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Dental Materials 3
Test 3
| Question | Answer |
|---|---|
| what is an impression? | negative replica of the teeth and or gingival tissue in the mandibular and maxillary arch |
| what are the requirements of impression materials? | accuracy: crisp and clear, spring back to original shape tear strength: flexibility on removal dimension stability handling: long working time, fast set |
| what are chemical impression materials? | chain lengthening, cross linking, thermoset (permanent) |
| what are physical change impression materials | solidification: waxes gelation: doesn't get solid like wax, more jello like thermoplastic (reversible) |
| what are the types of dental impression materials? | inelastic: rigid and inflexible elastic:; rubbery and flexible |
| what are the characteristics of inelastic impression materials? | rigid and inflexible used on edentulous patients (no undercut) |
| what are the examples of inelastic impression materials? | impression compound (thermoplastic) zinc oxide eugenol paste (chemical set) impression plaster (chemical set) impression waxes (thermoplastic) |
| what are the characteristics of inelastic impression material? | wax with added fillers, more brittle but flows mud less when compared to wax |
| what are the characteristics of inelastic impression plaster? | higher water/powder ratio set in mouth, taken out in pieces and glued back together |
| what are the characteristics of inelastic impression wax? | soften melted and formed to desired shape lack of accuracy and distort easily on removal from tissue undercuts |
| what are the characteristics of elastic dental impression material? | rubbery and flexible dentulous or edentulous |
| what are the types of elastic dental impression materials? | hydrocolloids (water based impression material) aqueous or non aqueous non aqueous: elastomers and cross linked polymers |
| what are the two hydrocolloids? | agar and alginate |
| what is agar characteristics? | thermoplastic and reversible hydrocolloid |
| what is alginate characteristics? | chemical set irreversible |
| what do agar and alginate change to? | both materials change from a viscous liquid state (sol) to a semisolid, rubbery state called a gel (gelation) |
| what is the change of agar? | changes from a rubber like gel to a liquid like sol by raising and lowering the temperature physical change no chemical reaction |
| what is the agar composition? | colloid suspension in water seaweed extract, agar agar, potassium sulfate ensure proper set of gypsum material, borax strengthener, alkyl benzoate anti fungal agent, water |
| what are the baths of the hydrocolloid condition for Agar? | conditioning chamber: makes more of a semi solid (71- 100C) storage bath:keeping it in fluid state so you can place it (60-65 C) temporing bath: cooling bath 5-10 minutes (45-46 C) |
| how do you remove agar impression from the mouth? | quick snap, DO NOT rock back and forth pour up impression immediately in gypsum |
| what is the hysteresis curve? | temperature at which hydrocolloid liquefies is not same temperature at which it solidifies |
| what temperature does sol transform into gel liquid to gel | 50 degrees C |
| what temperature does gel transform into sol? gel back to a liquid | 70 degrees C |
| what are the advantages of Agar? | very accurate impression, works well in a wet environment useful fro taking impressions when the margins of crown prep are subgingival |
| what are the disadvantages of agar? | require special equipment, unstable impression material, if stored must be 100% humidity, tears easily upon removal, can only be poured in gypsum ONE TIME |
| what are the applications of alginate impression material? | diagnostic casts, fabrication of provision restorations, fluoride and bleaching trays, mouth protectors |
| what are the characteristics of alginate? | irreversible hydrocolloid, chemical set |
| what are the composition of alginate? | potassium alginate, seaweed, calcium sulfate: ensures proper set of gypsum material, sodium phosphate: retarder to delay reaction, diatomaceous earth filler |
| what are the material characteristics of alginate? what happens to make it work | cross linking causes gelation to occur calcium sulfate dihydrate+ water = calcium alginate |
| what is the ideal water temperature for alginate? | 20 C 70 F |
| what is the working time for alginate | 2-3 minutes fast set 1.25-2 minutes |
| what is the setting time for alginate? | 2-5 minutes fast set 1-2 minutes |
| what are the advantages of alginate? | low cost, slight reaction with gypsum, good patient acceptance |
| what are the disadvantages of alginate? | minimal accuracy, low tear strength, poor dimensional stability |
| what is syneresis | alginate dries up and shrinks when left out in the air |
| what is imbibition? | alginate expands and sucks up water if they left it in a cup of water |
| steps for mixing alginate impression? | select appropriate try size, determine water/powder ratio, place water into rubber mixing bowl, fluff alginate and sift powder into water and stir until all particles wet, use stropping motion |
