mm 3 4 mm), and the same magnetic poles were fixed opposite to each other, in the middle of track-type housing boxes. Reactivation of the novel MPEA (Fig 1, D) was performed as follows: first, the limited bar was rotated 180 outwards, on one-side. Then the housing box was pushed on the same side by sliding it along the 2 tracks with the semicircular grooves for a distance of 1.5 mm, in order to make contact with the opposite box. The limited bar was then rotated 180 inwards to lock the box. After a 1.5-mm expansion was achieved, sliding was stopped using the semicircular grooves and limiting bar, which stopped the MPEA from applying an expansion force. The next time that force was applied, the above operation was repeated on the other side of the magnetic expander, resulting in reactivation of the left and right sides, performed alternately. The force and distance of each MPEA were measured using a universal testing machine (Instron model 2343, Canton, Mass). All animal work was performed using protocols approved by the Institutional Animal Care and Use Committee of the Medical College of Nanchang University. Thirty-six male beagle dogs (aged 6 months; equivalent to 9-11 years for humans) weighing 8 kg each were used in this study. Two percent sodium pentobarbital (1 ml/ kg, intraperitoneal) was used to anesthetize the dogs. After we drilled guide holes in the palatal bone at low speed, we inserted 4 pairs of nonmagnetic metal bone implants (diameter, 0.7 mm; length, 5 mm) approximately 3 mm and 6 mm lateral to the palatal suture and in line with bilateral first and fourth premolars, respectively. Bone markers were used to quantify palatal expansion during radiological procedures. The dogs were randomly divided into 3 groups, 12 of which were assigned to the magnetic expansion (ME) group and received magnetic expanders, 12 dogs were assigned to the screwed expansion (SE) group and received jackscrew expanders, and 12 dogs were assigned to the control group and received no expander but were
American Journal of Orthodontics and Dentofacial Orthopedics
February 2020 Vol 157 Issue 2
Tong et al
196
Fig 1. A, The former MPEA is composed of a squared housing box (a) to receive the magnets (b), pushing plates combined with crimped limiting grooves (c), pushing rods (d), central release links (e) and sliders (f). B, Procedure to reactivation the former MPEA. The release links have to be pushed with a band seating instrument in order to zero the distance between the 2 magnets. It is then crimped, and the weak point of the crimped limiting grooves are deformed using a plier, in order for them to serve as barriers. C, The novel MPEA consists of 4 magnets (b) and the same magnetic poles are fixed opposite to each other in the middle of the track-type housing boxes. D, Reactivation of the novel MPEA was done by rotating the limited bar 180 outwards on one-side, and then pushing the housing box on the same side by sliding it along the 2 tracks with semicircular grooves and the central track by a distance of 1.5 mm in order for it to make contact with the opposite box. The limited bar was then rotated 180 inwards, for the box to be locked. The next time that force is applied, the above operation is repeated on the other side of the magnetic expander, alternately performing activation of the left and right sides.
maintained under the same conditions as the other animals. The palatal expansion device was customized for each animal. The expansion appliance was placed at the midline between the left and right third premolars, about 3 mm away from the palatal mucosa (Fig 2, A and B). When the appliances were bonded, the housing boxes needed to be in full contact and were temporarily held in place using ligature wires. The animals were then anesthetized, and the palatal expansion devices were cemented to the abutment teeth, and the ligature wires were removed. Because anesthetization of the dogs was required during each reactivation, the frequency of activation for dogs of the SE group was limited to twice a week at 0.75 mm per activation, whereas the ME group expanders were loaded only once a week, in order to reduce the risk of anesthesia (Fig 2, C). After 5 weeks, expansion was completed, and 7.5-mm activation was achieved in both treatment groups. All 3 groups were further subdivided into 2 subgroups (A and B; n 5 6 per subgroup). The dogs in subgroup A were killed in order to evaluate the expansion effect after 5 weeks of
February 2020 Vol 157 Issue 2
activation, whereas the dogs in subgroup B continued to wear the expansion appliance (without activation) for 8 weeks, in order to evaluate the expansion and retention effect. All CBCT images were acquired at the Affiliated Stomatological Hospital of Nanchang University (Nanchang, Jiangxi, China). CBCT images were obtained at baseline (T0), after 5 weeks when expansion was achieved (T1), and after 8 weeks of retention (T2) using a three-dimensional (3D) exam CBCT scanner (KaVo Dental, Biberach, Germany). During the scan, the head of each dog was positioned in a customized plastic fixture. The scan parameters were 120 kVp, 20.27 mAs, 14.7 sec per revolution, pixel size 0.25 mm, and a reconstruction interval of 16 3 16 3 10 cm. All operations were performed by 1 technician. The CBCT images were saved as digital imaging and communications in medicine files and reconstructed into 3D volumes using in-vivo software (version 5.1.10; Anatomage, San Jose, Calif). The CBCT measurement method refers to the preliminary study conducted by our research group.8 The following parameters were measured:
American Journal of Orthodontics and Dentofacial Orthopedics
Tong et al
197
Fig 2. A, A mechanical screw palatal expansion appliance. B, A magnetic palatal expansion appliance. C, Reactivation of the magnetic expander.
1.
2.
3.
4.
5.
The distance between the bilateral canines (DC) was determined by measuring between the cusp tip of the bilateral maxillary canines in the 2D coronal images through the tip end of the left canine. The distances between the bilateral fourth premolar and the first molar (DPM4, DM1) were determined by measuring between the mesiobuccal cusp tip of bilateral teeth in the 2D coronal images through the mesiobuccal cusp tip of left fourth premolar or first molar. The distances between implants adjacent to the first premolar (DIPM1) were measured. Because the hard palate of beagle dogs was flat and had a low thickness (about 1 mm), horizontal images can be obtained using 3D adjustment by passing through the hard palate to a maximum extent. The distance between each marking implant on both sides of the midpalatal suture between the first premolars was measured. The distance of the same pair of implants can be measured and compared at different times. The distances between implants adjacent to the fourth premolar (DIPM4) were measured. The distance between each marking implant on both sides of the midpalatal suture between the fourth premolars was measured on a horizontal image. The distance between the same pair of implants was also measured and compared at different times. Angulations of the fourth premolar and first molar (APM4, AM1) were measured. The measurement of the angle was obtained through the long axis of the fourth premolar or the first molar (from the tip end to the apex). The axial plane of the 2D coronal images was made through the distal apex of the fourth premolar or the first molar. The bilateral
angles of the fourth premolar and the first molar were measured and then averaged. All CBCT measurements were taken 3 times by a single-blinded examiner (Z.M.) under the same conditions and then averaged. Intraexaminer reliability was within 6 5% for all measurements, as determined by data replications taken 2 weeks apart. Method error was calculated according to Dahlberg's formula. After 15 days, 29 CBCT radiographs were re-measured in order to evaluate intraexaminer measurement error (DC, 1.356 mm; DPM4, 1.65 mm; DM1, 1.675 mm; DIPM1, 0.863; DIPM4, 0.969; APM, P1.658; AM1, 1.743) using Dahlberg's formula, (S2 5 d2/2n).12 One percent tetracycline hydrochloride solution was intramuscularly injected (1 ml/kg) into dogs of subgroup A at the beginning of placement and 3 days before the end of expansion, whereas for dogs in subgroup B it was injected 3 days before the completion of extension and 3 days before the end of the retention period. Subgroup A was killed during the T1 phase and subgroup B during the T2 phase through an overdose of sodium pentobarbital. Bone tissue between the second and third premolars of the maxilla was quickly resected in order to remove surface mucosa and muscles and then fixed in 70% ethanol for 3 days. The tissue was gradually dehydrated using alcohol (80%, 90%, 95%, and 100% for 12 h). After the xylene was transparent, the tissue was immersed in permeates I, II, and III for 24 hours, then embedded in methyl methacrylate. An SP1600 hard tissue microtome was used to obtain slices that were perpendicular to the tarsal plate at the horizontal line of the third premolar, with a thickness of 30 mm. After polishing using P1200-P2000 sandpaper, the
American Journal of Orthodontics and Dentofacial Orthopedics
February 2020 Vol 157 Issue 2
Tong et al
198
Fig 3. Histomorphometric measurements. A, Distance between the 2 marker bands; B, sutural gap width; C, IOD of 5 randomly selected areas of the new bone area along the midpalatal suture.
