Three-Dimensional Evaluation of Skeletal and Dental Effects of Treatment with Maxillary Skeletal Expansion

Abstract

Objectives:

To determine the skeletal and dental changes with Microimplant Assisted Rapid Palatal Expansion (MARPE) appliances in growing and non-growing patients using cone beam computed tomography (CBCT) and 3D imaging analysis.

Methods:

The sample was comprised of 25 patients with transverse maxillary discrepancy treated with Maxillary Skeletal Expander (MSE, a type of MARPE appliance). CBCT scans were taken before and after maxillary expansion; the interval was 6 ± 4.3 months. The sample was divided into growing and non-growing groups using cervical vertebrae and midpalatal suture maturation. Linear and angular three dimensional dentoskeletal changes were assessed after cranial base superimposition. The treatment groups were compared with Independent Sample t-test (P < .05).

Results:

Both groups displayed marked transverse changes with a similar ratio of skeletal to dental transverse changes and parallel sutural opening from the PNSANS; 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 growing group had a significantly greater vertical displacement of the canines (1.7 ±1.0 mm in growing patients, 0.6 ±0.8 mm in non-growing patients; P = 0.02) and ANS (1.1 ±0.6 mm in the growing group, 0.5 ±0.5 mm in the non-growing group; P = 0.004).

Conclusions:

Treatment of patients with MARPE appliance is effective in growing and non-growing patients. While greater skeletal and dental changes were observed in growing patients, a similar ratio of skeletal to dental transverse changes was observed in both groups.

INTRODUCTION

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 in1860.¹ Later, in 1961, Haas introduced his version of the palatal expander which involved an acrylic coverage of the palate.² Despite Haas and Hyrax expanders not changing in design during the last 50 years, the recent addition of mini-screws 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 pre-pubertal 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.³ As patients begin their pubertal growth spurt and transition into early adulthood, the two palatal shelves begin to fuse.^4–6 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.^4,7–12 Another factor that should be taken into consideration when treating these patients is the resistance of circummaxillary sutures such as zygomaxillary buttress and sphenoidal sutures, which can hinder palatal expansion.^13–14

An alternative treatment modality for post-pubertal patients with transverse discrepancy that cannot be corrected by orthodontic tooth movement is with surgically assisted rapid palatal expansion (SARPE). 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.¹⁵

Studies have shown that the amount of expansion achieved with microimplant assisted palatal expander is similar to that of traditional expanders, such as Hyrax,^16–17 Haas,¹⁷ and quadhelix appliances,¹⁸ with about 40% of expansion being skeletal, 20% alveolar and about 40% dental tipping. ^19–20 In theory, when a MARPE is activated, the forces of the expander are transmitted through the mini-screws to the palate, resulting in more skeletal expansion and less dental expansion, a finding that suggests better long-term stability. Moreover, MARPE appliances have been found to be a clinically acceptable nonsurgical treatment option to correct mild-to-moderate transverse discrepancies for skeletally mature patients.²¹

For this study, an important distinction must be made. MARPE is an umbrella term for any appliance that uses mini-screws 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 mini-screws to achieve orthopedic expansion.²² With this distinction in mind, this study aimed to determine if MSE can achieve maxillary skeletal expansion in growing (GR) and non-growing (NG) patients. The significance of this study is that it compares maxillary and dental 3D displacements relative to the cranial base in growing and non-growing patients, using a well-defined 3D analysis.²³

METHODS

This study was a secondary data analysis of de-identified cone-beam computed tomography scans (CBCT) of patients with posterior unilateral or bilateral crossbites, who underwent treatment with MSE. The study was granted approval (ID: HUM 00146140) through the University of Michigan Institutional Review Board (IRB) process. The sample comprised of CBCT’s taken between 2015–2018 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. The transverse discrepancy was determined using the maxillomandibular differential index (>16.4 mm in GR patients and >19.6 mm in the NG patients).²⁴

2. No previous orthodontic treatment

3. No history of syndrome, trauma, or oral/craniofacial surgery

4. No congenital facial anomaly or dysmorphism

5. Pre- and post-expansion CBCT scans, with the field of view including the cranial base

6. Adequate scan quality, without movement artifacts

The final sample was comprised of CBCT scans from 25 subjects (12 females and 13 males) who met the inclusion criteria. Skeletal maturation of each subject was assessed using the cervical vertebra maturation (CVM)²⁵ and midpalatal suture maturation (MSM)²⁶ methods. The CVM stage was determined by two independent observers, JM and LF, using lateral cephalograms generated from CBCT images. The MSM stage was assessed by two independent observers, LC and FA, using standardized multiplanar views of the CBCT scans. Based on these classifications, the study sample was divided into two groups: 11 growing (GR; CVM 1–4 and MSM B-C) and 14 non-growing (NG; CVM 5–6 and MSM D-E) subjects (Fig 1).

