Sutures play a pivotal role in craniofacial growth and development.1 Tensile stress along the suture typically induces bone deposition, whereas compressive stress triggers bone resorption.2 Histological studies in animals have demonstrated increased cellular activity and deposition of immature bone tissue along the suture margins.3 Rapid maxillary expansion (RME) is typically the primary approach for addressing transverse maxillary deficiency in young patients. Fixed appliances, such as the Hyrax or Haas expander, are commonly used to achieve extensive maxillary expansion. However, RME uses teeth for anchorage; therefore, alveolar bone bending and shifting of the tooth roots towards the outer surface of the alveolar bone may occur during expansion.4,5 The midpalatal suture (MPS) gradually undergoes calcification with age; hence, adequate opening of the intermaxillary suture using RME is impossible in patients aged > 15 years,1,4 and miniscrew-assisted maxillary expansion devices are frequently used for patients with calcified MPSs.

Although the RME force primarily targets the intermaxillary suture, it also affects other surrounding sutures such as the frontomaxillary, zygomaticomaxillary, temporozygomatic, and pterygopalatine sutures.6,7 Scintigraphy studies have revealed significant increases in metabolic activity in various regions of the maxilla; zygomatic, sphenoid, and nasal bones; and MPS.8 Furthermore, during maxillary expansion, force is transmitted through the pterygomaxillary suture to the unpaired sphenoid bone of the skull base, generating stress. Although previous studies have explored the effects of RME on the craniofacial sutures and the sphenooccipital syndesmosis,7,9,10 few studies have investigated the effects of bone-borne expansion devices on the circum-maxillary sutures (CMS) and the sphenooccipital syndesmosis. Therefore, this retrospective study investigated the influence of two different bone-borne maxillary skeletal expanders (MSEs) on the CMS using cone-beam computed tomography (CBCT) images of patients before and after treatment.

Subjects

This study enrolled 85 patients (59 women and 26 men) with insufficient transverse maxillary development, defined as a distance between the maxillary and mandibular first molar resistance centers < –2 mm, treated using bone-borne maxillary expansion appliances at the Kyung Hee University Dental Hospital between 2017 and 2021. Patients were randomized into two groups: the C-expander group and the ATOZ expander group. The exclusion criteria were patients with congenital diseases; history of surgery, trauma, or orthodontic treatment; and lack of clear pre- and post-treatment CBCT images. Additionally, patients without cranial and CMS opening during orthodontic treatment or midpalatal expansion < 2 mm were excluded. After exclusion, 44 (32 women and 12 men, mean age, 17.95 ± 6.38 years) and 37 (26 women and 11 men, mean age, 18.16 ± 8.04 years) patients were included in the C-expander and ATOZ groups, respectively and informed consents were provided. This study was approved by the Institutional Review Board of Kyung Hee University Dental Hospital (IRB No.: KH-DT24106).

Experimental procedures

C-expander group

The C-expander used six miniscrews (1.6 × 8.0 mm; Bio-action screw, Jin-Biomed Co., Bucheon, Korea) as skeletal anchors. Three miniscrews were placed in the bilateral palatal bones, (Figure 1A). The miniscrews were connected to the expansion screws using a resin pad (Forestadent Co., Pforzheim, Germany) (Figure 1B). The activation protocol for the C-expander was as follows: two turns per week till MPS separation, and one turn per every two days thereafter (Figure 1C and 1D).

Figure 1. Representative pre- and post-treatment occlusal images of a 16 year-old girl treated with a tissue bone-borne type C-expander showing the treatment stages of C-expander: A, After placement of six 1.6-mm diameter, 8-mm length miniscrews, B, device placement, C, post-expansion, D, occlusal radiograph after expansion.

ATOZ expander group

The ATOZ expander (MK Meditech Inc., Seongnam, Korea) uses 6–10 miniscrews (1.6 × 10.0/13.0 mm; MK Meditech Inc.) as skeletal anchors (Figure 2). A major difference from the conventional jackscrew used for rapid palatal expansion and miniscrew-assisted rapid palatal expansion (MARPE, MSE2 Biomaterials Korea Inc., Seoul, Korea) is that the ATOZ jackscrew is positioned parallel to the MPS and is constructed such that the force generated by jackscrews activation is transmitted through four connector links tangential to the jackscrew axis creating tension across the MPS (Figure 2A). The width, length, and depth of the ATOZ are 8.2, 24.9, and 9.3 mm, respectively, and the width increases to 16.2 mm upon full activation. Because of its small width, the ATOZ can be applied to very narrow palates where conventional expanders cannot be used. Furthermore, on activation, it provides sufficient expansion to normalize such palates (Figure 2B–2D).

