MAXILLOFACIAL PROSTHETICS PART-II : MATERIALS AND TECHNOLOGY . A REVIEW OF PAST , PRESENT AND FUTURE TRENDS

1. Dental Dept. CGHS Wing, Safdarjung Hospital, Delhi. 2. MDS Oral and Maxillofacial Surgery, Visiting Consultant, Head and Neck Surgical Oncology, Fortis Escorts Hospital. 3. MDS Periodontology, Dept. of Dental Surgery, VMMC & Safdarjung Hospital. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 16 February 2020 Final Accepted: 18 March 2020 Published: April 2020


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Recently a copolymer of 5-20% vinyl acetate with the remaining percentage being vinyl chloride has been introduced. This copolymer is more flexible but, less chemically resistant than polyvinylchloride itself 14,25,26 .
The primary disadvantage is plasticizer migration and loss, resulting in discoloration and hardening of the prosthesis.
Edges tear easily and require reinforcement with nylon mesh. They get easily stained and degrade on exposure to ultraviolet light, peroxides and ozone and absorb sebaceous secretions. Their clinical service may extend from 1-6 months 26 .

Latex:
They were once used extensively forfabrication of life-like facial prosthesis. However, it changes color rapidly and the finished product is weak, degenerating rapidly with age. Hence its use as a prosthetic material is limited 11,12,13 .

Ter-PolymerLatex:
These materials are under investigation. It consists of two latexes based on acrylate monomers with formaldehyde as a cross-linking agent 27,28 . Prostheses are fabricated by dip-casting over male models. The mechanical properties achieved are in the desired range for an extra-oral prosthetic material 29 .
Ouellette reported that the unnatural milky, see-through appearance of prosthetic devices fabricated from terpolymer latex could be eliminated by secondary spraying of the outer surface with an oil dye 30 .

Chlorinated Polyethylene:
These are thermoplastic elastomers available as thin sheets in a variety of shades. The sheet is heated and placed between heated metal mould halves and the prosthesis is formed in a hand press.The prosthesis can be colored with oil-soluble dyes 31,32 . Chemically, CPE closely resembles polyvinyl chloride. It is resistant to environmental degradation and can be developed into soft, tough elastomers without the need for plasticizer 33,34 .
Lewis and Castleberry reported similarity of this material to polyvinyl chloride in both chemical composition and physical properties 6,34 .

Polyurethane Elastomers:
These serve a variety of commercial uses and can be synthesized with wide range of physical properties.
They arise from 2 major reactants. In the presence of a catalyst, polymer terminating with an iso-cyanate is combined with one terminating with a hydroxyl group. Varying number of iso-cyanates will change the physical properties of final product 13 .
Commercially, Epithane -3 is the only polyurethane used in facial restorations. It consists of a soft polyol component, a hardand toxic iso-cyanate component and an organotin catalyst. Their flexibility is wellsuited to defects with movable tissue beds. They can be colored both intrinsically and extrinsically and superior cosmetic results can be obtained 35 .
Extrinsic colors wear off rapidly on exposure to ultraviolet light and surface oxidation.
Clinical usefulness is generally less than 6 months. Polyurethanes have poor compatibility with existing adhesive systems 35 .

Isophorone Polyurethane:
A unique polyurethane elastomer based on a cycloaliphatic diisocyanate monomer is being developed and tested. It is a three-component kit comprising an isocyanate terminated prepolymer, a triol as the crosslinking agent, and an organotin catalyst. These have unusually high strength compared to other aliphatic polyurethanes. The aliphatic nature of the polymer improves resistance to sunlight degradation 36,37 .

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Other Polyurethanes: Goldberg et al, reported synthesis of a series of polyurethane elastomers based on aliphatic diisocyanate and a polyether macro-glycol. The elastomers displayed high strength and elongation necessary for maxillofacial applications and exhibited satisfactory resistance to ultraviolet light 35,36,37 .

