The Asymmetry of the Mandible in Patients with Unilateral Temporomandibular Joint Disc Displacement Confirmed by Magnetic Resonance Imaging

Abstract

Objectives

The study aimed to determine the relationship between the mandibular asymmetry index according to Kjellberg between patients with painful unilateral anterior disc displacement (DD) and asymptomatic volunteers without disc displacement. Vertical measurements were performed on a panoramic single-image radiograph, and the disc status was confirmed by magnetic resonance imaging (MRI).

Material and methods

Two groups of subjects were retrospectively selected, 40 patients (the overall mean age was 35.5 years; 75% female) with temporomandibular disorder symptoms confirmed by RDC/TMD axis I and manual functional analysis. Unilateral DD was determined by MRI. A comparative group of asymptomatic volunteers (20 dental students with a mean age of 23.4 years; 72% female) had the physiological position of the disc determined by MRI. The vertical asymmetry of the condyle was determined by the method of Kjellberg et al. The symmetry of the gonial angle of the mandible was also measured.

Results

A comparison of the mean of the asymmetry index between patients (average 90.89±7.08%) and asymptomatic volunteers (mean 95.86±4.44%) showed a statistically significant difference (p=0.0029). There was no difference (p=0.088) in gonial angle symmetry between the patients (mean 96.48±2.96°) and the asymptomatic volunteers (mean 97.52±2.31°). The distribution of the presence of individual DD diagnoses (partial and total displacement with reduction, displacement without reduction) in patients diagnosed with asymmetry of the mandible was without statistical significance (p>0.05).

Conclusion

This study actually points to the asymmetry of the mandible as a potential morphological risk of anterior DD.

Introduction

Temporomandibular disorder (TMD) is an umbrella term for musculoskeletal pathological conditions that may have similar signs and symptoms, and lead to disruption of normal function of the stomatognathic system. Within the term TMD, we recognize separately a muscle disorder and a temporomandibular joint (TMJ) disorder. In TMJ disorders, there are diagnoses of anterior disc displacement and degenerative joint changes (osteoarthritis). An overlapping situation may occur regarding diagnoses in the same patient. The main symptoms are a pain in the preauricular region (TMJ) and/or masticatory muscles, asymmetric and limited movements of the mandible. Symptomatology often includes headaches (1-3).

Of the potential etiological factors of TMD, a group of morphological factors, including occlusion, have a predisposing and perpetual role. For example, TMJ trauma is the most common precipitating factor in the pathogenesis of TMD (4, 5).

Orthopantomography scan (OPG exam) is a common dental imaging technique, or a basic radiological diagnostic and identification document, that is used in many polyvalent and specialist procedures in dentistry. Orthopantomography is used more frequently in the differential diagnosis of orofacial pain than in the targeted TMJ diagnosis. Apart from qualitative diagnostic possibilities, metric (quantitative) methods have also been developed (6-9).

The symmetry of the craniofacial region is not only an aesthetic imperative. The asymmetry of the mandible is a potential cause of dysfunction of the mandible and TMJ as a bilateral joint. Numerous studies have explored the association of vertical asymmetry of the condyle with the risk of TMJ disc displacement (10-16).

This study aimed to determine the relationship between the mandibular asymmetry index according to Kjellberg between patients with painful unilateral anterior disc displacement and asymptomatic volunteers without disc displacement. The disc status was confirmed by magnetic resonance imaging (MRI).

Material and methods

This research was approved in 2009 by the competent Ethics Committee of the School of Dental Medicine of the University of Zagreb with decision 05-PA-30-XXIV-2. The purpose and method of diagnosing TMJ were explained to all subjects according to the reported protocol. The included subjects of both subgroups had previously signed consent on voluntary participation, including MRI and orthopantomography.

82 consecutively collected patients with painful and unilateral TMJ disorders and 25 volunteers, students of dental medicine, were retrospectively included. Based on the clinical diagnostic criteria of RDC/TMD I (17) and manual functional analysis according to Bumann and Groot Landeweer (18), an indication for MRI was set. The patients were examined at the Department of Removable Prosthodontics of the School of Dental Medicine, University of Zagreb, and at the Department of Diagnostic and Interventional Radiology of Sestre milosrdnice University Hospital Center, where the Chair of Radiology, School of Dental Medicine, University of Zagreb is located.

Patients were definitively selected by MRI, which determined displacement of the TMJ disc. 19 patients with pronounced degenerative changes (osteoarthritis) with and without disc displacement were excluded from the study. Of the remaining 63 patients, 23 patients with bilateral disc displacement were excluded. The study finally included 40 patients (mean age 35.5 years; 75% female) with unilateral anterior displacement of the TMJ disc.