nondecalcified bone tissue grinding plate was made by removing the knife marks that were on the surface of the tissue. The grinding plate was magnified 40 times under a fluorescence microscope (with the wavelength of 360-400 nm) for observation and photographing. Image-Pro Plus image analysis software (version 6.0; Media Cybernetics, Inc, Md) was used to analyze the images, which were taken under the fluorescence microscope. A blinded evaluator (X.Z.) traced the bone labels. The widest 3 pairs of fluorescence bands were selected from the edge of the midline palatine suture and were measured in each sample, and each pair of fluorescence bands were automatically measured at each 10-mm interval between the leading edges of the label pairs. The distance between the frontal edges of 2 bands was measured in triplicate and averaged (Fig 3, A). By measuring the distance between the 2 marker bands, the mineralization and deposition rate of the new bone during the expansion period can be calculated. Therefore, the mineralization and deposition rate was equal to the distance between the 2 marker bands at 35 days. Sutural gaps were measured 3 times at each 10-mm interval between the 2 trailing edges of the bone labels, along the midpalatal suture (Fig 3, B), for an average to be obtained. Eventually, ImageJ 1.48 imaging analysis software (National Institutes of Health, Bethesda, Md) was used to measure the tetracycline fluorescence optical density (IOD) of 5 randomly selected areas (84 mm 3 84 mm) of new bone along the midpalatal suture in each bone slice (Fig 3, C), and then averaged. Statistical analysis
The SPSS 19.0 software (IBM, Armonk, NY) was used for statistical analysis in this study. The data are expressed as mean 6 standard deviation. One way analysis
February 2020 Vol 157 Issue 2
of variance was used to compare the CBCT measurements of the 3 groups, in order to obtain phase differences at T0-T1 and T1-T2. In addition, the independent-samples t test was used to compare tetracycline fluorescence markers between 2 groups and 1way analysis of variance was used to compare the 3 groups. Repeated measurements of indicators among multiple groups were compared using repeated measurements of analysis of variance. A P value of \0.05 was considered statistically significant. RESULTS
The force-deflection curves of MPEA after 5 loadings of a distance of 1.5 mm were similar. This suggests that the mechanical properties of the MPEA were approximately the same after each activation. The range of forces at intervals between 0 and 1.5 mm varied from 10.7 to 5.5 N, respectively. All 36 beagle dogs survived until the T2 period. In the experimental group, the dogs lost weight within 1 week of expansion, which may be related to eating during the initial period of wearing the expansion appliance, which returned to normal after the second week. A few implants fell off during the experiment. However, there were no dogs that lost both implants on the same side of the midpalatal suture, adjacent to the first or fourth premolars. Therefore, there was no effect on the measurements. According to the CBCT examination, the midpalatal sutures of the ME and SE groups had expanded during T1 (Fig 4). As shown in the Table I, the measurement parameters of both experimental groups had significantly increased after 5 weeks of expansion. Except for APM4 and AM1 in the control group, all other items had slightly increased during the T1 period. After 8 weeks, except for a small increase in DPM4 and APM4 of the
American Journal of Orthodontics and Dentofacial Orthopedics
Tong et al
199
Fig 4. CBCT images showing the widening of the midpalatal suture in the ME and SE groups after expansion. A, ME group at T0; B, SE group at T0; C, control group at T0; D, ME group at T1; E, SE group at T1; F, control group at T1. The yellow arrows show midpalatal sutures.
control group during the T2 period, the other groups showed no significant changes in measurement. A comparison of the changes in the value of parameters of the different periods shows that the difference between the 2 experimental groups during the T1-T0 period was larger than their difference with the control group (P \0.01), but no statistical difference was found between the ME and SE groups (Table II). A comparison between the 2 groups during the T2-T1 period showed that differences in DPM4 and APM4 of the control group were higher than when compared with the ME or SE groups. No statistical difference was observed in the values of other parameters between the groups. The midpalatal suture was significantly enlarged in the SE and ME groups during the T1 period, as seen under a fluorescence microscope. Several large continuous yellow-green or gold-yellow fluorescent bands were visible near the midpalatal suture, representing the frontier of new bone formation. The new bone at the midpalatal suture showed a long finger-like protrusion pointing to the middle (Fig 5, A and B), whereas the
midpalatal suture was curved and twisted and was small in width, and the tetracycline labeling band was narrower in the control group. The fluorescence intensity of tetracycline was less evident than that of the ME and SE groups (Fig 5, C). The width of the midpalatal suture decreased slightly in the control group, whereas it significantly decreased in the ME group and SE group during the T2 period (Fig 5, D-F). The measurements of the tetracycline fluorescence markers in each group are shown in Tables III and IV. The width of the palatal suture, mineralization and deposition rate of the bone, and IOD during the T0-T1 period were significantly different (P \0.05). Further comparison between the 2 groups showed that these 3 parameters were statistically different between the control and experimental groups as well. The value was minimum in the control group, and no significant difference was found between the ME and SE groups, while the width of palatal suture during the T1-T2 period was significantly different (P \0.05). Additional comparisons between the 2
American Journal of Orthodontics and Dentofacial Orthopedics
February 2020 Vol 157 Issue 2
Tong et al
200
Table I. CBCT measurements at different time point for ME, SE, and control groups DC (mm)
DPM4 (mm)
DM1 (mm)
DIPM1 (mm)
DIPM4 (mm)
APM4 ( )
AM1 ( )
Group Control SE ME Control SE ME Control SE ME Control SE ME Control SE ME Control SE ME Control SE ME
T0 31.26 6 1.49 32.21 6 1.67 32.51 6 2.17 43.86 6 2.84 46.32 6 2.56 46.25 6 1.52 46.07 6 2.87 48.08 6 2.73 47.87 6 1.32 13.08 6 0.31 11.9 6 1.13 11.53 6 1.88 7.83 6 4.65 13.55 6 2.62 12.68 6 1.55 86.17 6 3.70 86.69 6 2.66 86.83 6 2.83 79.00 6 6.81 74.83 6 2.28 77.01 6 1.87
T1 31.32 6 1.59 38.61 6 1.71 39.45 6 3.07 44.35 6 3.02 52.90 6 2.15 51.88 6 2.68 46.66 6 2.98 55.00 6 2.96 53.58 6 2.78 13.23 6 0.36 15.17 6 0.60 13.86 6 2.69 8.01 6 4.77 16.7 6 2.04 15.68 6 1.10 87.17 6 3.91 91.29 6 2.48 90.57 6 3.77 78.08 6 6.27 79.33 6 3.15 80.11 6 3.52
T2 31.31 6 1.67 38.44 6 1.86 39.89 6 3.05 44.89 6 3.22 53.13 6 2.12 52.87 6 2.85 46.89 6 3.03 55.14 6 2.96 54.5 6 2.70 13.23 6 0.29 15.19 6 0.63 14.05 6 2.71 8.04 6 4.71 16.81 6 2.20 15.92 6 1.66 88.73 6 4.46 90.86 6 2.70 90.4 6 3.32 78.83 6 6.04 79.08 6 3.32 80.53 6 2.89
Note. Values are mean 6 standard deviation.
Table II. The comparison of absolute changes (T1-T0 and T2-T1) among the 3 groups Variables T1-T0 Difference in DC (mm) Difference in DPM4 (mm) Difference in DM1 (mm) Difference in APM4 ( ) Difference in AM1 ( ) Difference in DIPM1 (mm) Difference in DIPM4 (mm) T2-T1 Difference of DC (mm) Difference of DPM4 (mm) Difference of DM1 (mm) Difference of APM4 ( ) Difference of AM1 ( ) Difference of DIPM1 (mm) Difference of DIPM4 (mm)
Control
SE
ME
0.06 6 0.19 0.54 6 0.52 0.5 6 0.5 0.43 6 1.84 0.1 6 1.38 0.16 6 0.2 0.17 6 0.18
6.4 6 0.9* 6.4 6 0.58* 6.58 6 0.73* 3.45 6 1.83* 2.66 6 2.55* 2.87 6 0.81* 3.34 6 1.83*
6.63 6 0.97* 5.85 6 0.94* 5.98 6 1.35* 3.59 6 1.35* 3.63 6 2.69* 2.54 6 0.94* 2.96 6 0.88*
0.11 6 0.21 0.57 6 0.39 0.06 6 0.76 1.56 6 1.37 0.44 6 0.89 0.02 6 0.11 0.01 6 0.08
0.02 6 0.06 0.14 6 0.18* 0.14 6 0.21 -0.43 6 0.87* -0.25 6 0.46 0.02 6 0.11 0.01 6 0.16
0.03 6 0.26 0.15 6 0.27* 0.92 6 0.89 -0.17 6 1.05* 0.09 6 0.66 0.05 6 0.51 0.04 6 0.55
F 271.6 250.2 156.5 13.4 7.6 46.2 70.4 0.374 4.385 2.893 5.637 1.452 0.017 0.014
P \0.001 \0.001 \0.001 \0.001 0.002 \0.001 \0.001 0.694 0.032 0.087 0.015 0.265 0.983 0.986
Note: Values are mean 6 standard deviation. *Indicates significant comparison with the control group, P \ 0.05.
groups showed that these 3 parameters were statistically different between the experimental group and control group, in which the value was minimum in the control group, and there was no significant difference between the ME group and SE group. There was no statistical difference in the mineralization and deposition rate of the bone and IOD between the control group and experiment group (P .0.05). It was shown that the width of the palatal suture,
February 2020 Vol 157 Issue 2
mineralization and deposition rate of the bone, and IOD of the ME and SE groups were significantly different during different periods and that these values were smaller in the ME and SE groups during the T1T2 period. In the control group, the width of the palatal suture was significantly different (P \0.05), with the minimum value seen at T2; however, there was no significant difference in the mineralization and deposition rate of the bone and IOD (P .0.05).
American Journal of Orthodontics and Dentofacial Orthopedics
Tong et al
201
Fig 5. Fluoroscopic images. A, ME group at T1; B, SE group at T1; C, the control group at T1, D, ME group at T2, E, SE group at T2; F, control group at T2 (S: midpalatal suture, N: new bone; Scale bar 5 200 mm).
