|
 
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| REVIEW ARTICLE |
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| Year : 2022 | Volume
: 9
| Issue : 4 | Page : 267-275 |
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A comprehensive review of extraoral maxillofacial material: Part II: Early extraoral maxillofacial materials
Darshana Choubisa
Department of Prosthodontics, Geetanjali Dental and Research Institute, Udaipur, Rajasthan, India
| Date of Submission | 28-Sep-2022 |
| Date of Acceptance | 10-Oct-2022 |
| Date of Web Publication | 12-Feb-2023 |
Correspondence Address:
Darshana Choubisa
Department of Prosthodontics, Geetanjali Dental and Research Institute, Udaipur, Rajasthan
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/jdrr.jdrr_152_22
This article is a continuation of previous article entitled as A Comprehensive Review of Extraoral Maxillofacial Material: Part I. Part I dealt with historical background; Part II dealt with review of some early extraoral maxillofacial materials; Part III dealt with majorly with silicone elastomers as an extraoral maxillofacial material; and Part IV dealt with recent advances.
Keywords: Extraoral maxillofacial material, facial defects, maxillofacial rehabilitation, prostheses
How to cite this article:
Choubisa D. A comprehensive review of extraoral maxillofacial material: Part II: Early extraoral maxillofacial materials. J Dent Res Rev 2022;9:267-75 |
How to cite this URL:
Choubisa D. A comprehensive review of extraoral maxillofacial material: Part II: Early extraoral maxillofacial materials. J Dent Res Rev [serial online] 2022 [cited 2023 Mar 26];9:267-75. Available from: https://www.jdrr.org/text.asp?2022/9/4/267/369583 |
| Extraoral Maxillofacial Materials | |  |
Earlier materials used were metals, usually gold or silver, paper, cloth, wood, leather, wrought alloys, and porcelain. These prostheses were color matched with oil paints. Later, materials used were cellulose nitrate, latexes, etc.
| Cellulose Nitrate | | |
Cellulose nitrate, also referred as nitrocellulose, is the polynitrate ester of cellulose (natural polysaccharide). Cellulose is a natural polymer collected from wood pulp or the short fibers (linters) that are attached to cotton seeds. It comprises repeating glucose units that have the chemical formula C6H7O2 (OH) 3 and the following molecular structure:
In pure cellulose, the X in the molecular formula represents hydrogen (H), indicating a presence on the cellulose molecule of three hydroxyl (OH) groups. Due to formation of strong hydrogen bond between cellulose by hydroxyl groups, resultant cellulose neither is softened by heat nor dissolved by solvents without causing chemical disintegration. Nonetheless, on treatment with nitric acid with sulfuric acid as a catalyst, hydroxyl groups are substituted by nitro (NO2) groups. In theory, all three hydroxyl groups can be substituted, resulting in cellulose trinitrate, which contains more than 14% nitrogen. Practically, yet, majority of nitrocellulose compounds are dinitrates. The solubility and flammability of the end product is decided by degree of nitration.[1]
Because of its unique physical properties such as easy solubility in many solvents, high refractive index, easy moldability, and low cost, it has been an important factor in many advances in the industrial arts and sciences.[2]
In dentistry, its use as extraoral maxillofacial material in practice was reported by several prosthetists, particularly the following: Coulioux, Grohnwalt, Junse, Michails, and Klinman, but often with disastrous results. Smoking tended to make nasal prostheses turn brown and on occasion catch fire. Celluloid was also used for cranioplasty usage in the second half of this century, again with unacceptable results, particularly because it was found to be carcinogenic. Tetamore appears to be the first in describing his prosthetic techniques to recognize the merits of using eyeglass frames as a direct source of retention for nasal and orbital facial prostheses and as a means of obscuring the edges of certain facial prostheses. The prosthesis also consisted of very light, nonirritating plastic material (possibly nitrated cellulose).[3],[4]
Its major drawback is dimensionally unstable, unpleasant odor, high water sorption, poor color stability, extremely flammable, and short lived. A modified version of this material, cellulose acetate, was later developed and used, especially in France, with more satisfactory results. Although used for a short time, this material indicated a general development of plastic and was later replaced by the introduction of latex.[2],[5]
| Latexes | | |
Natural latex products have continued since the 18th century. They are also contained in various materials that are used daily dentistry. It is a stable dispersion (emulsion) of polymer microparticles in an aqueous medium. Although natural latex is made from a white milky sap harvested from tropical tree, the "Hevea brasiliensis," many chemical products are added to the raw material, which determine the texture, color, and elasticity of the end product. Ammonia is added to the sap to preserve it, but at the same time, the ammonia hydrolyzes and degrades the sap proteins to produce allergens. Vulcanization is the process by which liquid latex is hardened into rubber through the use of sulfur compounds and heat. These chemicals may be allergenic themselves and are often present to some quantity in the end product. The manufacturing process leaches the allergens by soaking the rubber products in hot water. The leaching water is changed repeatedly to decrease the concentration of the allergens, but leaching brings other allergens to the surface and unfortunately places the highest concentrations near the skin of the wearer. Thus, the allergenicity of a given batch of latex will be dependent on how the latex was collected, preserved, and processed.[5],[6],[7]
