By Robert J. DeFatta, MD, Ph D
Facial implants are chosen based on their biocompatibility, composition, strength, biodegradability, & resistance to stress and fatigue. There are five main types of implants that will be discussed.
The first are metallic implants. These can either be based on a single metal or a combination of alloys. The three most common components are titanium, stainless steel, and tantalum. They differ from one another based on their elasticity, tensile/ compressive/ shear strength, percent elongation, & strain. In vitro loading studies are used to determine long-term corrosion, fatigue, & instability. These have shown that while stainless steel has minimal corrosion, it has high fatigability. Similarly, titanium has minimal corrosion but minimal fatigue. Conversely, tantalum has moderate corrosion and rapid fatigability.
The second group are the ceramics, of which, there are three main types. The glass based ceramics are actually combinations of silicone dioxide and crystalline lattice meterials embedded in glass. They are known for their thermal resistant and excellent biocompatibility. However, they are prone to stress cracks and fracture instead of bending. These ceramics are virtually only used for ossicular reconstruction. The aluminum based ceramics do not fracture and are therefore used for dental implants. Finally, you have hydroxyapatite based ceramics which are resorbable, osteoconductive, increase bone density, & promotes osteointegration. This ceramic is prepared as a paste and is composed of elements found in ground substance of bone (Ca & P). It is important to note that these are not the same as hydroxyapatite cement, which is not a ceramic, is used to fill bony defects, and is never allowed to contact mucosa of paranasal sinuses.
The third type of ceramics is the polymer group. By varying the polymers, you can change structural & chemical composition to very between soft and fragile & hard and brittle. The common examples include polyurethanes, silicones, & polymethyl methacrylate. They are known for their strength and biocompatibility. They are manufactured by thermoplastic molding or thermosetting.
The fourth group of ceramics is a broad group that includes the biologic materials. The first example of this group is bovine collagen which is nzymaticlly modified to diminish cutaneous sensitivity reactions and decrease resorption time. It is important to note that antibodies can be formed due to macrophage attack. Another example is human acellular dermis which allows for re-epithelialiazation on its surface. In addition, the presence of “ghost channels” allow for revascularization of the tissue. This is frequently used for septal perferations, parotidectomy, & protecting vessels after a radical neck dissection.
The final group is that of resorbable plates & screws. These are composed of polymeres and co-polymers of polylactide & polyglycolide. They are heat-malleable and are frequently used in pediatric patients where metallic plates can cause strss-shielding and loss of bone growth & remodeling capabilities.
An important topic in implants is that of biocompatibility. It is important to note that the most inert materials are those composed purely of elements nearest to carbon & calcium on periodic table. The most important influencing factor of biocompatibility is interface between implant and body tissue. With titanium, an oxide layer forms. This is a 10nm thick layer that slowly thickens after implantation and acts like a ceramic coating to improve biocompatibility and osteointegration. The extracellular matrix acts as bridge between cells and implanted material. It is required for initial cell adhesion and proliferation. Fibroblasts are the primary cells attaching to protein layer.
Two main factors that affect biocompatibility are the pore size and the particle size. The pore size must be >50µ in diameter to allow tissue ingrowth. In fact, soft-tissue implants are secured by fibrous capsule production or collagen fiber ingrowth, based on porosity of the implant. The particle size must be >60µ in diameter to prevent phagocytosis by macrophages which can lead to a chronic inflammatory response.
Preoperative Counseling needs to be extensive and cover all the major complications which include infection, rejection, and extrusion. Extrusion most commonly occurs by one of three methods, including, trauma, infection, and autoimmune rejection. Another complication includes macromotion, which needs to be differentiated from micromotion which allows the implant to closely simulate the characteristics of the host tissue.
Facial implants are very versatile and can be used in many various locations. The most common scalp implant is the tissue expander. It is important to note that the expanded skin has increased thickness and mitotic activity of the epidermal layer, 30-50% thinning of dermis, blood vessel proliferation, enhanced collagen synthesis, increased melanin production, increased blood flow. In fact, the v iability and blood flow is similar to that of delayed flaps. There is also no change in hair follicle number, however, hair density decreases. Muscle routinely thins and can atrophy. Tissue expanders are available in various shapes, with the rectangular, crescent, and round being the most common. The rectangular will give the largest increase in size (38%) while the round will give the least at 25%. The expander base should be 2.5-3.0x the area to be reconstructed. When inserting an expander, two pockets should be formed for the actual expander and the injection port. The site for the port should be located several centimeters away from expander. The port can be injected with a ÿ 23-guage needle every 4-14 days. Each injection can continue until skin blanches or patient discomfort occurs.
