Maxillofacial Prosthetic Materials Ideal Requirements

• Maxillofacial materials are used to correct facial defects or deformities resulting from cancer surgery, accidents or congenital deformities.
• The nose, ears, eyes or any other part of the head and neck may be reconstructed by these prostheses.
• They are also used in the movie industry for special effects.
• Ancient Chinese and Egyptians used waxes and resins to reconstruct missing portions of the face and head.

Read And Learn More: Basic Dental Materials Notes

• Since 1500 CE when facial prostheses were described by French surgeon Ambrose Pare (1575), they have evolved from prostheses made from gold, silver, paper, cloth, leather, wrought metals, ceramics, and vulcanite.
• Modern maxillofacial prosthetics saw a resurgence after the world war due to the severe nature of the war-related injuries.
• In spite of the advances in techniques and materials, there is still a lot of scope for further development in this field.

Ideal Requirements

• These materials must be biocompatible, easy and inexpensive to fabricate, strong and durable.
• The prosthesis must be skin-like in appearance and texture.
• It must be color stable as it is subjected to sunlight (including ultraviolet light) heat, and cold.
• It must be easy to clean and manage by the patient.
• Facial prostheses are often constructed with thin margins to enable blending to the skin.
• This is then attached to the skin with adhesives. On removing at night, the thin edges can tear. It must be resilient enough to prevent tearing.
• The water absorption of the prosthetic material is important since facial prostheses may absorb saliva or sweat from surrounding facial tissue.
• During washing, the prosthesis can absorb water. Any absorbed water may affect the physical properties and also affect the perception of color matching to the surrounding facial tissue.
• It should be hygienic and prevent the growth of microorganisms.
• It should have the translucent properties of the part it is replacing.

No material so far has all of these characteristics.

Evolution Of Maxillofacial Materials Poly 1940–1960

• It was once commonly used for maxillofacial prostheses.
• It is readily available, easy to manipulate, strong, color, stable, hygienic, and durable.
• Its usefulness in extra-oral prosthesis is limited because acrylic is hard and heavy, does not move when the face moves, and does not have the feel of skin.
• Particularly used in cases where there is the least movement of the tissue bed during function.

Latexes

• Latexes are soft, inexpensive, and easy to manipulate. They are realistic and form lifelike prostheses.
• However, the finished product is weak, degenerates rapidly with age, and changes color. Latex is no longer a major facial prosthetic material.

PlastIcIzed PolyvInyl chlorIde

• Polyvinyl chloride is a rigid plastic and is made more flexible by adding a plasticizer.
• Other ingredients added to polyvinyl chloride include cross-linking against (for strength) and ultraviolet stabilizers (for color stability).
• Color pigments can be incorporated to match individual skin tones.
• It is supplied as finely divided polyvinyl chloride particles suspended in a solvent.
• When the fluid is heated above a critical temperature, the polyvinyl chloride dissolves in the solvent.
• When the mix is cooled, an elastic solid is formed.
• The prosthesis becomes hard with age because the plasticizers are lost from the surface of the prosthesis.

Chlorinated Polyethylene

• This material was introduced in the 1970s and 1980s as an alternative to silicone.
• Processing involves high heat curing pigmented sheets in metal molds.
• Dow Chemicals’ chlorinated polyethylene elastomer is an industrial-grade thermoplastic elastomer.
• It is less irritating to the mucosa than silicone, less toxic than thermosetting silicone materials, and non-carcinogenic.
• Chlorinated polyethylene elastomer appears to be a suitable substitute for silicones for the fabrication of extraoral maxillofacial prosthesis in situations where the cost of silicone is prohibitive.
• Advantages Higher edge strength, permanent elasticity, and lower fungus growth

Polyurethane Polymers

• It is the most recent addition. One of its components is acrylate, which needs careful handling to prevent a toxic reaction to the operator.
• Although the material is cured at room temperature, it requires accurate temperature control because a slight change in temperature can alter the chemical reaction.
• A metal mold is used to avoid moisture in the air affecting the processing.
• It has a lifelike feel and appearance and the color stability is better than that of polyvinyl chloride.
• But it is susceptible to deterioration with time.

