prostho sheet # 9 - Nada Kadere

Prostho
Saturday (first lecture)

Determinants of Mandibular Movement: (revision of last lecture)

1. Posterior determinants
TMJ and its relationship with the glenoid fossa (we have to take the intervening soft tissues into consideration- what do we mean by this?....) If we took a radiograph of the glenoid fossa, will the inclination of the glenoid fossa be equal to the condylar guidance?
No, because there are soft tissues in between that do not appear in the radiograph.

2. Anterior Determinants

Any contact of teeth.
Normally, when the patient closes in centric relation, the closure posteriorly is stronger than anteriorly. So, the posterior teeth are what determine centric relation or the maximum intercuspal position. (Note that when we make a denture, we want the maximum intercuspation to be at centric relation – i.e, max intercuspation to be when the mandible is in the most physiologically retruded position.)
This is talking about centric, but what about eccentric? What determines anterior guidance?
If you close your mandible at maximum intercuspation and start to move it right and left, you will find a space between posterior teeth, so determination of the mandibular cranial relationship anteriorly is dictated by anterior teeth. This is what we call anterior guidance.
To sum up, the anterior determinant of mandibular movement is any contact between the teeth.
Some people have anterior open bite, even with eccentric movement of mandible the anterior teeth won’t come in contact so there will only be contact posteriorly. This is not the normal situation but it happens.

3. Neuromuscular System
The first 2 (ant and post determinants) gave us the limits of the movement. For example, the limits of a room are the walls, BUT we aren’t going to walk on every single area in a room. In our mouth, we have the muscle of mastication and the nervous system (which guides the muscles.) The border movements are the limits (i.e the walls) of all the possible movements of the mandible (or of a point fixed to the mandible in a 3D space.) But whether you can reach all the possible movements or not is determined by the neuromuscular system … the muscles: how they move the mandible
the nervous system: the orders they give to the muscles to move the mandible.

We have extreme movements in the mouth, but we do NOT take them into consideration when we do the occlusion of our patient simply because things will become too complicated. We are basically interested in a harmonious contact between upper and lower teeth at centric relation and 3-4 mm excursion (i.e lateral movement and protrusion of mandible).


The incisal pin-incisal table relation in the articulatior:
1. It determines the vertical dimension of occlusion
2. Gives temporary anterior guidance (we still don’t have teeth… only bite blocks!) – illeh b5alli il mandible tib3id w 3an il maxilla with eccentric movement or protrusion.
BUT this relationship is only in the lab. We always say that the patient’s mouth is the gold standard! The final anterior guidance is the teeth!

Page 21…
The cross section of the TMJ. The thinnest area is when the condyle is in the most anterio-superior position (centric relation).
Remember that the lateral pterygoid muscle is fixed to the neck of the condyle, and part is fixed to the anterior border of the disc for coordination of movement during excursion of the condyle.

You can see the ligaments that fix the mandible to the cranium which limit the possibilities of movement of the condyle in relation to the cranium (thus part of posterior guidance).
Note that when you know the limitations anteriorly and the limitations posteriorly then you will know the limitations of the 3D movement of the mandible.

Page 22….
When the patient closes in max intercuspation, there will be a little space in between ant teeth, because we said that the post closure is stronger. When the patient protrudes or does lateral movement, the mandible should go away from the maxilla. But what do we mean by go away? The mandible will move downward in relation to the maxilla (it doesn’t stay in the same horizontal location). But why does the mandible do this during excursions? For 2 reasons:
1. The TMJ is designed in a way that the condyle moves downwards with translation. When the condyle moves downwards, the mandible posteriorly will moved downwards in relation to maxilla.
2. The relation between anterior teeth is made in a way that the mandible has to move downwards with any eccentric movement. So the mandible will be a little bit inferior in relation to centric relation position with any excursion.
SOOO.. any eccentric movement is going to be accompanied by separation between the mandible and the maxilla. This separation causes disocclusion posteriorly. This happens in natural dentition and this is GOOD!
When we do eccentric movement, the feedback of the periodontal ligaments of the ANTERIOR teeth to the brain is NEGATIVE…so... Inhibition of temporalis and masseter muscles…so the force of closure of mandible at eccentric movement will be less. And this is a protective mechanism because the canine alone cannot handle all the force that the posterior teeth handle!
On eccentric movement, in natural dentition, there is still contact posteriorly (no disocclusion)! The feedback of the periodontal ligament of the posterior teeth is POSITIVE! More conttracion of masseter and temporalis… more force… more failure of masticatory system… TMJ disorders… we’re supposed to take this in occlusion later.
TO SUM UP… anterior teeth contact - inhibition of muscles
post teeth contact excitation of muscles
That’s why… at centric  force is strong
at eccentric  not too strong

