This retrospective study was approved by the Ethics Committee of Wuhan University, School and Hospital of Stomatology (approval No. WDKQ2024-B122). This study analyzed preexpansion and postexpansion CBCT records of 82 patients who underwent MARPE treatment by an experienced orthodontist.

Inclusion criteria were as follows:

• 1.

  Diagnosis of MTD requiring bone-borne expansion, presenting with unilateral or bilateral posterior crossbite.

• 2.

  Good oral hygiene and healthy periodontal status.

• 3.

  Achievement of successful maxillary skeletal expansion, defined as >1 mm of posterior skeletal expansion confirmed by measurement.

• 4.

  Availability of CBCT scans obtained preoperatively and within 3 weeks post-MARPE.

• 5.

  Aged ≥15 years at appliance placement.

Exclusion criteria included:

• 1.

  History of orthodontic treatment, maxillofacial trauma, surgery, or respiratory therapy.

• 2.

  Diagnosis of any systemic disease or pathologic jaw bone injury.

• 3.

  Presence of significant maxillofacial deformities (eg, craniofacial clefts).

• 4.

  Long-term use of medication known to affect bone metabolism (eg, glucocorticoids, antiepileptics, antituberculosis drugs, thyroid hormones, and heparin).

• 5.

  Noncompliance with the expansion activation protocol (inability to rotate the expander screw as instructed) or repeated appliance failure (screw loosening or detachment).

A retrospective review of the patient database identified 90 patients initially meeting the inclusion criteria. Among these, 2 patients exhibited unsuccessful MPS expansion, and 6 were excluded because of incomplete CBCT records. The final cohort consisted of 82 patients (19 males and 63 females) aged 15.2-32.4 years, treated at the Orthodontics Department of Wuhan University Stomatological Hospital, Hubei, China, between 2020 and 2024. The cohort included 25 patients with Class I malocclusion, 26 patients with Class II malocclusion, and 31 patients with Class III malocclusion.

The design and manufacturing workflow of customized MARPE appliances was shown in Figure 1 . Preexpansion CBCT images were imported into Dolphin Imaging 11.95 software (Dolphin Imaging and Management Solutions, Chatsworth, Calif) to evaluate palatal bone thickness and soft-tissue morphology, facilitating precise miniscrews positioning. After virtual localization of optimal insertion sites on CBCT scans, these coordinates were transferred to the patient’s 3-dimensionally reconstructed maxilla in Standard Tessellation Language format. Intraoral scans were obtained using an iTero Element intraoral scanner (Align Technology, San Jose, Calif). The customized expander designs, accompanied by CBCT data and digital intraoral scans, were submitted to the dental laboratory for fabrication.

Flowchart for the digital design and manufacturing of customized MARPE appliances.

Dental technicians used 3-dimensional (3D) registration of CBCT images and digital casts to simulate virtual implantation, ensuring avoidance of tooth root interference ( Fig 2 , B-D ). The digital design was accomplished using 3Shape ApplianceDesigner CAD software (version 2020, 3Shape A/S, Copenhagen, Denmark) ( Fig 2 , A ). The final design file was used for fabricating anatomically precise appliances via 3D metal powder printing (selective laser melting) with a 3D metal printer (PROFETA, Intelligent and Expert, IE 150H II, Nanjing, China). Subsequently, the central jackscrew (Dentaurum, Dusseldorf, Germany) and the bilateral palatal expander brackets were assembled via laser welding on the dental model, which was printed using a HeyGears A3D printer (HeyGears Inc, Guangzhou, China).

The customized MARPE device comprised a jackscrew unit with 4 miniscrew insertion slots, integrated with molar bands and bilateral metal connecting rods. Cementation was performed on the maxillary first premolars and first molars, with a connecting rod traversing the lingual surface of the maxillary second premolars. Four orthodontic miniscrews (A1, Syntec Scientific Corporation; collar diameter: 2 mm; lengths: 10, 12, or 14 mm, selected based on palatal bone and soft-tissue thickness) were inserted ( Fig 2 , E and F ). Activation protocol involved 2 daily turns (0.2 mm/turn) until target expansion was achieved. Mean treatment duration was 21 days (range, 14-28 days), yielding an average expansion of 7.2 mm (range, 5.5-10 mm) .

