Dental
plasters
Introduction:
- dental plaster is used from any thousands years
- as first plaster was used for dental practice the gypsum plaster
or „plaster of Paris“ ( the name is from period of renaissance Renaissance - the period of European history at the close of the Middle Ages
and the rise of the modern world "a cultural rebirth from the 14th through the
middle of the 17th centuries“)
- it was the better plaster for works of art
Chemical composition:
- from chemical view has a plaster simly composition:
- it is a beta polyhydrate of gypsum: CaSO4 . 1/2H2 O
- it is produced by the process of dehydration of gypsum
- chemical reaction is following
CaSO4 .2H2 O
CaSO4 . 1/2H2 O + 1/2H2 O
- the gypsum is common white or colorless mineral used to make
cements and plasters, especially plaster of Paris
- from chemical view can be divided :
1. plaster of Paris ( soft plaster)
2. hydrocal plaster ( hard plaster)
3. densite plaster ( super hard plaster)
Plaster of Paris – general information:
- it is the basic type of plaster, which is important for making
other types plasters
- the chemical base is the dehydrating of gypsum and so create
the beta polyhydrate
- this anhydric type is adapted plaster of paris for dental use
- the dehydration is by the teperature about 130°C
- after it, the beta polyhydrate is beating and separating
- for the adaptation of feeding speed is added the regulators of
fedding
Hydrocal plaster (alfa plaster) – chemical
characteristic:
- this type of plaster creates by the vulcanisation at the high
temperature and at the high atmospheric pressure
- the base of this plaster is the alfa polyhydrate
- this plaster is more hardness than plaster of paris
- chemical and physical properties are similar, but atomic lattice
is different
- the beta plaster has a high porosity, but hydrocal plaster has
nonporosity crystals
Hydrocal plaster (alfa plaster) – physical properties:
- the mixing rate is V/P 30-60 ml water for 100g plaster
- in prosthetic is used for producing of models
- these models are easy to produce, they stiffen with optimal
speed ( no quickly,no slowly), they have a good duplicating ability
and volume stability
Hydrocal plaster contain after dehydrating:
- 95-98 alfa polyhydrate of gypsum
- rest is insoluble mineral ingredients
Densite plaster ( superhard) :
- it is produced from the hydrocal plaster by the high temperature
115°C-130°C and high pressure
- or it is producwd by the temoerature 115°C and atmospheric
pressure in solution of calcium chloratum with ingredients of
mineralisating (mineralisators)
- they are alcalic citran and maeinat, which can change little
crystals on large square ( it is the base of atomic lattice of densite
plaster)
Setting of plaster:
- Setting is exotermic reaction, it is hydration:
CaSO4 . 1/2H2 O + 1,5 H2 O
CaSO4 .2H2 O + teplo
- by the setting is mixed the powder with water and can arise the
supersaturad solution
- from it can crystalizze dihydrat
- at this reaction plastering slush is setting and begin the heat z
- accelerating this reaction can be secured with the :
- hot water
- chemical regulators of setting
- change of mixing speed (intensive mixing)
- changen of mixing rate
-
Setting of plaster:
-chemical regulators can change the solubility, nucleous and
crystalizzing speed
- for the acceleration are used the na urýchlenie sa používajú
sírany, chloridy, vodné sklo alebo mletý sadrovec
- for the deceleration is added borax, citric acid, casein, alginates
and tanin
- these regulators can change the character of crystals and so
change the basic properties of plaster
- by the setting begin the volume expansion
- the condition of this expansion is mashing of rising crystals
- the name of this expansion is hydroscopic expansion, because
it is in water solution
Physical properties:
- at the drying, feeding of plaster models begin the softly
contraction and so is compensate the volume expansion at the
setting
- by this reaction is increased the hardness and strength of
plaster models
- the hardness is depend on character and large of crystals
Physical properties:
-porosity – the main physical properties, which can change the
hardness of plaster models
- can be divided: macroscopic porosity and capilary porosity
- macroscopic porosity is depend on creation of air pocket, which
begin at start of mixing water and powder
- for removing this air pocket can be used the vibration unit and
mixing in vac
- by the vibration can be decreased the macroscopic porosity
about 40%
Clinical distribution of plaster, ISO 6873:
- Clinical distribution of plaster according to ISO 6873 is
following:
- typ I: impression plastero
- typ ll: plaster of paris
- typ lll: hard plaster ( hydrocal plaster)
- typ lV: superhard plaster ( density plaster)
- typ V: superhard plaster with high expansion
Clinical characteristic:
-the cause of diverse mechanical properties of individual plaster
type is the makroscopic porosity, which is depend on free water
in first phase of mixing
- it was investigate that for hydration of 100g plaster of paris is
necessary 18,6ml of water
- but for origin of slurry plaster matter is necessary 60 ml water
(from this water volume is used only 18,6ml for own hydration
and rest of water, 41,4 ml stay without chemical change and this
rest make the macroscopic porosity , 50% porosity, and
hardness is only 6 Mpa
