Skeletal and Dentoalveolar Changes after Skeletal Anchorage-assisted Rapid Palatal Expansion in Young Adults: A Cone Beam Computed Tomography Study

Introduction

Maxillary transverse deficiency is a common orthodontic finding that is reported in 21% of children and 10% of adult population.^[1] Rapid palatal expansion (RPE) is an effective orthodontic approach for widening the maxilla and correct the transverse discrepancy in children. By exerting a rapid transverse force on the maxillary dentition, the midpalatal suture is disrupted and separated, leading to greater cellular activity resulting in bone remodeling along the sutural areas.^[2,3] In adults due to the maturation of the midpalatal sutures and resistance from the zygomatic buttress, the response to palatal expansion with RPE is less successful. The resulting widening of the maxillary width is more due to alveolar bone bending and dental tipping in adults.^[4]

In addition to its inability to produce complete skeletal expansion in adults, the RPE can bring about unwanted side effects such as arch expansion failure, buccal crown tipping, alveolar bone dehiscence, root resorption, reduction in buccal bone thickness, and marginal bone loss.^[5,6] Surgically assisted RPE (SARPE) is a treatment modality that helps to overcome the increased resistance from the bony plates and zygomatic buttress in adults.^[7] However, SARPE has been reported to suffer from several limitations including but not limiting to surgical morbidity, high cost, and periodontal complications.^[6,8] Recently, miniscrew-assisted RPE (MARPE) appliances that can localize the lateral forces to the midpalatal suture by using mini implants for anchorage have become available for treatment of maxillary constriction in adults.^[9]

A bone-borne expander that is purely skeletally anchored and free of alveolar and dental side effects have been utilized to bring about pure skeletal expansion in adults.^[10] Of interest is a particular study that compared a surgically assisted, bone anchored RPE and traditional tooth anchored RPE. It concluded that the bone anchored RPE produces more skeletal than dentoalveolar changes.^[11] However, it has been reported in an another study that both expansions produce similar results.^[12] Although bone-borne expanders are capable of producing greater skeletal expansion, there are concerns regarding its stability. The interlocked suture in mature patients undergoes significant torsion in three dimensions. As it splits the two halves of the maxilla twist away from each other, the implants used to anchor the RPE will experience additional strain and may tip. This will subsequently result either in loosening or breakage of the implant, thereby rendering the expansion device unstable.^[13]

Wilmes et al. suggested the use of the term hybrid expander to any device that is used for expanding maxilla that is skeletally and dentally anchored.^[14] These devices are reported to produce greater skeletal expansion ensuring minimal alveolar/dental tipping and at the same time offer greater stability. Maximization of the skeletal expansion through skeletal anchorage enhancement is by recruiting both palatal and nasal cortices. At the same time, a support wire connecting the expander to the I molars enhances the stability. This was a recent concept and such expanders are referred to as maxillary skeletal expander (MSE).^[13] There are only a few reports on the skeletal and dentoalveolar changes, with this appliance.

The aim of this pilot study was to evaluate the short-term skeletal and dentoalveolar changes before and after MSE in young adults with maxillary transverse discrepancies. We hypothesized that the parameters do not differ significantly between pre- and postexpansion.

Materials and Methods

The sample for the prospective study was recruited from the patients who reported to the Department of Orthodontics at Ragas Dental College and Hospital. Ethical approval was obtained from the Institutional Review Board and informed written consent was obtained from all the participants. The sample consists of ten patients (5 boys, 5 girls) within the age group of 19–24 and with the mean age of 21.5 years.

The inclusion criteria were as follows: maxillary transverse deficiency, skeletal Class 1 malocclusion, no developmental deformity, no cleft lip and palate, no history of previous orthodontic treatment, and need for nonextraction orthodontic treatment. Pretreatment study models, photographs, and cone beam computed tomography (CBCT) were taken at the start of treatment and after completion of stabilization of maxillary expansion.

Appliance design

The expansion device which was used is MSE (BioMaterials, Korea). As per the manufacturer’s instruction, the screw size was selected based on the constriction of the palatal wall; appliance was fabricated by sizing the molar bands, taking a pick-up impression, and pouring it in stone. Placing the appliance 1–2 mm anterior to the junction of hard and soft palate, the lateral arms were contoured to the curvature of the palatal shelves and soldered to the molar bands. The central jack screw expander was flush against the palate and the supporting arm had 2 mm clearance to the lateral wall of palate.^[15]

Four orthodontic mini screws (BioMaterials, Korea) measuring 1.8 mm × 11 mm were inserted into the palate using the Mini Handle Driver (BioMaterials, Korea). Care was taken to ensure that opposing screws were fitted first to prevent distortion or movement of the device and lifting up of the appliance on one side and the appliance was installed.

Maxillary expansion was initiated 2 days after insertion of the device. The appliance was then activated 2 turns per day until the required expansion was achieved. After achieving intended expansion, the expansion screw was blocked, and the appliance was left in place for 4 months [Figure 1], following which posttreatment CBCT, models, and photographs were taken.

