The objective was to determine the skeletal and dental changes with microimplant assisted rapid palatal expansion (MARPE) appliances in growing (GR) and nongrowing (NG) patients using cone-beam computed tomography and 3-dimensional imaging analysis.
The sample consisted of 25 patients with transverse maxillary discrepancy treated with a maxillary skeletal expander, a type of MARPE appliance. Cone-beam computed tomography scans were taken before and after maxillary expansion; the interval was 6.0 ± 4.3 months. The sample was divided into GR and NG groups using cervical vertebral and midpalatal suture maturation. Linear and angular 3-dimensional dentoskeletal changes were assessed after cranial base superimposition. Groups were compared with independent-samples t test ( P <0.05).
Both groups displayed marked transverse changes with a similar ratio of skeletal to dental transverse changes and parallel sutural opening from the posterior nasal spine–anterior nasal spine; a similar amount of expansion occurred in the anterior and the posterior regions of the maxilla. The maxilla expanded skeletally without rotational displacements in both groups. The small downward-forward displacements were similar in both groups, except that the GR group had a significantly greater vertical displacement of the canines (GR, 1.7 ±1.0 mm; NG, 0.6 ± 0.8 mm; P = 0.02) and anterior nasal spine (GR, 1.1 ± 0.6 mm; NG, 0.5 ± 0.5 mm; P = 0.004).
Treatment of patients with MARPE appliance is effective in GR and NG patients. Although greater skeletal and dental changes were observed in GR patients, a similar ratio of skeletal to dental transverse changes was observed in both groups.
Microimplant assisted palatal expansion is effective in growing (GR) and nongrowing (NG) patients.
GR patients presented greater dental and skeletal changes compared with NG patients.
A similar ratio of skeletal to dental changes was observed in GR and NG groups.
The use of palatal expanders to achieve maxillary expansion in patients with a transverse discrepancy has been described for over one and a half centuries. Angell was the first to develop and use a palatal expander to achieve maxillary expansion in 1860. In 1961, Haas introduced his version of the palatal expander, which involved an acrylic coverage of the palate. Despite Haas and Hyrax expanders not changing in design during the last 50 years, the recent addition of miniscrews to the Hyrax expander has allowed clinicians to explore a new realm of possibilities for correction of transverse maxillary discrepancies.
Traditionally, maxillary expanders were ideal for patients who were prepubertal and had a transpalatal width less than 33-35 mm, which is the average width for patients with mixed dentition as described by Spillane et al. As patients begin their pubertal growth spurt and transition into early adulthood, the 2 palatal shelves begin to fuse. The fusion of the palatal sutures challenges clinicians treating patients who already have gone through their pubertal growth spurt and yet have a transverse maxillary discrepancy. As these patients mature, the palatal shelves become more interdigitated, resulting in greater dentoalveolar expansion and less skeletal expansion when treated with a traditional expander. , Another factor that should be considered when treating these patients is the resistance of circummaxillary sutures such as zygomaxillary buttress and sphenoidal sutures, which can hinder palatal expansion. ,
An alternative treatment modality for postpubertal patients with a transverse discrepancy that cannot be corrected by orthodontic tooth movement is surgically assisted rapid palatal expansion. Although the success rate for the surgical correction of the transverse discrepancy is high, it is associated with the risks normally encountered with any surgical procedure.
Studies have shown that the expansion achieved with microimplant assisted palatal expander is similar to that of traditional expanders, such as Hyrax, , Haas, and quad-helix appliances, with about 40% of expansion being skeletal, 20% alveolar, and 40% dental tipping. , In theory, when a microimplant assisted rapid palatal expansion (MARPE) is activated, the forces of the expander are transmitted through the miniscrews to the palate, resulting in more skeletal expansion and less dental expansion, a finding that suggests better long-term stability. Moreover, MARPE appliances are a clinically acceptable nonsurgical treatment option to correct mild-to-moderate transverse discrepancies for skeletally mature patients.