Table III. Intergroup comparisons of the changes of tetracycline fluorescence detection parameters during from T0-
T1 and from T1-T2 Parameters T0-T1 Midpalatal suture width Bone mineral deposition rate IOD T1-T2 Midpalatal suture width Bone mineral deposition rate IOD
ME
SE
Control
F
P
294.55 6 62.34 26.41 6 2.48 1187.85 6 184.67
292.64 6 47.60 29.32 6 3.42 1128.58 6 141.52
65.44 6 9.63*,y 1.37 6 0.5*,y 844.48 6 136.19*,y
50.020 235.085 8.343
0.001 \0.001 0.004
124.81 6 15.64 2.36 6 0.76 938.58 6 71.26
123.78 6 15.64 1.84 6 0.18 934.07 6 134.02
51.538 2.986 0.632
\0.001 0.089 0.548
43.44 6 0.35*,y 1.59 6 0.19 866.22 6 116.87
Note: Values are mean 6 standard deviation. For T0-T1, the dyes were administered at the beginning of placement and 3 days before the end of expansion. For T1-T2, the dyes were administered 3 days before the completion of extension and 3 days before the end of the retention. *Indicates a significant comparison with ME group, P \ 0.05; yIndicates a significant comparison with SE group, P \ 0.05.
DISCUSSION
It is of vital importance to maintain a sustainable, stable, and gentle force during orthodontic procedures. The biggest difference between the ME and SE groups is that the magnetic force generated by the former is sustainable and gentle, whereas the mechanical force generated by the latter is an intermittent heavy force.12 However, the most important weakness of ME is that as the dental arch expands and the distance between the magnetic poles increases, causing the magnetic force to attenuate too fast to achieve expansion. In order to obtain a more stable magnetic expansion force, some
researchers have added resin rings or magnet blocks in order to reclose the distance between the magnetic poles and maintain a stable orthodontic force.4,7 However, these methods can be complicated for clinical application, and may also be unhygienic. Although the previous MPEA with a reactivation system simplified the repetitive loading operation, the size of the appliance was still large, resulting in the rapid attenuation of the arch force (range, 10.38-1.63 N), whereas the working range was limited (6 mm) and the reactivation operation was still complicated.8 In the present study, the novel MPEA was succinctly designed with smaller size, whereas the
American Journal of Orthodontics and Dentofacial Orthopedics
February 2020 Vol 157 Issue 2
Tong et al
202
Table IV. Comparison of the changes of tetracycline fluorescence measurements between T0-T1 and T1-T2 Grouping ME Midpalatal suture width Bone mineral deposition rate IOD SE Midpalatal suture width Bone mineral deposition rate IOD Control Midpalatal suture width Bone mineral deposition rate IOD
T0-T1
T1-T2
t
P
294.55 6 62.34 26.41 6 2.48 1187.85 6 184.67
124.81 6 15.64 2.36 6 0.76 938.58 6 71.26
6.469* 22.695 3.085
\0.001 \0.001 0.012
292.64 6 47.60 29.32 6 3.42 1128.58 6 141.52
123.78 6 15.64 1.84 6 0.18 934.07 6 134.02
8.160* 17.803 2.324
\0.001 \0.001 0.045
65.44 6 9.63 1.37 6 0.5 844.48 6 136.19
43.44 6 0.35 1.59 6 0.19 866.22 6 116.87
5.587* 0.84 0.26
0.003 0.425 0.801
Note: Values are mean 6 standard deviation. *Indicates significant corrected t test, P \ 0.05.
expansion force generated by the novel MPEA was more stable (10.7-5.5 N), with a larger working range (7.5 mm), and the reactivation operation was simplified. Its performance was comprehensively evaluated by combined CBCT measurements and tetracycline fluorescence labeling. According to the CBCT measurements, the maxillary width can be effectively expanded by the methods used in the ME and SE groups. The measurements of the ME and SE groups significantly increased during the expansion period, and this increase was larger than that of the control group. At the same time, no statistical differences were observed in the increase of DC, DPM4, and DM1 between the 2 experimental groups, indicating that the effects of these 2 methods were similar regarding the total amount of expansion. The maxillary expansion effects included skeletal and dental effects. In this study, 4 pairs of nonmagnetic metal bone marker implants were placed on either side of the midpalatal suture to prevent a loss of experimental data that may be caused by the loosening of implants. The change in the distance between bilateral implants reflects the amount of palatal suture expansion, that is, the skeletal effect. In the previous study, the skeletal effect of the ME appliance was smaller than that of the SE appliance, whereas in the present study, the increment of DIPM4 in the SE group was 3.34 mm, and was 6.40 mm for DPM4, indicating that the skeletal expansion effect (the expansion of midpalatal suture) was 52.2% of the total expansion effect. In the ME group, the increment of DIPM4 was 2.96 mm and DPM4 was 5.85 mm, showing that the skeletal expansion effect was 50.6% of the total expansion effect, indicating that there was no significant difference in comparison with the SE group, implying that the skeletal effect of the novel MPEA was similar to that of the
February 2020 Vol 157 Issue 2
SE appliance. This effect may be related to a relatively small magnetic force and faster attenuation of the previous magnetic expander, whereas the magnetic force exerted in this study was relatively stable. By simulating clinical expansion using the method of 3D finite element analysis, Gautam et al13 reported that the midpalatal suture opened in a wedged shape, with a wider anterior separation and narrower posterior separation, which was similar to results of studies conducted by Vardimon et al4 and Gabriel et al.14 However, in our study, the increase in DIPM1 was slightly lower than that of the DIPM4 in the ME and SE groups, indicating that the midpalatal suture expanded in parallel or in a wedged shape, with a narrower anterior separation and a wider posterior separation, similar to that reported by Habersack et al15 and Christie et al.16 In their opinion, linear expansion of the midline suture was related to rigidity, position of the expander, as well as the patient's age. The maximum expansion of the midline palatine suture was close to the center of the expander in this study. During the retention period, both the SE appliances and MPEA had a positive effect without releasing an expansion force. There was no statistical difference in the measurement of all parameters in the ME and SE groups, indicating that the SE and ME appliances can be used as retainers to maintain the expansion effect effectively. However, when Lione et al17 performed SE on 17 patients, the width of the midpalatal suture increased at the end of expansion; after 6 months of retention, the width had not significantly changed from what it was before treatment. Thus, they assumed that the skeletal expansion effect of the midpalatal suture had undergone a large number of relapses, which is inconsistent with the results of our study. These differences may be related to the method of activation used. Lione et al17 turned the screw expander for an expansion
American Journal of Orthodontics and Dentofacial Orthopedics
Tong et al
of 0.5 mm each day and continuously reactivated for 14 days, which resulted in a total expansion of 7 mm. However, in our study the SE and ME appliances were applied at longer intervals of 1.5 mm per week, and a total expansion of 7.5 mm occurred over 5 consecutive weeks, which resulted in slower expansion, and the expansion force was relatively gentler, thereby reducing trauma to the midpalatal suture, as well as being beneficial for new bone formation. Secondly, the measurement method was different. Lione et al17 used computed tomography to measure the width of the anterior nasal spine, the posterior margin of the incisor foramen, and posterior nasal spines at the horizontal level of the palate plate, in order to represent the width of the midpalatal suture. However, after 6 months of expansion, the decrease in width of the midpalatal suture was accompanied by new bone formation and mineral deposition and did not imply relapse. In addition, because it was difficult to locate the left and right anterior nasal spine, the posterior edge of the incisor foramen, and the posterior nasal spine, we used the implants as a bone marker on both sides of midpalatal suture, making the measurements more repeatable and comparable. In our study, tetracycline fluorescence labeling was used for the evaluation of the skeletal effect of expansion, to complement the CBCT measurements. Several large and continuous tetracycline fluorescent bands were observed in the ME and SE groups near the midpalatal suture under a fluorescence microscope, and the new bone at the midline of palatal suture showed a long finger-like protrusion pointing to the middle, whereas there was no expansion in the control group. In addition, the tetracycline fluorescent bands at the edge of the palatal suture were formed because of natural growth and reconstruction of bone tissue, so the fluorescence band was narrow. At the end of expansion, the width of the palatal suture was greater in the ME and SE groups, in comparison with that of the control group, indicating that these 2 methods can effectively expand the midpalatal suture, which was consistent with CBCT measurements. After the retention period, the width of the midpalatal suture reduced in all 3 groups, whereas the width of the control group was smaller than that of the experimental groups. When no major change in the distance between implants was observed using CBCT measurements, a decrease in the width of the palatal suture, as represented by the intertetracycline bandwidth was observed. This finding indicates the presence of mineralization and deposition of the bone on both sides of the midline suture during the retention period. At the same time, there was no significant difference between the midpalatal suture width of the SE and
203
ME groups during the retention phase, indicating that the amount of bone deposition on both sides of the midline of the magnetic group was similar. At the same time, tetracycline fluorescence IOD was also used to evaluate new bone formation at the edge of the palatal suture.18,19 In this study, during the expansion period, both the mineralization and deposition rate of the bone and IOD were significantly higher in the ME and SE groups compared with that of the control group, indicating that the mineralization and deposition rate of the bone at the edge of the midline suture can be accelerated through expansion, thereby promoting osteogenesis. There was no statistical difference in the mineralization and deposition rate of the bone between the ME and SE groups, suggesting that the 2 expansion methods have similar effects in stimulating osteogenesis. During the retention period, the mineralization and deposition rate of the bone in the 2 experimental groups decreased significantly, and there was no statistically significant difference with that of the control group, indicating that the rate of osteogenesis during the retention period had decreased to normal. However, Ekstrӧm20 used a radioisotope of Iodine as a radiation source to evaluate bone formation using occlusal radiographs. The measurements showed that the mineralization rate within the suture rose rapidly during the first month after expansion. Vardimon21 studied the mineralization pattern of the midpalatal suture in cats by measuring it on occlusal radiographs. The results indicate that reorganization of the mineralized tissue and acceleration of mineralization rate during the retention period had taken place. The results were different from that of our study. There are 2 possible reasons for the differences in the findings: first, the expansion frequency was different and the studies by Ekstrӧm20 and Vardimon21 applied rapid palatal expansion. In this study, palatal expansion was slow since the expanders were activated once a week with an expansion of 1.5 mm per activation. Second, the examination methods were different. In previous studies, mineralization rate was evaluated by measuring changes in the midpalatal width and the optical density using X-ray films. It is difficult to detect new bone through images at the beginning of the mineralization process, but tetracycline fluorescence labeling is comparatively a more sensitive detection method. One of the potential limitations of this study is that the sample size in each subgroup is relatively small. Therefore, further research with larger sample sizes is needed to obtain more information about the expansion and retention efficiency of MPEA. In addition, this is part of a series of studies, and follow-up studies on the comparison of the skeletal effect between an MPEA group
American Journal of Orthodontics and Dentofacial Orthopedics
February 2020 Vol 157 Issue 2
Tong et al
204
and a palatal expansion group with miniscrews will produce more interesting results. CONCLUSIONS
(1) (2)
(3) (4)
The novel MPEA with a reactivation system can effectively expand the midpalatal suture. The dental and skeletal expansion effects of the novel MPEA with a reactivation system are similar to those obtained with a screw expansion appliance. Wearing the appliance as a retainer can effectively maintain the expansion effect. The new bone formation rate was accelerated during the expansion process and decreased to normal levels during the retention period.
