Lecture 2 - Preliminary Impressions for Diagnostic Casts

what is the function of a diagnostic cast?

enable static and dynamic relationships of teeth to be examined without interferences present, allows dentist to view from all directions and reveal aspects not seen intraorally

what are some scenarios where diagnostic casts are necessary?

periodontal treatment and surgery planning, restorations such as veneers, diastema closings, indirect restorations on the most distal tooth in the arch, prosthodontic treatment planning, implant treatment planning, oral surgery such as tuberosity reduction

what impression material is used for preliminary impressions?

alginate

how is alginate impression material described?

made of Na/K salts of alginic acid, irreversible hydrocolloid where salts react with calcium sulfate to produce a one-way chemical reaction to form a gel

why should alginates be poured immediately after being taken?

readily absorbs and gives off liquid to the atmosphere, causing extreme distortion

what are the advantages of alginate?

low cost, accurate, faster than other materials, antimicrobial additives, dustless

what are the disadvantages of alginate?

influenced by environmental and mechanical factors, does not provide sufficient surface detail necessary for definitive casts and dies

what must the tray extensions capture when taking a preliminary impression?

all teeth, vestibule, frenum attachment, ridges, retromolar pads, tuberosities, hamular notches

what is the ideal space between the tray and teeth/soft tissue during preliminary impressions?

1/4 inch

what are some modifications that can be made to stock trays to help them fit better?

heating tray for reshaping, placement of periphery wax for lengthening, trimming with acrylic bur for shortening

what should be applied to the stock tray before adding alginate?

alginate tray adhesive

why is alginate tray adhesive used?

to prevent distortion of impression material and the cast

what does adding alginate to teeth and vestibule before placing tray in the mouth allow for?

prevention of air entrapment and helps capture extensions

how long does it take for alginate to set?

around 2 minutes

within how long should an alginate be poured after being taken?

12 minutes

what does wrapping alginate impressions in a damp paper towel and placing into a plastic bag allow for?

slower dehydration of the impression

what characteristics of an alginate impression should be evaluated after taking?

teeth and important structures present, no voids or bubbles, adequate extensions, no tray burn in critical areas

why should rubber bands not be placed around the impression and red wax when pouring up?

can distort casts

what type of stone is used for diagnostic casts?

silky rock (type IV) die stone

what is the working time of silky rock?

3-6 minutes

how long does it take for silky rock to set?

30 minutes

why should diagnostic casts be removed before 60 minutes of setting?

to prevent alginate from breaking down the silky rock

what are the criteria for diagnostic casts?

occlusal surfaces of posterior teeth properly captured with little to no voids and positive bubbles removed, anterior teeth contact areas must be pregnant (lingual of maxillary canines and facial/incisal or mandibular canines), check for distortion