L2 Principles of impression making and complete denture retention

Question 1

What are the 8 major principles of impression making?

Answer 1

• Must have a reason to take an impression
• Tissues should be healthy wherever possible
• Impression should include all teeth and supporting tissues (material should flow and fully reach relevant holes/grooves/sulci)
• Borders must be in harmony with the anatomical and physiological limitations of the oral structures
• Proper space for impression material in the tray
• Impression tray must be removed without damaging mucoca, teeth or residual ridges
• Guiding mechanism should be provided for correct positioning of the tray (handle)
• Tray and impression should be dimensionally stable materials

Question 2

Where should you stand for max. and mand. impressions?

Answer 2

• Maxillary: behind the patient
• Mandibular: in front of the patient

Question 3

How should the tray be placed in the mouth?

Answer 3

Should be rotated into the mouth.

Question 4

Why are impression trays perforated?

Answer 4

Improves retention of the material

Question 5

Why are adhesives used?

Answer 5

Placed on the tray to ensure the impression material adheres to it.

Question 6

What tips should be remembered when using adhesives?

Answer 6

• Don’t use adhesive directly from the bottle (cross infection)
• Adhesives are material specific e.g. alginate adhesive
• Universal adhesives are only universal to the specific material type they belong to

Question 7

Should you size up or down if you’re unsure of which stock tray to use?

Answer 7

Use larger size.

Question 8

How would you modify a stock tray for a partially dentate patient with free end saddles?

Answer 8

Place impression compound or clinical putty in the stock tray in the edentulous region.

Question 9

How is impression compound used?

Answer 9

Use hot water to alter shape of compound, line bowl with paper towel.
Place alginate on top of compound.

Question 10

What material is most commonly used for primary impressions in edentulous patients?

Answer 10

Silicone and compound materials more commonly used because they are thicker than alginate and can displace soft tissues easily.

Question 11

Why are primary impressions not sufficient to make dentures?

Answer 11

• Using stock trays means the impression will be either under or over extended
• There will be impression inaccuracies

Question 12

How do we assess an impression?

Answer 12

Check the following:
- Is it centred?
- Is it under or over extended?
- Is the gross detail correct?
- Are there any defects? Are these defects in critical areas?
- Does the impression need to be remade?

Question 13

How are impressions disinfected?

Answer 13

• Rinse with water to remove blood and debris
• Immerse in a suitable disinfectant (dependent on material) e.g. glutaraldehyde, chlorine-based products
• Rinse
• Label and dispatch

Question 14

What is the next stage after a successful primary impression?

Answer 14

Creating a master impression using a special tray.

Question 15

Describe the process of making a special tray impression.

Answer 15

• Special tray is made from models of the initial impression
• Assess tray, check it’s not over or under extended
• Overextended: trim
• Underextended: use greenstick to modify
• Modification often required around frenal attachments
• Repeat border moulding of the cheeks/lips, make them make “oo” and “ee” noises, lift tongue
• Mandibular arch: get patient to stick tongue out, move side to side and try to swallow
• Check material has set before removing
• Inspect impression for air blows, distortions and check all relevant anatomical landmarks are recorded

Question 16

How is greenstick used?

Answer 16

• Warm in hot waterbath (if material is to be applied directly to the mouth) or above a bunsen burner (useful for re-warming localised areas of material)
• Warm material until it’s softened, rotate as it’s heating so it doesn’t drip
• Stick should be rough and round (not sharp)
• Apply full thickness of greenstick to tray borders
• Encourage the material back around the tuberosities of the upper teeth

Material is generally placed on buccal and lingual areas outside of tray, or inside of tray in the retromolar pad and post dam areas.

Question 17

What is the working time of greenstick?

Answer 17

10-15 seconds from the time it is heated before it will cool

Question 18

Why are master impressions helpful?

Answer 18

• Record greater detail and accuracy than primary impressions
• Records functional depth of sulcus
• Chance to make sure impression is border moulded effectively

Question 19

How should you approach mobile teeth?

Answer 19

• Extract and allow healing if you beleive taking an impression will lead to extraction anyway
• Use alginate, less likely to cause issues
• Use perforated tray
• Smear petroleum jelly on mobile teeth
• Gently remove
• Block out undercuts with soft wax/silicone
• If mobility is extreme, splint the mobile tooth to firmer abutments (adjacent teeth) with composite resin

Question 20

Define mucostatic techniques.

Answer 20

Tehcniques that mean the denture fits better at rest (ie no occlusal contacts), but during occlusal contacts forces are not evenly ditributed across the ridge- risk of uneven resorption.
Making an impression of the teeth and tissues in an undistorted way.

Question 21

Define mucocompressive techniques.

Answer 21

Ridge is compressed as it would be during occlusion, so when there is occlusion the forces are evenly distributed across the ridge, but at rest the denture doesn’t fit exactly.

Question 22

Is mucocompressive or mucostatic better?

Answer 22

No definitive evidence for either option.
If there is mobile/flabby tissue mucostatic impression is recommended.

Question 23

How does retention differ between partial and complete dentures?

Answer 23

• Partial dentures are retained through direct retention of the alveolar undercuts or clasps on adjacent teeth
• Complete denture retentive forces act through the occlusal surfaces, impression surfaces and polished surfaces

Question 24

Describe each factor which complete denture retention depends on.

Answer 24

• Occlusal surface: even and balance occlusion is preferable
• Polished surfaces: denture flanges should be smooth so saliva can make a seal (xerostomia reduces retention)
• Impression surfaces: should be in close contact with alveolar ridge

Question 25

Describe the musuclar forces involved in complete denture retention.

Answer 25

• Forces exerted by lips, cheeks and tongue on polished surfaces, and muscles of mastication indirectly on the occlusal surfaces
• Teeth should be placed in neutral zone- place of balance between buccal and lingual sides of soft tissue
• If teeth aren’t in the neutral zone they will push against the muscle, and the muscle will push back thus displacing the denture

Question 26

Describe the physical forces involved in complete denture retention.

Answer 26

• Adhesion and cohesion increase with the quality and quantity of saliva
• Retention relies on the presence of an intact film of saliva between the fitting surface of the denture and mucosa
• Creates seal with sulcus
• Facial seal also important in keeping denture in place, not just the seal with the sulcus