The customized MARPE device: A, The customized design of MARPE on the digital cast; B-D, Virtually simulation of implantation based on 3D registration of CBCT images and digital casts; E, Installing the MARPE appliances; F , Intraoral photograph of postexpansion.

CBCT scans were taken before expansion (T0) and within 3 weeks after the final MARPE activation (T1). Scans were acquired using the NEWTOM VGi 9 scanner (Italy) in a 15 × 12 cm field of view with exposure parameters of 8.8 mA and 110 kV for 18 seconds (3.6 seconds emission time), a voxel size of 0.3 mm. Using a laser-guided system, the Frankfort horizontal plane (FH) was aligned parallel to the floor. The scans were saved in digital imaging and communications in medicine file format

Volumetric data from the T0 CBCT scan were imported into Dolphin Imaging to evaluate the stage of MPSM. Before evaluating each patient’s MPSM in this study, we established a standardized coordinate system, which serves as a critical prerequisite for the qualitative assessment of MPSM. First, we adjusted the patient’s head position by defining the midsagittal plane (z-plane) using the nasion (N) and the plane formed by the anterior nasal spine (ANS) and posterior nasal spine (PNS). Subsequently, we adjusted the angulation of the palatal plane in the sagittal orientation to ensure that both the ANS and PNS points lay on the horizontal plane (X-plane). The y-plane, representing the coronal plane, was positioned at the ANS point and perpendicular to both the x- and z-planes.The horizontal plane passing through ANS and PNS was calibrated to classify MPSM according to the 5-stage system proposed by Angelieri et al ( Fig 3 , A-D ). For this statistical analysis, the ordinal scale of MPSM was converted to the numerical variable in stages A = 1, B = 2, C = 3, D = 4, and E = 5.

A, Key radiologic morphologic characteristics specific to each MPSM stage based on the method of Angelieri et al Representative CBCT images of the MPSM stage C, D, and E in this cohort. B , Stage C: 2 radiopaque, scalloped, and parallel lines are separated by areas of low radiographic density; C , Stage D: 2 radiopaque, scalloped, and parallel lines can be visualized on the maxillary portion of the palate. The MPS cannot be visualized in palatine bone, and the palatine bones become more radiopaque than the para-sutural maxillary bone; D , Stage E: the suture cannot be identified along the maxillary and palatine bones. The parasutural bone density is the same as in other regions of the palate. No subjects were classified as typical stage A or B in this cohort.

To quantify maxillary expansion, axial cross-sectional images (T1) were standardized as depicted in ( Fig 4 , A ). In the transversal plane, the opening of the midpalatal suture was measured as the distance between the mesial edges of the intermaxillary suture, at the level of the ANS and PNS. Anterior palatal expansion (APE) and posterior palatal expansion (PPE) were calculated as the linear distances between landmarks 1-2 and 3-4 ( Table I ), respectively, in the axial plane. The MPS opening pattern was determined by the PPE-to-APE ratio. In addition, sagittal cross-sectional images were used to measure palatal length between ANS and PNS at T0 ( Fig 4 , B ).

A , Measurement of the postexpansion APE and PPE on the T1 CBCT images; B, Measurement of PL on the T0 CBCT images; C, Measurement of the PMS magnitude on the CBCT images.

