Diagnosing mandibular asymmetry using posteroanterior cephalograms in patients with unilateral and bilateral degenerative joint disease

INTRODUCTION

The Greek word symmetria which means “of like measure” is defined as correspondence in size, shape, and relative position of parts on opposite sides of a dividing line or median plane. Asymmetry is described as a lack of symmetry. When applying this to the human face, it exhibits an imbalance or disproportionality between the right and left sides. An average face illustrates an acceptable degree of asymmetry, which may be caused by a range of factors that affect the underlying skeletal structure or soft-tissue drape.^[1]

A mild degree of asymmetry in the craniofacial region is common in humans,^[2,3] including individuals with a normal facial appearance.^[4,5] Clinically, various degrees of mandibular asymmetry are observed in many patients.^[6] The importance of early diagnosis and the detection of progressive causative conditions is essential for the management of facial asymmetry.

Temporomandibular joint internal derangement (TMJ ID) is also widely reported to be associated with mandibular asymmetry.^[7-10] Some authors have reported TMJ ID itself as a primary cause of growth disturbance, including mandibular asymmetry, and the data indicate that mandibular asymmetry can influence the shape and function of the TMJ and vice versa.^[11,12]

The prevalence of a symptomatic side has been causally related to the degree of vertical asymmetry. Moreover, differences in bilateral TMJ morphology in patients with mandibular asymmetry have represented anatomic disorders and have been shown to predispose these patients to TMJ problems.^[13]

A study by Buranastidporn et al. categorized patients with mandibular asymmetry into three grades, mild, moderate, and severe. For patients with mild mandibular asymmetry, the facial occlusal plane or frontal occlusal plane (FOP) and frontal mandibular plane (FMP) were <4°, with a tendency for ipsilateral TMJ ID. Patients with moderate mandibular asymmetry had the FOP and FMP angle of 4–7°, with a tendency for bilateral TMJ ID; and patients with severe mandibular asymmetry had the FOP and FMP angle >7°, with a tendency for contralateral TMJ ID.^[14]

In a general orthodontic practice setting, patients presenting for initial screening and evaluation have records taken for comprehensive evaluation and treatment planning. These records typically include extra- and intraoral photographs, panoramic radiographs, lateral cephalograms, frontal cephalograms, and hand-wrist radiographs. Although panoramic radiographs are an excellent screening tool to identify gross osseous changes in the TMJ, it does not provide a thorough evaluation of the TMJ.

Magnetic resonance imaging (MRI) is considered to be the best method to assess suspected ID and for evaluating the soft-tissue structures of the TMJ (articular disc, synovial membrane, and lateral pterygoid muscle).^[15] MRI could also detect the early signs of TMJ dysfunction, such as thickening of anterior or posterior band, rupture of retrodiscal tissue, changes in shape of the disc, and joint effusion.^[16]

Computer tomography (CT) is considered a part of standard evaluation for bony elements and the adjacent soft tissues of the TMJ and is used for the diagnosis of fractures, degenerative changes, erosions, infection, invasion by tumor, as well as congenital anomalies.^[17] CT scans, however, should be judiciously considered due to the high risk of radiation exposure, particularly in young patients.^[18]

Posteroanterior (PA) cephalograms are widely used in radiographic evaluation of transverse asymmetry involving both skeletal and dentoalveolar components.^[19]

We hypothesized that mandibular lateral displacement measured as mandibular symmetry on the PA cephalogram in the patients with unilateral degenerative joint disease (UDJD) is greater than in bilateral degenerative joint disease (BDJD) and significantly greater than that in the controls and that the degree of displacement will be significantly related to the cant of the FOP and the FMP, indicating the reduced vertical dimension of the posterior occlusal level and the ramus height on the side to which the mandible is displaced.

MATERIALS AND METHODS

Cephalometric radiography and analysis

Posterior-anterior cephalogram, lateral cephalogram, and panoramic radiographs obtained routinely before orthodontic treatment, with the mandible in intercuspal position, were collected for every patient.

All PA cephalograms were traced and the landmarks identified and investigated for asymmetry by one investigator (SM). All PA cephalograms were traced manually on 0.003” thick matte acetate paper using a #3 pencil by one investigator (SM) after printing them from the Dolphin Imaging Software System (Dolphin Imaging and Management Solutions, 9200 Oakdale Ave. Suite 500, Chatsworth, CA 91311, U.S.A.).

The landmarks selected were based on previously published PA reproducibility study by Major et al.^[20] A line passing through crista galli and anterior nasal spine (ANS) perpendicular to the line between the intersections of the greater wing of the sphenoid bone and the orbital margin was constructed first to represent the vertical reference plane, or the facial midline, as shown in [Figure 1].

Close

Figure 1:

Tracing of posteroanterior cephalogram. Asymmetry was determined by frontal occlusal plane (FOP) and frontal mandibular plane angles (FMP) measured between the perpendicular line of the facial midline (vertical reference plane: Crista Galli and anterior nasal spine [ANS]). The angle between the facial midline and a line passing through ANS and Menton was defined as the degree of MMS. The distance in millimeters, between the facial midline and the midline of the mandibular incisors, was defined as the DMS.

