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M01.02.043 TMJ
The temporomandibular joint (TMJ) is a unique and complex structure that connects the mandible to the temporal bone of the cranium. Located just anterior to the tragus of the ear on the lateral aspect of the face, it facilitates essential functions like chewing, speaking, and opening the mouth.
This guide breaks down the anatomy, movement, and clinical relevance of the TMJ for USMLE Step 1 preparation.
Anatomy of the TMJ
Articulating Surfaces
The TMJ involves three articulating surfaces:
- Mandibular fossa (temporal bone)
- Articular tubercle (temporal bone)
- Head of the mandible
Unique Features:
- Articular Disk: Separates the joint into two synovial cavities, lined by synovial membranes.
- Fibrocartilage: Unlike most joints, the articular surfaces are covered by fibrocartilage instead of hyaline cartilage.
| Component | Description |
|---|---|
| Mandibular Fossa | Depression in the temporal bone. |
| Articular Tubercle | Rounded projection aiding stability. |
| Articular Disk | Divides the joint into two cavities. |
Ligaments
The TMJ is stabilized by three extracapsular ligaments:
- Lateral Ligament: Prevents posterior dislocation by connecting the articular tubercle to the mandibular neck.
- Sphenomandibular Ligament: Runs from the sphenoid spine to the mandible.
- Stylomandibular Ligament: Derived from the fascia of the parotid gland, supporting the jaw’s weight.
| Ligament | Origin | Insertion | Function |
|---|---|---|---|
| Lateral Ligament | Articular tubercle | Mandibular neck | Prevents posterior dislocation. |
| Sphenomandibular Ligament | Sphenoid spine | Mandible | Supports the jaw’s movement. |
| Stylomandibular Ligament | Parotid gland fascia | Mandible | Aids jaw stabilization. |
Movements of the TMJ
The TMJ permits a variety of movements, thanks to its dual-cavity structure.
1. Protrusion and Retraction
- Protrusion: Forward movement of the mandible.
- Muscles: Lateral pterygoid (main), medial pterygoid (assists).
- Retraction: Backward movement of the mandible.
- Muscles: Posterior fibers of the temporalis.
2. Elevation and Depression
- Elevation: Closing the mouth.
- Muscles: Temporalis, masseter, medial pterygoid.
- Depression: Opening the mouth.
- Muscles: Gravity (primary), digastric, geniohyoid, mylohyoid.
| Movement | Primary Muscles Involved | Function |
|---|---|---|
| Protrusion | Lateral and medial pterygoids | Forward jaw movement. |
| Retraction | Temporalis (posterior fibers) | Backward jaw movement. |
| Elevation | Temporalis, masseter, medial pterygoid | Closing the mouth. |
| Depression | Digastric, geniohyoid, mylohyoid | Opening the mouth (gravity aids). |
Neurovascular Supply
Arterial Supply
The TMJ is vascularized by branches of the external carotid artery:
- Superficial temporal artery (primary supply)
- Deep auricular, ascending pharyngeal, and maxillary arteries (secondary branches)
Innervation
The TMJ receives sensory input from branches of the mandibular nerve (CN V3):
- Auriculotemporal nerve
- Masseteric nerve
Clinical Relevance
1. Temporomandibular Joint Dislocation
- Causes: Trauma, yawning, or taking a large bite.
- Mechanism: The mandibular head slips out of the mandibular fossa, moving anteriorly.
- Presentation: Inability to close the mouth, pain, and possible nerve injury.
- Rare Posterior Dislocations: Require significant force to overcome the postglenoid tubercle and lateral ligament strength.
| Type of Dislocation | Cause | Mechanism |
|---|---|---|
| Anterior | Trauma, yawning | Mandibular head slips anteriorly. |
| Posterior | High-energy trauma | Overcomes postglenoid tubercle. |
Points to Remember
- The TMJ is unique due to its fibrocartilage-covered surfaces and articular disk dividing it into two cavities.
- Movements at the TMJ involve specific muscle groups, with distinct functions for its upper and lower divisions.
- Neurovascular supply is primarily derived from the external carotid artery and mandibular nerve.
- TMJ dislocations are most often anterior and can affect nearby nerves with severe trauma.