Comparison between basal arch width in normal occlusions and class Ⅱ malocclusions by using cone-beam computed tomography

Abstract

Objective

The purpose of this research was to investigate the measurements of maxillary and mandibular basal arch width in male and female with normal occlusion, and to compare dental arch width difference between normal occlusion and ClassⅡ malocclusion groups could be helpful in orthodontic diagnosis and treatment planning.

Methods

Cone-beam computed tomography (CBCT) images from 133 individuals (76 males and 57 females) with normal occlusion and 64 (25 males and 39 females) with skeletal Class II malocclusions were evaluated. The distances between canines, first molars (basal arch widths: BAW) and second molars were measured from CRs (center of resistance) of the teeth and the projection of first molars on buccal bones (WALA distance) were measured.

Results

There were significant differences in male and female maxillary and mandibular dental transverse widths. The normal range of the maxilla and mandible basal bone widths differences were −2 mm–2 mm (−0.05 ± 2.17 mm). The normal occlusion and Class Ⅱ groups exhibited significant differences in the width of the intercanine and first molars. Sella-nasion-A point angle (SNA) and Sella-nasion-B point angle (SNB) in the Class Ⅱ male group were positively correlated with the width between the maxillary canines. For individuals with normal occlusions, the width of the mandible at the second molar was greater than that of the maxilla, so more attention should be paid to the width of the second molar when considering clinical treatment.

Conclusion

Measuring the width of the maxilla and mandible basal bones from the resistance center of the first molar was a feasible and repeatable method can be used in clinical practice. The data could serve as a reference for orthodontic treatment planning. More consideration should be paid to the horizontal dental problems of the treatment plan for Class Ⅱ patients. And the width of the mandible at the second molar was greater than that of the maxilla, so more attention should be paid during treatment.

1. Introduction

Malocclusions usually occur in three dimensions and affect each other. The development of maxillofacial width started the earliest, stopped at the earliest time point and increased the least. The irregularity of the horizontal of the jaw may affect its vertical and sagittal development at an early stage [1]. Transverse coordination of the dental arches plays an important role in maintaining stable periodontal conditions and functional occlusion over the long term [2]. Malocclusion usually occurs at the mixed or the early permanent dental stage, when the patient's horizontal development of the jaw is nearing completion or has been completed [3,4]. The transverse growth of the maxilla in both the male and female reaches the adult level at the age of circa 12 years, and the width between the canines of the mandible basically do not change at the age of 9 years [5]. However, clinically, patient's chief complain often was sagittal or vertical problems, and if there was no obvious posterior crossbite, the problem of horizontal discrepancy was more likely to be ignored.

For transverse research, previous studies focused on 2D posteroanterior radiographs or dental casts to compare the dental and skeletal components of transverse dimensions in subjects with different sagittal facial patterns. Posteroanterior images enable the measurement of basal bone widths at the jugal points and antegonial notches, but the images provide a poor representation of bone width, especially for the mandible [[6], [7], [8]]. Dental casts analysis is used to measure intercanine and intermolar widths between the cusp tips or the fossae. Dentoalveolar compensation, however, will likely mask the underlying transverse deficiencies in patients with different malocclusions [[9], [10], [11]]. Usually, a narrow maxillary arch is accompanied by buccal tilted maxillary posterior teeth (Fig. 1A) and lingual tilted mandibular posterior teeth (Fig. 1B). In recent years, some studies have documented the accuracy and reliability of cone-beam computed tomography (CBCT), which overcomes the limitations of conventional 2D imaging and dental casts [[12], [13], [14], [15]]. The cross-sectional view allows to measure linear distance or angular values much more precisely. Kyung and colleagues used CBCT to make comparisons between dental and basal arch forms in normal and Class III malocclusions [16]. Bayome et al. used 3D virtual models and CBCT images to evaluate the relationship between dental and basal arches in normal occlusion subjects [17].

Fig. 1.

