occlusion sheet # 4- lama assayed

Occlusion lec # 4
Neuromuscular function and its interaction with occlusion morphology

This lec. Is divided into 2 parts: the first part is about the anatomy of muscles of mastication and the second is about closure interferences.
As you know that determinant of occlusion are 3: 1.Anterior determinant (teeth) 2.posterior determinant and (related to the TMJ) 3.and the neuromuscular system.

First part: Anatomy of muscles of mastication
Before we start you will notice that it’s difficult to say that this muscle dose this specific function; because it’s a complex interplay between the muscles to produce this movement some muscles need to contract and others need to relax and so on so usually one muscle dose more than one function and on the other hand one function involve more than one muscle.
Definition of muscles of mastication: these are the muscles that are directly responsible for movements & positions of mandible, roughly subdivided into muscles of propulsion, retraction, lateral movement, closure & opening.
They don’t have only a Masticatory functions but also involve in speaking, yawning, swallowing & phonetics all of these involve reflexes of contraction & relaxation of muscles of mastication whose activity is initiated voluntarily.

*** Capsule of the TMJ:
It is connected to articular disk at its entire circumference forming 2 separate compartments superior and inferior

Attachment of the TMJ capsule:
Anteriorly: Attached to the anterior margin of the articular eminence.
Medially: to the sphenosquamous suture.
Posteriorly: follows deep part of mandebular fossa in front of petrotympanic fissure
Laterally: extends on outer edge of mandebular fossa, the capsule is inserted into the condyle at some distance from its articular eminence.
*This means that the capsule encloses the entire TMJ compartment and it’s attached to the articular disk from entire periphery.

Ligaments of the TMJ:
In general the function of these ligaments is: to stabilize the joint and to limit the movement of the jaw so they will participate it the border movement along with other factors like the bone …act
1-tempomandibular ligament: it is the strongest and most important ligament.
Extension: lower margin of zygomatic process of temporal bone and follows an oblique course infero-posteriorly, it’s superficial strongest fibers are inserted into postero-lateral part of the neck of the mandible.
Function: play an important part in caudal limitation of terminal hinge movement and embraces the neck of the condyle.
This means once you go into a centric relation or terminal hinge position the ligament that stabilizes your joint is tempomandibular ligament
2-sphenomandibular ligament:
Origin: from the angular spine of sphenoid bone.
Insertion: to mandibular lingula, which is at the opening of the inferior dental canal.
3-stylomandibular ligament:
Origin : from the styloid process.
Insertion: to the angle of mandible
*These 3 ligaments are suspensory ligament which are fixed and influence the parameters of envelope of motion through which your mandible moves.

Muscles of mastication
1-temporalis:
Which is the biggest and its fan shaped.
Origin: temporal fossa & fascia. Start as a fan shape and then constrict when it goes downward.
Insertion: coronoid process and the anterior border of ramus and some fibers are inserted into articular disk up to distally last molar.
Function:
Its fan shape and it’s divided into 3 parts: Posterior horizontal, middle oblique and anterior vertical (these r the direction of the fibers).
Posterior: retraction of the mandible.
Anterior and middle fibers: elevation of mandible.

• Temporalis muscle is the principle positioned of your mandible during elevation. This means when you do elevation or closure of your mandible the muscle that is going to do final positioning of your mandible in its place is Temporalis muscle.
• It also has a role in teeth clinching. “Now the overlapping starts because the muscle that do forceful clinching of your mandible is masseter muscle“.
• On closing, the contraction of Temporalis holds the articular disk in place to allow condyle to return to the disk; this means it plays a role in the stability of the disk, this is because it passes mesially and anteriorly to the joint itself therefore its contraction will stabilize the disk so it doesn’t move
2-masseter:

It has superficial and deep portion.
Superficial part Origin: ant 2/3 of lower border of zygomatic arch
Deep part Origin: deep or inner part of zygomatic arch
Insertion: it runs obliquely, inferiorly and posteriorly to the lateral surface of the ramus, coronoid process and the angle of the mandible.
• Note :There is muscle that is parallel to it from the inside it’s medial pterygoid
Function: elevation of mandible and clenching of teeth (active during forceful jaw closure), also it can help in protrusion and extreme lateral movements of mandible.
• Note : usually contraction of lateral pterygoid bilaterally produces protrusion and contraction of medial and lateral pterygoid on the same side produces lateral movement of the mandible


3-lateral pterygoid:

Origin: superior Head: infratemporal surface of sphenoid greater wing
Inferior head: lateral surface of lateral pterygoid plate. (That’s why it’s called Lateral pterygoid).
Insertion: anterior portion of condyler neck, TMJ capsule and articular disk.
Function: protrusion of the condyle while drawing the disk forward; because it is inserted on the neck of the mandible and the disk at the same time contraction of this muscle will result in movement of the condyle and the disk together.

