
==== Front
Rom J Morphol Embryol
Rom J Morphol Embryol
RJME
Romanian Journal of Morphology and Embryology
1220-0522
2066-8279
Academy of Medical Sciences, Romanian Academy Publishing House, Bucharest

39020545
650224297307
10.47162/RJME.65.2.17
Original Paper
Modifications of the articular eminence inclination in association with the loss of occlusal support in the molar area. A cone-beam computed tomography study
Vîrlan Maria Justina Roxana 1
Nimigean Victor 2
Păun Diana Loreta 3
Bisoc Adriana 4
Nimigean Vanda Roxana 1
1 Department of Oral Rehabilitation, Faculty of Dentistry, Carol Davila University of Medicine and Pharmacy, Bucharest, Romania
2 Department of Anatomy, Faculty of Dentistry, Carol Davila University of Medicine and Pharmacy, Bucharest, Romania
3 Department of Endocrinology, Faculty of Medicine, Carol Davila University of Medicine and Pharmacy, Bucharest, Romania
4 Department of Prosthodontics, Faculty of Dentistry, Carol Davila University of Medicine and Pharmacy, Bucharest, Romania
Corresponding Author: Victor Nimigean, Professor, DMD, MD, PhD Department of Anatomy, Faculty of Dentistry Carol Davila University of Medicine and Pharmacy 17–23 Plevnei Avenue, Sector 1 010221 Bucharest Romania + 040722–368 849 victor.nimigean@umfcd.ro
Corresponding Author: Diana Loreta Păun, Associate Professor, MD, PhD Department of Endocrinology, Faculty of Medicine Carol Davila University of Medicine and Pharmacy 37 Dionisie Lupu Street, Sector 2 020021 Bucharest Romania + 4021–318 08 62 diana.paun@umfcd.ro
Apr-Jun 2024
30 6 2024
65 2 297307
10 3 2024
18 6 2024
Copyright © 2024, Academy of Medical Sciences, Romanian Academy Publishing House, Bucharest
2024
https://creativecommons.org/licenses/by-nc-sa/4.0/ This is an open-access article distributed under the terms of a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International Public License, which permits unrestricted use, adaptation, distribution and reproduction in any medium, non-commercially, provided the new creations are licensed under identical terms as the original work and the original work is properly cited.
The articular eminence (AE) is part of the temporal component of the temporomandibular joint (TMJ). The inclination of the AE (AEI) has an important role in TMJ biodynamics, influencing the path of movement of the disc–condyle complex. Although AEI values might change due to tooth loss, little is known about what effect could have the loss of occlusal support in the molar area on AE morphology. AEI was measured on cone-beam computed tomography (CBCT) images of 41 patients (82 TMJs) with or without occlusal support in the molar region. The patients included in this study were divided into four groups: (1) patients with maintained occlusal support in the molar areas on both sides (right and left side) (MM); (2) patients with loss of occlusal support in the molar areas on both sides (right and left side) (LL); (3) patients with loss of occlusal support in the molar area only on the right side, but with maintained occlusal support on the left molar area (LM); (4) patients with loss of occlusal support in the molar area only on the left side, but with maintained occlusal support in the right molar area (ML). AEI values were higher in the TMJs on the side with maintained occlusal support in the molar area (M) than the AEI values measured in TMJs from the side with loss of occlusal support in the molar area (L). AEI values on the right side were higher in the MM patients in comparison to the AEI values on the right side of LL patients. The loss of occlusal support in the molar areas changes the mandibular biodynamics, which might be reflected in the morphological changes at the AE level, where it might cause flattening of the articular slope predominantly on the right side.

articular eminence
temporomandibular joint
loss of occlusal support
edentulism
molar loss
==== Body
pmcIntroduction

The articular eminence (AE) of the temporal bone is a constituent part of the temporal component of the temporomandibular joint (TMJ) [1]. The AE represents the anterior border of the mandibular fossa, and has a convex shape with two slopes, the posterior slope being also called the articular slope [2, 3, 4, 5, 6]. The importance of the articular slope comes from the fact that the disc and the condyle slide on the articular slope of the AE during the mandibular movements [1, 7, 8].

The inclination of the articular slope of the AE (AEI) is defined as the angle between the articular slope of the AE and a horizontal reference plane, the most common landmark used being the Frankfurt Horizontal Plane [9, 10, 11, 12, 13, 14, 15]. AEI varies individually in humans and influences the trajectory of the condyle and the disc [1, 5, 7, 16, 17 18]. AEI has on average values of 30–60° in adults, angles lower than 30° indicating a flattened articular slope and values greater than 60° being usually considered a steep slope [7, 15, 19, 20]. AEI has an important role in TMJ biodynamics [5, 21]. For example, a steep AEI causes a rapid disocclusion of the posterior teeth during protrusive and lateral movements [21].

