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Introduction Over the years, many studies concerning the articular eminence (AE) of the temporomandibular joint (TMJ) have been conducted. Very few used the now available cone-beam computer tomography (CBCT) technology. Thus, the aim of the current study was to contribute with our own findings to the recent available literature and to compare results. Methods Forty-eight CBCT files (96 TMJs) were analyzed in mid-condyle orthogonal plane. AE inclination (AEI), length of condylar path (LCP), length of the glenoid fossa (FL), sex and occlusion class were defined as parameters. Results In the examined lot, 9% of the slopes were below 30°, 70% between 30° and 50° and 21% above 50°. A minimum value of 21.63° was found in a female patient with a LCP of 11.07 mm and a FL of 19.96 mm, and a maximum value of 59.62° in a male patient with a LCP of 7.72 mm and a FL of 14.39 mm. A decrease in mean LCP and FL values for each increase in AEI category was noticed. Averages of both sexes were similar. Asymmetry was found in all patients. There were some differences between the two occlusal groups. Conclusions CBCT is a useful, non-invasive tool in analyzing TMJ bone structures. There might be a connection between AEI, LCP and FL in dictating the geometrical shape of the glenoid fossa. Differences in the values of the occlusal groups denote that occlusion could act as a factor for slope steepness.
The articular eminence (AE) is the articular bony part of the temporal squama in the temporomandibular joint (TMJ) assembly.1 The AE inclination (AEI) dictates the gliding path for mandibular movement and the degree of disc rotation over the condyle (a steeper AE leads to greater vertical movement of the condyle upon opening).2 Anterior disk displacement with reduction is more frequent in cases of high articular tubercle or abrupt posterior path and, in time, flattening of the AE may occur, which can lead to anterior disk displacement without reduction.3
Thus, given its importance, the AE has been the topic of many studies and its inclination was defined as the angle formed by one of the lines passing through the AE and the horizontal plane (Frankfort horizontal plane, occlusal plane or palatal plane).2 Numerous attempts for correlating AEI with age, sex, race, condyle morphology, body height and weight, anterior and posterior facial dimensions, type of occlusion, missing teeth, degrees of abrasion, internal derangements, bony changes, articular dysfunction, etc. have been conducted.2,4
As few studies were made by cone beam computed tomography (CBCT) examination, more results on the subject should give more ground for discussion and comparison in order to clarify certain aspects that are still a matter of debate.
Patients
Fifty CBCT patient files were randomly selected from our database, in order to conduct a retrospective study. Patient files had to have a complete and clear visibility of the articular and periarticular bone structures. Based on these criteria, two files were excluded and the study was performed on 48 CBCT patient files summing a total of 96 TMJs (70.83% female and 29.17% male).
The lot contained only adult patients who had given written informed consent for their CBCT files to be used in research and teaching purposes.
Imaging procedures
Images were acquired and analyzed as in previous studies.5 As a reference plane the hard palate was used. The images in the sagittal and axial planes were tilted in order to place the anterior and the posterior nasal spines (ANS and PNS) on a line parallel to the horizontal and the mid-sagittal planes (Figures 1A and 1B). In the coronal plane the image was tilted so that the sagittal section crossed ANS and point nasion (located at the suture junction of the frontal bone with the nasal bone) (Figure 1C). The TMJs were assessed separately in an orthogonal plane perpendicular on the condyle’s long axis straight through its midpoint (Figure 1D).
The AEI was defined as the angle between the horizontal line and the lowest point of the AE (A) with the highest point of the glenoid fossa (B) (fossa roof – eminence top line method). The [AB] segment was measured and was termed length of condylar path (LCP). Antero-posterior glenoid fossa dimension was included in the study, represented by the [AC] segment, which stretches from the AE apex to the tip of the posterior articular tubercle (Figure 2).
A – articular eminence (AE) tip, B – glenoid fossa roof top point, C – tip of posterior tubercle, PP line – horizontal plane. Measurements were done regarding the angle between AB line and horizontal plane (AE inclination), the segment [AB] (length of condylar path) and the segment [AC] (glenoid fossa length).