Buccal and lingual inclination occur around the resistance center of teeth. A, The posterior teeth tilted to buccal side. B, The posterior teeth tilted to lingual side. Red dot shows the resistance center of multirooted tooth. The dashed line shows the teeth in the correct position without any compensation. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

In a free-floating body, the center of resistance (CR) represented by the center of mass, which has been considered a reasonable landmark to define tooth position and its displacement [18]. Unlike the cusp tips or root apices, the CR position is not affected by the tilting of teeth. The buccal or lingual tilting of the crown was carried out around the CR, when the CR of the molars served as a very stable indicator of the width of the basal arch. It is generally presumed to be located around one-third to halfway down the root of a healthy single-rooted tooth. For a multirooted tooth, the CR is between the roots, 1–2 mm apical to the furcation [5]. Unfortunately, using the location of CR points on 2D cephalograms to analyze the transverse dimension has many limitations due to overlapping structures in the posterior teeth area of images. Some research has evaluated maxillomandibular basal arch form differences at CR, located inside the basal bone anatomy using CBCT images in normal occlusion and different skeletal malocclusions [16,18,19].

To sum up, there are a paucity of studies on the transverse measurement of the maxillomandibular basal bone width in individuals with normal occlusions and Class II malocclusions using CBCT, and few reports on transverse comparisons between females and males. Similarly, there is a lack of a standard reference for transverse differences of the maxillomandibular basal arch widths (BAW) assessed using different methods. Therefore, the purpose of the present study was to evaluate maxillomandibular transverse differences measured at the CR to provide transverse reference values, and to compare the upper and lower BAW of individuals with normal occlusions and patients with Class Ⅱ malocclusions.

2. Materials and methods

2.1. Subjects

In this retrospective study, based on the six keys of Andrews [20], normal occlusion individuals recruited as approximately ideal occlusion acted as the control group, which consisted of 76 males (mean age 21.3 ± 2.68 years) and 57 females (mean age 22.7 ± 5.66 years) aged 18–26 years from Air Force Medical University, Shaanxi, China. They were classified as having the following features: a Class I skeletal and dental relationship, A point-nasion-B point angle (ANB) angle between 0° and 4°, with symmetrical maxillofacial features, maxillary and mandibular dental arches are symmetrical, and no external teeth, complete and permanent dentition with crowding <1 mm, a normal anterior and posterior overbite and overjet.

For the Class Ⅱ malocclusion group, which served as the experiment group, 64 patients (25 males, 39 females) with skeletal Class Ⅱ relationships were selected from the orthodontic department of the Stomatological Hospital of Air Force Medical University from 2019 to 2021. The age range was from 16 to 30 years (males: 20.4 ± 3.36 years, females: 20.93 ± 5.62 years). These patients had skeletal and dental Class Ⅱ malocclusions, with 4° < ANB ≤ 6° and crowding ≤4 mm (Table 1).

Table 1.

Sample characteristics of the normal occlusion and Class II malocclusion.

Variable	Normal occlusion	Class Ⅱ	Class Ⅱ - Normal occlusion	P-value
SNA (°)	83.51 ± 3.31	83.53 ± 3.81	0.02	0.970
SNB (°)	80.61 ± 3.91	77.47 ± 3.88	−3.14	< 0.001†
ANB (°)	2.99 ± 1.90	6.07 ± 1.47	3.08	< 0.001†
Wits (mm)	−0.13 ± 2.48	3.66 ± 3.69	3.79	< 0.001†
SN/MP (°)	30.25 ± 5.95	33.95 ± 6.69	3.70	< 0.001†
U1/SN (°)	105.57 ± 7.73	107.05 ± 12.04	1.48	0.019*
U1/L1 (°)	129.46 ± 9.81	116.19 ± 16.70	−13.27	< 0.001†
IMPA (°)	94.71 ± 6.17	100.87 ± 7.20	6.16	< 0.001†

Notes: Values are presented as the mean ± standard deviation. P < 0.05* and P < 0.01^† were considered to be significant differences.

Abbreviations: ANB, A point-nasion-B point angle; Class Ⅱ - Normal occlusion, mean difference between class Ⅱ malocclusion and normal occlusion groups; IMPA, incisor mandibular plane angle; SNA, Sella-nasion-A point angle; SNB, Sella-nasion-B point angle; SN/MP, Sella-nasion plane-mandibular plane angle; U1/L1, maxillary central incisor axis-mandibular central incisor axis angle; U1/SN, Sella-nasion plane-maxillary central incisor axis angle; Wits, distance between projection of points A and B to the occlusal plane.

