Disc Displacement

TMJ Disc Displacement: Types, Frequency, and Clinical Significance

Disc displacement is a central feature of temporomandibular joint (TMJ) internal derangement and is commonly evaluated using magnetic resonance imaging (MRI). Multiple patterns of disc displacement have been described in symptomatic TMJs, but clinical and imaging studies consistently show that only a small number of displacement types account for the vast majority of cases.

Understanding the direction and frequency of disc displacement is important for diagnosis, treatment planning, and interpretation of TMJ MRI studies.

Overview of Disc Displacement Patterns

As many as seven types of disc displacement have been described in MRI evaluations of symptomatic TM joints. However, research by Werther et al. demonstrated that approximately 74% of disc displacements fall into just two primary categories: anterior displacement and medial rotary displacement.

These findings highlight that while many theoretical displacement patterns exist, clinically relevant TMJ disc displacement is dominated by anterior and medial components.

Most Common Types of TMJ Disc Displacement

Anterior Disc Displacement (≈44–45%)

Anterior disc displacement is the most common form of TMJ disc displacement. In this pattern, the articular disc lies anterior to the mandibular condyle when the mouth is closed.

Anterior displacement is frequently associated with:

• Clicking or popping (when reducing)

• Locking and restricted opening (when non-reducing)

• Progression through Wilkes stages of internal derangement

This displacement pattern forms the basis for most clinical TMJ treatment algorithms.

Anterolateral / Medial Rotary Displacement (≈29%)

Medial rotary displacement occurs when the disc is displaced anteriorly with a medial rotational component. This pattern accounts for a substantial proportion of symptomatic joints and is often underrecognized on routine imaging unless specifically assessed.

Medial rotation reflects the influence of the lateral pterygoid muscle, which inserts on the disc and can pull it anteriorly and medially.

Less Common Disc Displacement Patterns

Anteromedial Displacement (≈6%)

This represents a smaller subset of cases where the disc is displaced anteriorly with a more pronounced medial component, often associated with more complex internal derangement.

Lateral Disc Displacement (≈2–3%)

Lateral disc displacement is rare. Werther et al. reported lateral displacement in approximately 2.5% of symptomatic joints. Similarly, Simmons found no cases of lateral disc displacement in a series of 58 symptomatic TMJs evaluated with MRI.

These findings emphasize that lateral displacement is exceptionally uncommon and should be interpreted cautiously when suggested on imaging.

Pure Medial Displacement (≈1%)

Pure medial disc displacement without an anterior component is extremely rare. Most medial displacements occur in combination with anterior displacement rather than as an isolated finding.

Frequency of TMJ Disc Displacement Types

Clinical and Imaging Implications

• Anterior and medial components dominate TMJ disc pathology

• Lateral disc displacement is rare and should prompt careful MRI review

• Disc displacement direction correlates with symptoms, joint mechanics, and progression of internal derangement

• MRI interpretation should focus on disc position in both closed and open mouth views, with attention to rotational components

Board Exam and Clinical Pearls

• ~75% of disc displacements are anterior or medial rotary

• Lateral disc displacement is extremely rare

• The lateral pterygoid muscle contributes to anterior and medial disc displacement

• Pure medial or lateral displacement is uncommon and often overdiagnosed

Conclusion

While multiple TMJ disc displacement patterns have been described, the vast majority of clinically significant cases involve anterior displacement with or without medial rotation. Lateral and isolated medial displacements are rare and should be interpreted cautiously. Accurate understanding of displacement patterns improves MRI interpretation, staging of internal derangement, and surgical decision-making.