Polymers

Commonly referred to as plastic material, polymers have arguably made the most significant impact on the science of implantation. Polymerization refers to the process by which carbon links are chained together in a repeating monomeric unit. The physical attributes of polymeric material are dictated by the degree of polymerization, cross-linking, the size of the repeating unit, and the position of the repeating unit in the chain (70). Polymers are classified into several groups based on their physical characteristics: solid, porous, woven, or injectable. In general, the lower -g the molecular weight of the material and the less cross-linking, the more fluid the polymer.

Solid polymers include solid silicones and polymethylmethacrylate. Silicones have been used in a variety of procedures in facial plastic surgery. Silicones have a repeating unit of dimethylsiloxane that dictates the viscosity of the material (71). >3

This may vary from a liquid injectable form to sheets of solid silicone manufactured |

under the name of Sialastic (Dow Corning, Midland, Michigan). When solid o silicone is implanted, a thin fibrous capsule is formed around the material. There is minimal to no ingrowth of host tissues into the implant. This minimal inflammatory response occurs only under the optimal conditions of small implants that have been well stabilized (72). In implants subject to migration, a dense reaction is noted resulting in a much thicker capsule. For silicone rubber and Sialastic, rare instances of resorption have been reported (73). Although the Food and Drug Administration (FDA) does not approve of injectable silicone, preformed silicone implants are used for malar, mental, nasal, and mandible augmentation. The use of silicone in nasal augmentation has fallen out of common use because of a relatively high degree of extrusion (74).

Polymethylmethacrylate is an acrylic polymer that possesses the advantages of great strength and rigidity and may be sculpted in the operating room during surgery. The polymer is supplied in a powder form that is mixed with a monomer liquid to form the final product. This process is highly exothermic and may reach temperatures in excess of 100° C. These high temperatures have rarely resulted in intraoperative complications (75). Such reactions are exceedingly rare and have never been reported in facial plastic procedures. The inflammatory response to implantation ofpolymethylmethacrylate has been described as intense and characterized by a multi-nucleated giant cell reaction (76). Because of its many potential advantages, it remains a viable choice for cranioplasty in neurosurgical procedures and in selected cases of orbital reconstruction.

The porous polymers include high-density porous polyethylene (HDPE) (Med-pore, Marlex) and polytetrafluorethylene. HDPE exhibits a minimal inflammatory response with the ingrowth of tissue into the porous implant (77). It is composed of chains of ethylene units with a high degree of side-chain branching. The size of the pores within the HDPE dictates the amount of fibrous ingrowth with the pore size of 150 mm being ideal.

Polytetrafluorethylene (PTFE; Proplast II, Vitek, Houston, TX) is also highly porous; the size of the pores range from 200 to 500 mm and the pore volume makes up to 90% of the total volume of the implant (78). Implantation has been shown to have few complications and a high degree of reliability. Within the implant, a dense inflammatory cell response with granulation occurs (79). Another advantage of Proplast II is its relative ease of sculpture and it has been used successfully in orbital, malar, nasal, mandibular, and ossicular chain reconstruction.

Gore-Tex (W.L. Gore, Flagstaff, AZ), a fibrillated polytetrafluorethylene, has been more recently introduced and met with wide success. It is produced as a soft tissue augmentation patch in thicknesses of 1, 2, and 4 mm and is easily cut and shaped. It has been used commonly for nasal, malar, lip, and brow augmentation (80). A pore size of 30 mm may delay the ingrowth of fibrous tissue; however, the inflammatory response is similar to that seen in Proplast. In general, it is a reliable and stable implant.

The meshed polymers include polyamide mesh (Nylamid Mesh, S. Jackson Inc, Mineapolis, MN), HDPE mesh (Marlex Mesh), polypropylene polymer mesh (Prolene, Ethicon, Inc, Somerville, NJ), and polyethylene terephthalate (Dacron, Merceline, Ethicon). The major advantages of mesh include its malleability and stability with implantation due to ingrowth of surrounding tissue. It may be folded, sutured, cut, and shaped with ease. The material can be problematic, however, if infected and removal is difficult due to its adhesion to surrounding tissue. Polymer mesh was initially thought to be chemically inert and highly resistant to breakdown. Due to evidence of structural breakdown and loss of material, meshes are now used infrequently for facial implantation (81). Several authors have found Mersiline mesh to be helpful in malar and chin augmentation, but by far the most common use of mesh polymers is in abdominal wall repair (82-84). Silver and Maas suggest in previously unpublished data that Mersiline also undergoes significant resorption and loss of structural support after several years of implantation and question its use in facial augmentation (76). As expected, a dense inflammatory response with multinucleated giant cells is seen surrounding the implanted mesh (85).

Injectable polymers include silicone and PTFE paste, and while initially promising, these materials have shown various problems in clinical use. Injectable sili-cone was widely used in facial augmentation when first released, however, major problems with migration and extrusion were noted. Case reports of systemic and severe local allergic and inflammatory reactions were published. As a result, injectable silicone is not approved for facial augmentation and is not widely used. PTFE paste (Polytef paste) has been used successfully in vocal cord augmentation, however, its use in facial augmentation is limited due to foreign body reaction and difficulties in injection due to its high viscosity. Injectable collagen and Alloderm have generally replaced injectable polymers in facial plastic surgery.

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