Exclusion criteria for both groups included: no missing or decayed teeth, no prosthetic crowns or gingival defects, and no history of maxillofacial trauma, surgery, orthodontic treatment or temporomandibular joint disorder. The Ethical Committee of the Stomatological Hospital of Air Force Medical University approved this study (IRB-REV-2022056).

2.2. CT measurements

Three-dimensional X-ray images were obtained using KaKo CBCT (KaKo Kerr, Orange, CA, USA). The CBCT images were saved as DICOM files, which were then imported into Dolphin Imaging software (Dolphin ver. 11.9, Patterson Dental Supply, Chatsworth, CA, USA) to reconstruct high-precision 3D images of the bones and teeth of the cranial and maxillofacial regions. Standard orientation of the craniofacial structure was established using the Frankfort horizontal plane, which was (1) the plane passing through the bilateral orbitales and the right porion as the axial plane; (2) the “coronal plane” was perpendicular to the axial plane, passing the nasion point; and (3) the sagittal plane was perpendicular to the axial and coronal planes passing through the nasion point.

Based on the findings of previous studies [16,18,21], the BAW from the CT scans (BAW-CT) were obtained by digitizing the CR at the level of the coronal one-third for a single rooted tooth and at the furcation for a multirooted tooth. The positions of the CRs were pinpointed on axial, sagittal and the coronal sections by using the slice locator of the Dolphin software. The resistance center of the canine was positioned at the sagittal slice (2/3 from the apical orifice) (Fig. 2 A, B), and the distance between the canines was measured in the coronal slice. The first molar was located in the coronal slice, and the distance between the left and right resistance centers (the bifurcation of the root of the first molar) was measured in the axial slice to represent the transverse width of the bone arch of the maxillomandibular base (BAW) (Fig. 2 C, D). In addition, the distances between the CRs and the intersecting point of the buccal bone cortex were measured. This level was selected to represent the horizontal level of the WALA (William Andrews and Larry Andrews) point [22], which is located at the most convex point on the mucogingival junction. We also measured the distance between the CRs of the upper and lower second molars.

Fig. 2.

Transversal widths measurements on three-dimensional computed tomography. A, B, The digitization of centers of resistance, a single-rooted tooth: at the level of the coronal one-third, a multirooted tooth: at the level of the furcation. C, Maxilla: 1, the width of canines, 2, the width of first molars, 3, the width of second molars, 7, the width of maxilla WALA points. D, Mandible: 4, the width of canines, 5, the width of first molars, 6, the width of second molars, 8, the width of mandible WALA points.

WALA point: projection of the resistance center of first molar on the buccal bone cortex, which is located at the most convex point on the mucogingival junction.

2.3. Reliability

All measurements were performed by 2 operators, and 15 randomly selected samples were assessed for a second time after an interval of 2 weeks. The correlation coefficients showed high reliability (0.943–0.990, P < 0.01).

2.4. Statistical analysis

All statistical analyses were performed using IBM SPSS Statistics ver. 22.0 (IBM Co, Armonk, NY, USA). The normality of variables was tested by the Kolmogorov-Smirnov test. Sex differences were analyzed using an independent-samples t-test or the Mann-Whitney U test when the sample was non-normally distributed. If there was a difference, the data for males and females were analyzed separately. A paired-samples t-test was used for comparison of maxillomandibular differences of the BAW values between the normal occlusion and Class Ⅱ malocclusion groups. The results are expressed as means ± deviation. Statistical significance was set at the 5% level.

3. Results

In this study, we found significant differences between the sexes for each group of subjects. Table 2 shows the sex differences of transverse measurements in the maxilla and mandible and their differences in males and females. The measurements included the distances between canines, first molars, second molars and WALA points. In the normal occlusion group, males had significantly greater width measurements at the intercanine, first molars and second molars, and the distance of WALA points compared to females whether in the maxilla or mandible. In the Class Ⅱ group, the canine values in the maxilla and mandible were not significantly different in males and females. In addition, males had greater width values for the first molar, WALA and second molar distance for both the maxilla and mandible. There were no differences in the maxillomandibular width for the first molars and second molars between males and females in the normal occlusion and Class Ⅱ groups. It is noteworthy that there was a significant difference in the maxillomandibular width of the canines and the width of WALA points in the normal occlusion group (Table 2).

Table 2.

Sex differences of transverse measurements according to sagittal facial pattern.