ACKNOWLEDGMENTS
The authors thank Associate Professor Huiqiang Yu of the Public Health College of Nanchang University for his assistance in planning and processing the statistical analyses of this study. REFERENCES [1]. Sun Z, Smith T, Kortam S, Kim DG, Tee BC, Fields H. Effect of bone thickness on alveolar bone-height measurements from cone-beam computed tomography images. Am J Orthod Dentofacial Orthop 2011;139:e117-27. [2]. Rungcharassaeng K, Caruso JM, Kan JY, Kim J, Taylor G. Factors affecting buccal bone changes of maxillary posterior teeth after rapid maxillary expansion. Am J Orthod Dentofacial Orthop 2007;132:428e1-8. [3]. Asanza S, Cisneros GJ, Nieberg LG. Comparison of Hyrax and bonded expansion appliances. Angle Orthod 1997;67:15-22. [4]. Vardimon AD, Graber TM, Voss LR, Verrusio E. Magnetic versus mechanical expansion with different force thresholds and points of force application. Am J Orthod Dentofacial Orthop 1987;92: 455-66. [5]. Kawata T, Hirota K, Sumitani K, Umehara K, Yano K, Tzeng HJ, et al. A new orthodontic force system of magnetic brackets. Am J Orthod Dentofacial Orthop 1987;92:241-8. [6]. Vardimon AD, Graber TM, Voss LR. Stability of magnetic versus mechanical palatal expansion. Eur J Orthod 1989;11:107-15.
February 2020 Vol 157 Issue 2
[7]. Darendeliler MA, Strahm C, Joho JP. Light maxillary expansion forces with the magnetic expansion device. A preliminary investigation. Eur J Orthod 1994;16:479-90. [8]. Tong F, Liu F, Liu J, Xiao C, Liu J, Wu J. Effects of a magnetic palatal expansion appliance with reactivation system: an animal experiment. Am J Orthod Dentofacial Orthop 2017;151:132-42. [9]. Linder-Aronson S, Lindgren J. The skeletal and dental effects of rapid maxillary expansion. Br J Orthod 1979;6:25-9. [10]. Hicks EP. Slow maxillary expansion. A clinical study of the skeletal versus dental response to low-magnitude force. Am J Orthod 1978;73:121-41. [11]. Lee K, Sugiyama H, Imoto S, Tanne K. Effects of bisphosphonate on the remodeling of rat sagittal suture after rapid expansion. Angle Orthod 2001;71:265-73. [12]. Noar JH, Evans RD. Rare earth magnets in orthodontics: an overview. Br J Orthod 1999;26:29-37. [13]. Gautam P, Valiathan A, Adhikari R. Stress and displacement patterns in the craniofacial skeleton with rapid maxillary expansion: a finite element method study. Am J Orthod Dentofacial Orthop 2007;132:5.e1-11. [14]. da Silva Filho OG, Lara TS, de Almeida AM, da Silav HC. Evaluation of the midpalatal suture during rapid palatal expansion in children: a CT study. J Clin Pediatr Dent 2005;29:231-8. [15]. Habersack K, Karoglan A, Sommer B, Benner KU. High-resolution multislice computerized tomography with multiplanar and 3-dimensional reformation imaging in rapid palatal expansion. Am J Orthod Dentofacial Orthop 2007;131:776-81. [16]. Christie KF, Boucher N, Chung CH. Effects of bonded rapid palatal expansion on the transverse dimensions of the maxilla: a cone-beam computed tomography study. Am J Orthod Dentofacial Orthop 2010;137:S79-85. [17]. Lione R, Ballanti F, Franchi L, Baccetti T, Cozza P. Treatment and posttreatment skeletal effects of rapid maxillary expansion studied with low-dose computed tomography in growing subjects. Am J Orthod Dentofacial Orthop 2008;134:389-92. [18]. Yu BH, Zhou Q, Wang ZL. Comparison of tissue-engineered bone from different stem cell sources for maxillary sinus floor augmentation: a study in a canine model. J Oral Maxillofac Surg 2014; 72:1084-92. [19]. Schliephake H, Neukam FW, Hutmacher D, W€ ustenfeld H. Experimental transplantation of hydroxylapatite-bone composite grafts. J Oral Maxillofac Surg 1995;53:46-51: discussion 52. [20]. Ekstr€ om C, Henrikson CO, Jensen R. Mineralization in the midpalatal suture after orthodontic expansion. Am J Orthod 1977;71: 449-55. [21]. Vardimon AD, Brosh T, Spiegler A, Lieberman MM, Pitaru S. Rapid palatal expansion: part 1. Mineralization pattern of the midpalatal suture in cats. Am J Orthod Dentofacial Orthop 1998;113:371-8.
American Journal of Orthodontics and Dentofacial Orthopedics
Treatment and posttreatment effects of a novel magnetic palatal expansion appliance with reactivation system in dogs Fei Tong, Zhanqi Miao, Xingchen Zhou, Chang Xiao, and Jianyong Wu Nanchang, China
Introduction: This study aimed to compare the effects of a novel magnetic palatal expansion appliance (MPEA) during the expansion and maintenance period with that of a screw expansion appliance. Methods: Based on previous research, the MPEA had a reactivation system that was modified for a broader working range and more stable expansion. Thirty-six male beagle dogs were assigned to a magnetic expansion (ME; n 5 12), screwed expansion (SE; n 5 12) or control (n 5 12) group. Half of the dogs from each group were evaluated only during 5 weeks of activation, whereas the rest were evaluated for 5 weeks of activation and 8 additional weeks of retention. Nonmagnetic metal marking implants were implanted on both sides of the midpalatal suture of all dogs. Three-dimensional assessment of treatment and posttreatment dental and skeletal effects were conducted using cone-beam computed tomography. The width of the midpalatal suture, mineralization and deposition rate of bone, and fluorescence integral optical density were calculated during the expansion and retention periods using tetracycline fluorescence labeling. Results: There were increases in the value of all cone-beam computed tomography parameters in the SE and ME groups during the expansion period, and the increase was significantly greater than that of the control group (P \0.01). However, there was no significant difference in the values of any parameters during the retention period. The width of the midline sutures, mineralization and deposition rate of bone, and integral optical density in the 2 experimental groups were significantly higher than those of the control group (P \0.01), and there was no significant difference between the SE and ME groups. After the retention period, the values of all tetracycline fluorescence evaluation parameters of the experimental groups decreased significantly. Conclusions: The novel MPEA with a reactivation system was able to expand the midpalatal suture effectively. Dental and skeletal expansion effects are similar to those of the screw expansion appliance. Wearing the appliance as a retainer can effectively maintain the expansion effect. The new bone formation rate was accelerated during the expansion process and decreased to normal levels during the retention period. (Am J Orthod Dentofacial Orthop 2020;157:194-204)
P
alatal expansion (PE) is one of the most frequently used orthodontic treatment strategies in the correction of malocclusion. It is applied for the
From the Department of Orthodontics, Affiliated Stomatological Hospital of Nanchang University, Stomatological and Biomedical Key Laboratory of Jiangxi Province, Nanchang, Jiangxi, China. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. This study was supported by funding from National Natural Science Foundation of China (No. 81360171) and Natural Science Foundation of Jiangxi province (No. 20151BAB215018). Address correspondence to: Jianyong Wu, 49# Fuzhou Road, Donghu District, Nanchang, Jiangxi 330006, China; e-mail, [email protected]. Submitted, August 2018; revised and accepted, March 2019. 0889-5406/$36.00 Ó 2019 by the American Association of Orthodontists. All rights reserved. https://doi.org/10.1016/j.ajodo.2019.03.020
194
correction of posterior crossbites occurring as a result of constricted maxilla, mainly mechanical and magnetic expansions. Mechanical expansion is widely used in clinical settings, but it requires frequent reactivation, complicated operations, and a considerable initial force, which may cause the patient to be prone to root resorption, alveolar dehiscence and fenestration, and additional side effects.1-3 On the other hand, magnetic force expansion is relatively gentle, possessing superior vector control and atraumatic periodontal remodeling.4,5 However, the magnetic energy product of early generation magnets was relatively weak, and the magnetic force generated was also inadequate. Furthermore, the force generated between the 2 magnets sharply decreased along with distance, which led
Tong et al
