Long-term evaluation of soft-tissue changes after miniscrew-assisted and conventional rapid palatal expansion using voxel-based superimposition of cone-beam computed tomography scans

Introduction

This study aimed to evaluate the soft-tissue changes in the long-term after miniscrew-assisted rapid palatal expansion (MARPE) and conventional rapid palatal expansion (RPE) appliances compared with a matched control group using voxel-based superimposition of cone-beam computed tomography (CBCT) scans.

Methods

A total of 180 CBCTs for 60 patients at 3-time points were evaluated: pretreatment (T1), postexpansion (T2), and posttreatment (T3) for 3 groups: (1) MARPE, (2) RPE, and (3) controls (time-period T1 to T3: MARPE, 2 years 8 months; RPE, 2 years 9 months; control, 2 years 7 months). The voxel-based superimposition technique was used to superimpose the CBCT scans, after which the soft-tissue surfaces were extracted from the superimposed T1-CBCT, T2-CBCT, and T3-CBCT scans. Nine landmarks were identified on the CBCT scans: nasion, A-point, pogonion, right and left alar base, right and left zygoma, and right and left gonion. The coordinates of the 9 parameters were obtained in the x-axis, y-axis, and z-axis for the CBCT scans and subjected to statistical analyses. The changes in the soft-tissue surfaces were also evaluated by color-coded maps for short-term (T2) and long-term (T3) changes. The mean changes from T1 to T2 and T1 to T3 were tested against no change within the groups by paired t test, and the mean changes among the 3 groups were compared with analysis of variance F test with Tukey’s Honest significant difference used for adjusting P values for multiple testing.

Results

In the short term, both MARPE and RPE led to a significant downward movement of pogonion, left gonion, and lateral movement of the right and left alar base compared with controls at T2 ( P <0.05). In addition, MARPE led to a significant downward movement of right gonion than controls at T2 ( P <0.05). Moreover, RPE led to a significant downward and forward movement of A-point and downward movement of the right and left alar base than controls at T2 ( P <0.05). However, in the long-term, there were no significant differences in the soft-tissue changes among the MARPE, RPE, and control groups.

Conclusions

MARPE and RPE do not lead to significant soft-tissue changes in the long term when compared with controls.

Highlights

•

MARPE and RPE caused downward movement of pogonion and left gonion than controls in the short term.
•

MARPE and RPE caused lateral movement of alar bases than controls in the short term.
•

There were no differences in the soft-tissue changes among the 3 groups in the long term.

Maxillary deficiency in the transverse plane is a common condition in the adolescent population seeking orthodontic treatment. It results in the development of posterior crossbite, which is found to be prevalent in 9% to 23%. ^, A frequent option used for the treatment of posterior crossbite is rapid palatal expansion (RPE). ^, RPE appliance consists of an expansion screw that is connected to posterior teeth, typically maxillary molars and maxillary premolars. The activation of the expansion screw results in a transverse force on the maxillary structures leading to the correction of the transverse maxillary deficiency. However, RPE has been reported to lead to certain side effects, such as root resorption, alveolar bone loss, and dental relapse. ^, Thus, to overcome such dental side effects, miniscrew-assisted rapid palatal expansion (MARPE) has been introduced in the quest for attaining more skeletal expansion. In the MARPE design, the expansion screw is connected to palatal mini-implants, and thus, it derives anchorage from the palatal bone as opposed to maxillary posterior teeth as in RPE. ^,

In addition to the skeletal and dental assessment of an orthodontic patient, the evaluation of soft-tissue esthetics plays an important role in comprehensive diagnosis and treatment planning in modern orthodontics. The esthetics of smile and face are one of the primary motivators for patients to undergo orthodontic treatment. ^, It has been reported that the social impact of the esthetics of smile and face is wide-ranging, and it affects patients’ self-perception and overall confidence. ^, Although soft-tissue diagnostics and esthetics of smiles and faces have been incorporated in orthodontic treatment planning for long, the methods to assess the soft-tissue dimensions and changes with orthodontic treatment have some limitations. Two-dimensional (2D) photographs and radiographs are popularly used in clinical practice; however, they provide a 2D representation of the 3-dimensional (3D) structures of the head and neck and thus are unable to provide accurate, reliable, and complete information regarding soft-tissue esthetics. ^, Direct anthropometric measurements can be performed reliably but are difficult and lengthy. In addition, 3D photography looks encouraging but is not very commonly used in orthodontics.