For this study, an important distinction must be made. MARPE is an umbrella term for any appliance that uses miniscrews to help achieve orthopedic expansion. The maxillary skeletal expander (MSE) is a hybrid appliance with bone and tooth anchorage. In contrast, bone-anchored expanders with absolute bone anchorage do not use any dental anchorage but rely solely on miniscrews to achieve orthopedic expansion. With this distinction in mind, this study aimed to determine if MSE can achieve maxillary skeletal expansion in growing (GR) and nongrowing (NG) patients. The significance of this study is that it compares maxillary and dental 3-dimensional (3D) displacements relative to the cranial base in GR and NG patients, using a well-defined 3D analysis.
Material and methods
This study was a secondary data analysis of deidentified cone-beam computed tomography scans (CBCT) of patients with posterior unilateral or bilateral crossbites who underwent treatment with MSE. The study was approved (ID no. HUM 00146140) through the University of Michigan Institutional Review Board process. The sample consisted of CBCT scans taken between 2015 and 2018 and was collected from clinical database archives at the University of West Virginia.
Inclusion criteria for the study included the following: (1) patients with transverse skeletal maxillary discrepancy treated with the MSE appliance. Transverse discrepancy was determined using the maxillomandibular differential index (>16.4 mm in GR patients and >19.6 mm in the NG patients) ; (2) no previous orthodontic treatment; (3) no history of syndrome, trauma, or oral/craniofacial surgery; (4) no congenital facial anomaly or dysmorphism; (5) preexpansion and postexpansion CBCT scans, with the field of view including the cranial base; and (6) adequate scan quality, without movement artifacts.
The final sample consisted of CBCT scans from 25 subjects (12 females and 13 males) who met the inclusion criteria. The skeletal maturation of each subject was assessed using the cervical vertebra maturation (CVM) and midpalatal suture maturation (MSM) methods. The CVM stage was determined by 2 independent observers (J.A.M and L.F), using lateral cephalograms generated from CBCT images. The MSM stage was assessed by 2 independent observers (L.H.S.C and F.A), using standardized multiplanar views of the CBCT scans. Based on these classifications, the study sample was divided into 2 groups: 11 GR (CVM 1-4 and MSM B-C) and 14 NG (CVM 5-6 and MSM D-E) subjects ( Fig 1 ).
The MSE used in this study consists of a central expansion jackscrew with 4 attached arms soldered to orthodontic bands placed on maxillary first molars ( Fig 2 ). The addition of 4 sheaths welded to the body of the central expansion jack screw allowed for the placement of the miniscrews in the roof of the mouth. The miniscrews were 1.8 mm in diameter and varied from 8 to 12 mm in length, depending on what was required to achieve bicortical engagement. The palatal miniscrews were placed symmetrically and parallel to the midpalatal suture (MPS). They were also positioned posteriorly without extending into the palatine processes; maintaining bony anchorage in the hard palate maximizes orthopedic forces to the pterygoid plates. The jackscrew was placed as close as possible to soft tissues, without impinging on them, to avoid entrapment of food particles.
Activation protocol for all subjects began 2 weeks after the placement of the miniscrews. The rate of activation was standardized according to the subjects’ chronological age, as shown in Table I . Patients were taught how to turn the jackscrew to activate the expansion. Furthermore, they were shown how to maintain proper oral hygiene. Patients were seen at regular orthodontic appointment intervals, in which the number of turns was recorded, and the patient could be monitored for any adverse events. The expansion was concluded when the lingual cusp of the maxillary molar contacted the tip of the mandibular molar buccal cusp. If the expansion occurred asymmetrically, it was stopped according to the side that expanded more.
|Age, y||Rate (turn/d)|
|Initial||After opening of the diastema|
The CBCT scans originally were obtained for clinical purposes with a scanning protocol that involved 17 × 23 cm extended field of view, with 0.3-0.4 mm voxel size. The scans were taken before treatment (T1) and after the maxillary expander (T2) removal. The generated digital imaging and communications in medicine files were converted to Guys Imaging Processing Laboratory files used in the open-source software ITK-SNAP. All image analysis steps were performed by 1 examiner (C.M), and each step was performed on all subjects before moving to the next step.
