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 Table of Contents  
ORIGINAL ARTICLE
Year : 2021  |  Volume : 8  |  Issue : 3  |  Page : 172-176

Prevalence and characteristics of mandibular divergency in class III patients


1 Department of Preventive Dentistry, Alfarabi Colleges, Riyadh, Saudi Arabia
2 Riyadh Elm University, Alfarabi Colleges, Riyadh, Saudi Arabia
3 Dental School, Alfarabi Colleges, Riyadh, Saudi Arabia
4 Dental School, Prince Sattam Bin Abdulaziz University, Al-Kharj, Saudi Arabia
5 Dental School, Majmaah University, Riyadh, Saudi Arabia

Date of Submission12-Sep-2021
Date of Decision06-Oct-2021
Date of Acceptance09-Oct-2021
Date of Web Publication30-Dec-2021

Correspondence Address:
Dr. Nada Esam Tashkandi
Department of Preventive Dentistry, Riyadh Elm University, King Fahad Service Road, Namouthajya, Riyadh 12734
Saudi Arabia
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/sjoralsci.sjoralsci_46_21

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  Abstract 


Introduction: The classical skeletal divisions of orthodontic patients are the Angle's classification of Classes I, II, and III and of mandibular inclination into normo-, hypo-, or hyperdivergent. Of these classifications, Class III and hyperdivergent are the least studied. This study aimed to investigate the prevalence and relationship between Class III and hyperdivergency.
Materials and Methods: Lateral cephalometric radiographs of Class III patients were collected from Riyadh Elm University and were traced by WebCephTM software and measured for 16 linear and angular measurements to determine anteroposterior, vertical, and dental associations. To determine the anteroposterior relationships, Sella-Nasion to A point Angle, Sella-Nasion to B point Angle, A point to B point Angle, and Wits appraisal were used. Class III status was determined by utilizing (A point to B point Angle <1) and (Wits appraisal <−2.5). The vertical divergency pattern was considered according to normal Frankfort horizontal plane and mandibular plane (FMA) (22°–28°) and SN-GoMe: Sella-Nasion to Mandibular plane (27°–36°) measurements. The dental correlation was considered according to upper incisor to maxillary plane (U1-Mx) and lower incisor to mandibular plane angle. Statistical analysis was performed with SPSS (version 25) software.
Results and Conclusions: One hundred and twenty-five Class III patients were included in the final analysis of this study. A significant correlation at 0.650 was discovered between ANB and Wits analyses (<0.001). Subjects data was grouped, based on their mandibular divergency, into hypodivergent, normodivergent, and hyperdivergent. The distribution, regardless of measurement used, was significantly higher in the prevalence of hyperdivergents (FMA 46.4% and SNGoMe 56%) over hypo- and normodivergents. Although FMA and SNGoMe were positively correlated at 0.887, there were significant differences in their classification of divergency with higher levels of hyperdivergency with SNGoMe.

Keywords: Class III, FMA, hyperdivergency, mandibular divergency, SN-GoMe


How to cite this article:
Tashkandi NE, Alshanbari SS, Almutairi NN, Al Hawsawi AA, Abuabah AA, Alanazi AA. Prevalence and characteristics of mandibular divergency in class III patients. Saudi J Oral Sci 2021;8:172-6

How to cite this URL:
Tashkandi NE, Alshanbari SS, Almutairi NN, Al Hawsawi AA, Abuabah AA, Alanazi AA. Prevalence and characteristics of mandibular divergency in class III patients. Saudi J Oral Sci [serial online] 2021 [cited 2022 Jan 23];8:172-6. Available from: https://www.saudijos.org/text.asp?2021/8/3/172/334299




  Introduction Top


Class III malocclusion represents an intricate three-dimensional facial skeletal imbalance between maxillary and mandibular development. It is a complicated disorder that is characterized by a concave profile, which may show mandibular protrusion, maxillary retrusion, or a combination of both along with varying degrees of dentoalveolar and soft tissue compensations.[1],[2],[3],[4],[5],[6] It is the result of genetic heredity, environmental factors, hormonal imbalances, resting tongue habits or size, history of prolonged sucking, mouth breathing, functional mandibular shifts for respiratory needs, or it could be a combination of these factors.[7],[8],[9],[10] Familial genetic heredity has a strong influence on skeletal craniofacial dimensions contributing to Class III malocclusions, and a significantly higher incidence of this malocclusion has been found to have a familial occurrence between members of many generations.[11],[12],[13],[14],[15]