Tooth loss changes the biomechanics of the masticatory system and consequently the forces that are transmitted to the TMJ [8], thus leading to alterations of the AE morphology [6]. Previous studies have shown that edentate patients display a flattening of the articular slope of the AE [8 9]. Moreover, the AEI values were statistically significantly different in edentulous patients in comparison to elderly patients with maintained occlusion [9].

Although previous papers have stated that AEI values could change as a result of edentulism [6], little is known about what effect might have the loss of occlusal support in the molar area on the morphology of the AE. Molar loss and the consequent loss of occlusal support in the molar area is a clinical situation frequently encountered in the day-to-day practice of the dental practitioners. The loss of occlusal support in the molar areas leads to variations in mandibular biodynamics and might also lead to remodeling in the temporal region of the TMJ, mainly at the level of the articular slope of the AE.

Aim

This study aims to investigate, with the help of the cone-beam computed tomography (CBCT) technology, the association between the loss of occlusal support in the molar areas and possible changes in the AEI values. The hypothesis is that the loss of occlusal support in the molar area of the dental arches leads to changes in the AE morphology, respectively to variations in the AEI values.

This research includes a comparative analysis between the AEI values measured on CBCT images of patients with loss of occlusal support in the molar areas versus the AEI values of patients with maintained occlusal support in the molar region of the dental arches. For this purpose, four groups of patients were investigated: (1) patients with maintained occlusal support in the molar areas on both sides (right and left side) (MM); (2) patients with loss of occlusal support in the molar areas on both sides (right and left side) (LL); (3) patients with loss of occlusal support in the molar area only on the right side, but with maintained occlusal support in the left molar area (LM); (4) patients with loss of occlusal support in the molar area only on the left side, but with maintained occlusal support in the right molar area (ML).

Patients, Materials and Methods

Selection of patients

Participants were selected from the patients of a private dental clinic. Participants were recommended to perform the CBCT analysis by the dentist due to the dental conditions they presented at the time of the examination. The patients’ informed consent in writing form on the use of their clinical and radiological data in scientific research was obtained. The study was approved by the Ethics Committee of Carol Davila University of Medicine and Pharmacy, Bucharest, Romania (Registration No. 1511/23 January 2024).

Inclusion criteria

Patients included in the study have fallen into one of the following two categories:

▪ Patients with complete dental arches – all the first and second molars were present, and the molar occlusal support was maintained;

▪ Patients with molar edentulous sites and with loss of occlusal support in the molar area(s).

Exclusion criteria

▪ Clinical symptoms of TMJ damage;

▪ Clinical signs or symptoms of temporomandibular disorders (TMDs);

▪ The presence of pathological conditions in the TMJ area: congenital craniofacial anomalies or any pathology that could influence TMJ morphology, e.g., rheumatoid arthritis;

▪ Previous trauma or pathological lesions in the AE region, which might interfere with the performance of measurements in this region;

▪ The existence of previous treatments in the TMJ region;

▪ Prosthetic reconstructions in the TMJ region;

▪ Pregnant patients.

Finally, a total of 41 patients were selected and the study included measurements performed on 82 TMJs.

The distribution of patients according to the four studied groups was as follows:

(1) Patients with maintained occlusal support in the molar areas on both sides (right and left side) (MM) = 18 subjects / 36 TMJs (43.9%);

(2) Patients with loss of occlusal support in the molar areas on both sides (right and left side) (LL) = 18 subjects / 36 TMJs (43.9%);

Patients with maintained occlusal support in the molar area on one side (right or left side), but with loss of molar occlusal support on the other side (left or right) = 5 patients / 10 TMJs (12.2%), of which:

(3) Patients with loss of occlusal support in the molar area only on the right side, but with maintained occlusal support in the left molar area (LM) = 2 patients / 4 TMJs (7.3%);

(4) Patients with loss of occlusal support in the molar area only on the left side, but with maintained occlusal support in the right molar area (ML) = 3 patients / 6 TMJs (4.9%).

The study group consisted of a homogenous sex ratio, with 53.7% female patients and 46.3% male patients (Table 1). The mean age was 61.83±11.29 years, with a median of 62 years (Table 2).

Table 1 Sex ratio of the study participants

Sex

	No. of cases

	Percent

	
Female

	22

	53.7%

	
Male

	19

	46.3%

	

Table 2 Age of the patients included in the study group

Parameter

	Mean ± SD

	Median (IQR)

	Minimum

	Maximum

	
Age [years]

	61.83±11.29

	62 (52.5–69.5)

	42

	82

	
IQR: Interquartile range; SD: Standard deviation

CBCT analysis

All CBCT imaging recordings were performed at the same dental imaging center. All measurements were conducted by the same examiner twice, one month apart. All measurements on the patients’ CBCT images were accomplished using the CBCT NNT Viewer analysis software. The AEI measurement procedure was quite similar to the one described in the studies by Moscagiuri et al. [22].

