TMJ morphology and Angle classes: the impact of occlusion on joint anatomy

The temporomandibular joint (TMJ) occupies a unique place in the body due to its direct dependence on dental occlusion. Occlusal balance regulates not only muscle function, but also the position of the condyle and the distribution of mechanical loads on the articular surfaces. While occlusal alterations are suspected of causing morphological adaptations of the TMJ, the literature still lacks precision to establish whether specific anatomical configurations are systematically associated with different types of malocclusions.

This retrospective study, conducted on 131 patients, aims to evaluate TMJ morphology using CBCT (cone beam computed tomography) imaging according to the occlusal relationship of the arches. The authors analyzed key parameters such as the height and inclination of the articular eminence, as well as the width and depth of the glenoid fossa, comparing Angle Classes I, II, and III.

The null hypothesis tested by the researchers postulated a total absence of association between occlusal class and morphological variations of the TMJ. For the practitioner, the challenge is to determine whether the orthodontic or prosthetic diagnosis should integrate predictable anatomical joint specificities according to the patient's occlusal profile.

A retrospective analysis using 3D imaging

This study involved a cohort of 131 patients to dissect the morphology of the temporomandibular joint (TMJ) according to occlusal relationships. The sample, drawn from a radiographic database, adheres to rigorous selection criteria: adult patients, full dentition (excluding third molars), and absence of prior pathologies or orthodontic treatments. Subjects were divided into three distinct groups according to Angle's classification: Class I, Class II, and Class III.

The imaging protocol relied on Cone Beam technology (CBCT) with standardized acquisition parameters (120 kVp, 5 mA, 0.3 mm voxel). The objective? To map the bone anatomy via several key morphometric variables:

• Articular eminence: measurement of height (AEH) and inclination via two geometric methods (Best-fit line and Top-roof line).
• Glenoid fossa: evaluation of width (GFW) and depth (GFD).
• Condylar dynamics: measurement of the intercondylar angle (ICA) and the condylar angle (CA) relative to the mid-sagittal plane.

To ensure the reliability of the results, a statistical power calculation validated a minimum sample size of 128 individuals. Data analysis was based on two-way and three-way ANOVA tests with repeated measures to isolate the influence of side (right/left), sex, and occlusal class on TMJ structures.

TMJ morphology: the impact of Angle's classification

Analysis of the 131 CBCT scans reveals that the morphology of the temporomandibular joint (TMJ) is closely linked to the occlusal relationship of the dental arches. The study highlights significant variations, particularly in the articular eminence, depending on whether the patient presents a Class I, II, or III malocclusion.

The results highlight a marked specificity for Class II patients: they present a significantly higher articular eminence height than Class I patients (p < 0.05). Similarly, the eminence inclination (measured via the best-fit line and top-roof line methods) reaches its maximum values in the Class II group, followed by Class I, then Class III.

Regarding the glenoid fossa and the condyles, the following observations have been reported:

• Glenoid fossa: No statistically significant difference was observed between the three occlusion groups for the width (GFW) or depth (GFD) of the fossa (p > 0.05).
• Asymmetries and biological variables: Significant variations related to the side (right vs left) were noted for the eminence inclination, as well as for the width and depth of the glenoid fossa (p < 0.05).
• Gender influence: The inclination of the articular eminence and the depth of the glenoid fossa show significant differences depending on the patient's gender.

3D CBCT imaging has confirmed that while the glenoid fossa remains relatively constant between Angle classes, the articular eminence adapts to or reflects the mechanical stresses specific to each type of malocclusion, thereby rejecting the null hypothesis of an absence of association between occlusion and TMJ morphology.

Clinical decoding: occlusion shapes the joint

The results of this CBCT study on 131 patients confirm that the morphology of the temporomandibular joint (TMJ) is not fixed, but closely linked to the occlusal relationship. The major finding concerns Angle Class II patients: they present a significantly higher articular eminence (6.93 mm ± 1.07) and a steeper inclination (up to 51.74°) compared to Class I (6.27 mm ± 1.22). Clinically, this suggests that the sagittal position of the mandible imposes specific mechanical constraints leading to adaptive remodeling of the temporal bone.

The study also highlights variations related to sex and side (right/left) for the eminence inclination and the glenoid fossa depth. In contrast, the width and depth of the fossa do not appear to vary significantly between Angle classes. Contrary to popular belief, while malocclusion influences the slope and height of the condylar path, it does not necessarily alter the overall dimensions of the glenoid cavity itself in this sample.

Limits and perspective

Although robust due to its three-dimensional analysis, this retrospective study has limitations. The use of CBCT, with a voxel size of 0.3 mm, is ideal for bone but does not allow for the evaluation of soft tissues or disc position, elements that are nevertheless inseparable from TMJ dynamics. Furthermore, patients with tooth loss or joint pathologies were excluded, which limits the extrapolability of the results to the most complex clinical cases encountered in dental surgery practice.

Consequences for daily practice

For the practitioner, these data serve as a reminder that occlusal diagnosis does not stop at the dental arches. A Class II is not just a dental discrepancy; it is also an articular environment with steeper slopes, which must influence the adjustment of your articulators in prosthetics or the assessment of the risk of temporomandibular disorders (TMD) in orthodontics.

Summary of results

This study conducted on 131 patients demonstrates that TMJ morphology is directly correlated with Angle's classification: Class II patients present a significantly higher (6.93 ± 1.07 mm) and more inclined (51.74° ± 5.77°) articular eminence than Class I (6.27 mm / 48.54°) and Class III (47.30°) patients. While malocclusion influences the eminence, the width and depth of the glenoid fossa depend more on gender and laterality than on the inter-arch relationship.

In concrete terms, for the practitioner:

• Kinematic anticipation: In your Class II patients, expect a steeper condylar guidance due to the marked inclination of the eminence, which may influence your prosthetic reconstructions and dynamic occlusion adjustment.
• Differential diagnosis: Do not consider a high eminence or a steep slope as an isolated anomaly; systematically analyze them in light of the patient's sagittal occlusal relationship.
• CBCT Precision: During the imaging examination, evaluate each joint independently, as the study confirms significant morphological variations between the right and left sides in the same individual.

Technical lexicon

Articular eminence height: Vertical distance measured between the deepest point of the glenoid fossa and the summit of the articular eminence.

Articular eminence inclination: Angle formed by the posterior slope of the eminence relative to a horizontal reference plane, measured either by a tangent to the slope (*best-fit line*) or by a line connecting the summit of the eminence to the roof of the fossa (*top-roof line*).

Width of the glenoid fossa: Linear distance measured between the highest point of the articular eminence and the posterior point of the glenoid fossa.

Glenoid fossa depth: Perpendicular distance measured between the deepest point of the fossa and the reference line defining its width.

Intercondylar angle: Angular value formed by the intersection of the longitudinal axes of the right and left mandibular condyles.

Condylar angle: Measurement of the orientation of the long axis of each condyle relative to the median sagittal plane.

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