Grafts and Implants in Facial, Head, and Neck Surgery
Richard G. Holt
Christian L. Stallworth
The biotechnology of implants in otolaryngology and head and neck surgery has expanded rapidly, and facial, head, and neck implants now include autologous biomaterials, metals, polymers, synthetics, and tissue-engineered materials. Since the range of requirements for implants ranges from bone replacement and stabilization implants to soft tissue fillers, the science of surgical implantology now involves cell surface physics, molecular level biochemistry, and tissue engineering. Biomaterials, both biologic and synthetics, for osseous and soft tissue augmentation are becoming increasingly important in reconstructing defects of traumatic, congenital, extirpative, and aging etiologies.
BIOCOMPATIBILITY OF IMPLANTS
Cells do not adhere directly to the surface of implanted synthetic materials. A substance in the extracellular matrix binds the cell to the surface. This substance is essential for initial cell adhesion and proliferation. The substrate needed varies with the type of cell. Well-differentiated cells, such as chondroblasts, osteoblasts, and epithelial cells, require substrate characteristics distinct from those needed by less differentiated cells, such as fibroblasts. Focal contacts represent adhesion sites to specific extracellular matrix proteins adsorbed on the implant surface. Focal contacts typically occur in low-motility cells, such as fibroblasts and epithelial cells. The composition of the substrate (the adsorbed layer of protein on the implant surface) is crucial for tight cellular adhesion. Proteins such as fibronectin, vitronectin, cold-insoluble globulin, and possibly proteoglycans provide the necessary substrate for this adhesion.
The extracellular matrix contains collagen, elastin, and fibronectin interwoven into a hydrated network of glycosaminoglycan chains. The glycosaminoglycan chains are long, negatively charged polysaccharide chains that link proteins to form giant proteoglycan molecules. Interaction with cell membrane receptors provides linkage for cellular attachment to adsorbed extracellular matrix on the surface of a biomaterial. Tissue cells do adhere to the implant surface —not directly but by means of a complex series of protein attachments.
When implants are placed in facial soft tissue, the primary tissue reaction includes protein adsorption and cellular attachment. The predominant cell attaching to the protein layer is the fibroblast. Within the first week, the fibroblast lays down immature collagen on the implant surface, or interstices. The usual response to a soft tissue implant is production of a fibrous capsule or collagen fiber ingrowth, which secures the implant. A smooth implant such as silicone more often elicits dense capsule formation than does a porous implant. If an implant is too reactive, has surface contamination, or is biodegradable, the host tissue response usually is aggressive macrophage activity, increased vascularity, breakdown of the overlying skin, and extrusion of the implant. The presence of inflammatory cells such as neutrophils and macrophages suggests poor tissue response to the implanted material.
After placement of an implant, protein adsorption occurs. As a hole is drilled into bone to receive the implant, the bone must not be heated to more than 45°C to 50°C, or osteoblasts die. An implant in bone induces a rapid host response. The first stage is formation of a small hematoma and a cascade of chemical breakdown products. These substances act on blood vessels and attract cells from surrounding tissue. Because cortical bone is avascular, most blood products come from the marrow-containing spaces of the bone.
The second stage is tissue organization, regeneration, and repair. The duration is related to the extent of injury and implantation site geometry. Extracellular processes and cell functioning can be affected by soluble and insoluble particles from the implant and by the mechanical influence of the implant itself. The third stage of repair is
remodeling, which affects the implant-host tissue interface and occurs over weeks or months. Appropriate stress levels must be imposed on the bone adjacent to the implant. Bone-binding intensity can be measured according to the shear or torque forces needed to produce failure. Bone is the main contributor to tensile strength of bonding; other tissues are less important. The basal lamina in contact with a bone implant contains type IV collagen, laminin, and proteoglycans. These constituents of the ground substance are deposited in or adjacent to the mineralized layer. Mineralization of the ground substance seems to be important for transmission of compression and for shear and tensile loads.
remodeling, which affects the implant-host tissue interface and occurs over weeks or months. Appropriate stress levels must be imposed on the bone adjacent to the implant. Bone-binding intensity can be measured according to the shear or torque forces needed to produce failure. Bone is the main contributor to tensile strength of bonding; other tissues are less important. The basal lamina in contact with a bone implant contains type IV collagen, laminin, and proteoglycans. These constituents of the ground substance are deposited in or adjacent to the mineralized layer. Mineralization of the ground substance seems to be important for transmission of compression and for shear and tensile loads.
