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Implant Materials
The history of implant materials can be traced to 3000 BC, when the Incas of Peru used gold and silver to repair trephination defects.
Originally, implants were devised to restore function,
and aesthetics was secondary. Soldiers suffering from the
ravages of World War II and later conflicts cried out for
effective therapies.
The selection of a particular implant depends on the specific requirement for its use. Implants can be used as alternatives to autogenous tissue in selected cases and, as such, have the advantages of avoiding operative time for graft harvesting, the absence of donor site morbidity, and an unlimited supply. Unlike autologous tissue, implants can be fabricated such that they undergo no resorption, and may therefore be preferable to autologous grafts in certain cases. In particular, implants have been used as bone graft substitutes with much success in orbital floor reconstruction and as interpositional materials in maxillofacial reconstruction.
The use of autologous tissue, particularly vascularized autologous tissue, may be more appropriate in many circumstances, especially when conditions for implantation are not optimal. These include a history of radiotherapy, marginal blood supply of the surrounding tissue, or tenuous soft tissue coverage over the implant. In these circumstances, the risk of implant-related complications, including infection and implant extrusion, are significant, and the use of an alloplastic implant should be avoided. Implants are less likely to tolerate overlying wound healing problems than autologous tissue, and in such circumstances may require removal of the implant when a reconstruction with autologous tissue might have been salvaged. In addition, it should be recognized that the complications of alloplastic materials may develop over the long term and therefore one must not make early assumptions about the safety of these implants.
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Metals
Metals are primarily used in plating systems for craniomaxillofacial internal fixation. Stainless steel, cobalt-chromium, pure titanium, and titanium alloys are the principal metals currently available. Characteristics of a desirable metal implant include biocompatibility, strength, resistance to corrosion, and imaging transparency.
Gold has also been used as an implant material, but with limited applications. Although gold is resistant to corrosion, its lack of strength has generally made it a suboptimal choice as an implant. It is, however, used as an upper eyelid weight in cases of facial nerve dysfunction.
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Calcium Phosphate Ceramics
Calcium phosphate ceramics have been extensively used as bone graft substitutes. They are biocompatible, can be fabricated into different shapes, and are osteoconductive, providing a scaffold for bone ingrowth. The primary calcium phosphate ceramics in clinical use are hydroxyapatite and tricalcium phosphate.
he principal disadvantages of calcium phosphate ceramics are that they are brittle, have low tensile and compressive strength, and may be difficult to fixate. However, because they are osteoconductive, they are incorporated into surrounding tissue by both bone and fibrous tissue ingrowth, and become vascularized.
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Polymers
Polymers are the most extensively used alloplastic materials and have applications in both bone and soft tissue reconstruction and augmentation. The structure of polymers consists of long chains of repeating basic units that can reach high molecular weights. Commonly used polymer biomaterials include silicone, polyurethane, polymethylmethacrylate, polyesters, nylon, polyethylene, polypropylene, cyanoacrylates, and polytetrafluoroethylene.
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Polytetrafluoroethylene
The basic unit of the polytetrafluoroethylene (PTFE) polymer consists of an ethylene monomer backbone with four covalently bound fluorine molecules. This material is inert and highly biocompatible.
Teflon (Dow Chemical Company, Wilmington, DE) is synthesized from the polymerization of tetrafluoroethylene gas under high temperature and pressure. It is a chemically inert polymer with favorable handling characteristics. The main application for Teflon has been in the reconstruction of orbital floor defects. |
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Cyanoacrylates
The cyanoacrylates are quick-setting, biodegradable, polymeric tissue adhesives that have become useful tissue-bonding agents11. They form a strong, durable bond with most human tissues, particularly those that contain a large amount of protein such as skin and tendon. In addition to their role in bonding tissues, these polymers have also been used as hemostatic and embolic agents.
The cyanoacrylate tissue adhesives polymerize by an exothermic reaction in the presence of water and hydroxyl groups on the wound surface, and thus are effective on moist surfaces.
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Polyethylene, Polypropylene
Polyethylene is an inert material with a high degree of biocompatibility. Several types of polyethylene compounds are available with different biochemical properties, based on the density of the material. Chemical resistance, tensile strength, and hardness increase with increasing density from the low-density polyethylene to the ultra high molecular weight polyethylene.
Medpor (Porex Surgical, College Park, GA) is a high-density, porous polyethylene implant that is widely used in facial reconstruction. It is nonantigenic, nonallergenic, nonresorbable, highly stable, and easy to fixate, and is available in a wide variety of preformed shapes. |
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Polyamide
It consists of long chains of amide units that are twisted and then woven. Nylamid is biocompatible, can be easily shaped and sutured, possesses stability as a result of fibrous tissue ingrowth, and has been used successfully as an implant for the repair of orbital floor defects. Polyamide compounds do, however, undergo resorption over time and their applications in facial reconstruction and augmentation are therefore limited.
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Silicone
Silicone, or dimethylsiloxane, consists of a monomer backbone of interlinked silicone and oxygen molecules, with methyl and occasionally vinyl or phenyl side groups, in a varying number of repeating units. This material is very stable, highly biocompatible, nontoxic, and insoluble in body fluids. While silicone is considered biologically inert, it does elicit a mild foreign body reaction, which is followed by encapsulation, but without tissue ingrowth.
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Bioplastique
Bioplastique is a non biodegradable, relatively inert material used as an in jectable liquid for soft tissue augmentation8. It consists of solid, textured silicone rubber microparticles, mixed with water and a hydrogel carrier.
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Polyurethane
The polyurethanes are a group of polymers consisting of a diisocyanate and an alcohol. Concerns regarding the effect of toluene-diamine, the breakdown product of polyurethane, resulted in the FDA request for a voluntary delay of production and sales of polyurethane-covered breast implants in 1991, and production was subsequently discontinued.
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Polymethylmethacrylate
Polymethylmethacrylate (PMMA) is a high molecular weight polymer used as a replacement for bone. It is biocompatible, biologically inert, and rigid. When liquid methylmethacrylate monomer is added to powdered granules of methylmethacrylate polymer, a moldable dough forms as the monomer polymerizes and binds together preexisting polymer particles, and hardens in about 10 minutes.
Most of the potential complications are related to the exothermic reaction produced during the curing process, which can result in local tissue damage, including bone necrosis and soft tissue injury. Local and systemic allergic reactions have on occasion been observed and are attributed to the toxic effects of the unbound monomer.
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Biodegradable polyester
In contrast to the aromatic polyester, Dacron, the biodegradable polyesters are aliphatic compounds that are more susceptible to hydrolysis. Polyglycolic acid (PGA) and poly-L-lactic acid (PLLA) are two of these polymers that are degraded in the body at physiologic pH over the course of several months. These resorbable polymers are available as mesh sheets for body wall reconstruction, and as rods for the internal fixation of fractures and osteotomies.
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