Basic Categories:

1. Metal
   • Most commonly pure titanium or titanium alloy with aluminum and vanadium
     • Most commonly used for craniofacial reconstruction due to favorable properties.
     • Light, strong, totally biocompatible
     • Has some osteoconductive properties but also has a modulus of elasticity. 
   • Platinum and Gold: High-density metals useful in upper eyelid to address paralytic lagophthalmos
     • Gold has higher incidence of hypersensitivity reaction (up to 10% in the population)
     • Platinum eyelid implants are higher in density allowing for lower profile compared to gold.
   • Most metals are not compatible as implant materials due to the relatively corrosive environment within the body combined with the poor tolerance of the body to minimal concentrations of most metallic corrosion.

2. Polymers
   • Long molecules composed of repeating subunits.
   • Polysiloxane (silicone) interlinking silicone and oxygen with organic side groups. The only form of non-carbon chain polymer in clinical use. 
     • Solid silicone: 
       • Benefit is it can be sterilized by steam or irradiation, carved easily, and stabilized with suture or screw. It is removed easily due to smooth surface and capsule formation.
       • Highly hydrophobic and no bonding to its capsule and has potential to migrate if not fixed. Additionally, can be associated with persistent seromas. Associated with high extrusion rates of silicone implants in nasal and auricular application sites
       • There is a tendency to cause bone resorption and potential for visible capsule formation
     • Injectable silicone was first introduced by Dow Corning in 1960. 
       • Medical grade liquid silicone has been associated with adverse reactions such as inflammatory nodules or “siliconoma” reported up to several years following injection.
       • Migration along lymphatic system to regional lymph nodes, liver, and spleen have been reported. 
       • In 1991, FDA banned use of pure injectable silicone in the United States.
   • Polytetrafluoroethylene (PTFE), Gore-Tex. Carbon-Ethylene Backbone with Fluorine molecules
     • Very chemically stable, non-adherent surface, very flexible because not crosslinked and soft.
     • Easily sterilized, can be fixed to underlying structures. 
     • Pore size 5-30 microm with minimal tissue ingrowth.
     • Complications include infection, extrusion, migration, shrinkage, and scarring. Modifications of expanded PTFE have been developed to reduce complication risks. 
   • Acrylics: polymerized esters of acrylic of methyl acrylic acids.
     • Smooth surfaces, strong, inflexible
     • Polymethylmethacrylate (PMMA): 
       • Radiolucent, strong, rigid, unaffected by temperature. 
       • Encapsulated by host and not incorporated, believed to be more susceptible to late complications. 
       • Liquid and Powder packaged and mixed to become a solid resin. May be molded in situ. 
       • Common uses in cranioplasty and can be reinforced with a mesh for moderate-sized defects. For large defects, CT guided porous custom implants can be fabricated to facilitate more accurate reconstruction. 
       • Requires good quality soft tissue without previous infection or connection to mucosal surfaces. 
   • Polyethylene: Ethylene monomer, Omnipor, Medpor. 
     • High density, porous variety most commonly used in craniofacial surgery. Porosity between 125 to 250 microm. Allows more extensive fibrous ingrowth. 
       • Porous high density poly ethylene solid implants with pore size 100-150 microm encourages osseous tissue ingrowth with mature bony ingrowth into surface pores at 1 year. 
     • Advantages: extensive soft tissue ingrowth, lessening tendency to migrate or erode underlying bone. Firm consistency allows it to be easily fixed with screws and contoured with scalpel or drill without fragmenting
     • Disadvantages: Soft tissue adherence making placement more difficult and requiring larger pockets to be made. Soft tissue ingrowth makes implant removal more difficult. 
     • Common uses: Used in facial augmentation for reconstructive or cosmetic purposes. 
   • Polyether ether ketone (PEEK): 
     • Organic, semicrystalline, high-temperature engineering thermoplastic polymer. Excellent mechanical and chemical resistance properties. 
     • Smooth surface, exhibits chemical, hydrolysis, and high-temperature resistance. Preferred in cranial vault defect. 
     • Radiolucent with low artifact on MRI scans

3. Ceramics: Crystal-like components, heated to high temperatures
   • Hydroxyapatite: most commonly found in craniofacial surgery. Calcium Phosphate salt found as majority component of bone matrix. Dense hydroxyapatite can be produced synthetically. 
     • Theoretical advantage of being osteoinductive and osteoconductive. 

References:

1. Yaremchuk MJ. Implant materials. In: Atlas of Facial Implants. Elsevier Health Sciences; 2019. Chapter 3, p. 23-40.
2. Kim JJ, Evans, GRD. Applications of Biomaterials in Plastic Surgery.  Clin Plast Surg. 2012 Oct;39(4):359-76.
3. Thrivikraman G, Athirasala A, Twohig C, Boda SK, Bertassoni LE. Biomaterials for Craniofacial Bone Regeneration. Dent Clin North Am. 2017 Oct; 61(4): 835–856.
4. Boahene KDO. Synthetic and Biologic Implants. In: Facial Plastic and Reconstructive Surgery, 4th ed. New York: Thieme Medical Publishers; 2016 Chapter 4, p. 25-37.