Figure 1.

Classification of growing and non-growing groups using cervical vertebrae and midpalatal suture maturation. A Patient in the growing group presenting with CVM 3 and MSM B. B Patient in the non-growing group presenting with CVM 5 and MSM D.

The Maxillary Skeletal Expander (MSE) used in this study consists of a central expansion jackscrew with four attached arms soldered to orthodontic bands placed on maxillary first molars (Fig 2). The addition of four sheaths welded to the body of the central expansion jack screw allowed for the placement of the mini-screws in the roof of the mouth. The mini-screws 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 mini-screws were placed symmetrically and parallel to the mid-palatal suture. They were also positioned most 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.

Figure 2.

A Upper occlusal view depicts the design of the Microimplant Assisted Rapid Palatal Expansion (MARPE) appliance at the day of the placement (T1) in a 19 yrs. male patient, CVM 5. B Upper occlusal view at the day of the removal of MARPE after completion of the expansion for the same patient (T2).

Activation protocol for all subjects began 2 weeks after the placement of the mini-screws. 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.²⁷ The expansion was concluded when the lingual cusp of the upper molar contacted the tip of the lower molar buccal cusp. If the expansion occurred asymmetrically, it was stopped according to the side that expanded more.

Table I.

Recommended rates of expansion for patients based on their age

Age (year)	Rate (turn/day)
	Initial	After opening of the diastema
8-l3	2	2
13–15	2–3	2
16–17	3	2
18	3–4	2–3

Data Collection & Analysis Overview

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 removal of the maxillary expander (T2). The generated DICOM files were converted to GIPL files that were used in the open-source software ITK-SNAP.²⁸ All image analysis steps were performed by one examiner (CM), 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 re-sampled to 0.3 mm isotropic voxel size for proceeding segmentation of anatomic structures of interest (CMF Registration, 3D Slicer Software²⁹). Subsequently, 3D image analysis was performed through the following steps:³⁰

1. 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, using 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).

2. Head orientation using ‘Transforms’ extension: 3D Slicer provides a fixed 3D coordinate system with three orthogonal planes denoted by yellow, red, and green colors, 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 and Porion (Frankfort horizontal plane) utilized to define the axial plane (Fig 3).

3. Manual approximation of T1 and T2 using ‘Transform’ extension in 3D Slicer: The .gipl scans of T2 were translated and rotated manually to superimpose the T1 and T2 anterior cranial bases.

4. Construction of 3D volumetric label maps of the approximated T2 scan: The same procedure described in step #1 was used to construct T2 segmentations.

5. Voxel-based registration of scans using the cranial base as reference: The 3D voxel-based registration (‘CMF Reg’ extension) was used to align the T1 and T2 scans automatically by utilizing 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.

6. Pre-labeling landmarks in ITK-SNAP: Anatomic landmarks of interest were “pre-labeled” 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.

7. Generation of 3D Models (.vtk files) in 3DSlicer: Using the ‘Model Maker’ extension in 3DSlicer, 3D surface models were generated from the segmented head and pre-labeled landmarks at T1 and T2 for each patient.

8. Landmark based quantitative assessments: Using the ‘Q3DC’ extension, landmarks were placed on the pre-labeled models, and displacement of landmarks listed in Table III were reported in anterior-posterior, superior-inferior and 3D directions. When needed, mid-points were generated for bilateral landmarks (Fig 5; Table IV).

9. Generation of semitransparent overlays and color maps for visualization: Semitransparent overlays of the T1 and T2 models were created for visualization. Additionally, using the Model-to-Model Distance and Shape Population Viewer, color maps were generated to visualize the changes from T1 to T2 (Fig 6).

Figure 3.

Head orientation for T1 in 3Dslicer. Yellow line represents the midsagittal plane. Red line represents the axial plane. Green line represents the coronal plane.

Figure 4.

