The Temporomandibular Joint (TMJ)

💡 Core Concept: The TMJ is a compound, synovial joint. It is unique because it contains a moveable Articular Disc that divides the joint into two distinct cavities, each with a different function (Rotation vs. Translation).

1. Structure and Articular Surfaces

A. Bones

• Mandibular Condyle: Convex. Shape allows rotation.
• Mandibular Fossa (Temporal Bone): Concave (Posterior part).
• Articular Eminence (Temporal Bone): Convex (Anterior part). This is the surface upon which the condyle slides during full opening.

B. The Articular Disc

A biconcave fibrocartilage structure that increases congruency.

• Posterior Band: Thickest part. Attaches to the Retrodiscal Laminae (Bilaminar Zone).
• Intermediate Zone: Thinnest part. This is the weight-bearing surface. Avascular and Aneural (No pain sensation).
• Anterior Band: Attaches to the Superior Head of the Lateral Pterygoid.

⚠️ Retrodiscal Tissue (Bilaminar Zone): Located behind the disc. Highly vascular and highly innervated. If the condyle compresses this tissue (due to disc displacement), it causes significant pain.

2. Functional Biomechanics

The joint is divided into two functional cavities:

Joint Cavity	Location	Primary Motion	Biomechanics
Lower Joint	Between Condyle & Disc	Rotation (Roll)	Occurs in the first 50% of opening (Early Phase).
Upper Joint	Between Disc & Eminence	Translation (Glide)	Occurs in the last 50% of opening (Late Phase).

Mandibular Motions

• Depression (Opening): Posterior Rotation of Condyle (Lower Joint) → Anterior Translation of Condyle/Disc (Upper Joint).
• Elevation (Closing): Reverse of opening.
• Protrusion: Pure Anterior Translation (Upper Joint).
• Lateral Deviation:
  • Ipsilateral Condyle: Spins/Rotates in place.
  • Contralateral Condyle: Translates Anteriorly/Inferiorly.

3. Muscular Control

A. Primary Muscles of Mastication

Muscle	Action	Innervation
Masseter	Elevation (Closing), some protrusion.	Trigeminal (V3)
Temporalis	Elevation, Retrusion (Posterior fibers), Ipsilateral Deviation.	Trigeminal (V3)
Medial Pterygoid	Elevation, Protrusion, Contralateral Deviation.	Trigeminal (V3)
Lateral Pterygoid	Opening (Depression), Protrusion, Contralateral Deviation.	Trigeminal (V3)

B. Accessory Muscles

• Suprahyoids (Digastric): Assist in depression (opening).
• Infrahyoids: Stabilize the hyoid bone.

4. Pathomechanics & Dysfunction

A. Anterior Disc Displacement

• With Reduction: "Clicking." The condyle slips over the posterior border of the disc onto the center (Click 1) during opening, and slips back off during closing (Click 2).
• Without Reduction (Closed Lock): The disc stays anterior to the condyle. No click, but opening is severely limited (blocked by the disc).

B. Forward Head Posture

Forward head posture stretches the infrahyoid muscles, which pull the mandible into Retrusion and Depression. This increases compression on the Retrodiscal tissues.

🏆 Key Points: Clinical Gems

• Freeway Space: The normal resting position of the jaw (2-4mm opening between teeth). Essential for tissue rest.
• Deflection vs. Deviation:
  • Deflection: Mandible moves to one side and stays there (C-curve). Points to the hypomobile side.
  • Deviation: Mandible moves to one side but returns to center (S-curve). Usually indicates motor control issue or disc reduction.
• Disc Attachment: The Superior Head of the Lateral Pterygoid attaches directly to the disc, pulling it forward during contraction.

📝 20 High-Yield MCQs

Test your knowledge for Exams.

Q1. The TMJ is structurally classified as which type of joint?

A. Ball and Socket B. Synovial, Compound, Modified Ovoid C. Cartilaginous D. Suture

Rationale: It is synovial, compound (3 bones involved: temporal, mandible, disc), and modified ovoid (convex on concave/convex).

Q2. Which part of the TMJ articular disc is avascular and aneural?

A. Anterior Band B. Posterior Band C. Intermediate Zone D. Bilaminar Zone

Rationale: The intermediate zone bears the weight/load and therefore cannot have nerves or blood vessels (which would be compressed).

Q3. Motion in the Lower Joint cavity of the TMJ is primarily:

A. Translation B. Rotation C. Lateral Deviation D. Compression

Rationale: The condyle rotates against the inferior surface of the disc in the lower compartment.

Q4. The primary muscle responsible for Opening (Depression) of the mandible is:

A. Masseter B. Lateral Pterygoid (Inferior Head) C. Medial Pterygoid D. Temporalis

Rationale: The Lateral Pterygoid pulls the condyle anteriorly, initiating opening. The other choices are closers (elevators).

