The temporomandibular joints (TMJ) is a complex articular system which is located between the mandible and the temporal bone. ¹ The articular eminence forms the anterior limit of the glenoid fossa ² and is a part of the temporal bone on which the condylar process slides during mandibular movements. ¹

The angle formed by the articular eminence and the Frankfort horizontal (FH) plane or any other horizontal plane, such as the occlusal or palatal plane, is defined as the articular eminence inclination. ³ The articular eminence inclination varies among individuals and dictates the path of condylar movement as well as the degree of rotation of the disc over the condyle. The posterior wall of the articular eminence is an important element in the biomechanics of TMJ and the entire masticatory system. As the steepness of articular eminence increases, the movements of the disc and the condylar process also increases. ⁴ The steeper the articular eminence the more the condyle is forced to move inferiorly as it shifts anteriorly which results in greater vertical movement of the condyle, mandible, and mandibular arch on opening. ⁵

The morphology of the articular eminence, glenoid fossa, and condyle depends on numerous factors. Temporomandibular disorders (TMDs) are frequently associated with degenerative bone changes, which may occur in the subarticular surfaces of the condyle and fossa during TMJ disorders. ⁶ Erosions appear as pitted and irregular contours of the bony surfaces, and in some instances, the bony surfaces become flattened, and small bone projections may form known as osteophyte. ⁴

There are various methods which are used to examine the inclination of the articular eminence and assessment of the morphological changes, but cone-beam computed tomography (CBCT) was used in the present study to assess the TMJ morphology as owing to its higher resolution the visualization of subtle bony changes is much better than conventional techniques. Furthermore, it allows for shorter scanning time and a lower radiation dose than conventional computed tomography (CT). ¹ , ⁴

It has been hypothesized that steeper articular eminence is a predisposing factor for TMDs. However, the association of articular eminence inclination and height with TMDs is a matter of controversy and needs to be further evaluated. ⁷ Furthermore, there has been no clear evidence of association of morphology of TMJ complex as a probable predisposing factor in the pathogenesis of TMDs. Therefore, this study was conducted to evaluate the correlation between the TMJ articular eminence inclination, height, condylar bone changes, condyle, and fossa shapes with symptomatic TMDs using CBCT.

The study design was a cross-sectional, case–control study. The present study was conducted on patients visiting the Outpatient Department of a reputed dental college of North India after taking the Ethical Clearance from the Institutional Ethical Committee.

The study sample comprised fifty patients within the age range of 20–40 years. The sample was allocated in two groups with thirty patients in Group I (TMJ disorder group) and twenty healthy patients in Group II (Control group). CBCT scans and evaluation was performed for sixty joints in thirty patients with symptomatic TMDs in Group I and for forty healthy joints of 20 age-matched patients in Group II.

Informed consent was taken and the patients were subjected to a detailed case history of the “American Association of Orthodontics” for the analysis of TMDs and thorough clinical examination of the patients was done followed by the recording of findings. All of the patients in the Group I had clinical signs and symptoms of TMJ dysfunction, such as TMJ sounds, clicking, pain, subluxation, trismus, hypomobility, or hypermobility. Any patient with craniofacial abnormalities or any systemic conditions that may affect the TMJ, patients with a history of trauma to orofacial complex, under any systemic corticosteroids, undergoing orthodontic appliance therapy or any intervention from other previous TMJ treatment, scans with artifacts, and pregnant females were excluded from the study. The patients for control group were selected from CBCT scans, which were performed for reasons other than TMJ dysfunction. The assessment was made by three oral and maxillofacial radiologists at an interval of 2 weeks with a minimum experience of 5 years in CBCT interpretation who were blinded to eliminate the inter-observer and intra-observer bias.

Imaging procedure

The CBCT images were taken with a NewTom GiANO (CEFLA-SC, CEFLA DENTAL GROUP, ITALY) with a maximum output of 90 kvp, 3 mAs, and typical exposure time of 3.6 s, voxel resolution of 150 μ and the field of view (FOV) of 8 cm × 8 cm. The patients were made to stand in the gantry with head in the horizontal position and hence that the FH plane was perpendicular to the table.

All the CBCT scans were taken using a standard exposure and patient positioning protocol to ensure standardization. Two scout images, i.e., sagittal and coronal view were taken in accordance with the patient's position and a 360° scan was acquired afterward. The NNT software version 7.0 program (New Net Technologies Ltd. Naples, USA) was used for the analysis. The orientation and area of interest were determined in Multiplanar reformation (MPR) format. First, the condylar head was oriented and triangulated in all the three planes. Then, in axial window, the widest mesiodistal dimension of the condyle was identified which acted as the reference point for secondary reconstruction. On this plane, parasagittal and para-coronal cross-sections were created. The parasagittal sections were evaluated perpendicular to the long axis of the condyle and para-coronal cross-sections were created parallel to the long axis of condyle with slice thickness and interval of 1 mm each, respectively.

Measurements

The points used for measurements of the articular eminence inclination and heights in this study are as follows Figure 1: Figure 1

articular eminence (a) inclination (b) height.