A comparison group of volunteers was obtained from a sample of 25 undergraduate students who were without any previous symptoms of TMD. In 5 students, an asymptomatic disc displacement was detected by MRI; hence they were excluded from the study. The remaining 20 volunteers (mean age 23.4 years; 72% female) with physiological disc positions were included in the study. Patients and asymptomatic volunteers were not undergoing orthodontic therapy, had no facial anomalies or previous treatment of facial and jaw anomalies, did not suffer from polyarthritis and rheumatic forms of arthritis, and did not undergo major oral surgery procedures.

MRI evaluation

Right and left TMJs were recorded simultaneously in closed-mouth and open-mouth positions when the patient was in a supine position. MRI recording began with fast sequences (pilots), by which the recording object was brought to the center of the magnetic field. Subsequently, its incomplete alignment concerning the midsagittal plane was corrected. The recording angle of the parasagittal sections was determined individually by the angle shown on the individually angled layers of the axial and coronal sections. The layers in the spin-echo sequences (Table 1, Figure 1) with a thickness of 3 mm were used for a parasagittal projection of the TMJ.

Table 1. Magnetic resonance imaging parameters for the parasagittal view of the temporomandibular joint in different spin-echo sequences.

imaging parameters and sequences	T1-weighted image	T2-weighted image
time of echo (TE) (ms)	12	13–15
time of repetition (TR) (ms)	450	460–770
field of view (FoV) (dot/cm)	160x160	160x160
matrix (pixel)	256x192	256x128

ms, millisecond; cm, centimeter

Figure 1.

Magnetic resonance imaging of the temporomandibular joint of a patient with anterior disc displacement (closed mouth position)

The normal superior position of the disc is differentiated from the anterior displacement of the disc in patients depending on the presence of pain. The criteria for asymptomatic disc displacement are identical to those of painful (symptomatic) joints. However, anamnestic and clinical disease indicators (pain, sounds) of the TMJ are absent. The criterion for the normal superior position of the disc is: the pars intermedia of the disc lies in the shortest distance between the posterior slope of the articular tubercle and the anterior edge of the head of the condyle in the closed mouth position. The pars posterior lies on the head of the condyle. Both criteria should be met for all parasagittal layers of an individual joint. When the mouth is open, the disc maintains its normal superior position. The criterion for partial disc displacement with reduction is as follows: while the mouth was closed the disc was normally positioned in some parasagittal layers and anteriorly displaced in other parasagittal layers. In the open mouth position, there is a reduction of partial anterior displacement, i.e. in all layers of the same joint, the disc is without displacement. In the case of complete displacement of the disc with the reduction in all parasagittal layers of the same joint, the disc is displaced anteriorly when the mouth opens completely and it is repositioned on the condyle. A complete disc displacement without reduction means that the disc is displaced in the closed-mouth position, and in the open-mouth position the anterior displacement is still not reduced.

Evaluation of mandibular asymmetry

All orthopantomographs were recorded using a standard protocol on the same X-ray orthopantomographic device Orthophos D3200 (Siemens, Erlangen, Germany) which was operated at 12 mA and 90 kV, in the X-ray room of the School of Dental Medicine, University of Zagreb. High-quality recordings were used with a standardized patient position according to the manufacturer's instructions. The patient's lips were relaxed and the head was oriented towards the Frankfort horizontal plane and the midsagittal plane using the tripod of the device in order to avoid distortion. Patients were positioned with their back and spine as straight as possible and their necks extended. The patient's anteroposterior position was achieved by placing the incisal edges of their maxillary and mandibular incisors in a notch positioning device.

All measurements (40 orthopantomographs of patients and 20 orthopantomographs of volunteer subjects) were performed by one researcher (S.V.) on orthopantomographs in digitized form using the computer program Adobe Photoshop, Adobe Systems.

The methodology of symmetry of the mandible using the method according to Kjellberg et al. (19) is focused on the vertical distances of the distal edge of the mandible (Figure 2). The reference points of the ramal tangent on the lateral edge of the ramus of the mandible are at the height of the highest point of the condyle (point C) and the mandibular notch (point I). The tangential line following the marginal edge of the body (mandibular line) forms a gonial angle with the ramal line. The intersection of the ramal and gonial lines in the area of ​​the angle of the mandible forms point G. The calculation of the index is obtained according to the formula shown in Figure 3 as the ratio of the height of the condyle (CH) and the height of the entire ramus (MH).

Figure 2.

Schematic representation of asymmetry measurement using point projections on the distal edge of the ramus of the mandible. The height of the condyle (CH) is the distance between the projections of the condylar (C) and incisura (I) points. Height of the mandible (RH) between point I and point gonion (G). The gonial angle is formed by the lines RH and the tangent of the lower edge of the body of the mandible.

Figure 3.