Images that did not have a voxel size of 0.3 mm were resampled to 0.3 mm isotropic voxel size for proceeding segmentation of anatomic structures of interest (CMF Registration, 3D Slicer Software ). Subsequently, 3D image analysis was performed through the following steps:
Construction of 3D volumetric label maps (segmentations) and 3D surface models of T1 scans: Automatic segmentations were generated in 3D Slicer using the intensity segmenter extension. Then, the ITK-SNAP software, contours of the segmentation were edited, cropped, and cleaned. Next, using the extension model maker in 3D Slicer, the T1 segmentations were converted to 3D surface models (.vtk files).
Head orientation using transforms extension: 3D Slicer provides a fixed 3D coordinate system with 3 orthogonal planes denoted by yellow, red, and green , representing sagittal, axial, and coronal planes, respectively. These planes were used as a reference to orient (translate and/or rotate) the T1 model of each patient using glabella, crista galli, and basion used to define the midsagittal plane and the bilateral structures of orbitale (Or) and porion (Frankfort horizontal plane) used to define the axial plane ( Fig 3 ).
Manual approximation of T1 and T2 using transform extension in 3D Slicer: The Guys Imaging Processing Laboratory file scans of T2 were translated and rotated manually to superimpose the T1 and T2 anterior cranial bases.
Construction of 3D volumetric label maps of the approximated T2 scan: The same procedure described in step no. 1 was used to construct T2 segmentations.
Voxel-based registration of scans using the cranial base as reference: The 3D voxel-based registration (CMFReg extension) was used to align the T1 and T2 scans automatically by using corresponding voxels in the cranial base to achieve a reliable and reproducible superimposition of the 2 time points. Once this automated voxel-based registration was completed, the registered files (scans and segmentations) were used for subsequent steps.
Prelabeling landmarks in ITK-SNAP: Anatomic landmarks of interest were prelabeled in ITK-SNAP on the registered T1 and T2 segmentations simultaneously. Sagittal, axial, and coronal slices of the greyscale image and the 3D reconstruction of the image were used for landmark positioning ( Fig 4 ). Landmarks’ definitions are displayed in Table II .
Landmark Location Nasion Most anterosuperior junction of the nasofrontal suture Or Most inferior point of the orbital concavity in a frontal view, centered anterior-posteriorly on the orbital rim from the superior view Zygomatic The greatest point of convexity in which the horizontal and sagittal components of the zygomatic arch intersect in an inferior view Nasal cavity The central point in a frontal view and the most anterior portion of the inferior contour of the NC in a lateral view ANS Most anterior point of the anterior nasal spine in a lateral view PNS Most posterior point of the posterior nasal spine in a lateral view PF The most central point of the PF canal in anteroposterior direction in an inferior view at the palatal level A-point Most posterior point of anterior concavity of maxilla in a lateral view Canine incisal tip Center of the canine tip at most occlusal level Canine root apex Center of the root canal at the most apical level Maxillary molar incisal tip Center of mesiobuccal cusp at occlusal level Mesiobuccal root apex maxillary first molar Center of mesiobuccal root at the most apical level Alveolar bone level at first molar region (buccal) Center of alveolar bone at the gingival margin level
Generation of 3D models (.vtk files) in 3D Slicer: Using the model maker extension in 3D Slicer, 3D surface models were generated from the segmented head and prelabeled landmarks at T1 and T2 for each patient.