For PMS magnitude analysis, T0 and T1 CBCT images were reoriented to a consistent natural head position in Dolphin Imaging ensuring that the Frankfurt plane was parallel to the horizontal plane and the midsagittal plane was centered. Subsequently, the exported digital imaging and communications in medicine data were imported into MIMICS (version 21.0; Materialise NV, Leuven, Belgium). A horizontal measurement plane (plane A) parallel to the FH plane and a sagittal plane perpendicular to the FH plane (parallel to the midsagittal plane) was constructed through points 5 or 6 ( Table I ). The PMS width was quantified as the linear distance between points 7 and 8 ( Table I ) according to the same anatomic images of the pterygoid process in sagittal images ( Fig 4 , C ). The PMS images were magnified 4-fold in both sagittal and axial planes in full-screen view on a 14-inch laptop (Lenovo, Beijing, China) and the interpolation function of MIMICS software was applied to enhance spatial resolution ( Fig 5 ). PMSS magnitude was defined as the difference between T1 ( Fig 5 , B ) and T0 ( Fig 5 , A ) measurements. In the sagittal CBCT image, a parallel measurement line (plane B) was designed at position 10 mm above the plane A. Two sets of PMSS data were obtained on 2 parallel measurement planes using the same measurement method ( Fig 6 ).

A , Measurement of the PMS magnitude on the T0 CBCT images; B, Measurement of the PMS magnitude on the T1 CBCT images at the same plane across the same anatomic marks.

Schematic representation of the measurement planes for superior and inferior PMS width: Plane A , location of the inferior measurement plane; Plane B , location of the superior measurement plane, positioned 10 mm above the inferior plane.

Lateral cephalometric tracings at T0 were conducted by a single operator using Dolphin Imaging. The sagittal skeletal jaw relationship was classified using the ANB angle as Class I (1°≤ ANB <5°), Class II (ANB ≥5°), or Class III (ANB <1°). The vertical classification was defined using the SN-GoGn angle as normodivergent: 27°≤ SN-GoGn ≤36°, hyperdivergent: SN-GoGn >36°, or hypodivergent: SN-GoGn <27°.

On the basis of preliminary experiments, the overall mean PMSS magnitude was approximately 0.3 mm, whereas the mean PMSS magnitude in the strong positive group was approximately 0.6 mm. Therefore, PMSS was categorized using these cutoff values as follows: Split group was defined as an observed PMS split at T1 with a significant sutural split ≥0.6 mm. Nonsplit group was defined as no PMS split >0.3 mm bilaterally. Patients were classified as limited group if 1 or both sides exhibited PMSS ≥0.3 mm but <0.6 mm.

The ICC results demonstrated good intraexaminer reliability (ICC, 0.872-0.994) and good interexaminer reliability (ICC, 0.870-0.998; Table II ). Dahlberg’s formula indicated minimal measurement error (PMSS Dahlberg error: 0.06-0.12 mm, n = 82; Table II ).

This retrospective study included 82 patients (mean age, 20.7 ± 3.7 years; range, 15.2-32.4 years) diagnosed with MTD. The sample size (n = 82) met the minimum requirement (n = 75) with power = 0.85. TableIII presents the baseline characteristics of all patients, including the group split (n = 19), limited (n = 23) and nonsplit (n = 40). Table Ⅳ showed the PMSS magnitudes of the split group and limited group.

Table III

Baseline characteristics stratified by PMSS status

Characteristics	Patients (n = 82)
	Split (n = 19)	Limited (n = 23)	Nonsplit (n = 40)
Age (y), mean ± SD	19.88 ± 3.73	20.41 ± 4.19	21.23 ± 3.31
APE (mm), mean ± SD	4.51 ± 1.49	3.71 ± 1.05	3.15 ± 0.95
PPE (mm), mean ± SD	3.43 ± 0.95	2.70 ± 0.69	2.06 ± 0.48
MPS opening pattern, mean ± SD	0.79 ± 0.18	0.74 ± 0.11	0.68 ± 0.14
PL (mm), mean ± SD	46.44 ± 3.82	47.94 ± 3.44	47.54 ± 3.37
Sex, n
Male	2	7	10
Female	17	16	30
MPSM stage, n
C	15	8	5
D	4	9	14
E	0	6	21
Horizontal skeletal pattern, n
Class I	4	9	12
Class II	3	7	16
Class III	12	7	12
Vertical skeletal pattern, n
Hypodivergent	2	2	9
Normodivergent	11	8	13
Hyperdivergent	6	13	18

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