Anteroposterior status is classified according to ANB angles or the Wits Appraisal. Patients are classified as Class III if either angle is below the standardized norms. The cephalometric measurements used to evaluate mandible divergency are the angle formed between the cranial base and mandibular plane (SN-GoMe) and the angle formed between Frankfort horizontal plane and mandibular plane (FMA). Low Sn-GoMe or FMA angles show a horizontal growth pattern and are described as hypodivergent or short faces. On the other hand, the high SN-GoMe or FMA angles have a vertical growth pattern and are described as hyperdivergent or long faces.[16]

When considering treatment of Class III hyperdivergent patterns, there could be ambivalence, in that treatment of one may worsen the other. There are three main treatment options for skeletal Class III malocclusion: growth modification, dentoalveolar compensation, and orthognathic surgery.[17] However, traditional treatments for growth modification (facemask or chincup) tend to increase the lower facial height, which in cases of hyperdivergency, worsens the appearance.[18],[19] This poses a unique problem in orthodontic case management and was a driving force in this research. The purpose of the present study was to investigate Class III and hyperdivergent and their relationships and to look into Class III mandibular divergency and the associated dental and skeletal features.


  Materials and Methods Top


This cross-sectional study was conducted using records from Riyadh Elm University Orthodontics Department. Cephalometric radiographs of 140 subjects with Class III malocclusion were collected.

Exclusion criteria resulted in fifteen records being excluded after the tracing process due to the following:

  • Class I or II subjects
  • Participants with craniofacial disorders or abnormalities
  • Previous orthodontic treatment
  • Radiographs with patient positioning errors.


This study used WebCephTM software, which is an AI-driven online tracing software, but which was manually corrected for the landmarks and tracing by the researchers in this study. All tracings were reviewed by the first author in this paper to ensure reliability with 10% of the samples retracted to a reliability of 92%.

Lateral cephalometric radiographs were traced and measured for 16 linear and angular measurements. SNA, SNB, ANB, and Wits appraisal used to determine the anteroposterior relationship. Class III status determined by using (ANB <1) and (Wits Appraisal<-2.5). The vertical relationship considered using the FMA (22°−28°) and SN-GoMe (27°−36°) to determine normal mandible divergency. Subjects above or below the normal range were considered hyperdivergent or hypodivergent, respectively. The occlusal plane was measured using the occlusal plane cant as well as the SN-Occlusal Plane (SN-OccPl). Facial height ratio (Posterior facial height/Anterior facial height) measured to determine where the face is longer in the anterior or posterior. The gonial angle was measured to determine the steepness of the mandibular border. Incisor to mandibular plane angle and U1 to maxillary plane angle were measured to define the relationship between incisal inclinations to the jaws.

This study was approved by the Riyadh Elm University IRB with registration number SRS/2020/20/201. All the data were analyzed using SPSS (IBM-SPSS version 25, Armonk, NY: USA). Normality of the data was checked, and data were found to be normally distributed. Descriptive statistics of mean and standard deviations were calculated for the cephalometric variables and compared between gender and age groups by using independent sample's t-test. Divergence based on cut-off values of SNGoMe and FMA were classified and compared across various cephalometric variables. ANOVA (Analysis of Variance) and Tukey's multiple comparison tests were applied to compare divergence among different groups. Pearson's correlation test was applied to assess the relationship between cephalometric variables. A value of P < 0.05 was considered significant for all statistical purposes.


  Results Top


Class III records according to both ANB and Wits were considered in the final analysis. There was a significant correlation between ANB and Wits in these measurements with a Pearson coefficient of 0.650 (P < 0.001). The mean age was 18.78 ± 5.48 and the gender distribution was 73 males and 52 females. There were no significant gender or age differences noted in any of the measurements except with facial height (both total and PFH/AFH ratio).

Subjects were grouped according to their mandibular divergency into hypodivergent (FMA <22°, and SNGoMe <27°]), normodivergent (22°>FMA >28° and 27°>SNGoMe >36°), and hyperdivergent (FMA >28° and SNGoMe >36°). Although FMA and SNGoMe were positively correlated according to Pearson's coefficient at 0.887, there were significant differences in their classification of divergency with higher levels of hyperdivergency with SN-GoMe. The distribution, regardless of measurement used, was significantly higher in the prevalence of hyperdivergents (FMA 46.4% and SNGoMe 56%) over hypo- and normodivergent with the distribution as shown in [Figure 1].
Figure 1: Box and whisker plot showing the distribution of hypodivergent, normodivergent, and hyperdivergent groups