Width (mm)		Normal occlusion			Class Ⅱ
		Male (n = 76)	Female (n = 57)	P-value	Male (n = 25)	Female (n = 39)	P-value
Maxilla (Mx.)	Canine	33.68 ± 2.21	31.54 ± 1.53	< 0.001†	32.26 ± 2.62	31.64 ± 1.93	0.284
	1st molar	50.28 ± 2.97	46.94 ± 2.60	< 0.001†	48.51 ± 3.00	46.83 ± 2.66	0.022*
	WALA	65.21 ± 3.75	61.26 ± 3.06	< 0.001†	64.56 ± 3.55	61.72 ± 3.43	0.002†
	2 nd M	53.63 ± 3.25	50.90 ± 2.45	< 0.001†	52.45 ± 4.09	49.48 ± 2.65	0.003†
Mandible (Mn.)	Canine	25.20 ± 1.51	24.32 ± 1.91	0.027*	23.40 ± 2.62	23.21 ± 2.20	0.289
	1st molar	50.18 ± 2.8	47.19 ± 2.14	< 0.001†	48.98 ± 2.56	46.75 ± 2.45	0.001†
	WALA	61.96 ± 3.32	59.04 ± 2.89	< 0.001†	62.66 ± 4.02	59.53 ± 3.72	0.002†
	2 nd M	57.11 ± 2.76	55.68 ± 3.21	0.042*	58.42 ± 4.42	54.24 ± 3.18	< 0.001†
Mx.-Mn.	Canine	8.49 ± 2.12	7.21 ± 2.14	0.015*	8.86 ± 3.52	8.43 ± 2.44	0.600
	1st molar	0.10 ± 2.53	−0.24 ± 1.56	0.341	−0.47 ± 4.00	0.08 ± 2.81	0.515
	WALA	3.25 ± 3.38	2.22 ± 2.41	0.033*	1.90 ± 4.65	2.19 ± 4.10	0.795
	2 nd M	−3.47 ± 3.19	−4.78 ± 3.05	0.091	−5.96 ± 6.53	−4.75 ± 3.68	0.404

Notes: Values are presented as mean ± standard deviation. Non-normally distributed variables are marked in italics and non-parametric statistical methods were used. P < 0.05* and P < 0.01^† were considered to be significant differences.

Abbreviations: Mx., maxilla; Mn., mandible; WALA point: projection of the resistance center of first molar on the buccal bone cortex, which is located at the most convex point on the mucogingival junction.

In terms of jaw differences, in the normal occlusion group, no matter whether the subject was male or female, there were significant differences in the width values at the intercanine, in the widths of second molars, and the distance of WALA points, except for the first molar values. There was no difference in the width of the upper and lower first molars, indicating that the transverse width at the base bone of the upper and lower mandible was almost the same. In the Class Ⅱ group, there were no significant differences between the width of the first molars and WALA measurements in males (Fig. 3B), and no significant differences in the distance of first molars in females (Table 3, Fig. 3C). For the normal occlusion and Class Ⅱ groups, when the data from males and females were combined, there was no significant difference between the upper and lower jaw, except at the width of the first molar, but other measurements revealed significant differences (Table 3, Fig. 3A).

Fig. 3.

Differences of width between normal occlusion and Class Ⅱ malocclusion as well as differences of width between maxilla and mandible. A, The width differences between maxilla and mandible different areas of normal occlusion and Class Ⅱ malocclusion of all subjects. B, The width differences between maxilla and mandible different areas of normal occlusion and Class Ⅱ malocclusion of males. C, The width differences between maxilla and mandible different areas of normal occlusion and Class Ⅱ malocclusion of females. P < 0.05* and P < 0.01** were significant differences. P ≥ 0.05 were no statisticant difference, annotated as “ns”.

For the differences between normal occlusion and Class Ⅱ malocclusion, all significant differences are annotated. For maxillomandibular differences, all nonsignificant differences are annotated.

Abbreviations: Mx., maxilla; Mn., mandible.

Table 3.

Difference of transverse measurements between maxilla and mandible.

