prostho- sheet #4 -by haitham alnoti

In The Name Of God
First of all, this lecture is a continuation of the previous two lecture. As you know:
- the materials that are used in the impression preparation procedure( the primary impression specially ) have a high viscosity(like alginate , silicone putty, impression compound) .
- impression compounds for secondary impression like ZNO eugenol , silicon like putty, poly sulfide ).
- when the tray is inserted into the patient's mouth ( the tray is on the jaw), we need to exert a pressure( pressure difference between the interior of the tray(positive P) and the external environment which is zero(= atmospheric P) ) on it to overcome the resistance of the impression material to flow . the pressure is also required to remove the excess material by pushing it to the escape way( edge) .
- if the viscosity of the material increases , we need more pressure to overcome its resistance to flow .
- as we know the special tray may be:
1) close- fitting tray
2) spaced tray
3) selectively spaced tray : mix of the previous two types.
Here, the amount of hydrostatic pressure required varies according to the procedure that is used;

- In the spaced tray, we don’t need to apply considerable pressure to distribute the material because the presence of enough space for such movement . this will give the chance to the compressed tissue to recover ( so ……..we take an accurate impression for them)

- This is not true for the close- fitting tray ( coz the small available distance makes the material movement difficult),and the tissues come in contact here are the compressed one on the primary impression(such contact doesn’t occur in the spaced tray)
- In the spaced tray we need to use stoppers to ensure that the tray will set properly in its place that is determined by the previous impression because there is no contact here)
- When the tray takes its place properly, we don’t need to keep the same pressure ( we have already used it) because the pressure required to keep the tray in place is less. (this doesn’t mean to leave it without any pressure until it reaches the sitting state).
- According to the ( response of the denture bearing tissue to the pressure):
# the elasticity of the oral tissues(viscoelastic behavior) varies with age.
# note the tissue reaction to the applied pressure ( according to its thickness, it compressibility will be determined). Note: tissue reaction is time dependent in part and independent in another part.
# the first drop in the tissue thickness is elastic behavior ,and it is time independent.
# so, we can say that (this diagram somewhat contain 2 phases :
-time independent phase
- time dependent phase( the compressibility increases with time if the pressure remains)
# with age the time required for tissue to recover increases because the rapid fluids exchange between the circulation and tissues in young .( note the figure in the doctor's slides). So, dentists ask elderly patient to move their denture out for at least 24 hours before the engagement to give the tissue a chance to relax before the secondary impression.
# viscoelastic behavior : the tissue recovery after the compressed force removing .
- If we look carefully on the impression, we will note the following:
# in the upper arch, the higher pressure( we exert) is at the center ( rogue area, ……)
# in the lower arch , the higher pressure is on the (tray) periphery .
- Depend on the amount of pressure exerted on the mucous membrane surface by impression compound, There are two impression techniques for mucous membrane: ( by which we can record the soft tissue to be suitable with the denture)
1) Muco-static( roughly without any considerable pressure).
2) Muco- compressive.( with a pressure that comes in different degrees range from low to high pressure).
- The reaction of the mucous membrane to the pressure depend on:
1) Firmness of the mucous membrane.
2) Thickness of the mucous membrane.
3) Availability of the bone support.
- If we make the impression at rest ….. the resultant denture(impression) will have all the normal contact points with the oral tissues ( so, we get a good retention =cohesion + adhesion and peripheral sealing ) .
- If we take it at the compressive state of the patient's mouth …….the contact will be with all of the compressed structures ……(there are some spaces in the incompressible tissues areas).

- Actually we need to leave spaces for the highly compressible areas (to give a chance for the tissue recovery and to avoid trauma )as well as the highly incompressible area( to give the denture the ability to be stable in its seating).
- Relieve ( previously defined) can be:
1) Direct relieve by put the impression inside the patient mouth to take the impression and leave the required space ( you control the impression to leave spaces by carving the in that area to leave a space).
2) Indirect relieve : in this procedure we record the compressed areas( so the incompressible areas will be away from the tray without any addition steps to leave the space like the previous method).
- That's good, but the question is how to determine the suitable method to record the mucous membrane surface?
# if the patient tissue is highly ( or relatively) compressible …..we use the muco-static procedure .
# if the patient tissue is highly incompressible ……..we use the muco-compressive procedure.
# if the patient tissue locate in an intermediate state the decision relates the operator to choose the suitable method in his opinion.
- We can reduce the required pressure by venting the tray( make another escape way for the material in the middle of the tray( this is useful to reduce pressure on the highly compressible areas when it presents with in compressible one) .
- In our working, we need to remember that:
# the shape of the tissue we record is a 3D shape.
# when you choose the critical bearing tissue for the denture , you need to put their function and movement in your consideration( e.g.\ we can't use tongue coz its wide range of movement which will cause the denture to omit its place).
- Also, we should take the undercut area in our consideration when we choose the impression material so, that;
# if the undercut area of the soft tissue is minor ……. We can use a rigid material.
# if it is severe …………. We should use flexible material.
# if it is bilateral……….. We should use flexible material.
- Also ,we should know that:
# alginate is a one shot material( can't be edited ) .
# in the spaced tray we can use stopper anywhere except at the periphery ( it must be free for connection).
- Note: go back to the slides to study the 10 points that is required to achieve a successful impression( any additional points to them are concluded in this sheet).
- some notes on the slides :
# slide 60: very compressible fat tissue results from long period of wearing and eliciting of the denture.
# slide 61: compressible tissue on the lower arch that can be treated surgically to give more support to the ridge , on the other hand it causes a reduction in the retention, developing of the arch.
# slide 62: very compressible parus(check it's spelling please)palatine.
Slide 63: torus mandibularis.
Slide 68: we put alginate on areas that is difficult to be reached .
Slide 72: beading and boxing.
Note : 1)we may use wax( or polyether) to make the tray long enough to reach all the region we want just with alginate impression material coz other thermoplastic materials cause its melting.
2)we make windows in the wax for spacing goals

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To have the knowledge the human need to study ,but to have the wisdom he need to observe.




Done by : haitham alnoti