Figure 2. Measurements done in orthogonal section of the temporomandibular joint
Additional information was noted: patient sex (M-male, F-female) and class of occlusion. The occlusion was classified as follows: CLASS 0 included patients with no terminal edentation and no vertical teeth migrations (type 0 occlusion) and NON CLASS 0 which included unilateral terminal edentation, bilateral terminal edentation, advanced occlusal wear and lateral edentation with important vertical teeth migration.
Data analysis
The lot was subdivided into 3 categories based on the AEI measurements, as follows: angle below 30° (considered flat), between 30° and 50° (moderate) and angle equal to or above 50° (protuberant) (Figure 3). All data was sorted based on AEI values. AVERAGE, MAX and MIN functions were used to obtain the data for analysis of AEI, LCP and FL.
A1, A2 – AEI below 30°, B1, B2 – AEI between 30° and 50°, C1, C2 – AEI above 50°, White arrow – articular eminence, Blue arrow – roof of mandibular fossa, Orange arrow – posterior articular tubercle.
Female and male groups were analyzed separately for differences in MAX, MIN and means regarding AEI, LCP, FL and occlusion.
The symmetry between joints was evaluated for each patient based on AEI values.
Results
Various results were obtained. AEI, LCP and FL means for each subgroup of the study lot are presented in Table 1.
In the examined lot, 9% of the posterior slopes had an inclination below 30°, 70% between 30° and 50° and 21% above 50°. A female patient presented the flattest slope, in the left TMJ, measuring an AEI of 21.63° (under 30° category, Figure 4A), a LCP of 11.07 mm and a FL of 19.96 mm. The steepest was found in the left TMJ of a male patient, with an AEI value of 59.62° (above 50° category, Figure 4B), a LCP of 7.72 mm and a FL of 14.39 mm. Both patients presented asymmetry between left and right TMJs; the female’s contralateral AE was in the moderate (30°-50°) category, while the male’s opposing joint remained in the same category (above 50°).
A – flattest articular eminence slope, B – steepest articular eminence slope, Arrow– indicates the posterior slope of the articular eminence.
Figure 4. Sagittal three-dimensional render slice, through the midline of the condyle and perpendicular to its long axis
Average, maximum and minimum values of AEI, LCP and FL for CLASS 0 and NON CLASS 0 are presented in Table 2.
Differences in means, MAX and MIN values between male and female participants were observed. Although the minimum AEI was measured in a left female joint, the minimum value measured in a right joint was determined in a male patient. Table 1 values indicate that males in the NON CLASS 0 had lower values than females regarding the AEI, LCP and FL.
Asymmetry between right and left TMJs was found in all patients. The difference in condylar path inclination ranged between 0.05° and 29°. The mean right AEI was 40.6°±7.55 and the left one was 42.65°±9.29.
Table 3 contains data on LCP and FL for all AEI groups and occlusion classes. Results indicated that the higher the AEI value was, the smaller the LCP and FL values were. The CLASS 0 occlusion group had a lower LCP and a higher FL than NON CLASS 0 group, regardless of the AEI category.
Table 3. Length of condylar path (LCP) and fossa length (FL) means, based on occlusion and articular eminence inclination (AEI)
A literature review revealed many AEI studies made on plain radiographs, such as panoramic radiographs and transcranial lateral oblique radiographs.6 But the temporomandibular joint is a complex three-dimensional structure and its skeletal surfaces are irregular, thus it is difficult to precisely assess the bony morphology using plain radiography.7 The CBCT imaging is a useful tool for precise in-vivo examination of the TMJ’s bony structures, in a non-invasive matter.6
According to Paknahad,8 most studies used the central sagittal slice of the condylar process because they demonstrated that this slice showed the steepest part of the articular eminence, giving the best representation of the joint. This study aimed to follow the same methodology as to obtain terms for comparison. Wish-Baratz objectively observed that "a review of the literature reveals both support and refutation of almost any claims put forth regarding the nature of the temporomandibular joint (TMJ) morphology”9 and Meng et al.7 concluded that the difference in methodology and parameter selection between various studies impairs the possibility to compare the results obtained from them.
