Evaluation of condylar cortical bone thickness in patients with different vertical facial dimensions using cone-beam computed tomography.

Background The aim of this study is to evaluate through computed tomography differences in cortical plate thickness of condyle in patients with different facial vertical skeletal pattern. Methods The final sample of this retrospective study included CBCT exams of 60 adult subjects (mean age 33.2 ± 5.6), selected from the digital archive of a private practice. The subjects were assigned to 3 different groups according to the values of the Frankfurt-mandibular plane angle: hyper- normo- and hypodivergent group. The volume rendering of the mandible was obtained and three condylar points were marked on it: median pole, lateral pole and the most cranial point. For each considered reference point the minimum distance between external and internal cortical surface was measured, obtaining three different outcomes: median pole, lateral pole and cranial point cortical bone thickness. The measurement was executed by means of Mimics software by the same expert operator in specific scan views. Results cortical bone thickness of hyperdivergent patients was found to be statistically thicker than normodivergent patients and hypodivergent patients. Cortical bone thickness of normodivergent patients was found thicker than hypodivergent patients. All the differences were statistically significant (p<0.05). The Person correlation coefficient showed a statistically significant correlation (p<.001) between the Frankfurt-mandibular plane angle and the evaluated cortical bone thickness outcomes. Facial biotype characteristics that define vertical facial skeletal pattern affect the cortical bone thickness of mandibular condyle. variance evaluation. variance, parametric tests Analysis of variance (ANOVA), Tukey post-hoc test. In case data showed non-normal distribution, following non-parametric tests were used: Kruskal-Wallis test and Dunn-Bonferroni post-hoc test. Inferential statistics was performed applying multiple comparisons and post hoc tests among patients with different vertical mandibular patterns (different facial bio-type) for the cortical bone thickness of evaluated condylar sites: superior pole, medial pole and lateral pole. Person correlation coefficient was calculated between the Frankfurt-mandibular plane angle and each considered cortical bone thickness.


Background
The interaction of masticatory muscles and craniofacial skeleton plays an important role in craniofacial growth development and masticatory system function. Several studies showed a relationship between muscle activity and vertical skeletal pattern [1][2][3]. Literature showed that during mandibular function and in particular during closing, biting, and chewing, the mandible can distort, bend, and stretch [4][5][6][7]. Cortical bone thickness can react directly to the load exerted by the masticatory muscles to the mandibular bone [8] or it can react indirectly to the muscular load if it is applied by the articulating surfaces of the dentition and condyles [9].
The thickness of the cortical bone responds to this functional loading, changing his characteristics. As a consequence, the evaluation of cortical bone characteristics could provide clinicians and researchers a deeper understanding of mandibular function [10][11][12].
However, to date no study evaluated mandibular cortical bone condyle thickness in patients with different vertical facial dimension patterns.
Computed tomography (CT) is a diagnostic imaging technology that is considered the gold standard in order to investigate skeletal anatomy of oral cavity and facial complex [19].
Comparing it to conventional radiologic techniques, the use of computed tomography allows the measurement of anatomical craniofacial structures with greater accuracy [19].
In the last decades dose reduction was obtained with the use of Cone Beam Computed Tomography (CBCT) or with the introduction of Low Dose Multislice Computed Tomography protocols allowed the three-dimensional diagnostic information provided by CT become more frequently available for clinicians and researchers [20][21][22]. The aim of this study was to evaluate through computed tomography, the differences in cortical plate thickness of condyles in patients with different facial vertical skeletal pattern.

