Facial implants and biomaterials

The decision on the choice of biomaterial for implantation requires an understanding of the histopathology of the material-tissue interactions as well as the host response. All implant materials induce the formation of a connective tissue capsule that creates a barrier between the implant and the host. Adverse reactions are the result of an unresolved inflammatory response to the implanted material. Implant behavior also depends on the configurational characteristics of the implantation site, such as the thickness of the overlying skin, the scarring of the tissue bed, and the architecture of the underlying bone, which may create conditions for implant instability. For example, implants located deeper and covered by a thick layer of soft tissue are less likely to be exposed or displaced. Other important factors, such as the prevention of hematoma, seromas, and infection, both intraoperatively and postoperatively, contribute to the prevention of implant-host interactions and to increasing implant stability.

The ideal implant

The ideal implant material should be cost-effective, non-toxic, non-antigenic, non-carcinogenic, acceptable to the recipient, and resistant to infection. It should also be inert, easily moldable, malleable, easy to implant, and capable of permanently maintaining its original shape. It should be easily reshaped and adjusted to the needs of the recipient site during surgery, without compromising the integrity of the implant, and be resistant to thermal sterilization.

Favorable surface characteristics are essential for implant placement and stabilization; paradoxically, this also greatly facilitates removal and replacement without damaging the surrounding tissues. Immobilization of an implant means that it will be fixed in place for the life of the patient. Implant materials such as silicone elastomer induce the formation of a surrounding capsule that holds the implant in place, while porous polytetrafluoroethylene (ePTFE), which is less encapsulated, is fixed with minimal tissue ingrowth. Each type of material interaction with the recipient organism offers specific advantages in different clinical situations. Materials that induce significant tissue ingrowth and permanent fixation are often undesirable, especially if the patient wishes to change the correction in subsequent years. The natural encapsulation process of silicone and minimal surface ingrowth in ePTFE implants ensure immobility while allowing the implants to be replaced without damaging the surrounding soft tissues.

An ideal implant shape should have tapered edges that merge with the adjacent bone surface, creating a non-palpable, imperceptible transition to the surrounding recipient zone. A plastic implant that adapts well to the underlying structures becomes even less mobile. The shape of its outer surface should imitate the natural anatomical configuration of the area. The new silicone implant Conform (Implantech Associates, USA) is designed to improve compatibility with the underlying bone surface. For example, implants cast with a new type of mesh surface reduce the shape memory of the silicone elastomer and improve its flexibility. Better adaptability to uneven bone surfaces reduces the likelihood of displacement and prevents the formation of dead space between the implant and the underlying bone. Renewed interest in biomaterial research and development has led to the development of composite implants (consisting of silicone and ePTFE) that promise to combine the advantages of both biomaterials when used in facial surgery (personal communication, Implantech Associates and Gore, 1999).

Biomaterials for implants

• Polymer materials/ monolithic polymers
  • Silicone polymers

Since the 1950s, silicone has a long history of widespread clinical use with a consistent, excellent safety/efficacy profile. The chemical name for silicone is polysiloxane. Currently, only silicone elastomer can be individually processed using 3D computer modeling and CAD/CAM (computer-aided design/computer-aided manufacturing) technology. Manufacturing characteristics have an impact on the stability and purity of the product. For example, the harder the implant, the more stable it is. An implant that has a durometer hardness of less than 10 approaches the properties of a gel and, over time, "etches" or loses some of its internal molecular content. However, most recent studies of silicone gel breast implants have shown no objective links between silicone and the development of scleroderma, systemic lupus erythematosus, systemic vasculitis, collagenoses, or other autoimmune diseases. Dense silicone elastomer has a high degree of chemical inertness, is hydrophobic, extremely stable and does not cause toxic or allergic reactions. The tissue reaction to a dense silicone implant is characterized by the formation of a fibrous capsule without tissue ingrowth. In case of instability or placement without adequate soft tissue coverage, the implant may cause moderate low-grade inflammation and possibly seroma formation. Capsular contracture and implant deformation are rare unless the implant is placed too superficially or has migrated toward the overlying skin.

• • Polymethyl methacrylate (acrylic) polymer

Polymethyl methacrylate polymer is supplied as a powder mixture and when catalyzed, it becomes a very hard material. The rigidity and hardness of acrylic implants is a problem in many situations where large implants need to be inserted through small holes. The finished implant is difficult to fit to the contour of the underlying bone.