Export to PPT

FOP was defined and traced as the horizontal line that passed through the occlusal surface of the bilateral lower first molar, the angle was measured between the perpendicular line of the facial midline and the horizontal plane. Another line was traced connecting the right and left antegonial notches, the angle was measured between the horizontal plane and the perpendicular line of the facial midline and was defined as the FMP.

The FOP and FMP were used to represent the deviation noted as asymmetry in the vertical dimension. Positive values for FOP and FMP indicated that these planes were inclined superiorly toward the short side. The angle between the facial midline and a line passing through ANS and Menton was defined as the degree of mandibular midline shift (MMS). The distance in millimeters, between the facial midline and the midline of the mandibular incisors, was defined as the dental midline shift (DMS). A DMS on the left and right side was termed as the absolute value for diagnostic criteria for transverse asymmetry.

The lateral cephalogram and panoramic radiographs were used with PA cephalograms to identify valid landmarks. The error of the method was determined by retracing each cephalogram on separate occasions, 2 weeks after the first tracing.^[14]

RESULTS

FOP, FMP, DMS, and MMS measures were compared pairwise between the study groups, with multiple comparisons using Tukey–Kramer procedure. Significant differences are found between normal and bilateral, normal and unilateral, but not between unilateral and bilateral patients [Table 2].

Correlation analyses were performed to evaluate the relationship among the measures. Mantel–Haenszel Chi- square test was used to evaluate the association between group and severity of the cant.

The outcome measures are significantly different between severity groups, with moderate/severe patients having higher values [Table 3].

All the measures were positively correlated, with Pearson correlation coefficients and p-values listed in Table 4.

The presence of TMJ ID was directly proportional to the values of FOP and FMP, with the mandibular displacement being notably higher when the asymmetry was >3° (i.e., the higher values associated with a higher prevalence of cant). A similar tendency of a higher prevalence of mandibular displacement with a higher value of DMS and MMS was observed [Table 3].

Mantel–Haenszel Chi-square test was used to evaluate the association between group and severity of mandibular displacement. There are significant differences in mandibular displacement and therefore asymmetry between normal and bilateral, normal and unilateral. However, no significant difference was detected between bilateral and unilateral groups [Tables 5-7].

DISCUSSION

Asymmetry of the mandible influences normal TMJ structure and function. Pathological conditions that affect the TMJ can manifest themselves as facial asymmetry, including congenital disorders such as hemifacial microsomia,^[21,22] condylar hyperplasia,^[23] ID,^[24] rheumatoid arthritis,^[25] and osteoarthritis.^[26]

ID, that is, disc displacement of the TMJ, is a common intra- articular disorder characterized by an abnormal relationship of the articular disc relative to the mandibular condyle, fossa, and articular eminence. Almost 80% of patients with TMJ disorders (TMD) have ID.^[27,28]

The prevalence of a symptomatic side has been a matter of controversy. Trpkova et al. evaluated frontal radiographs of female orthodontic patients with unilateral or bilateral temporomandibular joint disc displacement and compared them to female controls without disc displacement. They concluded that subjects with bilateral TMJ-ID had significantly greater asymmetry in the vertical position of the antigonion, and greater the disc displacement on one side, the shorter the ipsilateral ramus, resulting in significant asymmetry of the mandible.^[24]

Tallents et al. conducted a study on 12 consecutive subjects presenting with facial asymmetry thought to represent unilateral condylar hyperplasia. However, half of these patients were found to have unilateral disc displacement that presented on the short side.^[13] Another study by Fushima et al. reported that TMJ ID was more prevalent on the short side of the mandible.^[9] One may argue that both these studies involved small sample size.

In contrast, Kurihara et al., in a study of 433 orthodontic patients, found that the symptoms were more frequently bilateral. However, there was no clear evidence of how symptoms were observed on each side of the TMJ.^[10]

The data from our retrospective study suggest that severity of the vertical mandibular displacement was associated with significant differences between BDJD and control patients, and between UDJD groups and control patients. However, they were no significant differences found between UDJD and BDJD patients, and this may be attributable to a different pattern of load on the ipsilateral and contralateral sides of the TMJ when they were compared to the controls. Moreover, the higher values of FOP and FMP correlated with the mandibular displacement being notably greater when the asymmetry was >3° (i.e., the higher values associated with a higher prevalence of symptoms). A similar tendency of a higher prevalence of mandibular displacement with a higher value of DMS and MMS was observed.

We acknowledge that not being able to confirm the diagnosis for TMJ ID or arthritic changes though MRI and CT scan may pose as a limitation to the study and further posteroanterior cephalometric studies would be valuable to confirm its use an alternative diagnostic tool for TMD diagnoses. We also think that future studies could benefit from increasing the sample size.

CONCLUSION

Overall, our findings suggest a relationship between vertical mandibular asymmetry in patients with temporomandibular degenerative joint disease, both UDJD and BDJD, when they were compared to controls and therefore recommend thorough dental, skeletal as soft-tissue evaluation including intraoral, extraoral, and radiographic examination of transverse asymmetry, by identifying condylar changes, antegonial notching, dental compensation, midline discrepancy, and comparing ramus heights, when planning an orthodontic or orthognathic surgical case.