Figure 2. Schematic illustration of the ATOZ expander. A, The ATOZ expander is composed of the base, a hexagonal nut, screw holes formed in the base, and a screw hole for the wire arm. B, Comparison of the sizes of the tooth-bone hybrid MARPE (MSE2, Biomaterials Korea Inc., Seoul, Korea) and ATOZ (MK Meditech Inc., Seongnam, Korea). C, MSE2 applied to a model with a narrow palate. D, ATOZ applied to the same model as in (C). Both devices facilitate expansion up to 8 mm.

The ATOZ can generate approximately 7 and 12 kgf of force with moderate and maximum activation, respectively (Figure 3). The amount of expansion decreases as the number of rotations of the expansion screw increases. Thus, the ATOZ provides a rapid initial expansion, which gradually slows down towards the end of the activation process. This unique parabolic pattern of the expansion rate is caused by the pan-type movement of the four-link system (Figure 4). This allows for the generation of sufficient force to distract the MPS and CMS with minimal rotation initially.

Figure 3. Graph showing the amount of expansion achieved with the ATOZ. Initially, the expansion occurs rapidly owing to the effective expansion force; however, expansion gradually decreases over time. At this stage, treatment can be completed quickly with rapid expansion, or the expansion can be slowed to keep the circum-maxillary sutures continuously activated, depending on the treatment goals.
*The ATOZ can withstand up to 50 kgf of compression force without permanent deformation and can easily revert to its original shape. This unique capability is due to its distinctive 4-link structure, similar to that of a beam spring, allows for the continuous dissipation of heavy orthopedic forces (ranging from 7–12 kgf) for > 4 weeks or until the next appointment.

Figure 4. A, B, ATOZ placement using a thermoplastic installation guide. After administering lidocaine local anesthesia, the installation guide is placed on the palate and a motor-driven screwdriver is used to insert the anchoring miniscrews. After inserting the four anchoring miniscrews, the ATOZ achieves initial stability. The installation guide is removed, and the remaining miniscrews are inserted. C, D, Intraoral photographs taken before and after key activation, respectively.

In this study, miniscrews for the ATOZ expanders were placed in the midpalatal region within 3 mm on either side of the MPS. On the day of implantation, the ATOZ was activated with seven to eight turns using a spanner-type key coupled with a hexagonal nut (Figure 4). The force was retained because of the spring-back property of the ATOZ. Subsequent activations were performed every 2–4 weeks at the doctor’s discretion until resistance was encountered. Expander activation was stopped once the desired expansion was achieved (Figure 5).11

Figure 5. Representative pre- and post-treatment images of a 15-year-old girl treated with pure bone-borne type ATOZ expander. Intraoral photographs and cone-beam computed tomography images showing the treatment stages of the pure bone-borne ATOZ expander: A–C, Expansion started; D–F, After 8 mm expansion.

Cone-beam computed tomography

CBCT (Alphard-3030, Asahi Roentgen, Kyoto, Japan) images were obtained before treatment (T0) and after the active expansion phase (T1) using the following settings: current, 10 mA; voltage, 80 kVp; exposure time, 17 seconds; voxel size, 0.39 mm; and scan area, 20.0 × 17.9 cm. The ON3D program (3D ONS Inc., Seoul, Korea) was used to evaluate the images with millimeter-level precision; reference planes (Table 1) and coordinate systems were established, and all images were reoriented in three dimensions (Figure 6), The widths of 10 CMS (Figure 7) at T0 and T1 were measured at the bone-surface level at the midpoint of each suture in the sagittal, coronal, and axial planes as follows:

Figure 6. Schematic diagram showing the 10 circummaxillary sutures measured in this study: A, Coronal plane; B, Sagittal plane; C, Axial plane.

Figure 7. Orientation of the radiographs in the coronal, sagittal, and axial planes.

Table 1 . Definitions of reference planes.