It is one of the oldest materials used in maxillofacial prosthetics. Although the finished prosthesis is weak, deteriorates rapidly with age and discolors, can cause allergic reactions, and is satisfactory for a short period of time, its use has continued because it is soft, inexpensive, easy to use, and forms realistic prostheses.[5],[7],[8]
While natural latex occurs naturally, synthetic latexes can be made by polymerizing a monomer-like styrene that has been emulsified with surfactants. In other words, synthetic latexes are aqueous dispersions of polymers that are obtained by emulsion polymerization. It produces the same problems as natural rubber except that naturally occurring proteins and their degradation products are not present.[5]
Before mentioning some synthetic latexes, we should know what is terpolymer? Terpolymer is a polymer (like a complex resin) that results from the copolymerization of three discrete monomers. These types of latex materials, studied at George Washington University, offered a chemical approach and manufacturing technique that was significantly different from the more classic materials. The components of the system were two latexes based on acrylate monomers with formaldehyde as a cross-linking agent. Normally, the latexes are bound together in a solid content of about 50% and precross-linked with formaldehyde. Prostheses were usually made by immersion casting over male models. The manufacturing process was unusual compared to other materials and hence delayed the broad acceptance of the terpolymers.[9]
One of the synthetic latexes, terpolymer of butyl acrylate, methyl methacrylate, and methacrylamide, was developed and handed down as a maxillofacial material that was comparatively better than natural latex. This material is used by soaking a gypsum cast that is similar to the final prosthesis. Then, this cast with a layer of synthetic latex is coated with a layer of soluble plaster, known as plastogum (gypsum and corn starch) to counter the latex from warping when it solidifies. On solidification, noticeable shrinkage occurs in latex due to evaporation of the large volume of water from the mixture. Once the latex is completely cured, coated gypsum cast is placed in boiling water, which ruptures the plastogum layer and enables the latex molding to be recouped. However, the thickness of final product is only a few mils and does not have adequate strength to support itself; therefore, fabrication of silicone foam rubber casting (Silastic S-5370) is done on a refractory gypsum mold to design a framework on which latex skin can rest.[5],[10],[11]
Relatively, latex skin is translucent or slight milk white in color. The skin colors are simulated by spray painting the material on the tissue side with vinyl-based paints. The colors appear through the translucent latex in a similar way as pigments and blood beneath natural skin, resulting in a very lifelike prosthesis. An artist's airbrush is ideal for spraying, but concerns arise due to the generation of fumes from the spraying process which can be toxic and explosive; therefore, it is mandatory to use an explosion-proof hood during spraying process.
Although the final prosthesis (framework of foam rubber with overlying latex skin) has advantages of being airy, esthetic, and pliable, the factors such as lengthy, tedious, and time-consuming fabrication technique and comparatively short lifespan of only 3–4 months of final prosthesis have limited latex acceptance as extraoral maxillofacial material.[11]
| Acrylic Resins | | |
Acrylic resin was proposed to the dentistry in 1937 and interchanged with the older vulcanite rubber in both intraoral and extraoral prostheses.[12] It is often seen that notable archival events were often a driver in material modernization. In the 20th century, Second World War led to great urging for prostheses and reconstruction, which led to the exhaustion of the glass supply. Therefore, procedures for using ocular prostheses have been instituted using acrylic resin in place of glass, which led to the use of acrylic resin as an extraoral maxillofacial material.[13],[14] During this time, basically two varieties of acrylic resin were used to fabricate ocular prosthesis, essentially the same as denture base resin and the procedure for using it were also essentially the same. These two varieties were as follows: (i) white acrylic resin (sclera formation) and (ii) clear acrylic resin (cornea formation). These led to practically eradication of the glass use for ocular prosthesis fabrication.[15],[16]
Principally, acrylics are the derivatives of ethylene and contain a vinyl group (-C = C-) in their constitutional formula. In terms of dental interest, these can be derived by cross-linking esters of acrylic (CH2 = CHCOOH) or methacrylic acid (CH2 = C (CH3) COOH). These two compounds are cross-linked by addition reaction. Polyacids are hard and clear but had polarity, which is akin to the carboxyl group, leading them to imbibe water. The water tends to detach the chains, resulting in general softening and loss of strength. Therefore, they are not used in the mouth.