Silicone Elastomers:
These were introduced in 1946, but are being used as prosthetic materials only for the past few years.
Silicones consist of alternate chains of sodium and oxygen which can be modified by attaching various organic side groups to the silicon atoms or by cross linking the molecular chains.They exhibit good physical properties over a range of temperatures and can be cured at room temperature or under high temperatures 38,39 .
Silicones are classified into 4 groups according to their applications 40  HTV silicone is usually a white, opaque and viscous material. The catalytic agent is dichlorobenzoyl peroxide or platinum salt depending on condensation or addition polymerizationrespectively 2,7 . Pure, finely divided silica particle of about 30 microns are added as fillers. A small amount of methyl vinyl or methyl phenyl siloxyradical varies the relative softness and tear strength 8,11,12,13 .
After milling to blend the catalyst and coloring pigments, the material is packed under pressure and cured at 180ºC for 30 minutes 13 .

SE-4524U:
This silicone requires moderate to high temperatures for initiation of cross-linking reaction 42 . The Silastic 44514 and 44515 available from Dow Corning are of the same type. Organic peroxide initiates the cross-linking reaction, therefore, prosthesis must be fabricated at a temperature high enough to cause decomposition of the peroxide catalyst 42,45 . Lontz et al have found that when bis-2,4-dichlorobenzoyl peroxide is used as catalyst, prosthesis can be processed at 100°C. Amount of catalyst and fabrication temperature affect the properties of the finished prosthesis 42 .

PDMSiloxane:
This HTV silicone has physical and mechanical properties that exceeds values considered clinically acceptable 45,47 . 920

Siliastic S-6508:
In the raw state it is similar to sticky modeling clay. It must be vulcanized at 260°F and formed in pressure moulds 45,47 .

R.T.V. Silicones:
RTV silicones are designed for rapid room temperature curing. They are short-chain silicone polymers with crosslinking agent such as tetra-ethoxysilane,withstannous octoateas catalyst.
After addition of catalyst and careful introduction into the mould, it is allowed to cure for 30 minutes. The prosthesis is removed from the mould and thoroughly cleaned with chloroform before proceeding towards extrinsic coloration and characterization 2,7,8,11,48,49 . It includes 12,13 -A filler-Diatomaceous earth particles. A catalyst-Stannousoctoate. A cross linking agent-Tetra-ethoxysilane.

Silastic 382,399:
Thesepolymerize by condensation reaction. These are colorstable, biologically inert, and retain their physical and chemical properties at wide temperature ranges 50 . These are much easier to process and moulds of dental stone can be used. Silastic 399 is translucent and Silastic 382 is opaque and white 51 .

Silastic 891:
It is also known as SilasticMedical Adhesive Silicone Type A and is a methyltriacetoxysilane-cross-linked silicone. It is a translucent, non-flowing paste, which polymerizes at room temperature on contact with moisture 52,53 . It does not contain solvents, plasticizers, or catalyst and can be processedin gypsum moulds. Metal moulds are not recommended as it may react with acetic acid, which is liberated as a by-product of polymerization. The color stability is good and is compatible with a wide range of colorants 53,54 .

Cosmesil:
Cosmesil is an RTV silicone which can be processed to a varying degree of hardness. This material shows higher tear strength at failure than MDX 4-4210 50,51 .

A-2186:
It is a recently developed material whose physical and mechanical properties are inferior compared to MDX 4-4210 50,51 .

Polyderm:
Polyderm is an RTV silicone consisting of short chain dimethyl polysiloxane polymers, cross-linking agent and highly dispersed fillers. It is specially formulated for facial and body prosthetics. It is supplied as basic skin shade and clear base, which can be characterized by addition of intrinsic pigment. Extrinsic pigments are supplied with the kit 50,51 .

Materials of the 3rd Millennium:
According to Remerdale E.H. these are expected to be translucent with pigmentation ability to match any skin color 9 . They should have following characteristics-1. Increased elongation. 2. Increased tear strength. 3. Should be easily mouldable (clay like consistency). 4. Cured with light. 5. They should readily accept extrinsic coloration. 6. High temperaturemetal moulds should not be necessary.

