Abstract
Conventional rapid palatal expansion (RPE) has been proven to be a reliable treatment for correcting transverse maxillary deficiency in young patients. However, side effects including dental tipping and risk of periodontal problem limited its application to young patients after the pubertal growth spurt. Surgically assisted rapid palatal expansion (SARPE), a supplement to RPE, could be applied in skeletally mature patients. However, SARPE was an invasive method, and the morbidity, risks and cost related to surgical treatment might discourage many adult patients. The use of Microimplant-Assisted Rapid Palatal Expansion (MARPE) appliance, which can potentially avoid surgical intervention, is gaining popularity in treatment of maxillary transverse deficiency (MTD) in young adolescent patients. However, the literature on the skeletal and dentoalveolar changes with this appliance is scarce. To evaluate the immediate skeletal and dentoalveolar changes in the transverse dimension with the maxillary skeletal expander (MSE), a MARPE appliance with hybrid anchorage, using cone-beam computed tomography (CBCT). Twenty-two patients (11 males and 11 females, mean age 14.97 ± 6.16 years) with transverse maxillary deficiency were treated using the MSE (Biomaterials Korea, Inc., Seoul, Korea). The appliance consisted of a central expansion screw that were welded to four tubes that served as guides for microimplant placement. The microimplants were 1.8 mm in diameter and 11 mm in length. The longer length of microimplants permitted bicortical engagement of the palatal and nasal floor, reducing the force transmitted to the anchored teeth during expansion. The appliance activation varied with age and skeletal maturity of the patient. The expansion was terminated when 2–3 mm of overexpansion was achieved. CBCT scans were taken before treatment (T1) and immediately after expansion (T2). Measurements were taken to evaluate the amount of total expansion, skeletal expansion, and angular dental tipping at the first molar region. A total expansion of 5.41 ± 2.18 mm was achieved, 59.23 ± 17.75% of which was attributed to skeletal expansion (3.15 ± 1.64 mm) with the first molars exhibiting buccal tipping of 2.56 ± 2.64°. The use of MARPE appliances such as MSE can be used to correct transverse maxillary deficiency in adolescent patients with minimal dentoalveolar side effects.
Introduction
The use of maxillary expansion appliance could be dated back to 1860s, when Angell came up with the concept “maxillary expansion”, which opened the mid-palatal suture laterally by a palatal expansion appliance. Haas introduced the Haas Expander to United States in 1956 and was the first to report on the increase in nasal width and arch perimeter with maxillary expansion. This technique was soon accepted by clinicians in patients with growth potential. However, the use of RME was less predictable in patients over 15-years-old due to the fact that this was a tooth-borne anchorage device. Expansion performed after the peak pubertal growth spurt will lead to more dental than skeletal changes with side effects of buccal dental tipping and often a downward and backward rotation of the mandible. Moreover, the skeletal effect was limited to around 4 mm due to the fact that transverse expansion would be inevitably compensated by dental tipping. It was reported that skeletal expansion only accounts for about 38% of total expansion, and the recurrence rate was 35%∼50%.
Brown introduced the concept of surgically assisted rapid palatal expansion (SARPE) in 1938 and then SARPE procedure gradually became the main treatment modality for adult patients with maxillary transverse discrepancy. The advantages of treatment with SARPE were predictable skeletal and dental changes, and a low rate of relapse (5%∼25%). However, many adult patients were discouraged from choosing this treatment due to the invasive nature, risks, complications and cost of the surgical procedure.
