Temporomandibular Joint and Styloid Apparatus

TEMPOROMANDIBULAR JOINT

– The temporomandibular joint (TMJ) is formed between the condyle of the mandible and the temporal bone at the base of the skull.

Type of Joint

• Complex joint: It contains an intra-articular disc.
• Bicondylar joint: The right and left mandibular condyles articulate with the temporal bones.
• Synovial joint: Its articular cavities contain synovial fluid.
• It is a complex, bicondylar synovial joint that permits both hinge and gliding movements.

Articular Surfaces

Superior articular surface

• It is formed by the mandibular fossa and articular eminence of the squamous part of temporal bone.
• It is concavoconvex from posterior to anterior.
• The posterior part of the mandibular fossa, adjoining the tympanic plate, is non-articular.

Inferior articular surface

• It is formed by the elliptical head (condyle) of the mandible.
• Its transverse diameter is greater than its anteroposterior diameter.
• Its long axis is directed medially and posteriorly. The axes of both condyles converge towards the anterior margin of the foramen magnum.

Note: The articular surfaces are covered by fibrocartilage, unlike most synovial joints, which have hyaline cartilage.

Joint Cavity and Articular Disc

– The intra-articular disc divides the joint cavity into two separate compartments:

1. Upper (meniscotemporal) compartment: It mainly permits gliding or translatory movements.
2. Lower (meniscomandibular) compartment: It mainly permits rotational or hinge movements.

Articular disc

• It is an oval, biconcave fibrocartilaginous disc situated between the temporal bone and mandibular condyle.
• Superior surface: It is concavoconvex and conforms to the articular eminence and mandibular fossa.
• Inferior surface: It is concave and accommodates the convex mandibular condyle.
• Peripheral margin: It is attached to the fibrous capsule of the joint.

Structure

– From anterior to posterior, the disc presents five regions:

1. Anterior extension.
2. Anterior thick band.
3. Thin intermediate zone.
4. Posterior thick band.
5. Posterior bilaminar zone.

Attachments

• Anteriorly: The disc is continuous with the joint capsule and receives fibres from the superior head of lateral pterygoid.
• Posteriorly: It continues into the bilaminar (retrodiscal) tissue, which consists of:
  1. Superior lamina: It contains elastic fibres and attaches to the temporal bone near the squamotympanic fissure.
  2. Inferior lamina: It consists mainly of collagen fibres and attaches to the posterior aspect of the mandibular condyle.
  3. Intermediate tissue: It contains loose connective tissue, blood vessels, and a venous plexus.

Functions

• It reduces friction between the articular surfaces.
• It absorbs shock and distributes mechanical loads.
• It improves joint stability by increasing articular congruence.
• It facilitates coordinated rotational and translatory movements.

Note: The central portion of the disc is avascular and may undergo degenerative changes or perforation.

Ligaments of Temporomandibular Joint

The ligaments associated with the temporomandibular joint are:

1. Capsular ligament.
2. Lateral temporomandibular ligament.
3. Sphenomandibular ligament.
4. Stylomandibular ligament.
5. Pterygomandibular ligament (raphe), an adjacent fibrous structure.

Fibrous Capsule (Capsular Ligament)

• It is a fibrous envelope enclosing the joint cavity.
• Superior attachment: It attaches around the mandibular fossa and articular eminence of the temporal bone.
• Inferior attachment: It attaches around the neck of the mandible.
• The capsule is relatively loose above the articular disc and tighter below it.
• The peripheral margin of the disc is attached to the capsule.
• The synovial membrane lines the non-articular internal surfaces of the capsule, forming separate upper and lower synovial cavities.
• Fibres of the superior head of lateral pterygoid attach to the anterior capsule and disc.

Lateral Temporomandibular Ligament

• It is a strong thickening of the lateral part of the fibrous capsule.
• Superior attachment: It attaches to the articular tubercle and adjoining root of the zygomatic arch.
• Inferior attachment: It attaches to the posterolateral aspect of the mandibular neck.
• Direction: Its fibres run downwards and backwards.
• Function: It reinforces the joint laterally and limits excessive posterior displacement of the mandibular condyle.

Sphenomandibular Ligament

• It is an accessory ligament of the temporomandibular joint.
• Superior attachment: It attaches to the spine of sphenoid.
• Inferior attachment: It attaches to the lingula near the mandibular foramen.
• Development: It represents a fibrous derivative associated with the dorsal part of Meckel’s cartilage.

Relations

• Laterally: It is related to lateral pterygoid, auriculotemporal nerve, maxillary artery, and inferior alveolar nerve and vessels.
• Medially: It is related to medial pterygoid, chorda tympani nerve, and pharyngeal wall.
• Near its lower attachment: The mylohyoid nerve and vessels pass close to or through the inferior part of the ligament.

Stylomandibular ligament

• It is an accessory ligament formed by thickening of the deep cervical fascia.
• Superior attachment: It attaches to the styloid process of temporal bone.
• Inferior attachment: It attaches to the angle and adjoining posterior border of the mandibular ramus.
• Function: It limits excessive protrusion of the mandible.
• Clinical significance: It separates the parotid gland from the submandibular gland.