Cone-beam computed tomography (CBCT) scans provide a 3D representation of both the hard and soft tissues of the patient without magnification and distortion. The superimposition of CBCT scans on stable reference structures enables the evaluation of the changes in the soft tissues of the face occurring because of orthodontic treatment modalities and growth. Superimposition of CBCT scans recorded at different time intervals can be undertaken with different techniques such as landmark-based superimposition, surface-based superimposition, and voxel-based superimposition techniques. However, because of certain drawbacks, such as a lengthy process required for plotting a high number of landmarks and the errors during segmentation with landmark- and surface-based superimposition techniques, respectively, the voxel-based technique has gained more popularity in recent times for CBCT superimposition. In the voxel-based technique, the best-fit approach is used for the superimposition of the original CBCT volume (voxel gray scale values). The voxel-based superimposition techniques have been reported to be highly reliable and user-friendly. ^,

The effects of MARPE and RPE on skeletal and dental tissues have been investigated previously. ^, However, the literature on the effects of the different expansion appliances on soft tissues is scarce. Furthermore, the effects of MARPE and RPE have not been investigated in the long term. The lack of controls and inadequate follow-up period in the current literature makes it difficult to have meaningful conclusions that can be applied clinically. Thus, the important clinically relevant question is whether the different expansion appliances can lead to a difference in the soft-tissue esthetics of the patients in the long term.

Thus, the objective of this study was to evaluate the effects of MARPE and RPE on soft-tissue changes in the short term and long-term in comparison with controls using 3D CBCT superimposition. Our null hypothesis was that there are no differences in the soft-tissue changes with MARPE, RPE, and controls in the long term.

Material and methods

This retrospective study was undertaken at the University of Connecticut (Institutional Review Board no. SM1168). The records of the patients enrolled in a randomized controlled clinical trial at the University of Alberta were evaluated retrospectively to assess the effects of expansion appliances on soft-tissue changes of the face. All the patients were treated in the orthodontic clinic at the University of Alberta. The randomization of the patients led to the creation of 3 groups: (1) MARPE, (2) RPE, and (3) controls. The inclusion criteria for this study consisted of patients aged 11-15 years with bilateral posterior crossbite, which was assessed by clinical examination, no systemic diseases, no temporomandibular disorder, no orthodontic treatment before the study, and no surgical interventions such as adenoidectomy or tonsillectomy.

RPE appliance was designed so that the expansion screw was anchored to the maxillary molar and premolars. For the MARPE appliance, 2 miniscrews (12 mm × 1.5 mm; Straumann GBR System, Andover, Mass) were inserted into the palatal bone adjacent to the maxillary first molars, and the expansion screw was anchored on the miniscrews ( Fig 1 ). The activation of the expansion appliance was performed by opening the expansion screw by 2 turns/d (1 turn = 0.25 mm/turn; 2 turns/d = 0.5 mm/d). The expansion was performed until the posterior crossbite was corrected, and overexpansion was achieved until the palatal cusps of the maxillary first molar were in contact with the buccal cusps of the mandibular first molars. After active expansion treatment, the expansion screw was fixed with light-cured acrylic.

All the CBCT scans were recorded with the same machine (iCAT Imaging Sciences International, Hartfield, Penn; 120 kVp, 20 mA, large field of view, 8.9 seconds exposure time, and 0.3 voxels) at 3 different periods: Pretreatment (T1: CBCT was recorded before initiating orthodontic treatment) postexpansion (T2: CBCT was recorded 6 months after the expansion in MARPE and RPE groups and 6 months after the T1 in controls. After T2, all 3 groups were bonded with fixed preadjusted edgewise orthodontic appliances), and posttreatment (T3: CBCT was recorded at the end of orthodontic treatment when the patients were debonded). All the patients were informed regarding the exposure to radiation with CBCT. The potential risks from radiation exposure with CBCT scans were minimal. For CBCT scans, the radiation dose can be as low as 50 μSv, and the yearly limit of effective dosage for infrequent radiation exposure is 5 mSv. The length of treatment from T1 to T3 was 2 years 8 months for MARPE, 2 years 9 months for RPE, and 2 years 7 months for controls. The average age of the patients was 13.69 ± 1.74 years (n =20 patients) in the MARPE group, 13.9 ± 1.14 years (n = 21 patients) in the RPE group, and 13.3 ± 1.49 (n = 19 patients) in the control group.

