Introduction
This study aimed to evaluate mandibular asymmetry in unilateral posterior crossbite (UPXB) patients and compare the asymmetry between adolescents and adults with UPXB.
Methods
This study included and analyzed cone-beam computed tomography scans of 125 subjects. The subjects were divided into a UPXB group and a control group according to the presence or absence of UPXB, and each group included adolescent patients (aged 10-15 years) and adult patients (aged 20-40 years). Linear, angular, and volumetric measurements were obtained to evaluate the asymmetries of the mandibles.
Results
Both adolescent and adult patients in the UPXB group presented asymmetries in condylar unit length, ramal height, body length, and mediolateral ramal inclination ( P <0.05). Adult patients with UPXB showed greater asymmetries than adolescents. Differences with condylar unit length, condylar unit width, ramal height, condylar unit volume, and hemimandibular volume were significantly greater in adult UPXB patients than adolescent UPXB patients ( P <0.05).
Conclusions
The worsening of mandibular asymmetries in UPXB adults suggests that asymmetry in UPXB patients may progress over time; therefore, early treatment should be considered for UPXB adolescent patients. Further studies are still needed to evaluate the effectiveness of early treatment.
Highlights
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We aimed to evaluate the mandibular asymmetry in unilateral posterior crossbite (UPXB) patients.
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We aimed to compare the degree of asymmetry between adolescents and adults with UPXB.
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Both adolescent and adult UPXB patients presented remarkable mandibular asymmetry.
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Adult patients with UPXB showed significantly greater asymmetries than adolescents.
Posterior crossbite (PXB) is a kind of malocclusion related to the transverse discrepancy of the dental arches, mainly manifesting as the buccal cusps of the mandibular dentition occluding the buccal cusps of the maxillary dentition. Only 1 side of the dental arch is affected in patients with unilateral posterior crossbite (UPXB), the most common type of PXB, and the mandible usually shifts toward the crossbite side when occluded. The prevalence of UPXB ranges from 1.00% to 11.65% in patients with deciduous dentition, , from 8.40% to 9.13% in those with mixed dentition, , and 12.37% in those with permanent dentition. A narrow maxilla resulting from genetic and/or environmental influences, nonnutritive sucking, mouth breathing, low tongue position, and so on, is a possible cause of UPXB.
Early treatment for UPXB is recommended by most clinicians because of the possible adverse effect of malocclusion on maxillofacial structures and function. The asymmetrical activity of the masticatory muscle and an asymmetrical temporomandibular joint (TMJ) space in UPXB patients might have long-term effects on the growth and development of the mandible. In some previous studies, researchers have suggested a close relationship between UPXB and mandibular asymmetry. In contrast, mandibular asymmetry was not associated with UPXB in other studies that were mainly based on an adolescent population. , , Björk analyzed longitudinal profile radiographs of 45 Danish boys and found that the growth of mandibular condyles continued to age 22. Sherwood et al reported that the age at which the increase in mandibular height ceased was 24.58 years and 23.37 years for males and females, respectively. Accordingly, the continuous growth of the mandible in UPXB adolescents may worsen the deformation of the mandible. However, Evangelista et al found 3 different age groups (children, adolescents, and adults) in patients with a skeletal Class I relationship with UPXB had surprisingly similar slight mandibular asymmetries. Hence, it is important to understand the effect of growth on mandibular symmetry by grouping UPXB patients according to biological age and developmental stages.
Various methods have been proposed to evaluate mandibular asymmetry. Lopatienė and Trumpytė and Kasimoglu et al used panoramic radiographs to assess condylar and ramal height asymmetry in UPXB patients. Although less invasive, a major limitation of panoramic radiographs is the unequal magnification of the right and left sides in the horizontal dimension if the midsagittal plane (MSP) of the patient’s head is not positioned in the rotational midline of the machine. Submentovertex radiographs were also used to evaluate structural, positional, and dentoalveolar asymmetry of the mandible in patients with PUXB. , However, 2-dimensional measurement tools cannot compensate for distortion, magnification, and superimposition. Cone-beam computed tomography (CBCT) has been widely used in dental research as a reliable and accurate method for quantitative analysis of jaw deformity.