Pre-labeling of the posterior nasal spine (PNS) landmark in ITK-SNAP software. A-D Images show positioning of the PNS at T1 in A axial, B sagittal, C coronal, and D 3D rendering views. E-H Images depict labeling of the PNS at T2 in E axial, F sagittal, G coronal and H 3D rendering views. Note two red points used to mark the PNS at T2 because of the separation of the palatal floor.

Table II.

Definitions of the 3D landmarks

Landmark	Location
Nasion	Most anterosuperior junction of the nasofrontal suture
Orbitale	Most inferior point of the orbital concavity in a frontal view, centered anterior-posteriorly on the orbital rim from the superior view
Zygomatic	Greatest point of convexity where the horizontal and sagittal components of the zygomatic arch intersect in an inferior view
Nasal Cavity	Central point in a frontal view and the most anterior portion of the inferior contour of the nasal cavity 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
Palatine foramen	Most central point of the palatine foramen 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 canine tip at most occlusal level
Canine root apex	Center of root canal at most apical level
Maxillary molar incisal tip	Center of mesio-buccal cusp at occlusal level
Mesiobuccal root apex maxillary first molar	Center of mesio-buccal root at most apical level
Alveolar Bone Level at first molar region (buccal)	Center of alveolar bone at the gingival margin level

Table III.

Measurements used for analysis including lateral displacements, anteriorposterior/superior-inferior/3D displacements, and angular changes.

Measurements
1.Maxillary Lateral Displacements Difference between T2-T1 measurements	Distance between right and left orbitale (OrR-OrL)
	Distance between right and left zygomatic ( ZygR-ZygL)
	Distance between right and left nasal cavity (NCR-NCL)
	Distance between right and left palatine foramen (PFR-PFL)
	Distance between right and left canine cusp tip (CR-CL)
	Distance between right and left molar cusp tip (MR-ML)
2.Maxillary Anterior-posterior (AP), Superior-inferior (SI) and 3D Displacements Midpoints were generated for each bilateral landmark and then the difference was taken from T1-T2	Orbitale midpoint (Orm)
	Zygomatic midpoint (Zygm)
	Nasal cavity midpoint (NCm)
	Palatine foramen midpoint (PFm)
	Canine cusp tip midpoint (Cm)
	Molar cusp tip midpoint (Mm)
	Anterior nasal spine (ANS, T2 only)
	Posterior nasal spine (PNS, T2 only)
	A-point (T2 only)
3.Angular Changes	Palatine plane: anterior and posterior nasal spine (ANS-PNS)
	Angle formed by the right and left orbitale-zygomatic lines, in the anterior view (OrR-ZygR – OrL-ZygL)
	Molar torque: long axis of the molars ( M-M′ - M″-M‴)
	Canine torque: long axis of the canines ( CR-C′R - C″R-C‴R)

Figure 5.

3D models showing the pre-labeled landmarks (red) utilized in the study. A Frontal view displays orbitale (Or), nasal cavity (NC), infrazygomatic (IZyg), and permanent canine cusp tip (C). B Right lateral view depicts canine root apex (C′), first molar root apex (M′), anterior nasal spine (ANS), and anterior concavity of the maxilla (A-point). C Inferior view shows palatine foramen (PF), first molar cusp tip (M) and posterior nasal spine (PNS).

Table IV.

Error study assessment using Intraclass Correlation Coefficients (ICC) with 95% confidence interval (CI)

