Comparison of condylar size among different anteroposterior and vertical skeletal patterns using cone-beam computed tomography

A complicated combination of skeletal and denture component incongruities in the craniomaxillofacial region contributes to malocclusion.¹  Condylar morphology plays an important role in orthodontic treatment planning.²  Moreover, skeletal pattern is involved in orthodontic diagnosis, treatment, or therapeutic response;²  for example, control of the vertical dimension during orthodontic treatment is of major importance in subjects with hyperdivergence.³ 

Conventional radiographic images used in orthodontic treatment (cephalometric and panoramic radiographs) do not provide clear or multidirectional views of the temporomandibular joints (TMJ).⁴  Cone-beam computed tomography (CBCT) is beneficial for dental and maxillofacial use because of its shortened scan time and high-resolution images. CBCT is used in various fields, including for the diagnosis of TMJ disorders and orthodontics.^5–7  Saccucci et al.⁸  attempted to determine the condylar volume in subjects with different anteroposterior and vertical skeletal patterns and found that greater condylar volume was a common characteristic of subjects with hypodivergence compared with the subjects with normodivergence and hyperdivergence. They also reported that skeletal class was associated with the condylar volume and surface. However, the morphological features that can cause differences in the condylar volume have not been clarified. Park et al.²  compared condylar morphology among different vertical skeletal patterns and found that the hypodivergent and hyperdivergent groups showed significant differences in several condylar linear measurements. The subjects of their study included those with flattened condyles or osteophytes. The study examined 20 subjects in each group, with a combination of men and women in each group. The condylar morphology in subjects with temporomandibular disorder (TMD)^9,10  and in subjects with facial or mandibular asymmetry^11–13  has also been observed. However, it is surprising that little is known about the condylar morphology in subjects with different anteroposterior and vertical skeletal patterns.

The purpose of this study was to compare condylar size among adult subjects with different anteroposterior and vertical skeletal patterns considering sex differences. The null hypothesis was that condylar size does not differ considerably among subjects with different anteroposterior and vertical skeletal patterns.

Three-dimensional scans of 332 TMJ of Japanese adults (61 men, mean age 27.2 ± 7.6 years; 105 women, mean age 27.4 ± 9.2 years) were retrospectively examined. They visited the Showa University Dental Hospital and consented to participation in the study. CBCT examination was carried out for all patients as part of the planning stage for orthodontic treatment. Subjects with congenital or systemic disease were excluded. Subjects had no symptoms of TMD, including joint pain, limited opening, or occurrence of joint sounds. Subjects with flattened condyles or osteophytes were also excluded. There were no evident mandibular deformities. None of the subjects had received orthodontic or orthopedic treatment. The study was approved by the Ethics Committee of Showa University.

Lateral cephalograms were traced and Power Cephalo software (ReazaNet, Tokyo, Japan) was used to derive the measurements. The anteroposterior skeletal pattern was classified as skeletal Class I (−1° ≤ A point–nasion–B point angle (ANB) < 4°; 24 male subjects and 45 female subjects), Class II (ANB ≥ 4°; 18 male subjects and 39 female subjects), or Class III (ANB < −1°; 19 male subjects and 21 female subjects).^6,7  The vertical skeletal pattern was classified as hypodivergent (mandibular plane angle [MP] ≤ 23°; 13 male subjects and 13 female subjects), normodivergent (23° < MP < 30°; 27 male subjects and 43 female subjects), or hyperdivergent (MP ≥ 30°; 21 male subjects and 49 female subjects).⁷ 

Dental and maxillofacial CBCT images were acquired using a cone-beam X-ray CT system (CB MercuRay, Hitachi Medico Technology, Tokyo, Japan, and KaVo 3D eXam, KaVo, Biberach, Germany). The scanning conditions in the CB MercuRay CT system were 100 kVp, 10 mA, F-mode 512 slices/scan (slice width: 377 μm); the voxel size was 0.378 mm; and the acquisition time was 9.6 s. The scanning conditions in the KaVo 3DeXam CT system were 120 kVp, 5 mA, 432 slices/scan (slice width: 400 μm); the voxel size was 0.4 mm, and the acquisition time was 17.8 s. It has been confirmed that calibration between the two systems is unnecessary.⁷  Data were stored in Digital Imaging and Communications in Medicine format and imported into Invivo 5 (Anatomage, San Jose, Calif) for further processing and analysis.

The condylar size on either side of the condyle was measured as described by Hilgers et al.¹⁴  and Al-koshab et al.¹⁵  The method used to assess condylar morphology was based on the delimitation and measurement of the distance between anatomical landmarks. The condylar length was determined by taking the posterior mandibular condyle point (PCo) and the anterior mandibular condyle point (ACo) on the front and rear sides 4 mm below the most superior mandible condyle point (SCo) and measuring the length of the line connecting them (Figure 1). The condylar height was determined by measuring the distance between the intersection point of a tangent from the most inferior point of the sigmoid notch (InfSig), parallel to the true horizontal line, in the sagittal plane and the posterior border of the ramus and the SCo (Figure 2). The medial mandible pole (MCo) and the lateral mandible pole (LCo) were determined corresponding to the largest dimension of the mandibular condyle in the coronal plane (Figure 3). Condylar width was determined by measuring the length of the line connecting the MCo and LCo in the coronal plane. The condylar length, height, and width were measured using CBCT images by one investigator.

Thirty CBCT images were randomly chosen, and all measurements were re-evaluated in separate sessions at 2-week intervals under identical conditions by one investigator to assess intraoperator error. Measurement error was estimated according to Dahlberg's formula (S2 = Σd2/2n).¹⁶  The statistical significance of differences between the left and right sides in each measurement was determined using the Student's t test.