Q5. During "Lateral Deviation" to the RIGHT, what is the Left Condyle doing?

A. Rotating in place B. Translating Anteriorly and Inferiorly C. Retracting D. Elevating

Rationale: The contralateral (left) condyle must slide forward to push the chin to the opposite (right) side. The ipsilateral (right) condyle spins.

Q6. Which muscle attaches directly to the articular disc?

A. Temporalis B. Masseter C. Lateral Pterygoid (Superior Head) D. Digastric

Rationale: The superior head of the lateral pterygoid attaches to the anterior band of the disc, controlling its position during closing.

Q7. The "Retrodiscal Tissue" (Bilaminar Zone) is known for being:

A. Highly vascular and innervated (Pain sensitive) B. Avascular and Aneural C. Purely cartilaginous D. Bone-like

Rationale: This tissue sits behind the disc. If the disc displaces anteriorly, the condyle presses on this sensitive tissue, causing pain.

Q8. A "Click" sound during opening usually indicates:

A. Disc Displacement WITH Reduction B. Disc Displacement WITHOUT Reduction C. Fracture D. Normal mechanics

Rationale: The click occurs when the condyle "recaptures" the anteriorly displaced disc (pops back onto it).

Q9. The "Closed Packed Position" of the TMJ is:

A. Teeth tightly clenched B. Mouth slightly open (Rest) C. Maximum opening D. Protrusion

Rationale: Maximal intercuspation (teeth clenched) is the occlusal closed packed position. (Note: Max opening is sometimes considered the capsular close packed, but clenching is the standard answer for stability).

Q10. Anterior translation of the condyle occurs in the:

A. Lower Joint Cavity B. Upper Joint Cavity C. Both cavities equally D. Neither

Rationale: The disc slides (translates) along the articular eminence in the Upper compartment.

Q11. The normal range of mouth opening (Interincisal opening) is approximately:

A. 10-20 mm B. 40-50 mm C. 70-80 mm D. 100 mm

Rationale: 40mm is generally considered functional (roughly 3 finger widths).

Q12. If a patient has a "Closed Lock" (Disc Displacement Without Reduction) on the Right, they will:

A. Deflect to the Right during opening B. Deflect to the Left during opening C. Open straight D. Deviate in an S-curve

Rationale: The mandible deflects *towards* the hypomobile (stuck) side because the healthy side continues to translate, pushing the chin over.

Q13. The Temporalis muscle is unique because its posterior fibers can perform:

A. Protrusion B. Retrusion (Retraction) C. Depression D. Anterior Translation

Rationale: The posterior fibers run horizontally backward, making them the primary retractors of the mandible.

Q14. Which ligament limits excessive mouth opening?

A. Temporomandibular (Lateral) Ligament B. Stylomandibular Ligament C. Sphenomandibular Ligament D. Periodontal Ligament

Rationale: The Lateral Ligament (outer oblique portion) prevents excessive rotation and posterior displacement.

Q15. Forward Head Posture affects the TMJ by causing:

A. Protrusion of the mandible B. Retrusion and Depression of the mandible C. Anterior Disc Displacement D. Relaxation of the masseter

Rationale: Forward head creates tension in the infrahyoid muscles, which pull the jaw back and down, compressing the retrodiscal tissues.

Q16. The "Freeway Space" is:

A. The resting space between upper and lower teeth (2-4mm) B. The space inside the joint capsule C. The maximum opening width D. The space between the disc and condyle

Rationale: This is the physiological rest position where muscles are relaxed and tissues are not compressed.

Q17. During the first phase of opening (0-10mm), the primary motion is:

A. Rotation B. Translation C. Glide D. Deviation

Rationale: Early opening is pure rotation in the lower compartment. Translation happens later.

Q18. Which nerve innervates the muscles of mastication?

A. Facial Nerve (CN VII) B. Trigeminal Nerve (Mandibular Branch V3) C. Hypoglossal Nerve (CN XII) D. Glossopharyngeal Nerve (CN IX)

Rationale: The Mandibular branch of the Trigeminal nerve (V3) is the only branch with motor function for mastication.

Q19. What provides nutrition to the central part of the articular disc?

A. Blood vessels B. Synovial fluid diffusion (Intermittent compression) C. Lymphatics D. Retrodiscal tissue

Rationale: Since it is avascular, the disc relies on the "pumping action" of movement to circulate synovial fluid for nutrition.

Q20. The articular surface of the Mandibular Condyle is lined with:

A. Hyaline Cartilage B. Fibrocartilage C. Elastic Cartilage D. Periosteum only

Rationale: Unlike most synovial joints (Hyaline), the TMJ is lined with Fibrocartilage, which is better at self-repair and handling shear forces.