However, the esters of these polyacids are of substantial interest in dentistry. For example, if R typifies an alkyl radical, the formula for a polymethacrylate would be:
Since R can be almost any organic or inorganic radical, it is obvious that thousands of divergent acrylic resins can be formed. Such a consideration does not include the possibilities of copolymerization, which are even greater. In dentistry, the first member of the series, methyl methacrylate, is of paramount relevance.
Methyl methacrylate is a clear liquid at room temperature with the following physical properties: MP of − 48°C, BP of 110.8°C, density of 0.945 g/cm3 at 20°C, and heat of polymerization of 12 kCa per molecule. It has a high vapor pressure and is a very good organic solvent. The polymerization of methyl methacrylate can be initiated by ultraviolet (UV) radiation or heat as well as by chemical initiations. The degree of polymerization varies with the polymerization conditions such as temperature, activation process, initiator type, initiator concentration, and purity. Approximately 21% of volume shrinkage occurs during the polymerization of pure methyl methacrylate, which may be obstacle in fitting accuracy.
Another member of the series is polymethyl methacrylate (PMMA). In pure state, it is a colorless and clear solid and transmits light in the UV range to a wavelength of 250 nm with a Knoop hardness number 18–20. Its properties have proven to be sufficient for use in dentistry [Table 1]. It is extremely stable, does not change color in UV radiation, and shows exceptional aging properties. The disinfectant solution used for regular cleaning of the prosthesis can affect the surface roughness and color of acrylic resins.
It is chemically heat resistant and softens at 125°C and can be molded as a thermoplastic material. A depolymerization takes place between 125°C and 200°C. At approximately 450°C, 90% of the polymers depolymerize to form the monomer. PMMA with a higher molecular weight is degraded by the development of monomer and at the same time forms a polymer with a lower molecular weight.[5],[10],[17]
Like all resins, PMMA tends to absorb water by imbibition. Its noncrystalline structure has a higher internal energy, hence less activation energy is required for molecular diffusion in the resin. In addition, although esterifies, the carboxyl group can form a hydrogen bond with water to a limited extent. Because both absorption and adsorption are involved, the term "sorption" is commonly used to encompass the overall phenomenon. It has been reported that typical dental methacrylate resins have an increase of approximately 0.5% by weight after 1 week in water. Higher values have been reported for a number of methyl methacrylate polymers. However, laboratory tests have shown that the linear expansion induced by water absorption is approximately equal to the thermal shrinkage that occurs as a result of the polymerization process. Therefore, these processes are almost balanced. It is also soluble in a number of organic solvents such as chloroform and acetone due to its linear structure.[10],[18]
Even though acrylic resins are used in the processing of intraoral and extraoral prostheses, it is more successfully used as an extraoral maxillofacial material for distinctive types of extraoral defects, particularly those where there is slight shifting while activity in the tissue bed, e.g., orbital/ocular prostheses or as a framework for silicone prostheses to improve retention and orientation.[18],[19]
Advantages are its availability and familiarity of most dentists with its physical and chemical properties as well as the processing techniques. Both intrinsic and extrinsic coloring can be processed as it is colorless and transparent. Extrinsic coloring is easily processed with acrylic-based colors using solvent such as chloroform, acetone, or monomer. Exceptional cosmetic outcomes can be accomplished with acrylic resin. Facial prostheses made from this material can be maintained for up to 2 years, but occasionally external coloring is required. However, with aging, the prostheses become shiny and hairline cracks are occasionally found. Its lifespan can extend, if the processed prosthesis has a well-stippled surface. Application of surface stain to such a surface is easy to apply and lasts longer. The strength of this material allows the clinician to feather exposed edges. If necessary, changes can be easily done. It is compatible with the majority of adhesive systems and simple to clean of adhesives and dirt. It can also be easily repaired or relined with a tissue conditioner or a temporary prosthesis reliner and can be processed quickly and easily.[8],[12]
Due to the absence of free toxic tertiary amines and better color stability on exposure to UV radiation, heat-polymerized methyl methacrylate is preferred over the autopolymerizing resin. Even visible light-cured resin is also being used as an extraoral maxillofacial material. It has matrices of urethane dimethacrylate, microfine silicon dioxide, and high molecular acrylic resin monomers with inclusion of acrylic resin beads as organic fillers. Visible light is the activator, while a photosensitizer such as camphorquinone serves as an initiator for the polymerization. It is a single-component denture-based resin supplied in sheet and rope form and packaged in light-tight bags to prevent accidental polymerization.[5],[20],[21]