Enlow and Hans proposed a theory of craniofacial growth, which suggested that different regional areas have different local developmental and functional and structural conditions; therefore, regional signals activate local osteogenic tissues to adapt as a part of overall growth. It is reasonable to assume that functional changes in UPXB patients might cause a continuous imbalanced signal that perpetually alters the development and growth of the mandible. Therefore, this study aimed to compare the shape and morphology of mandibles between adolescent and adult patients with or without UPXB. The following null hypothesis was tested: there are no significant differences in the shape and morphology of the mandible between adolescent and adult patients with UPXB.
Material and methods
This study is a cross-sectional study. CBCT scans of 125 subjects were included in this study and analyzed. The subjects were divided into a UPXB group and a control group according to the presence or absence of UPXB, and each group included adolescents (aged 10-5 years; n = 29 in the control group and n = 38 in the UPXB group) and adult (aged 20-40 years; n = 29 in each group) subjects. Adolescents with UPXB were then subdivided according to different cervical vertebral maturation stages (CVMSs) proposed by Baccetti et al. All subjects were selected from a large pool of patients who were sequentially admitted for orthodontic treatment from 2015 to 2023 in our institution. This study was approved by the Research and Ethics Committee of the Affiliated Stomatology Hospital of Guangzhou Medical University (no. LCYJ2022005).
The inclusion criteria for the UPXB group were as follows: (1) UPXB involving at least 2 posterior teeth, (2) no history of orthodontic treatment, (3) no prostheses, (4) no absence of permanent teeth (except third molars) and no early loss of primary teeth. The exclusion criteria were as follows: (1) signs of a TMJ disorder, (2) history of orthodontic or facial surgical procedures, (3) pathologic conditions in the craniomaxillofacial complex, (4) congenital craniofacial deformities, (5) signs of maxillofacial trauma, and (6) diagnosed with a systemic disease. The inclusion and exclusion criteria of the control group were the same as those for the UPXB group, except for the absence of PUXB. The patient characteristics of the control and UPXB groups are listed in Table I .
| Characteristics | Adolescents (aged 10-15 y) | Adults (aged 20-40 y) | UPXB | ||
|---|---|---|---|---|---|
| Control | UPXB | Control | |||
| CVMS I+II | CVMS III | ||||
| Number | 29 | 19 | 19 | 29 | 29 |
| Age (y) | 12.97 ± 1.16 | 10.79 ± 0.98 | 13.84 ± 1.07 | 24.97 ± 4.69 | 23.75 ± 3.14 |
| Sex | |||||
| Male | 6 | 6 | 5 | 11 | 11 |
| Female | 23 | 13 | 14 | 18 | 18 |
| Crossbite side | |||||
| Right | – | 8 | 8 | – | 16 |
| Left | – | 11 | 11 | – | 13 |
| Mandibular deviation (mm) | |||||
| Mean ± SD | 0.91 ± 0.58 | 2.75 ± 1.84 | 3.39 ± 2.32 | 0.88 ± 0.49 | 4.73 ± 2.40 |
| P value | 0.001 ∗∗∗ | 0.001 ∗∗∗ | 0.001 ∗∗∗ | ||
| Mandibular rotation (°) | |||||
| Mean ± SD | 1.24 ± 0.99 | 2.30 ± 1.63 | 2.31 ± 1.94 | 0.92 ± 0.72 | 2.97 ± 1.93 |
| P value | 0.019 ∗ | 0.015 ∗ | 0.001 ∗∗∗ | ||
The CBCT scans were obtained using NewTom (QR srl, Verona, Italy), and the imaging parameters were as follows: 110 kV, 3.07 mA, a scan time of 18 seconds, a voxel size of 0.3 mm and a focal spot of 0.3 mm. Images were saved in digital imaging and communications in medicine format. These data were reconstructed into 3-dimensional (3D) images using Mimics Research software (version 20.0; Materialise NV, Liege, Belgium). The mandibles were separated from the whole image, and the teeth above the alveolar bone in the mandibles were removed. All landmark identifications and measurements were made by this software.