The comparison of the distribution of condylar bone changes was compared between TMJ disorder and control groups Table 1using the Chi-square test. The incidence of erosion and osteophyte was more in the TMJ disorder group and the difference was statistically significant (P = 0.001). The distribution of condyle and fossa shapes was compared between the two groups using the Chi-square test Table 1and there was no statistically significant difference (P = 0.482, P = 0.689, respectively).{Table 1}

The articular eminence inclination and height were compared in both the groups using unpaired or independent t-test Table 2. There was a statistically significant difference of articular eminence inclination (P = 0.044) and height (P = 0.035) between the TMJ disorder and control group. The articular eminence inclination was more in control group, whereas the articular eminence height was more in the TMJ disorder group.{Table 2}

The association of articular eminence inclination and height with condylar bone changes, condyle shapes, and fossa shapes was evaluated using one-way anova Table 3and Table 4. There was no significant association of articular eminence inclination and height with condylar bone changes and condyle shapes in both the groups. The articular eminence inclination was not associated with fossa shapes in both the groups and articular eminence height was associated with fossa shapes only in the TMJ disorder group.{Table 3}{Table 4}

Interclass correlation coefficient for inter-observer reliability varied from 0.953 to 0.990 and for intraobserver reliability varied from 0.927 to 0.987, which suggested excellent agreement.

TMDs are the major cause of nonodontogenic pain in the oral and maxillofacial region and include various groups of disorders, which include psychological, masticatory, and muscular components with overlapping features. ⁸

The etiology of TMDs is complex and multifactorial, ⁹ and in spite of multitudinous research activities, the etiology of TMDs is still an enigma. ¹⁰ Because of varied and unexplainable etiology, the management of TMDs patients is a particular challenge. The key to successful management is the diagnosis of all predisposing, initiating, and perpetuating factors. ¹⁰ Since alterations in the articular eminence morphology have been stated as an etiological factor for TMDs, ⁷ , ¹¹ we tried to explore the association between articular eminence morphology and TMDs.

The articular eminence is situated in front of the glenoid fossa and its posterior surface slope varies among people. Although it is an anatomical structure belonging to the cranium, its morphological shape is mainly influenced when it is exposed to functional load arising from chewing forces. ¹ The normal value of the articular eminence in adults has been reported to be 30°–60°. If the articular eminences inclination is <30° it is characterized as flat, whereas if the value is >60° it is characterized as steep. ³

Articular Eminence Inclination is studied using different materials and methods as it is an important element in the biomechanics of TMJ. ¹² The various methods include measurements on models taken from an impression of the fossa or direct measurement carried out on dry skulls, measurements taken by interocclusal protrusive records, measurements performed on arthrograms, panoramic radiographs, tomographic radiographs, ³ cephalometric radiographs, and scaled photographs. The articular eminence and glenoid fossa have a complicated structure due to many small and complex curved surfaces that make these measurements difficult. ³ , ¹²

It has been demonstrated that CBCT is an appropriate method for the measurements of the articular eminence and offers a dose and cost-effective alternative to conventional CT for the diagnostic evaluation of osseous abnormalities of the TMJ. The effective dose of the currently available large FOV CBCT units is higher than that of conventional panoramic imaging but several to many times lower than the reported doses for conventional CT. CBCT also allows linear measurements with real dimensions and without superimposition or distortion, which is the main concern in conventional radiography. ¹ It provides sub-millimeter spatial resolution images because of which minute details can be observed with markedly shorter scanning times (10–70 s). ¹³ Therefore, CBCT was used in this study to determine the articular eminence inclination and height, condylar bone changes, fossa, and condyle shapes in patients. In the present study, the central sagittal slice of the condyle for measurements was chosen as the view of the eminence in the central slice reflects the steepest part of the eminence and gives the best representation of the eminence inclination. ¹ The major growth of the articular eminence inclination is completed by the age of 20 years ^7,and as the age advances, there are more chances of osteoarthritic changes which can affect the morphology of the TMJ. Therefore, participants <20 years and >40 years of age were excluded from this study.

A pronounced steepness of the articular eminence has been suggested as a predisposing factor in the development of the TMJ disorders as with a steep articular eminence, the disk would have to rotate forward on the condyle to maintain a proper condyle-disk relationship during mandibular movements. ¹⁴ Meanwhile, the masseter and temporalis produce a stabilizing force, which with a steeper eminence, places a greater relative distalizing force relative to the disc which might result in laxity of the ligaments that attach the disc to the condyle. ¹¹ As a consequence, during mouth opening, the disk articulating against a steep eminence would achieve a gradually increasing anterior position in relation to the condyle, predisposing the disk to anterior displacement. This has been stated as the biomechanical theory of the TMJ anterior disk displacement. ¹⁴

