Steps Before Taking The Impression
- Examine patient = Assess intraoral hard and soft tissues
- Allows selection of appropriate impression tray
- Determine need for modification depending on anatomical features
- Select stock tray
- Fit loosely over dental arches (3mm gap between teeth and tray)
- Modify tray
For Maxillary, this may include- Use of blue periphery wax to place posterior seal
- Labial flange extension (but be aware of labial frenum)
- Bunsen burner - widen arch by heating walls of tray where needed
For Mandibular - Use of blue periphery wax to extend posteriorly for coverage of
all teeth and retromolar soft tissues. - Extension of tray lingually in posterior portion to capture
retromylohyiod fossa
Also we maybe required to cut tray back when exotoses are present
- Practice seating
- Apply thin layer of tray adhesive. Ensure that adhesive does not pool and limited to within the tray where the alginate will be filled.
- When ready, ask DA to mix alginate powder and water according to manufacturer’s instructions and fill tray incrementally from lateral to lateral and do not overload.
IMPRESSIONS
What?
Impressions are the negative imprint of an oral structure used to produce a positive replica of the structure to be used as a permanent record or in production of a dental restoration or prosthesis.
For complete RDP impressions, we require a negative registration of the entire denture bearing, stabilising and border seal areas. However, for fixed prosthodontics, we only need to record the abutment teeth and 2-3mm from the gingival ridge. There is no need to record oral structures as they are not required for retention of prosthesis.
Taking the Mandibular Impression
- Approach from front and insert tray into mouth with a rotational movement.
- To seat the tray we:
Anteriorly first –> ask patient to raise tongue –> reflect lower lip to prevent entrapment of air in labial sulcus –> seat posteriorly –> patient may rest tongue.
To encourage flow of alginate, jiggle tray to discourage air bubbles. - Hold firmly until set
- Break seal of impression by either running finger around margin of impression or asking patient to puff cheeks.
- Remove tray in one decisive movement vertically.
- Disinfect and store.
Taking a Maxillary Impression
- Approach patient from behind and insert tray into mouth with rotational movement.
- Seat try posteriorly first to seal impression, then anteriorly and lift lip outwards to prevent air voids.
- Massage upper lip to register vestibular anatomy
- Disinfect and store.
Some Tips
1) Why the decisive snap removal?
2) Increase ease of removal by?
3) Reduce mixing time by?
1) We remove tray using decisive movement as alginate is strain-rate dependent. By doing so in a decisive movement, we increase tear strength and reduce deformation. Easing the impression out will lead to deformation and ‘strain-dependent softening - Mullins Effect.
2) Increase of ease of removal by leaving impression in mouth a little longer as after gelation, gel strength continues to increase for a while.
3) By mixing alginate in pre-poured water, we ensure that all particles of the powder are wet evenly. The reverses can inhibit water penetration to powder at the bottom of the bowl.
Impression Taking Techniques:
Single Step
- For Primary Impressions
Single viscosity monophase technique where single type putty is inserted into tray and seated immediately after using material to ‘wash’ around prepared tooth/teeth (in order to obtain all sulcular anatomy and occlusal anatomy)
Impression Taking Techniques:
Two Step
- For Primary Impressions
aka: Putty Wash Technique
When a putty is used to take an initial preparation. The putty is placed into the impression tray and modelled so that it resembles an impression itself (rises at palate and deeper where teeth are). The tray is then seated and jiggled back and forth, side to side. Once set, remove from mouth. A light bodied material is then syringed over impression and reinserted. Once set, remove and disinfect.
Impression Taking Techniques:
Altered Cast Tray Technique/Pick Up Technique/Window Technique
- For Primary Impressions
A combination of two impression materials where we can selectively placed or minimise pressure to the anatomy being recorded.
E.g. Flabby Ridge
- A hole/window is cut on the tray over the area of the flabby ridge
would be located (most commonly on the maxillary anterior region).
- The tray is then filled with the impression material whilst avoiding the
hole. Impression is seated and the flabby ridge is moved through the
hole. Another, low viscosity, material is then used to record the
flabby ridge without disturbing the loose tissue.
E.g. Partially Edentate Patient
- Free saddles are liable to be displaced under occlusal pressure due
to displacibility of mucosa. Altered cast try technique can be used to
minimise such displacement but taking impression of mucosa under
controlled pressure….jokes i ahve no idea how this works. figure it out.
Impression Taking Techniques:
Closed Mouth Impression Technique
- For Primary Impressions
aka: Dual Arch or Triple Tray Technique
Used to take an impression of both arches at the same time. This technique can be used selectively or completely over arches.
