First: outline



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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 .

FIRST: OUTLINE:

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)



THIRD: CONNECTERS:

Like class 3 we put the minor connectors and the major connector.



FOURTH:RETENTION:

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 .

NOTES:

* 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 .

Afaf Haddad …


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