Anatomical Structure and Location
Superiorly, the ramus of the mandible terminates in two processes separated by the mandibular notch (Incisura mandibulae):
- The condylar process (Processus condylaris);
- The coronoid process (Processus coronoideus), which is anterior and pointed.
The condylar process comprises:
- The head of the mandible (Caput mandibulae);
- The neck of the mandible (Collum mandibulae).
The pterygoid fossa (Fovea pterygoidea) is located on the medial surface of the neck.
Involvement in the TMJ
The condylar process forms the temporomandibular joint (articulatio temporomandibularis). The articular surfaces of the TMJ include:
- The head of the mandible;
- The mandibular fossa and articular tubercle of the temporal bone.
- Joint type: Complex, combined (the right and left joints act synchronously).
- Articular disc (discus articularis): Divides the joint cavity into two compartments. The inferior compartment allows rotational movements, while the superior compartment permits translational (sliding) movements.
- Ligamentous apparatus: The capsule is strong and attaches along the margins of the articular surfaces. TMJ ligaments include the lateral ligament (lig. laterale) as the primary stabilizer, the stylomandibular ligament (lig. stylomandibulare), and the sphenomandibular ligament (lig. sphenomandibulare).
Biomechanics of Movement
Movements of the mandible at the TMJ include:
- Depression and elevation;
- Protrusion and retraction;
- Lateral displacement (side-to-side movements).
Laterotrusive (working-side) and mediotrusive (non-working/balancing-side) pathways: During lateral excursion, the condyle on the balancing (non-working) side moves inferiorly, anteriorly, and medially.
Bennett angle: The angle formed in the horizontal plane between the sagittal plane and the path of the advancing balancing condyle during lateral mandibular movement.
Clinical Significance and Traumatology
- Palpation of the mandible: Performed systematically from the condylar process down the ramus and forward along the body of the mandible.
- Diagnosis of condylar fractures: Palpation of a "bony step" indicates a fracture with medial displacement of the proximal fragment.
- Palpation of the TMJ: Assesses the mandibular head and its articulation within the mandibular fossa.
- Radiography: In a posteroanterior (PA) projection, fractures of the condylar process classically show lateral displacement of the fractured segment.
- Differential diagnosis: Acute arthritis of the TMJ must be differentiated from a fracture of the condylar process.
Joint Pathologies and Displacements
Alterations in the position of the condylar process and articular disc are observed in various TMJ disorders:
- Chronic mandibular head dislocation: During mouth opening, the condyle excessively translates anteriorly, moving past the articular tubercle of the temporal bone.
- Chronic dislocation with disc subluxation: The condyle moves past the apex of the articular tubercle, accompanied by an early "click" within the articular fossa.
- Chronic TMJ dislocation: A click occurs as the condyle exits the fossa and moves onto the anterior slope of the articular tubercle, repeating in reverse during closure.
- Articular disc subluxation: Anterior displacement of the disc with reduction within the mandibular fossa. Clicking occurs during both opening and closing; at maximum opening, the condyle does not translate past the fossa.
- Recurrent disc displacement with reduction: The displaced disc acts as a mechanical obstruction blocking condylar translation, with locking episodes lasting from minutes to hours.
- Chronic disc displacement without reduction: The disc assumes a permanent anteromedial position, clinically presenting as limited mouth opening (interincisal distance less than 2.5 cm) and deviation of the mandible toward the affected side.
- Chronic disc displacement with secondary osteoarthritis: Characterized by disc deformation, cartilage defects, bone remodeling, and osteophyte formation.
- Chronic posterior disc displacement: The disc becomes entrapped between the condyle and the posterior wall of the mandibular fossa upon initiation of movement.