Pterygomandibular ligament

• It is a fibrous band extending between the pterygoid hamulus and the posterior part of the mandible.
• Superior attachment: It attaches to the pterygoid hamulus at the lower end of the medial pterygoid plate.
• Inferior attachment: It attaches to the posterior end of the mylohyoid line, behind the mandibular third molar.
• It provides attachment to the buccinator anteriorly and superior constrictor of pharynx posteriorly.

Note: The pterygomandibular raphe is not a true supporting ligament of the temporomandibular joint. It is an adjacent fibrous structure.

Relations of Temporomandibular Joint

Lateral Relations

• The joint is superficial and lies beneath the skin.
• Its lateral relations include:
  • Skin and fascia.
  • Parotid gland.
  • Temporal branches of facial nerve.

Medial Relations

– The following structures lie medial to the joint:

1. Auriculotemporal nerve.
2. Middle meningeal artery.
3. Spine of sphenoid.
4. Sphenomandibular ligament.
5. Chorda tympani nerve.

Anterior Relations

• Lateral pterygoid muscle.
• Masseteric nerve and vessels.

Posterior Relations

• Parotid gland.
• Superficial temporal vessels.
• Auriculotemporal nerve.
• External acoustic meatus.

Superior Relations

– The middle cranial fossa lies above the joint, separated from it by a thin plate of temporal bone.

Inferior Relations

– Maxillary artery and vein lie inferior and medial to the joint.

Blood Supply, Lymphatic Drainage, and Innervation

Blood supply

– The temporomandibular joint receives arterial branches from:

1. Maxillary artery.
2. Superficial temporal artery.

Lymphatic drainage

– Lymph from the joint drains into:

1. Superficial parotid (preauricular) lymph nodes.
2. Deep parotid lymph nodes.
3. Upper deep cervical lymph nodes.

Innervation

– The joint receives sensory fibres mainly from:

1. Auriculotemporal nerve.
2. Masseteric nerve.

Resting and Occlusal Positions

• Resting position: The mandible remains relaxed, with a free space of approximately 2–4 mm between the upper and lower teeth. The lips may remain in contact.
• Occlusal position: The upper and lower teeth come into contact during closure of the mouth.

Movements of Temporomandibular Joint

– The temporomandibular joint permits five principal movements:

1. Depression: Opening of the mouth.
2. Elevation: Closing of the mouth.
3. Protrusion: Forward movement of the mandible.
4. Retrusion: Backward movement of the mandible.
5. Lateral movements: Side-to-side movements during chewing and grinding.

Muscles Producing Movements

1. Depression

• The lateral pterygoid assists in depressing the mandible.
• Digastric, mylohyoid, and geniohyoid assist in opening the mouth when the hyoid bone is stabilised.
• Gravity also contributes to depression.

2. Elevation

– Masseter, temporalis, and medial pterygoid elevate the mandible to close the mouth.

3. Protrusion

• Bilateral contraction of lateral and medial pterygoid muscles moves the mandible forwards.
• Superficial fibres of masseter also assist in protrusion.

4. Retrusion

• Posterior fibres of temporalis are the principal retractors of the mandible.
• Deep fibres of masseter assist in retrusion.

5. Lateral or Grinding Movements

• Alternate contraction of the medial and lateral pterygoid muscles on opposite sides produces side-to-side movements.
• Unilateral contraction of the pterygoid muscles moves the chin towards the opposite side.
• These movements are coordinated with the elevator muscles during mastication.

Mechanism of TMJ movements

– Movements of the temporomandibular joint occur in two compartments:

1. Upper (meniscotemporal) compartment: It mainly permits gliding movements during protrusion, retrusion, and chewing.
2. Lower (meniscomandibular) compartment: It mainly permits rotational movements during opening and closing of the mouth.

Axes of Rotation

• Transverse axis: It passes mediolaterally through the mandibular condyles and permits rotation during depression and elevation.
• Vertical axis: It passes approximately through the condyle of one side during lateral movements of the mandible.

Mechanism of Depression

Opening of the mouth involves three overlapping phases:

1. Initial phase: The mandibular condyle rotates forwards beneath the articular disc within the lower compartment.
2. Middle phase: The condyle and disc glide forwards together along the articular eminence within the upper compartment.
3. Final phase: Further rotation and translation allow maximal mouth opening. Movement is limited by the joint capsule, ligaments, and surrounding muscles.

Mechanism of Elevation

• Elevation involves the reverse sequence of depression. The mandibular condyles and discs glide backwards towards the mandibular fossae while the condyles rotate to close the mouth.

Side-to-Side Movements

• During lateral movement, the condyle on one side glides forwards and medially, while the opposite condyle mainly rotates around a vertical axis. Alternate movements on both sides produce the grinding action required for mastication.

Range of Mandibular Movements

• Maximum mouth opening: Approximately 50 mm.
• Normal mouth opening: Approximately 40 mm.
• Rotational component: It contributes approximately 25 mm to mouth opening.
• Translational component: It contributes approximately 15 mm to mouth opening.
• Maximum protrusion: Approximately 10 mm.
• Maximum lateral movement: Approximately 10 mm.

Factors maintaining stability of TMJ

• Bones: The articular eminence guides condylar movement, while the postglenoid region limits posterior displacement.
• Ligaments: The lateral temporomandibular ligament strengthens the capsule and restricts excessive posterior displacement.
• Muscles: The muscles of mastication maintain joint alignment and control mandibular movements.
• Position of mandible: The joint is generally more stable when the mouth is closed and the teeth are in occlusion.