Silicone Rubber

• Silicones were introduced as a maxillofacial material in the 1950s.
• Currently, silicone-based maxillofacial materials are the most widely used.
• Based on the curing mechanism, two types of silicone rubber are available. Both types are widely used.
  • RTV – Room Temperature Vulcanized
  • HTV – Heat Vulcanized Silicones

Silicone Rubber supplied as

• Both HTV and RTV silicones are available as fluid, semisolid, gel-like or putty-like material.
• They are generally supplied as clear or translucent materials though occasionally they may be supplied pre-pigmented (for example flash colored – 2009 by Factor 2).
• They are usually provided as base and catalysts where the base-catalyst ratio is 10:1.
• However, the base-catalyst ratio of 1:1 is also available.

Room temperature vulcanized (RTV) silicones

• Room vulcanizing silicones \ are available as fluid, semisolid or putty-like material.
• They may be transparent or translucent. The prosthesis can be easily fabricated in the dental laboratory with little special equipment using RTV silicones.
• However, such silicones are not as strong as heat-vulcanized silicones and the intrinsic color is monochromatic.

Room temperature vulcanized Types

1. Both condensation (tin catalyzed), and
2. Addition types (platinum catalyzed) are available.

Working time

Ranges from 10 min to 2 hours depending on the product.

Curing

• Curing time varies with products and can range from 25 minutes to 24 hours depending on the product.
• The optimal room temperature for curing is around 25 °C. Very high temperatures and humidity reduce working time.
• Lower temperatures are also not recommended (below 20 °C) as it can cause retardation of setting and affect the properties.

Heat Vulcanized Silicones

• Heat vulcanizing silicone is available as semisolid or putty-like material.
• They are supplied as a two-component system—a base (vinyl and hydride-containing siloxanes) and a catalyst (chloroplatinic acid catalyst).

Commercial examples

Cosmesil M511, Nusil, Factor II, TechSil S25, etc.

• Fabrication involves milling, packing under pressure, and curing.
• Pigments are milled into the material for intrinsic coloring.
• This is the material of choice, particularly in terms of strength and color stability.
• The coloring procedure is faster. Both intrinsic and extrinsic stains can be used making it polychromatic.
• Curing One current silicone (Cosmesil M511) has a working time of approximately 1 hour and a curing time of approximately 1 hour at 100 °C.

Heat Vulcanized Silicones Advantages

• Better strength and color stability.

Heat Vulcanized Silicones Disadvantages

• A milling machine and press is required.
• A metal mold is normally used and the fabrication of the mold is a lengthy procedure.

Coloring pigments and effects

• To simulate natural skin and body appearance, various cosmetic grade pigments both intrinsic and extrinsic may be applied.
• They include a range of colors as well as basic skin shades. Pigments and fiers are available as
  • Dry powder
  • Liquid stains
  • Flocking microfibers are added to silicones to provide the increased appearance of depth and light scattering
  • Short veining fiers
  • Longer fiers for creating the appearance of more complex veining structures

Skin Adhesives

• Adhesives are often used to attach the prosthesis to the skin.
• Various forms of adhesives used are water-based gels, creams, liquids or silicone-based pastes.
• Water-based adhesives are easily washed of with soap and warm water.

Properties Of Silicones

1. Shore hardness

• In order to achieve a lifelike feel, the material must be soft and compressible like flesh.
• Most materials have a shore hardness value ranging from 15 to 40.
• Hardness can be controlled through the use of softening agents supplied by the manufacturer.
• Depending on the tissue being replaced, tissue consistency may be soft (Shore A:10), medium (Shore A:30) or hard (Shore A:40).

2. Elongation

• Elongation ranges from 340 to 630%.

3. Bonding to acrylic

• Bonding to acrylic is required on occasion to attach dentures an other acrylic prostheses to the silicone prostheses.
• Bonding may be achieved by the use of a special primer/bonding agent.

4. Tear Strength

• Tear strength ranges from 84 to 120 ppi.