The study protocol consisted of the following stages:

(1) Axial sections were performed at the TMJ level on the CBCT images [22], as shown in Figure 1;

(2) The axial section on which it could be visualized the mandibular condyle with the largest medio-lateral diameter was chosen (the central axial section);

(3) Then several sagittal cross-sections were performed at a distance of 0.5 mm each on the axial cross-section selected at step 2 [22], as can be seen in Figure 2;

(4) The central sagittal cross-section was chosen from all the sagittal sections obtained at step 3 [22] (the central sagittal cross-section);

(5) The AEI angle was measured on the central sagittal CBCT section obtained at step 4 by drawing a line parallel to the Frankfurt Horizontal Plane and a line constructed along the posterior slope of the AE [22], from the most superior and anterior point of the mandibular fossa in the direction of the most inferior and posterior point of the AE, but following the best-fit line to the articular slope of the AE (Figure 3). The angle between the tangent to the articular slope of the AE and the Frankfurt Horizontal is the AEI (Figure 3).

Figure 1 Performing the axial sections at the level of the TMJ on the CBCT images. CBCT: Cone-beam computed tomography; TMJ: Temporomandibular joint

Figure 2 Performing the sagittal sections at a distance of 0.5 mm each at the level of the TMJ. Selection of the central sagittal cross-section. TMJ: Temporomandibular joint

Figure 3 Measuring the AEI on CBCT central sagittal cross-section at the TMJ level. The AEI was measured as the angle between the parallel to the Frankfurt Horizontal and the line constructed along the posterior slope of the AE, from the most superior and anterior point of the mandibular fossa in the direction of the most inferior and posterior point of the AE but following the best-fit line to the articular slope of the AE. AE: Articular eminence; AEI: Articular eminence inclination; CBCT: Cone-beam computed tomography; TMJ: Temporo-mandibular joint

Statistical analysis

Statistical analysis was performed using IBM Statistical Package for Social Sciences (SPSS) Statistics 25 and Microsoft Office Excel/Word 2013, and the Shapiro–Wilk test, the Mann–Whitney U-test, the Wilcoxon test, the Student’s t-test, the Levene’s test and paired samples t-test.

The independent variable used was the value of the angle of the AE of the temporal bone with the Frankfurt Horizontal (qualitative numerical variable) (AEI). Quantitative variables were tested for distribution using the Shapiro–Wilk test and were expressed as means with standard deviations or medians with interpercentile intervals. Independent quantitative variables with non-parametric distribution were tested using the Mann–Whitney U-test while paired quantitative variables were tested using the Wilcoxon test. Independent quantitative variables with normal distribution were tested using Student’s t-test or Welch t-test against equality of variances observed by Levene’s test while paired qualitative variables were tested using paired samples t-test. In the case of significant comparisons, mean differences with 95% confidence intervals (CIs) were illustrated. Qualitative variables were expressed in absolute form or as a percentage.

Results

AEI values were significantly higher in TMJs on the side with maintained molar occlusal support (58.16±11.93°) than in TMJs on the side with loss of occlusal support in the molar area (49.71±12.89°), the mean difference being significant between groups (8.448°) (95% CI: 2.987–13.909) (Table 3; Figure 4).

Table 3 Comparison of the AEI values measured on CBCT between the TMJs on the side with maintained molar occlusal support and the AEI values measured on the TMJs on the side with molar loss and loss of occlusal support in the molar area

TMJ type

	Mean ± SD [°]

	Median (IQR) [°]

	p * ( p =0.441***)

	
TMJ on the side with maintained occlusal support in the molar area ( n =41) ( p =0.124**)

	58.16±11.93

	57.2 (46.85–68.3)

	0.003

	
TMJs on the side with molar loss and loss of occlusal support in the molar area ( n =41) ( p =0.143**)

	49.71±12.89

	49.4 (40.35–61.45)

	
AEI: Articular eminence inclination; CBCT: Cone-beam computed tomography; IQR: Interquartile range; n: No. of cases; SD: Standard deviation; TMJ: Temporomandibular joint. *Student’s t-test; **Shapiro–Wilk test; ***Levene’s test for equality of variances

Figure 4 Box-plot chart for the comparison of the AEI values measured on TMJs on the side with maintained molar occlusal support and the AEI values measured on TMJs on the side with molar loss and loss of occlusal support in the molar area. AEI: Articular eminence inclination; TMJ: Temporomandibular joint

Data in Table 3 and Figure 4 show the results of the comparison between the AEI values measured on TMJs on the side with molar occlusal support and the AEI values measured on the TMJs on the side with molar loss and loss of occlusal support in the molar area. The data distribution was normal in both groups according to the Shapiro–Wilk test (p>0.05). According to the Student’s t-test, the differences observed between the types of joints are statistically significant (p=0.003), the AEI value being significantly higher in TMJs with maintained occlusal support in the molar area (58.16±11.93°) than in TMJs with loss of occlusal support in the molar area (49.71±12.89°), the mean difference being significant between groups (8.448°) (95% CI: 2.987–13.909).