Landmarks and measurements were selected according to previous studies. Landmarks ( Table II ) were designated on the surface of reconstructed 3D images and were verified on the axial, coronal, and sagittal views. The following planes were identified: (1) the Frankfort horizontal plane (FH plane): the plane passing through bilateral Or and left Po; (2) the MSP: the plane passing through N, Ba, and perpendicular to the FH plane; (3) the frontal plane: the plane passing through N and perpendicular to the FH plane and the MSP; (4) the Sig plane: the plane passing through the sigmoid notch (Sig) and parallel to the FH plane; (5) the Gomid-Jlat-Jmed plane: the plane passing through Gomid, Jlat, and Jmed; (6) the B-Me-G plane: the plane passing through B, Me and G; (7) the Mid-mandibular plane (MMP): the plane passing through the MGo and Me and perpendicular to the mandibular plane.
| Landmark | Definition |
|---|---|
| Consup (condylion superius) | The most superior point of the condylar head |
| Conmed (condylion medialis) | The most medial point of the condylar head |
| Conlat (condylion lateralis) | The most lateral point of the condylar head |
| Corsup (coronoid superius) | The most superior point of the coronoid process |
| Sig (sigmoid notch) | The most inferior point of the sigmoid notch |
| F (fossa of mandibular foramen) | The most inferior point on the fossa of the mandibular foramen |
| Jlat | The most lateral and deepest point of the curvature formed at the junction of the mandibular ramus and body |
| Jmed | The most medial and deepest point of the curvature formed at the junction of the mandibular ramus and body |
| Gopost (gonion posterius) | The most posterior point on the mandibular angle |
| Gomid (gonion midpoint) | The midpoint between the Gopost and Goinf on the mandibular angle |
| Goinf (gonion inferius) | The most inferior point on the mandibular angle |
| MGo | The midpoint between the left and right Gomid |
| MF (mental foramen) | The entrance of the mental foramen |
| Me (menton) | The most inferior midpoint on the symphysis |
| Pog (pogonion) | The most anterior midpoint on the symphysis |
| B (supramentale) | The midpoint of the greatest concavity on the anterior border of the symphysis |
| G (genial tubercle) | The midpoint on the genial tubercle |
| N (Nasion) | Most anterior and median point of the frontonasal suture |
| Or (Orbitale) | The deepest point on the infraorbital margin |
| Po (Porion) | The highest point on the roof of the external auditory meatus |
| Ba (Basion) | The middle point on the anterior rim of the occipital foramen |
All the measurements are shown in Figure . Linear measurements ( Fig ) were as follows: (1) mandibular deviation: distance from Me to the MSP; (2) condylar unit length: Consup-F; (3) coronoid unit length: Corsup-F; (4) angular unit length: F-Gomid; (5) body unit length: F-MF; (6) chin unit length: MF-Pog; (7) condylar width: Conmed-Conlat; (8) ramal height: Consup-Gomid; and (9) body length: Gomid-Me. Angular measurements ( Fig ) were as follows: (1) mandibular rotation: the intersection angle between the MSP and the MMP; (2) gonial angle: the angle between the Me-Gomid and Consup-Gomid vectors of both sides; (3) mediolateral ramal inclination: the inner angle between the right and left Consup-Gomid and the FH plane projected on the frontal plane; (4) anteroposterior ramal inclination: the inner angle between the right and left Consup-Gomid and the FH plane projected on the MSP plane; (5) condylar angle to the MSP: the inner angle between the right and left Conmed-Conlat and the MSP plane projected on the FH plane. Volumetric measurements ( Fig ) were as follows: (1) hemimandibular volume: the mandibular volume was divided into 2 hemimandibular volumes by the B-Me-G plane; (2) body unit volume and hemiramal volume: the hemimandibular volume was divided into the body unit volume and the hemiramal volume by the Gomid-Jlat-Jmedplane; (3) condylar unit volume: coronoid unit volume and ramal unit volume: the hemiramal volume was divided into the condylar unit volume, the coronoid unit volume and the ramal unit volume by the Sig plane.
