The use of alloplastic materials in tympanomastoid surgery is well established. They are divided into three main types based on the tissue response. Bioinert materials elicit no tissue response, biodegradable materials dissolve over a period of time, and bioactive materials encourage a particular host tissue response (e.g., bonding to surrounding tissue, encouragement of bone in-growth, fibrosis). Alloplastic materials can be metals, polymers, ceramics, or a composite of these. Metals are typically bioinert, polymers could be bioinert or biodegradable, and ceramics could be bioinert, biodegradable, or bioactive. The advantages of using alloplastic materials in tympanomastoid surgery include general biocompatibility, lack of disease transmission, user-friendliness, and unlimited material supply. The typical disadvantages include additional surgical cost and risks of secondary infection or extrusion. Other specific advantages and disadvantages for each type of material and some common applications in tympanomastoid surgery are given in ▶ Table 8.1.
Materials | Advantages | Disadvantages | Applications in Ear Surgery |
Polymers Silastic Teflon Polyethylene (Plastipore and Polycel) PTFE | Easy and relatively cheap to fabricate; easily trimmed | Not strong; deform with time; may degrade | Ventilation tubes; stapedotomy pistons; prosthetic ossicles; splints in middle ear |
Metals Stainless steel Platinum Titanium Nitinol | Mechanical strength; versatility in prosthetic designs | Higher cost of fabrication; rigid | Prosthetic ossicles; stapedotomy pistons; mesh/plate for bony defect repair |
Ceramics Bioglass Aluminium oxides Calcium phosphates/HA | Corrosion-resistant; low thermal and electrical conductivity | High cost of fabrication; brittle | Prosthetic ossicles; filler materials in mastoid obliteration |
8.2 Types of Alloplastic Materials
8.2.1 Polymers
The commonly used polymers in middle ear surgery are Silastic® (Dow Corning Corp.), polyethylene, and polytetrafluorethylene (PTFE). The most well-known brand name of PTFE is Teflon™ (Chemours). The most popular polyethylene used in ossicular prostheses are Polycel (trademark refers to porous polyethylene) and Plastipore (trademark refers to high-density porous polyethylene). The main advantages of these polymers are a low production cost and the ease with which they can be easily trimmed by scalpel or scissors. Most tympanostomy tubes are made from polymers.
8.2.2 Metals
Because metals are typically strong and good conductors of sound, they are commonly used in middle ear prostheses; however, metals are also often used in the construction of tympanostomy tubes. Metallic strength enables very thin shafts, hooks, and claws to be engineered into a prosthetic design without the risk of breaking. This characteristic allows unique designs that can promote secure coupling between the prosthesis and the ossicular chain, and the thin shaft of the metal ossicular reconstruction prosthesis affords placement even through tiny gaps. The most common metals used in tympanomastoid surgery are titanium, platinum, and stainless steel. Titanium is particularly popular because of its outstanding biocompatibility, strength, low weight, and high resistance to corrosion. In addition to being used in middle ear prostheses, titanium mesh and plates are used by some surgeons to reconstruct bony defects in the mastoid cavity.
In recent years, Nitinol (Fort Wayne Metals), an alloy composed of titanium and nickel, has been used in the design of middle ear implants due to its unique property of shape memory. This material undergoes deformation at one temperature and then recovers its original shape when heated above its “transformation temperature.” Nitinol is mainly used in the loop of stapes pistons because of this self-crimping property that can be initiated with heat application.
One concern raised by the use of metal is delayed hypersensitivity reaction. This is alleged to be extremely rare for titanium, gold, or stainless steel because they are noncorrosive. Although Nitinol is in part composed of nickel, which is known to be frequently involved with Type IV (delayed) hypersensitivity reactions, there is allegedly very little potential for a hypersensitivity reaction to Nitinol because a very stable protective titanium dioxide (TiO2) layer forms that acts as a barrier against ion exchange. There presently is not a reliable diagnostic test available for the determination of hypersensitivity to implanted metallic devices, and the most common test for delayed hypersensitivity is still the skin patch test. As the amount of metal used in middle ear prostheses is tiny, it is difficult to separate clinically an allergic reaction from other inflammatory conditions.
8.2.3 Ceramics
Bioceramics include a variety of biomaterials, such as carbon, alumina, hydroxyapatite (HA), bioglass, and tricalcium phosphate. In general, they show high biocompatibility, high resistance to corrosion and compression, and low electrical and thermal conductivities.1 Ceramics are mainly used as bone substitutes because the mineral component of bone is calcium phosphate micro-crystals, mainly as HA. Bioactive ceramics react with physiological fluids, forming bonds to bone through the formation of bone-like HA layers, leading to effective biological interaction.
The biological behavior of bioceramics depends on many factors, such as the chemical composition and microstructure pore size. Dense ceramics are more resistant than porous ceramics to infection and reabsorption and are mainly used in ossicular prostheses. With increasing porosity and pore interconnectivity, bone ingrowth becomes more effective. Porous ceramics are mainly used as filler materials for mastoid obliteration.