Although acrylic resins have so many advantages, the principal disadvantage is rigidity; therefore, its practicability compromises in highly mobile tissue beds, leading to local discomfit and marginal exposure. Its relatively high thermal conductivity can cause discomfort in cold climates. Moreover, from a psychological point of view, acrylic is less acceptable for the patient.[5],[12]
During processing, molds are made from dental stone, which are put under pressure, necessitating the use of dental flasks. One factor that must be considered in this process for long-term patients is that the duplication of the prosthesis must be dealt with at the level of the mold production, since the mold is destroyed after the prosthesis has been deflasked. This is more particularly for patients who are not expected to experience major tissue changes near the defect, and regular remakes can be part of the treatment protocol.[12]
Furthermore, the introduction of visible light-cured resins was a useful tool for intraoral and extraoral provisional prostheses; however, its characteristic brittleness, staining inability, and poor thermal conductivity indicate the use of methyl methacrylate as the acrylic of choice for most acrylic prostheses.[20],[21]
In general, acrylic resin is particularly more suitable for temporary facial restorations. Despite this, some clinicians still preferred it as a permanent material due to its durability, color stability, and good cosmetic outcomes. Recent surveys have shown that an approximately 6% of clinicians are using an acrylic material.[12],[22]
| Acrylic Copolymers | | |
There is an immense possibility in polymer chemistry to enhance the presently accessible materials that are clinically more suitable in the field of maxillofacial prosthetics. New materials were developed at the 1992 conference on material exploration in maxillofacial prosthetics, which varied the polymerization processes and made clinical applications more adaptable. In a venture to avert the difficulties experienced with the original acrylic copolymers, combining high-molecular-weight acrylic polymers with blocks of other polymers was done.[23]
Acrylic copolymers are although being soft and elastic in characteristics but have not been widely preferred due to undesirable properties such as (i) poor edge strength and poor durability, (ii) deterioration and loss of retention when exposed to UV radiation and environmental conditions, (iii) difficulty in processing and coloring, (iv) require frequent repairs, (v) difficulty in processing, and (vi) the finished prostheses often become sticky and predisposed to dirt collection and coloring. It has been suggested in the literature that the addition of VeoVa-10 monomer and vinyltrimethoxysilane monomer to acrylate polymer emulsions increases the coating resistance to UV radiation, water, and various types of solvents, enhancing the durability of the coating.[12],[24],[25]
Palamed is one of the early acrylic copolymers used, which is a plasticized methyl methacrylate resin formulated with a foaming agent. During curing, as a result of heat or a triggering chemical, foaming agent releases gas that is integrated into the material; therefore, the final prostheses are spongy and have a firm skin wherever the material touches the mold surface. The size of the pores varies directly with the cross-sectional thickness of the mold, resulting in a nearly solid material on the peripheries where the mold is thin. The size of the pores also varies with the amount of material that is placed in the unvented mold. The mold is underfilled (by 10%) to allow the material to enlarge and the foam-like center to form. Therefore, Palamed must be dosed carefully, as too much leads to a stiff, heavy unusable product or too little leads to an incompletely filled form with large pores. The material (wax, clay, etc.) used for sculpturing must be recovered and weighed to calculate the correct amount of Palamed for that particular mold. It has disadvantages such as plasticizer migration, stiffness with age, and inadequate life expectancy.[26]
Cantor and Hildestad stated a well-documented discussion of the properties and fabrication of prostheses using Palamed, while Cantor also delineated about mechanical and reflective spectrophotometric properties of Palamed.[27],[28]
Antonucci and Stansbury delineated the development of a new generation of acrylic monomers, oligomers, and macromeres. They stated that these materials can be easily polymerized using various polymerization methods such as thermal, chemical, photo-initiated, or even dual-curing initiators. Their approach was to incorporate high molecular acrylic polymers into molecular blocks of other types of polymers, e.g., polyetherurethane, hydrocarbon, fluorocarbon, or siloxane, that erase the drawbacks of conventional acrylic copolymers and also to achieve a broad range of physical and mechanical properties that can reach the prerequisites of extraoral maxillofacial materials and their applications.[12],[23]
| Vinyl Polymers and Copolymers | | |