• • Polyethylene

Polyethylene can be produced in a variety of consistencies; currently the most popular form is porous. Porous polyethylene, also known as Medpore (WL Gore, USA), is stable with minimal inflammatory reaction. However, it is dense and difficult to mold. The porosity of polyethylene allows significant fibrous tissue ingrowth, which provides good implant stability. However, it is extremely difficult to remove without damaging the surrounding soft tissue, especially if the implant is located in areas with thin soft tissue coverage.

• • Polytetrafluoroethylene

Polytetrafluoroethylene encompasses a group of materials that have their own history of clinical use. A well-known brand name was Poroplast, which is no longer manufactured in the United States because of complications associated with its use in temporomandibular joints. Under significant mechanical stress, the material was subject to disintegration followed by intense inflammation, infection with formation of a thick capsule, and eventual expulsion or explantation.

• • Porous polytetrafluoroethylene

This material was initially produced for use in cardiovascular surgery. Animal studies have shown that it allows limited ingrowth of connective tissue, without capsule formation, and with minimal inflammatory response. The time-tracked inflammatory response compares favorably with many materials used for facial contouring. The material has been found to be suitable for subcutaneous tissue augmentation and for the fabrication of shaped implants. Because of the lack of significant tissue ingrowth, ePTFE has advantages in subcutaneous tissue augmentation because it can be re-modified and removed in the event of infection.

• Crosslinked polymers

Mesh polymers such as Marlex (Davol, USA), Dacron - and Mersilene (Dow Corning, USA) have similar advantages - they are easy to fold, suture and shape; however, they allow ingrowth of connective tissue, which makes mesh removal difficult. Polyamide mesh (Supramid) is a nylon derivative that is hygroscopic and unstable in vivo. It causes a weak foreign body reaction involving multinucleated giant cells, which over time leads to degradation and resorption of the implant.

• Metals

Metals are mainly stainless steel, vitalium, gold and titanium. Except for a few cases, such as upper eyelid springs or dental restorations, where gold is used, titanium is the metal of choice for long-term implantation. This is due to its high biocompatibility and corrosion resistance, strength and minimal attenuation of X-ray radiation during computed tomography.

• Calcium phosphate

Calcium phosphate-based materials, or hydroxyapatites, do not stimulate bone formation, but they do provide a substrate onto which bone can grow from adjacent areas. The granular form of hydroxyapatite crystals is used in maxillofacial surgery to augment the alveolar process. The block form of the material is used as an interposition implant in osteotomies. However, hydroxyapatite has been shown to be less suitable for augmentation or onlay applications due to its fragility, difficulty in molding and contouring, and inability to adapt to irregularities in the bone surface.

Autografts, homografts and xenografts

The use of autografts such as autologous bone, cartilage, and fat is hampered by donor site complications and limited availability of donor material. Processed cartilage homograft is used for nasal reconstruction but is subject to resorption and fibrosis over time. Other materials and injectable forms are commercially available.

Tissue engineering and creation of biocompatible implants

In recent years, tissue engineering has become an interdisciplinary field. The properties of synthetic compounds are modified to deliver aggregates of separated cells into recipients, which can create new functional tissue. Tissue engineering is based on advances in many fields, including the natural sciences, tissue culture, and transplantation. These techniques allow cells to be suspended, providing a three-dimensional environment for the formation of a tissue matrix. The matrix entraps the cells, promoting the exchange of nutrients and gases, with the subsequent formation of new tissue in the form of a gelatinous material. A number of cartilaginous implants have been created based on these new principles of tissue engineering. These have included articular cartilage, tracheal ring cartilage, and ear cartilage. Injections of alginate, administered with a syringe, have been successfully used to create cartilage in vivo for the treatment of vesicoureteral reflux. This resulted in the formation of irregularly shaped nests of cartilage cells that prevented the backflow of urine. Tissue engineering can provide precisely shaped cartilage, and various types of contoured facial implants are currently being developed, consisting of immune-compatible cells and interstitial substance. The introduction of such technologies will reduce the number of complications in donor areas and, as with alloplastic implants, reduce the duration of operations.

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