to an insufficient and unstable expansion force in early expanding appliances, thereby restricting their development.6,7 The new magnetic palatal expansion appliance (MPEA) developed through our previous research applies repulsion forces from third generation high-energy rareearth permanent magnetic alloy neodymium-ironboron magnets, combined with a reactivation system, in order to overcome the limitation of the magnetic force decreasing along with distance. Reactivation can be performed through limiting grooves and central release links. Thus, the operation is more convenient, and the orthodontic force is more stable than that of earlier magnets. Indeed, this method has been shown to expand the maxilla in animal experiments effectively.8 However, the appliance still suffers from numerous deficiencies, including a large size, insufficient working range (expansion amount), and the requirement of a complicated reactivation operation. In this study, we have made some improvements to the magnetic expansion appliance, based on the results of our previous study. The squared housing box was changed into a middle track-type housing box, in order to make it smaller and easier to clean, whereas the limiting grooves were replaced with a locking clasp device so that reactivation was more convenient and reliable during clinical applications. Long-term stability after maxillary expansion is one of the research hot spots of orthodontics. The stability of PE has been studied and shown that without retention, relapse immediately occurs and may reach 45%.9,10 Long-term interventions have revealed the negative influence of cumulative relapse over an extended posttreatment period. The effect of many factors, including patient's age, severity of the maxillary constriction, rate and amount of expansion, design of the device, structures surrounding the maxilla, duration of the retention period and response of the midpalatal suture, and adaptation of soft tissues to the new positions, have been detailed. Lee et al11 found that expansion of the midline suture was the most important part of PE. After deposition and mineralization of a new bone in the midpalatal suture, it can resist reconvergence of the palate plate, which is directly related to the stability of the PE. As such, evaluation of bone reconstruction at the midpalatal suture is of considerable significance in evaluating the stability of PE. In this animal experiment, we aimed to compare the treatment and posttreatment effects of the novel MPEA with that of a mechanical screw expansion device using cone-beam computed tomography (CBCT) and investigate the mineralization and deposition rate of new bone in the midpalatal suture, during the expansion
195
and retention phases, using a tetracycline fluorescence labeling method. MATERIAL AND METHODS
The novel MPEA was designed and manufactured with a reactivation system. The former MPEAs (Fig 1, A) consists of 2 magnets (2.7 mm 3 11.5 mm 3 2.3 mm) placed in a square housing box with the same pole facing each other, and reactivation is applied by zeroing the distance of the 2 magnets through the squeezing of the central release links, followed by crimping the weak points of the limiting grooves in the pushing plate (Fig 1, B).8 The novel MPEA (Fig 1, C) is composed of 4 magnets (6 mm 3 4 mm 3 4 mm), and the same magnetic poles were fixed opposite to each other, in the middle of track-type housing boxes. Reactivation of the novel MPEA (Fig 1, D) was performed as follows: first, the limited bar was rotated 180 outwards, on one-side. Then the housing box was pushed on the same side by sliding it along the 2 tracks with the semicircular grooves for a distance of 1.5 mm, in order to make contact with the opposite box. The limited bar was then rotated 180 inwards to lock the box. After a 1.5-mm expansion was achieved, sliding was stopped using the semicircular grooves and limiting bar, which stopped the MPEA from applying an expansion force. The next time that force was applied, the above operation was repeated on the other side of the magnetic expander, resulting in reactivation of the left and right sides, performed alternately. The force and distance of each MPEA were measured using a universal testing machine (Instron model 2343, Canton, Mass). All animal work was performed using protocols approved by the Institutional Animal Care and Use Committee of the Medical College of Nanchang University. Thirty-six male beagle dogs (aged 6 months; equivalent to 9-11 years for humans) weighing 8 kg each were used in this study. Two percent sodium pentobarbital (1 ml/ kg, intraperitoneal) was used to anesthetize the dogs. After we drilled guide holes in the palatal bone at low speed, we inserted 4 pairs of nonmagnetic metal bone implants (diameter, 0.7 mm; length, 5 mm) approximately 3 mm and 6 mm lateral to the palatal suture and in line with bilateral first and fourth premolars, respectively. Bone markers were used to quantify palatal expansion during radiological procedures. The dogs were randomly divided into 3 groups, 12 of which were assigned to the magnetic expansion (ME) group and received magnetic expanders, 12 dogs were assigned to the screwed expansion (SE) group and received jackscrew expanders, and 12 dogs were assigned to the control group and received no expander but were
American Journal of Orthodontics and Dentofacial Orthopedics
February 2020 Vol 157 Issue 2
Tong et al
196
Fig 1. A, The former MPEA is composed of a squared housing box (a) to receive the magnets (b), pushing plates combined with crimped limiting grooves (c), pushing rods (d), central release links (e) and sliders (f). B, Procedure to reactivation the former MPEA. The release links have to be pushed with a band seating instrument in order to zero the distance between the 2 magnets. It is then crimped, and the weak point of the crimped limiting grooves are deformed using a plier, in order for them to serve as barriers. C, The novel MPEA consists of 4 magnets (b) and the same magnetic poles are fixed opposite to each other in the middle of the track-type housing boxes. D, Reactivation of the novel MPEA was done by rotating the limited bar 180 outwards on one-side, and then pushing the housing box on the same side by sliding it along the 2 tracks with semicircular grooves and the central track by a distance of 1.5 mm in order for it to make contact with the opposite box. The limited bar was then rotated 180 inwards, for the box to be locked. The next time that force is applied, the above operation is repeated on the other side of the magnetic expander, alternately performing activation of the left and right sides.
maintained under the same conditions as the other animals. The palatal expansion device was customized for each animal. The expansion appliance was placed at the midline between the left and right third premolars, about 3 mm away from the palatal mucosa (Fig 2, A and B). When the appliances were bonded, the housing boxes needed to be in full contact and were temporarily held in place using ligature wires. The animals were then anesthetized, and the palatal expansion devices were cemented to the abutment teeth, and the ligature wires were removed. Because anesthetization of the dogs was required during each reactivation, the frequency of activation for dogs of the SE group was limited to twice a week at 0.75 mm per activation, whereas the ME group expanders were loaded only once a week, in order to reduce the risk of anesthesia (Fig 2, C). After 5 weeks, expansion was completed, and 7.5-mm activation was achieved in both treatment groups. All 3 groups were further subdivided into 2 subgroups (A and B; n 5 6 per subgroup). The dogs in subgroup A were killed in order to evaluate the expansion effect after 5 weeks of
February 2020 Vol 157 Issue 2
activation, whereas the dogs in subgroup B continued to wear the expansion appliance (without activation) for 8 weeks, in order to evaluate the expansion and retention effect. All CBCT images were acquired at the Affiliated Stomatological Hospital of Nanchang University (Nanchang, Jiangxi, China). CBCT images were obtained at baseline (T0), after 5 weeks when expansion was achieved (T1), and after 8 weeks of retention (T2) using a three-dimensional (3D) exam CBCT scanner (KaVo Dental, Biberach, Germany). During the scan, the head of each dog was positioned in a customized plastic fixture. The scan parameters were 120 kVp, 20.27 mAs, 14.7 sec per revolution, pixel size 0.25 mm, and a reconstruction interval of 16 3 16 3 10 cm. All operations were performed by 1 technician. The CBCT images were saved as digital imaging and communications in medicine files and reconstructed into 3D volumes using in-vivo software (version 5.1.10; Anatomage, San Jose, Calif). The CBCT measurement method refers to the preliminary study conducted by our research group.8 The following parameters were measured:
American Journal of Orthodontics and Dentofacial Orthopedics
Tong et al
197
Fig 2. A, A mechanical screw palatal expansion appliance. B, A magnetic palatal expansion appliance. C, Reactivation of the magnetic expander.
1.
2.
3.
4.
5.
