Preliminary Edentulous
Impressions
Conventional Complete Denture
for Edentulous Patient
1. Diagnostic Impression
- Edentulous anatomy
- Custom tray
2. Final impression
- Border molding
- Final impression
- Box and Pouring master cast
3. Jaw relation record
4. Wax try in
5. Denture delivery
1. Diagnostic Impression
2. Border molding and final impression
3. Jaw relation record
- Record base, Occlusal Rim
- Articulator and Facebow
- Maxillomandibular Relationship (MMR)
4. Wax try in
- Denture Esthetic, Setting Anterior Teeth
- Denture Occlusion
- Setting Posterior Teeth
- Waxing and Festooning, Posterior Palatal Seal
5. Denture delivery
- Denture Insertion, Adjustments
Preliminary Edentulous
Impressions
Custom Tray
Preliminary impressions are needed to fabricate Diagnostic
casts. In Removable Prosth, we then use these diagnostic
casts to fabricate custom trays.
Custom Trays
• Required to accurately
register moveable mucosa
• Provides ability to create
seal for retention
Objectives
• Review materials and instrumentation needed for preliminary
impressions and fabrication of diagnostic casts for the
completely edentulous patient
• Briefly review the composition and properties of Alginate
(Irreversible hydrocolloid)
• Learn how to handle and use alginate when taking
completely edentulous preliminary impressions
• Briefly review the composition and properties of Gypsum
• Learn how to properly pour a preliminary impression
Develop the ability to self-assess the quality and accuracy of
your own alginate impressions and/or diagnostic casts
Materials &
Instrumentation
Our goal
Dentate Impression Trays
Edentulous Impression
Trays
Please do NOT use Massad
Trays for Alginate Impressions
in Pre-Clinic
Use the Regular COE-Spacer Stock trays
instead. You can take good preliminary
alginate impressions with these trays if you
mix the alginate to a dough-like consistency.
Don’t forget to apply Alginate
Adhesive!
The goal of a preliminary impression of a completely edentulous arch is
to obtain an over-extended impression of the edentulous foundation
area and adjacent tissues recording all accessible landmarks.
Alginate
Composition & Properties
Alginate Powder Composition
Component
Function
Weight %
Potassium alginate
Soluble alginate
15
Calcium Sulfate
Reactor
16
Zinc Oxide
Filler
4
Potassium titanium
fluoride
Accelerator
3
Diatomaceous earth
Filler
60
Sodium phosphate
Retarder
2
Controlling Setting Time
• The setting time of alginate can be altered by changing
the water power ratio or the mixing time. However, this
may alter the dimensional stability of the impression.
• Setting time is BEST regulated by the amount of the
retarder (sodium phosphate), i.e. Regular set vs. Fast set
• Altering the water temperature can safely influence the
setting time (One minute reduction in setting time occurs
for each 10˚C (18˚F) increase in temperature. (Favorable
method to increase setting time if Regular set is not
available).
Controlling Setting Time
*When used with Room Temperature water: 23°C / 73°F
Dimensional Stability
• Alginate has a tendency, after it sets, to lose water
(syneresis) or absorb water (imbibition). This depends
on the atmospheric conditions surrounding it.
• If conditions are dry alginate will lose water and
shrink
• If immersed in water, alginate will absorb moisture
and swell.
Alginate
Handling & Instructions for use
Irreversible Hydrocolloid:
Dispensation
Pre-weighed pouches
Bulk Material
• Easier dispensing
• Minimizes contamination
• More $$$
• Store in cool, dry airtight
containers
Irreversible Hydrocolloid:
Storage
Deteriorate if:
• Stored above 54˚C (129.2˚F)
• Repeated openings of container
Deterioration results in:
• Thin mixtures
• Reduced strength
• Permanent deformation
Measuring Powder
• Fluff (shake) the alginate
powder, scoop, tap and
flatten the scoop of powder
• Use 3 scoops for stock tray
impressions
Mixing Alginate
• Fill water measure to 3rd line
• Add powder to liquid
• Quickly mix alginate until smooth,
dough-like consistency is
achieved. (Mix should not look
spongy).