There are three main implants designed for the skull. The first was polymethyl methacrylate (PMMA) which was invented in 1902 by Dr. Otto Rohm. It was first used to fill skull defects in humans in 1939. It is formed from several monomers in the presence of a catalyst. Its advantages include that it can be molded to fill a specific defect shape. In addition, it can be drilled, sculptured, and secured to surrounding bone. Another implant that can be used to fill skull defects are titanium and tantalum. These can be obtained in either plates or mesh-sheets. The advantages of these include being able to be bent and cut to any specific size. Also, they slowly deform with external trauma for better brain protection, whereas PMMA is brittle and fractures easily. The third type of implant is hydroxyappatite cement which is especially useful in children. One of the advantages of this implant is that computer-aided designs can be used for large defects.
Microtia is a very difficult problem to correct. When attempting to use implants, the choices include the design of a prosthetic ear versus reconstruction using either medpore. A prosthesis uses osseointegrated titanium fixtures and is attached by magnets. The medpore implant can be covered by either a temperoparietal flap or a thin full thickness skin graft. Advantages of the medpore implant includes that it requires only one to two stages for reconstruction and that individual helix and helical rim pieces can be used together for better symmetry.
The two most common causes for orbital implants are from orbital fractures and following an orbital exenteration. For orbital fractures, many different implants can be effectively used. Which implant is used is highly dependent upon the extent of the fracture and the surgeon’s preference. For small fractures, silicone sheeting, medpore, or gilfilm can all be used. For larger fractures, especially blow-out fractures, orbital floor titanium plates, silicone sheets, medpore, or multiple layers of gelfilm can all be effectively used. Following an orbital exenteration, either osseointegrating titanium fixtures can be used with a prosthesis or medpore can be used to reconstruct the orbit. The advantages of medpore include connective tissue integration and implant vascularization which reduces the risk of infection and extrusion.
For the perioral region many different implants can be used to increase the size and projection of lips. In addition, they also add the benefit of lessening the depth of the nasolabial folds. The most common types of implants that are used include injectable collagen and autologous fat and tubed acellular dermis and SMAS.
The indications for malar augmentation include the aging face, congenital defects, post-traumatic bony deficiencies, cheek soft tissue ptosis, and liposystrophy. Many different types of surgical approaches can be used. These include the subcilliary, transconjunctival, rhytidectomy, coronal, and intraoral, of which the last is the most common. In this technique, the implant is placed in subperiosteal pocket and attached to zygomaticomaxillary complex with 2 screws.
The final location that will be discussed for facial implants is the chin. It is important to note that mentoplasty is only for horizontal (AP) bony defects. Conversely, vertical deficiencies are only corrected by osteotomies. During the preoperative analysis, it is important to observe the patients at rest and animation to evaluate mentalis soft tissue mound and its support, as aging patients frequently have ptosis of soft tissue pad. Another pitfall that needs to be recognized preoperatively are open bite deformities and lip incompetence, which can be caused by mentalis hyperactivity. The normal preoperative photos that are taken include the AP, lateral, & oblique views.
Chin implants can either be placed through the intraoral approach or the submental approach, of which the latter is the more common. In the submental approach, a 2-3 cm incision is made in the submental crease and the mentalis is divided and a plane is created. The implant is then placed at the inferior mandible border. The mentalis muscle is then closed with a 4-0 vicryl in interrupted fashion. A jaw braw is then placed for 3 days to ensure immobility of the implant. The patient is placed on 48 hours of perioperative antibiotics to prevent infection and extrusion of the implant.
Witch’s chin or senile chin deformity is caused by weakening of the muscular attachments of mentalis and depressor labii inferioris. With this deformity, the chin soft tissue pad falls below mandibular line & a deep horizontal crease develops in submental region. This deformity is caused or worsened if mentalis is not appropriately reapproximated after implant insertion.
The two most common types of implants include the button and anatomic implants. The latter implant is the more common implant. In addition, it allows for lateral mandibular augmentation, step-offs are harder to appreciate, and a one to two mm displacement will not be apparent.