Statistical analysis
PASS software (version 15.0; NCSS, LLC, Kaysville, Utah) was used to estimate the minimum sample size (power = 0.8; α = 0.05) on the basis of the pilot and previous studies.
The intraclass correlation coefficient (ICC) was used to assess the intraoperator and interoperator error. To determine the intraoperator error, thirty-five 3D images were randomly selected, and the entire workflow, including separation of the mandible and landmark identifications, was repeated by the same operator (M.H.) 2 weeks apart. To evaluate the interoperator error, a second blinded experienced operator (B.C.) analyzed 35 randomly selected CBCT images and repeated the workflow sequence.
The statistical analysis was performed with SPSS software (version 26.0; IBM, Armonk, NY). Descriptive statistics were calculated for all variables, including mean and standard deviation. The difference between the measurements of the 2 sides was the right side minus the left side in the control group and the noncrossbite (non-XB) side minus the crossbite (XB) side in the UPXB group. Positive difference values in the control group indicated that the measurements on the right side were larger than those on the left side, and in the UPXB group, the measurements on the non-XB side were larger than those on the XB side. The data were then checked for normal distribution by the Shapiro-Wilk test. As all the data were normally distributed, parametric tests were used. The paired t test was performed to compare the intragroup differences between the measurements. An independent Student t test was used to compare the mandibular measurement differences between sides among groups. Because no significant difference could be detected for any measurement differences between CVMS I + CVMS II and CVMS III subjects, all the measurements were pooled in subsequent analysis. P values <0.05 were considered statistically significant.
Results
All 3D measurements showed high intraoperator and interoperator agreements. The intraoperator ICC ranged from 0.982% (for body unit volume) to 0.998% (for body unit length), and the interoperator ICC ranged from 0.816% (for body unit volume) to 0.977% (for coronoid unit length). Table I shows that patients in the UPXB group presented significantly greater mandibular deviation and rotation than subjects in the same age range in the control group.
A significantly larger coronoid unit length and a shorter body length were detected on the XB side than on the non-XB side in both CVMS I + CVMS II and CVMS III subjects. Although the condylar unit length was larger on the non-XB side in CVMS I + CVMS II patients, the statistical significance for condylar unit length was found only in CVMS III patients. None of the measurement differences significantly differed between CVMS I + II and CVMS III subjects ( Table III ).
