Implants and biomaterials for face

The decision to choose a biomaterial for implantation requires an understanding of the histopathology of the interaction of the material with the tissues, as well as the response of the recipient organism. All materials for implantation cause the formation of a connective tissue capsule, which creates a barrier between the implant and the host's body. Adverse reactions are a consequence of an unresolved inflammatory response to the implanted material. The behavior of the implant also depends on the configuration characteristics of the implantation site, such as the thickness of the covering skin, the scarring of the tissue bed and the architecture of the underlying bones, which can create conditions for instability of the implant. For example, implants located more deeply and covered with a thick layer of soft tissue are less often exposed or displaced. Other important factors, such as preventing the formation of hematomas, gray and infection, both during surgery and in the postoperative period, contribute to the prevention of implant interactions with the host organism and to an increase in the stability of the implant.

Ideal implant

The ideal material for implantation should be cost-effective, non-toxic, non-antigenic, non-carcinogenic, perceived by the recipient organism and resistant to infection. It must also be inert, easily moldable, supple, easily implantable and capable of constantly maintaining the original shape. It should be easy to change and adapt to the needs of the recipient zone during surgery, without compromising the integrity of the implant, and to be stable with thermal sterilization.

For the installation and stabilization of the implant, it is important to have favorable surface characteristics; paradoxically, but it also significantly facilitates removal and replacement without damaging surrounding tissues. Immobilization of the implant implies that it will be fixed at the installation site throughout the life of the patient. Materials for implantation, such as silicone elastomer, cause the formation of the surrounding capsule, which holds the implant in place, while porous polytetrafluoroethylene (pTTPE), which is encapsulated to a lesser degree, is fixed with minimal tissue ingrowth. Each type of interaction of the material with the recipient organism gives certain advantages in various clinical situations. Materials that cause significant tissue ingrowth and permanent fixation are often undesirable, especially if the patient wants to change the correction in subsequent years. The process of natural silicone encapsulation and minimal surface ingrowth into implants from pPTPE provides immobility, while allowing replacement of implants without damaging surrounding soft tissues.

The ideal implant in the form should have wedge-shaped edges that merge from the adjacent bone surface, creating a non-palpable, insensible 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 must mimic the natural anatomical configuration of the area. A 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 memory of the shape of the silicone elastomer and improve its flexibility. Better adaptability to uneven bone surfaces reduces the likelihood of bias and prevents the formation of a dead space between the implant and the underlying bone. The renewed interest in research and development in the field of biomaterials has led to the emergence of composite implants (consisting of silicone and PPTFE), which promise the combination of the benefits of both biomaterials when used in face surgery (personal communication. Implantech Associates and Gore, 1999).

Biomaterials for implants

• Polymer materials / monolithic polymers
  • Silicone polymers

Since the 50s of the last century, silicone has a long history of wide clinical application with a constant, excellent ratio of safety and efficacy. The chemical name of silicone is poly-siloxane. Currently, only the silicone elastomer can be processed individually using three-dimensional computer modeling and CAD / CAM technology (computer-aided design / automated manufacturing). The features of production are important for the stability and purity of the product. For example, the harder the implant, the more stable it is. An implant that has a hardness (on a durometer) of less than 10 approaches the characteristics of the gel and, over time, "etches out" or loses some of its internal molecular content. However, most recent studies of breast implants with silicone gel have shown no objective links between silicone and scleroderma, systemic lupus erythematosus, systemic vasculitis, collagenoses, or other autoimmune diseases. The dense silicone elastomer has a high degree of chemical inertness, is hydrophobic, extremely stable and does not cause toxic or allergic reactions. Tissue reaction to a dense silicone implant is characterized by the formation of a fibrous capsule without tissue ingrowth. In the case of instability or installation without adequate soft tissue coverage, the implant may cause mild lethargic inflammation and, possibly, the formation of seroma. Capsule contraction and implant deformation occur rarely if it is not placed too superficially or migrated to the skin covering it.

