Alloplastic Materials

Introduction

Advances in medical technology have allowed plastic surgeons to utilize synthetic materials as an alternative to autologous tissues when performing many of today’s aesthetic and reconstructive surgeries. Although autologous materials are generally preferred, synthetic materials provide several advantages over tissues obtained from the patient:

• Not resorbed over time (unless they are designed to do so)
• Do not require a second surgical donor site
• Provide more material than can often be obtained from the patient
• Can be custom-tailored to the individual patient
• Reduce operating time since graft harvesting is not performed

Because of the many benefits to using alloplastic materials, there is currently a strong interest in developing the ideal implant material which would possess the following characteristics: it should (1) be chemically inert; (2) be incapable of producing hypersensitivity or a foreign body reaction; (3) be easily contoured; (4) retain stable shape over time (except when desired); (5) be noncarcinogenic; (6) become ingrown or replaced by living tissue; (7) be easy to remove and sterilize; and (8) not interfere with radiographic imaging. Despite much effort and ingenuity, creation of the ideal implant material has yet to be accomplished. However, various alloplastic materials are being used today in plastic and reconstructive procedures, and many of them have proven quite promising.

Pre- and Intraoperative Considerations
The vascularity of the recipient site and the ability to provide sufficient soft tissue coverage of the implant must be assessed preoperatively. Decreased vascularity secondary to scar tissue (from previous surgeries) or radiation impairs the establishment of normal fibrovascular tissue encapsulation and may interfere with the normal inflammatory response if the implant were to become infected.

In order to prevent implant exposure or extrusion, soft tissue coverage over an implant should be as thick as possible. The size of the implant should be comparable to that of the tissue pocket or wound cavity in order to avoid tension of the overlying soft tissue, and the implant should be fixated to a stable adjacent structure to prevent migration of the implant. All patients should receive perioperative intravenous antibiotics followed by a postoperative oral course, although the optimal antibiotic choice and duration have yet to be determined for most implants. What is clear is that intraoperative handling of the implant should be minimized in order to prevent bacterial transmission, and strict adherence to sterile technique is essential.

Classification of synthetic materials used in plastic and reconstructive surgery
Silicone-based materials:	BioPlastique Injectable silicone Silastic sheets Silicone Silicone gel
Polytetrafluoroethylene:	Gore-Tex Proplast I and II Teflon
High density polyethylene:	Medpor
Polymer mesh:	Dacron (Mersilene) Dexon Prolene Supramid Vicryl
Biological glasses:	Bioactive glasses (Bioglass) Glass ionomer
Tissue adhesives:	Cyanoacrylate
Acrylics:	HTR Polymer Methylmethacrylate

Choice of Alloplastic Material
The type of procedure as well as the size and character of the defect being augmented often dictate the type of implant material. In the preantibiotic era, inert materials such as gold, silver, platinum and paraffin were used with little success and were quickly abandoned. Currently, there are numerous implantable materials being used today (Table 14.1). These materials are used in a wide range of procedures, such as aesthetic procedures, craniofacial surgery, maxillofacial trauma, breast reconstruction and hand surgery. Table 14.2 lists the common uses for the various allopastic implants.

Silicone
Silicone-based prosthetics have been used as medical implants since the 1950s due to their chemically inert nature, resistance to degradation, and lack of significant allergic reactions. Silicone is useful for a variety of aesthetic surgeries, complex contouring and reconstructive procedures. Silicone comes in the form of silicone gels, silicone rubber or solid silicone implants. Silicone gels can provide a more natural feel, as seen with breast implants, but the risk of rupture requiring capsulectomy is a distinct disadvantage. The use of silicone gel has been surrounded by controversy related to concerns about migration, toxicity and an unproven association with systemic disease, leading to restriction of the use of silicone gel implants by the FDA in 1992. This ban was recently lifted after an extensive unbiased review by the Institutes of Medicine. Silicone rubber is used for tissue expanders, the outer shell of both saline-filled and silicone gel-filled breast implants, and as an onlay material for the augmentation of the bony skeleton and soft tissues. However, silicone rubbers are relatively weak and tend to tear, leading to implant failure. Solid silicone implants are commonly used for chin and malar augmentation, and have been used in nasal, chest and calf augmentation, as well as in joint replacement and tendon reconstruction.