Vinyl chloride (and copolymers) in the form of plastisol were introduced into dentistry in the mid 1940's, and for several years they were the most common and may be the best materials for extraoral maxillofacial prostheses.[8] The most generally accepted were Realistic (polyvinyl chloride [PVC]) and Mediplas (polyvinyl acetate chloride).[12]
In pure form, conventional PVC is a relatively rigid, clear, tasteless, and odorless resin with a glass transition temperature (80°C) higher than room temperature. It is a partially syndiotactic polymer with sufficient irregularity of structure to exhibit a crystallinity that is quite low. Several other chemicals such as plasticizers, UV stabilizers, cross-linking agent, and catalyst are added for maxillofacial applications. The PVC is soluble in the plasticizer at elevated temperatures. However, addition of plasticizer also increases processing time and is predisposed to undesirable shrinkage. Thus, plastisol preparations contain small particles of solid polymer (PVC) which are dispersed within plasticizer (solvent), producing a viscous liquid.[29],[30] This moderately viscous mixture is thoroughly mixed, and when heated at approximately 170°C, it gels and fuses into an elastomer. The viscosity of the mix is lowered by the temperature increase and elevated by the swelling and gelation of the PVC particles. Suitable pigments are incorporated within the plastisol during manufacture, and therefore, the clinician can modify the color by mixing different color preparations within the laboratory. When the plastisol dispersion is heated, the solid particles are partially dissolved by the plasticizer; producing a solid, pliable end product on cooling. The viscosity of the mixture is reduced by the rise in temperature and increased by the swelling and gelation of the PVC particles, while the pliability and softness vary directly with the quantity of plasticizer used.[11]
Desirable properties are its considerable pliability, compliant with both intrinsic and extrinsic coloring, and show a satisfactory initial semblance when properly manipulated.[12]
Even though plastisols have general acceptance, they also have definite drawbacks. In spite of the fact that plasticizer makes the material viable, it is also the material's gigantic shortcoming. Since the curing reaction is physical and not chemical, the plasticizer migrates through the material with aging. Plasticizers that are on the surface of the material can be lost through evaporation or dissolve with contacting materials, making prostheses hard, rigid, deformed, and blemished. Even, plastisol preparation stain easily due affinity of many coloring agents with plasticizer.[11] Thin edges tear easily so may necessitate nylon reinforcement. On exposure to UV radiation, peroxides, and ozone, it easily colors and degrades. Even though UV stabilisers are added to counteract the UV radiation to the maximum amount as possible and crosslinking agents for added strength, still color change and degradation may occur. Lacking lifelike translucency, they tend to soak up sebum secretions, cosmetics, and solvents, further influencing their physical properties. It is easily tainted due to its surface stickiness. A very high percentage of prostheses are retained by the use of adhesives that contain a solvent that easily amalgamates with the plasticizer. Thus, the clinical benefit of a prosthesis can be between 1 and 6 months.[12],[31]
As this polymer is a thermoplastic material and supplied as a solid suspension in a solvent, the quality of the cured plastisol depends on the time it takes to reach its curing temperature. Since curing takes place at high temperature, metal molds are required, which is additionally one among the disadvantages mentioned before. In the trade, thin electroplated molds, which cause a fast-curing cycle, are submerged in heated liquid, while the clinician's molds are made of linotype metal, which is unevenly thick and heated in a dry heat oven, leading to a slower curing cycle, and in particular the material is cured unevenly due to its different thickness. Hence, commercially made vinyl plastisol products are superior to customized prostheses.[11],[12]
In clinical use, the linotype metal mold is preheated within the oven and handled with asbestos gloves. A thin layer of skin-colored material is applied to the surface of the mold with a brush. The heat in the mold causes the material to partially cure or solidify. Then, different colored plastisol layers can be applied without mixing the colors, followed by mold filling with skin-colored plastisol. On completion, the entire mold is placed in the dry heat oven for final curing. The final prostheses can be esthetic due to clarity of the deeper colors through the translucent outer layer.[11],[12] Bulbulian delineated excellent discussions regarding the processing of this material and the manufacture of metal molds.[8]
Endeavor has been made to search out a plasticizer that might chemically bond to a vinyl molecule to attenuate migration and loss at the edge of the prostheses, as reported by Castleberry.[32] With these changes, the lifespan of PVC prostheses has been extended to 9–11 months. However, there are still serious problems with polymer deterioration and darkening of the material due to UV radiation. The inferior dimensional stability of PVC is another disadvantage.[12],[33] There have been some successes in this endeavor, but currently even the most effective vinyl prostheses have a very short life expectancy. Since vinyl plastics have been used for a long time and are one of the most frequently used materials, the technical knowledge is vast. Despite this extensive knowledge, the clinician's product remains about the same as it was 20 years ago.[11]
