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 = 12), screwed expansion (SE; n = 12) or control (n = 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.
Highlights
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Modified magnetic palatal expansion appliance with reactivation system was tested in dogs.
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Expansion and retention were evaluated with cone-beam computed tomography and tetracycline fluorescence labeling.
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Dental and skeletal expansion effects were similar to those of screw expansion appliance.
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The magnetic palatal expansion appliance can be worn as a retainer to maintain the expansion effect.
Palatal expansion (PE) is one of the most frequently used orthodontic treatment strategies in the correction of malocclusion. It is applied for the 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. On the other hand, magnetic force expansion is relatively gentle, possessing superior vector control and atraumatic periodontal remodeling. , 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 to an insufficient and unstable expansion force in early expanding appliances, thereby restricting their development. ,
The new magnetic palatal expansion appliance (MPEA) developed through our previous research applies repulsion forces from third generation high-energy rare-earth permanent magnetic alloy neodymium-iron-boron 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. 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%. , 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 al 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 re-convergence 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 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 × 11.5 mm × 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 ). The novel MPEA ( Fig 1 , C ) is composed of 4 magnets (6 mm × 4 mm × 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 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 = 6 per subgroup). The dogs in subgroup A were killed in order to evaluate the expansion effect after 5 weeks of 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 × 16 × 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. The following parameters were measured:
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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.
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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.
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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.
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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.
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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 ± 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, 1.658; AM1, 1.743) using Dahlberg’s formula, (S 2 = ∑d 2 /2n).
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 μm. After polishing using P1200-P2000 sandpaper, the 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-μm 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-μm 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 μm × 84 μm) 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 ± standard deviation. One way analysis 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 1-way 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 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 ).
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