Advantages Of Maxillofacial Silicones Over Other Materials

• Polyvinyl siloxane is the most successful maxillofacial prosthetic material to date and new advances are being made to this material to overcome its weaknesses.
• Silicones became more popular than other materials for the following reasons.
  • They have a range of good physical properties (such as excellent tear and tensile strength) over a greater range temperature range.
  • Easier to manipulate.
  • The high degree of chemical inertness.
  • Low degree of toxicity.
  • The high degree of thermal and oxidative stability.
  • They can be stained intrinsically and/or extrinsically to give them a more life-like natural appearance.
  • When adequately cured, silicone elastomers resist absorbing organic materials that lead to bacterial growth and so with simple cleaning these materials are relatively safe and sanitary compared to other materials.

Disadvantages Of Maxillofacial Silicones

• Susceptible to fungus growth.
• Susceptible to fraying at the edges.

3D PrInted MaxIllofacIal Prostheses

• Conventional maxillofacial prostheses are incredibly laborious and expensive to produce can take up to 10 weeks to complete.
• The process involves taking an impression from the area of trauma, casting a plaster positive, then making a mold, carving the desired form in wax, and finally casting in silicone.
• The end result of this handmade process is normally expensive ranging between £1,500 to £3,000.
• Among others, Sheffid-based Fripp Design has developed a system for fast and low-cost manufacture of facial prostheses such as nose and ear replacements for accident victims.
• Working with researchers at the University of Sheffield, the company developed a process that can print a customized nose or ear within 48 hours.
• First, the patient’s face is 3D-scanned, then the specific contours are added to a digital model of the new prosthetic part for a perfect fit.
• These features are either taken from the scan of the patient’s relatives or the patient’s own file, for example, one ear can be scanned and mirrored to replicate another.
• Work is also in progress on 3D-printed eyes. A handmade eye can cost between $ 1800 to 8300, but a 3D-printed one will only cost around $160.
• The parts are printed in full color in starch powder using a Z Corp Z510 color 3D printer.
• The lightweight model is then vacuum-infiltrated with medical-grade silicone, binding it together.
• The cost of making such a part is almost the same as a handmade prosthetic, but Fripp says once the file is created, it can be used infinitely and the cost can be lowered to £150.
• The main barrier is the high cost of 3D scanning technology as well as getting approval from the health authority.

3D Bioprinting

A natural evolution of artificial prostheses would ultimately be the ultimate goal of creating replacements of missing facial parts using living substitutes.

• The technology of 3D bioprinting—the medical application of 3D printing to produce living tissue and organs is rapidly advancing.
• In August 2013, the Hangzhou Dianzi University in China announced it had invented the biomaterial 3D printer Regenovo, which printed living cells that survived for up to four months.

• San Diego medical research company Organovo announced last year it had created slices of functioning, long-lasting human liver which can survive for 40 days, using a 3D printer.
• Dr. Faiz Y Bhora, Director of Thoracic Surgical Oncology at the St Luke’s-Roosevelt Hospital Center in New York, focuses his work on producing 3D printed tracheas from completely biological materials primed with stem cells for growth.
• Bioengineering involves the creation of scaffolds on which the tissues can grow.
• The scaffolds must meet some specific requirements. A high porosity and an adequate pore size are necessary to facilitate cell seeding and diffusion throughout the whole structure of both cells and nutrients.
• Biodegradability is often an essential factor since scaffolds should preferably be absorbed by the surrounding tissues without the necessity of surgical removal.
• A commonly used synthetic material is polylactic acid (PLA). This is a polyester which degrades within the human body to form lactic acid, a naturally occurring chemical which is easily removed from the body.
• Other materials are polyglycolic acid (PGA) and polycaprolactone (PCL). Scaffolds may also be constructed from natural materials.
• Derivatives of the extracellular matrix have been studied to evaluate their ability to support cell growth.
• Protein materials such as collagen or firing and polysaccharides like chitosan or glycosaminoglycans (GAGs) have all proved suitable in terms of cell compatibility, but some issues with potential immunogenicity still remain.
• Among GAGs hyaluronic acid, possibly in combination with cross-linking agents (for example glutaraldehyde, water-soluble carbodiimide, etc.) is one of the possible choices as scaffold material.
• Another form of scaffold under investigation is decellularised tissue extracts whereby, the remaining cellular remnants/extracellular matrices act as the scaffold.
• Problems to overcome in the field of bioengineering remain problems with revascularization and debate on the ethical ramifications (see also a chapter on additive manufacturing).