• Note: lateral pterygoid muscle is concerned in all degrees of protrusion and rotary movement (hinge movement) of the mandible it’s always active.
At the same time (protrusion of the mandible pulls articular disk forward & assists rotatory movement of the mandible)
• Its superior head is active during various jaw closing movement only, presumably to stabilizes condyler head and disk against articular Eminence during mandibular closure. “So there are two muscles that stabilize the jaw during closure Temporalis and the superior head of lateral pterygoid”.
• Its inferior head is active during jaw opening movements & protrusion only; it is also active during lateral mandibular movements but still assisted by masseter, medial pterygoid, & temporalis.
4-Medial pterygoid:

Origin: medial surface of lateral pterygoid plate, pyramidal process of palatine bone and palatine tuberosity.
Insertion: posterior and lower part of medial surface of ramus and the angle of the mandible.
Function: protraction and elevation of the mandible, it also helps in lateral movement of the mandible and also has a role in closure of the mandible.


Suprahyoids:

There function in general is to open the mandible when the hyoid bone is stabilized, and what stabilize the hyoid bone are the infrahyoid muscles.

1-geniohyoid:
Origin: genial tubercle on the inner surface of mandibular symphysis
Insertion: anterior surface of body of hyoid bone
Function: elevation hyoid bone and tongue.


2- Mylohyoid:
It looks like a diaphragm and it forms the floor of the mouth.
Origin: inner surface of the mandible from the last molar root to the symphysis
Insertion: median raphe and the hyoid bone.
Function elevation of hyoid bone and base of tongue raising floor of mouth, it depresses the mandible when the hyoid is fixed.


3-digastric muscle:
Origin: posterior belly: mastoid notch of temporal bone.
Anterior belly: digastric fossa of the mandible.
Insertion: at intermediate tendon attached to hyoid bone
4- Stylohyoid:
Origin: posterior border of the styloid process
Insertion: body of hyoid bone at the junction between the greater and smaller horns
Function elevation of hyoid bone & base of tongue

• All suprahyoids depress the mandible when hyoid bone is fixed, mainly the mylohyoid


Infrahyoids:
thyrohyoid, sternohyoid, sternothyroid, omohyoid
Function as a group: lower hyoid bone & larynx, also stabilizing hyoid bone so that suprahyoids will assist in mouth opening.


Platysma: could help in depressing the jaw.


Buccinators: helps in mastication by pushing the bolus of the food back to the teeth.
Function compresses the cheek to help in mastication.



Innervations:

Mandibular nerve of trigeminal gives motor supply to temporalis, masseter, medial and lateral pterygoids.
Whereas the facial is sensory and give motor supply to muscles of facial expression not muscles of mastication

Vascularization:
external carotid artery gives : facial and maxillary arteries







Now the second part of the lecture
closure interferences:

first we need to know what happens during normal clouser .
The muscles that are involved during normal closure of the mandible are: anterior fibers of temporalis , medial pterygoid , and masseter all of these muscle will work in order to elevate your mandible , and at the same time suprahyoids and lateral pterygoid would relax.

Definition of closure interferences
Premature contact occurs mainly between posterior teeth causing the mandible to shift during closure.
This shift can occur either anteriorly or laterally on the same side or on opposite sides, but not posteriorly because there is nothing posterior to centric relation. :s

So now when you close your jaw you will close in centric occlusion but during doing that you will face some interferences “which are mainly posteriorly as the doctor said before) and then you will pass these interferences and close in centric occlusion without noticing anything due to memory as the doctor mentioned in previous lecture.

If we have slight interference we won’t have problems due to the adaptive capacity of the muscles but for example if we have big difference between centric occlusion and centric relation like 1cm( meaning big shift) we would exceed the adaptive capacity of our muscles, and the person will suffer from TMJ problems.

There are other types of interferences like working side and non -working side interferences we are not talking about them we are talking about closure interferences that cause forward displacement ,,,,
Forward displacement happens because when we have interference lateral pterygoid must protrude the mandible to relief the closure interference and at the same time posterior and medial fibers of temporalis cannot complete retraction of the mandible and because of that the mandible will move forward.

Closure interferences that cause forward displacement of the mandible always happen when the mesial slope of your upper cusps and the distal slope of your lower cusps interfere with your closure.

Closure interference toward the opposite (opposite to Ur interference) side that cause lateral displacement of the mandible happens when the inner incline of the maxillary buccle cusp and outer incline of the mandibular buccle cusps.

Another case that cause your mandible do deviate laterally to the opposite side when there is a premature contact with the outer incline of the maxillary lingual cusp and the inner of the mandibular lingual cusp.

Now what happens to the condyle during this shift to the opposite side?
Try to imagine that this small shift is like lateral movement of the mandible, so on the same side of the interference the condyle will move anterior to its normal closing position and on the other side the condyle will move lateral to its normal closing position.

So the muscles that are involved are:
On the opposite side of the interference: medial and lateral pterygoid.
On the same side: masseter and temporalis

Closure interference that would cause your mandible to deviate to the same side happens when there is a premature contact between the inner incline of the maxillary lingual cusp and the inner incline of the mandibular buccle cusp.

Palpation of the muscles:
You should always palpate the patient muscles of mastication and TMJ, and you palpate bilaterally to compare, extra-orally palpate masseter muscle when the patient is clenching his teeth, and intra- orally palpate medial & lateral pterygoid usually posterior to the tuberosity.


Good luck

Occlusion lec # 4
Done by lama alsayed