-at the superhard plaster is mixed rate the 22ml water for 100g
plaster
- only 3,4 ml is free water from it result, that porosity is 10% and
hardness is 49Mpa
- different in mechanical properties between plaster of paris and
superhard plaster is tenfold
Clinical characteristic:
- the cause of these differents is diverse of large and the shape
atomic crystals both type of plasters
- the plaster of paris is adorned with a crystals in shape of
needles and this atomic lattice need for mixing a lot of water
- superhard plaster is adorned with a squars and for mixing on
the slurry plaster matter is need a very little volume of water
- if is used a lot of water for mixing plaster , the hardness is
decreased and in mixed plaster is a lot of free water
Clinical characteristic impression plaster:
- the impression plaster, type I – is not used
- it is replaced by elastomers
Plaster of Paris:
- it si necessary for preparation situeted, diganostic a working
models in prosthodontic
- models for the base of complete denture
- hardness of this type plaster is increased adition of 20-25%
hydrocal plaster by
- the ixing rate is V/P 50 ml , hardness is 15 Mpa
Hard plaster, typ III:
- it is alfa polyhydrate os gypsum
- mixing rate is V/P 30
- volume expansion at setting is 0,15 %
- hardness is 20-30 Mpa
- this plaster is used for producing of workimg models for
complete and partial denture, orthodontic aparats, individual
impressions tray and wax rims
Superhard plaster, type IV:
- it is the best plaster according to mechanical properties
- it is produced from gypsum and into is added mineralisators
(elements for working large crystals) and regulators of setting
- mixing rate is V/P 18,5-24
- hardness is 60 MPa
- volume expapnsion at the setting is 0,1%
- according to high hardness and mechanical resistance it is a
good type for working models at fixed bridge and crown
Plaster type V:
- it is superhard plaster with high expansion
- it is used in USA for compensation of melting contraction of
cobalt and nickel alloys
- mixing rate is 18,6
- high expansion at setting is 0,3%
Producing of plaster models
in prosthodontic
Modelová technika:
Požiadavky na kvalitu modelovej sadry:
a)
- objemová stabilita
b)
- dostatočný čas na spracovanie
c)
- presná reprodukcia detailov
d) - žiadne dodatočné zmeny po stuhnutí spôsobené kontaktom
s odtlačkovým materiálom
e)
- hladký, neporézny povrch
f)
- dostatočná pevnosť v tlaku a ohybe
g)
- dostatočná tvrdosť materiálu ako ochrana pred
poškodením pri manipulácii s kovovými odliatkami
Zhotovovanie modelov:
Modelový materiál sa musí vyberať tak, aby súčet objemových
zmien
odtlačkových
a
modelových
materiálov
bol
vykompenzovaný.
Odtlačkové materiály sa spravidla zmršťujú a preto modelové
materiály by mali byť expandujúce.
K modelovým materiálom patria:
- sadra
- formovacie hmoty (na zhotovenie lejacích modelov pri
odlievaní kovových konštrukcií snímateľných náhrad)
- syntetické živice s plnivami
- galvanoplastická meď a striebro
- ľahkotaviteľné zliatiny
Zhotovovanie modelov:
Vlastnosti modelových materiálov:
- rozmerová stabilita
- presná reprodukcia detailov
- odolnosť voči odretiu
- pevnosť v ohybe a lome
- jednoduché spracovanie
- kompatibilita s odtlačkovými a modelovacími hmotami
- svetlá farba kontrastujúca s modelovacími hmotami
Zhotovovanie modelov:
- model je nenahraditeľnou pomôckou pri plánovaní najmä
zložitejších protetických náhrad, pri čeľustnoortopedickej liečbe
a taktiež aj pri niektorých chirurgických výkonoch.
Rozlišujeme model:
1. študijný – je zhotovovaný z mäkkej alabastrovej sadry
2. pomocný - z odtlačku antagonistov
3. pracovný – z detailného odtlačku
Zhotovovanie modelov:
- pre potreby fixnej protetiky sa využíva delený model, ktorý je
rozdelený na jednotlivé vysúvateľné kýptiky
- umožňujú pohodlnejšiu a detailnejšiu modeláciu
- vysúvateľné časti modelu a zuby sú zhotovené zo super tvrdej
sadry, pričom báza modelu by mala byť z tvrdej sadry
- vysúvateľné modely sa zhotovujú pomocou prefabrikátov
vodiacich čapov, alebo pomocou repozičnej skrinky
Zhotovenie modelu :
Metóda vodiacich čapov:
- továrenský vyrábané vodiace čapy sa zafixujú do špeciálneho
fixačno-paralelizačného prístroja
- odtlačok sa vyplní namiešanou sadrou len do výšky hrebeňa
alveolárneho výbežku
- do miest preparovaných zubov, do ešte mäkkej sadry, sa
zavedú vodiace čapy a do ostatných častí sa zavedú retenčné
krúžky
- po stuhnutí tejto vrstvy sadry sa namieša sadra na zhotovenie
bázy modelu
- takto pripravený model laborant po stuhnutí materiálu, pílkou
rozreže na jednotlivé kýptiky
Zhotovenie modelu – vodiace čapy (dowel pins):
Dvojité (double)
Jednoduché (simple)
Zhotovenie modelu – systém Pindex laser:
-slúži na navŕtanie dier a následné zavedenie vodiacich čapov do
sadrového modelu
- najnovší prístroj pracuje s laserovým lúčom
Zhotovenie modelu:
Metóda repozičnej skrinky:
-tento postup je jednoduchší.
- repozičná skrinka slúži na fixáciu jednotlivých častí modelu. Je
továrensky zhotovovaná z kovu alebo z plastu.
- zabezpečuje vertikálnu a horizontálnu stabilitu jednotlivých častí
deleného modelu.
- pre potreby snímateľnej protetiky sa zhotovuje model zo
zmesi alabastrovej a tvrdej sadry, z formovacej hmoty ( model na
zhotovovanie kovových konštrukcií snímateľných náhrad). Nie je
to delený model.
Zhotovenie modelu – repozičná skrinka:
Zhotovenie modelu – repozičná skrinka:
End.