ALL THIS HAPPENS IN NATURAL DENTITION!
But not in complete dentures! Because we don’t have perio ligaments!
In complete dentures…. If there is contact anteriorly.. we will have dislodgement posteriorly.
If there is contact on the right side… we will have dislodgement on the left side
We don’t want this to happen!
So…. In complete dentures we want complete contact everywhere (at centric AND eccentric movements).
How are we going to make this happen in COMPLETE dentures???
The separation cannot go completely… but we can REDUCE it. We (the dentists) determine it. You can determine the anterior guidance and you can make it shallower to decrease the space btwn the maxilla and the mandible.
The doctor then mentioned an equation about approximation and separation that we will discuss later…

Page 23…
Neural mechanisms for the control of muscles of mastication
Final station for the orders…. Is the motor nucleus! It has the cell body of the motor nerve and the impulse will go to the motor unit. The final output is affected by all brain centers that will coordinate movements. We have reflexes that come directly to motor nerves, some through brain stem , some through interneruons some through the cortex and etc… anyhow, the movements of the mandible that we normally make are in a set of orders (ingrams) and most of it is subconscious. When you want to start chewing, you tell the cortex start chewing and it happens subconsciously. This is because we have ingrams! Our complete denture patients need new ingrams to develop because his/her intraoral conditions have changed! This is why there is a learning process that must occur. Remember that the older the patient, the longer the time for learning and the slower the adaptation.
The chewing center is what controls the chewing process… it has the package of orders (which nerve is fired at any moment of time).
Page 24 and 25….
Envelope of Motion
Habitual Opening and Closing
Habitual Opening is normally a little bit vertical and some people say it has a small posterior component. Closing is more posterior than opening; this is in Saggital Plain.
In frontal plain… the opening is more or less vertical, sometimes it moves a little bit to the working side or a little bit to the non-working side. The closing is a little more lateral. If you are chewing on the right side: the mandible moves down, moves a little toward the right side, and then moves up, there is contact between teeth and finally it moves towards the midline. So the final closing of the mandible is toward the medial!
This has an imp factor on the direction of force acting on our denture! When you close your mouth, the lower denture has a force acting on it toward the buccal and the upper denture has a force acting on it toward the lingual. Is this good?
Yes it is! But why?
When the maxilla resorbs it becomes narrower, so it is good to have a force acting on it toward the lingual (medial).
When the mandible resorbs, it becomes wider, so it is good to have a force acting on it toward the buccal (lateral). Note that this is another factor which makes the buccal shelf of bone of the mandible a suitable area of support of the lower denture because the force is toward the buccal.


Let us revise the mandibular movements…
Terminal Hinge opening… the mandible opens and closes when the condyle is in the most retruded position. **Is this a functional movement? No! During chewing, we said that there is always translation of condyles. So instead of the mandible moving downwards and backwards, it moves more vertically (more downwards as opposed to downwards and backwards).** The terminal hinge movement, however, is important for us in our clinical work (even though it doesn’t happen during chewing!). During terminal hinge movement, centric relation is present! (remember we said that we want maximum intercuspation of the denture at centric relation!). But, the range of this movement is limited because there will be tension in the ligaments that connect the mandible to the cranium. This tension particularly occurs in the oblique temporomandibular ligament which comes from the temporal point and behind the neck of the condyle. This ligament will force the condyle to move forward (further translation of the condyle!) This is especially occurs during yawning when we want further translation of the condyle. This terminal hinge movement is limited to around 12 mm.