Patients with loss of occlusal support in the molar areas (LL) display much lower values of the AEI (49±13.71°) values on the right side compared to the AEI right side values of the patients with maintained occlusal support in the molar regions on both sides (MM) (59.67±11.38°) (Table 4; Figure 5).

Table 4 Comparison of AEI values measured on the right side between patients with MM and patients with LL

Group

	Mean ± SD [°]

	Median (IQR) [°]

	p * ( p =0.332***)

	
MM ( n =18) ( p =0.625**)

	59.67±11.38

	57.45 (52.37–68.75)

	0.016

	
LL ( n =18) ( p =0.677**)

	49±13.71

	48.35 (39.62–60.02)

	
AEI: Articular eminence inclination; IQR: Interquartile range; LL: Loss of occlusal support in the molar areas on both sides (right and left side); MM: Maintained occlusal support in the molar areas on both sides (right and left side); n: No. of cases; SD: Standard deviation. *Student’s t-test; **Shapiro–Wilk test; ***Levene’s test for equality of variances

Figure 5 Box-plot chart for the comparison of AEI values measured on the right side between patients with MM and patients LL. AEI: Articular eminence inclination; LL: Loss of occlusal support in the molar areas on both sides (right and left side); MM: Maintained occlusal support in the molar areas on both sides (right and left side).

In Table 4 and Figure 5 are shown the comparisons of the AEI values on the right sides between patients with maintained occlusal support in the molar areas on both sides (MM) and patients with loss of occlusal support in the molar areas on both sides (LL). The distribution was normal in both groups according to the Shapiro–Wilk test (p>0.05). According to the Student’s t-test, the differences observed between the groups are statistically significant (p=0.016), patients with loss of occlusal support in the molar areas (LL) displaying much lower values of the right AEI (49±13.71°) values compared to patients with maintained occlusal support in the molar regions on both sides (MM) (59.67±11.38°), the mean difference between groups being 10.672° (95% CI: 2.1369–19.2075).

Discussions

The importance of the AE derives from the fact that, during mandibular movements, the head of the mandibular condyle together with the articular disc slide along the posterior (articular) slope of the AE [7, 23]. Consequently, the inclination of the slope (AEI) on which the condyle moves influences the movement and the trajectory of the mandibular condyle [2, 7, 8, 11, 13, 16, 23]. AEI refers to how steep the articular slope of the EA is relative to a reference plane, the most commonly used being the Frankfurt Horizontal Plane [7].

AEI shows important variations throughout the population, some papers indicating angles between 30° and 94° [23, 24]. In general, AEI values increase until the maximum value they reach in the third decade of life, after which they start to gradually decrease [7, 13, 25, 26 27]. Although in some studies it was mentioned that the biological sex or the age of the patients could have the possibility to influence the AEI values [5, 8, 9, 13, 28, 29, 30], tooth loss is the main factor that has the most significant role in decreasing AEI values and not the physiological phenomenon of aging [6, 23].

It has long been known that the TMJ undergoes morphological changes continuously throughout life and that these changes are also influenced by the dental function [31]. Previous research has shown that tooth loss, as well as tooth wear, have the possibility to cause bone resorption at the AE level, leading to flattening of the articular slope and the decrease of the values of the AEI angles [2, 32, 33]. As early as 1981, Hinton observed a lower inclination of the articular slope of the AE in skulls with a high incidence of loss of posterior teeth compared to specimens in which all molars were preserved [31]. Some articles have indicated that even a single missing tooth could generate changes in the articular slope of the AE [2, 34]. Chiang et al. determined lower AEI values (but not statistically significantly lower) on the TMJ side with unilateral loss of posterior teeth, regardless of patients’ age or gender [2]. Furthermore, in 2020, Fang et al. measured statistically significantly lower AEI values in subjects who have posterior edentulous sites as compared to subjects who have all posterior teeth present [34]. Starting from the data of previous studies regarding the flattening of the articular slope of AE in association with tooth loss [2, 6, 8, 9, 34, 35] we wanted to investigate if also the loss of occlusal support in the molar area of the dental arches might be associated with morphological changes at the AE level. Recent articles stated that the loss of occlusal support is one of the causes of dysfunction of the human stomatognathic system [23].

This is the first study that aimed to investigate, with the help of the CBCT technology, the association between loss of occlusal support in the molar area(s) and changes in the AEI angle values.