| steps fro placing alginate in tray and making an impression? | load impression with alginate place a small amount of alginate on the occlusal surface of posterior teeth place tray in mouth and make sure centered seat tray posterior first then anterior portion and press lightly and pull lip over, hold until set |
| how do you remove the alginate tray? | leave tray in mouth approx 2-3 minutes break buccal seal by placing finger at side of tray while pulling tray while pulling tray away from the teeth with snap (do not rock) rinse and spray with disinfectant and place in plastic bag |
| how soon must the impression be poured up in gypsum? | 1 hour |
| what is the cause of grainy alginate? | improper mixing, prolonged mixing, water to powder ratio too low |
| what is the cause of tearing alginate? | premature removal from mouth |
| what is the cause of external bubbles in alginate? | air incorporated during mixing, moisture or berks on tissue |
| what is the cause of irregularly shaped voids in alginate? | inadequate cleaning of impression |
| what is the cause of distortion of alginate? | impression not poured immediately, movement of the tray during elation, renature removal from mouth, improper removal from mouth by rocking |
| what are the types of gypsum? | plaster (white), lab stone (yellow), die stone (green blue, pink) |
| what is the most expensive gypsum? | die stone |
| what is model plaster? | mostly white, most porous form, irregular shape usually used to make diagnostic casts, treatment planning and study models |
| what is lab/dental stone? | yellow or orange, stronger, less dense more uniformly shaped resist stresses usually used on working casts or master casts in fabricating removable prosthesis |
| what is die stone? | high strength or improved stone, most uniform, densest form, hard, abrasive resistant |
| what is gypsum rock naturally made out of? | calcium sulfate dihydrate |
| what is gypsum rock in dentistry made out of? | calcium sulfate hymihydrate produced as a result of heating gypsum and remove part of water of crystallization which is called calcination |
| the larger more irregular and porous particles of powder in gypsum are... | weaker and less resistant to abrasion |
| what are the properties of plaster? | porous irregular particles, weakest and least expensive used mainly when strength is not a critical requirement |
| what is the chemistry of plaster? | calcium sulfate hemihydrate it goes from calcium sulfate hemihydrate to a calcium sulfate dihydrate |
| what is made by direct and rapid heating process in open air? | plaster |
| what is made by steam pressure in a closed container? | stone |
| what are the properties of stone? | regular and uniform in shape, less porous, stronger and more expensive than plaster and referred to as alpha hemihydrate used for casts of diagnostic purposes |
| what are the properties of high strength improved stone? | strongest, less expansive, more expensive strobe used mainly for making casts or dies for fabrication of crowns, bridge onlay/inlay |
| which stone requires the least amount of water? | die stone |
| what does surface hardness depend on in gypsum? | densitry and compressive strength |
| which stone has the greatest density? | die stone, lab stone, plaster |
| how much water do you mix for 100 g plaster powder? | 45-50 ml |
| how much water do you mix for 100 g of stone powder? | 28-30 ml |
| how much water do you mix for 100 g of improved stone powder? | 19-24 |
| if you increase the amount of water to plaster/stone... | thin mix and more time to set |
| if you decrease the amount of water to plaster/stone... | difficult to mix and may set too quickly |
| an increase in spatulation time will... | shorten setting time and increase the rate of setting expansion |
| does temperature affect dental stone? | no, not noticeable like alginate |
| what does slurry water do to dental stone? | accelerate the chemical reaction by acting as established site for crystallization |
| what does borax do in dental stone? | retarder, slows down reaction |
| what does potassium sulfate do in dental stone? | accelerator, speeds up reaction |
| what is the working time or initial setting time? | length of time from the start of the mix until the setting mass reaches a ski hard stage represents the available time for manipulating the product |
| what is the final setting time? | represents the length of time from the start of the mix until the setting mass becomes rigid and can be separate from the impression (30-45 minutes) indicates the major completion of the hydration reaction |
| what can you do to increase setting time of dental stone? | decrease mixing, higher water to powder ratio (thinner mix), add borax a retarder |
| what can you do to decrease setting time of dental stone? | increase mixing (longer mixing time, shorter setting time) lower water powder ratio (creates a thicker mix) add potassium sulfate (accelerator) |
| what are preliminary impressions used for? | models for custom trays, educational aid, permanent record for patients mouth, fabrication of temporary crowns and removable dental appliances |
| what are final impressions used for? | more accurate detail of tooth structure, used by laboratory clinician to construct restorations, dentures and implants |