Impression Taking Techniques:
Bite/Jaw Registration
- For Primary Impressions
To record the occlusal relationship between maxillary and mandibular teeth/jaws to articulate cast models.
Material used can include:
- Waxes (moyco)
- VPS bite registration paste (GC exabite II NDS)
- PE bite registration paste (3M ramitec)
- ZOE
Impression Taking Techniques:
Closed Mouth Technique (Neutral Zone Concept)
- For Primary & Secondary Impressions
- Requires occlusal rims to be produced from primary impression
- Patient closes onto rim and manipulates their tissue by closing,
sucking, swallowing to form the peripheral borders of the tray
Impression Taking Techniques:
Border Moulding
- For Primary & Secondary Impressions
The shaping of border areas of the impression material by function or manual manipulation of the soft tissues adjacent to the border to duplicate the contour and the size of the vestibule. This allows determination of extension of prosthesis.
Note: Unsure whether we use greenstick material or simply add impression material to vestibule before seating impression.
Demo: www.youtube.com/watch?v=1GXdMkujnz4
IMPRESSION OF COMPLETE RDP
1) Objectives
2) Requirements
1) Objectives
- Preservation of remaining natural structures
- Retention
- Esthetics
- Support
- Stability
2) Requirements
- Oral tissues must be healthy
>E.g. Is tissue inflamed? Tissue hyperplasia? As these can lead to
ill-fitting of denture
- Material must be dimensionally stable
- Impression must be removed without damaging mucosa
- Stops and handle must be incorporated to aid correct positioning of
custom tray
Impression Taking Techniques:
Open Mouth Technique
- For Primary & Secondary Impressions
- No pressure is applied
- Tray carries impression material into contact with supporting tissues
- Moved into approximate relation to peripheral tissues when mouth
is open
PRIMARY IMPRESSIONS
1) Why?
2) Materials
1) Why?
Taken for diagnostic or for construction of custom tray.
2) Materials
- Impression compound and waxes
- Irreversible hydrocolloids (alginate)
Note: In some cases, impression compound is used in combination with alginate. Though impression compound can be used alone to take a primary impression, it does not provide the best surface detail. In such situations where surface detail must be immaculate (e.g. very resorbed mandibular ridge can make the residual ridge crest difficult to distinguish from sulcus), a thin spray of alginate (alginate wash) can be used to refine the detail. (Devlin, H 2002)
SECONDARY IMPRESSIONS
1) Materials
2) Principle Approaches to taking the Impression - Mucostatic
1) Materials
- PE
- VPS
- ZOE
- Alginate
- Impression plaster and waxes
2) Principle Approaches (Devlin, H 2002)
Mucostatic Impression
- To take impression of tissues in their resting state with minimal
displacement (Zarb, GA 2004)
- Taken with minimal application of pressure while oral mucosa and
jaws in relaxed, resting position.
- No border moulding is required
- Low viscosity material is used e.g. alginate or impression plaster.
SECONDARY IMPRESSIONS
2) Principle Approaches to taking the Impression - Mucocompressive
2) Principle Approaches
Mucocompressive Impression (Devlin, H 2002)
- Impression that displaced the tissues (Zarb, GA 2002)
- To take impression of denture bearing tissues while compressed to
mimic displacement of tissues under functional/occlusal loading of
the denture
- Compression of tissues is achieved by
> High viscosity material e.g. impregnum or
> Closely adapted secondary impression tray with ZOE
> Closed mouth impression technique where load is applied to
tissues using wax record rims with acrylic baseplates
- Denture produced from this approach will:
> Cause tissue to stay compressed for period of time due to
viscoelasticity of tissues
> Retention given during function is in closest contact with tissues
> May lead to further resorption due to constant pressure
SECONDARY IMPRESSIONS
2) Principle Approaches to taking the Impression
- Selective Mucocompressive
2) Principle Approaches
Selective Mucocompressive Impression
- Restriction of forces acting on denture to the stress bearing areas.
- Achieved by special tray design where non-stress bearing areas are
relieved and stress bearing areas come into contact with tissues
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1. What & Relevance Of Prosthodontics6
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Interdisciplinary Considerations1
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2. Classification of Partially Edentulous Arches14
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3. Examination of PE Patients7
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Prosthodontic Materials32
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4. Conditions That Affect Denture Design & Wearing11
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5. Impression Taking Techique18
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Producing A Light Cured (Secondary) Tray3
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Designing an RDP27