Width (mm)	Normal occlusion			Class Ⅱ
	Maxilla (Mx.)	Mandible (Mn.)	P-value	Maxilla (Mx.)	Mandible (Mn.)	P-value
	Male (n = 76)			Male (n = 25)
Canine	33.68 ± 2.21	25.20 ± 1.51	< 0.001†	32.26 ± 2.62	23.40 ± 2.62	< 0.001†
1st molar	50.28 ± 2.97	50.18 ± 2.80	0.738	48.51 ± 3.00	48.98 ± 2.56	0.561
WALA	65.21 ± 3.75	61.96 ± 3.32	< 0.001†	64.56 ± 3.55	62.66 ± 4.02	0.052
2 nd M	53.63 ± 3.25	57.11 ± 2.76	< 0.001†	52.45 ± 4.09	58.42 ± 4.42	< 0.001†
	Female (n = 57)			Female (n = 39)
Canine	31.54 ± 1.53	24.32 ± 1.91	< 0.001†	31.64 ± 1.93	23.21 ± 2.20	< 0.001†
1st molar	46.94 ± 2.60	47.19 ± 2.14	0.244	46.83 ± 2.66	46.75 ± 2.45	0.852
WALA	61.26 ± 3.06	59.04 ± 2.89	< 0.001†	61.72 ± 3.43	59.53 ± 3.72	0.002†
2 nd M	50.90 ± 2.45	55.68 ± 3.21	< 0.001†	49.48 ± 2.65	54.24 ± 3.18	< 0.001†
	Total (n = 133)			Total (n = 64)
Canine	33.20 ± 2.26	25.00 ± 1.64	< 0.001†	31.88 ± 2.22	23.28 ± 2.35	< 0.001†
1st molar	48.85 ± 3.26	48.90 ± 2.93	0.796	47.49 ± 2.90	47.62 ± 2.71	0.749
WALA	63.52 ± 3.97	60.71 ± 3.45	< 0.001†	62.83 ± 3.72	60.75 ± 4.11	< 0.001†
2 nd M	53.02 ± 3.29	56.79 ± 2.91	< 0.001†	50.64 ± 3.57	55.87 ± 4.21	< 0.001†

Notes: Values are presented as mean ± standard deviation. Non-normally distributed variables are marked in italics and non-parametric statistical methods were used. P < 0.05* and P < 0.01^† were considered to be significant differences.

Abbreviations: Mx., maxilla; Mn., mandible; WALA point: projection of the resistance center of first molar on the buccal bone cortex, which is located at the most convex point on the mucogingival junction.

For the sagittal facial pattern difference in males, the width of intercanine and the BAW in maxilla were found to be greater in the normal occlusion group than in the Class Ⅱ group. The width of intercanine in the mandible was greater too (Table 4, Fig. 3B). When female and male data were combined, the normal occlusion group had significant differences in the width values at the intercanine, the BAW and the widths of the second molars in the upper jaw compared to the Class Ⅱ group. In addition, there were significant differences in the width values at the intercanine and the BAW in the mandible (Table 4, Fig. 3A).

Table 4.

Difference of transverse parameters.