When the mandible does lateral movement, the working condyle (the condyle toward which the mandible is moving) normally stays in the same position (there is only some variation). While the non working side and the rest of the mandible will travel in circles around this axis of lateral movement. Draw a straight line to the axis and project it 90 degrees (i.e 90 degrees to the line that connects the center of rotation to the moving point). SOOO… lateral movement is a kind of ROTATIONAL MOVEMENT and NOT TRANSLATIONAL MOVEMENT because it happens around an axis of rotation.
Page 28…
When the mandible protrudes, both condyles travel forward. Will they stay in a horizontal position? NO! They will move DOWNWARDS! When they go downwards the posterior teeth will disocclude (there will be space between posterior teeth.)

Speaking about orientation of occlusion plain (remember the mandible is protruding):
If anterior guidance is steeper than posterior guidance:

If posterior guidance is steeper than anterior guidance:
(our case is bite blocks-because the anterior guidance is
zero in bite blocks)

If they are equal:
(the mandible moves down but the orientation of occlusion plain stays the same)

Note that in these 3 cases, we are not talking about what is normal or abnormal. These these are simply different cases that we have to know how to deal with and thus how to set the artificial teeth in each case (how the teeth need to be adjusted to cope with these factors.)
A question asked by a student:
When the posterior teeth are in contact.. what do the anterior teeth do? Do they give any impulses?
If there is no contact anteriorly, the perio ligament is not stimulated, so no impulses. When u close in centric, the mechanisms of the coordination of the mandible are made in a way to cause positive feedback to increase the force. The mandible slowly returns to centric relation and the force is increased in the end. When moving in eccentric movement, if you hit the canines, then there will be negative forces telling the brain to not let the mandible increase its force and WAIT! When the impulses from the posterior are greater than anteriorly THEN the force will increase and the maxilla and mandible close against each other (il mandible bishid.) Note that a lot of the TMJ disorders are due to overstimulation of muscles! This overstimulation is bad and causes problems to the stomatognathic system and can cause fracture of teeth, muscle pain, and pain to the joint and etc…

Page 29…
Right Lateral Movement.
One condyle is in its position and the other moves down.
Where is the contact between teeth during lateral movement towards the right? On the right side! (either on the canine or on all the right posteriors). On the left side there is disocclusion (because as we said, the left condyle moves downwards).
We just said that the condyle on the right stays in its position, but is this entirely true? NO!
It moves a little towards the right. We call this BENNETT SHIFT! When the non working condyle protrudes, it moves a little towards the medial. Why? Because there is a space that allows it to move medially; there is space between the condyle and the medial wall of the glenoid fossa. But what makes the condyle move medially? The lateral pterygoid muscle (which is active during translation of condyle). (remember: the insertion is on the neck of the condyle and its origin is the lateral suraface of the lateral pterygoid plate… which is located MEDIALLY!) so the muscle pulls the mandible medially…
When the non working side moves medially, will the working side stay in its place? NO… or else the mandible will bend! So the condyle will be forced to move laterally. This is called BENNET SHIFT! The Bennet shift happens to the whole mandible (the whole mandible will shift toward the working side)
the angle between the sagittal plain and the path travelled by the condyle is called the Bennet angle.
Does the Bennet angle affect the teeth? YES! Anything that affects the mandibular movement will be reflected on the teeth. (we will talk about this more later.)
Notes:
• The horizontal condylar guidance is the inclination of the condylar guidance in relation to the horizon.
• The lateral condylar guidance is the same as the Bennet angle and is condylar guidance in relation to the sagittal plain.
How do we measure the horizontal condylar guidance on the artriculator?!
THE PROTRUSIVE RECORD! We put recording medium between the upper and lower bite blocks and protrude the mandible. We then record the space between them posteriorly.
How do we measure the latereal condylar guidance??
We do lateral check bite. We move the mandible to the right and left by 4 mm. If we move the mandible to the right, then we are measuring the lateral condylar guidance of the left condyle (the translating condyle) and vice versa. So the lateral check bite records the lateral condylar guidance of the CONTRALATERAL CONDYLE (which is the translating condyle).

We need to understand all this in order to understand the correct way to set the artrifical teeth.


The End
Done by: Nada Kadri
Corrections are more than welcomed.
Good Luck!
Special thanks to Sarah Waia 