Reference plane	Definition
Horizontal reference plane (FH plane)	The plane passing through the bilateral orbital and the midpoint of the bilateral portion points
Sagittal reference plane	The sagittal reference plane uses the mid-sagittal plane passing through the nasion
Coronal reference plane (Frontal plane)	Perpendicular to the FH and sagittal reference planes and passing through the point of nasion

FH, Frankfurt Horizontal..

Sagittal plane: Frontonasal, frontomaxillary, and pterygomaxillary sutures (Figure 8).

Figure 8. Sutures measured in the sagittal plane. A, B, Frontonasal suture; C, D, Frontomaxillary suture; E, F, Pterygomaxillary suture. Red arrow, actual suture measured.

Coronal plane: Frontozygomatic, zygomaticomaxillary, and temporozygomatic sutures (Figure 9).

Figure 9. Sutures measured in the coronal plane. A, B, Frontozygomatic suture; C, D, Zygomaticomaxillary suture; E, F, Temporozygomatic suture.

Axial plane: internasal, nasomaxillary, intermaxillary, and MPSs (Figure 10).

Figure 10. Sutures measured in the axial plane. A, B, Internasal suture; C, D, Nasomaxillary suture; E, F, Intermaxillary (IM), midpalatal canine (MPC), midpalatal premolar (MPP), midpalatal molar (MPM) and midpalatal posterior nasal spine (MPPNS) sutures.

For the intermaxillary suture and MPS, the axial plane was adjusted to simultaneously pass through the anterior nasal spine (ANS) and posterior nasal spine (PNS). The intermaxillary width was measured at the ANS, whereas the MPS width was measured at the canine, premolar, molar, and PNS levels.

Statistical analysis

The widths of the sutures at T0 and T1 were measured twice by the same orthodontist at one-week intervals, and intraclass correlation coefficient values were calculated. The average of the two measurements was used for analysis. Because the measurements for the left and right sides were not significantly different, the averages of the values for the left and right sides were used. The Wilcoxon signed-rank test was used to compare suture widths between T0 and T1 in both the C-expander and ATOZ groups. Statistical significance was set at P < 0.05. All analyses were performed using SPSS software, version 25.0 (IBM Corp., Armonk, NY, USA).

The intraclass correlation coefficients for all measurements were > 0.8. The suture widths before (T0) and after (T1) expansion in the C-expander and ATOZ groups are shown in Tables 2 and 3, respectively. In the C-expander group, the frontonasal (0.22 ± 0.11 mm), frontomaxillary (0.21 ± 0.12 mm), and pterygomaxillary (0.18 ± 0.14 mm) suture widths significantly increased in the sagittal plane. In the axial plane, the internasal and intermaxillary suture width increased by 0.12 ± 0.14 mm and 2.07 ± 0.80 mm, respectively (P < 0.001). The MPS width in the canine, premolar, molar, and PNS regions increased by 2.27 ± 0.96 mm, 2.63 ± 1.13 mm, 2.66 ± 1.06 mm, and 1.98 ± 0.92 mm, respectively (all P < 0.001). In the coronal plane, the frontozygomatic, zygomaticomaxillary, and temporozygomatic suture widths decreased by –0.04 ± 0.07 mm (P < 0.01), –0.02 ± 0.05 mm (P < 0.05) and –0.02 ± 0.07 mm (P < 0.05), respectively. In the ATOZ group, significant increases in the frontonasal (0.22 ± 0.16 mm), frontomaxillary (0.19 ± 0.13 mm), and pterygomaxillary (0.15 ± 0.13 mm) suture widths were observed in the sagittal plane. In the axial plane, the internasal and intermaxillary suture widths increased by 0.17 ± 0.22 mm and 0.95 ± 0.59 mm, respectively (both P < 0.001). The MPS width in the canine, premolar, molar, and PNS regions increased by 1.05 ± 0.53 mm, 1.23 ± 0.64 mm, 1.28 ± 0.72 mm, and 1.23 ± 0.71 mm (all P < 0.001), respectively. In the coronal plane, the frontozygomatic suture width increased by 0.12 ± 0.13 mm (P < 0.01).

Table 2 . Comparison of suture widths between T0 and T1 in the C-expander group (n = 44).