A copolymer of 5% to 20% vinyl acetate has also been introduced, the remaining percentage being vinyl chloride.[12] The polyvinyl acetate is comparatively light and heat resistant but has an unusually low softening point (35°C–40°C). Therefore, when the monomer of vinyl chloride and vinyl acetate is copolymerized in different proportions, many useful copolymer resins develop.[18] When amalgamated correctly, the Vinyl in elastomeric form have properties that are remarkable to those of natural rubber in terms of durability and resistance to UV radiation and aging. Copolymers of vinyl chloride and vinyl acetate are more pliable, but less chemically stable than PVC itself.[12]
| Chlorinated Polyethylene | | |
Principally, the chlorinated polyethylene (CPE) system incorporates an industrial CPE from Dow Chemical Co. that is amalgamated with an array of additives, including low-density polyethylene, calcium stearate, soybean oil, and low-molecular-weight CPE.[31]
Although CPEs are chemically and physically similar to PVC, some significant differences exist. For example, CPEs are (i) often more resistant to environmental degradation; (ii) can be made as soft, tough elastomers without the need for plasticizers; (iii) adaptability with many polymers and can be easily alloyed with them to further expand their scope of usable properties; and (iv) stable thermoplastics and can be formed into complex configurations at low temperature. CPEs thus have the possibilities to demonstrate the main benefits of PVC with advancement over PVC. Lewis and Castleberry delineated testing of CPEs, a material that is alike in its chemical composition and physical properties to PVC.[31],[34],[35]
They had pretty good mechanical properties. Because of the high tear strength, thin cross-sections of the CPE materials can be fabricated. Prostheses of the CPE materials are fabricated from pigmented foils of the thermoplastic polymer alloy by processing them at high temperatures in linotype metal molds and were molded in a hand press. Occasionally, the heating and pressing were repeated. No chemical reactions are involved in the fabrication process. After the prosthesis has been removed from the mold, the requisite alterations can be made and the molding process can be repeated as much as required due to its thermoplasticity. The prostheses can be hand colored with oil-soluble paints. Relatively, CPEs are simple and inexpensive to fabricate and the properties are good. Yet, fabrication in metal molds is undeniably a weighty disadvantage of this system. The prostheses are similar in appearance to those made from vinyl plastisols.[12],[34],[35]
CPEs can have advantages over conventional silicone rubber materials in its ability to be repaired, relined, or reconditioned, thereby extending the lifetime of the prosthesis. In addition, it can be used with any type of adhesive. It has a substantial edge strength, does not assist fungal growth, and is often cheaper compared to silicone materials. It irritates the mucous membrane less than silicone is less toxic than thermosetting silicone materials and is not carcinogenic. It appears to be an acceptable replacement for silicone for the manufacture of maxillofacial prostheses in situations where the cost of silicone is exorbitant.[5],[36]
Gettleman delineated the evaluation of thermoplastic chlorinated polyethylene, as a possible maxillofacial material. The processing technique using steam autoclaves with gypsum molds has been developed, and a laminated coloring technique has also been described.[37]
| Polyurethane Polymers | | |
Polyurethane polymers (PUPs) constitute a gigantic family of polymers that, as per Juan B. Gonzalez, have often been used with great smash hit in an array of day-to-day merchandise. However, they can be arduous to process in most dental laboratories. The polyurethane (PUE) polymers utilized in maxillofacial rehabilitation consist of three components and are cured at room temperature, stamped by the linkage or presence of urethane.[11] However, other groups such as esters, ethers, and biuret can also be present within the molecule.[38] In general, this synthetic preparation has a segmented structure comprising a soft segment formed with a polymer glycol and a hard segment formed with a diisocyanate and an organotin catalyst so that the polymerization process can take place. Because these segments are varied in proportion to each other, the softness of the final product also varies for the intended application, as the maxillofacial prostheses lean to necessitate greater softness and pliability.[35],[39] Prostheses can be formulated softer and extra pliable by amplifying the ratio of polyol to diisocyanate in the polymerization concoction. These may be thermoplastic or thermosetting polymer systems. Both the microphase-separated structure and the mechanical properties of the PUEs are altered by a chemical structure and weight fraction of both constituents, a molecular weight, the polydispersity of the soft segment component, thermal, etc.[40],[41],[42]