The introduction of the Microimplant Assisted Rapid Palatal Expansion (MARPE) appliances provided a new alternative treatment modality for clinicians and adolescent patients with maxillary transverse deficiency. Three dimensional finite element analysis showed that these devices had greater skeletal and less dental side effects than traditional RME. Lin et al. compared tooth-borne versus bone-borne rapid maxillary expanders and concluded that bone-borne expanders produced greater orthopedic effects in late adolescent patients. Carlson et al. reported a 19-year-old male with severe constricted maxillary arch treated with the MARPE technique. Cunha et al. reported success in treating a 24-year-old female with maxillary transverse deficiency and Class II malocclusion using the MARPE appliance therefore avoiding the need of a surgical procedure. Choi et al. conducted a larger study using MARPE in 69 young adult patients and reported success in opening the mid-palatal sutures in 86.96% of the subjects. Park et al. reported success in obtaining sutural expansion in 14 young adult patients treated with the MARPE appliance and found a decrease in buccal bone thickness and height of alveolar crest using CBCT. These studies suggested that MARPE appliances can correct transverse maxillary deficiency in young adolescent patients but there was a lack of literature about the skeletal and dental effect of these appliances. The objective of this study was to evaluate the immediate skeletal and dentoalveolar changes in the transverse dimension with the maxillary skeletal expander (MSE), a MARPE appliance with hybrid anchorage, using CBCT.
Materials and methods
Subjects
A total of 22 patients from the Department of Orthodontics, School of Dentistry, West Virginia University and the Department of Orthodontics, Hospital of Stomatology, Wuhan University (11 males and 11 females, average age 14.97 ± 6.16 yrs.) were included in the study. All patients initially presented in the clinic with transverse maxillary deficiency and had undergone maxillary expansion by MARPE between February 2016 and June 2018. CBCT images were taken before treatment (T1) and immediately after completion of maxillary expansion (T2). This study was approved by the Institutional Review Board of the Hospital of Stomatology, Wuhan University (Reference #: 2018-B01) and the West Virginia University (Reference #:1501557557).
The inclusion criteria consisted of young adolescent patients with transverse maxillary discrepancy of greater than 4 mm and less than 10 mm treated with the MSE appliance. Consent was obtained from all the subjects. CBCT images were taken before and after expansion with the MSE appliance. The exclusion criteria consisted of subjects diagnosed with congenital facial anomaly or dysmorphism; obvious facial asymmetry; waiving the MARPE therapy for any reasons; with incomplete pre- and post-treatment CBCT scans.
Appliance design
The MSE appliance consisted of a central expansion jackscrew and four attached arms soldered to orthodontic bands on the anchor teeth to facilitate placement of the appliance. Four tubes were welded to the central expansion jack screw that served as guides for microimplant placement ( Fig.1 ). The microimplants were 1.8 mm in diameter and 11 mm in length. The longer length of microimplants allowed bicortical engagement of the palatal and nasal floor, reducing the force transmitted to the anchored teeth during expansion. The palatal microimplants were placed as posterior as possible to maximize the orthopedic force to the pterygoid plates efficiently but never to exceed the palatine process ( Fig.2 ). The microimplants were placed parallel and symmetrical to the mid-palatal suture. The jackscrew was placed as close to the palatal vault as possible without pressing on the soft tissues.
Activation protocol
Every subject began maxillary expansion 2 weeks after the placement of microimplants. The rate of activation depended on patient’s chronological age ( Table1 ). Maxillary expansion should be suspended if patients complained of unbearable pain; if jackscrew was too tight to turn; if one or more microimplants were loose; and if microimplants or linkages were wrapped by soft tissue and there was obvious localized inflammation. Patients were instructed how to activate the expansion and how to flush the palatal region after meal to keep oral hygiene. The turns of expansion were recorded, regular dental visits were required, and every adverse event was reported. The expansion was terminated when 2–3 mm of overexpansion was achieved.
| Age(year) | Rate(turn/day) | |
|---|---|---|
| initial | after opening of the diastema | |
| 13 | 2 | 2 |
| 13–15 | 2–3 | 2 |
| 16–17 | 3 | 2 |
| ≥18 | 3–4 | 2–3 |
CBCT analysis
CBCT scans were taken before treatment (T1) and immediately after expansion with MSE appliance (T2) by the NewTom VGi 9 (Imola, Italy) at Wuhan University and the Carestream Kodak CBCT 9300 (Atlanta, Georgia, USA) at West Virginia University. The exposure time was 3.6 s, scanning time was 18 s, and scanning view was 20 × 25 cm, with minimal layer thickness of 0.3 mm. Patients were instructed to sit up keeping the Frankfort plane paralleled to the ground and clenching on intercuspal position during scanning. The Dicom files were extracted from the CBCT disks using NNT software. All documents were then introduced to the Dolphin 11.9 software.