Formula for calculating the percentage of symmetry of the mandible according to Kjellberg. CH is the distance of the height of the condyle, and RH is the distance of the entire ramus of the mandible. Regardless of the left or right side, the smaller value (A) is always divided by the larger value (B).

The symmetry index was calculated by dividing the values of CH and RH regardless of whether it was on the left or right side, and the numerator had to be smaller than the value of the denominator (Figure 3). The symmetry of the gonial angle was also measured by dividing the value of the gonial angle of the left and right sides of the same person. It was done in such a manner that a higher value than one on the opposite side was put into the numerator. In order to obtain the percentage of the symmetry index of the mandible and the gonial angle, the obtained value was multiplied by 100. For the limit criterion of asymmetry, a percentage value greater than 6% was taken, i.e. if the symmetry index was ≤93 (19), then it was a true skeleton asymmetry of the mandible. The same criterion was used for the symmetry values of the gonial angle.

Statistical analysis

The collected data were encrypted and organized as a file using the Microsoft Office Excel 2003 program on a personal computer. Statistical analysis of the data was performed using the STATISTICA and SAS programs. The normality of the data distribution of the numerical variables of the symmetry index of the mandible and the gonial angle was tested with the Shapiro–Wilk normality test (p < 0.05). The t-test for independent samples was used to compare numerical variables between the group of patients and asymptomatic volunteers. Fisher's exact test was used within the group of students to analyze the frequency of established asymmetry depending on the diagnosed types of unilateral disc displacement: partial disc displacement with reduction, complete disc displacement with reduction, and disc displacement without reduction.

An analysis of the accuracy of the MRI reading and the concordance of the diagnosis of disc displacement was performed between two examiners (specialists in medical radiology (DZ) and dental prosthetics (TB), and by the examiner himself (TB). High reliability of the findings was determined (κ = 0.8 – 1) on the scans of 12 patients by independent analysis of disc displacement findings, whether by a single examiner (TB) or by two examiners (TB and DZ). Cohen's kappa was used for this purpose. The significant difference for statistical testing was 5% and 1% (20, 21). The Dahlberg error method (22) was used to check the reliability of the measurement of metric values. The error between two independent measurements of orthopantomographs of 12 randomly selected patients was calculated. The procedure was performed on the same image in two different periods. When there is no measurement error, then the Dahlberg error (ME) is zero. The expression ME=√∑d2/2n is used for calculation (d – the difference between two measurements; n – the number of double-measured values on the orthopantomograph). It was shown that the repeated measurement of ME amounted to as high as 0.6 to 0.08.

Results

A comparison of the mean asymmetry index between patients (n=40, mean 90.89±7.08%, minimum-maximum 72.38%–99.92%) and asymptomatic subjects (n=20, mean 95.86±4.44%, minimum-maximum 83%–99.98%) showed a statistically significant difference (t-test (df58)=286491, p=0.0029). The symmetry of the gonial angle was also analyzed, which for patients (mean 96.48±2.96°, minimum-maximum 97.37°-99.73°) and asymptomatic volunteers (mean 97.52±2.31°, minimum- maximum 90.99°–100°) was not statistically significant (t-test(df58)=-1.36762 with p=0.088).

23 patients (symmetry index on average 86.32±5.95%) and 5 asymptomatic volunteers (symmetry index on average 89.63±4.62%) had less than the limiting 94% symmetry index value. Their comparison was not statistically significant (t-test (df26) =-1.16082 with p=0.1281). Furthermore, the gonial angle (subjects of both groups with a Kjellberg index <94%) was compared between patients (n=23; mean gonial angle symmetry index 96.14±3.28°) and asymptomatic volunteers (n=5; mean gonial symmetry index angle 96.30±2.21°). No statistically significant difference was found (t-test (df26) =-0.10925, with p=0.457).

Associations between the vertical asymmetry of the mandible and the presence of certain diagnoses of disc displacement within the group of patients with unilateral disc displacement were analyzed using Fisher's exact test (p>0.05), but without statistical significance (Table 2).

Table 2. Distribution of the frequency of certain diagnoses of disc displacement within the group of patients depending on whether they have symmetry or established asymmetry of the mandible.

variables /        subgroups of patients	Partial DDwR	Total DDwR	DDwoR	total        hp
n of patients - asymmetry	5	5	13	23
hp        vp	12.5%        55.56%	12.5%        58.33%	32.5%        31.58%	57.5%        100%
n of patients - symmetry	4	7	6	17
hp        vp	10%        44.44%	17.5%        58.33%	15%        31.5%	42.5%        100%
n of patients - total	9	12	19	40
vp	22.5%	30%	47.5%	100%
Fischer's exact test, p=0.388

n, number of patients; DDwR, disc displacement with reduction; DdwoR, disc displacement without reduction; hp, the sum of the column percentages; vp, the sum of the row percentages

Discussion

A panoramic X-ray image of the jaws, teeth, and TMJ is used in qualitative initial dental diagnostics. In addition, the panoramic X-ray image also shows the surrounding bone structures and pathological changes in the imaging zone. Furthermore, it is used in the quantitative analysis of jaw asymmetry in orthodontics and TMD diagnostics (19, 23).