• Completed mix should not be
granular or lumpy
Mixing Alginate
Why is it important to add powder into liquid in the alginate
mixing bowl, especially when mixing fast-set alginate
impression material?
The water is added first to wet the mixing bowl and ensure
complete wetting of powder particles. If the powder is
placed first in the mixing bowl, penetration of the water to
the bottom of the bowl is inhibited and greater mixing time
may be required to ensure a homogenous mix.
Alginate Impression
Technique
• Load the alginate tray
• Use an intraoral mouth mirror instead of a finger to retract the
cheeks/lips. This provides improved visibility and increased
maneuverability.
In Clinic
Video in class only
In Clinic
Video in class only
Completed Alginate Impressions
Preliminary Impression:
Evaluation Criteria
• All detail is reproduced, including the peripheral roll and the retromolar
pads or tuberosities
• Detail is sharp, not blurred or indistinct
• Free of void in critical areas
• Free of large folds of alginate
• No areas where alginate has pulled away from the tray
• Free of rips and tears (except in interproximal areas if patient still has
teeth)
• Alginate thoroughly covers the tray, no tray visible through the alginate
• Free of bulked areas or depressions that indicate a sub-surface bubble
• Alginate is smooth, not sponge-like
Gypsum
Composition & Properties
Gypsum (Dental Plaster &
Stones)
• Powder = Calcium Sulfate Hemihydrate
• Various gypsum products require different amounts of
water. These differences are accounted for principally by
the shape and compactness of the crystals. These factors
are regulated by the manufacturer.
Mixing Gypsum
• We should always use manufacturer specifications and
measure mL of water and weigh g of gypsum powder.
• During mixing of a gypsum product with water, [to a
certain extent] extra water may be added to facilitate the
mixing and pouring processes.
Mixing Gypsum
What adverse changes may occur when the recommended
water:powder ratio has been exceeded?
As the water:powder ratio increases:
1. The setting time increases
2. The flow of the mixture increases
3. The setting expansion decreases
4. The compressive strength of the gypsum product
decreases
How to fabricate
Diagnostic Casts
Steps to obtain a Dx Cast
• An alginate impression is made directly in the patient’s
mouth producing a negative mold.
• The impression is poured in dental stone, producing a
positive cast.
• The cast is inspected for defects, repaired if possible, and
trimmed to the proper size and shape on a model
trimmer.
Pouring a cast
• Weigh powder, measure water
• Vacuum mix (less time, increase
strength of cast)
• Flow stone into alginate
impression from one end
• Modulate speed of pouring by
tilting back and forth, or pressing
the tray firmly into the vibrator
• Use a two-pour technique
Two Pour Technique
Preliminary Impressions ONLY
Two Pour Technique
Two Pour Technique
Trimming Casts
• Casts should be a minimum of 1015mm in the thinnest part
• Separate the alginate impression from
the stone cast after 45 minutes
• Trim the base on the model trimmer
parallel to the residual ridges
• Leave the vestibular reflection intact
for making a custom tray
• All anatomical surfaces should be
included with minimum voids
Finalized Diagnostic Casts
Conclusions
• Complete denture fabrication is a multi-step process that begins with
really good preliminary alginate impressions
• Handling/mixing alginate or gypsum products should be done according
to manufacturer specifications. A general understanding of the material’s
composition is necessary to understand how to manipulate the product.
• When handling/mixing alginate or gypsum, proper instruments should be
used to achieve homogenous mixes with favorable properties. Work
smarter, not harder.
• Over-extended preliminary impressions must be obtained when taking
preliminary impressions for the completely edentulous patient. This
ensures that when we fabricate custom trays, they will have adequate
extensions.
• Diagnostic casts should be accurate, clean and reflect a positive cast of
the negative impression mold
Questions?
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