Variable		1st measurement	2nd measurement	Difference	ICC
		Mean (SD)	Mean (SD)
Lateral Displacements (distance)	OrR-OrL	1.2 (0.9)	1.1 (0.9)	−.07	.994
	ZygR-ZygL	2.1 (1.5)	2.1 (1.4)	.03	.987
	NCR-NCL	3.5 (1.4)	3.5 (1.3)	.02	.994
	PFR-PFL	3.3 (1.2)	3.4 (1.1)	.14	.992
	CR-CL	3.9 (2.1)	3.9 (2.1)	.01	1.000
	MR-ML	3.2 (1.5)	3.2 (1.6)	−.05	.999
Sagittal Displacements (AP)	Orm	0.5 (0.5)	0.5 (0.5)	−.02	.994
	Zygm	0.9 (1.1)	0.9 (1.1)	−.02	.998
	NCm	0.6 (0.4)	0.7 (0.4)	.03	.992
	PFm	−0.2 (0.1)	−0.2 (0.1)	−.01	.852
	Cm	1.5 (0.7)	1.6 (0.7)	.02	.990
	Mm	1.8 (0.5)	1.8 (0.6)	−.01	.986
	ANS	0.4 (0.5)	0.4 (0.4)	−.01	.978
	PNS	1.4 (1.0)	1.4 (1.1)	.04	.996
	A-point	0.8 (0.5)	0.7 (0.5)	.04	.997
Vertical Displacements (SI)	Orm	−0.1 (0.3)	−0.1 (0.3)	−.02	.966
	Zygm	−0.2 (0.8)	−0.3 (0.8)	−.04	.987
	NCm	−1.1(1.2)	−1.1 (1.3)	.02	.996
	PFm	−1.3 (0.6)	−1.3 (0.6)	.02	.998
	Cm	−1.1 (1.1)	−1.2 (1.1)	−.06	.994
	Mm	−0.5 (1.1)	−0.5 (0.1)	.00	.999
	ANS	−1.2 (0.6)	−1.3 (0.5)	−.10	.969
	PNS	−1.1 (1.3)	−1.0 (1.2)	.08	.997
	A-point	−1.6 (1.1)	−1.5 (1.1)	.04	.997
Angular Changes	ANS-PNS	1.4 (1.0)	1.5 (0.9)	.10	.989
	OrR-ZygR – OrL-ZygL	3.2 (1.5)	3.6 (1.2)	.45	.896
	MR-M′R - M″R-M‴R	2.4 (2.5)	2.4 (2.4)	.06	.995
	ML-M′L - M″L-M‴L	4.7 (3.5)	4.8 (3.7)	.06	.999
	CR-C′R - C″R-C‴R	2.9 (2.8)	3.0 (2.1)	.15	.997
	CL-C′L – C″L-C‴L	1.9 (1.8)	1.7 (1.9)	.14	.989

Abbreviations: SD, standard deviation; AP, anterior-posterior; SI, superior-inferior; Or, orbitale; NC, nasal cavity; PF, palatine foramen; Zyg, zygomatic; M, first permanent molars cusp tip; C, permanent canines cusp tip; ANS, anterior nasal spine; PNS, posterior nasal spine; M′, first permanent molars root apex; C′, permanent canines root apex; M″, T2 permanent molars cusp tip; C″, T2 permanent canines cusp tip, M‴, T2 permanent molars root apex; C‴, T2 permanent canines root apex, _m, midpoint; _R, right; _L, left.

Figure 6.

Semi-transparent overlays of T1 and T2 show skeletal changes in a growing patient from pre-treatment (T1; shown in white) to post-treatment (T2; shown in red), the maxilla expanded skeletally without rotational displacements.

Statistical Analysis

Sample size calculation estimated that a minimum of 11 patients in each group was needed based on a power of 0.86, an alpha of 0.05, a mean difference of 1.0mm (SD of 1.0mm) for lateral displacement of the nasal cavity (NC, defined in Table II).

To measure the study error, the same operator (CM) performed all measurements on 20% of the sample after a 30-day interval. Intraclass Correlation Coefficient (ICC) was used to assess intra-observer repeatability. Chi-square tests assessed differences in gender distribution between growing and non-growing groups. Differences in skeletal and dental measurements also were tested between the two groups, at baseline and after expansion, using Independent Sample t-test.

All statistical analyses were performed using IBM SPSS Statistics, version 26.0 (SPSS Inc., Chicago, IL). The level of significance was set at 5% (P < .05).

RESULTS

The sample was comprised of 11 growing and 14 non-growing 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 midpalatal suture closure while 12 patients were classified as stage D. The study error and the ICC are presented in Table IV. The male and female distribution and all maxillary baseline measurements were similar in both groups (Table V).

Table V.

Sample demographics, maturational stage, and maxillary widths at baseline (T1)

Variable		Growing n=11	Non-growing n=14	P value
		Mean (SD)	Mean (SD)
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
Sex	Female	4 (36.4%)	8 (57.1%)	.428§
	Male	7 (63.6%)	6 (42.9%)
Or R -Or L		65.5 (6.6)	67.3(5.8)	.481†
Zyg R -Zyg L		86.9 (4.2)	84.1 (4.6)	.260†
NC R -NC L		21.6 (1.7)	21.6 (2.4)	.980†
PF R -PF L		29.2 (1.7)	29.9 (1.8)	.354†
M R -M L		53.6 (4.1)	53.2 (7.1)	.862†
C R -C L		36.9 (4.8)	35.8 (7.0)	.637†

^†t-test

^§Chi-square test; n/a, non-applicable; Or, orbitale; Zyg, zygomatic; NC, nasal cavity; PF, palatine foramen; M, first permanent molars; C, permanent canines; _R, right; _L, left. No tested variables were significantly different at baseline.