A list of the procedures that commonly employ allopastic materials
Procedures	Materials Used
Cranioplasty and forehead augmentation	Glass ionomer and bioactive glass Hard-Tissue-Replacement (HTR) polymer Methylmethacrylate Medpor Poly(L-lactide) and polyglycolic acid plates and screws Silicone
Anterior mandibular augmentation	Medpor Polyamide mesh Silicone
Mandibular body and angle augmentation	Glass ionomer and bioactive glass Medpor Methylmethacrylate Poly(L-lactide) and polyglycolic acid plates and screws
Malar and maxillary reconstruction	Glass ionomers Gore-Tex Medpor Methylmethacrylate Polyamide mesh Silicone Teflon
Zygomatic reconstruction	Glass ionomers Medpor Gore-Tex Poly(L-lactide) and polyglycolic acid plates and screws Silicone
Orbital reconstruction	Gore-Tex HTR Polymer Medpor Poly(L-lactide) Silicone Teflon
Ear reconstruction	Medpor Silicone
Tendon repair	Gore-Tex Cyanoacrylate
Soft tissue augmentation	BioPlastique Gore-Tex
Breast augmentation and tissue expansion	Silicone (saline or silicone gel filled)
Wound repair and scar revision	Cyanoacrylate Silastic sheets
Chest and abdominal wall reconstructions	Dacron mesh Gore-Tex Prolene mesh Vicryl mesh
Nasal augmentation	Gore-Tex Polyamide mesh Silicone

Because silicone is not porous, tissue ingrowth does not occur. A fibrous capsule forms around the implant that is relatively avascular and can contract which may lead to implant migration. This avascular capsule is a potential space for infection and in the setting of infection may require removal of the implant.

BioPlastique
BioPlastique® is a nonbiodegradable, relatively inert injectable liquid used for soft tissue augmentation. The textured surface of the particles allows for tissue ingrowth, and the particle size is large enough to prevent engulfment by macrophages but small enough to become encapsulated within 3 to 6 weeks. Studies on the use of BioPlastique demonstrate good-to-excellent results in augmenting small defects on the dorsal nose, malar area, cheeks and chin with no adverse immunologic reactions. Although the clinical results with Bioplastique have been encouraging, it is not FDA approved at this time.

Polymethylmethacrylate
Polymethylmethacrylate (PMMA) is an acrylic polymer used as a bone substitute in plastic surgery and neurosurgery. PMMA is radiolucent, extremely durable and completely biocompatible, making it a widely used material for cranial bone reconstruction-alone or in combination with wire or mesh reinforcement. When powdered granules of methylmethacrylate polymer are mixed with methylmethacrylate liquid monomer, a moldable dough forms as the monomer polymerizes and hardens in about ten minutes. Near the end of the polymerization process, an exothermic reaction occurs that can potentially damage the local tissues, the major complication associated with the use of PMMA. This can be avoided by continually irrigating the implant bed with cool saline during the polymerization. A rare, but serious complication is the inadvertent entry of the PMMA into the venous or arterial systems. If this occurs it can cause complete heart block, cardiac arrest and other arrhythmias. This complication is most often seen during orthopedic procedures where PMMA is used for joint replacements or fracture repair. Hard-tissue-replacement (HTR) polymer is a porous form of PMMA that allows for fibrous ingrowth, leading to an implant that is nonresorbable and very stable. Applications for HTR include chin and malar augmentation, with potential for additional uses in craniofacial reconstruction.

Polyester (Dacron®, Mersilene®)
Polyethylene terephthalate (Dacron) is a biocompatible, flexible, nonabsorbable polymer that is used as a suture material, as a prosthetic material for arterial replacement, and as a mesh (Mersilene) in abdominal and chest wall reconstruction. Its use has also been described for chin and nasal augmentation. Biodegradable Polyester (Polyglycolic Acid, Poly-L-lactic Acid) Polyglycolic acid (PGA) and Poly(L-lactide) (PLLA) are polymers that are degraded in the body at physiologic pH over the course of weeks to months. These resorbable polymers are available as mesh sheets for body wall reconstruction and as rods for the internal fixation of fractures and osteotomies. They have also been fashioned into resorbable miniplates and screws for the fixation of bones of the craniofacial skeleton. Although they do not appear to have any cytotoxic effects, they do provoke an inflammatory or foreign body response after implantation.

Polyamide Mesh (Supramid®, Nylamid®)
Polyamide mesh is a woven, polymer mesh implant that is biocompatible, can be easily shaped and sutured, allows for fibrous tissue ingrowth, and has been used for the repair of orbital floor defects. It seems to be well tolerated and has a low rate of extrusion, even in areas of thin skin such as the nasal dorsum. However, polyamides do undergo resorption and induce an inflammatory response, making their use in facial augmentation and reconstruction somewhat limited.

