Problems with impressions for indirect restorations.

Question 1

The ideal impression?

Answer 1

1. Uniform, homogeneous mix
2. Tray sufficiently filled
3. Thoroughly applied adhesive
4. Rigid, sturdy tray
5. No voids or pulls at the margin
6. Detailed margins with no tears or rough surfaces
7. No tray show-through
8. Good blend with heavy and light body
9. Strong bond between material and tray
10. No tooth contact with the tray
11. Lab aware of impression material

Question 2

What is the clinical technique for taking an impression of an indirect restoration? (preparation of the tray)

Answer 2

Choose correct stock tray size
note you can use lower stock tray to take upper or lower impression for indirect restoration
Apply PVS adhesive to tray
Wait few minutes/use air from 3 in 1 until adhesive dries as per manufacturer instructions
Locate and apply the:
Heavy bodied PVS impression material

Light bodied impression material:
Light bodied PVS impression material cartridge and dispensing gun
Standard yellow tip plus fine yellow tip

Question 3

What is the clinical technique for taking an impression of an indirect restoration? (taking the impression)

Answer 3

Dispense light body around crown prep margins
At same time, assistant to dispense heavy bodied impression material from machine into the previously prepared stock tray
Seat tray into mouth (either upper or lower arch). To be seated over previously applied light bodied material and all teeth in that arch
Do not remove the impression until the material has fully set

Question 4

Common faults?

Answer 4

Lack of impression detail
Inhibited or slow setting impression material
Voids on the margin
Tearing at the margin
Tray tooth contact
De-lamination
Poor bond of impression to tray
Stone model discrepancies

Question 5

Causes of lack of impression detail?

Answer 5

Impression material stored at elevated temperature
Material stored at too low temperature
Thick saliva/blood around the prep
Inadequate retraction of gingival sulcus
Excessive working time
Inadequate disinfection affects surface quality
E.g. incompatible disinfectant solution

Question 6

solution to lack of detail?

Answer 6

Store impression material at room temperature
Remove blood and saliva prior to impression
Good retraction technique* (see next slide)
Follow manufacturers working time specification (work a bit faster!)
Choose material with longer working time if required
Use water-based disinfectants

Question 7

Good retraction technique?

Answer 7

Gingival retraction cord kit:
Cords of different widths
Astringent/Haemostatic agent

Gingival retraction cord packer instrument
Note: use a flat plastic if none of these available

Question 8

Inhibited or slow setting impression material cause?

Answer 8

For PVS materials:
Contamination with latex gloves
Residues from temporary cements
Inadequate mix
Incompatible light and heavy body materials
Material past expiry date

Question 9

Solution to inhibited or slow setting impression material

Answer 9

Do not wear gloves which may contain chemicals likely to interfere with setting
Check expiration date of material
Ensure the materials are compatible

Question 10

Causes of voids

Answer 10

Air trapped in light-bodied syringe
Air trapped while filling tray
Improper syringe technique
Inadequate coverage of margins with light-bodied material
Blood and saliva contamination around the prep

Question 11

Solution to cause of voids

Answer 11

Impression technique:
Properly ‘bleed’ light body syringe before use
Keep tip immersed in material when loading on to prep
Continuous loading of syringe when working i.e keep material flowing out of tip
Stir while syringing
Push material forward
Cover prep and abutments with wash material
Good moisture control techniques
Use appropriate haemostatic agents/astringent around the prep

Question 12

Causes of tearing at the margin

Answer 12

Expired impression material
Inadequate mix
Insufficient retraction

Question 13

Solution to tearing at the margin

Answer 13

Ensure the materials are streak free
Check expiration date of impression material
Bleed before applying mix tip to ensure even dispensing
Displace tissue to allow material to access prepared area
Use proper/ appropriate mix tip
Consider two cord retraction if required

Question 14

Causes of tray tooth contact

Answer 14

Prep contacting side of tray
Insufficient impression material

Question 15

Solution to tray tooth contact

Answer 15

Use proper tray size
Try in trays before selecting
Fill tray adequately
Correct positioning and placement within mouth

Question 16

Causes of de-lamination

Answer 16

Exceeding the working time
Impression material stored at elevated temperature
Light body and heavy body materials incompatible

Question 17

Solution to de-lamination

Answer 17

Follow manufacturer`s working time specification
Choose material with longer working time
Store material at correct temperature
Check the material compatibility and best before date

Question 18

Causes of bond of impression to tray

Answer 18

No tray adhesive used
Incompatible tray adhesive used
Inadequate drying time

Question 19

Solution to poor bond of impression to tray

Answer 19

Use appropriate adhesive
Follow manufacturer`s instructions for drying time

Question 20

Causes of stone model discrepancies

Answer 20

Tooth contact with impression tray
Cast not made properly and lacking detail

Question 21

Solution-stone model discrepancies

Answer 21

Fill tray adequately
Provide lab with as much information as possible, type of impression material and, if required, disinfectant.