Measurement	T0 (mm)					T1 (mm)					Difference between T1 and T0 (mm)				P value
	Mean	SD	Min	Max		Mean	SD	Min	Max		Mean	SD	Min	Max
Frontonasal	0.67	0.15	0.37	0.91		0.89	0.20	0.58	1.27		0.22	0.11	−0.01	0.47	< 0.0001***
Frontomaxillary	0.58	0.11	0.35	0.84		0.79	0.16	0.55	1.24		0.21	0.12	0.02	0.52	< 0.0001***
Frontozygomatic	0.66	0.11	0.52	1.02		0.63	0.11	0.41	0.92		−0.04	0.07	−0.32	0.08	0.0003***
Nasomaxillary	0.29	0.19	0.00	0.68		0.49	0.25	0.00	0.96		0.20	0.16	−0.27	0.56	< 0.0001***
Zygomaticmaxillary	0.55	0.08	0.31	0.67		0.53	0.08	0.30	0.71		−0.02	0.05	−0.12	0.09	0.0124*
Temporozygomatic	0.50	0.11	0.22	0.81		0.48	0.10	0.23	0.73		−0.02	0.07	−0.32	0.16	0.0203*
Pterygomaxillary	0.42	0.16	0.00	0.73		0.60	0.20	0.25	1.20		0.18	0.14	−0.02	0.72	< 0.0001***
Internasal	0.11	0.13	0.00	0.50		0.23	0.19	0.00	0.62		0.12	0.14	−0.04	0.52	< 0.0001***
Intermaxillary	0.15	0.18	0.00	0.64		2.22	0.84	0.53	3.86		2.07	0.80	0.53	3.71	< 0.0001***
Midpalatal suture (canine)	0.17	0.19	0.00	0.73		2.44	0.97	0.42	5.69		2.27	0.96	0.42	5.33	< 0.0001***
Midpalatal suture (premolar)	0.21	0.20	0.00	0.59		2.84	1.13	0.64	5.71		2.63	1.13	0.63	5.61	< 0.0001***
Midpalatal suture (molar)	0.46	0.17	0.00	1.01		3.12	1.11	0.76	5.95		2.66	1.06	0.54	5.30	< 0.0001***
Midpalatal suture (PNS)	0.44	0.19	0.00	0.84		2.42	1.01	0.53	5.02		1.98	0.92	0.21	4.19	< 0.0001***

Wilcoxon signed-rank test was used to compare the changes between T0 and T1..

SD, standard deviation; Min, minimum; Max, maximum; T0, before expansion; T1, after expansion; PNS, posterior nasal spine..

Significant level of *P < 0.05 and ***P < 0.001..

Table 3 . Comparison of suture widths between T0 and T1 in the ATOZ expander group (n = 37).

Measurement	T0 (mm)					T1 (mm)					Difference between T1 and T0 (mm)				P value
	Mean	SD	Min	Max		Mean	SD	Min	Max		Mean	SD	Min	Max
Frontonasal	0.53	0.18	0.00	0.83		0.75	0.20	0.31	1.12		0.22	0.16	−0.06	0.71	< 0.001***
Frontomaxillary	0.47	0.12	0.17	0.70		0.66	0.15	0.39	1.06		0.19	0.13	−0.04	0.66	< 0.001***
Frontozygomatic	0.55	0.14	0.21	0.93		0.67	0.17	0.33	1.15		0.12	0.13	−0.14	0.46	0.002**
Nasomaxillary	0.23	0.21	0.00	0.67		0.44	0.28	0.00	0.97		0.21	0.16	−0.07	0.63	0.001**
Zygomaticmaxillary	0.43	0.14	0.17	0.66		0.49	0.18	0.15	0.84		0.07	0.11	−0.19	0.30	0.064
Temporozygomatic	0.41	0.15	0.00	0.66		0.48	0.14	0.21	0.83		0.07	0.13	−0.20	0.36	0.118
Pterygomaxillary	0.35	0.15	0.00	0.72		0.49	0.18	0.23	1.02		0.15	0.13	−0.03	0.44	< 0.001***
Internasal	0.13	0.21	0.00	0.83		0.30	0.30	0.00	0.93		0.17	0.22	−0.26	0.73	0.007**
Intermaxillary	0.19	0.23	0.00	0.80		1.14	0.57	0.00	2.18		0.95	0.59	−0.39	2.03	< 0.001***
Midpalatal suture (canine)	0.18	0.20	0.00	0.80		1.22	0.57	0.00	2.51		1.05	0.53	−0.29	2.18	< 0.001***
Midpalatal suture (premolar)	0.18	0.22	0.00	0.80		1.40	0.65	0.00	2.96		1.23	0.64	0.00	2.49	< 0.001***
Midpalatal suture (molar)	0.36	0.23	0.00	0.96		1.64	0.74	0.00	3.55		1.28	0.72	0.00	3.23	< 0.001***
Midpalatal suture (PNS)	0.27	0.25	0.00	0.79		1.50	0.77	0.00	3.44		1.23	0.71	0.00	3.16	< 0.001***