CHARACTERISTICS OF IMPLANTS
Implant material is characterized by composition, strength, biodegradability, and resistance to stress and fatigue (1, 2). The properties of bulk material, however, can differ from those of the implant surface at the tissue-implant interface because of surface alterations by design or physicochemical reaction. The materials and clinical applications for facial, head, and neck implants are summarized in Table 172.1.
TABLE 172.1 FACIAL, HEAD, AND NECK IMPLANTS | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Metallic Implants
Metallic devices can be composed of a single metal or an alloy of several metals. Alloys are developed to improve qualities of the original metal by adding other metals with characteristics that improve biocompatibility or mechanical attributes. The principal metals used in facial implants are titanium, stainless steel, and tantalum. Chromium, aluminum, cobalt, copper, nickel, and tungsten are included in alloys.
Metals are crystalline materials with well-defined, orderly, three-dimensional arrangements of atoms that form a microscopic lattice characteristic of each metal. The lattice can be modified by means of heating, cooling, hardening, or altering the physical properties of the metal to achieve a particular result. Lattice defects can modify the characteristics of the metal. Large structural defects can cause failure to withstand external stresses. Metallic biomaterials are characterized by elastic modulus, tensile strength, percentage elongation, compressive strength, shear strength and modulus, and strain. Stress is the ability of a material to withstand a given load per cross-sectional area. The material must be designed to meet the functional requirements of the dental or maxillofacial implant.
Stress versus strain curves are generated experimentally for implant materials. They provide information about the bulk material independent of shape or thickness. These can be used to predict the response of the material to mechanical forces on an implant in a particular use. The forces of shear, compression, tension, torsion, and bending must be considered in selection of a material for an implant. In vitro loading studies are performed to assess how a material responds to long-term wear. Most metals relax with time, and the relaxation can cause metal fatigue and implant failure. A relatively brittle metal, such as stainless steel, can function well initially but with longterm use can fail because of fatigue. All metals corrode when exposed to living tissue; the gradual result is failure of many metal implants. Stainless steel, an alloy of iron, chromium, nickel, molybdenum, and manganese, resists corrosion well. It can, however, undergo gradual plastic deformation.
Titanium and its alloys are among the most biocompatible metallic implants used today. Titanium is lightweight and corrosion resistant and has high tissue acceptance. It is rather soft and when not anchored to bone can be deformed by loading forces. Used in mandibular reconstruction and for anchoring screws in facial applications, titanium performs well. Tantalum and vanadium have been used as bone trays for mandibular reconstruction, but the mechanical properties are not as good as those of titanium. Tantalum and vanadium are not strong, can fatigue rapidly, and must be removed after the mandible heals. Some metallic implants, such as stainless steel, have a better stress response than does bone. This can cause stress shielding of the bone and impede formation of new bone. Metal implants may have to be removed after the bone is stabilized to allow growth and development.
Ceramics
Ceramics have a microscopic lattice structure. Glass ceramics, on the other hand, have an amorphous atomic structure. Most biologic implants are glass ceramics—combinations of silicon dioxide (SiO2) and crystalline lattice materials embedded in this glass. Glass ceramics are thermally resistant and can be used when thermal shock can occur. Glass ceramics last well in the body. They are well tolerated and biocompatible. Because of a peculiar grain size and distribution, however, glass ceramics are susceptible to cracking from stress concentration. Clinically, these are considered brittle materials; they fracture rather than bend when subjected to excessive stress. This limits the use of glass ceramic implants in the head and neck to areas with minimal force loading, such as a tympanic ossicle.
