Jaw and Temporomandibular Joint

The jaw is made up of the mandible, which comprises the lower jaw, and the maxilla, which comprises the upper jaw. The mandible articulates with the temporal bone via the temporomandibular joint (TMJ). The 4 muscles of mastication produce the movements of the TMJ to ensure the efficient chewing of food. Blood is supplied to this region by the external carotid system, and the TMJ is primarily innervated by branches of the trigeminal nerve.

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Development

The pharyngeal arches are embryonic structures that serve as precursors for many structures of the face, neck, and head. The arches are composed of mesoderm and neural crest cells and are accompanied externally by ectoderm and internally by endoderm.

  • The jaw arises from the 1st pharyngeal arch.
  • Neural crest cells create the 1st pharyngeal arch.
  • The 1st pharyngeal arch gives rise to the mandible, maxilla, and muscles of mastication.
First pharyngeal arch

The first pharyngeal arch is the most cephalad of the six pharyngeal arches.

Image by Lecturio. License: CC BY-NC-SA 4.0

Bones of the Jaw

The jaw consists of 3 bones: the maxilla, the mandible, and the temporal bone.

Maxilla

  • Paired bone, fused at the midline (intermaxillary suture)
  • The upper jaw is composed mainly of the alveolar process, which forms the maxillary dental arch, housing the upper teeth
  • Does not articulate with the mandible or contribute to movements of mastication
Maxilla, mandible, and temporal bone

Anterior view of the skull:
Note the locations of the maxilla, the mandible, and the temporal bone.

Image: “The cranial bones” by Charles Molnar and Jane Gair. License: CC BY 4.0

Mandible

  • Single, horseshoe-shaped bone
  • Site of attachment for all 4 mastication muscles—temporalis, masseter, lateral pterygoid, and medial pterygoid (see below)
  • Ramus: vertical projection, site of protrusion of 2 processes separated by the mandibular notch
    • Coronoid process: anterior, attachment for the temporalis
    • Condylar process: posterior, component of the temporomandibular joint (TMJ)
  • Body: forms the chin (mental protuberance), site of attachment for muscles of facial expression
    • Base: the inferior portion of the body, features the mental protuberance (chin), mental tubercle (2, on either side of the protuberance), and mental foramen (2, opening in the mandible located below the 2nd premolar for the passage of the mental nerve)
    • Alveolar process: superior border of the body of the mandible, forms the mandibular dental arch
  •  Angle: the angle between the body and the ramus of the mandible
The mandible Jaw joint

The mandible

Image by Lecturio. License: CC BY-NC-SA 4.0

Temporal bone

The mandibular fossa is the articular surface in which the condylar process of the ramus of the mandible articulates with the skull, forming the TMJ.

Lateral view of the skull

Lateral view of the skull

Image: “Lateral view of the skull” by OpenStax College. License: CC BY 3.0

Temporomandibular Joint

Joint type and surfaces

  • The TMJ is a hinge-and-plane synovial joint
    • Hinge motion allows the jaw to open with a set axis (rotational movement).
    • Plane motion allows the jaw to move side to side (translational movement).
    • Note: The articular surfaces of synovial joints are usually covered in hyaline cartilage. The TMJ has fibrocartilage instead, to prevent degeneration.
  • Articular surfaces:
    • Condylar process (or mandibular condyle) of the mandible
    • Mandibular fossa in the squamous portion of the temporal bone
    • Articular disc: biconcave, divides the joint cavity into superior and inferior compartments

Compartments

  • Separated by the articular disc 
  • Each lined by separate synovial membranes and filled with synovial fluid
  • Superior compartment
    • Also called discotemporal space
    • Allows for translational movements
  • Inferior compartment
    • Also called discomandibular space
    • Allows for rotational movements

Supporting structures

  • Joint capsule: from the border of the mandibular fossa to the neck of the mandible 
  • Temporomandibular ligament
    • A horizontal band that covers the lateral portion of the capsule
    • Prevents lateral/posterior displacement of the condyle
  • Stylomandibular ligament
    • A thickened band of deep cervical fascia that extends from the styloid process to the mandibular angle 
    • Does not limit movement of the jaw 
  • Sphenomandibular ligament
    • From the spine of the sphenoid to the mandibular lingula
    • Limits extensive protrusion/opening
    • Note: the only ligament of the TMJ not fused to the joint capsule
  • Collateral ligaments
    • Medial and lateral
    • Connect the articular disc to the ipsilateral condyle