Wilcoxon signed-rank test was used to compare the changes between T0 and T1..

SD, standard deviation; Min, minimum; Max, maximum; T0, before expansion; T1, after expansion; PNS, posterior nasal spine..

Significant level of **P < 0.01 and ***P < 0.001..

A comparison of sutural width changes after transverse maxillary expansion between the ATOZ and C-expander groups is shown in Table 4. The changes in the frontozygomatic, zygomaticomaxillary, temporozygomatic, pterygomaxillary, internasal, intermaxillary, and MPSs (canine, premolar, molar, and PNS regions) were significantly different between the two groups (all P < 0.05). The ATOZ group showed greater changes in the frontozygomatic, zygomaticomaxillary, temporozygomatic and internasal suture widths, whereas the C-expander group showed greater changes in the pterygomaxillary, intermaxillary, and midpalatal (canine, premolar, molar and PNS) suture widths.

Table 4 . Comparison of the changes in suture widths between the ATOZ and C-expander groups after expansion.

Measurement	C-expander (mm)			ATOZ (mm)		Difference between ATOZ and C-expander (mean, mm)	P value
	Mean	SD		Mean	SD
Frontonasal	0.22	0.11		0.22	0.16	0.00	0.591
Frontomaxillary	0.21	0.12		0.19	0.13	−0.02	0.155
Frontozygomatic	−0.04	0.07		0.12	0.13	0.16	< 0.001***
Nasomaxillary	0.20	0.16		0.21	0.16	0.01	0.894
Zygomaticmaxillary	−0.02	0.05		0.07	0.11	0.09	< 0.001***
Temporozygomatic	−0.02	0.07		0.07	0.13	0.09	< 0.001***
Pterygomaxillary	0.18	0.14		0.15	0.13	−0.03	< 0.001***
Internasal	0.12	0.14		0.17	0.22	0.05	0.042*
Intermaxillary	2.07	0.80		0.95	0.59	−1.12	< 0.001***
Midpalatal suture (canine)	2.27	0.96		1.05	0.53	−1.22	< 0.001***
Midpalatal suture (premolar)	2.63	1.13		1.23	0.64	−1.40	< 0.001***
Midpalatal suture (molar)	2.66	1.06		1.28	0.72	−1.38	< 0.001***
Midpalatal suture (PNS)	1.98	0.92		1.23	0.71	−0.75	< 0.001***

Wilcoxon signed-rank test was used to compare the changes between ATOZ and C-expander..

SD, standard deviation; PNS, posterior nasal spine..

Significant level of *P < 0.05 and ***P < 0.001..

Previous studies have shown that after maxillary expansion, the nasomaxillary complex (NMC) splits laterally in a pyramidal pattern in the coronal plane, with the frontonasal suture as the center of rotation.12,13 During expansion, the force is transmitted not only to the maxilla but also to other craniofacial structures. A three-dimensional finite-element study revealed that during rapid orthopedic expansion, high-pressure stresses occur in the maxillary first molar region; additionally, significant stresses are observed around the frontal process of the maxilla, and the nasomaxillary, frontonasal, frontomaxillary, and zygomaticomaxillary sutures.14 Animal studies have indicated that during expansion, the dominant strain polarity is compression at the zygomaticomaxillary and temporozygomatic sutures, and tension at the maxillo-premaxillary suture.15,16 Growth at the craniofacial sutures, which are soft connective tissue joints between the mineralized skull bones, is influenced by genetic and mechanical signals; in other words, mechanical stimulation can effectively regulate suture growth.17 Therefore, tension and compression during expansion may affect the sutures. Correspondingly, changes in the CMS during transverse maxillary expansion, from activation and modification to repositioning of the NMC, also affect expansion stability. As early as the mid-20th century, Isaacson and Ingram18 suggested that expansion forces are transmitted to the facial bones through alterations in the skull’s periosteal envelope. This process guides the repositioning of the maxillary complex, and the stability is directly related to the amount of reconstruction at the CMS.18 Prado et al.19 reported limited retention of transverse maxillary expansion with a bonded metal device after surgically assisted rapid palatal expansion, which only separates the MPS. Furthermore, de Oliveira et al.20 reported that MARPE had a more pronounced activation effect on the CMS than surgically assisted rapid palatal expansion and resulted in better skeletal stability.