| Measurements | CVMS I + CVMS II (n = 19) | CVMS III (n = 19) | P value † | ||||||
|---|---|---|---|---|---|---|---|---|---|
| XB side | Non-XB side | Difference | P value † | XB side | Non-XB side | Difference | P value ƚ | ||
| Linear measurements (mm) | |||||||||
| Condylar unit length | 39.58 ± 3.99 | 40.46 ± 3.55 | 0.89 ± 2.13 | 0.086 | 42.43 ± 3.77 | 43.74 ± 4.17 | 1.31 ± 2.22 | 0.019 ∗ | 0.553 |
| Body unit length | 53.10 ± 3.27 | 53.65 ± 3.66 | 0.55 ± 1.68 | 0.172 | 54.43 ± 2.17 | 54.78 ± 2.71 | 0.35 ± 2.06 | 0.468 | 0.749 |
| Coronoid unit length | 35.7 ± 3.53 | 35.02 ± 3.51 | −0.68 ± 1.27 | 0.031 ∗ | 39.55 ± 3.59 | 38.79 ± 3.00 | −0.75 ± 1.42 | 0.033 ∗ | 0.864 |
| Angular unit length | 17.01 ± 2.28 | 17.18 ± 2.98 | 0.16 ± 1.71 | 0.679 | 19.25 ± 2.48 | 19.18 ± 2.52 | −0.07 ± 1.25 | 0.814 | 0.634 |
| Chin unit length | 28.47 ± 2.03 | 28.31 ± 2.46 | −0.16 ± 1.12 | 0.542 | 28.69 ± 1.86 | 29.42 ± 1.29 | 0.73 ± 1.36 | 0.072 | 0.062 |
| Condylar width | 17.29 ± 1.95 | 17.35 ± 2.31 | 0.05 ± 1.39 | 0.870 | 17.22 ± 2.29 | 17.47 ± 1.89 | 0.25 ± 1.20 | 0.375 | 0.642 |
| Ramal height | 50.34 ± 4.86 | 51.17 ± 6.12 | 0.83 ± 2.66 | 0.190 | 54.53 ± 4.55 | 55.68 ± 3.40 | 1.14 ± 3.01 | 0.114 | 0.735 |
| Body length | 78.33 ± 4.71 | 79.21 ± 4.77 | 0.88 ± 1.62 | 0.030 ∗ | 81.47 ± 4.03 | 82.67 ± 4.41 | 1.20 ± 2.30 | 0.036 ∗ | 0.622 |
| Angular measurements (°) | |||||||||
| Gonial angle | 124.34 ± 4.67 | 125.11 ± 4.84 | 0.76 ± 1.95 | 0.106 | 122.32 ± 4.58 | 122.96 ± 4.51 | 0.64 ± 2.86 | 0.350 | 0.874 |
| Mediolateral ramal inclination | 81.15 ± 4.23 | 79.99 ± 3.92 | −1.16 ± 3.29 | 0.143 | 83.54 ± 3.72 | 82.69 ± 4.22 | −0.85 ± 2.40 | 0.140 | 0.746 |
| Anteroposterior ramal inclination | 80.90 ± 3.94 | 80.19 ± 4.88 | −0.71 ± 2.21 | 0.179 | 82.49 ± 4.34 | 81.56 ± 4.12 | −0.93 ± 2.44 | 0.110 | 0.772 |
| Condylar angle to MSP | 72.58 ± 6.52 | 72.18 ± 7.74 | −0.40 ± 5.81 | 0.768 | 70.63 ± 9.32 | 71.93 ± 8.29 | 1.30 ± 6.43 | 0.389 | 0.398 |
| Volumetric measurements (10 3 mm 3 ) | |||||||||
| Condylar unit volume | 1.33 ± 0.27 | 1.42 ± 0.40 | 0.09 ± 0.23 | 0.110 | 1.40 ± 0.40 | 1.50 ± 0.35 | 0.10 ± 0.28 | 0.138 | 0.922 |
| Coronoid unit volume | 0.18 ± 0.06 | 0.17 ± 0.06 | −0.01 ± 0.05 | 0.640 | 0.29 ± 0.11 | 0.28 ± 0.10 | −0.01 ± 0.06 | 0.388 | 0.707 |
| Ramal unit volume | 5.48 ± 1.40 | 5.55 ± 1.64 | 0.08 ± 0.52 | 0.530 | 6.33 ± 1.39 | 6.34 ± 1.49 | 0.02 ± 0.52 | 0.881 | 0.732 |
| Body unit volume | 16.47 ± 2.59 | 16.39 ± 2.71 | −0.08 ± 0.62 | 0.596 | 18.32 ± 1.98 | 18.46 ± 2.17 | 0.14 ± 0.58 | 0.299 | 0.269 |
| Hemiramal volume | 6.62 ± 1.71 | 6.77 ± 1.96 | 0.15 ± 0.64 | 0.335 | 7.65 ± 1.79 | 7.78 ± 1.83 | 0.13 ± 0.57 | 0.336 | 0.940 |
| Hemimandibular volume | 23.17 ± 3.68 | 23.24 ± 4.10 | 0.07 ± 1.01 | 0.758 | 25.97 ± 3.41 | 26.22 ± 3.65 | 0.25 ± 0.87 | 0.223 | 0.562 |
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