The patient's intermaxillary relation achieved through protrusive movement utilizing the bite block leads the TMJs to the position of the condyle outside the articular fossa. Therefore, panoramic imaging, in contrast to MRI, is not applicable for analyzing the basic position of the condyle behind the articular fossa in the closed mouth position. In this study, panoramic imaging was used for vertical measurement of the structure of the ramus and condyle of the mandible. MRI is the gold standard in determining disc status and it is used for the analysis of soft and hard tissues, depending on whether the mouth is closed (in bite) or maximally open, and for quantitative analysis of intra-articular structures (24-26).

There was no predominance of any form of anterior displacement and jaw asymmetry in our study. The difference in the reference positions of the mandible and TMJ in the diagnosis of MRI and panoramic radiograph did not affect the results of this study, because they are two completely different radiological methods (27-30). In this study, MRI was used to differentiate TMJ depending on the physiological and displaced disc, which is not possible with classical X-ray diagnosis, including panoramic radiography.

The issue of TMJ and mandible image distortion on orthopantomographs can have an impact on the accuracy of measurements. The validity of vertical measurements, as well as horizontal measurements that do not cross the medial line, has been proven on the same device on which our research was conducted (31, 32).

The method developed by Kjellberg et al., which was applied in this research (19), implies measuring the scale and not the actual sizes of the distances, including the magnification factor. The relationship between the relative height of the condyle and the height of the ramus of the mandible is determined separately for each joint of the same patient. The comparative method of calculating mandibular asymmetry according to Habets et al. (23) differs from the Kjelberg et.al, method. The advantage of the method according to Kjellberg et al. (19) is that only unilateral linear measurements are included in the calculation. They are not taken into account when measuring the left and right sides at the same time.

Iturriaga et al. (10) did not confirm patients' vertical condylar symmetry as a risk factor for the development of TMD, using both measurement methods (19, 23). Xie et al. (11), similar to this study, used MRI to confirm disc displacement and compared vertical measurements on the posterior-anterior cephalogram with control subjects. The asymmetry of the mandible was present in 72.12% of patients and 25.64% of control subjects. Since the patients also had unilateral disc displacement, the results obtained by the aforementioned researchers are in line with ours. Mandibular asymmetry is more common in subjects with unilateral disc displacement.

Ahmed (13) used CBCT scans to measure the correlation of the left and right condyles of subjects without a diagnosis of TMD, thus detecting the asymmetry of the mandible. Toh et al. (12) investigated patients with clinically confirmed TMD and dentofacial deformities, which was contrary to the patients in our research sample. They detected mandibular asymmetry in 61.2% of subjects.

Chandhok et al. (14) took a sample of patients with myalgia and of those with myalgia and clicked in the TMJ. The control group included subjects without pain and without clicking. Although the highest index of asymmetry was in the group of patients with myalgia and clicking, there was no statistical significance: height and contour of the condyle, as well as mandibular asymmetry, cannot be a predisposing factor for clicking in the TMJ. In contrast, Piancino et al. (15) showed the results obtained in their research. They used MRI to determine the position of the disc. They stated that orthopantomography can be a screening method because they believe that condylar asymmetry increases in individuals who have disc displacement. Mendoza-García et al. (16) found on orthodontic patients that asymmetry was greater in patients with TMD, but without a statistically significant difference. Therefore, they concluded that TMD is not related to mandibular asymmetry.

Radhakrishnan et al. (33) found on the orthopantomographs of orthodontic patients, likewise in our study, that the differences in the symmetry of the gonial angle were insignificant. Habib et al. (34) did not find statistically significant differences in vertical measurements, including mandibular asymmetry index and gonial angle values in pre-orthodontic patients.

So far, the direct impact of certain etiological factors in the process of TMD formation has not been sufficiently explained. Therefore, disc displacement and/or osteoarthritis are the most common diagnoses of TMJ (35). Although the application of the measurement method did not show a correlation of mandibular asymmetry in different profiles of dental patients, patients with TMD, and control subjects, it can be concluded that there is a correlation between unilateral disc displacement and facial asymmetry, which is contrary to our results (9, 36).

Conclusions: In conclusion, with proven disc status using MRI, it was shown that there was a statistically significantly higher index of mandibular asymmetry in patients with unilateral disc displacement compared to asymptomatic subjects who were proven to have a physiological disc position. The asymmetry of the mandible can be considered part of the morphological risk for the occurrence of anterior disc displacement.