In this study, the C- and ATOZ expanders primarily affected the sutures in the naso-maxillo-zygomatic and circumscribed zygomatic regions. The suture widths significantly increased in the naso-maxillo-zygomatic region in both the C-expander and ATOZ groups. This could be attributed to the separation of the sutures in this region owing to the tension generated by the expansion devices. The frontonasal, frontomaxillary, pterygomaxillary, internasal, nasomaxillary, intermaxillary, and MPS widths increased significantly after expansion in both groups. This pattern resembled the effects of conventional RME and MSEs on the sutures in the previous studies.7,21 The expansion patterns in the frontonasal region with RME and MSEs, such as the C- and ATOZ expanders are similar.7,21 Both appliances can cause suture widening in the maxillary frontonasal region during expansion. The tension generated by the expansion screw is transmitted to other sutures through the maxilla, resulting in widening of the sutures in the areas under tension. However, in this study, compared to the ATOZ expander, the C-expander caused more pronounced changes in the MPS immediately after activation in late adolescent and young adult patients. Interdigitation of the mature MPS is the primary anatomical reason for resistance to the expansion force exerted by the expansion device. Conventional hyrax type C-expanders use heavy interrupted forces to rupture sutures; however, the ATOZ expander continuously applies 7–12 kgf of force to the MPS and CMS over an extended period.11 Even after the MPS has separated, the CMS resist expansion, with the posterosuperior CMS often being the last to yield.22

The greater changes in MPS width may be related to the anchorage position of the C-expander; the miniscrews located farther from the MPS result in a longer force arm.23 In contrast, the ATOZ expander produced a more evenly distributed expansion force between the bilateral NMC, resulting in a more parallel expansion of the maxilla in the coronal plane. These results are consistent with those of a previous study on the skeletal expansion.11

In the circumscribed zygomatic region, the C-expander group showed significant reduction in the widths of the frontozygomatic, zygomaticomaxillary, and temporozygomatic sutures. The ATOZ group exhibited a slight increase in the width of the frontozygomatic sutures, but no significant changes in the widths of the zygomaticomaxillary and temporozygomatic sutures (Tables 2 and 3). These findings contradict the findings of recent studies on MSE, in which the sutures in the circumscribed zygomatic region showed a tendency for widening during MSE-induced expansion.24 Isaacson et al. suggested that the structure of the sphenoid and zygomatic bones might influence maxillary expansion.18,25 Mao17 suggested that under compressive loading, the interosseous intersections of the premaxillomaxillary suture may be physically removed owing to the removal of microfractured bone, creating a larger suture space that is subsequently replaced by fibrous suture tissue, ultimately resulting in increased suture width.26 Naso-maxillo-zygomatic repositioning during expansion involves a complex process, with the maximal bone effects exerted to separate the bilateral parts. In the ATOZ group, the circumscribed zygomatic region primarily experienced compressive stress, which increased the frontozygomatic suture width. In contrast, the C-expander group showed decreased suture widths in the circumscribed zygomatic region. This can be attributed to the more pronounced effect of the ATOZ expander on the bone.11 The actual repositioning of the naso-maxillo-zygomatic complex is achieved through the deposition of new bone in the sutures. However, the C-expander likely induced greater rotation, with the frontonasal suture as the center of rotation. Therefore, the suture was compressed on one side and stretched on the other rather than increasing the volume of the entire suture.