Ceramics made with alumina compounds also are used in dental implants. The device is designed so the shape facilitates biomechanical stress application without fracture. Hydroxylapatite is another form of ceramic; it is characterized as bioreactive. It comes as a powder and is reconstituted as a paste for dental and bone replacement. Hydroxylapatite is resorbable and osteoconductive, and it increases bone density. It is composed of elements that exist in the ground substance of bone, that is, calcium and phosphorus. Hydroxylapatite can provide a substrate for osseointegration and osseoconduction when used as a bone replacement material for facial, head, and neck defects.
Facial augmentation with calcium hydroxylapatite (Radiesse, Merz USA, Greensboro, NC) microspheres in a gel carrier of carboxymethylcellulose can provide soft tissue enhancement for a correction period of 12 to 18 months when injected subdermally. Because the hydroxylapatite microsphere is in a gel suspension, the material can be massaged after injection into the tissues to create a smooth appearance (3). When used in porous granular form for facial skeleton augmentation, the material has been found to maintain its bony skeletal projection for at least 2 years (4).
Polymers
No synthetic implant material can exactly reproduce the biomechanical properties of bone. Ceramics and metals are stronger than human bone, and polymers are more flexible. Polymers are useful in implantation because the mechanical properties can be altered to suit the application. These properties are derived from the structural and chemical composition, which are related to length and cross-linking. Varying these two characteristics can produce a wide range of polymer properties, from soft and fragile to hard and brittle. The implant designer can choose a polymer that provides the characteristics needed for a particular situation.
The most commonly used medical polymers are polyurethanes, silicones, and polymethyl methacrylate (PMMA). These polymers are reasonably strong and biocompatible. When supplied as porous fibers (polytetrafluoroethylene [PTFE], nylon, polylactic acid, and polyglycolic acid), these materials can be woven fabric as well as suture material. Expanded PTFE (ePTFA) fabric (Gore-Tex, Gore Medical Co., Flagstaff, AZ) has excellent biocompatibility when used for soft tissue augmentation or vascular repair. Mechanical stresses on polymer implants usually are small. When used for mandibular replacement, a polymer is tested for the same mechanical tolerances as are metals, including tensile strength, modulus of elasticity, stress, and strain. Impact testing is important when a material is used for skull reconstruction. Internal defects that occur during molding and processing can cause cracks and implant failure.
Polymers are manufactured by means of thermoplastic molding (the material is formed in a heat-softened state in a mold) or by means of thermosetting (the insoluble polymers are cured by cross-linking). Suture material is formed by means of extrusion of the polymer through small holes in a die to produce a fiber thinned to the proper diameter
before cooling. Used as a glue, PMMA, ethyl-2-cyanoacrylate, and butyl-2-cyanoacrylate produce histotoxic cellular reactions and an exothermic reaction. A permanent injectable polymer of cleaned microspheres of PMMA suspended in bovine collagen (ArteFill, Suneva Medical, San Diego, CA) is FDA approved for augmentation of the nasolabial folds. Serious complications such as granuloma formation have not been reported with this synthetic injectable, allegedly owing to the minimalization of the number of microspheres smaller than 20 µm (5).
before cooling. Used as a glue, PMMA, ethyl-2-cyanoacrylate, and butyl-2-cyanoacrylate produce histotoxic cellular reactions and an exothermic reaction. A permanent injectable polymer of cleaned microspheres of PMMA suspended in bovine collagen (ArteFill, Suneva Medical, San Diego, CA) is FDA approved for augmentation of the nasolabial folds. Serious complications such as granuloma formation have not been reported with this synthetic injectable, allegedly owing to the minimalization of the number of microspheres smaller than 20 µm (5).
Polydioxanone (PDS) is a resorbable material utilized in suture material and in thin sheets for support of osseous defects, usually an orbital wall fracture. Recently, PDS foil (PDS Flexible Plate, Mentor, San Diego, CA) has been utilized to support nasal cartilaginous structures during septoplasty and rhinoplasty, which can be particularly useful because of its low extrusion rate, structural strength during healing, and ability to support multiple free fragments of septal cartilage as a scaffold for reimplantation (6, 7).