For the PUPs, the soft segments are often divided into either an ether or an ester series, including carbonate series. On the whole, it is known that polyether glycol features a good hydrolysis resistance and inferior properties for UV radiation, and that polyester glycol has the opposite tendency for these properties.[43] Polycarbonate (PC) glycol is comparatively new and has both features. Thus, they are used to betting on things of the sensible usage.[44],[45],[46],[47]
The PUPs have a variety of fantastic properties. When processed correctly, they are chemically inactive, impervious to solvent and ozone; inodorous, abrasion resistant, have high tear and tensile strength, with low modulus of elasticity (even without plasticizer use), and a broad spectrum of pliability and softness. Thus, a prosthesis with softness and elasticity close to the tissues to be a substitute or a prosthesis with two divergent softness and proportions of pliability, e.g., in the nose, can be fabricated. The bridge of the nose is hard and stiff, while the tip of the nose and the alae are soft and pliable. Hence, it is possible to accurately simulate the divergent elastic modulus of living tissue. Materials are often made adequately pliable to allow the functioning of the tissue to which the prostheses are attached. The materials do not harden through wear and are dimensionally stable during processing and subsequently. They can be colored readily, intrinsically, and extrinsically, and deliver remarkable cosmetic results. Easy to process, yet, they compel great accuracy and caution in processing because poor elastomers may form and be degraded.[12],[38] Gonzalez et al. investigated the various physical and mechanical properties of polyurethane with the criterions such as surface hardness, modulus of elasticity, strength, percentage of elongation, and ratio of strength to modulus of elasticity. The conclusions of the study proved that these physical and mechanical properties can be changed and customized to the prosthetic situation by altering the ratio of polyol (part [A]) to isocyanate (part [B]) and adding catalysts. Compositions with small amounts of part (B) and without a catalyst reached or approximated the parameters suggested as ideal targets for simulating living tissues.[48] Goldberg et al. scrutinized in more detail the aliphatic diisocyanate and polyether macro glycol polymerizations with different cross-linking densities and OH/NCO ratios (glycol polymer). Stoichiometries, which gave between 8600 and 12,900 g/mol/cross-linking and an OH/NCO ratio (glycol polymer) of 1:1, ensued in polymers with a low modulus but high strength and elongation, which are compelled for maxillofacial applications. The significance of proper mixing was also emphasized to avert air entrapment and phase uncoupling of the catalyst. These are conditions that can generate the less durable prosthesis.[48],[49] An et al. found that there was no statistically notable divergence in elastomeric properties with respect to variation in curing temperatures and times.[50]
Polyurethane and other polymers have been used as a thin varnish over vinyl plastisol prostheses. The basic principle is that the varnish would avert the plasticizer from migrating and act as a barricade between the environs and the vinyl plastisol. While this process extends the lifespan of a vinyl plastisol prosthesis, it is not adequate to make the process agreeable. In 1987, Udagama instituted a procedure for securing a thin, prefabricated polyurethane film with silicone prostheses to extend tear resistance and adhesive affinity. Among the various lining materials tested, polyurethane was chosen due to its transparency, high tear resistance, moldability, and affinity with water-based skin adhesives.[51],[52]
However, there are still serious shortcomings. These materials are different to process consistently. Only a small error rate is possible when measuring the components. The temperature of the material should be regulated as just a few degrees Fahrenheit can alter the chemical reaction notably. Furthermore, the isocyanates are susceptible to moisture, and if moisture contamination occurs, gas bubbles (CO2) cause deformity and poor curing of the material. Moreover, proportioning, temperature, and moisture can be easily controlled in large commercial establishments, but these factors can be difficult to regulate within the usual laboratory. Therefore, linotype metal molds (if higher temperatures are used so that higher tensile strengths are obtained after the polymerization) are almost a requirement to avert moisture contamination.[11],[12] However, stone, urethane, and epoxy molds can be also used. If stone molds are used, they must be dry out assiduously ahead of processing. As soon as the prosthesis is contaminated with such contaminants, the outer staining no longer coheres so well.[25] The particulars of the casting technique of these materials are discussed by Gonzalez.[38]
