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Presented by
Dr Rinu Sharma
1st year PG resident
Dept of Prosthodontics &
Maxillofacial Prosthetics
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Contents
Introduction
Chemistry & manufacturing process
History of Maxillofacial prosthetic material
Silicone elastomers
Classification
Properties
Advantages/ disadvantages Common problems associated
Conclusion
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Introduction
Silicon Chemical element with the symbol Si
Atomic number 14
Rarely occurs as pure free element but in its combined formaccounts for about 25% of earths crust
Elemental silicon is dark grey in color with metallic shine
It is hard & brittle octahedric material.
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Silicon and oxygen have a strong chemical affinity.
Occurs naturally only in stable form of Si-Ocompounds such as calcium, magnesium and ironsilicates.
As well as silicon dioxide (silica) as sand and quartz.
The tetravalent structure is common to all compounds
in which silicon is surrounded by oxygen atoms.
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Silicone Not to be confused with metalloid element silicon.
Silicone includes silicon together with carbon, hydrogen,oxygen & sometimes other chemical elements.
Silicon to oxygen bond forms the
backbone & in addition bonded to
organic group typically methyl.
Thus , silicones are combination of
organic & inorganic compounds
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Chemistry
Chemically Silicone is :
Polydimethylsiloxane(PDMS)
The basic repeating unit is known as siloxane.
Polymerized siloxane or polysiloxane with chemical formula[R2SiO]n, where R is an organic group eg. methyl .
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Manufacturing Process
( Silicon to Silicone)1. Silicon from Quartz :
Thermal reduction of quartz(SiO2) with carbon.
SiO2 + C ------------> Si + CO2
Thus obtained silicon is a solid metal and needs to becrushed into powder for further reaction.
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Rochow Process
2. Chlorosilanes from Silicon
Most commercially important reaction .
Finely ground and well mixed Silicon is reacted with Methylchloride in gaseous form in presence ofcopper catalyst &certain promoters to form Methylchlorosilanes.
300 Cent
2 CH3-Cl + Si ---------------> Me2SiCl2Catalyst
(Mueller-Rochow synthesis)
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Produces a silane mixture
According to the number of chlorine atoms on the basicsilane molecule,
Mono-, di-, tri- or other silane units with Si-O bonds.
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The predominant material obtained is
dimethyldichlorosilane (approx. 80% by weight).
All silicone fluids, emulsions and rubbers arebased on dimethyldichlorosilane. This is thereforethe decisive base product.
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3. Silicone from Chlorosilanes
Hydrolysis
The reaction of the dimethyldichlorosilanes with water(hydrolosis) or methanol (methanolysis) producessiloxane.
Linear siloxanes with OH groups as well as Cyclicsiloxanes with normally between three and six chainunits are formed.
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Organochlorosilanes react violently with waterreleasing hydrochloric acid.
Using HCl catalysis, thus formed siloxane directlyleads to formation of further reacted oligomers orpolymer siloxanes.
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QUARTZ
SILICON
CHLORO-SILANES
DIMETHYL
DICHLORO-SILANES
SILOXANE
POLYDIMETH
YL -SILOXANE(PDMS)
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Polydimethylsiloxane (silicone)
Polydimethylsiloxane (PDMS) is the most commonlyavailable silicone.
By adjusting -Si-O- chain lengths, the functionality ofthe side groups and the cross-linking betweenmolecular chains, silicones can be synthesized intoalmost infinite varietyof materials.
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Polydimethylsiloxane
Wherein n is an integer ,
Higher n = more elastic
Lower n = more plastic
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Grades of silicone
Fluids Emulsions Resins Elastomers
Silicone elastomers, or rubbers, are made fromlinear polymers that bear hydroxyl, vinyl or otherreactive side chains.
They can be cross-linked in various ways to yieldhighly elastic, more or less open-pored structures.
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Apart from wide range of Uses in Prosthodontics,
Most importantly,
Silicone Elastomers are used as MaxillofacialProsthetic Material for Rehabilitation of patients
with Oral or Facial defects.
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History
Auricular, nasal, and even ocular prosthesis fabricatedwith various materials, have been found in EgyptianMummies.
According to Beder, the first obturator was describedin 1541 byAmbrose pare. It consisted of a simple discattached to sponge.
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Tycho Brache (1546-1601), who used an artificial nosemade from gold to replace his own nose.
1600 to 1800:- Pierre Fauchard (1678) fabricated a silvermask for a French soldier. It was painted with oil
paints & margins covered with facial hair.
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1800 to 1990:-William Morton was credited with
fabrication of a nasal prosthesis using enameledporcelain to match the complexion of a patient.
In 1880:- Kingsleydescribed a combination of a nasalpalatal prosthesis in which the obturator portion was anintegral part of the nasal prosthesis.
In 1900 to 1940:- In the nineteenth century,vulcaniterubberwas widely used in dental profession and wasalso adopted for use in facial prosthesis.
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Upham described the fabrication of nasal andauricular prosthesis made from vulcanite.
In 1905, Ottofy, Baird and Baker all reported usingblackvulcanized rubber.