Width (mm)		Male (n = 101)			Female (n = 96)			Total (n = 197)
		Normal occlusion	Class Ⅱ	P-value	Normal occlusion	Class Ⅱ	P-value	Normal occlusion	Class Ⅱ	P-value
Maxilla (Mx.)	Canine	33.68 ± 2.21	32.26 ± 2.62	0.009†	31.54 ± 1.53	31.64 ± 1.93	0.828	33.20 ± 2.26	31.88 ± 2.22	< 0.001†
	1st molar	50.28 ± 2.97	48.51 ± 3.00	0.012*	46.94 ± 2.60	46.83 ± 2.66	0.836	48.85 ± 3.26	47.49 ± 2.90	0.005†
	WALA	65.21 ± 3.75	64.56 ± 3.55	0.442	61.26 ± 3.06	61.72 ± 3.43	0.492	63.52 ± 3.97	62.83 ± 3.72	0.244
	2 nd M	53.63 ± 3.25	52.45 ± 4.09	0.144	50.9 ± 2.45	49.48 ± 2.65	0.044*	53.02 ± 3.29	50.64 ± 3.57	< 0.001†
Mandible (Mn.)	Canine	25.20 ± 1.51	23.40 ± 2.62	0.003†	24.32 ± 1.91	23.21 ± 2.20	0.051	25.00 ± 1.64	23.28 ± 2.35	< 0.001†
	1st molar	50.18 ± 2.80	48.98 ± 2.56	0.062	47.19 ± 2.14	46.75 ± 2.45	0.352	48.90 ± 2.93	47.62 ± 2.71	0.004†
	WALA	61.96 ± 3.32	62.66 ± 4.02	0.394	59.04 ± 2.89	59.53 ± 3.72	0.467	60.71 ± 3.45	60.75 ± 4.11	0.941
	2 nd M	57.11 ± 2.76	58.42 ± 4.42	0.173	55.68 ± 3.21	54.24 ± 3.18	0.087	56.79 ± 2.91	55.87 ± 4.21	0.131
Mx.-Mn.	Canine	8.49 ± 2.12	8.86 ± 3.52	0.624	7.21 ± 2.14	8.43 ± 2.44	0.055	8.20 ± 2.18	8.60 ± 2.89	0.323
	1st molar	0.10 ± 2.53	−0.47 ± 4.00	0.405	−0.24 ± 1.56	0.08 ± 2.81	0.591	−0.05 ± 2.17	−0.13 ± 3.31	0.853
	WALA	3.25 ± 3.38	1.90 ± 4.65	0.190	2.22 ± 2.41	2.19 ± 4.10	0.963	2.81 ± 3.04	2.08 ± 4.29	0.223
	2 nd M	−3.47 ± 3.19	−5.96 ± 6.53	0.077	−4.78 ± 3.05	−4.75 ± 3.68	0.702	−3.77 ± 3.19	−5.23 ± 4.98	0.040*

Notes: Values are presented as mean ± standard deviation. Non-normally distributed variables are marked in italics and non-parametric statistical methods were used. P < 0.05* and P < 0.01^† were considered to be significant differences.

Abbreviations: Mx., maxilla; Mn., mandible; WALA point: projection of the resistance center of first molar on the buccal bone cortex, which is located at the most convex point on the mucogingival junction.

In males, Sella-nasion-A point angle (SNA) and Sella-nasion-B point angle (SNB) in the Class Ⅱ group were positively correlated with the width between maxillary canines (Pearson correlation coefficient of SNA = 0.542, P = 0.005; Pearson correlation coefficient of SNB = 0.481, P = 0.015. Fig. 4). In the normal occlusion group, the correlation between SNA and maxillary canine width was weak (P = 0.038, Pearson correlation coefficient = 0.238). However, there was no significant correlation between the width of the maxillary canines, SNA and SNB in females in the normal occlusion or Class Ⅱ group.

Fig. 4.

Correlation between sagittal measurements and the width between maxillary canines. SNA and SNB were positively correlated with the width between maxillary canines. The SNA was marked with dots, and the dark blue line was the linear correlation fitting between the SNA and the width between maxillary canines. SNB were marked by triangles, and the lines in purple were linear correlation fitting between SNB and width between maxillary canines. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

4. Discussion

Clinically, most patients complain about sagittal and vertical problems, so most clinicians mainly focus on improving the patient's profile or vertical discrepancies. Unless the patient has obvious posterior crossbites in the mouth, transverse problems are often ignored. However, ignoring the transverse problems will not only lead to the failure of the correction of the sagittal and vertical discrepancies, but also reduce the long-term stability of the bite and damage the periodontal tissue [2]. Thus, it is important to access the balance of the maxillomandibular transverse dimension in each case before treatment. However, there have been few studies carried out on appropriate maxillary and mandibular BAW in normal occlusion and/or class Ⅱ malocclusion groups. In particular, the horizontal relationship of the second molar region has been difficult to access.

In today's dentistry, CBCT is already a routine examination method, so the diagnosis of lateral problems has become relatively easier. Different methods have been used to study the maxillomandibular width differences. Some researchers predict the difference in the width of the basal arch by a measurement model, but this method is greatly affected by the teeth, and dental compensation will cover the differences in bone. Some researchers measured the distance of the root furcation of the first molar as the width of the basal arch. The resistance center of multirooted tooth is almost located at the root furcation of the tooth, and the buccal-lingual tilt of the tooth has very little effect on the position of the resistance center. Research has also found that the distance between the root furcation of the first molar and the width of the basal arch have a strong correlation [16,18]. Therefore, one of the methods used in the present study was to measure the distance of the root furcation of the first molar. There are also studies that used the distance of the resistance center projected on the buccal cortex to represent the width of the basal arch [23]. This measurement method is also very commonly used in the clinical, but it is more susceptible to individual differences and the thickness of the buccal cortex of patients. Moreover, there are very few reports using this method, so this study also used this method to study the maxillomandibular differences.