Apart from the articular eminence inclination, TMJ morphology is also related to TMDs. Richards ¹⁵ has stated that the morphology of the TMJ is related to the morphology of the system with which the joint is functionally linked which includes environmental and oro-functional factors such as mechanical loading, genetic factors, para-functional habits that may influence the growth and development of skeletal components of the TMJ. The patterns of forces generated during the mastication lead to the remodeling of the glenoid fossa, and with stronger forces, the fossa becomes deeper, whereas the AEI becomes steeper. ¹² , ¹⁶

In the present study, the articular eminence inclination was greater in the control group (34.97 ± 7.69) than in patients with TMJ dysfunction (32.23 ± 7.68), and the difference were statistically significant. These results were in accordance with Sümbüllü et al. ¹ and Ren et al. ¹⁷ On the contrary, Gökalp et al. ¹⁸ and Sülün et al. ¹⁹ reported that a greater articular eminence inclination in patients with TMJ disorders. The differences may be attributed to the varied and wide age range of patients included in their study.

The articular eminence height was more in the TMJ disorder group (6.30 ± 1.61) in comparison to the control group (5.66 ± 1.23), and the difference was statistically significant. Similar results were reported by Paknahad et al. ⁷ In a study by Caglayan et al. ⁴ eminence height was more in the TMJ disorder group, but the difference was not statistically significant.

We evaluated morphological changes and observed condylar bone changes in only 28 (46.6%) of sixty joints in the TMJ disorder group, and in the control group, we observed only 1 (2.5%) joint of forty joints with condylar osteophyte formation. A significant difference was found between condylar bone changes in two groups as the occurrence of erosion and osteophyte was significantly more among the TMJ disorder group. Similarly, dos Anjos Pontual et al. ⁶ and Koyama et al. ¹³ found the prevalence of bone changes in 71% and 63.7% of the joints of patients with TMDs using CT. However, in the study of Capurso et al., ²⁰ degenerative bone changes were seen only in 19.6% of the 461 patients.

Osteophytes occur at an advanced stage of degenerative change and appear to stabilize and widen the surface in an attempt to improve the overload resulting from occlusal forces, representing areas of neo-formed cartilage, whereas erosion is the initial stage of degenerative changes, indicating that the TMJ is unstable and changes in bone surfaces will occur, probably resulting in changes in occlusion. ⁶ According to Alexiou et al., ²¹ patients in older age groups are expected to have more frequent and severe bone changes than those in younger age groups. The degenerative changes are the long-term consequences of TMJ disorders, but sometimes, they can be detected incidentally on radiographs. ¹³

There was no significant difference of condyle and fossa shape between the TMJ disorder and control group. The triangular condyle shape was observed more frequently, i.e., in 24 (40%) joints in the TMJ disorder group and oval condyle shape was observed in 14 (35%) joints in the control group. Oval fossa shape was found most frequently in both the groups (41%). Our results were not in accordance with Caglayan et al. ⁴ as they reported significant differences between the condyle and fossa shapes in the TMJ disorder and control groups.

Pirttiniemi et al. ²² affirmed a strong functional dependence between the articular eminence and the condyle. According to Wang et al., ²³ the initial development of the glenoid fossa is independent of the condyle, but its continuous development relies on an interaction with the condyle. If the interaction of the glenoid fossa and the condyle interferes with the growth pattern of these structures, then it also interferes with its morphology and is unique for each patient. ²⁴

Association of the articular eminence inclination and height with condyle and fossa morphology was also assessed and there was no association with the condylar bone changes and condyle shapes. Our results were in accordance with Caglayan et al. ⁴ as they also found no significant association of the articular eminence inclination and height with condylar bone changes. However, they found a significant association of the articular eminence inclination and height with condyle shapes only in the control group.

In our study, we did not find any association of the articular eminence inclination with fossa shapes in both the groups. However, there was a significant association of the articular eminence height with fossa shapes in the TMJ disorder group. Our results were not in accordance to Caglayan et al., ⁴ as they found a significant association of the articular eminence inclination with fossa shapes in both the TMJ disorder and control group, whereas articular eminence height was only associated with fossa shapes in control group. Katsavrias and Halazonetis ²⁵ stated that the variability of the fossa shape and condyle shape was related to the eminence inclination and condylar inclination, respectively.

The reason for the variations in the results could be due to factors such as different methods of measurements, sample size, age range, and other differences between the populations. Further studies are required with a larger sample size to determine an association between TMJ complex anatomy and etiopathogenesis of specific TMD.

The present study shed light on the relationship between TMJ morphology and the incidence of TMDs. The articular eminence inclination was steeper in patients without TMJ dysfunction and thus, the steep articular eminence inclination could not be considered as a predisposing factor in etiology of TMDs in the present study population. There was increased articular eminence height in the TMJ disorder group, and it can be proposed that increased articular eminence height might be a predisposing factor in the etiology of TMDs. The condylar bone changes had an association with TMJ disorders in the present study population, and there was no association of articular eminence inclination and height with the condylar bone changes and condyle shapes. Furthermore, the articular eminence inclination had no association with fossa shapes, but eminence height was associated with fossa shapes only in the TMJ disorder group. Thus, the results depicted that there was no significant association between TMJ complex anatomy and TMJ disorders.

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The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.