In 1913Gelatin-glycerin compounds were introduced
for use in facial prosthesis in order to mimic thesoftness and flexibility.
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Kazanjian described the use ofcelluloid prints forcoloring vulcanized rubber facial prosthesis.
From 1940 to 1960:-Acrylic resin was introduced in thedental profession.
From 1960 to 1970:- The introduction of various kindsofelastomers resulted in major changes
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1960 to 1970 Barnhart was the first to use silicone rubber for
construction and coloring of facial prosthesis bycombining silicon rubber base material with acrylic
resin polymer.
Tashma used dry earth pigments dispersed in colorlessacrylic resin polymer powder for intrinsic coloring of a
silicon facial prosthesis.
Thus , silicon elastomers came into practice.
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Silicone elastomers
Probably the most widely used material for facialprosthesis now a days.
Introduced in mid 19th century, but has been used in thefabrication of maxillo-facial prosthesis only for the pastfew years. 24
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Silicone elastomer = Silicone polymer + Filler+ Crosslinker (vulcanizing agent)
Thus, Silicone elastomers are crosslinked linearsilicone (fluids or gums )with a three-dimensional
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Cross-linkingofSilicones
There are three different types of cross-linking reactions:
Peroxide (-initiated) curing --where polymer containsvinyl groups.
Addition curing --where polymer contains vinyl groupsand crosslinking agent contains Si-H groups
catalyst- platinum , palladium , rhodium etc
Condensation curing betweendihydroxypolydimethylsiloxanes and silicic acid esters
Catalyst Tin.
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ClassificationSilicones are classified into 4 groups according totheir applications:
Class I: - Implant grade, which requires the material to
undergo extensive testing and must meet FDArequirements.
Class II: - Medical grade, which is approved for external use.This material is used for fabrication of maxillofacial
prosthesis. Class III: - Clean grade
Class IV: - Industrial grade commonly used for industrial
applications.
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Composition Inorganic
silicon , carbon, oxygen ,hydrogen
Organicmethyl / phenyl / vinyl group
Fillers1. Reinforcing fillers :
- Pyrogenic (fumed) silicas- Modifies properties( Modulus, Tensile strength or
Elongation at break )
2. Non Reinforcing fillers:
-Needed for bulking up silicone rubberseg. Quartz, diatoms or metal oxide
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Crosslinking / Vulcanizing agent
Additives / Colorants: organic or inorganicpigments.eg. Oxidative degradation can be retarded by iron oxides
Catalysts : platinum, tin and titanium complexes etc.
Surfactants / Antioxidants
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Ideal properties of MFP material
Ideal Physical and Mechanical Properties:
- High edge strength.
- High elongation.
- High tear strength. - High tensile strength
- Softness, compatible to tissue.
- Translucent. - Low coefficient of friction
- Low glass transition temperature
- Low specific gravity
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Ideal Biological Properties:
- Non-allergenic.
- Cleansable with disinfectants. - Color stability.
- Dimensionally stable
- Inert to solvents and skin adhesives.
- Resistance to growth of microorganisms.
-Resistance to environmental discoloration
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Why a MFP material ?
Silicone elastomers have been widely used to fabricatefacial prostheses primarily because of their chemicalinertness, strength, durability, ease of manipulation,and biocompatibility
Still far from ideal
The chemical combination of the organic groupsattached to the polysiloxane backbone leads toproducts with outstanding chemical and physicalproperties.
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Properties of Silicones
Thermally stable :
The thermal stability of silicones stems from the Si-O andSi-CH3 bonds which are themselves thermally stable.
(standard grades of silicone elastomers can handle
temperatures ranging from -40 to 200C/-40 - to 392F)
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Electric insulation
Methyl groups regularly spaced throughout siliconemacromolecules are non-polar and do not allow electriccurrent to pass through them.
Silicones are therefore non-conductive and are excellent
electrical insulators.
They are chemically inert after processing.
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Tensile strength Measurement of resistance resulting from stretching
till it breaks.Measured as pounds per square inch(psi).
Depends on amount & type of filler used.
High elongation is desirable when peeling prosthesis
from tissue surface
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The polymer chains, silica fillers, and the interactionsbetween these two components affect the overallsilicone strength and bonding .
(Muhanad M. Hatamleh, David C. Watts . Mechanical properties andbonding of maxillofacial silicone elastomers. Dent Mater 2 6 ( 2 0 1 0 )185191.)
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Tear strength :Measurement of material by pulling until it tears.
Measured as (psi) Silicone elastomers posses high tear resistance due to
cross-linking of material.
This property helps to overcome adhesive resistance &
allows the fabrication of thin margins.
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Viscosity Measured as internal resistance of material to flow.
Measured in centipois(cps) or millipascals(mPas).
Lower the viscosity, higher the flow.
Can be increased with both cross linker & fillers.
AdhesionPosses excellent adhesion propertiesSilicon based adhesives & sealants can withstand
highly acidic / basic chemicals, water and organicsolvents.
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Hardness:Measure of flexibility
Measurement of hardness is done by instrumentDurometer.
Shore A Scale most common (0-100)
Higher the value , harder is the rubber.