Polyurethane materials accessible for maxillofacial rehabilitation are not color stable, apparently as a consequence of UV radiation and surface oxidation. In addition, there is a tendency of quick deterioration of surface coloring. Thus, the clinical longevity of this material is usually but <6 months and more often approaches 3 months.[11],[12],[53]
Another dissuading aspect is their poor compatibility with prevailing adhesive systems. Scrubbing the adhesive from the prosthesis is arduous and exasperating for innumerable patients. The outer coloring is often peeled off during this process. Wariness should be taken when managing the isocyanate as these compounds are noxious. Although the final prosthesis has the isocyanate in bound and presumably nonnoxious form, there is proof that the ternary composition for maxillofacial prosthesis is noxious to human excised donor orofacial tissue cells.[12],[25],[54] Lontz and Schweiger have outlined free isocyanate in cured restorations, which is an evident issue related to cytotoxicity and tissue irritation after prosthetic wear.[55]
Initial PUPs for extraoral maxillofacial prostheses were epithane-3 and Calthane ND2300. Epithane-3 is a three-component polyurethane system, officially retailed by MIP Industries as dermathane.[38],[48] The kit consists of a polyol component (a mixture of polyesters), a diisocyanate component, and an organotin catalyst (tin octoate or dibutyltin dilaurate).[35] In addition, it also contains a thermosetting deglossing emulsion of a polyurethane polymer with silica powder, which serves as a protective layer to insulate or seal the surface colorants. In particular, inorganic colorants (pigments and inks) that have been specially developed for this system are known to be innocuous to humans and resistant to UV radiation.[38],[56] The material has been used clinically by a number of clinicians; however, the most comprehensive studies have been delineated by Gonzalez et al.[48] In general, the material provides well-characterized and realistic prostheses. Studies have shown that the mechanical properties of the polymer can be calibrated by handling the ratios of the polyol and diisocyanate components. Softer, additional pliable prostheses can be acquired by increasing the ratio of polyol to diisocyanate in the vulcanization mixture.[35]
Calthane ND2300 is distributed by Cal Polymer C0.I4 (California). It is a two-component system and contains neither 4,4-methylenebis-2-chloroaniline nor toluene diisocyanate. Part A is an aliphatic diisocyanate-terminated prepolymer and Part B is a polyester with hydroxyl termination. The two components are colorless and react when mixed with acceptable antioxidants and UV stabilizers to form a polymer that is suitable as a maxillofacial material.[53],[57]
Isophorone polyurethane is next in the series that has been developed in the PUP series. The material is formulated as a three-component kit that includes an isocyanate-terminated prepolymer, a triol as a cross-linking agent, and an organotin catalyst. The main component of the system, the prepolymer, is made by the controlled combination of isophorone diisocyanate, butanediol, and a polyether polyol. Exceptionally, it has a higher strength compared to other aliphatic polyurethanes, resulting from the cycloaliphatic isophorone unit in the vulcanized network. The system has not been studied toxicologically in animals and requires further clinical trials.[35],[58]
Divergent studies were conducted in the family of polyurethanes to find a polymer that has higher durability and color stability, as well as better processing properties. Additives that enhance light stability are used in commercially available polyurethane, but some of them are noxious or mutagenic. Comprehensive tests are, therefore, requisite to assess individual components. The structural stability of the urethane chains is not adequate to avert detaching chemical bonds when exposed to UV radiation. Deterioration without these light stabilizers is thus inevitable.[12]
Lewis and Castleberry delineated the development of an aliphatic polyurethane prepolymer, isophorone, and preliminary data on its physical and mechanical properties.[35] Turner, who had assessed the mechanical properties of the polyurethane polymer before and after 900 h of accelerated aging, reported that the material did not disintegrate and that it showed many desirable properties for usage as a maxillofacial prosthesis.[59] However, further assessment of biocompatibility and clinical trials with the material are still required.
Therefore, polyurethane materials can have a considerable possibility for future use as new formulations are developing quickly. Any one of these could be ideal for maxillofacial prostheses.
| Conclusion | | |
Researches have made significant advances in extraoral maxillofacial prosthodontics. The form of prosthesis that rehabilitates the facial tissue, principally the overlying skin, must meet strict caliber to simulate living moving tissue. Although numerous extraoral maxillofacial materials are currently available, none is ideal for simulating and rehabilitating lost facial structures. Each material has its own advantages and disadvantages. In the next Part III of this article, silicone elastomers as an extraoral maxillofacial material will be discussed.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Table 1]
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