Porous Polyethylene (Medpor®)
Medpor is a high-density, porous polyethylene implant used frequently in facial surgery because it is nonantigenic, nonallergenic, nonresorbable, highly stable and easy to fixate. In addition, Medpor is available in a wide variety of preformed shapes for its use as a malar, chin, nasal, orbital rim, orbital floor and cranial implant, as well as an auricular framework in postburn ear reconstruction. Overall, complications of Medpor, such as exposure or infection, are rare.

Polytetrafluoroethylene (Teflon®, Gore-Tex®, Proplast®)
Polytetrafluoroethylene (PTFE) is an inert and highly biocompatible polymer that is extremely useful in soft tissue augmentation but has limited use in bony repair due to its low tensile and compressive strength. Teflon, the first PTFE graft to be used in plastic surgery, is a chemically inert polymer used for soft tissue augmentation in the past, but the main application for Teflon has been orbital floor reconstruction. Gore-Tex is a pliable, durable, inert, biocompatible material that has some tissue ingrowth, little inflammatory reaction and almost no encapsulation. In addition to being used in abdominal fascial reconstruction, chest wall reconstruction and soft tissue reconstruction, Gore-Tex has also been utilized for lip, nasal, chin and malar augmentation. It has also been utilized for the treatment of nasolabial and glabellar creases. Proplast I is a highly porous, black graphite/PTFE composite with a spongy consistency. Because Proplast I led to discoloration of the surrounding soft tissues when implanted, Proplast II-a more rigid, white PTFE/alumina compound-was developed as an alternative. Proplast, with a wide variety of applications including the reconstruction of the chin, zygoma, orbital rim, maxilla, mandible, skull and rib cage, was originally regarded favorably. However, reports of biomechanical failure, intense inflammation, infection and extrusion related to the Proplast temporomandibular joint implant, led to the removal of all Proplast implants from American markets by the FDA in 1990.

Calcium Phosphate Ceramics
Calcium phosphate implants have been available as bone replacement/augmentation materials for 20 years. The primary calcium phosphates in clinical use are hydroxyapatite and tricalcium phosphate. These materials are osteoconductive (providing a scaffold for bone ingrowth) thus allowing for integration into the recipient site after placement. As a result, calcium phosphates are very well tolerated with essentially no inflammatory response, minimal fibrous encapsulation, and no adverse effects on local bone mineralization.

Metals
Metals have been used for the past 35 years for skull reconstruction and repair, in addition to reconstruction of craniofacial and upper extremity skeletal injuries. Stainless steel, cobalt-chromium (vitallium), 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.

Postoperative Considerations
Although numerous potential complications may occur with any implant-related procedure (e.g., migration, extrusion, palpability), the one common denominator shared by all alloplastic implants is their inherent risk of infection. The majority of postoperative infections appear within weeks to months after the initial surgery. Low-grade infections manifested only by fevers and signs of mild cellulitis are managed by intravenous antibiotics. More serious infections involving wound breakdown, implant exposure, gross purulence or systemic spread of the infection require prompt removal of the implant as antibiotics and drainage alone are usually insufficient. Reimplantation should not be performed for at least 3 to 6 months to allow for complete treatment and resolution of both the infection and the inflammation in the surrounding tissues. Several studies suggest that smooth, nonporous, nonresorbable implants have lower rates of infection, but it remains to be seen whether any true infectious risk differences exist among the various alloplastic implant materials available today.

Pearls and Pitfalls

• Incisions should be placed as far as possible from the final position of the implant. This will decrease the risk of implant exposure or extrusion in the setting of a minor wound infection.
• The implant should be covered with as much soft tissue as possible. The pocket should be of adequate size; too large and the implant will shift position, too small and the implant will be at risk for extrusion due to tension on the closure.
• Whenever possible, always try and close a second layer of tissue between the skin and implant. This is critical if the implant lies directly beneath the incision.
• Implants with sharp corners should be smoothed down, since sharp edges can erode through the skin with time.
• The implant should be touched as little as possible. Clean, powder-free gloves should be worn and instruments should be used to handle the implant whenever feasible. The risk of infection and abnormal capsule formation is increased by the presence of any bacteria or foreign material on the implant.
• Do not use an implant composed of a rigid material to replace soft, pliable tissue.
• Keep an organized registry of all alloplastic implants in the event that the device fails or has to be removed. Give the patient a copy of the device name, model, manufacturer and serial number, in case failure occurs in the care of another physician.