Similar hardness to missing facial tissue desired(shore A 5-20).
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Surface tension PDMS have a low surface tension (20.4 mN/m) & are
capable of wetting most of the surfaces.
Methyl group points outward making it hydrophobic withgood release properties.
Glass Transition Temperature is low i.e. (146 K forPDMS )
Excellent weatherability
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Biocompatible The material has minimal adverse impact on host
Hydrophobicity, its chemical & high molecular weight
polymeric nature provides the theoretical basis forlack of toxicity.
Biodurable The Host has minimal side effects on the material
Related to exceptional thermal & chemical stabilityproperties.
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Silicone elastomers
According to the type of vulcanisation (crosslinkingagents, temperature) and base-polymer viscosity
employed , it can be classified as
1. High Temperature Vulcanizing silicones (HTV)
2. Room Temperature Vulcanizing silicones (RTV)
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(HTV)
Usually white ,opaque material
Basically made from reactive silicone gums, or straight
chains with high molecular weight.
Available in semisolid or putty like consistency in 1 or 2component system
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Catalyst/vulcanizing agent : dichlorobenzoyl peroxideor platinum salt depending on type of polymerisationused
Copolymerisation with methyl vinyl or methyl phenylsiloxy radical varies the relative softness & tear
strength
HTV requires milling, packing under pressure & a 30min heat application cycle at 180 degrees.
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Advantages
Pigments are milled into these material so intrinsiccolor can be achieved.
Better color stability than RTV.
It exhibits better mechanical strength and can be usedat temperatures ranging from -50C to +300C. hencethermally stable
It is chemically & biologically inert Better Ageing Resistance & photo-oxidative stability
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Disadvantages
Requirement of a milling machine & a press
Metal mold is normally used & fabrication of mold isa lengthy procedure (stone mold increases risk ofdamage to the material during de-flasking)
It lacks sufficient elasticity to function in movabletissue.
Low edge strength (nylon can be reinforced)Has opaque & lifeless appearance .Do not readily accept extrinsic coloration
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Room Temperature Vulcanizing
(RTV)Available as single paste system or clear solution
Filler : Stannous octoate
Cross linking agent: Orthoalkyl silicate
Polymerisation by condensation or addition reaction
They are used more often than any other maxillofacial
material due to good physical properties & favorableprocessing characteristics.
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Advantages:
Can be polymerised by artificial stone molds but moredurable molds can be made from epoxy resins ormetals
Original RTV silicone are biologically inert & retaintheir physical & chemical properties at widetemperature ranges.
Excellent resistance to weathering
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Disadvantages:
Not as strong as HTV & generally monochromatic Poor edge strength & difficult to color
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Common problems of
Maxillo-facial Prosthetic material Discoloration over time
1. Intrinsic color change of elastomers
2. Intrinsic color change of colorants(pigment flocking)
Discoloration due to loss of external coloration
1. loss of adhesion of extrinsic coloration to prosthesis
2. primers & adhesives3. Poor patient handling
4. Staining
5. Medical adhesives & cleansers
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The degree of the color change observed clinicallyin maxillofacial prostheses may result fromstaining rather than aging of the polymers orpigments.
(RG.C.raig, k Koraann d R.Yu. Elastomers for maxillofacialapplications, Review. Biomaterials 1980, Vol 1)
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Degradation of physical & mechanical properties
1. Tear at margins ( tear strength , fatigue)
2. Change in surface texture3. Elongation at margins
4. Compatibility with medical adhesives
5. Weakening of margins by colorants, adhesives,solvents, cleansers(colorants do not adhere chemically)
6. Deterioration of static & dynamic mechanicalproperties.
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Regardless of type of maxillofacial prosthetic materialused in fabrication,
The service life time of a facial prosthesis is usuallyfrom 6 months to 2 years
The average wearing time of a facial prosthesis is 10months.
( Chen M, Udagama A, Drane JB : Evaluation of facial prostheses for head& neck cancer patients. J Prosthet Dent .46: 538 ; 1981 )
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Conclusion
Polydimethylsiloxane, commonly known as Siliconeare used in many applications including Maxillofacialprosthesis.
The ultimate challenge of a maxillofacial prostheticmaterial lies in
1. Improving physical & mechanical properties so thatit will behave more like human tissue & increase
service life of prosthesis2. Finding color stable coloring agents & developing
sceintific method of color matching to human skin.
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References1. John Beumer III, Maxillofacial rehabilitation: prosthodontics and
surgical considerations. Ishiyaku Euro-American. Inc. Pulblishers.
2. Restorative Dental Materials, Robert G. Craig 11th edition
3. Silicon Biomaterials : History & chemistry , Reprinted fromBiomaterials Science 2nd edition
4. Udita S Maller , Karthik K S , Sudhakara V Maller . MaxillofacialProsthetic Materials - Past and Present Trends. JIADS VOL -1 Issue 2
April - June,2010 |25|
5. Muhanad M. Hatamleh, David C. Watts . Mechanical properties andbonding of maxillofacial silicone elastomers. Dent Mater 2 6 ( 2 0 1 0 )185191.
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THANK YOU
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