Analysis of covariance (ANCOVA) was used to compare the three measurements among anteroposterior skeletal patterns and among vertical skeletal patterns using sex as a covariate. Nine groups were further divided according to the anteroposterior and vertical groups, and two-way ANCOVA was used to estimate the composite effect of skeletal pattern in both directions. All statistical analyses were conducted using SPSS Statistics Version 23 (IBM Corporation, Armonk, NY), and Power calculation was conducted using Minitab 16 (Minitab, Inc., State College, PA). Statistical significance was defined at P < .05.

The intraobserver measurement errors for all three measurements were within 3%.¹⁷  The left and right sides in all three measurements had no statistical difference.

Table 1 shows the descriptive statistics and ANCOVA results for the condylar size. Sex as a covariate showed statistical significance in seven among 12 examinations. Female condyles were smaller than the male ones. After adjusting for sex, the condylar height and the condylar width on both sides had statistical differences in the anteroposterior skeletal pattern (P < .001). The condylar height and width on both sides increased from Class II, Class I, and Class III. After adjusting for sex, the condylar width on both sides had statistical differences in the vertical skeletal pattern (P < .001). The condylar width on both sides increased from the hypodivergent, normodivergent, and hyperdivergent groups.

Table 2 shows the descriptive statistics for condylar size in the nine groups. Table 3 shows the ANCOVA results for condylar size to estimate the composite effect of skeletal patterns in both directions. The condylar height and width on both sides had statistically different anteroposterior and vertical skeletal patterns, respectively (P < .01 for both). No composite effect of skeletal patterns in both directions was observed.

Power calculation of this study was examined. In the anteroposterior skeletal patterns, the basic statistics of Class III as the largest average value and Class II as the smallest value were used to calculate the power using the unpaired t-test. The power of the right and left sides was 92% and 93%, respectively. In the vertical skeletal patterns, the basic statistics of hypodivergent for the right side and normodivergent for the left side were used as the largest average values, and hyperdivergent as the smallest value for both sides to calculate the power using the unpaired t test. The power was 47% and 78% on the right and left sides, respectively.

Condylar size was compared among different anteroposterior and vertical skeletal patterns using CBCT. The null hypothesis was that condylar size does not differ considerably according to the anteroposterior and vertical skeletal pattern. The condylar height and width differed considerably among subjects with different anteroposterior or vertical skeletal patterns. Therefore, the null hypothesis was rejected. Additionally, it was confirmed that condylar size had sex differences.

Condylar size measurements were determined based on the methods described by Hilgers et al.¹⁴  and Al-koshab et al.¹⁵  Al-koshab et al.¹⁵  found that the similarity in measurements for Malays and Chinese may be due to their common origin. A similarity in the measurements for the Japanese subjects in this study and their subjects was found because both are East Asian populations. Therefore, the validity of the measuring method was confirmed. In contrast, there are no data obtained on another population with the same measuring method. The observation in different ethnicities would be interesting as well.

Park et al.²  reported that hypodivergent and hyperdivergent groups showed significant differences in anteroposterior and mediolateral condyle widths (smaller in the hyperdivergent than in the hypodivergent groups). Their study included subjects with flattened condyles or osteophytes. Additionally, their statistical analysis combined both sexes. It has been reported that the frequency of osteophytes in the condyle may poorly correlate with age, sex, and dental and occlusal conditions.^18,19  To the contrary, there was a report that condyle size was larger in male subjects than in female subjects.¹⁵  Female subjects had signs and symptoms of TMD more frequently than male subjects²⁰  and mandibular volume had sex differences.⁷  Therefore, subjects with flattened condyles or osteophytes were excluded in the current study, and sex was used as a covariate. Although sex as a covariate showed statistical significance in most examinations, the findings by Park and colleagues²  were still replicated in this study.

Saccucci et al.⁸  reported that the difference was not significant although Class III subjects tended to show a higher condylar volume and surface than Class I and Class II subjects. Katayama et al.⁶  also reported no statistical difference in the mandibular volume among anteroposterior skeletal patterns. In the current study, a statistical difference in the condylar height and width was found among anteroposterior skeletal patterns. The difference in condylar size among anteroposterior skeletal patterns may be site specific.

Similar to the study conducted by Park et al.,²  the current study also demonstrated that subjects with hyperdivergence had short condylar widths, whereas those with hypodivergence had large widths. Larger condylar volumes were reportedly a common characteristic of subjects with hypodivergence compared with subjects with normo- and hypodivergence.⁸  Condylar volume and mandibular volumes had common characteristics in subjects with hypo- compared with those with normo- and hypodivergence.⁷  The results of these studies, including the current study, were consistent. The masseter volume and thickness significantly correlate according to the vertical skeletal pattern.²¹  Interestingly, positive correlations between masseter muscle weight and condylar size have been reported.²²  It has been shown that a significant relationship exists between masticatory muscle activity and vertical skeletal growth pattern.²³  The differences in masseter and medial pterygoid orientation and volume present in subjects with different underlying vertical skeletal patterns²⁴  may be important.

• Subjects with Class II or hyperdivergent skeletal patterns had small condylar sizes, and subjects with Class III or hypodivergent skeletal patterns had large condylar sizes.

• The anteroposterior and vertical skeletal patterns independently affected the condylar size.

• Female condylar sizes were smaller than those of males.

We are deeply grateful to the subjects who participated in the present study. The authors would like to thank all participants in this study. This work was supported by KAKENHI Grant Numbers 17K11947. The authors would like to thank Enago (www.enago.jp) for the English language review. We also wish to thank the Japan Institute of Statistical Technology (www.jiost.com) for the technical support and advice on statistical analyses.