The Temporomandibular joints, muscles, and teeth, and their Functions

Primary teeth should be in normal alignmentAnd occlusion shortly after age of 2, with allthe roots fully formed by the time the child is3 years old.

After a teeth have fully erupted and have assumed their respective positions in the arches, the rapid development of the jawsis sufficient to create a slight space between some of them.

Separation of anterior teeth because of theprocess that is caused by the growth of thejaws and the approach of the permanentteeth from the lingual side.

TEMPOROMANDIBULAR ARTICULATION- An example of diathrosis and its movements are a combination of gliding movements and a loose hinge movement.

- The osseous portions of the joints are the anterior portions of the mandibular fossa and articular eminence of the temporal bone and the condyloid process of the mandible.

-Articular disc: interposed between the condyle and temporal bone. * acts as a cushion; for support * dense collagenous connective tissue

MANDIBULAR FOSSA- Oval or oblong depression in the

temporal bone just anterior to the auditory candl.

- Bounded anterior by the eminentia articularis externally by the middle root of the zygoma and the auditory process and posteriorly by the zygopanic plate of the petrous portion of the bone.

CONDYLOID PROCESS- Convex on all bearing surfaces, althoug

somewhat flattened posteriorly, and its knoblike form is wider lateromedially than anteroposteriorly.

- The condyle is perpendicular to the ascending ramus of the mandible.

JOINT CAPSULE- TMJ is enclosed in a capsule that is

attached at the border of the articulating surfaces of the mandibular fossa and eminence of the temporal bone and to the neck of the mandible.

- Anterolateral side of the capsule may be thickened to form a band

→ temporomandibular ligament

- It appears to originate on the zygomatic arch and to pass backward to attach on the lateral and/ or distal surface of the neck of the mandible.

MANDIBULAR LIGAMENT- Accessory ligament are considered part of

the masticatory apparatus, including yellow stylomandibular and sphenomandibular articulation, although they may stabilize the articular system during jaw movements.

- Sphenomandibular ligament: from the angular spine of the sphenoid bone and from the petrotympanic fissures and ends broadly at the lingual of the mandible.

- Otomandibular ligaments: connect the middle ear and temporomandibular joint.

- Articular disc: interarticular dick consists of fibrous tissue shaped to accommodate the shape of the condyle and concavity of the mandibular fossa.

- Thicker anteriorly and posteriorly and a central zone.

- Superior ad inferior heads of the lateral pterygoid fovea of the mandible w/ a part of the superior head inserting into the disk and capsule.

MANDIBULAR POSITIONS- Tooth determined position- Centric relation: defined as maximum

intercuspation of the teeth- Centric relation: jaw to jaw relation

determined by the condyles position of the mandible in w/c the condyles are in their upperment midmost position in the mandibular fossae and related anteriorly to the distal slope of the articular eminence.

- Rest position: postural position of the mandible determined largely by neuramuscular activity and to a lesser degree by the viscoelastic properties of the muscles.

Mastication functions, as well as speaking and swallowing, involve reflex contraction and relaxation of the muscles of mastication whose activity is initiated voluntarily.

The masticatory muscles concerned with mandibular movements include the lateral pterygoid, digastric, masseter, medial pterygoid, and temporalis muscle and also the mylohyoid and the geniohyoid muscles.

Origin:› Inferior head: outer

surface of lateral pterygoid plate

› Upper or superior head: greater sphenoid wing

Insertion: anterior surface of the neck of the condyle.

Note! There is an insertion of some fibers to the capsule of the joint and to the anterior surface of the articular disc.

Inervation: nerve V Action:

› The Lateral Pterygoid is anatomically suited for protraction, depression and contra lateral abduction and may also be active during other movements for joint stabilization.

Superior Head Inferior Head1. Active during various jaw-

closing movements only as chewing, clenching of teeth and during swallowing.

2. Positions or stabilize the condylar head and discs against the articular eminence during mandibular closing

1. Active during jaw-opening movements and protrusion only

2. Assists the translation of the condyle downward, anteriorly, and contra laterally during jaw opening.

Origin: from zygomatic arch to ramus and body of the mandible.

Insertion: from the region of the second molar on the lateral surface of the mandible to the posterior lateral surface or the ramus.

The superficial part of the masseter muscle is separated distinctly only from the deeper layer of the muscle at the posterior upper part of the muscle.

Partial coverings:› Platysma Muscle

Activated during firm clenching in some individuals and sometimes active in facial expressions (because it has some insertion in the orbicular muscle)

› Risorius Muscle Is affected by emotions

and is active in facial expressions.

The masseter muscle is covered partly and to a variable degree with the parotid gland tissue.

Action:› Active during forceful jaw

closing and may assist in protrusion of the mandible.

The center of lower third of the masseter muscle is about 2 to 3 cm from the anterior border sternocleidomastoid muscle, which contracts during clenching in some individuals.

Innervation: fifth nerve (masseter nerve)

Zygomaticomandibular Muscle› Origin: Inner surface of the

zygomatic arch› Insertion: Coronoid process› It may be an antagonist to

the posterior temporalis and a synergist for the lateral pterygoid muscle.

Origin: Medial surface of the lateral pterygoid plate and from the palatine bone

Insertion: medial surface of the angle of the mandible and on the ramus up to the mandibular foramen.

Action:› Elevation and lateral positioning of the

mandible› Active during protrusion

Innervation: a branch of the mandibular division of the fifth nerve.

Fan shaped muscle that passes the zygomatic arch

Origin: temporal fossa Insertion: anterior

border and mesial surface of the coronoid process of the mandible and along the anterior border of the ascending ramus of the mandible.

The anterior fibers extends along the anterior ramus almost to the third molar.

Innervation: temporal branches of the mandibular division of the fifth nerve.

The muscle has 3 component parts and appears to behave as if it consisted of 3 distinct parts.

Action:› Principal positioner of the mandible during

elevation.

Posterior Part Anterior Part

1. Active in retruding the mandible

2. Act as an antagonist to the masseter in retruding jaw

1. Active in clenching

2. Act as a synergist with the masseter in clenching

There is a tendon between the anterior and posterior digastric muscle that is attached by a hooplike strip of fascia to the hyoid bone

The attachment of the anterior digastric muscle is at or near the lower border of the mandible and near the midline

The anterior digastric muscle is covered by platysma muscle, and beneath lie the mylohyoid and geniohyoid muscles.

All of these muscles are considered to be active during various jaw opening.

Innervation:› Anterior digastric muscle: A mylohyoid branch of the

mandibular division of the fifth nerve› posterior digastric muscle: The digastric branch of the fascia

nerve