• • Polymethyl methacrylate (acrylic) polymer

Polymethyl methacrylate polymer is supplied as a powder mixture and, catalysed, turns into a very hard material. The stiffness and hardness of acrylic implants are a problem in many situations, if necessary, introduce large implants through small holes. A ready implant is difficult to adjust to the contour of the underlying bone.

• • Polyethylene

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

• • Polytetrafluoroethylene

Polytetrafluoroethylene covers a group of materials that have their own history of clinical use. A well-known trademark was Poroplast, which is no longer produced in the United States due to complications due to its use in the temporomandibular joints. With considerable mechanical loading, the material was disintegrated with subsequent intense inflammation, infection with the formation of a thick capsule and, ultimately, expulsion or explantation.

• • Porous polytetrafluoroethylene

This material was first produced for use in cardiovascular surgery. Studies in animals have shown that it allows limited ingrowth of connective tissue, without the formation of a capsule and with a minimal inflammatory response. Traceable in time inflammatory reaction favorably differs from that of many materials used for correction of the face. The material was found to be acceptable for increasing the volume of subcutaneous tissues and for the production of implants with a predetermined shape. Due to the lack of significant tissue ingrowth, pPTFE has advantages in increasing subcutaneous tissues, since it can be re-modified and removed in case of infection.

• Mesh polymers

Mesh polymers such as Marlex (Davol, USA), Dacron - and Mersilene (Dow Corning, USA) have similar advantages - they are easily folded, sewn and molded; However, they allow the ingrowth of connective tissue, which makes it difficult to remove the nets. Polyamide mesh (Supramid) is a nylon derivative that is hygroscopic and unstable in vivo. It causes a weak reaction to a foreign body involving multinucleated giant cells, which eventually leads to degradation and resorption of the implant.

• Metals

Metals are mainly represented by stainless steel, vitallium, gold and titanium. In addition to individual cases, for example, in the manufacture of springs for the upper eyelids or for 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-rays in computed tomography.

• Calcium Phosphate

Materials based on calcium phosphate, or hydroxyapatite, do not stimulate the production of bone substance, but they are a substrate on which bone can grow from adjacent areas. The granular form of hydroxyapatite crystals is used in maxillofacial surgery to increase the alveolar process. The material in the form of blocks is used as an interposition implant in osteotomies. However, it has been proven that hydroxyapatite is less suitable for increasing or creating linings due to brittleness, difficulty in molding and contouring, and also because of the inability to adapt to uneven bone surfaces.

Autotransplant, homotransplant and xenograft

The use of autografts, such as autologous bone, cartilage and fat, is hampered by complications from the donor bed and limited availability of donor material. The processed cartilaginous gomotransplant is used for the reconstruction of the nose, but over time it undergoes resorption and fibrosis. Other materials and injectable forms are commercially available.

Tissue engineering and the creation of biocompatible implants

In recent years, tissue engineering has become an interdisciplinary field. The properties of synthetic compounds vary so that it is possible to deliver to the recipient organism aggregates of separated cells that can create a new functional tissue. Tissue engineering is based on the scientific achievements of many areas, including natural sciences, tissue cultivation and transplantation. These techniques allow the cells to be transferred to a suspension providing a three-dimensional medium to form a tissue matrix. Matrix captures cells, developing the exchange of nutrients and gases, followed by the formation of a new tissue in the form of a gelatinous material. Based on these new principles of tissue engineering, a number of cartilaginous implants were created. These were articular cartilages, cartilages of tracheal rings and ear cartilage. For the formation of cartilage in vivo, the injection of alginate was successfully used, which was injected with a syringe to treat vesicoureteral reflux. This led to the formation of nests of cartilaginous cells of irregular shape, which prevented the return flow of urine. Tissue engineering can ensure the growth of cartilage of precisely specified shape, now various types of contour face implants are being developed, consisting of immunocompatible cells and interstitial substance. The introduction of such technologies will reduce the number of complications in the donor zones and, like with alloplastic implants, reduce the duration of operations.

[1], [2], [3], [4], [5]