| when selecting size of tray for impressions the width should... | allow 1/4 inch of alginate to flow between the tray and oral structure |
| when selecting size of tray for impression the length should... | cover the retromolar pad, maxillary tuberosity and have 1/4 inch anterior clearance of the most protruded incisor |
| what should you watch out for when taking impressions? unusual impressions? | vaulted palate, labial frenum, peripheral roll, palatine fovea, impression of uvula |
| what are the pre trimming streps? | rinse casts under water, check drain in sink and hoses on model trimmer, make sure trimming platform is parallel to the counter top, make sure trimmer wheel is wet, remember to rinse periodically |
| how should the back of the model be trimmed? | angles is perpendicular to the midline of the arch |
| how should the base be trimmed? | the base portion of the cast should be about 1/2 to 1 inch thick and trimmed parallel to the occlusal plane |
| how should the backs be trimmed? | trim the backs of both models together so that they are perpendicular to the midline of the mouth use median raphe or labial frenum as guide |
| how do you trim the maxillary cast? | anterior portion to a point angle for the anterior portion of the model should extend from the canine eminence and end at a point between the central incisors |
| how do you trim the mandibular cast? | round off the anterior portion of the cast in the shape of an arch arch should extend from the canine eminence to canine eminence |
| how do you trim the posterior angles or heels of maxillary and mandibular arch? | posterior angles or heels of the cast at right angles to an imaginary line through the cuspids |
| what should the total height of both casts in occlusion be? | 2 inches |
| how do you trim the sides of each individual cast? | trim the sides of each parallel to a line through the central groove of the premolar teeth lowest depth of the vestibule (2-3 mm buccal to the lowest depth of the vestiuble on each side) |
| what are the advantages of the rubber dam? | provides a dry, clean visible field, dark colors provide contrasting field and reduces eye fatigue, allows for more refined tooth preparation in absence of moisture from saliva while retraction cheeks, lips and gingival tissue |
| what is patient protected from while wearing rubber dam? | swallowing, aspirating small objects and debris |
| what is tissue protected from while patient wears rubber dam? | damage from rotary instruments and from dental materials which may be damaging |
| what is the clinician protected from while patient wears rubber dam? | provides barrier for prevention of microbial transmission from patient to dental team |
| what are the disadvantages to rubber dams? | placement can be time consuming, should only take 3-4 minutes to apply if experienced |
| what are the contraindications from rubber dams? | partially erupted teeth, third molars, malposed teeth class V lesions on molars, asthmatic patients or psychological reasons |
| what side should be facing the operator in rubber dams? | dull or powdered side for less reflection |
| what is the purpose of rubber dam retainers and how should it be placed? | anchors the dam to the most posterior tooth to be isolated with bow of clamp placed at distal aspect retainer should not extend into the medial or distal embrasure |
| why should the retainer not extend into the medial or distal embrasure? | edge placement difficult, seal around tooth difficult and tissue trauma may occur |
| how should you punch the hole for the rubber dam? | the hole for the tooth to be restored is punched 1.0 to 1.5 mm facial to the alignment of the other holes clamp is positioned 1 mm below lesion to avoid gingival floor collapse |
| how are wingless clamps identified | designated by a W in their identification |
| how should you sterilize the rubber dam punch? | never autoclave, bag and dry heat sterilize |
| what should you never use as rubber dam lubricant? | vaseline or cocoa butter because it contaminates the field and may cause breakdown of dam causing leakage, makes dam inversion more difficult |
| how many teeth should you isolate with rubber dam minimally? | 5 teeth |
| how do you position holes of the rubber dam? | lingually cusps of posterior teeth and cingulum of anterior teeth |
| what should you do with higher/fuller papillae in rubber dam placement? | require holes that are further apart to avoid strangulation |
| what happens if the holes are placed too close together? | cause excessive stretching of the rubber dam resulting in leaking or loss of moisture control |
| what happens if the holes are placed too far apart? | bunching of the dam inter proximally and limited access to proximal surfaces |
| what is the number 1 hole on the rubber dam punch used for | smallest incisors |
| what is the number 2 hole on the rubber dam punch used for | incisors and cuspids |
| what is the number 3 hole on the rubber dam punch used for | cuspids and premolars |
| what is the number 4 hole on the rubber dam punch used for | premolars and molars |
| what is the number 5 hole on the rubber dam punch used for | molars and clamps |
Created by:
Chobchi
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