Click here to view


When considering the measurements regarding the mandibular divergency, there were considerable differences in almost all measurements between the groups both when using the FMA and Sn-GoMe as shown in [Table 1] and [Table 2]. There was a negative correlation between the SNGoMe and the upper incisor to maxillary plane and the lower incisor to mandibular plane with a correlation coefficient of − 0.553 and −0.495, respectively. The upper and lower incisors were positively correlated at a correlation coefficient of 0.522. The occlusal plane was considerably affected with the changes in mandibular divergence. Results of the occlusal plane cant as well as SN-OccPl showed significant changes between the hypo, normo, and hyperdivergent groups, especially when considered using SNGoMe. There were also, significant differences found in the gonial angles of the mandible among all three divergency patterns with respect to the FMA but not the SNGoMe as seen in [Table 1] and [Table 2].
Table 1: One-way analysis of variance showed a statistically significant difference in the mean and standard deviation values of the cephalometric variables compared across hypodivergent, normodivergent, and hyperdivergent groups based on FMA norms

Click here to view
Table 2: One-way analysis of variance showed a statistically significant difference in the mean and standard deviation values of the cephalometric variables compared across hypodivergent, normodivergent, and hyperdivergent groups based on SN-GoMe norms

Click here to view



  Discussion Top


The main objectives of this study were to examine the prevalence and characteristics of mandibular divergency in a Saudi population. The overall prevalence of Class III malocclusion reported in the literature varies from 0% to 26.7% for different populations.[20],[21],[22],[23] Patients with Class III malocclusions mostly display hypodivergent patterns whereas Class II cases are closely linked with hyperdivergent patterns.[24],[25] This is one of the reasons, it is unusual for a study like this to find a large percentage of hyperdivergent Class III patients regardless of the method used for measurement where we found that the majority of subjects were classified as hyperdivergent when considering both the FMA (46.4%) and the SNGoMe (56%).

Regarding the Class III status of subjects in this study, both Wits and ANB were considered. Wits appraisal can be altered by the cant of the occlusal plane whereas ANB is affected by the sagittal location of N (nasion) landmark.[26] Investigations have varied in their reported correlations between the two measurements.[26],[27],[28],[29],[30],[31] In this study, a significantly positive 0.650 (P < 0.001) correlation was found between the Wits and ANB measurements by Pearson coefficient which supports the use of either measurement bearing in mind the correct landmark positions.

In Class III cases, incisors are usually more upright than in Class I or II but lower incisors are normally inclined.[26] In the present study, both upper and lower incisors retroclined as the hyperdivergency increased but they retroclined at a similar rate with the upper slightly less than the lower. This phenomenon has also been supported in the literature.[32],[33],[34]

Hyperdivergent patterns exhibit increased angles between the occlusal plane and both the maxillary and mandibular planes with the opposite seen in hypodivergent patterns.[35],[36] The present study shows that the occlusal plane was considerably affected with changes in mandibular divergence. With respect to the occlusal plane cant as well as SN-OccPl, the changes were significantly affected between the hypo-, normo-, and hyperdivergent groups. Hyperdivergent subjects have higher inclinations of occlusal planes which suggest that the occlusal plane is pointedly affected by the mandibular plane angle. This advocates the use of treatment mechanics that reduce the inclination of the occlusal plane will significantly affect the degree of mandibular divergency which backs the philosophy that Class III treatment such as facemask therapy (which further steepen the occlusal plane) thereby increasing the mandibular plane angle and should be avoided in patients with a hyperdivergency tendency.

Hypodivergent patterns have been found in the literature to show highly decreased gonial angles whereas hyperdivergent patterns displayed increased gonial angle.[6],[37] There were significant differences found in the gonial angles of the mandible among all three divergency patterns in relation to the FMA but not to considering the SNGoMe. This may lead to the supposition that the FMA is more likely to be affected by changes in the body of the mandible itself rather than the SNGoMe because it considered the Gnathion point rather than Menton. This may be significant in that chin prominence and chin morphology can affect the mandibular plane angles and the subsequent measurements. One of the limitations of this study was the sample size. Furthermore, all the included subjects were collected from a single institution. Hence, further studies with large and diverse sample sizes are required to confirm our findings.


  Conclusions Top


Precautions need to be taken when treating Class III patients as the prevalence of hyperdivergency may be higher than previously expected. Furthermore, treatment modalities that increase the mandibular plane angle should be used with caution in Class III patients due to the higher chance of hyperdivergency.

Acknowledgments

We would like to thank the Riyadh Elm University Summer Research School for the opportunity and resources to conduct the present study. Deepest gratitude is also extended to Dr. Mohammad Abdulbaseer for his statistical analysis.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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