Biologic Materials
Grafts of nonhuman biologic material and xenografts are considered implants because they often are used for tissue augmentation. Bovine collagen in injectable solution or sheets (Zyderm, Allergan Medical, Irvine, CA) is enzymatically modified to diminish cutaneous sensitivity reactions and to decrease resorption time. When the macrophage system of the host identifies this collagen as foreign, immunologic defenses form antibodies to the collagen. Because collagen is similar in many ways among species, this problem can be diminished but not eliminated with biochemical alteration of unique proteins. Synthesis of components of human dermal collagen and basement membrane, such as polyglycolic acid and polylactic acid, has produced suture and implant material that is slowly resorbed by means of acid hydrolysis. These materials do not induce the intense immunologic response of animal collagen. They also are used as the carriers of sustainedrelease drugs in implantable drug delivery systems.
Human acellular dermis matrix, harvested from cadavers, is utilized primarily as a soft tissue augmentation material for the face. It can also serve as a “filler” or “scaffolding” for repair of a nasal septal perforation, where this material is placed between opposing flaps of nasal mucosa; if exposed, it allows for reepithelialization on its surface. Foreign antigenicity of this human allograft is achieved by leaching out the cells; however, there are “ghost channels” of preexisting vascular structures that may serve to support revascularization of the tissue. There is a tendency of this biomaterial to resorb with time in some patients.
Biodegradable (resorbable) plates and screws are not dissimilar to certain biodegradable suture materials. These firm implants are generally composed of biosynthetic polymers and copolymers of polylactide and polyglycolide and are heat malleable to fit the contour of the bony surface. They should be especially considered in pediatric fracture patients, where metallic plates can cause stress shielding and loss of bone growth and remodeling capabilities. Such minimally invasive bioabsorbable bone plates are being increasingly utilized in facial skeleton fractures, and they have been shown to be as strong as a titanium plate when fixating fractures of the mandibular body (8). A so-called “interflex” design allows for decreasing the volume of the plate while still maintaining a low stiffness and an absorption rate that allows for gradual reduction of mechanical stability commensurate with a gradual increase in load bearing of the healing mandible.
Patient-Specific Implants
With the growing availability of three-dimensional radiographic computer modeling, surgeons can preoperatively analyze bony and soft tissue defects through virtual manipulation. This can even be done for existing defects, but also prospectively to forecast a defect that will result from an ablative procedure. With this technology comes the ability to fabricate patient-specific implants (PSIs). Using the patient’s native contralateral anatomy or gender- and age-specific norms, implants can be constructed to replace tissue loss in multiple dimensions. Examples include preoperative fabrication of mandibular reconstruction plates to be used following composite resection of the mandible or multidimensional implants needed to replace a complex midface defect resulting from maxillectomy. In this way, manufactures are able to tailor their implant materials to precisely restore unique, patient-specific defects.
Polyetheretherketone (PEEK) (Synthes Craniomaxillofacial, West Chester, PA) is a semicrystalline polyaromatic linear polymer that is inert, nonporous, and customizable. More than 20 years of reliable use in the aerospace, automotive, and electrical industries led to its ultimate use as substitute for bone grafting and titanium cages in the treatment of cervical disc disease. PEEK has proven biocompatible, strong, and stable. It has the benefits of bone-like stiffness, can be secured with plates and screws, is lightweight, and can withstand the sterilization process. It also holds the advantage of customization (9, 10). With computer modeling, a PSI can be manufactured. Digital renderings and a tangible model of the patient’s skull and the fabricated implant are then provided to the surgeon for manipulation and approval preoperatively.
ASSESSMENT OF PATIENT NEEDS
As with all reconstructive procedures, it is imperative to evaluate each patient’s individual needs, the goals for reconstruction, and whether or not an implant will be needed to achieve these goals. Before an implant is used, a patient’s general operative candidacy must be considered. Patients with severe cardiopulmonary or other systemic diseases may be poor candidates for general anesthesia, especially if
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