Table VI shows the means, standard deviations, and significance of all tested variables between the GR and the NG groups. No significant differences were found between the male and female subjects.

Table VI.

Comparisons of skeletal and dental changes in growing and nongrowing patients (t-Test).

Variable		Growing n=11 Mean (SD)	Nongrowing n=14 Mean (SD)	P value
Lateral Displacements (distance)	OrR-OrL	1.6 (1.2)	0.9 (0.7)	0.06
	ZygR-ZygL	3.5 (2.2)	2.8 (1.8)	0.4
	NCR-NCL	3.6 (1.5)	1.9 (1.2)	.006*
	PFR-PFL	3.9 (1.3)	2.1 (1.3)	.022*
	MR-ML	5.5 (2.8)	3.6 (2.1)	0.1
	CR-CL	3.6 (2.4)	2.7 (1.9)	0.4
Sagittal Displacements (AP)	Orm	0.5 (0.6)	0.2 (0. 7)	0.3
	Zygm	0.4 (0.1)	0.01 (0.8)	0.2
	NCm	0.7 (0.5)	0.3 (0.6)	.05
	PFm	−0.9 (0.3)	.002 (0.4)	0.1
	Mm	1.1 (1.1)	0.1 (1.3)	0.9
	Cm	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)	Orm	−0.03(0.2)	0.01 (0.3)	0.8
	Zygm	−0.2(0.6)	−0.1 (0.6)	0.6
	NCm	−0.7(1.4)	−0.8 (1.2)	0.8
	PFm	−1.0(0.7)	−1.01(1.1)	0.9
	Mm	−0.4 (1.1)	−0.3 (0.9)	0.8
	Cm	−1.7(1.0)	−0.6 (0.8)	.02*
	ANS	−1.2(0.6)	−0.5 (0.5)	.004*
	PNS	−11(15)	−0.8 (1.2)	0.7
	A-point	−15(1.4)	−1.04 (0.7)	0.3
Angular Changes	ANS-PNS	1.7 (0.9)	1.1 (0.9)	0.1
	OrR-ZygR – OrL-ZygL	5.1 (3.7)	4.5 (4.6)	0.7
	MR-M′R - M″R-M‴R	4.03 (4.4)	3.2 (2,9)	0.6
	ML-M′L - M″L-M‴L	4.2 (3.6)	3.5 (3.4)	0.6
	CR-C′R - C″R-C‴R	3.1 (2.9)	1.5 (0.1)	0.02*
	CL-C′L – C″L-C‴L	2.70 (2.0)	1.2 (0.8)	0.04*

.Values were compared to the control group and statistical significance was determined to be present at

^*P value < 0.05.Abbreviations: SD, standard deviation; AP, anterior-posterior; SI, superior-inferior; Or, orbitale; NC, nasal cavity; PF, palatine foramen; Zyg, zygomatic; M, first permanent molars cusp tip; C, permanent canine cusp tip; ANS, anterior nasal spine; PNS, posterior nasal spine; M′, first permanent molars root apex; C′, permanent canines root apex; M″, T2 permanent molars cusp tip; C″, T2 permanent canine cusp tip, M‴, T2 permanent molars root apex; C‴, T2 permanent canines root apex, _m, midpoint; _R, right; _L, left.

For lateral displacements, an increase in the transverse skeletal dimension was found in all the tested variables. The transverse skeletal expansion at the nasal cavity 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 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 nasal cavity (NC_R-NC_L, P <.05) and palatine foramen (PF_R-PF_L, P <.05). The transverse dental changes did not vary significantly between the groups; greater expansion was observed at the level of the molars compared to 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. While 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 palatine foramen was divided by the amount of expansion at the first molar tips. Similar ratio of skeletal to dental transverse changes were observed in growing (62% skeletal and 32% dental) and non-growing patients (59% skeletal and 41% dental).

For vertical and sagittal displacements, small and insignificant forward and downward movement of the orbitale, zygoma, nasal cavity, maxillary first molars, PNS and A-point were found in both groups. Maxillary permanent canine cusp tips (C) and ANS 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 ANS; P values were 0.02 and 0.004, respectively).

