Impressions

Question 1

What are some common undercut areas that should be blocked out

Answer 1

• Buccal sulcus
• When there is periodontal bone loss
• Large interdental spaces
• Beneath bridge pontics

Question 2

Why should we try to block out undesirable undercut areas

Answer 2

As addition cured silicones and polyethers are rigid once set and will flow into and become lodged into these undercut areas

Question 3

At what point in the crown procedure should we check for and block out undercuts

Answer 3

Before taking an impression

Question 4

What should we use to block out undesirable undercuts before impression taking

Answer 4

Soft wax e.g. ribbon wax

Question 5

What are some advantages of using normal stock trays

Answer 5

They’re commercially available and cheap

Question 6

Why should you not use stock trays made from flexible plastic

Answer 6

These will flex under load and distort the impression

Question 7

When are adapted stock tray used

Answer 7

Sometimes required when taking an impression of a last standing molar

Question 8

When do we need to use a custom/special tray

Answer 8

When the shape of the arch, tooth alignment or other anatomical features prevents a stock tray from seating properly

Question 9

What are the disadvantages of using a custom/special tray

Answer 9

• Additional lab procedures and costs

Question 10

Why should you not use heavy bodied silicones for crown impressions with special trays

Answer 10

If the material sets in undercuts, removal will be v difficult and may require sectioning the tray while its in the patients mouth to salvage the situation - can take hours

Question 11

What are the uses of using a triple tray for crown impressions

Answer 11

• Taking an impression of a single prepared tooth where the patient has a stable occlusion and adjacent teeth present

Question 12

What are the disadvantages of using a triple tray for crown impressions

Answer 12

• Dynamic occlusion/articulation and guidance movements for the whole arch are not replicated
• Results in a less accurate restoration morphology requiring more occlusal adjustment at the fit appointment

Question 13

Why is moisture control important

Answer 13

• Patient comfort
• Improved operator vision
• To help obtain an accurate impression

Question 14

What can be used to ensure good moisture control

Answer 14

• Wide bore suction
• Salivary ejector
• Cotton wool rolls in buccal/lingual sulcus
• Cellulose pads
• Rubber dams
• 3 in 1 air

Question 15

What are the aims of a good impression

Answer 15

• Create an exact replica of the prepared and adjacent teeth from the patient mouth to the dental lab
• Free of air bubbles, especially on the finishing line
• There must be no drags or distortion in the impression material
• Include accurate occlusal surfaces of all teeth in the arch

Question 16

Why should the mouth and tooth be dry when using addition cured silicones (elastomers) for crowns

Answer 16

The addition cured silicone is hydrophobic

Question 17

What is a light bodied elastomer used for

Answer 17

this is a fluid material that is for syringing around the tooth preparation to obtain great surface detail

Question 18

What is a medium bodied elastomer used for

Answer 18

Consistency is suitable for syringing and mass filling of the tray

Question 19

What is a heavy bodied elastomer used for

Answer 19

Only suitable as a tray borne material

Question 20

Why is a putty less accurate than a heavy bodied elastomer

Answer 20

• Putty is less flowable

Question 21

Why do polyethers need to be stored dry

Answer 21

As this material takes up water

Question 22

Why are polyethers only used in a mono phase single stage impression technique

Answer 22

As polyethers, unlike silicones, only come in one viscosity

Question 23

What are polyethers particularly suited for

Answer 23

For implant prosthodontics

Question 24

AY BAWS CAN I HABE DE NOTE PLZ

Answer 24

Polyethers have good elastic properties

Also Hydrocolloids aren’t used for crowns

Question 25

What are hydrocolloids used for

Answer 25

Opposing model and study casts (not crowns)

Question 26

What are some of the disadvantages of hydrocolloid materials

Answer 26

- Poor tear resistance - Dimensionally unstable - if stored dry, they shrink, if stored in water, they swell - Inferior impression detail to addition silicones

Question 27

What is the preferred technique for impression taking

Answer 27

2 phase, single stage

Question 28

Describe the 2 phase, single stage method of impression taking

Answer 28

2 materials of different viscosity are used to take a single impression with a light bodied wash being syringed around the tooth prep to record fine detail and a heavy bodied loaded into the tray to take up the bulk of the space between the prep and the tray Take the impression in the supine position

Question 29

What is a good tip to use if you are using a new cartridge of I assume light bodied ting but maybe the heavy one im not sure

Answer 29

Syringe a small amount of the impression material onto a mixing pad as there may be some air bubbles present leading to some of the material being in an unmixed state

Question 30

What should be done with the tip of the impression gun when using it and why

Answer 30

Always keep it in the syringed impression to help avoid air bubble formation

Question 31

How should you syringe the impression material onto the tooth prep

Answer 31

- Start around the gingival sulcus and finish lines of the prep and work your way around and up the tooth like mr.whippy ice cream

Question 32

How should you try to remove the impression from the mouth

Answer 32

Don't just lever on the mouth and try to break the seal posteriorly first and then remove the impression

Question 33

What are the main differences between the 2 phase, 1 stage and the 2 phase, 2 stage techniques of impression material

Answer 33

1. before tooth prep, take a heavy bodied impression of the arch 2. After tooth prep, syringe light bodied around prepped tooth and inside the tooth concerned in the original impression, then re-seat the impression and what for the light bodied silicone to set

Question 34

What are the main differences in the 2 stage injection moulding technique and the 2 phase, 1 stage technique

Answer 34

1. Before tooth prep take a heavy bodied impression of the arch 2. Once set, remove and drill a hole/channel through the impression material, in the area of the tooth which is to be prepared 3. After preparation, seat the adjusted heavy bodied impression tray and inject the light bodied through the channel and wait for it to set

Question 35

Where can the 2 stage injection moulding technique be useful

Answer 35

On lower posterior impressions where moisture control is v difficult to achieve or in patients who have a v active tongue

Question 36

What should you check for in the impression of the prepared tooth

Answer 36

- All prep margins should be clearly visible | - No drags or air bubbles on the prep margins, axial walls, in slots, grooves or boxes

Question 37

What should you check for in the impression of the other teeth besides the prepped one

Answer 37

- Adequate impression of the entire arch to ensure the occlusion and shape of the other teeth are recorded - Check for air bubbles on occlusal surfaces

Question 38

What can happen if there are air bubbles on the occlusal surface of the teeth

Answer 38

- It will make the cast rock and will inevitably lead to a crown being made that doesnt fit into the patient's occlusal scheme