RPD Manual 11 - Removable Prosthodontics - Dalhousie University

Irreversible Hydrocolloid Preliminary Impressions
Preliminary Impressions - 11
Selection of a stock tray:
1. A space of 5 -7 mm should exist between the tray and the tissues to provide bulk for strength
and accuracy of the material.
2. The tray should be just short of the labial vestibule and slightly beyond the vibrating line
3. Compound may be placed on peripheries of stock tray to extend borders if needed.
Extension should be made only to provide coverage of critical anatomy, not for the purpose
of displacing or distorting the vestibular tissues, which should be registered accurately to
obtain a peripheral seal on a denture.
Handling the Material:
1. Pre-measure material – do not take containers to your operatory. Do not handle containers
with contaminated gloves/hands. This makes infection control easier.
2. Do not leave containers open in a humid environment - humidity and high temperatures can
cause deterioration of the powder.
3. Do not mix in a bowl contaminated with dental stone - gypsum can cause acceleration of the
alginate. Conversely alginate contamination of a bowl used to mix stone can diminish the
strength of the cast or model produced. Keep separate bowls and spatulas for alginate and
stone.
Measuring and Mixing alginate
1. Fluff the powder before measuring, making sure there are no large voids in the scoop. Do
not tap the scoop more than once or twice, since this will compact the powder, and result in a
thicker mix.
2. Measuring by weight is more accurate than by volume.
3. Ratio of 1 scoop powder : l measure water.
4. 3 scoops of powder is sufficient for most arches.
5. For adjusting the setting time, regulate water temperature rather than the water/powder ratio,
which can affect strength of the impression.
6. Mix for up to 45 seconds, until a smooth creamy consistency is reached. No lumps or
powder should remain visible in the mix.
Making the impression
1. Lightly dry the teeth and mucosa. Don’t desiccate the teeth or the alginate may stick to
them.
2. Wipe alginate onto the occlusal surfaces of any teeth.
3. When seating the tray, don't bottom out on the teeth or the residual ridge, as this will result in
distortions of the tissue or movement of the teeth.
4. Wait to remove the impression until the material is firm (approximately one minute after
initial set).
5. Pull the lip up to allow air to break the seal with the tissues. This will make the impression
easier to remove. Several drops of water placed in the vestibule can also aid in breaking the
seal. Remove rapidly, to prevent significant permanent deformation.