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Figure 1

Skeletal expander. (a) Pre-expansion (b) Postexpansion

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Measurements

Pre- and posttreatment CBCT images were obtained on a Kodak equipment (Model CS 9300, Carestream Health, Inc, Rochester, NY, USA) which was set at 8.0 mA and 70 kV and images were acquired for 6.15 s and with an axial slice thickness of 0.18 mm. During image acquisition, the patients were oriented to ensure that the Frankfort horizontal plane was parallel to the floor.

The Digital Imaging and Communications in Medicine images were imported, and cross-sectional slices were made with the help of Dolphin imaging software (version 11.5, Dolphin Imaging and Management Solution, Chatsworth, CA, USA). Three-dimensional image reconstructions were standardized using the Frankfort horizontal plane (represented by a line on the image) as the X-axis, the transporionic plane as the Y-axis, and the midsagittal plane as the Z-axis [Figure 2].

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Figure 2

Cone beam computed tomography image orientation

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Transverse expansion was evaluated at the level of maxillary I molars, II premolars (PM), I PM, canines, and incisors on the coronal slice.

Transverse expansion at the sutural level was evaluated at the medial limits of left and right palatine process in the region of maxillary incisor (S1, S2), canine (S3, S4), I PM (S5, S6), II PM (S7, S8), and I molar (S9, S10).

Expansion at the alveolar level was evaluated from the most coronal medial limits of left and right alveolar process in the region of canine (AL1, AL2), I PM (AL3, AL4), II PM (AL5, AL6), and I molar (AL7, AL8).

Dental expansion was evaluated as the distance between the crown tips of I molars, II PM, I PM, and canines to calculate the intercanine width (ICW), inter-PM width (I and II IPMW), and intermolar width (IMW).

Tooth angulation was evaluated by calculating the angle between palatal root axis and the nasal floor at the region of canine (TOR1, TOL2), I PM (TOR3, TOL4), II PM (TOR5, TOL6), and I molar (TOR7, TOL8).

For evaluation of transverse cranial measurements, a posteroanterior view was reconstructed perpendicular to the midsagittal plane, and the bilateral landmarks of nasal cavity (N), zygoma (Z), and frontonasal (F) areas were identified in these images.

The landmarks evaluated in the study and their definition is summarized in Table 1.

Table 1 Landmarks for transverse maxillary evaluation

Landmarks	Description
S	Medial limits of the palatine process at left and right central incisors - S1, S2 canine (S3, S4), I premolars (S5, S6), II premolars (S7, S8), I molar (S9, S10)
AL	Medial limits of the alveolar process at left and right canine (AL1, AL2), I premolars (AL3, AL4), II premolars (AL5, AL6), and I molar (AL7, AL8)
IC	Medial points of palatal crown tip at canine
I PM	Medial points of palatal crown tip at I premolars
II PM	Medial points of palatal crown tip at II premolars
IM	Medial points of palatal crown tip at molar
TOL and TOR	Inclination between the palatal root axis and NF at left (TOL) and right (TOR)
	Canine (TOR1, TOL2), I premolars (TOR3, TOL4), II premolars (TOR5, TOL6), and I molar (TOR7, TOL8)
N	Lateral most border of nasal cavity
Z	Lateral most border of zygoma
F	Lateral most border of frontonasal level

NF – Nasal floor; IC – Inter-canine

All transversal linear and angular measurements were recorded for each scan before insertion of expansion device (T1) and after stabilization (T2). One investigator performed the measurements. All points measured are depicted in Figures 3 and 4.

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Figure 3

Cone beam computed tomography coronal slice landmarks. S – Sutural level expansion at the medial limits of the palatine process; AL – Alveolar level expansion at coronal most medial limits of the alveolar process; TOR and TOL – Tooth inclination of palatal root to nasal floor (TOR: Right side, TOL: Left side); ICW – Inter canine width; I PMW – I Inter premolar width; II PMW – II Inter PM width; IMW – I Intermolar width

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Figure 4

Landmarks for nasal and cranial structures. Cone beam computed tomography at the nasal and cranial level landmarks. N – Lateral most border of nasal cavity; Z – Lateral most border of zygoma; F – Lateral most border at the frontonasal level

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Suture angulation

Using the same software, a section running through the center of the maxillary palatal bone was selected, generating a slice in the horizontal plane to pass the middle of the palatal bone, allowing for the best visualization of the opening of the midpalatal suture.

This axial slice image was used to measure the relative parallelism between the left and right sides of the open palatal suture [Figure 5].

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Figure 5

Axial view showing suture opening before and after expansion. (a) Pre-expansion. (b) Postexpansion

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A suture that demonstrated parallel opening (when the amount of expansion achieved in the anterior portion of the palatal suture was the same as the amount of expansion achieved in the posterior portion of the palatal suture) would have a 0° opening.

A suture that achieved more expansion in the anterior portion as compared to the posterior portion of the palatal suture would have a positive degree. The results for patients treated with the expander were then averaged to derive the degree of skeletal opening.

Results

The midpalatal suture was separated in all the patients, and the maxilla exhibited significant lateral movement [Tables 2-4].

