|Design of RPD :
CLASS 1 AND 2 :
The difference btn class 1 or 2 and class 3 is that class 3 RPD is tooth supported but both class 1 and 2 are mucosa tooth supported.
In designing , we follow the same steps ..
OSCAR outline , support, connectors and retention .
Missing teeth + primary abutment+GPs+tilt determination .
*we make the tilt according to the edentulous area to avoid any interference during insertion and removal of PRD. Tilting does not create new undercuts
* GP preparation should be equal to 2-3 mm why ??
Preparation is much simpler.
Tooth substance is preserved.
Gingival health is maintained.
RPD can be inserted more easily.
Does not interfere with physiological movement of abutment tooth.
When the GP =2-3 mm contact btn the abutment & the prosthesis =2-3mm (guiding plaTe ) no firm grip bcoz the touch surface is less .
Assume that the GP take the whole surface of the tooth ! then all the prosthesis will be in contact with the abutment any movement of the prosthesis will transfer the load to the abutment and we do not want this to happen .
So in class 1 & 2 (free end saddle ) we make GPs as minimal as possible to minimize the stress and the load on the abutments , this is called stress breaking effect .
Full tooth length preparation of the GP = over load and interference .
SECOND : SUPPORT:
Derived from both : teeth & mucosa
# support derived from the soft tissue (mucosa):
Any load on the prosthesis will affect the mucosa MORE than the teeth so we should try to minimize the load that acts on the mucosa , and this is achieved by 4 ways:
1- decrease the occlusal table surface so the penetration of the food will be better and the load will be less .
How ?? use narrow teeth with small MD and BL dimentions.
Instead of setting canine then 2 premolars then 2 molars .. we just add canine then 1 premolar or 1 molar so the whole surface will be less
2-wide coverage of the residual ridge by the denture base within the functional limit wide base = distribution of the load =less stress on mucosa
The concept is just like the idea of the foot of the camel which is wide to distribute its load and avoid sinking in the sand .
3-mucofunctional concept (altered cast technique ) :
The denture base fabricated to fit accurately against functionally displaced mucosa will sink less under the masticatory load than if fabricated to fit accurately against undisplaced form of the mucosa.
-we fabricate the base to be under certain load so the sinking of the prosthesis later on (when chewing) will be 1mm instead of 2 mm for example. We'll take it nxt year .
4- indirect support : by using the clasps ( 1ry function of the clasp is retention but they may also give indirect support as a secondary function )so when we put 2 clasps we avoid the sinking of the prosthesis .
Support from the teeth:#
v.imp: if we put the rest on the distal side of the abutment as in class 3 , three problems would happen :
*class 1 lever action that is not preferable
نقطة الارتكاز = rest
When the stress loading (arrow) increase it will lead to prosthesis displacement .
*Distal displacement of the abutment tooth rotation effect on the abutment will lead to displace the abutment and create a space food impaction and periodontal disease
*less utilization of the bone for support: bcoz the bone that needed for support lies distal to supporting element which is the rest .
So the solution of these problems in this case is to put the rests on the mesial side (mesio-occlusally)
If we put the rest mesio-occlusally:
* occlusal forces will deactivate the lever action creation of class 2 lever
Here we moved the rest to the mesial side to decrease or eliminate the displacement of the prosthesis .
*Mesial displacement will enhance the mesial contact point less rotation effect on the abutment.
*more alveolar utilization for support, bcoz the bone will be mesial to the rest so it will be used more
* stress breaking effect: minimal preparation of the GPs and less contact surface btn the GPs and the abutment tooth decrease the load and disengagement of the primary abutment.
RULE: WHEREVER THERE IS DETAL EXTENTION (FREE END SADDLE) WE PUT THE RESTS MESIO-OCCLUSALLY
After we put the rests we have to draw the fulcrum lines that are established btn any 2 rests
From the fulcrum access we draw a line perpendicular to it and extends it to the teeth and put another rest (indirect retention )
v.imp: if we draw the line and it extended to the lower ant teeth making rests on this area is not acceptable , it is annoying to the patient and bcoz the lower ant teeth are steep so they will not provide that much retention .
what to do in this case is what is called trapezoidal arrangement by putting 2 rests on more capable teeth which are the premolars.(slide 36)
Like class 3 we put the minor connectors and the major connector.
Direct ;by using clasps and indirect; by preparing the rests on the line that is perpendicular to the fulcrum line.
Then the dr. showed us a case of unmodified class 2 (slide 40)
Outline: class 2 mod.: none , tilt: mainly lat. Tilt and we determine the GP.
Support: rest mesio-occlusaly ( bcoz there is free end saddle ) on 2nd premolar .
Then we draw the fulcrum line and a line perpendicular to it so the other rest will be on the 3rd molar.
Another fulcrum line btn the 2 rests line perpendicular another new rest on the 1st premolar .
Now , if we take any other fulcrum (for example: btn rest 2 and 3 the perpendicular line will be on the prosthesis ,, )
Then we draw the connectors and the clasps .
* the first choice of the lower major connector is the lingual bar but in order to use it the sulcus depth should be at least 8 mm (8mm= 5mm the width of the major connector + 3mm the distance btn the connector and the free gingival margin).
* the 2nd choice is lingual plate,
v.imp : lingual plate does NOT act as indirect retainer unless we made rests at the ends of the plate.
* indirect retention should be as far as possible from the fulcrum line
* RPI system = on the ANT. Teeth = mesial rest + plate + I bar and the reciprocal arm is the minor connecter
* from the components of the RPD we draw the minor connectors and join btn them by the major connector.
* draw the finish line btn the major connector and the acrylic base.
* never draw clasps with the I bar .
*when drawing the clasps the should surround the tooth more than 180 degrees.
* draw the tissue stop as a closed square on the 6 or 7 teeth
* the depth of the retentive component ( clasps) = 0.25 mm unless we have an I bar then the depth = 0.15 mm
*keep your design as simple as possible.
*we can make rests without clasps ( for indirect retention ) but we can not put clasps without rests !!
* the dr showed us a case:
In this case we make more than 2 rests !! why ??
- because the remaining teeth is only from 4 to 4 we need much support.
- may be the roots of the teeth are short and not enough to provide support or the anatomy of the teeth is not that helpful to give a good floor of the rest
So we tried to distribute the forces by preparing many rests .
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