Landmark-based quantitative assessments: Using the Q3DC extension, landmarks were placed on the prelabeled models, and displacement of landmarks listed in Table III were reported in anteroposterior, superior-inferior and 3D directions. When needed, midpoints were generated for bilateral landmarks ( Fig 5 ; Table IV ).Table III
Type of measurement Measurements
Maxillary lateral displacements
Difference between T2 − T1 measurements
Distance between right and left Or (Or R -Or L ) Distance between right and left zygomatic (Zyg R -Zyg L ) Distance between right and left nasal cavity (NC R -NC L ) Distance between right and left PF (PF R -PF L ) Distance between right and left canine cusp tip (C R -C L ) Distance between right and left molar cusp tip (M R -M L )
Maxillary anteroposterior, superior-inferior, and 3D displacements
Midpoints were generated for each bilateral landmark, and then the difference was taken from T1 − T2
Or midpoint (Or m ) Zygomatic midpoint (Zyg m ) Nasal cavity midpoint (NC m ) PF midpoint (PF m ) Canine cusp tip midpoint (C m ) Molar cusp tip midpoint (M m ) Anterior nasal spine (ANS, T2 only) Posterior nasal spine (PNS, T2 only) A-point (T2 only)
Palatine plane: anterior and posterior nasal spine (ANS-PNS) Angle formed by the right and left Or-zygomatic lines, in the anterior view (Or R -Zyg R – Or L -Zyg L ) Molar torque: long axis of the molars (M-M′ − M′′-M′′′) Canine torque: long axis of the canines (C R -C′ R − C′′ R -C′′′ R ) Fig 5
Variable First measurement Second measurement Difference ICC Lateral displacements (distance) Or R -Or L 1.2 (0.9) 1.1 (0.9) −0.07 0.994 Zyg R -Zyg L 2.1 (1.5) 2.1 (1.4) 0.03 0.987 NC R -NC L 3.5 (1.4) 3.5 (1.3) 0.02 0.994 PF R -PF L 3.3 (1.2) 3.4 (1.1) 0.14 0.992 C R -C L 3.9 (2.1) 3.9 (2.1) 0.01 1.000 M R -M L 3.2 (1.5) 3.2 (1.6) −0.05 0.999 Sagittal displacements (AP) Or m 0.5 (0.5) 0.5 (0.5) −0.02 0.994 Zyg m 0.9 (1.1) 0.9 (1.1) −0.02 0.998 NC m 0.6 (0.4) 0.7 (0.4) 0.03 0.992 PF m −0.2 (0.1) −0.2 (0.1) −0.01 0.852 C m 1.5 (0.7) 1.6 (0.7) 0.02 0.990 M m 1.8 (0.5) 1.8 (0.6) −0.01 0.986 ANS 0.4 (0.5) 0.4 (0.4) −0.01 0.978 PNS 1.4 (1.0) 1.4 (1.1) 0.04 0.996 A-point 0.8 (0.5) 0.7 (0.5) 0.04 0.997 Vertical displacements (SI) Or m −0.1 (0.3) −0.1 (0.3) −0.02 0.966 Zyg m −0.2 (0.8) −0.3 (0.8) −0.04 0.987 NC m −1.1(1.2) −1.1 (1.3) 0.02 0.996 PF m −1.3 (0.6) −1.3 (0.6) 0.02 0.998 C m −1.1 (1.1) −1.2 (1.1) −0.06 0.994 M m −0.5 (1.1) −0.5 (0.1) 0.00 0.999 ANS −1.2 (0.6) −1.3 (0.5) −0.10 0.969 PNS −1.1 (1.3) −1.0 (1.2) 0.08 0.997 A-point −1.6 (1.1) −1.5 (1.1) 0.04 0.997 Angular changes ANS-PNS 1.4 (1.0) 1.5 (0.9) 0.10 0.989 Or R -Zyg R – Or L -Zyg L 3.2 (1.5) 3.6 (1.2) 0.45 0.896 M R -M′ R – M′′ R -M′′′ R 2.4 (2.5) 2.4 (2.4) 0.06 0.995 M L -M′ L – M′′ L -M′′′ L 4.7 (3.5) 4.8 (3.7) 0.06 0.999 C R -C′ R – C′′ R -C′′′ R 2.9 (2.8) 3.0 (2.1) 0.15 0.997 C L -C′ L – C′′ L -C′′′ L 1.9 (1.8) 1.7 (1.9) 0.14 0.989
Generation of semitransparent overlays and color maps for visualization: Semitransparent overlays of the T1 and T2 models were created for visualization. In addition, using the
Model-to-model distance and shape population viewer, color maps were generated to visualize the changes from T1 to T2 ( Fig 6 ).