Our data showed that, in this study group, the AEI value was statistically significantly higher in the TMJs on the side with maintained occlusal support in the molar area (58.16±11.93°) compared to the TMJ on the side with lost occlusal support in the molar area (49.71±12.89°), the mean difference being significant between the two groups (8.448°) (95% CI: 2.987–13.909) and the p-value was 0.003. Comparison of the AEI value measured on CBCT images of TMJs on the side with maintained occlusal support in the molar area (M) and the TMJs on the side with missing molars and loss of occlusal support in the molar area (L) revealed that the AEI value was significantly higher in the joints on the side with maintained occlusal support in the molar region of the dental arches (58.16±11.93°) than in the joints on the side with loss of occlusal support in the molar region (49.71±12.89°), the mean difference being significant between groups (8.448°) (95% CI: 2.987–13.909). These results are consistent with other research papers. Oruba et al. reported in 2020 a significant decrease of AEI in patients over 60 years who presented the loss of all occlusal support areas, in contrast to patients of similar ages who still had at least one occlusal support area on each side [35]. Moreover, a 2012 study described lower AEI values in edentulous patients compared to subjects in which dental occlusion was still present [9]. AEI values measured on panoramic radiographs were the lowest in the group of edentulous patients aged over 60 years (AEI right: 29.8±6.0°, AEI left: 31.9±5.2°, p<0.01), in comparison to the patients from the group over 60 years but with maintained occlusal support zones (AEI right: 38.9±4.7°, AEI left: 39.5±7.4°) or compared to the group of young patients and with maintained dental occlusion (AEI right: 39.8±5.4°, AEI left: 43.0°) [9].

Several papers have investigated the association between tooth loss and the flattening of the AE [2, 8, 9, 34, 35]. Unal Erzurumlu & Celenk showed that fully edentulous patients had lower AEI values (35.10±4.7°, p<0.001) than dentate patients (42.60±4.3°) [8]. Also, a CBCT investigation of 60 individuals (120 TMJ) in India described statistically significantly lower AEI values in edentulous patients (51.43±3.630°) compared to partially edentate participants (53.40±7.91°) or dentate participants (60.98±5.42°) [27]. However, Zabarović et al. found no statistically significant differences between the AEI values measured in edentulous human skulls (59 skulls) and the AEI values found in dentate specimens (78 dry human skulls) [24]. Jasinevicius et al. also did not find any statistically significant correlation between the value of the AEI angle and edentulism (especially in the posterior area) following measurements made on dry human skulls from the 20th century from the American continent [36, 37]. However, these investigations are based on measurements performed on dry human skulls [24, 36, 37] in contrast to the present study in which the angles were measured on CBCT sections of contemporary adults.

The mean angles observed in this study are comparable to the results of other scientific articles on AEI of the temporal bone. Measurements made on 60 patients from India revealed values comparable to those determined in this research, namely mean AEI values of 60.98±5.42° in dentate patients, 53.40±7.91° in partially edentate patients and 51.43±3.63° in edentulous patients [27]. Moreover, Verner et al. also reported relatively similar mean values: AEI values of 57.50° on the left side and 58.89° on the right side in patients with oval-shaped dental arches; 61° and 65.18° on the left side and 61.33° and 64.72° on the right side in patients with tapered and square shaped dental arches [19]. At the same time, Zabarović et al. reported a close average AEI value of 61.90°, resulting from direct craniometry on 137 dry human skulls from Croatia [24]. Bilgin et al. also found an average AEI value of 60.33° [38].

However, some research groups have described lower mean AEI values than in this study. Unal Erzurumlu & Celenk measured a mean AEI value of 35.10±4.70° in edentulous patients and 42.60±4.30° in dentate patients, in a radiological study conducted on a Turkish population [8]. Oruba et al. determined the following mean AEI values on radiographs of Polish patients: 34.05±5.17° for young patients aged 18–35 years with maintained occlusal support zones, 36.68±5.65° in patients aged 36–60 years, with all areas of occlusal support zones maintained, 34.86±6.26° in participants over 60 years old, with at least one occlusal contact maintained on each side and mean AEI values of 26.31±5.12° in radiographs of subjects over 60 years old who presented simultaneously the loss of all occlusal support areas [35]. Also, in 2012, Csadó et al. measured lower mean values of AEI on the panoramic radiographs of their investigated Hungarian patients: AEI right: 29.8±6.0°, AEI left: 31.9±5.2° in edentulous patients aged over 60 years, AEI right: 38.9±4.7°, AEI left: 39.5±7.4° in patients aged over 60 but with maintained occlusion and AEI right: 39.8±5.4°, AEI left: 43±5.9° in young patients with maintained occlusion [9]. At the same time, measurements performed by direct craniometry indicated average AEI values lower than those reported in this study, respectively 21–64° in the research by Gilboa et al. on dry human skulls [39]. Kranjcic et al. (2021) reported mean AEI values of 31.56–38.72° or 44.14–58.37° due to AEI determination by two different methods performed on sagittal sections made on silicone molds of the AE [40]. It should be noted that these papers that reported lower values of AEI than those in our research, used as methodology the measurement of the AEI on panoramic radiographs [8, 9, 3535], or by direct craniometry [39] on dry human skulls, in contrast to the present study where it was chosen to measure AEI on CBCT sections of contemporary patients. An extremely useful observation is that on the panoramic radiograph only the most lateral part of the AE can be visualized and thus only the angle of the posterior most lateral slope of the AE can be determined [15, 41]. In fact, a 2000 study that investigated two different methods of measuring AEI in sections taken medially, centrally, or laterally at the level of the mandibular fossa, on 64 dry human skulls, found that the steepest AEI angles were measured on the central sections at the AE level (p<0.05) (lateral section: 58.28±10.1°, medial section: 58.26±8.6° and central section: 60.33±10.19°) [38]. For this reason, in this research, we chose to measure the AEI angle on central sagittal CBCT cross-sections at the AE level. Central sagittal sections at the level of the AE can only be visualized by CBCT technology; sagittal sections cannot be performed on the panoramic radiographs, and neither the medial nor the central portions of the AE of the temporal bone can be seen on panoramic radiographs [15, 41]. Therefore, AEI measurements performed on panoramic radiographs would not truly reflect the AEI, the measurements performed on central sagittal CBCT sections at the level of the AE being more appropriate to the real values of the AEI angles [41, 42, 43, 44].