MDX 4-4210:
This is a medical-grade silicone elastomer most popular among clinicians. Moore reported that it exhibits improved qualities of coloration and edge strength 54 .

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It is based primarily on a modified polydimethylsiloxane structure;polymerization involves addition of Si-H groups to Si-vinyl units. A platinum catalyst initiates the cross-linking reaction;sensitive to amines, sulphur and tin compounds, inhibiting curing of the material. It is not as opaque as other highly filled silicones 55 . It is available as a two-component kit. It has chloroplatinic acid catalyst and hydro-methylsiloxane as cross-linking agent 56 .

Recent Advances: Silicone Block Copolymers:
These have been introduced to improve tear strength, low percent elongation and susceptibility to bacterial growth 56 .

Foaming Silicones:
A gas forms bubbles within the polymerizing silicone which is eventually released; leaving a spongy material causing an increase in volume by as much as sevenfold 42,43,44 .
The purpose is to reduce the weight of the prosthesis; however, it has reduced strength and is susceptible to tearing, which, can be partially overcome by coating it with another silicone.

SiphenylenePolymer:
It is another siloxane copolymer with phenyl and methyl groups and exhibits significant improvement with respect toedge strength, coloration, and lowmodulus of elasticity relative to other RTV silicones 10,56 .

Primers:
Various primers such as 1200, 1205, S-22602, 4040, Z6032, Z6076 are available. Primers are used to promote bond between silicone and other maxillofacial material. S-2260 and A-4040primer were found most effective in bonding Medical Adhesive Type A to polyurethane sheets. A-4040 primer has greatest bond strength when bonding Silastic 891 to Lucitone 199 denture-based resin andZ-6032 produce the highest bond strength between silicone elastomers and light activated resin 10 .

Printable Silicones:
A 3D color image reproduction protocol has been developed for 3D printing of facial prostheses. It involves 3D scanning of the face using a photogrammetry system that captures both 3D topography and color information. Using a combination of software suites, raw scanned data can be modified and corrected by removing noisy polygons, along with color adjustment 57,58,59 .
In order to add realism, fine textures (e.g. pores, wrinkles) are added over the 3D mesh using high-field mapping. Finally, thickness is added to get a solid printable model 57,58,59 .
As with the surface topographic information, the color images may also require further processing before final colour printing, which involves, management of the 2D color image from the camera RGB to printer RGB pixel by pixel. When the color is finalised, surface texture mapping is done to map the new color image onto the 3D model. The penultimate step involves printing to produce the 3D model 57,58,59 . Which can be done by two methods-

Silicone Infiltration:
A full color 3D printingtechnology called 3DPTM printing, also known as powder-binder printing has beendeveloped at MassachusettsInstitute of Technology and is licensed to Z Corporation and 3D Systems, it is based on inkjet printing, with a powder being deposited in consecutive layers, which is then selectively joined by ink-jetting with coloredbinderto print powder material in a fullcolor spectrum, layer by layer 57,59,60 .
The powder can be made of gypsumin combination with plastic powder, starch, ceramic, glass or otherpowdered materials.

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After printing color into the powder, infiltration with elastomeric polymer is carried out to produce aflexible, lightweight and lifelike prosthesis 60 .

Direct Printable Silicones:
This technology is called as Drop-On-Demand (DOD) printing and is basedon platinum-catalysed addition polymerization, meaning the crosslinking Si-H groups react with the vinyl groupto form a 3D network 60,61,62 .
A puresilicone free of solvents isused. This technologyuses single satellite droplets that are dosed onto theworking surface according to the STL mesh, and eachlayer (0.4 mm) is polymerized with ultravioletlight. After this the finished prosthesis is then coated with a suitable silicone polymer, is colored extrinsically and is finally cured at 200ºC 60,61,62 .