In both groups, males had significantly greater transverse widths at the maxillary first and second molar widths as well as maxillary WALA widths compared to females. The widths of the mandibular first molar and WALA distances were also significantly greater in males than females. Although there were some differences between the groups, most skeletal and dental transverse measurements exhibited greater values in males. However, these findings are inconsistent with the results of a previous study [18], most likely due to the sample size. In the present study, the normal occlusion sample size was much larger and the results were in good agreement with those of previous studies that reported that craniofacial and dental widths were significantly affected by gender [[23], [24], [25], [26]]. Therefore, basal bone arch widths in different groups were described for both sexes (Table 2).

The transverse dental dimension in subjects with Class Ⅱ malocclusion and those with normal occlusions varied between studies. Ball et al. found that the mandibular arch morphology of Class I and Class Ⅱ groups were basically the same, except for the canine width segment [27]. Uysal and colleagues demonstrated that the maxillary canine, premolars width, premolars and molars alveolar widths, and mandibular premolars and molars alveolar widths were significantly narrower in Class Ⅱ division 1 malocclusion patients compared with normal occlusions [28]. Fu et al. found that the maxillary molars width of skeletal class Ⅱ was smaller, and the length of the dental arch was significantly longer compared with normal occlusions. The mandibular intermaxillary widths of skeletal Class Ⅱ malocclusions were smaller than normal occlusions [29]. Our study found that the width of the upper and lower intercanines and maxillary BAW in the male normal occlusion group was larger than in the Class Ⅱ group, but no such difference was found for females. However, when male and female data were combined, it was obvious that the width of the upper and lower intercanines and the BAW in the normal occlusion group was greater than that in the Class Ⅱ group (Table 4, Fig. 3A). We also found that Class Ⅱ canine width was related to sagittal SNA and SNB values. This finding indicated that the jaw width of Class Ⅱ subjects was different from the normal occlusion group. Therefore, for Class Ⅱ subjects undergoing clinical treatment, attention should first be paid to whether there is a horizontal problem. And clearly, further studies are needed to prove whether different vertical skeletal patterns correspond to different basal arch widths and whether horizontal patterns influence vertical growth or vertical patterns determine horizontal growth. It is important to note that Class Ⅱ children growth appears to be very different in males and females and should be studied separately.

However, the subjects of most studies carried out to date have been on patients with malocclusions, and few studies have focused on the appropriate maxillary and mandible basal arch dimensions in ideal occlusion male and female groups. Therefore, the present research evaluated the width of the upper and lower canine, the width of the basal arch, and the width between the second molars of 133 individuals with approximately ideal occlusions to find the maxillomandibular differences and provide ideal references for clinical diagnosis of horizontal problems and a convenient and repeatable horizontal measure method by using CRs.

The present study found that maxillomandibular basal bone width differences measured from CRs of the first molar were 0.10 ± 2.53 mm for males and −0.24 ± 1.56 mm for females. There were no significant differences between the sexes. After combining males and females together, the maxillomandibular basal bone width difference was −0.05 ± 2.17 mm (Transverse index of Air Force Medical University)(Fig. 5A), similar to the Yonsei Transverse Index [18]. In normal occlusion group, if the basal bone width of the maxilla and mandible were coordinated, the teeth will not have been compensated by labial or lingual tilt. When the maxillomandibular basal bone width differences were more than −0.05 ± 2.17 mm, different arch expansion methods should be considered first based on the ages of the patients.

Fig. 5.

The maxillomandibular differences of basal arch width (BAW) in normal occlusion. A, The maxillomandibular basal bone width differences measured from the first molar CRs. B, The maxillomandibular basal bone width differences measured from the first molar WALA points.

CR: center of resistance, Mx: maxilla, Mn: mandible.

The maxillomandibular basal bone width differences measured from the WALA points were 3.25 ± 3.38 mm for males and 2.22 ± 2.41 mm for females (Fig. 5B), and the difference was statistically significant. Clinically, it can be used as an auxiliary method to measure the maxillomandibular basal bone width difference, but it should be remembered that it may be affected by the thickness of the buccal cortex [23].

