INTRODUCTION
A chemist from Scotland noticed that certain brown sea weed (algae)
yielded a peculiar mucous extraction. He named it "algin" .
In England 40 years later another chemist S.Willing Wilding received a
basic patent for the use of algin as a dental impression material.
TYPES
• 1. Type I - fast setting
• 2. Type II - Normal setting
ADVANTAGES
1) It is easy to mix and manipulate
2) Minimum requirement of equipment
3) Flexibility of the set impression
4) Accuracy if properly handled.
5) Low cost
6) Comfortable to the patient.
7) It is hygienic, as fresh material must be used for each impression
8) It gives a good surface detail even in very wet mouths.
DISADVANTAGES
It is not accurate enough for crown and bridge final
impression
It cannot be corrected
Distortion may occur if the material is not held stationary
in relation to the tissues throughout its setting period.
MANIPULATION
Selection of a Stock Tray
5 -7 mm between tray & tissues
Just short of vestibule
Slightly beyond vibrating line
Compound on peripheries if needed
Extension only to provide coverage of critical anatomy
Handling the Material
Do not leave containers open in a humid environment
• Humidity & high temperatures cause deterioration
Recommend shaking the sachet or the jar well before using
the material
• – homogenizing the material
COLOUR INDICATION
Mixing
Working
30-45 s
Working time 1:50 min
Setting in
the mouth
MIXING RATIO
1:1
(9g of powder :20 ml of water)
+
10
Preparing - Dosage
1. water
2. powder
Upper jaw
3 scoops + 3 cups
(powder)
(water)
Lower jaw
2 scoops + 2 cups
(powder)
11
(water)
Preparing - Mixing
1. VIOLET phase –
MIXING
30-45 s
 the mixing is done by long strokes with a plastic
spatula along the mixing jar walls.
Smooth creamy consistency .
No lumps or powder should remain.
Preparing - Application
2. DARK PINK phase –
Application of substance to
the impression tray
 fill the impression tray to the
edge
 optimum impression layer is
4 – 6 mm
13
Impression – Tray insertion
3. LIGHT PINK phase –
Inserting the tray
into patient’s
mouth
14
Impression - Fixing
4. WHITE phase Fixing the impression in the mouth –
setting in the mouth
Wait to remove impression until the material is
firm
Approximately one minute after initial set
15
Removal
Lift lip up to break the seal against tissues
Remove rapidly, to prevent significant permanent deformation
Impression – Disinfection
Rinse the impression
Place in zip lock bag, pour in disinfectant to cover all surfaces
Seal for 10 minutes
Remove and rinse prior to pouring stone
Regulating Set Time
Regulate with water temperature
• recommended temperature is 23 ºC
Water/powder ratio can affect tear strength & viscosity
Impression Storage
Wrap in a damp towel
Pour within 12 minutes ; Minimizes distortion
Support impression by handle or tray until cast poured
Evaluating Irreversible Hydrocolloid Impressions
• Tray not centered over ridge
• Large voids in the impression
Evaluating Irreversible Hydrocolloid Impressions
• All critical anatomy not recorded
• No significant tissue/tooth
contact
– Hamular notches
– Retromolar pads
– Vestibules, etc.
Mixing Stone
• Weigh powder
• Measure water
• Vacuum mix
– Less time to complete than hand
mixing
– Stronger cast
Two pour technique
Pour stone into impression, Use
vibrator to flow. Slowly wait for
stone to set (~ 30 min)
Invert impression with set first
pour onto base stone.
Trimming Casts
Model moist for trimming
• Debris from trimmer will not attach
Soak the model by immersing
• in slurry water
• just base of cast contacting tap water
• Prolonged immersion in tap water can lead to erosion of the cast
Trimming Diagnostic Casts
Cast thickness; 10-12mm (0.5 inch) in thinnest part
Trim the base parallel to ridges
10-12
mm
Leave the mucous membrane reflection intact for making a custom
tray.
Boxed and trimmed with a 3mm wide by 3mm deep land area
Diagnostic casts (no land area)
• Used for making custom trays
• Land area omitted so material is easier to trim & remove from the cast
3mm
3mm
Land 10-12
Area mm
Outline of Bases for Trimmed Casts
Mandibular
Maxillary
Follow the contour of the ridges, with rounded angles.
A diagnostic cast is an accurate replication of the anatomic form
of the maxillary or mandibular dental arch showing the
relationships of the remaining teeth and the surrounding tissues.
Importance of diagnostic cast.
Diagnostic casts are supplement to the oral examination,
They permit inspection from perspectives that are
impossible to obtain when looking in the patient’s mouth.
Diagnostic casts permit extended observations and
comparasions in patients.
Diagnostic casts are used in educating the patient about
his/her dental needs and corresponding treatment plan.
Student ____________________
Preliminary Alginate Impressions
Date
/
q
q
/
Initials_______ Instructor #_____
Unacceptable
Unacceptable
q
q
q
q
q
q
q
Correctable
Correctable
q
q
Acceptable
Student
Acceptable
Instructor __________
Accurate recording of all critical anatomy
Material properly mixed as indicated by impressions
Minimum of voids
Trays properly selected and centered over the ridges
Minimal areas where trays have contacted tissues
Diagnostic Cast Evaluation
Date
q
q
q
q
q
q
q
/
/
Initials_______ Instructor #_____
Unacceptable
Unacceptable
q
q
Correctable
Correctable
q
q
Acceptable
Student
Acceptable
Instructor __________
No significant bubbles or flaws in stone
Base approximately parallel to ridge and approximately l/2 inches thick (minimum)
Evidence of a dense stone surface
Clean and well trimmed
Includes all anatomical surfaces
THANK YOU