Regarding the angular measurements, changes in 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 orbitale-zygomatic lines from pre- to post-treatment 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 maxillary permanent canine presented slight buccal tipping of approximately 3° in the GR group and 1.5° in the NG group (Table VI).

Figures 7 and 8 illustrate the treatment changes in growing and non-growing patient, respectively.

Figure 7.

Treatment outcomes for a growing patient. A 3D rendering of the CBCT scan at T1. B 3D rendering of the CBCT scan at T2, 6 months following the initial CBCT scan. Yellow arrows are pointing to midpalatal suture before (T1) and after expansion (T2). C Overlays of T1 (white) and T2 images (red) show skeletal and dental changes. The black arrows are pointing to maxillary and dental expansion.

Figure 8.

Treatment outcomes for a non-growing patient. A 3D rendering of the CBCT scan at T1. B 3D rendering of the CBCT scan at T2, 6 months after the initial CBCT scan. Yellow arrows are pointing to midpalatal suture before (T1) and after expansion (T2). C Overlays of T1(white) and T2 images (red) depict skeletal and dental changes. The black arrows are pointing to zygomatic, maxillary, and dental expansion.

DISCUSSION

The significance of the present study is two-fold. First, the cervical vertebrae and midpalatal suture maturation were used to classify the study groups. Second, 3D cranial base superimposition was utilized to evaluate the ratio of skeletal and dental responses in growing and non-growing subjects to a tooth-bone anchored MSE appliance.

Rapid maxillary expansion (RME) has been used as the treatment of choice for patients who are in their pre-peak pubertal stage or early adolescent stage of growth and have transverse maxillary discrepancy. However, as patients progress through adolescence and enter early adulthood period, the circummaxillary sutures become more interdigitated.^13–14 Consequently, treatment with conventional RME appliances causes variable responses within the skeletal structures as it widens the maxillary arch, primarily through opening the midpalatal suture. Additionally, RME may induce buccal tipping of the tooth crown, decrease the thickness of the buccal bone, and reduce the level of the marginal bone.³¹

To overcome these unfavorable side effects, microimplant assisted expansion has been used recently as an alternative method of treatment; MARPE transmits the activation forces directly to the maxilla and decreases the need for dental anchorage. Bone-borne expanders also can achieve up to three times greater expansion in the midpalatal suture area in adolescents and induce less negative side effects than the traditional expanders.³²

Evaluation of Skeletal and Dental Responses in 3D Superimpositions

In the current study, the reported results are based on cranial base superimpositions. Maxillary regional superimpositions were used to determine if the growth of adolescent patients affected the results. However, due to the skeletal expansion produced by the expansion treatment, no stable regions suitable for maxillary superimposition could be identified. Moreover, many studies have reported the average rate of growth of the maxillary transverse width ranges between 0.4 mm to 0.9 mm per year. These studies noted that the transverse width growth of the maxilla is completed by the age of 14–16, in males and females.^33–36 Because small changes in the transverse width are expected to occur within 6 months, the time interval between T1 and T2, we believe that the effects of growth on our study findings are negligible.

Lateral Skeletal Displacements (Skeletal Expansion)

Maxillary width increased from pre-treatment to post-treatment in both groups, as shown in Table VI. This increase in width corroborates the findings of previous studies that have agreed that the circummaxillary structures such as the zygomatic buttress, and palatine bones of the midfacial complex provide resistance to sutural separation with the apex toward the nasal cavity and the base at the level of the palatine processes.^37–40

Our study showed that after the expansion, orbitale displaced laterally with the least amount, 1.6 ± 1.2 mm and 0.9 ± 0.7 mm in the GR and the NG groups, respectively. Furthermore, a greater lateral displacement was observed for the infrazygomatic point and the palatine foramen compared to the orbitale. At the level of the suture, molars and canines showed marked lateral displacement; the amount of expansion is consistent with other studies.^19–20 Overall, MSE treatment resulted in a superior-inferior pyramidal opening with less lateral displacement at the orbital level and greater lateral displacement close to the site of forces application.