Discussion

The present pilot study was designed to evaluate the dentoalveolar and skeletal changes of a modified miniscrew-supported palatal expander in young adults using CBCT. The expander is designed in such a way that it delivers the expansion forces to 4 mini implants that are capable of anchoring the device to both nasal and oral cortical layer.^[17-19] In addition, the maxillary I molars are used to stabilize the position of the expansion screws rather than for anchorage.^[13] The appliance is positioned in the IM area so as to direct the lateral forces against pterygomaxillary buttress which offers resistance to expansion in adults.^[20] The participants recruited for the present study were young adults in whom the midpalatal suture and the adjacent articulation would have matured, and same being confirmed as observed in pretreatment CBCT. The study has used CBCT to measure the parameters considered, as the versatility of the CBCT and imaging software^[21-23] for identifying the dimensional changes has been reported.

To determine the difference between two time points for each parameter, a paired sample t-test was calculated because of the small sample size, and the level of significance was defined as P < 0.05. The paired sample t-test is considered robust to violations of normality as it is reported to have inbuilt mechanism to account for minor variation in normality distribution.^[18]

Clinical translation of the result, as observed by the effect size [Table 2], indicates that the appliance produces a consistent, predictable result. The MSE effectively achieved dentoalveolar as well as skeletal expansion by separating the midpalatal suture in all the participants. The suture expansion was greater along the incisor region than in the molar region. This triangular pattern of sutural expansion is similar to that reported with RPE by Garrett et al.^[19] However, the divergence from the posterior to the anterior of the suture in the present study was relatively less compared to those reported with RPE in literature. The reason for difference in degree of divergent opening of suture in the present sample could be due to the more posterior position of jack screw and the parallel placement of the four miniscrews that probably are more effective in overcoming the resistance offered by the zygomatic buttress. A similar but more parallel expansion of suture has been reported with the use of bone-borne expansion device.^[21]

The skeletal gain in our sample accounts for 61% of total expansion and is higher than the 50% skeletal expansion that has been reported by Profitt and 40% by Kartalian et al.^[24] and 55% reported by Garrett et al.^[6] The greater percentage of skeletal expansion in our sample can be attributed to the better anchorage potential of the four miniscrews that are capable of bicortical engagement in this design.^[23] Tausche et al.^[7] using CT has documented that in their sample, 85%–91% of expansion achieved was skeletal in nature. However, in their participants, a surgical intervention was employed along with distraction unlike in our sample where no surgeries were performed.

The alveolar width increase was significant and contributed to nearly 20% of total expansion. This is similar to 22.2% expansion at cementoenamel junction, as reported with mini implant-assisted RPE (MARPE) by Mary.^[25] Handlemen has reported that the quantum of alveolar expansion to be greater in adults than in adolescents and contribute to nearly 33% of total expansion when RPE was used in adults.^[4]

The greatest changes following expansion were at the IM level; this reflects the combined skeletal and dentoalveolar expansion. Of the total IM increase of 7.33 ± 1.96 mm, the dental expansion at the level of the molar was found to be 2.83 ± 1.96 mm and accounted for 19% of total expansion. This increase in the IM width at dental level is relatively less when compared to other reports employing MARPE device. The most probable reason could be due to the nature of the design of the MSE, especially the bicortical engagement with the long implants offering greater skeletal anchorage.

Despite the fact that the appliance was skeletally anchored, there was evidence of buccal tipping of the PM and molars in a few cases. This is similar to prior reports employing MARPE and RPE.^[12] The molars experienced great tipping than the PM and can be attributed to the inability of the stabilizing wire from transmitting expansion forces to the molars. The crown tipping can also be due to the play between the miniscrew and the insertion slot of miniscrew, as reported by Carlson et al.^[15]

The skeletal expansion observed in the present study includes the possible expansion of the nasal cavity, zygomatic arch, and frontonasal area; this reflects a pyramidal pattern of expansion with the base at the nasal cavity and similar to expansion pattern reported by conventional RPE,^[2,3] SARPE,^[7] and MARPE.^[26] The extent of the influence of this pyramidal expansion on the sutural expansion and the dentoalveolar width warrants further study.

Based on the present results and observations, the null hypothesis is rejected. The present pilot study identifies the effectiveness of the new MSE design for maxillary widening, even with small sample size. The extent of the effectiveness and difference in parameters considered, especially those relating to the width of the arch, at several positions as well as those that of dental tipping still remain relatively elusive. Studies using larger sample size perhaps would help to understand the complete dynamics of the expansion when employing MSE.

Conclusion

Within the limitations of this study, our results suggest the following: The skeletal expansion device can be an effective treatment modality for the correction of maxillary transverse deficiency in young adults through separation of the midpalatal suture. Expansion achieved with the appliance exhibits a pyramidal pattern. The degree of skeletal, alveolar, and dental expansion was 61%, 20%, and 19%, respectively. There was evidence of mild buccal tipping of the maxillary teeth. The expansion also caused widening of the nasal cavity, the zygomatic arch, and frontonasal area.