A total of 180 CBCT scans were analyzed in this study, and 3 CBCT scans were excluded because of motion artifacts. Digital imaging and communication in medicine data were used to reconstruct the CBCT images with Dolphin Imaging software (version 11.9; Dolphin Imaging and Management Solutions, Chatsworth, Calif). The voxel-based superimposition technique, as described by Kanavakis et al ^, was used to superimpose the CBCT scans from T1 and T2. The anterior cranial base was used as the reference for the CBCT superimpositions as it is considered to be a stable reference. ^, ^, The anterior cranial base was identified on the multiplanar reconstruction views on the T1 CBCT scans by defining a box incorporating the voxels to be used for the superimposition. ^, Once the box was defined, repositioning of the T2 CBCT scan volume was performed using the multiplanar view as a guide to approximate and superimpose the T2 CBCT scans over the T1 CBCT scans ( Fig 2 ). The outcome of the superimposition was assessed by visually inspecting in multiplanar view whether there is adequate overlapping of the structures of the anterior cranial base reference structures among the CBCT scans at different periods. If the overlap was found to be inadequate, then the automatic registration process of the superimposition was repeated until the 2 CBCT scans were found to be reasonably overlapped by the investigator. Similarly, the voxel-based superimposition technique was then used to perform the superimposition between the CBCT scans at T1 and T3. The T1 CBCT scans were used as the reference scans, and the CBCT taken at T2 and T3 were reoriented and saved with the newly obtained coordinates in the x-axis, y-axis, and z-axis after superimposition on T1 CBCT scans ( Fig 3 ).

Once the superimposition was done, the soft-tissue surfaces were extracted from the superimposed T1, T2, and T3 CBCT scans. The soft-tissue segmentation of the reoriented CBCT scans was performed with the automated function in Dolphin Imaging software that selects the same threshold for a single dataset. Thus, the soft-tissue surface obtained from T1 CBCT scans was not affected by the segmentation factor. The segmentation of T2 and T3 CBCT scans was carried out manually through real-time visual inspection of the surface models so that the resultant surface was smooth and continuous with the least possible holes and artifacts, and the soft-tissue surfaces were extracted in stereolithography file format. The soft-tissue surfaces stereolithography files were imported into the Viewbox 4 software (version 4.1.0.10, 64-bit, dHAL software), which has been thoroughly investigated for surface-modeling processing. ^, Nine landmarks were selected for evaluating the treatment effects: nasion, A-point, pogonion, right and left alar base, right and left zygoma, and right and left gonion as shown by Kanavakis et al. The coordinates of the 9 parameters were obtained for the T1, T2, and T3 CBCT scans ( Fig 4 ). The changes in the soft-tissue surfaces on CBCT scans from T1 to T2 and T3 were evaluated by color-coded maps ( Fig 5 ). The changes in the soft-tissue surfaces in the x-axis, y-axis, and z-axis were calculated and subjected to statistical analyses.

All the procedures and measurements were performed by 1 investigator. Twenty random CBCT pairs were analyzed again with the whole superimposition procedure, segmentation procedure, and measurements after a washout period of 4 weeks for intraexaminer reliability.

Statistical analysis

Analysis of variance F test determined that 19 samples per group would allow detection of 0.7 standard deviations mean the difference among the groups in change from T1 to T3, for 80% power at a 5% significance level. The normality of the distribution of data was verified through QQ plots and the Shapiro-Wilk test. The data were observed as the mean and standard deviation of the variables with a normal distribution. Intraclass correlation coefficient and Dahlberg’s method were used to calculate the intraexaminer reliability and the method error for all 9 parameters. The mean changes from T1 to T2 and T1 to T3 were tested against no change within the groups by paired t test, and the mean changes among the 3 groups were compared with repeated measures analysis of variance F test. In addition, 3 between-group comparisons and Tukey’s Honest significant difference were used for adjusting P values for multiple testing. All statistical analyses were performed using GraphPad Prism (version 9; GraphPad Software, La Jolla, Calif) with a significance level of 0.05.