The distance between the bilateral canines (DC) was determined by measuring between the cusp tip of the bilateral maxillary canines in the 2D coronal images through the tip end of the left canine. The distances between the bilateral fourth premolar and the first molar (DPM4, DM1) were determined by measuring between the mesiobuccal cusp tip of bilateral teeth in the 2D coronal images through the mesiobuccal cusp tip of left fourth premolar or first molar. The distances between implants adjacent to the first premolar (DIPM1) were measured. Because the hard palate of beagle dogs was flat and had a low thickness (about 1 mm), horizontal images can be obtained using 3D adjustment by passing through the hard palate to a maximum extent. The distance between each marking implant on both sides of the midpalatal suture between the first premolars was measured. The distance of the same pair of implants can be measured and compared at different times. The distances between implants adjacent to the fourth premolar (DIPM4) were measured. The distance between each marking implant on both sides of the midpalatal suture between the fourth premolars was measured on a horizontal image. The distance between the same pair of implants was also measured and compared at different times. Angulations of the fourth premolar and first molar (APM4, AM1) were measured. The measurement of the angle was obtained through the long axis of the fourth premolar or the first molar (from the tip end to the apex). The axial plane of the 2D coronal images was made through the distal apex of the fourth premolar or the first molar. The bilateral
angles of the fourth premolar and the first molar were measured and then averaged. All CBCT measurements were taken 3 times by a single-blinded examiner (Z.M.) under the same conditions and then averaged. Intraexaminer reliability was within 6 5% for all measurements, as determined by data replications taken 2 weeks apart. Method error was calculated according to Dahlberg's formula. After 15 days, 29 CBCT radiographs were re-measured in order to evaluate intraexaminer measurement error (DC, 1.356 mm; DPM4, 1.65 mm; DM1, 1.675 mm; DIPM1, 0.863; DIPM4, 0.969; APM, P1.658; AM1, 1.743) using Dahlberg's formula, (S2 5 d2/2n).12 One percent tetracycline hydrochloride solution was intramuscularly injected (1 ml/kg) into dogs of subgroup A at the beginning of placement and 3 days before the end of expansion, whereas for dogs in subgroup B it was injected 3 days before the completion of extension and 3 days before the end of the retention period. Subgroup A was killed during the T1 phase and subgroup B during the T2 phase through an overdose of sodium pentobarbital. Bone tissue between the second and third premolars of the maxilla was quickly resected in order to remove surface mucosa and muscles and then fixed in 70% ethanol for 3 days. The tissue was gradually dehydrated using alcohol (80%, 90%, 95%, and 100% for 12 h). After the xylene was transparent, the tissue was immersed in permeates I, II, and III for 24 hours, then embedded in methyl methacrylate. An SP1600 hard tissue microtome was used to obtain slices that were perpendicular to the tarsal plate at the horizontal line of the third premolar, with a thickness of 30 mm. After polishing using P1200-P2000 sandpaper, the
American Journal of Orthodontics and Dentofacial Orthopedics
February 2020 Vol 157 Issue 2
Tong et al
198
Fig 3. Histomorphometric measurements. A, Distance between the 2 marker bands; B, sutural gap width; C, IOD of 5 randomly selected areas of the new bone area along the midpalatal suture.
nondecalcified bone tissue grinding plate was made by removing the knife marks that were on the surface of the tissue. The grinding plate was magnified 40 times under a fluorescence microscope (with the wavelength of 360-400 nm) for observation and photographing. Image-Pro Plus image analysis software (version 6.0; Media Cybernetics, Inc, Md) was used to analyze the images, which were taken under the fluorescence microscope. A blinded evaluator (X.Z.) traced the bone labels. The widest 3 pairs of fluorescence bands were selected from the edge of the midline palatine suture and were measured in each sample, and each pair of fluorescence bands were automatically measured at each 10-mm interval between the leading edges of the label pairs. The distance between the frontal edges of 2 bands was measured in triplicate and averaged (Fig 3, A). By measuring the distance between the 2 marker bands, the mineralization and deposition rate of the new bone during the expansion period can be calculated. Therefore, the mineralization and deposition rate was equal to the distance between the 2 marker bands at 35 days. Sutural gaps were measured 3 times at each 10-mm interval between the 2 trailing edges of the bone labels, along the midpalatal suture (Fig 3, B), for an average to be obtained. Eventually, ImageJ 1.48 imaging analysis software (National Institutes of Health, Bethesda, Md) was used to measure the tetracycline fluorescence optical density (IOD) of 5 randomly selected areas (84 mm 3 84 mm) of new bone along the midpalatal suture in each bone slice (Fig 3, C), and then averaged. Statistical analysis
The SPSS 19.0 software (IBM, Armonk, NY) was used for statistical analysis in this study. The data are expressed as mean 6 standard deviation. One way analysis
February 2020 Vol 157 Issue 2
of variance was used to compare the CBCT measurements of the 3 groups, in order to obtain phase differences at T0-T1 and T1-T2. In addition, the independent-samples t test was used to compare tetracycline fluorescence markers between 2 groups and 1way analysis of variance was used to compare the 3 groups. Repeated measurements of indicators among multiple groups were compared using repeated measurements of analysis of variance. A P value of \0.05 was considered statistically significant. RESULTS
The force-deflection curves of MPEA after 5 loadings of a distance of 1.5 mm were similar. This suggests that the mechanical properties of the MPEA were approximately the same after each activation. The range of forces at intervals between 0 and 1.5 mm varied from 10.7 to 5.5 N, respectively. All 36 beagle dogs survived until the T2 period. In the experimental group, the dogs lost weight within 1 week of expansion, which may be related to eating during the initial period of wearing the expansion appliance, which returned to normal after the second week. A few implants fell off during the experiment. However, there were no dogs that lost both implants on the same side of the midpalatal suture, adjacent to the first or fourth premolars. Therefore, there was no effect on the measurements. According to the CBCT examination, the midpalatal sutures of the ME and SE groups had expanded during T1 (Fig 4). As shown in the Table I, the measurement parameters of both experimental groups had significantly increased after 5 weeks of expansion. Except for APM4 and AM1 in the control group, all other items had slightly increased during the T1 period. After 8 weeks, except for a small increase in DPM4 and APM4 of the
American Journal of Orthodontics and Dentofacial Orthopedics
Tong et al
199
Fig 4. CBCT images showing the widening of the midpalatal suture in the ME and SE groups after expansion. A, ME group at T0; B, SE group at T0; C, control group at T0; D, ME group at T1; E, SE group at T1; F, control group at T1. The yellow arrows show midpalatal sutures.
control group during the T2 period, the other groups showed no significant changes in measurement. A comparison of the changes in the value of parameters of the different periods shows that the difference between the 2 experimental groups during the T1-T0 period was larger than their difference with the control group (P \0.01), but no statistical difference was found between the ME and SE groups (Table II). A comparison between the 2 groups during the T2-T1 period showed that differences in DPM4 and APM4 of the control group were higher than when compared with the ME or SE groups. No statistical difference was observed in the values of other parameters between the groups. The midpalatal suture was significantly enlarged in the SE and ME groups during the T1 period, as seen under a fluorescence microscope. Several large continuous yellow-green or gold-yellow fluorescent bands were visible near the midpalatal suture, representing the frontier of new bone formation. The new bone at the midpalatal suture showed a long finger-like protrusion pointing to the middle (Fig 5, A and B), whereas the
midpalatal suture was curved and twisted and was small in width, and the tetracycline labeling band was narrower in the control group. The fluorescence intensity of tetracycline was less evident than that of the ME and SE groups (Fig 5, C). The width of the midpalatal suture decreased slightly in the control group, whereas it significantly decreased in the ME group and SE group during the T2 period (Fig 5, D-F). The measurements of the tetracycline fluorescence markers in each group are shown in Tables III and IV. The width of the palatal suture, mineralization and deposition rate of the bone, and IOD during the T0-T1 period were significantly different (P \0.05). Further comparison between the 2 groups showed that these 3 parameters were statistically different between the control and experimental groups as well. The value was minimum in the control group, and no significant difference was found between the ME and SE groups, while the width of palatal suture during the T1-T2 period was significantly different (P \0.05). Additional comparisons between the 2
American Journal of Orthodontics and Dentofacial Orthopedics
February 2020 Vol 157 Issue 2
Tong et al
200
Table I. CBCT measurements at different time point for ME, SE, and control groups DC (mm)
DPM4 (mm)
DM1 (mm)
DIPM1 (mm)
DIPM4 (mm)
APM4 ( )
AM1 ( )
Group Control SE ME Control SE ME Control SE ME Control SE ME Control SE ME Control SE ME Control SE ME
T0 31.26 6 1.49 32.21 6 1.67 32.51 6 2.17 43.86 6 2.84 46.32 6 2.56 46.25 6 1.52 46.07 6 2.87 48.08 6 2.73 47.87 6 1.32 13.08 6 0.31 11.9 6 1.13 11.53 6 1.88 7.83 6 4.65 13.55 6 2.62 12.68 6 1.55 86.17 6 3.70 86.69 6 2.66 86.83 6 2.83 79.00 6 6.81 74.83 6 2.28 77.01 6 1.87
T1 31.32 6 1.59 38.61 6 1.71 39.45 6 3.07 44.35 6 3.02 52.90 6 2.15 51.88 6 2.68 46.66 6 2.98 55.00 6 2.96 53.58 6 2.78 13.23 6 0.36 15.17 6 0.60 13.86 6 2.69 8.01 6 4.77 16.7 6 2.04 15.68 6 1.10 87.17 6 3.91 91.29 6 2.48 90.57 6 3.77 78.08 6 6.27 79.33 6 3.15 80.11 6 3.52
T2 31.31 6 1.67 38.44 6 1.86 39.89 6 3.05 44.89 6 3.22 53.13 6 2.12 52.87 6 2.85 46.89 6 3.03 55.14 6 2.96 54.5 6 2.70 13.23 6 0.29 15.19 6 0.63 14.05 6 2.71 8.04 6 4.71 16.81 6 2.20 15.92 6 1.66 88.73 6 4.46 90.86 6 2.70 90.4 6 3.32 78.83 6 6.04 79.08 6 3.32 80.53 6 2.89
Note. Values are mean 6 standard deviation.