CBCT data orientation
Three-dimensional model of CBCT was reconstructed in Dolphin software. Specific landmarks which formed the reference planes were determined in the model ( Table2 ). The three dimensional model was then adjusted based on these reference planes ( Fig.3 ). Accurate orientation could enhance the reliability of superimposition.
| Landmarks and reference planes | Definition |
|---|---|
| Nasion | anterior point on nasofrontal suture in the mid-sagittal plane |
| Sella | central point in Sella turcica |
| Basion | anterior point of foramen magnum |
| Porion | superior point on external auditory canal |
| Orbit | The inferior point on orbit |
| ANS | Anterior nasal spine |
| PNS | Posterior nasal spine |
| Horizontal plane | Frankfort plane (FH plane) Or(left)-Po-Or(right) |
| Mid-sagittal plane | Perpendicular to the FH plane, and pass the ANS-PNS |
| Coronal plane | Perpendicular to the FH and sagittal plane |
Diagnosis
PAC (posterior-anterior cephalogram) was reconstructed after orientation. Transverse maxillary deficiency was diagnosed and measured using the method of Betts et al. ( Fig.4 ). The estimated maxillary transverse discrepancy was measured by comparing the distance of JR-JL and AG-GA. The proper size of MSE (8 mm, 10 mm, 12 mm) was selected according to the diagnosis.
CBCT superimposition
Pre- and post-treatment CBCT data were superimposed via Dolphin software by voxel. The anterior cranial base was regarded as the most reliable reference structure, thus in this study the voxel of anterior cranial base was used for superimposition ( Table3 , Fig.5 ). Through superimposition, pre- and post-treatment CBCT data were shown in different colors ( Fig.6 ).
| Selection boundary | |
|---|---|
| Superior(mid-sagittal) | Superior border of ethmoid sinus |
| Inferior(mid-sagittal) | Inferior border of hypophyseal fossa |
| Anterior(mid-sagittal) | Posterior border of frontal sinus |
| Posterior(mid-sagittal) | anterior clinoid process |
| Exterior(horizontal) | Internal ethmoid sinus |
CBCT measurements
All measurements were performed by one of the investigators. Error measurements were performed by repeating the measurements 3 weeks later to test for its reliability. No significant differences were found between the first and second measurements using Intra-Class correlation ( Table4 ).
| Parameter | Intra-class correlation coefficient |
|---|---|
| Pre-treatment maxillary width | 0.987 |
| Post-treatment maxillary width | 0.983 |
| Pre-treatment maxillary molar width | 0.945 |
| Post-treatment maxillary molar width | 0.957 |
| Pre-treatment Torque of right maxillary first molar | 0.963 |
| Pre-treatment Torque of left maxillary first molar | 0.978 |
| Post-treatment Torque of right maxillary first molar | 0.976 |
| Post-treatment Torque of left maxillary first molar | 0.987 |
| Vertical skeletal expansion (nasal floor) | 0.989 |
| Vertical skeletal expansion (palatal floor) | 0.989 |
| Horizontal skeletal expansion (anterior) | 0.988 |
| Horizontal skeletal expansion (posterior) | 0.991 |
Distance of mesiolingual cusps of maxillary first molars
Landmarks were determined in the coronal slice where both the mesiolingual cusp and palatal root apex of the maxillary first molar could be firstly seen when the transverse section moves from the mesial to the distal ( Fig.7 ). The pre- and post-treatment distance was compared. The differences in value was defined as the “ total expansion ” acquired at the maxillary first molar. Total expansion includes skeletal expansion (true extended distant of mid-palatal suture) and dental expansion (dental tipping and alveolar bending).