Sample size calculation estimated that a minimum of 11 patients in each group was needed on the basis of a power of 0.86, an alpha of 0.05, a mean difference of 1.0 ± 1.0 mm for lateral displacement of the nasal cavity (NC, defined in Table II ).
To measure the study error, the same operator (C.M) performed all measurements on 20% of the sample after a 30-day interval. The intraclass correlation coefficient was used to assess intraobserver repeatability. Chi-square tests assessed differences in gender distribution between GR and NG groups. Differences in skeletal and dental measurements also were tested between the 2 groups, at baseline and after expansion, using an independent-samples t test.
All statistical analyses were performed using IBM SPSS software (version 26.0; IBM, Armonk, NY). The level of significance was set at 5% ( P <0.05).
The sample comprised 11 GR and 14 NG patients. The mean age at T1 for the GR and NG sample was 11.9 ± 3.1 years and 19.9 ± 4.8 years, respectively. The mode CVM was 3 for the GR group and 5 for the NG group. Only 2 patients in the NG group were classified as stage E of MPS closure, whereas 12 patients were classified as stage D. The study error and the intraclass correlation coefficient are presented in Table IV . The male and female distribution and maxillary baseline measurements were similar in both groups ( Table V ).
|Variable||GR (n = 11)||NG (n = 14)||P value|
|Initial age, y||11.9 (3.1)||19.9 (4.8)||N/A|
|MPS||B (n = 9), C (n = 2)||D (n = 12), E (n = 2)||N/A|
|CVM||1 (n = 1), 2 (n = 2), 3 (n = 4), 4 (n = 4)||5 (n = 7), 6 (n = 7)||N/A|
|Female||4 (36.4%)||8 (57.1%)|
|Male||7 (63.6%)||6 (42.9%)|
|Or R -Or L||65.5 (6.6)||67.3 (5.8)||0.481 ∗|
|Zyg R -Zyg L||86.9 (4.2)||84.1 (4.6)||0.260 ∗|
|NC R -NC L||21.6 (1.7)||21.6 (2.4)||0.980 ∗|
|PF R -PF L||29.2 (1.7)||29.9 (1.8)||0.354 ∗|
|M R -M L||53.6 (4.1)||53.2 (7.1)||0.862 ∗|
|C R -C L||36.9 (4.8)||35.8 (7.0)||0.637 ∗|
Table VI shows the mean, standard deviation, and significance of all tested variables between the GR and the NG groups. No significant differences were found between the male and female subjects.