In the anterior-posterior direction, previous studies have reported a parallel and uniform sutural opening at the level of the palate in adults treated with MARPE.^13,21 Celenk-Koca et al.³² described a similar pattern of parallel suture opening in adolescents; most subjects had a slight triangular opening with the suture being wider anteriorly. Our study results are in line with prior findings in adolescents and adults who received maxillary skeletal expansion. The palatine foramen in the GR and the NG groups showed a lateral displacement of 3.4 ±1.3 mm and 2.1 ±1.3 mm, respectively, and a displacement of 3.6 ± 1.5 mm and 1.9 ± 1.2 mm, respectively, at the level of the nasal cavity that shows a relatively parallel sutural opening.

Although both groups showed a similar pattern of sutural separation, they had significantly different responses from skeletal expansion at the level of the palatine foramen (P =.006) and the nasal cavity (P =.02), with greater skeletal expansion in the GR group. The observed differences between the two groups in our study are likely related to the fact that the circummaxillary structures constrain the sutural separation and their resistance increase with patient maturation.^37–40

In the GR group, there was less resistance from these surrounding structures. However, it is important to note that MSE is more invasive than conventional RME, and RME is the therapy of choice for children and adolescents.⁴¹ The findings of this study suggest that MSE may be indicated to prevent undesirable dentoalveolar effects and optimize dentoskeletal expansion in adolescent patients who have thin gingival biotype or need significant bony expansion. MSE can achieve greater skeletal expansion, especially in comparison to traditional RME; MSE produces 1.5 to 2.8 greater skeletal expansion than tooth-borne expanders.³²

Sagittal and Vertical Skeletal Changes

All variables measured in this study demonstrated a downward and forward movement in the GR and the NG groups except for the palatine foramen which moved backward by 0.9 ± 0.3 mm and downward by 1.0 ± 0.7 mm in the in the GR group. The small, not statistically significant, backward displacement of the palatine foramen may be explained by bone apposition at the transverse palatine suture in growing patients.

Dental Changes

Marked transverse dental changes were observed in both groups at the molars and canines, with greater expansion at the molar level. These changes were larger in the GR group; however, the changes were not significantly different between the GR and NG groups. The vertical displacement of the canine varied significantly between the two groups. The greater downward displacement of the canines in the GR group, may be due to the fact that they were not completely erupted before expansion, however, natural development and eruption of the canines occurred during the treatment.

Dental tipping was noted in the GR and the NG groups with 3–4° of buccal crown tipping at the first molars on the right and left sides. The buccal crown tipping of the upper first molars in this study is consistent with other studies^19,21,30,40 that showed an increase of buccal crown tipping by 2.5°−4° with a bone anchored RPE in skeletally mature patients. Likewise, a recent study by Ruellas reported 2.6° of buccal crown tipping at the maxillary molars in adolescents,²⁹ similar to our findings. Interestingly, the amount of dental tipping in adolescent subjects in the present study is less than those treated with traditional RPE; RPE causes 19– 24° of buccal crown tipping.^42–43

Skeletal vs. Dental Expansion and Final Considerations

The amount of expansion at the palatine foramen was divided by the amount of expansion at the first molar tips to calculate the ratio of skeletal to dental expansion. The GR group had 62% skeletal expansion and 38% dental expansion, consistent with other studies reporting approximately 60% skeletal expansion and 40% dental expansion in the adolescent population.^30,32 Maxillary skeletal expansion in the NG group also presented a similar ratio with 59% skeletal and 41% dental expansions. This may be explained by the fact that in our study, 12 out of the 14 patients in the NG group were classified as MSM stage D and fusion of the midpalatal suture had occurred only at the palatine portion. Although two patients in the NG group were classified as MSM stage E, they had fusion only in the central portion of their midpalatal sutures. Accordingly, the fusion index of the midpalatal suture in the NG group was low⁴⁴ and that can explain the skeletal expansion observed in these patients.

The results of this study showed that MARPE corrected transverse maxillary discrepancy in growing and non-growing patients. It also provides additional insights into the dentoskeletal effects that patients may experience when treated with the MARPE. Hence, this study findings may help clinicians to determine which cases would benefit from the MARPE appliance.

CONCLUSION

The use of MARPE appliance is an effective way of treating patients with transverse maxillary discrepancy, regardless of their sex or maturation stage. Although greater skeletal and dental changes were found in growing patients, the growing and the non-growing patients had similar ratio of skeletal transverse changes compared to dental changes.

Highlights.

• Treatment of patients with transverse maxillary discrepancy using MARPE appliance is effective in growing and non-growing patients.

• Growing patients presented greater dental and skeletal changes compared to non-growing patients.

• A similar ratio of skeletal to dental changes was observed in growing and non-growing groups.