Methods
The sample of this retrospective study included cone beam computer tomography exams of 135 adult subjects, selected from the digital archive of a private practice. The exams were consecutively selected from a digital archive according to a specific inclusion criteria: Caucasian subjects, aged between 22 and 42y.o., have a complete permanent dentition with or without third molars, all sagittal class of skeletal and dental malocclusion were included. Exclusion criteria were: genetic syndromes or craniofacial dysmorphism, history of facial trauma and previous orthognathic surgery treatment.
The protocol of this study was approved by the Human Research Ethical Committee (103/16).
All exams were performed with i-CAT scanner (Imaging Sciences International, Hatfield, Pa) after setting the acquisition parameters as follows: 120 Kv, 5mA and 4-6 second to exposure time. On volume-derived cephalograms conventional cephalometric analysis was performed and Frankfurt-mandibular plane angle was calculated (figure 3).
The subjects were assigned to 3 different groups according to the values of the Frankfurtmandibular plane angle. Patients with an angle greater than 27° were included in the high angle facial group, others with an angle ranged from 27° to 19° were included in the normal angle group and patients with an angle smaller than 19° were included in the low angle group.
At the end of this subdivision we obtain 26 subjects with high angle facial group, 72 patients with a mandibular plane angle ranged from 27° to 19°, and 37 patients with a mandibular plane angle smaller than 19°.
A random sequence generator (http://www.randomizer.org) was employed to generate three lists of randomized numbers of 26, 72 and 37 numbers. The first 20 numbers of each random lists were selected, an adjustment was performed in order to have equal numbers of male and female in each group. Finally 60 CBCT were selected and included in the study. In order to assess the methodological error, rendering landmarks selection and outcomes measurements were repeated for 10 randomly selected patients one week apart. Paired ttest and Intra-class correlation coefficient (ICC) were used to assess the intra-operator reliability. The magnitude of the random error was assessed using the Dahlberg formula.
No differences (p<0.05) were found between the two readings, all measurements were highly reliable with the ICC varying from 0.81 to 0.92. Random error ranged from 0.2 to 0.4mm. Statistical analysis was performed with IBM SPSS Statistics Version 20 for Windows (IBM Corporation, NY, USA. SPSS, Inc., an IBM Company), the level of significance was set at P<.05. Table 1 reports the characteristics of the three-considered sample selected according On average the cortical bone thickness of hyperdivergent patients was found thicker than normodivergent patients and hypodivergent patients, cortical bone thickness of normodivergent patients was found thicker than hypodivergent patients. All the differences were statistically significant (p<0.05).

Discussion
This is the first study that has evaluated, by means of CBCT, mandibular condylar cortical Tsai and coworkers reached the conclusion that muscular function is able to affect both the morphology of the overall mandible (gonial angle dimension, ramus height) as well as bone characteristics (cortical thickness and bone mineral content).
The results of our study seemed to show that patients with increased vertical skeletal growth pattern showed a significantly greater cortical condylar bone thickness compared to patients with normal and reduced vertical skeletal growth pattern. In order to explain this finding we analyzed forces geometries produced by the masticatory muscles in patients with different vertical growth patterns ( Figure 6).
In figure 6 we represented with the red color the forces produced by the masseter muscle on mandibular skeleton and teeth. We know that masticatory forces are released on the occlusion and on temporomandibular joints, so we decomposed the masticatory forces of the masseter into two forces directed towards the anatomical structure (TMJ and teeth) on which masticatory forces are directed.
Analyzing the geometry of the vectors of masticatory force and its components, it is possible to observe that the masticatory forces of high angle patients are closer to the temporomandibular joint compared to the low angle patients, consequently the component It is known that condylar resorption can occur as an unfortunate sequela of orthognathic surgery.
Kerstens et al. [32] observed radiographic evidence of condylar atrophy in 12 patients out of 206 that underwent surgical orthodontic treatment. All 12 patients had the same dentofacial deformity: high-angle mandibular retrognathia. They considered the dentofacial deformity to be the main reason for condylar resorption and hypothesized that orthognathic surgery acts as a factor that stimulates the progress of the disease by increasing the load on TMJs.
The results of this study and the explanation provided by its authors can help explain the cortical differences observed among patients with different vertical skeletal pattern, and they could help orthognathic surgeons and orthodontists to evaluate the role of masticatory function in patients with different skeletal characteristics. Different aspects must be taken into consideration when evaluating the results of this investigation including its limitations. This study did not evaluate the possible differences between male and female subjects. However, in a recent study by Farnsworth et al., no sex differences in cortical bone thickness in the mandible were found between both sexes [33].
Limitations of CBCT imaging should also be considered in terms of spatial resolution and in terms of bone density evaluation [34]. CBCT images do not have consistent Hounsfield values and consequently it does not provide an accurate bone density evaluation [35].
Further research will be necessary in order to directly correlate cortical condylar bone and masticatory function.      Figure 1 Volume rendering of Region of Interest.

Figure 2
Transparency was applied to create a conventional radiographic cephalogram in latero-lateral projection imagines.    Forces geometries produced by masticatory muscle in patients with different vertical growth pattern. Low angle mandibular patients (A), Hi angle mandibular patient (B). The force of masseter was decomposed into two forces directed towards TMJ and teeth (for teeth a direction a line passing through the center of mandibular first molars was represented).