A clear positive relation between the sex and AEI was found by Hinton10 in his studies of ancient populations that lived more than 5000 years ago; he explained that, since then, the robusticity of the masticatory system changed as an adaptive response to introduction of farming and agriculture and that the current result was TMJ losing its sexual dimorphism. There were some authors who continued to state the existence of sexual dimorphism regarding AE steepness.1,6,8,11,12 Others disagreed, stating that the difference was too small to be of statistical significance or that the connection couldn’t be proven.9,13,14 The current study indicated small differences between the two groups, but without a pattern that would suggest either sex to have a steeper AEI.
It would seem that age correlated with occlusion are important contributing factors in the TMJ development period (throughout childhood) and the remodeling done afterwards.9,15 In the current lot, CLASS 0 was exhibited by only 56% of the patients, while the remaining 44% had tooth loss and occlusal disturbance. The most frequently encountered form of occlusal alteration was bilateral terminal edentation (25%). Dawson et al.6 indicated that, in time, due to teeth attrition, the inclination of the incisal guidance decreases while that of the AE increases through bone remodeling. Hinton10 supported the idea that posterior tooth wear and loss implied changes in the articular slopes. A moderate tooth wear (age-function appropriate) seemed consistent with an increase in AE height, while a severe one, especially on the molars, determined a loss in fossa depth and a more gently sloped eminence. Many more confirmed a positive connection between occlusion and AEI,12,13,16 but still there were some who did not.9,17,18 Although the primary aim of the study was not a correlation between TMJ components and occlusion, results from Table 1 would indicate that males in the NON CLASS 0 occlusion have had lower values than females for all 3 components (AEI, LCP and FL). Tables 2 and 3 also showed differences between occlusal groups. This could be attributed to chance, given the sample size, and further investigation into the subject is required.
The posterior slope of the articular eminence may vary from 89° to 16°.19 In the current study the range was from 21.63° to 59.62°. Since we found no value over 60° we considered that for our population lot all AEI above 50° are steep.
The wide variation of measuring methods and of environmental and ethnic factors is of great importance. Mean AEI values are not to be taken as the actual condylar path, nor to be used for articulator setup.20 Also, during opening of the mouth, the condylar path in the sagittal direction differs in inclination due to the AE profile.2 Thus, the statistical data obtained from measurements is of orientational purpose only, and it is not advised to be used in everyday practice.21 Our results can be compared to those of Sa et al.,11 for they were obtained by using similar methodology. Mean values in their study were 37.6° (females) and 39.3° (males), while in ours they were 41.71° (females) and 41.29° (males).
Since the condyle tends to move forward, beside the AE tip, during wide opening,15 we considered that the LCP and FL could define the maximum antero-posterior functional excursion distances of the condyle (excluding anterior luxation). The analysis done for the LCP and FL based on the three categories of AEI (<30°, 30°-50°, >50°) indicated a certain pattern between the steepness of the eminence and the functionally constricted range of movement for the condyle. Table 3 contains data suggesting that the AE steepness is somewhat connected to the antero-posterior length of the glenoid fossa and the length of the condyle’s excursion. The same data also indicated that the CLASS NON 0 occlusion group exhibited a longer LCP and a shorter FL than the CLASS 0 group regardless of AEI category, which could lead to the conclusion that the occlusal factor is of importance in the TMJ design.
The AEI values of left and right joints suggest asymmetrical design in variable proportions. The difference in means between the two sides was 6.76°±6.32. Most authors agree on the asymmetrical state of the TMJ, statistically significant or not, some stating that the left side is steeper12 while others stating the opposite.9,14 The preference for one side in chewing has been considered as a possible factor for the difference between the left and right joint.22 Asymmetry could also be the result of teeth loss impact on the remodeling and reshaping of the articular slope.23
CBCT is a useful, non-invasive tool in analyzing TMJ bone structures.
A decrease in mean LCP and FL values for each increase in AEI category was noticed, indicating that there might be a morphological link between them in regard of glenoid fossa design. AEI values had a wide range of variations. Both sexes had comparable results. Bilateral asymmetry was seen in all patients. There were some differences between the two occlusal groups indicating that occlusion could have an impact as a remodeling factor for the posterior slope.
Perhaps the divergence in opinions on each and every aspect regarding the AEI is attributed to the many methods employed in research and the general characteristics of the study lot (age, race, environment, etc.).
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