RPD Basic Design principles

Question 1

5 Components of any RPD :

Answer 1

1. Saddle
2. Support
3. Retention & Indirect retention
4. Bracing and reciprocation
5. Connector

Question 2

Saddle

Answer 2

The component or components that provide the area of where the teeth are placed and provide the flange extensions in acrylic of an RPD.

Question 3

Support :

Answer 3

The function of a component or components to provide resistance to displacement towards the oral tissues on function .

Force is transmitted to:

• Mucosa via palatal areas, saddle areas.
• Teeth via rest seats, occulsal, cingulum, incsical.

Question 4

Retention:

The function of a component or components to provide resistance to displacement away from the oral tissues on function via:

Answer 4

• Mucosa coverage: Giving aheshion / cohesion
• NB Muscular effort: Tongue, Cheeks, Lips
• Clasps – C clasps, roach clasps, I bar

Question 5

Ideal clasps:
cobalt chromium alloy

Answer 5

A retentive clasp cast in cobalt chromium alloy should be at least 15mm in length:
1. This length allows flexing by 0.25mm without permanent deformation
2. Can be achieved with occlusally approaching clasps on molars and gingivally approaching clasps on any tooth

Question 6

Ideal clasps:
wrought wire

Answer 6

A retentive clasp made by wrought wire should engage a 0.5mm undercut
1. Stainless steel and gold wire clasps are more flexible
2. Increased proportional limit allows engaging a deeper undercut

Question 7

Depth of undercuts

Answer 7

Deep undercut do provide more retention but co/cr clasps that engaged into a deeper undercut have to be more flexible to prevent permanent deformation.

• Stainless steel wrought clasps are more flexible and is favoured in this situation. If there is undercuts, e.g due to short teeth or conical teeth, it’s possible to create an undercut by the use of composite bonded or trim the teeth to create an undercut .

Question 8

Indirect Retention

Answer 8

Rest seats on the RPI system on a free end saddle will have the rest seat on the mesial aspect of the tooth. On a bounded saddle it’s common to place the rest seat next to the saddle area.

Question 9

What is the RPI system in Co/Cr ?

Answer 9

It’s short for rest, plate and I bar clasp. The rest seat is placed on the mesial aspect of the tooth with an I bar clasp placed on the tooth and retention mesh for acrylic.

Question 10

Bracing and reciprocation

Answer 10

The function of a component or components to provide resistance to lateral displacement during function via extension of flanges into the full depth of the sulcus and the reciprocal arms and non retentive components of clasps.

Question 11

What are guide planes?

Answer 11

They are prepped areas on teeth adjacent to an edentoulous area and assist with bracing and reciprocation and path of insertion .