Type and accuracy of Impression technique:

Question 1

RPD impression techniques:

Answer 1

• Soft denture bearing tissues are displaceable at varying degrees
• Displaceability is determined by several factors, such as location, histology & anatomy, thickness, condition etc.
• At rest, the soft tissue contours are in the so-called ‘anatomic form’
• In function, occlusal loads are transferred through a tooth & mucosa supported denture base to the soft tissues, which are compressed into the so-called ‘functional form’

Question 2

RPD impression techniques-Types:

Answer 2

• Muco-static impression technique
• Muco-compressive impression technique
• Sélective pressure impression technique

Question 3

Muco-static impression technique:

Answer 3

Aim: to record the soft tissues in their ‘anatomic form’

• Spaced impression tray
• Material of choice: impression plaster*
• Other low viscosity materials (e.g. light bodied silicone)
• No border moulding

* Impression plaster is not any more used

Question 4

Anatomic form impression materials:

Answer 4

• The anatomic form is recorded by a soft impression material, such as a metallic oxide impression paste if the entire impression tray is uniformly relieved
• Depending on the viscosity of the particular impression used and rigitity of the impression tray the ridge form can also be recorded by mercaptan ruber, silicone and hydrocolloid impression materials

Question 5

Functional form impression materials:

Answer 5

• Distortion and tissue displacement by pressure may result from confinement of the impression material within the tray and from insufficient thickness of impression material between the tray and the tissues, as well as from the viscosity of the impression material.
• Therefore, the recorded form of the residual ridge which is achieved by exerting some loading whether this is done by occlusal loading, finger loading or by special designed individual trays, or depending on the recording medium is called the functional form

Question 6

Muco-compressive impression technique:

Answer 6

Aim: to record the soft tissues in their ‘functional form’

• More viscous impression material
• Close fitting custom tray
• Material of choice: zinc oxide and eugenol paste or impression wax
• Final impression may be recorded with the denture teeth set up and in occlusion (closed mouth technique)
• Custom tray is uniformly spaced to equalize compression of the denture bearing tissues throughout the area of the denture base

Question 7

Selective pressure impression technique:

Answer 7

Aim: to compress the soft tissues only in the main load bearing areas

• Viscous impression material
• Spacing of the custom tray is different depending on location
• Material of choice: zinc oxide and eugenol paste
• Alternatively: impression wax, polyether

Question 8

Selective pressure impression technique:

Selective Pressure Theory:

Answer 8

• Combines the principles of both pressure and minimal pressure techniques
• Tissue preservation + mechanical factor achieving retention with minimum pressure, which is within the physiologic limits of tissue tolerance

Philosophy of the Selective Pressure Theory:

• Certain areas of the maxilla and mandible, are by nature better adapted for withstanding extra loads from the forces of mastication
• These tissues can be recorded under slight placement of pressure while other tissues must be recorded at rest

Question 9

Accuracy of fit of the denture base:

Answer 9

Intimate contact between the denture base tissue surface and the supporting soft tissues throughout the entire area of contact

Question 10

Accuracy of fit of the denture base:

Possible sources of error (inaccuracy):

Answer 10

1. Impression procedure
2. Impression material
3. Custom tray fabrication
4. Handling of the impression
5. Handling of the gypsum during casting
6. Casting procedure
7. Gypsum setting expansion
8. Errors during processing of the acrylic base
9. Processing deformation of the acrylic
10. Errors during finishing of the prosthesis
11. Errors during handling of the prosthesis up to delivery
12. Errors in maintaining the prosthesis after placement

Question 11

Accuracy of fit of the denture base:

Reduce risk of significant errors:

Answer 11

1. Correct handling of the materials (according to instructions)
2. Careful execution of clinical stages
3. Careful execution of laboratory stages
4. Very conservative adjustments on tissue side if needed

Question 12

RPD framework design:

Answer 12

• Some movement of tooth and mucosa supported RPDs towards the tissues at the distal end under functional loading is inevitable
• This will be a rotational movement occurring around a fulcrum on the distal most abutment tooth (occlusal rest)
• By moving the fulcrum (rest) more anteriorly, a proportionally greater area of the residual ridge will support the denture base

Question 13

Total occlusal load applied:

Factors to consider:

Answer 13

• Number of artificial teeth
• Width of the occlusal surfaces
• Morphology of the occlusal surfaces

Question 14

Factors to consider:

Number of artificial teeth:

Answer 14

1. Not all missing teeth need to be replaced!
2. Consider the presence or absence of antagonists
3. Consider the horizontal space available (mesio-distally)
4. Consider the vertical space available (interarch space)
5. Consider aesthetics
6. Confirm final decision with the patient!
7. In many cases, replacing up to the first molar tooth is sufficient and acceptable

Question 15

Factors to consider:

2. Width of the occlusal surfaces:

Answer 15

Question 16

Factors to consider:

3. Morphology of the occlusal surfaces:

Answer 16