Results

The intraexaminer reliability showed good reliability for all parameters with intraclass correlation values of >0.931.

In both the MARPE and RPE groups, a significant change was observed ( P <0.05) in the downward movement of A-point (mean change 0.86 mm and 1.11 mm for MARPE and RPE respectively), pogonion (1.77 mm, 1.39 mm), right gonion (0.59 mm, 0.54 mm) and left gonion (0.61 mm, 0.75 mm); lateral movement of right alar base (0.92 mm, 0.87 mm), left alar base (1.11 mm, 1.07 mm), right zygoma (0.31 mm, 0.23 mm), and left zygoma (0.42 mm, 0.24 mm) at T2 compared with T1 ( P <0.05, Tables I and II ). In addition, in the MARPE group, a significant posterior movement of the right alar base (0.83 mm), left alar base (1.44 mm), and left gonion (1.14 mm) was observed at T2 than T1 ( P <0.05, Table I ). Moreover, in the RPE group, a significant anterior movement of A-point (1.22 mm), downward movement of right alar base (0.64 mm), and left alar base (0.60 mm) was observed at T2 compared with T1 ( P <0.05, Table II ). In the control group, no significant changes were observed at T2 compared with T1 ( Table III ).

Table I

Parameters for the MARPE group at T1, T2, and T3

Parameters	T2 − T1		T3 − T1		P values (T2 vs T1)	P values (T3 vs T1)
Parameters	Mean (95% CI)	Min, max	Mean (95% CI)	Min, max	P values (T2 vs T1)	P values (T3 vs T1)
Naison
x	−0.13 (−0.36 to 0.11)	−1.7 to 0.6	−0.10 (−0.60 to 0.40)	−1.7 to 3.3	0.281	0.680
y	0.04 (−0.07 to 0.15)	−0.7 to 0.3	−0.08 (−0.30 to 0.14)	−1.2 to 0.7	0.442	0.460
z	−0.46 (−1.40 to 0.49)	−7.4 to 2.0	0.10 (−1.09 to 1.29)	−9.2 to 2.2	0.325	0.862
A-point
x	0.01 (−0.21 to 0.22)	−0.7 to 1.1	0.06 (−0.24 to 0.35)	−1.7 to 1.4	0.961	0.683
y	−0.86 (−1.55 to −0.16)	−4.9 to 1.5	−2.64 (−3.45 to −1.83)	−5.7 to 0.6	0.018 ^∗	<0.001 ^∗
z	0.65 (−0.08 to 1.38)	−1.3 to 5.0	0.83 (−0.22 to 1.88)	−4.6 to 5.4	0.078	0.113
Pogonion
x	−0.09 (−0.50 to 0.32)	−3.4 to 0.9	0.04 (−0.19 to 0.27)	−0.9 to 0.9	0.654	0.740