The mean AEI values measured in this study were also higher than those described in the 2015 study by Chiang et al., who measured a mean AEI value of 37.7° [2]. It should be noted that the study by Chiang et al. was conducted on 53 Taiwanese patients aged between 20 and 85 years [2], unlike the present research in which we included a comparable group of 41 patients (82 TMJs) from Romania with an average age of 61.83±11.29 years. The gender distribution in this research was more balanced (53.7% of the participants were female and 46.3% were male), in contrast to the Taiwanese study which included much more female participants (66%, respectively 30 female participants and 33%, respectively 20 male participants) [2]. The reason for variations in results between different studies could be due to factors such as different measurement methods, sample size, age range and other differences between populations [45]. It is very important to mention that it is very difficult to compare the observed anatomy of the AE and its morphological parameters described in different studies, due to the differences in the study methodology used in different publications [24, 31, 40, 46, 47, 48, 49]. Describing the AEI, some researchers refer to the angle between the Frankfurt Horizontal and the tangent to the posterior slope of the AE, while another way of measurement refers to the angle between the Frankfurt Plane and the line that joins the most superior point of the mandibular fossa to the most inferior AE point [40]. In this investigation we chose to use a combination of the two methods, measuring the angle formed between the Frankfurt Horizontal and the tangent to the articular slope of the AE from the most anterior and superior point of the mandibular fossa following the posterior wall of the AE in the direction of the most inferior and posterior point of the AE, combining the methodologies from the studies by Kranjcic et al. (2021) [40], Unal Erzurumlu & Celenk [8] and Moscagiuri et al. [22]. The study methodology of the current research was based on the analysis of the images obtained with the help of CBCT, a technology that allows for clearly superior measurements to those performed on panoramic radiographs. Several methods have been implied in order to study the flattening of the articular slope of the AE, such as measurements performed on dry human skulls [36, 37, 50], traditional radiographic techniques [39, 51, 52], magnetic resonance imaging (MRI) analysis [14, 43, 53, 54], helical computed tomography (CT) [55] or CBCT [15]. Panoramic radiographs do not highlight the anatomy of AE very well, due to the overlap of the adjacent dense temporal bone [2, 15, 56, 57, 58]. The accuracy of panoramic radiographs can be affected by various factors, from projection geometry to incorrect patient positioning [15 59].

CBCT allows the three-dimensional (3D) visualization of the hard structures of the TMJ, reducing possible superpositions that may appear on two-dimensional (2D) radiographs and also allowing more accurate measurements of angles [13, 15, 27, 42, 57, 60, 61, 62, 63, 64, 65]. Therefore, in the present study, we chose to measure the AEI values on central sagittal sections at the AE level, because the central portion of the AE is the steepest area of the AE, where the highest AEI value will be found [15].

Another relevant finding of this paper is that, in our patients group, participants with loss of occlusal support in the molar areas on both sides (LL) displayed statistically significant much lower values of the right AEI (49±13.71°) compared to patients with maintained occlusal support in the molar regions on both sides (MM) (AEI right: 59.67±11.38°), the mean difference between the groups being 10.672° (95% CI: 2.1369–19.2075) (p=0.016). The fact that in this study lower values of the right AEI angles were observed in patients with loss of occlusal support in the molar areas on both sides (LL) in comparison to the AEI right values in patients with maintained occlusal support in the molar regions bilaterally (MM) might indicate a probable association of the loss of occlusal support in the molar areas with a possible decrease of the AEI predominantly on the right side.