|Variable||GR (n = 11)||NG (n = 14)||P value|
|Lateral displacements (distance)|
|Or R -Or L||1.6 (1.2)||0.9 (0.7)||0.06|
|Zyg R -Zyg L||3.5 (2.2)||2.8 (1.8)||0.4|
|NC R -NCL||3.6 (1.5)||1.9 (1.2)||0.006∗|
|PF R -PF L||3.9 (1.3)||2.1 (1.3)||0.022∗|
|M R -M L||5.5 (2.8)||3.6 (2.1)||0.1|
|C R -C L||3.6 (2.4)||2.7 (1.9)||0.4|
|Sagittal displacements (AP)|
|Or m||0.5 (0.6)||0.2 (0. 7)||0.3|
|Zyg m||0.4 (0.1)||0.01 (0.8)||0.2|
|NC m||0.7 (0.5)||0.3 (0.6)||0.05|
|PF m||−0.9 (0.3)||0.002 (0.4)||0.1|
|M m||1.1 (1.1)||0.1 (1.3)||0.9|
|C m||0.6 (1.3)||0.9 (0.9)||0.6|
|ANS||0.5 (1.4)||0.3 (1.2)||0.7|
|PNS||1.5 (1.5)||1.4 (1.9)||0.9|
|A-point||0.4 (0.8)||0.3 (0.5)||0.7|
|Vertical displacements (SI)|
|Or m||−0.03 (0.2)||0.01 (0.3)||0.8|
|Zyg m||−0.2 (0.6)||−0.1 (0.6)||0.6|
|NC m||−0.7 (1.4)||−0.8 (1.2)||0.8|
|PF m||−1.0 (0.7)||−1.01 (1.1)||0.9|
|M m||−0.4 (1.1)||−0.3 (0.9)||0.8|
|C m||−1.7(1.0)||−0.6 (0.8)||0.02∗|
|ANS||−1.2 (0.6)||−0.5 (0.5)||0.004∗|
|PNS||−1.1 (1.5)||−0.8 (1.2)||0.7|
|A-point||−1.5 (1.4)||−1.04 (0.7)||0.3|
|ANS-PNS||1.7 (0.9)||1.1 (0.9)||0.1|
|Or R -Zyg R – Or L -Zyg L||5.1 (3.7)||4.5 (4.6)||0.7|
|M R -M′ R – M′′ R -M′′′ R||4.03 (4.4)||3.2 (2.9)||0.6|
|M L -M′ L – M′′ L -M′′′ L||4.2 (3.6)||3.5 (3.4)||0.6|
|C R -C′ R – C′′ R -C′′′ R||3.1 (2.9)||1.5 (0.1)||0.02∗|
|C L -C′ L – C′′ L -C′′′ L||2.70 (2.0)||1.2 (0.8)||0.04∗|
For lateral displacements, an increase in the transverse skeletal dimension was found in all the tested variables. The transverse skeletal expansion at the NC was 3.6 ± 1.5 mm and 1.9 ± 1.2 mm in the GR and NG groups, respectively. The transverse skeletal expansion at the palatine foramen (PF) was 3.4 ±1.3 mm in the GR group and 2.1 ± 1.3 mm in the NG group. The increase in the transverse maxillary skeletal dimension after treatment with MSE was significantly different between the GR and the NG groups at the NC (NC R -NC L , P <0.05) and PF (PF R -PF L , P <0.05). The transverse dental changes did not vary significantly between the groups; greater expansion was observed at the level of the molars compared with the canines. The transverse dental expansion at the molars was 5.5 ± 2.8 mm and 3.6 ± 2.1 mm in the GR and NG groups, respectively. Although at the canines, the transverse dental expansion was 3.6 ± 2.4 mm and 2.7 ± 1.9 mm in the GR and NG groups, respectively.
To calculate the ratio of skeletal to dental expansion, the amount of expansion at the PF was divided by expansion at the first molar tips. A similar ratio of skeletal to dental transverse changes was observed in GR (62% skeletal and 38% dental) and NG patients (59% skeletal and 41% dental).
For vertical and sagittal displacements, small and insignificant forward and downward movement of the Or, zygoma, NC, maxillary first molars, posterior nasal spine and A-point were found in both groups. Maxillary permanent canine cusp tips (C) and anterior nasal spine presented small insignificant forward displacement in both groups, with greater downward displacement in the GR group (the difference between the groups at the C was 1.1 ± 1.0 mm and 0.7 ± 0.1 mm at the anterior nasal spine; P values were 0.02 and 0.004, respectively).
Regarding the angular measurements, changes in the palatal plane were small in both groups (1.7° ± 0.9° in the NG group and 1.1° ± 0.9° in the GR group). Both groups displayed an increase in the angles between right and left Or-zygomatic lines from pretreatment to posttreatment by around 5°. After expansion, the angular change of the maxillary first molar resulted in buccal tipping of the crown by about 4° in the GR group and 3° in the NG group. In addition, the permanent maxillary canine presented slight buccal tipping of approximately 3° in the GR group and 1.5° in the NG group ( Table VI ).