There was a statistically significant difference in the basal bone width of the upper second molar between males and females, but there was no statistical difference in the width of the mandibular second molar. Whether male or female, the basal bone width of the mandible is wider than that of the maxilla, with male values being −3.47 ± 3.19 mm and females values −4.78 ± 3.05 mm. These results show that even if the width of the basal bone at the first molar is coordinated, the mandibular was usually wider than maxillary at the second molar region, which is similar to findings reported by other researchers [16,18]. This may be the reason why the scissor bite or buccal tilt of the second molar is often observed in clinical practice, and the palatal tip of the second molar droops during the correction process. It is a timely reminder to pay more attention to the width of the basal arch at the second molar.

Although, compared with the measurement of model of cast, measurement based on Dento-maxillofacial CBCT data could more truly and objectively reflect the width of the dental arch base bone, the specific positions(2/3 from the apical orifice of canine's root and the bifurcation of the root of the first molar) of the tooth root were still manually positioned，which were used as the landmark of the measurement plane in this study. On the one hand, these landmarks may be affected by factors such as abnormal tooth position, and the landmarks position and measurement were both determined by manual visual inspection, which may lead to errors in the measurement results. Furthermore, the dentition selected in this study were symmetrical. For asymmetric dentition, we need to find simple and stable landmarks and method independent of teeth to measure the width of the dental arch. In recent years, artificial intelligence based on machine learning with big data has gradually developed in the clinical and scientific research field of orthodontics and has been widely used in the measurement of oral and maxillofacial soft and hard tissues, the formulation of treatment plans and the prediction of treatment effects, etc., achieving credible results and high accuracy [[30], [31], [32], [33]]. In future research, we will essay to use artificial intelligence technology based on deep learning to collect and measure 3D data of jaws, to obtain more comprehensive and real results.

5. Conclusions

There were significant differences in the width of the upper and lower jaw between males and females. The normal occlusion group and Class Ⅱ group have significant differences in the width of the intercanine and BAW. This suggests that more attention should be paid to the horizontal dental problems of the treatment plan for Class Ⅱ patients, and the width of the canine and the first molar segment equally importance for treatment. Measuring the width of the maxilla and mandible basal bones from the resistance center of the first molar was a feasible and repeatable method could be used in clinical practice. For individuals, the normal range of the maxilla and mandible basal bone widths differences were −2 mm–2 mm (−0.05 ± 2.17 mm) (Fig. 5A). When measured from the WALA points the maxillomandibular width difference were 3.25 ± 3.38 mm for males and 2.22 ± 2.41 mm for females (Fig. 5B). Which means in patients with abnormal measurements, arch expansion was required firstly. The width of the mandible at the second molar was greater than that of the maxilla, so more attention should be paid to the width of the second molar when considering clinical treatment.

Funding

This study was supported by the National Clinical Research Center for Oral Diseases (LCB202202), CSA Clinical Research Fund (CSA-02022-01), New Technologies and New Business of School of Stomatology, Air Force Medical University Fund (LX2022-401, LX2023-402), China Oral Health Foundation (A2023-013), Key Research and Development Program of Shaanxi Province (2022SF-227).

Ethics statement

This study was reviewed and approved by The Ethical Committee of the Stomatological Hospital of Air Force Medical University, with the approval number: IRB-REV-2022056.

All participants/patients (or their proxies/legal guardians) provided informed consent to participate in the study.

All participants/patients (or their proxies/legal guardians) provided informed consent for the publication of their anonymised case details and images.

Informed consent was not required for this study because informed consent was obtained by telephone, by introducing the study to the patient or family.

Data availability statement

Data will be made available on request.

CRediT authorship contribution statement

Haolin Zhang: Project administration, Investigation, Data curation. Donghui Guo: Software, Methodology, Investigation, Formal analysis, Data curation. Yanning Ma: Methodology, Funding acquisition, Formal analysis, Data curation. Yuerong Xu: Validation, Software, Methodology. Zuolin Jin: Supervision, Resources, Project administration, Methodology, Funding acquisition. Hao Zhang: Writing – review & editing, Validation, Supervision, Project administration, Methodology, Funding acquisition, Formal analysis, Conceptualization. Jie Gao: Writing – review & editing, Formal analysis, Data curation, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.