Several research papers have indicated a high percentage of patients’ preference for unilateral chewing, mainly on the right side [66, 67, 68, 69, 70, 71]. As early as 1994, the results of international research highlighted the fact that the majority of people are right-handed (85% of study participants) and prefer the right side for performing several activities [36, 72]. The chewing preference for a certain side could be considered similar to the dominant hand use (right-handed, left-handed), the possible origin stemming from the dominant cranial hemisphere [69]. In two Israeli studies from 2004 and 2011, each involving 189 patients, a preference for right-sided chewing was observed in 78% of participants [70, 71]. Similarly, an analysis on 146 patients in Barcelona showed that among the patients with preference for unilateral chewing, 77% preferred the right side [73]. Also, the research on 19 healthy young adults in Iran indicated a highly significant preference for right-sided chewing in both hard foods (nuts) and soft foods (cakes) [74]. Moreover, a cross-sectional epidemiological study from Pomerania (Poland) of 4086 adults concluded that almost half (45.4%) of the participants showed a preference for unilateral chewing, especially on the right side (64%) [75]. However, there are also studies that have reported different results. A 2022 study conducted at the Karolinska Institute observed bilateral chewing in the majority of investigated participants (13 out of 15 patients) [76]. Also, this research conducted on a small sample group observed no correlation between handedness and the objectively assessed chewing side [76]. Quite interestingly, in the case of edentate patients, even though the lost teeth are replaced by prosthetic restorations, no bilateral chewing is evident [75]. Chewing side preference does not seem to be influenced by the location of the edentulous site(s), gender, type of occlusion, lateral guidance or the type of prosthetic restoration (e.g., complete dentures or implant-supported restorations) [70, 71]. Also, relevant to our results are the findings of some Japanese researchers regarding the fact that patients with unilateral and bilateral missing posterior teeth chewed on one side more predominantly than healthy control individuals [70, 71, 72, 73, 74, 75, 76, 77]. Moreover, a significant difference in masticatory performance was observed between control dentate adults and individuals with bilateral missing posterior teeth [77, 78, 79]. Considering these findings [77, 78, 79], we might assume the possibility that patients with bilateral loss of occlusal support in the molar areas (LL) could more frequently present preference for unilateral chewing, mostly on the right side. This would be a possible hypothesis and might explain the lower AEI values on the right side in the patients included in this research. The predominant use of a part of the dental arches (right or left) during chewing leads to an uneven distribution of biomechanical forces [2] acting on the TMJ and implicitly on the articular slope of the AE, a fact that might possibly explain the decrease in AEI values predominantly on one side. Chewing side preference could affect the way the remodeling process takes place at the level of the TMJ [36]. The contact area of the teeth on the preferred chewing side is larger than that on the contralateral side and the center of force in the maximum intercuspation position is mainly oriented towards the preferred chewing side [80]. Haralur et al. found a strong correlation between the center of force and the predominant part of mastication [80]. Furthermore, a 2015 paper supports the hypothesis that preferential unilateral chewing affects bone morphology of the TMJ in asymptomatic subjects [81]. The research, that investigated 121 asymptomatic dentate patients, concluded that a lower AEI was observed on the preferred chewing side compared to the contralateral non-preferred chewing side (p<0.05) [81].

Most likely, the remodeling that occurs at the AEI level does not take place very rapidly and in this study, we could not record, for each individual patient, the length of time that had passed from the loss of occlusal support in the molar area. Presumably in patients in whom the loss of occlusal support in the molar region occurred a short time ago, the decrease in AEI values is not yet clinically evident. In the current study, patients’ chewing side preference (bilateral, right or left) was not recorded.

Also, another factor worth considering is the influence of the orofacial muscles. Most people can perform a larger maximum lateral movement to the left side than to the right side due to the stronger development of the right lateral pterygoid muscle [36, 82]. As early as 1921, the German dentist Hans Wertheim noticed that more patients could move their mandible more to the left than to the right [82]. Furthermore, in a 2005 study of 141 healthy individuals and 141 TMD patients, it was observed that most healthy individuals and most TMD patients made maximum left laterotrusions to the left side, this being due to the stronger development of the lateral pterygoid muscle on the right side compared to the lateral pterygoid muscle on the left side [36, 82]. Considering that a higher percentage of patients with unilateral or bilateral missing posterior teeth chewed on one side more predominantly than healthy dentate subjects [77, 78 79], most frequently on the right side, corroborated with a much more pronounced development of the muscles on the right side [36, 82], it could be somehow explainable that the cumulative effect of these factors over time leads to a more pronounced remodeling at the level of the TMJ on the right side, respectively to a possible reduction of the AEI predominantly on the right side in the case of patients with bilateral loss of occlusal support in the molar areas.

Moreover, individual variations in muscle tone should not be ignored. People with brachycephaly are usually characterized by a higher muscle tone, leading to greater masticatory muscle forces, unlike people with dolichocephaly [30, 83, 84, 85, 86].