Muscles of Mastication

Table: Muscles of mastication
MuscleOriginInsertionNerve supplyFunction
TemporalisBroad origin from the floor of the temporal fossaCoronoid process and anterior border of the ramus of the mandibleBranches of the mandibular division of the trigeminal nerve (due to their common origin: mesoderm of 1st pharyngeal arch)Elevates and retracts the mandible
MasseterMedial surface of the maxillary process of the zygomatic bone and zygomatic archAngle and lateral surface of ramus of the mandibleElevates the mandible
Lateral pterygoidInfratemporal surface and crest of the greater wing and lateral plate of the pterygoid process of the sphenoidPterygoid fossa of the mandible
  • Bilateral action: protracts mandible and depresses chin
  • Unilateral action: swings jaw to the contralateral side (alternating unilateral contractions produce mastication)
Medial pterygoid
  • Deep, larger portion: lateral plate of the pterygoid process of the sphenoid
  • Superficial, smaller portion: palatine bone
Medial surface of the ramus of the mandible, inferior to the mandibular foramen
  • Bilateral: pushes mandible forward
  • 1 side pushes mandible to the opposite side
  • Also helps elevate the mandible

Neurovasculature

Blood supply

  • TMJ: 
    • Deep auricular artery
    • Superficial temporal artery
    • Anterior tympanic artery
  • Muscles of mastication: branches from the maxillary artery
  • Venous drainage: 
    • Superficial temporal vein
    • Maxillary vein
Lateral view of the major arteries and veins of the skull

Lateral view of the major arteries and veins of the skull

Image by BioDigital, edited by Lecturio

Nerve supply

  • Mandibular nerve:
    • 3rd branch of the trigeminal nerve
    • Motor supply to muscles of mastication
  • Otic ganglion: 
    • Parasympathetic fibers 
    • Stimulate synovial fluid production
  • Superior cervical ganglion: 
    • Sympathetic fibers 
    • Pain reception within TMJ
Branches of the trigeminal nerve

Branches of the trigeminal nerve

Image by BioDigital, edited by Lecturio

Clinical Relevance

The following conditions can affect the TMJ:

  • TMJ dysfunction: syndrome of pain and dysfunction of the TMJ and the muscles of mastication. The most significant feature of TMJ dysfunction is pain, followed by restricted mandibular movement and possibly “cracking” or “popping” noises. The etiology of TMJ dysfunction is multifactorial, being attributed to musculoskeletal, psychological, and/or neuromuscular factors. Symptomatology may be chronic and difficult to manage. Management includes analgesics, physical therapy, and cognitive-behavioral therapy.
  • Trismus: also called lockjaw. Trismus is a limited range of motion of the jaw that may be caused by a spasm of the muscles of mastication or by an inferior alveolar nerve block injection leading to hemorrhage within the medial pterygoid muscle. Trismus significantly affect the patient’s quality of life by interfering with eating, speaking, and maintaining proper oral hygiene. The condition can present with an altered facial appearance and may be distressing and painful for the patient, but in most cases it is temporary.
  • Dislocation of the TMJ: painful condition that occurs as a result of trauma or excessive opening of the mandible (e.g., yawning, dental procedures), in which the mandibular condyle becomes fixed in the anterosuperior aspect of the articular eminence. Dislocation of the TMJ most commonly occurs bilaterally. Subsequent spasms of the masseter, temporalis, and internal pterygoid muscles may result in trismus, preventing the return of the condyle to the temporal fossa.
  • Arthritis of TMJ: unilateral, degenerative disease of the TMJ. Arthritis of the TMJ is characterized by the breakdown of the articular cartilage, architectural changes in bone, and degeneration of the synovial tissues causing pain and/or dysfunction during functional movements of the jaw, leading to joint clicking or crepitus.

References:

  1. Cerny R. et al. (2004). Developmental origins and evolution of jaws: new interpretation of “maxillary” and “mandibular”. Developmental Biology. https://www.sciencedirect.com/science/article/pii/S0012160604006177
  2. Drake, R.L., Vogl, A.W., Mitchell, A.W.M. (2014). Gray’s Anatomy for Students, 3rd ed. Philadelphia: Churchill Livingstone.

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