In this study, suture changes were measured on a slice containing the entire suture; however, evaluation of volume change in the sutures requires further three-dimensional spatial research. The expansion protocols of the C and ATOZ expanders used in this study differ from those of MSEs that are commonly used for RME. These devices provide slow-to-semi-slow expansion that can maintain sutural integrity.27 The MARPE technique is a non-surgical distraction osteogenesis technique involving deposition of new bone and overlying soft tissue by applying a gradual and controlled traction force.28 This process involves separating the suture in a short period to create regional microfractures and form calluses. The expansion protocol and activation principle of the ATOZ expander are consistent with this process.29,30 However, for MSE, rapid expansion is necessary to transmit the force from the teeth to the skeletal sutures as quickly as possible. Therefore, the expansion protocol should be adjusted according to anchorage type and locations of skeletal anchorages in the various types of MARPE appliances.

Our understanding of the physiology of the MPS and CMS is still evolving. However, the sutures and periodontal ligament (PDL) exhibit considerable histological similarity, and their response to force occurs through mediation by fibroblasts and osteoblasts, which is also similar.31 Figure 11A shows tooth movement with a lag period of 2–10 weeks.32 The threshold force for tooth movement is determined by the PDL area associated with the specific movement of tooth. A threshold force of 20–50 g is required for the tipping movement shown in Figure 11A. During tipping movement, the entire PDL is not involved; extremely limited areas in the cervical and apical regions act as the compression sides. The suture area is significantly larger than the PDL area involved in tooth movement. Precisely calculating the surface area of the suture resisting the expansion force exerted by the expander is challenging owing to variations in interdigitation and individual differences. However, it is estimated to be hundreds of times larger than the PDL area involved in the tipping movement of a single tooth. As the suture interdigitation becomes more pronounced, more time is required for effective bone remodeling. After the bony undercuts are disengaged, the suture can be widened by tensile forces. This process is analogous to the lag period in orthodontic tooth movement, where teeth do not move during the indirect bone resorption phase, but movement occurs during the subsequent direct bone resorption phase. The severity of sutural interdigitation increases with age; therefore, the lag period required for sutural distraction increases as patients get older. Clinical trials across diverse age groups have shown that the lag period typically ranges from 1–2 weeks in children, 4–6 weeks in adolescents, and 6–12 weeks or more in adults. This progression can be illustrated in a figure analogous to the well-known graph depicting tooth movement over time (Figure 11B).

Figure 11. Schematic illustrations of tooth displacement following moderate orthodontic force application (A) and midpalatal suture (MPS) and circummaxillary suture (CMS) separation after ATOZ expander activation (B).
Adapted from the book of Graber et al. (Orthodontics: current principles and techniques. 6th ed. St. Louis: Elsevier; 2017. Chapter 4, Figure 4-49 with original copyright holder’s permission.32

This retrospective study had inherent limitations. Our results showed that the changes in each suture are minimal, underscoring the need for more precise measurement methods. Additionally, a larger sample size is essential to increase the effect size and ensure that any small but clinically meaningful changes can be accurately detected. These improvements would strengthen the validity of the findings and provide a clearer understanding of the true impact. Further studies with long-term retention are required to evaluate the actual expansion effects of the ATOZ at the MPS and CMS. Another limitation of this study is that the accuracy of the measurements was limited by the voxel size of the CBCT. However, CBCT with a voxel size of 0.2 and 0.3 mm provides reasonable accuracy at lower radiation doses.21

1. C and ATOZ expanders affect the CMS during expansion.

2. Both the C and ATOZ expanders widen the bone sutures in the naso-maxillo-zygomatic region.

3. In the circumscribed zygomatic region, the C-expander decreases the width of the bone sutures, whereas the ATOZ expander does not affect any bone suture except for the frontozygomatic suture, in which it increases the suture width.

The authors thank Dr. Heon Jae Cho, CEO of 3DONS Company, and Sung-Chul Moon, Clinical Adjunct Professor of the Department of Orthodontics, Seoul National University, School of Dentistry, Seoul for editing during the article preparation.

Conceptualization: All authors. Data curation: BX. Formal analysis: BX, JJP, JB. Investigation: BX, JJP. Methodology: All authors. Project administration: JJP, SHK. Resources: SHK. Software: BX. Supervision: JJP, SHK. Validation: BX, JJP, JB. Visualization: BX. Writing–original draft: BX. Writing–review & editing: JJP, SHK.

No potential conflict of interest relevant to this article was reported.

None to declare.