Biomaterials- Chapter 6

gypsum

• most frequently used to make replicas of the patients mouth; called diagnostic casts, working casts and dies

• ideal for making replicas of the oral cavity (+ replication of the oral cavity)

diagnostic casts

• “study models”

• used to plan treatment, and observe the oral structure of the mouth

working casts

• “working models”

• used to fabricate appliances such as ortho retainer or bleaching trays

dies

• strongest- used for crowns and resistance to abrasion

• replicas of individual teeth or groups of teeth and are used to fabricate crowns and bridges

• generally removable from the working cast

• an impression is a negative reproduction and once poured is ?

uses of gypsum products

• diagnostic casts, working casts, and dies are not required to meet the same use stresses; therefore, they do not have the same physical property requirements

these qualities can depend on the use: gypsum

• accuracy

• reproduction of fine detail

• dimensional stability

• hardness

• strength and resistance to abrasion

• solubility

• ease of use

• cost

• color

production of gypsum products

• model plaster is the weakest- commonly used for diagnostic casts (study models)

• dental stone is stronger- is used for working casts

• die stone is the densest form of gypsum product: also known as high strength

gypsum- strength, hardness, and resistance to abrasion

• factors affect the strength of gypsum products also affect their hardness

gypsum- 2 factors contribute to the strength and abrasion resistance of the final product

• the shape of the particles and their porosity

• amount of water (the more water used, the weaker the cast will be)

dimensional accuracy

• setting expansion occurs with all gypsum products

• plaster- expands the most and high strength

• stone products- expand the least

• controlling setting expansion is critical for the production of accurate models and dies

reproduction of detail

• the greater the porosity of the final gypsum product, the less surface detail is produced

• contamination of an impression with blood, food debris, or saliva will affect the surface detail

  • all water should be thoroughly removed before pouring the impression

impression plaster

• (type 1)

• not used as much- dentures

• rarely used, but if selected, would be used as final impression for edentulous arches

model plaster

• (type 2)

• frequently used for diagnostic casts

• durable, but relatively weak cast when compared with the stone categories

• white in color to distinguish from dental stones

• least costly of all the gypsum products

dental stone

• (type 3)

• ideal for making full or partial denture models, ortho models, and casts requiring higher strength and abrasive resistance

• require less water

• around 2.5 times stronger than plaster

• easy to use and traditionally colored yellow or white

dental stone, high strength/low- expansion

• (type 4)

• “die stones”

• least amount of water for mixing

• has highest final density

dental stone, high-strength/high expansion

• (type 5)

• higher strength and can withstand the high temps

• good to use with the newer base metals bc of casting shrinkage

• “gypsum based investment)

gypsum: proportioning

• properties are linked to their water/powder ratio

• an increase in water- thinner mix- takes longer to set- so will be weaker and less accurate

• water should be measured with a graduated cylinder and powder weighed on a scale

gypsum: mixing

• “spatulation”

  • flexible rubber bowl w a metal spatula

• we want icing consistency

• measured amount of water is placed into the mixing bowl and the measured powder slowly sifted into the water

• spatulation continues until smooth, homogenous mix w a glossy surface is produced

gypsum: setting time

• mix for 1 minute- after working time begins

• as viscosity of the mix increases, the flow characteristics are decreased, and the product loses its glossy appearance

  • loss of gloss- shows it reached its initial set

gypsum: final setting time

• reached when material can be handled safely, minimal hardness

• 45-60 minutes before material can be safety separated from impression

• continue to harden and are 2-3 times harder after 24 hours

gypsum: working casts

• 2 parts

• anatomic portion

• art portion or base

anatomic portion

• replicates the hard and soft tissues/structure

• 2/3 of model

art portion or base

• aids in handling and articulating the casts

• 1/3 of model

gypsum: storage

• airtight, moisture proof container as they can absorb water from the environment

gypsum: cleanup

• equipment must be kept meticulously clean

• set particles inadvertently included with freshly mixed gypsum will accelerate the setting time

gypsum: trimming

• produce an attractive symmetric model

• bases from dental stone- soak on water for 5-10 minutes before trimming, so easier to trim

• anatomic portion, should never be soaked

• saturation of the teeth may lead to change in surface texture and may make the teeth more susceptible to chipping

study model: what should the size be

• should be trimmed proportionately

• base- 1/3

• anatomic- 2/3 of total depth

• total height- no more than 3 inches

pattern waxes

• inlay

• casting

• baseplate wax

impression wax

• corrective impression and bite registration wax

pattern and impression wax

• manufacturers produce them in several ways

• sticks, blocks, and tins

• unique colors to distinguish them in use

2 processing waxes

• boxing wax

• utility wax

boxing wax

• used to form the base portion of a gypsum model

• wax is easily manipulated at room temp

• slightly tacky at room temp, allowing it to adhere to itself to secure the boxed form

utility wax

• sticks, long square ropes, and round strips that are easily manipulated at room temp

• used in lab on impression trays

impression waxes

• used to obtain impressions of the oral structure

• for temp crowns

bite registration wax

• used to produce wax bite registrations for articulation of models

• susceptible to distortion at temps slightly higher than mouth temp and must be carefully monitored

• use in lab w study models