y	−1.77 (−2.50 to −1.03)	−4.5 to 0.9	−2.96 (−4.29 to −1.64)	−9.2 to 0.9	<0.001 ^∗	<0.001 ^∗
z	0.09 (−0.99 to 1.17)	−4.7 to 3.9	1.67 (0.21-3.13)	−5.2 to 7.9	0.864	0.027 ^∗
Right altar base
x	−0.92 (−1.32 to −0.52)	−2.3 to 0.7	−1.23 (−1.66 to −0.80)	−2.8 to 0.3	<0.001 ^∗	<0.001 ^∗
y	−0.19 (−0.65 to 0.28)	−2.8 to 0.9	−0.86 (−1.37 to −0.36)	−3.7 to 0.9	0.415	0.002 ^∗
z	−0.83 (−1.60 to −0.05)	−4.8 to 1.7	−1.21 (−2.40 to −0.02)	−5.9 to 3.0	0.038 ^∗	0.047 ^∗
Left altar base
x	1.11 (0.69 to 1.52)	−0.8 to 3.2	1.27 (0.99 to 1.56)	0.2 to 2.7	<0.001 ^∗	<0.001 ^∗
y	−0.18 (−0.69 to 0.34)	−3.2 to 1.8	−0.82 (−1.50 to −0.14)	−4.2 to 1.3	0.489	0.021 ^∗
z	−1.44 (−2.76 to −0.12)	−9.2 to 2.1	−2.24 (−4.07 to −0.42)	−11.1 to 4.6	0.035 ^∗	0.019 ^∗
Right zygoma
x	−0.31 (−0.61 to 0.00)	−1.7 to 0.9	−1.28 (−1.58 to −0.99)	−3.1 to −0.6	0.047 ^∗	<0.001 ^∗
y	−0.04 (−0.15 to 0.08)	−0.3 to 0.7	−0.61 (−0.75 to −0.46)	−1.4 to −0.1	0.545	<0.001 ^∗
z	0.25 (−0.74 to 1.23)	−5.3 to 5.6	−0.63 (−2.29 to 1.03)	−11.9 to 3.3	0.608	0.435
Left zygoma
x	0.42 (0.16 to 0.67)	−0.5 to 1.3	1.27 (1.02 to 1.52)	0.3-2.2	0.003 ^∗	<0.001 ^∗
y	−0.06 (−0.16 to 0.05)	−0.4 to 0.3	−0.39 (−0.69 to −0.10)	−1.4 to 0.8	0.275	0.012 ^∗
z	0.03 (−0.64 to 0.69)	−2.3 to 2.1	−1.34 (−3.32 to 0.65)	−15.6 to 1.9	0.938	0.174
Right gonion
x	1.37 (−0.58 to 3.33)	−5.70 to 16.90	0.99 (−1.24 to 3.23)	−4.00 to 16.80	0.159	0.363
y	−0.59 (−0.98 to −0.20)	−2.70 to 0.50	−1.33 (−1.94 to −0.72)	−3.70 to 1.20	0.005 ^∗	<0.001 ^∗
z	−1.91 (−4.07 to 0.25)	−19.10 to 5.70	−2.32 (−4.79 to 0.15)	−19.80 to 4.10	0.080	0.064
Left gonion
x	−0.91 (−2.08 to 0.26)	−5.00 to 3.70	−0.28 (−2.00 to 1.43)	−6.10 to 6.10	0.122	0.732
y	−0.61 (−0.90 to −0.31)	−1.80 to 0.40	−1.27 (−1.82 to −0.71)	−3.30 to 0.70	<0.001 ^∗	<0.001 ^∗
z	−1.14 (−2.24 to −0.03)	−6.00 to 3.40	−1.06 (−2.91 to 0.78)	−8.00 to 5.10	0.045 ^∗	0.242

CI , confidence interval; Min , minimum; Max , maximum.

∗ Significant at P <0.05.