However, there are also studies that had different results compared to the data from the present research, such as Zabarović et al. who found that the right AEI had a higher value than the left AEI in their study group, but with no statistical significance (p>0.05) [24]. Similarly, Jasinevicius et al. also observed that right AEI values were consistently higher than left AEI values in their studies on dry human skulls [36, 50]. However, population variations must be considered because the measurements by Zabarović et al. were made in Croatia, on 137 dry human skulls, of which 78 with no missing teeth and 59 toothless, and Jasinevicius et al. performed their analysis on dry 20th century skulls from the American continent [24, 36, 50]. We do not know to what extent data extracted from measurements made on human skulls is comparable to the data obtained in the present research which included 41 contemporary patients. Interestingly, AEI values can be influenced by craniofacial morphology and the shape of the dental arches [19, 30]. da Costa et al. reported that male brachycephalic patients presented higher AEI values, a conclusion drawn after a CBCT investigation on 76 patients [30]. Verner et al. observed in 2017, after analyzing CBCT images of 60 patients, that AEI values were typically higher in patients with oval arch shape and lower in patients with square arch shape (p<0.05) [19]. One of the limitations of our research is the fact that the facial profiles and muscle tone of the participants in the study group have not been recorded.

Moreover, considering the findings that showed that AE asymmetries are not observed at birth, we can conclude that the possible AE asymmetries that appear during life are the result of a process of functional remodeling, of course excluding certain pathologies [7, 26, 36, 37, 50]. Masticatory forces could differ very much, considering the fact that the masticatory efficacy does vary a lot between dentate and edentulous subjects [87, 88].

Some researchers even correlated the absence of posterior teeth on the mandibular arch with displacements of the articular disc [23, 32]. According to some researchers, AEI may be a factor in the etiopathogenesis of TMD [15, 20]. Patients with TMD or with degenerative conditions at the level of the TMJ, who also associate preference for a certain chewing side, more frequently show higher AEI values [1, 89]. Same papers have advanced the hypothesis that a steep AEI could predispose to certain TMD, especially disorders or displacements of the articular disc [13, 14, 19, 43, 52, 54, 90, 91, 92, 93]. On the other hand, many investigators believe that TMDs are associated with smaller AEI [13, 45, 53, 94, 95]. However, other research groups found no correlation between the AEI angle and the existence or the severity of TMD [14, 15, 20, 45, 50, 96, 97].

The inclusion of several factors is essential for understanding the remodeling processes at the TMJ level [50]. The results obtained in the current research find their meaning especially in prosthodontics and oral rehabilitation. Understanding mandibular dynamics is essential and necessary for all dentists as AEI has a significant role in mandibular dynamics. AEI has the potential to influence not only mandibular dynamics [21] but, together with other factors, has the potential to affect the anatomy of the cusps of posterior teeth [37, 98]. Moreover, for the dentist performing oral rehabilitation procedures, it is essential to replicate the path of the condyle in the dental articulator [17, 99, 100]. The condylar guide is constituted by the posterior slope of the AE, which restricts and guides the movements of the mandible [21]. Therefore, using arbitrary averages for the condylar guidance angles of the dental articulator may be not suitable in some clinical cases [36]. Data indicate the significant variation of AEI values in adults, as well as the existence of some cases of asymmetry between the right AEI and the left AEI, emphasizing the need for correct individualized programming of the dental articulators and the need to use fully programmable articulators.

Future studies should include information regarding patients’ history, facial pattern, type of mastication, muscle tonicity, time since loss of occlusal support in the molar region, or whether patients wear removable prosthetic appliances. Moreover, as the activity of the masticatory muscles can be influenced by bruxism [101, 102], it would be important to also incorporate data regarding any type of parafunctional behaviors. Future longitudinal research should investigate the possibility of TMD in individuals with loss of occlusal support in the molar region.

Conclusions

This is the first research paper studying the association between the loss of occlusal support in the molar area and AEI values.

1. In this study, the AEI values were significantly higher in the TMJs on the side with maintained occlusal support in the molar area than the AEI values measured on TMJs from the side with molar loss and subsequent loss of occlusal support in the molar area.

2. In this research, it was observed that the AEI on the right side was significantly higher in the group of patients with maintained occlusal support in the molar areas on both sides (right and left side) (MM) in comparison to the AEI values on the right side measured in patients with molar loss and loss of occlusal support in the molar areas on both sides (right and left side) (LL).

3. The loss of occlusal support in the molar areas changes the mandibular biodynamics, which might be reflected in the morphological changes at the AE level, where it might cause flattening of the articular slope predominantly on the right side.

4. Future in vivo studies with a higher number of participants might be beneficial to elucidate the relationship between the loss of occlusal support in the molar area and modifications at the AE level.

Conflict of interests

The authors declare that they have no conflict of interests.

Compliance with ethical standards

The patients’ informed consent in writing form on the use of their clinical and radiological data in scientific research was obtained.
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