Table II. The comparison of absolute changes (T1-T0 and T2-T1) among the 3 groups Variables T1-T0 Difference in DC (mm) Difference in DPM4 (mm) Difference in DM1 (mm) Difference in APM4 ( ) Difference in AM1 ( ) Difference in DIPM1 (mm) Difference in DIPM4 (mm) T2-T1 Difference of DC (mm) Difference of DPM4 (mm) Difference of DM1 (mm) Difference of APM4 ( ) Difference of AM1 ( ) Difference of DIPM1 (mm) Difference of DIPM4 (mm)
Control
SE
ME
0.06 6 0.19 0.54 6 0.52 0.5 6 0.5 0.43 6 1.84 0.1 6 1.38 0.16 6 0.2 0.17 6 0.18
6.4 6 0.9* 6.4 6 0.58* 6.58 6 0.73* 3.45 6 1.83* 2.66 6 2.55* 2.87 6 0.81* 3.34 6 1.83*
6.63 6 0.97* 5.85 6 0.94* 5.98 6 1.35* 3.59 6 1.35* 3.63 6 2.69* 2.54 6 0.94* 2.96 6 0.88*
0.11 6 0.21 0.57 6 0.39 0.06 6 0.76 1.56 6 1.37 0.44 6 0.89 0.02 6 0.11 0.01 6 0.08
0.02 6 0.06 0.14 6 0.18* 0.14 6 0.21 -0.43 6 0.87* -0.25 6 0.46 0.02 6 0.11 0.01 6 0.16
0.03 6 0.26 0.15 6 0.27* 0.92 6 0.89 -0.17 6 1.05* 0.09 6 0.66 0.05 6 0.51 0.04 6 0.55
F 271.6 250.2 156.5 13.4 7.6 46.2 70.4 0.374 4.385 2.893 5.637 1.452 0.017 0.014
P \0.001 \0.001 \0.001 \0.001 0.002 \0.001 \0.001 0.694 0.032 0.087 0.015 0.265 0.983 0.986
Note: Values are mean 6 standard deviation. *Indicates significant comparison with the control group, P \ 0.05.
groups showed that these 3 parameters were statistically different between the experimental group and control group, in which the value was minimum in the control group, and there was no significant difference between the ME group and SE group. There was no statistical difference in the mineralization and deposition rate of the bone and IOD between the control group and experiment group (P .0.05). It was shown that the width of the palatal suture,
February 2020 Vol 157 Issue 2
mineralization and deposition rate of the bone, and IOD of the ME and SE groups were significantly different during different periods and that these values were smaller in the ME and SE groups during the T1T2 period. In the control group, the width of the palatal suture was significantly different (P \0.05), with the minimum value seen at T2; however, there was no significant difference in the mineralization and deposition rate of the bone and IOD (P .0.05).
American Journal of Orthodontics and Dentofacial Orthopedics
Tong et al
201
Fig 5. Fluoroscopic images. A, ME group at T1; B, SE group at T1; C, the control group at T1, D, ME group at T2, E, SE group at T2; F, control group at T2 (S: midpalatal suture, N: new bone; Scale bar 5 200 mm).
Table III. Intergroup comparisons of the changes of tetracycline fluorescence detection parameters during from T0-
T1 and from T1-T2 Parameters T0-T1 Midpalatal suture width Bone mineral deposition rate IOD T1-T2 Midpalatal suture width Bone mineral deposition rate IOD
ME
SE
Control
F
P
294.55 6 62.34 26.41 6 2.48 1187.85 6 184.67
292.64 6 47.60 29.32 6 3.42 1128.58 6 141.52
65.44 6 9.63*,y 1.37 6 0.5*,y 844.48 6 136.19*,y
50.020 235.085 8.343
0.001 \0.001 0.004
124.81 6 15.64 2.36 6 0.76 938.58 6 71.26
123.78 6 15.64 1.84 6 0.18 934.07 6 134.02
51.538 2.986 0.632
\0.001 0.089 0.548
43.44 6 0.35*,y 1.59 6 0.19 866.22 6 116.87
Note: Values are mean 6 standard deviation. For T0-T1, the dyes were administered at the beginning of placement and 3 days before the end of expansion. For T1-T2, the dyes were administered 3 days before the completion of extension and 3 days before the end of the retention. *Indicates a significant comparison with ME group, P \ 0.05; yIndicates a significant comparison with SE group, P \ 0.05.
DISCUSSION
It is of vital importance to maintain a sustainable, stable, and gentle force during orthodontic procedures. The biggest difference between the ME and SE groups is that the magnetic force generated by the former is sustainable and gentle, whereas the mechanical force generated by the latter is an intermittent heavy force.12 However, the most important weakness of ME is that as the dental arch expands and the distance between the magnetic poles increases, causing the magnetic force to attenuate too fast to achieve expansion. In order to obtain a more stable magnetic expansion force, some
researchers have added resin rings or magnet blocks in order to reclose the distance between the magnetic poles and maintain a stable orthodontic force.4,7 However, these methods can be complicated for clinical application, and may also be unhygienic. Although the previous MPEA with a reactivation system simplified the repetitive loading operation, the size of the appliance was still large, resulting in the rapid attenuation of the arch force (range, 10.38-1.63 N), whereas the working range was limited (6 mm) and the reactivation operation was still complicated.8 In the present study, the novel MPEA was succinctly designed with smaller size, whereas the
American Journal of Orthodontics and Dentofacial Orthopedics
February 2020 Vol 157 Issue 2
Tong et al
202
Table IV. Comparison of the changes of tetracycline fluorescence measurements between T0-T1 and T1-T2 Grouping ME Midpalatal suture width Bone mineral deposition rate IOD SE Midpalatal suture width Bone mineral deposition rate IOD Control Midpalatal suture width Bone mineral deposition rate IOD
T0-T1
T1-T2
t
P
294.55 6 62.34 26.41 6 2.48 1187.85 6 184.67
124.81 6 15.64 2.36 6 0.76 938.58 6 71.26
6.469* 22.695 3.085
\0.001 \0.001 0.012
292.64 6 47.60 29.32 6 3.42 1128.58 6 141.52
123.78 6 15.64 1.84 6 0.18 934.07 6 134.02
8.160* 17.803 2.324
\0.001 \0.001 0.045
65.44 6 9.63 1.37 6 0.5 844.48 6 136.19
43.44 6 0.35 1.59 6 0.19 866.22 6 116.87
5.587* 0.84 0.26
0.003 0.425 0.801
Note: Values are mean 6 standard deviation. *Indicates significant corrected t test, P \ 0.05.
expansion force generated by the novel MPEA was more stable (10.7-5.5 N), with a larger working range (7.5 mm), and the reactivation operation was simplified. Its performance was comprehensively evaluated by combined CBCT measurements and tetracycline fluorescence labeling. According to the CBCT measurements, the maxillary width can be effectively expanded by the methods used in the ME and SE groups. The measurements of the ME and SE groups significantly increased during the expansion period, and this increase was larger than that of the control group. At the same time, no statistical differences were observed in the increase of DC, DPM4, and DM1 between the 2 experimental groups, indicating that the effects of these 2 methods were similar regarding the total amount of expansion. The maxillary expansion effects included skeletal and dental effects. In this study, 4 pairs of nonmagnetic metal bone marker implants were placed on either side of the midpalatal suture to prevent a loss of experimental data that may be caused by the loosening of implants. The change in the distance between bilateral implants reflects the amount of palatal suture expansion, that is, the skeletal effect. In the previous study, the skeletal effect of the ME appliance was smaller than that of the SE appliance, whereas in the present study, the increment of DIPM4 in the SE group was 3.34 mm, and was 6.40 mm for DPM4, indicating that the skeletal expansion effect (the expansion of midpalatal suture) was 52.2% of the total expansion effect. In the ME group, the increment of DIPM4 was 2.96 mm and DPM4 was 5.85 mm, showing that the skeletal expansion effect was 50.6% of the total expansion effect, indicating that there was no significant difference in comparison with the SE group, implying that the skeletal effect of the novel MPEA was similar to that of the
February 2020 Vol 157 Issue 2
SE appliance. This effect may be related to a relatively small magnetic force and faster attenuation of the previous magnetic expander, whereas the magnetic force exerted in this study was relatively stable. By simulating clinical expansion using the method of 3D finite element analysis, Gautam et al13 reported that the midpalatal suture opened in a wedged shape, with a wider anterior separation and narrower posterior separation, which was similar to results of studies conducted by Vardimon et al4 and Gabriel et al.14 However, in our study, the increase in DIPM1 was slightly lower than that of the DIPM4 in the ME and SE groups, indicating that the midpalatal suture expanded in parallel or in a wedged shape, with a narrower anterior separation and a wider posterior separation, similar to that reported by Habersack et al15 and Christie et al.16 In their opinion, linear expansion of the midline suture was related to rigidity, position of the expander, as well as the patient's age. The maximum expansion of the midline palatine suture was close to the center of the expander in this study. During the retention period, both the SE appliances and MPEA had a positive effect without releasing an expansion force. There was no statistical difference in the measurement of all parameters in the ME and SE groups, indicating that the SE and ME appliances can be used as retainers to maintain the expansion effect effectively. However, when Lione et al17 performed SE on 17 patients, the width of the midpalatal suture increased at the end of expansion; after 6 months of retention, the width had not significantly changed from what it was before treatment. Thus, they assumed that the skeletal expansion effect of the midpalatal suture had undergone a large number of relapses, which is inconsistent with the results of our study. These differences may be related to the method of activation used. Lione et al17 turned the screw expander for an expansion
American Journal of Orthodontics and Dentofacial Orthopedics
Tong et al