Table II

Parameters for the RPE group at T1, T2, and T3

Parameters	T2 − T1		T3 − T1		P values (T2 vs T1)	P values (T3 vs T1)
Parameters	Mean (95% CI)	Min, max	Mean (95% CI)	Min, max	P values (T2 vs T1)	P values (T3 vs T1)
Nasion
x	−0.16 (−0.33 to 0.02)	−1.3 to 0.6	−0.32 (−0.59 to 0.04)	−2.0 to 0.6	0.072	0.080
y	0.06 (−0.02 to 0.14)	−0.2 to 0.4	−0.11 (−0.85 to 0.64)	−5.6 to 2.0	0.114	0.770
z	0.25 (−0.42 to 0.92)	−4.2 to 3.9	0.54 (−1.07 to 2.14)	−4.9 to 11.5	0.440	0.491
A-point
x	0.24 (−0.31 to 0.80)	−0.5 to 4.8	0.14 (−0.08 to 0.58)	−0.8 to 2.3	0.372	0.135
y	−1.11 (−1.94 to −0.27)	−3.8 to 5.5	−2.75 (−4.52 to −0.99)	−10.6	0.012 ^∗	0.004 ^∗
z	1.22 (0.60 to 1.84)	−0.7 to 3.7	2.54 (0.96-4.13)	−1.9 to 12.7	0.001 ^∗	0.003 ^∗
Pogonion
x	0.14 (−0.44 to 0.71)	−1.7 to 5.3	−0.03 (−0.30 to 0.23)	−1.2 to 0.9	0.622	0.806
y	−1.39 (−2.21 to −0.56)	−4.6 to 3.9	−3.58 (−5.70 to −1.47)	−12.2 to 3.4	0.002 ^∗	0.002 ^∗
z	0.30 (−0.60 to 1.20)	−4.4 to 3.4	2.82 (1.44-4.20)	−0.4 to 8.3	0.493	<0.001 ^∗
Right alar base
x	−0.87 (−1.13 to −0.61)	−2.4 to 0.0	−0.78 (−1.16 to −0.40)	−2.4 to 0.3	<0.001 ^∗	<0.001 ^∗
y	−0.64 (−1.01 to −0.26)	−2.70 to 0.6	−1.50 (−2.72 to −0.28)	−8.9 to 1.6	0.002 ^∗	0.019 ^∗
z	−0.19 (−0.61 to 0.24)	−2.60 to 1.5	−0.05 (−0.77 to 0.68)	−2.5 to 3.3	0.373	0.893
Left alar base
x	1.07 (0.80 to 1.34)	0.0-2.7	1.08 (0.77-1.39)	0.0-2.3	<0.001 ^∗	<0.001 ^∗
y	−0.60 (−0.97 to −0.24)	−2.5 to 0.5	−1.47 (−2.58 to −0.37)	−6.7 to 0.5	0.002 ^∗	0.012 ^∗
z	−0.71 (−2.02 to 0.59)	−6.6 to 6.0	−0.29 (−1.95 to 1.37)	−7.8 to 7.6	0.266	0.718
Right zygoma
x	−0.23 (−0.44 to −0.01)	−1.4 to 0.3	−0.90 (−1.32 to −0.48)	−3.1 to 0.5	0.038 ^∗	<0.001 ^∗
y	−0.14 (−0.28 to 0.00)	−0.8 to 0.5	−0.96 (−1.51 to −0.41)	−4.8 to 0.0	0.052	0.002 ^∗
z	0.90 (0.11 to 1.69)	−1.5 to 5.9	1.47 (0.45-2.49)	−2.3 to 4.9	0.028 ^∗	0.007 ^∗
Left zygoma
x	0.24 (0.09 to 0.40)	−0.3 to 1.1	1.26 (0.70-1.82)	−0.4 to 4.4	0.004 ^∗	<0.001 ^∗
y	−0.09 (−0.20 to 0.03)	−0.6 to 0.3	−0.81 (−1.51 to −0.12)	−5.1 to 0.2	0.128	0.025 ^∗
z	0.44 (−0.21 to 1.10)	−2.9 to 3.5	0.86 (−0.21 to 1.93)	−3.8 to 5.5	0.175	0.109
Right gonion
x	0.18 (−0.58 to 0.93)	−3.0 to 2.80	−1.90 (−3.91 to 0.12)	−10.9 to 5.5	0.633	0.064
y	−0.54 (−0.87 to −0.21)	−2.1 to 0.90	−1.43 (−2.07 to −0.78)	−4.5 to 0.6	0.003 ^∗	<0.001 ^∗
z	−0.51 (−1.36 to 0.33)	−3.3 to 3.30	0.72 (−1.06 to 2.49)	−5.5 to 8.3	0.220	0.408
Left gonion
x	−0.43 (−1.49 to 0.63)	−4.9 to 3.5	0.74 (−1.01 to 2.48)	−8.4 to 6.8	0.409	0.386
y	−0.75 (−1.02 to −0.49)	−1.7 to 0.3	−1.86 (−2.53 to −1.19)	−6.0 to 0.0	<0.001 ^∗	<0.001 ^∗
z	−0.91 (−2.01 to 0.18)	−5.5 to 3.6	−0.11 (−2.20 to 1.98)	−9.9 to 11.7	0.097	0.913