Prostho sheet # 8 - Shadi Jarrar

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Prostho sheet # 8 - Shadi Jarrar

Post by Shadi Jarrar on 27/11/2010, 2:15 am

بسم الله الرحمن الرحيم

note : figures are not included in the topic, please download the file if u want to view the figures
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In the previous lecturers we discussed how to prepare all the surfaces of the denture (fitting ,polished and occlusal surfaces).We also determined the relationship between the maxilla and the mandible, but we need to keep this relation in the lab without any that’s why we use the articulator in the lab.

We mounted the upper trial denture on the upper member of the articulator, and the lower denture on the lower member using plaster as a separating medium between the casts and the members of the articulator.

The most primitive articulator was used to keep the models in the centric relation. (this is kept simply by adjusting the upper model on the lower just like LEGO cubes!.. no hinge is used)

Door hinge then was developed, it's a simple hinge with upper and lower extensions to fit the models on them. It is more convenient than the previous one as a hinge was used, even it keeps only one relation(the centric relation).

This articulator also doesn't represent the actual relationship between the maxilla and the mandible as it represent only one condyle while we have two condyles for the mandible.

A new addition was added to this articulator, which is the incisal pin to keep the vertical relation between the to models.(When the incisal pen is adjusted in zero, we always would have the same vertical dimension between the maxilla and the mandible)

But we can't use this with fully edentulous patient but with partially edentulous patients because wax is flexible and can be changed as we remove the models from the articulator. That’s why the articulator must be rigid to keep the relationship fixed.

The movement of the mandible is complex due its three-dimensional nature. It must be studied in sagittal, horizontal and frontal planes.

The mandible contacts with the cranium posteriorly and interiorly. The posterior guidance is the contact between the condyles and the cranium, while the anterior guidance is the contact through the teeth.

Posterior guidances are present through all movements of the mandible as there is always a contact there(no separation). But the anterior guidance is not present except if there is a contact between the teeth.(teeth are separated during mouth opening)

The occlusal plane is affected by the the posterior guidances and the anterior guidance.

But unfortunately, the movements of the mandible are not pure rotational movements, as we open our mouths for chewing for example, the condyles move from their positions, this is why they don't represent pure hinge rotation.(even though this true, the patient can achieve a pure rotational movement by rotating the mandible in the most retruded position because the condyles of the mandible there would be in the borders of contact with the cranium.. which would make the positions of the condyles unchangeable. But this movement is an opening movement and not the actual functional movement)

We can now conclude into that the movement of the mandible is a combination of a pure rotational movement and translational movements.

Newer articulators contain two condyles(like the mandible), which will consequently result in lateral movements.

Average value articulator is made according to Bonwill triangle. (read the following.

Bonwill (a scientist) measured the distance between the two condyles of several mandibles and found that the average value is 4 inches. He also found that the distance between each condyle of the mandible and the middle incisal point is 4 inches. This can be represented in a equilateral triangle which is called Bonwill triangle.

So when we mount the models using average value articulator, we use the average values that Bonwill has got, meaning that the incisal edges of the lower incisers will match about 4 inches from the condyles.

Of course this average value doesn't fit all patients, so we need to do facebow record.

We have two types of facebows: (1) Arbitrary facebow: which is less accurate, but it is more convenient and hence the most common. (2) Kinematic facebow : which is more accurate but less convenient in use because it needs two steps; the first step is to determine the rotational axis of the condyle, and the second step is to record it by the arbitrary facebow. It has two pieces one on the maxilla and the other is on the mandible, and we use a stylus to adjust it. (Dr : u can imagine it!! :P)

Another addition to the articulator is addition of magnetic rings (as in the articulators we use in our lab. ) we use them for mounting instead of using screws.

The articulators we use in the lab. are semi-adjustable articulators. All semi-adjustable articulators are able to fix facebows on them, but not all average value articulators we can fix articulators on them.(only some)

Mounting can be arbitrary ( such as mounting on most average value articulators) or by using a facebow.

Semi-adjustable articulators also have another function, which is changing the condylar angle.(condylar guidance is changeable)
Condylar guidance has two components : horizontal condylar angle which is with the horizon. And lateral condylar angle.

Protrusive record is the record used to record the horizontal condylar angle.( between the bite blocks we put a material and then ask the patient to protrude his/her mandible, then the material will have a shape of a wedge, the wedge will be thicker if the codylar guidance is steeper).

Another record is used to determine both the lateral condylar angle, which is asking the patient to move his mandible laterally, then a space will be recorded on the nonfunctional side.

Note here that we deal with an equations of 4 variables, these variables are :

-the space we've recorded on the nonfunctional side.
-the horizontal condylar angle.
-the lateral condylar angle.
-anterior guidance.(that I already have on the bite blocks)

Another type of articulators is the fully adjustable articulator, which accepts what we call Pantographic tracing; which is a more complex three dimensional tracing of the movements of the mandible done according to fixed points on the mandible moving in more than one plane.

In fully adjustable articulators, glenoid fossae are not straight as what we have in the semi-adjustable articulators, but instead, we make a different roof for movement for every patient.

The following figure shows Posselt’s Figure

While :

RCP retruded contact position
ICP intercuspal position
Pr maximum protrusion
T maximal mandibular opening
E edge to edge position incisers

Note here that the guidance between RCP and ICP is posterior guidance, but it's anterior guidance in E, but after E to Pr it's posterior guidance again.

But we also need to consider the lateral movements to make the figure three dimensional.

During the patient moves his mandible laterally, one condyle orbits around the other one in rotational movement, so that, the path of movement will be at right angle with the straight line drawn between the two condyles.(remember what we studied about the rotational movement in physics).

This is because we have two condyles inside patients' mouths, which differs from what is in the simple hinge articulator(one hinge is present). And because there is a difference between the two situations, interference in mandible movement should happen.

In other worlds, the movement inside the patient mouth(two condyles) differs from what that we have in the simple hinge articulator. Because of that, we use another accurate articulators that accept facebows to record the relations more accurately and achieve more accurate center of mandible closure.

Among all records we make for the jaws, the most important record is centric relation (75% of accuracy depends on this record). It is more important than the type of articulator.

Actually the most accurate articulator ever is the patient, but we can't work inside his mouth for days. Also we have problem inside patients' mouths, such as saliva and compressibility of the mucosa which will hinder the accuracy of our work.

In the final 10 minutes of the lecture the Dr started a revision for our mid exam. I know that the lecture is somehow complex, but I did my best to simplify it… don't forget to refer to the slides.
Shadi Jarrar
Prosto # 8
Dr. Ahmad Abdulaziz

Shadi Jarrar
مشرف عام

عدد المساهمات : 997
النشاط : 12
تاريخ التسجيل : 2009-08-28
العمر : 26
الموقع : Amman-Jordan

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