of 0.5 mm each day and continuously reactivated for 14 days, which resulted in a total expansion of 7 mm. However, in our study the SE and ME appliances were applied at longer intervals of 1.5 mm per week, and a total expansion of 7.5 mm occurred over 5 consecutive weeks, which resulted in slower expansion, and the expansion force was relatively gentler, thereby reducing trauma to the midpalatal suture, as well as being beneficial for new bone formation. Secondly, the measurement method was different. Lione et al17 used computed tomography to measure the width of the anterior nasal spine, the posterior margin of the incisor foramen, and posterior nasal spines at the horizontal level of the palate plate, in order to represent the width of the midpalatal suture. However, after 6 months of expansion, the decrease in width of the midpalatal suture was accompanied by new bone formation and mineral deposition and did not imply relapse. In addition, because it was difficult to locate the left and right anterior nasal spine, the posterior edge of the incisor foramen, and the posterior nasal spine, we used the implants as a bone marker on both sides of midpalatal suture, making the measurements more repeatable and comparable. In our study, tetracycline fluorescence labeling was used for the evaluation of the skeletal effect of expansion, to complement the CBCT measurements. Several large and continuous tetracycline fluorescent bands were observed in the ME and SE groups near the midpalatal suture under a fluorescence microscope, and the new bone at the midline of palatal suture showed a long finger-like protrusion pointing to the middle, whereas there was no expansion in the control group. In addition, the tetracycline fluorescent bands at the edge of the palatal suture were formed because of natural growth and reconstruction of bone tissue, so the fluorescence band was narrow. At the end of expansion, the width of the palatal suture was greater in the ME and SE groups, in comparison with that of the control group, indicating that these 2 methods can effectively expand the midpalatal suture, which was consistent with CBCT measurements. After the retention period, the width of the midpalatal suture reduced in all 3 groups, whereas the width of the control group was smaller than that of the experimental groups. When no major change in the distance between implants was observed using CBCT measurements, a decrease in the width of the palatal suture, as represented by the intertetracycline bandwidth was observed. This finding indicates the presence of mineralization and deposition of the bone on both sides of the midline suture during the retention period. At the same time, there was no significant difference between the midpalatal suture width of the SE and
203
ME groups during the retention phase, indicating that the amount of bone deposition on both sides of the midline of the magnetic group was similar. At the same time, tetracycline fluorescence IOD was also used to evaluate new bone formation at the edge of the palatal suture.18,19 In this study, during the expansion period, both the mineralization and deposition rate of the bone and IOD were significantly higher in the ME and SE groups compared with that of the control group, indicating that the mineralization and deposition rate of the bone at the edge of the midline suture can be accelerated through expansion, thereby promoting osteogenesis. There was no statistical difference in the mineralization and deposition rate of the bone between the ME and SE groups, suggesting that the 2 expansion methods have similar effects in stimulating osteogenesis. During the retention period, the mineralization and deposition rate of the bone in the 2 experimental groups decreased significantly, and there was no statistically significant difference with that of the control group, indicating that the rate of osteogenesis during the retention period had decreased to normal. However, Ekstrӧm20 used a radioisotope of Iodine as a radiation source to evaluate bone formation using occlusal radiographs. The measurements showed that the mineralization rate within the suture rose rapidly during the first month after expansion. Vardimon21 studied the mineralization pattern of the midpalatal suture in cats by measuring it on occlusal radiographs. The results indicate that reorganization of the mineralized tissue and acceleration of mineralization rate during the retention period had taken place. The results were different from that of our study. There are 2 possible reasons for the differences in the findings: first, the expansion frequency was different and the studies by Ekstrӧm20 and Vardimon21 applied rapid palatal expansion. In this study, palatal expansion was slow since the expanders were activated once a week with an expansion of 1.5 mm per activation. Second, the examination methods were different. In previous studies, mineralization rate was evaluated by measuring changes in the midpalatal width and the optical density using X-ray films. It is difficult to detect new bone through images at the beginning of the mineralization process, but tetracycline fluorescence labeling is comparatively a more sensitive detection method. One of the potential limitations of this study is that the sample size in each subgroup is relatively small. Therefore, further research with larger sample sizes is needed to obtain more information about the expansion and retention efficiency of MPEA. In addition, this is part of a series of studies, and follow-up studies on the comparison of the skeletal effect between an MPEA group
American Journal of Orthodontics and Dentofacial Orthopedics
February 2020 Vol 157 Issue 2
Tong et al
204
and a palatal expansion group with miniscrews will produce more interesting results. CONCLUSIONS
(1) (2)
(3) (4)
The novel MPEA with a reactivation system can effectively expand the midpalatal suture. The dental and skeletal expansion effects of the novel MPEA with a reactivation system are similar to those obtained with a screw expansion appliance. Wearing the appliance as a retainer can effectively maintain the expansion effect. The new bone formation rate was accelerated during the expansion process and decreased to normal levels during the retention period.
ACKNOWLEDGMENTS
The authors thank Associate Professor Huiqiang Yu of the Public Health College of Nanchang University for his assistance in planning and processing the statistical analyses of this study. REFERENCES [1]. Sun Z, Smith T, Kortam S, Kim DG, Tee BC, Fields H. Effect of bone thickness on alveolar bone-height measurements from cone-beam computed tomography images. Am J Orthod Dentofacial Orthop 2011;139:e117-27. [2]. Rungcharassaeng K, Caruso JM, Kan JY, Kim J, Taylor G. Factors affecting buccal bone changes of maxillary posterior teeth after rapid maxillary expansion. Am J Orthod Dentofacial Orthop 2007;132:428e1-8. [3]. Asanza S, Cisneros GJ, Nieberg LG. Comparison of Hyrax and bonded expansion appliances. Angle Orthod 1997;67:15-22. [4]. Vardimon AD, Graber TM, Voss LR, Verrusio E. Magnetic versus mechanical expansion with different force thresholds and points of force application. Am J Orthod Dentofacial Orthop 1987;92: 455-66. [5]. Kawata T, Hirota K, Sumitani K, Umehara K, Yano K, Tzeng HJ, et al. A new orthodontic force system of magnetic brackets. Am J Orthod Dentofacial Orthop 1987;92:241-8. [6]. Vardimon AD, Graber TM, Voss LR. Stability of magnetic versus mechanical palatal expansion. Eur J Orthod 1989;11:107-15.
February 2020 Vol 157 Issue 2
[7]. Darendeliler MA, Strahm C, Joho JP. Light maxillary expansion forces with the magnetic expansion device. A preliminary investigation. Eur J Orthod 1994;16:479-90. [8]. Tong F, Liu F, Liu J, Xiao C, Liu J, Wu J. Effects of a magnetic palatal expansion appliance with reactivation system: an animal experiment. Am J Orthod Dentofacial Orthop 2017;151:132-42. [9]. Linder-Aronson S, Lindgren J. The skeletal and dental effects of rapid maxillary expansion. Br J Orthod 1979;6:25-9. [10]. Hicks EP. Slow maxillary expansion. A clinical study of the skeletal versus dental response to low-magnitude force. Am J Orthod 1978;73:121-41. [11]. Lee K, Sugiyama H, Imoto S, Tanne K. Effects of bisphosphonate on the remodeling of rat sagittal suture after rapid expansion. Angle Orthod 2001;71:265-73. [12]. Noar JH, Evans RD. Rare earth magnets in orthodontics: an overview. Br J Orthod 1999;26:29-37. [13]. Gautam P, Valiathan A, Adhikari R. Stress and displacement patterns in the craniofacial skeleton with rapid maxillary expansion: a finite element method study. Am J Orthod Dentofacial Orthop 2007;132:5.e1-11. [14]. da Silva Filho OG, Lara TS, de Almeida AM, da Silav HC. Evaluation of the midpalatal suture during rapid palatal expansion in children: a CT study. J Clin Pediatr Dent 2005;29:231-8. [15]. Habersack K, Karoglan A, Sommer B, Benner KU. High-resolution multislice computerized tomography with multiplanar and 3-dimensional reformation imaging in rapid palatal expansion. Am J Orthod Dentofacial Orthop 2007;131:776-81. [16]. Christie KF, Boucher N, Chung CH. Effects of bonded rapid palatal expansion on the transverse dimensions of the maxilla: a cone-beam computed tomography study. Am J Orthod Dentofacial Orthop 2010;137:S79-85. [17]. Lione R, Ballanti F, Franchi L, Baccetti T, Cozza P. Treatment and posttreatment skeletal effects of rapid maxillary expansion studied with low-dose computed tomography in growing subjects. Am J Orthod Dentofacial Orthop 2008;134:389-92. [18]. Yu BH, Zhou Q, Wang ZL. Comparison of tissue-engineered bone from different stem cell sources for maxillary sinus floor augmentation: a study in a canine model. J Oral Maxillofac Surg 2014; 72:1084-92. [19]. Schliephake H, Neukam FW, Hutmacher D, W€ ustenfeld H. Experimental transplantation of hydroxylapatite-bone composite grafts. J Oral Maxillofac Surg 1995;53:46-51: discussion 52. [20]. Ekstr€ om C, Henrikson CO, Jensen R. Mineralization in the midpalatal suture after orthodontic expansion. Am J Orthod 1977;71: 449-55. [21]. Vardimon AD, Brosh T, Spiegler A, Lieberman MM, Pitaru S. Rapid palatal expansion: part 1. Mineralization pattern of the midpalatal suture in cats. Am J Orthod Dentofacial Orthop 1998;113:371-8.
American Journal of Orthodontics and Dentofacial Orthopedics