For ossicular reconstruction, the most popular bioceramic is HA. Although carbon, bioglass, and aluminium oxide middle ear prostheses have also been used, they are not as popular.2 For mastoid obliteration, the most popular bioceramics are HA and bioglass. Hydroxyapatite has a chemical composition that includes calcium (Ca) and phosphate (PO4). Bioglass has a chemical composition that includes silicon dioxide (SiO2), calcium oxide (CaO), and sodium oxide (Na2O). The bioactivity of these bioceramics, whether they are bioinert, biodegradable, or bioactive, depends on the ratio of their chemical components, degree of crystallinity, temperature, and surface area.3 SerenoCem is a hybrid glass polymer composite consisting of an alkaline inorganic glass reacted with an organic polyacid. It chemically bonds to bone mineral and metals and does not undergo appreciable shrinkage. Its granular form is used for mastoid obliteration, and its cement form is used as bone cement in middle ear surgery. BonAlive (Boston Medical Products Inc., Westborough, Massachusetts) is a bioglass that can be utilized for mastoid obliteration that takes advantage of an osteo-inductive host interaction, thereby promoting new bone formation within the obliterated cavity.
8.3 Use of Alloplastic Materials
8.3.1 Ossicular Reconstruction
Most new-generation ossicular prostheses are made of HA and/or titanium, virtually all of which demonstrate magnetic resonance imaging (MRI) compatibility. The clinical outcomes for HA in middle ear reconstruction were first reported by Grote,4 and so far it has stood the test of time. To make the HA prosthesis more user-friendly, various composite HA prostheses have been subsequently developed consisting of an HA head plate and shaft made from other materials, with advantages ranging from malleability or the potential to be trimmed easily with a scalpel. These materials include Plastipore, PTFE, FLEX HA (HA and silicone composites), and HAPEX (HA-reinforced polyethylene composites). Some prostheses even incorporate a stainless steel core in the shaft to aid sound conduction,5 whereas others have been designed with a pure titanium shaft.
Hydroxyapatite and SerenoCem in cement form are also being used in middle ear surgery, mainly to repair the long process of the incus6,7 or to reinforce placement of an ossicular prosthesis in challenging cases. Whereas the short-term results are promising, there are still doubts over their long-term benefits8 and a need for further long-term studies.
Titanium was first introduced as an alloplastic material for ossiculoplasty in 1993.9 Modern engineering techniques enable many innovative designs to be incorporated to make the prosthesis more user-friendly and secure within the middle ear. For example, an open head plate allows the surgeon to observe the position of the bottom end during the placement of the prosthesis. The bottom end of the titanium partial ossicular replacement prosthesis (PORP) can take the shape of a cup, a clip, or claws to accommodate the stapes head. The design of a separate shoe helps to stabilize the distal end of the titanium total ossicular replacement prosthesis (TORP) on the footplate.
There is currently a myriad of prosthetic designs used for ossicular reconstruction. The factors that influence choice are the user-friendliness of the design,10 cost of the prosthesis, and the personal preference of the surgeon. Because new designs of HA and titanium prostheses are being introduced all the time, direct comparison of clinical results reported in the literature are difficult. In particular, some manufacturers are producing composite prostheses using one material desirable for the head plate and another material desirable for the shaft. Even though HA has been alleged to be compatible for direct interface with the tympanic membrane and because all ceramic and metal prostheses are rigid compared with the thin tympanic membrane, they are all at risk of extrusion, unless there is a cartilage buffer between the head plate and the undersurface of the drumhead,11 especially in the setting of negative middle ear pressure and tympanic membrane retraction.
There is no evidence to date showing that alloplastic material is better than autologous bone or sculpted native ossicles in ossicular reconstruction. Even though computer model experiments of ossicular reconstruction with PORPs suggest a slight acoustic advantage in the speech frequencies with HA12 compared with other alloplastic materials, including titanium, there is in fact very little comparative in vivo data showing one alloplastic material to be more successful than all others. However, certain alloplastic materials have already been found to be less favorable than others as middle ear implants for a variety of reasons. Reparative granuloma was reported when gold pistons were used in stapes surgery.13 Multinucleated foreign body giant cells were present in large numbers in Plastipore and Proplast explants from the middle ear, and with histological evidence of the prostheses breaking down.14 There is a high rate of extrusion of the Plastipore prosthesis in long-term follow-up studies, even with the presence of cartilage between the implant and the tympanic membrane.15
Sometimes, unfavorable results only become apparent with time. Ceravital prostheses became resorbed over a long period, leading to poor long-term results in ossiculoplasty.16 ▶ Table 8.2 provides a summary of the long-term outcome of ossiculoplasty. Only ossiculoplasty studies with 5-year follow-up or longer and a follow-up rate of at least 50% were included. For the purpose of comparison, the outcomes of autologous and homologous materials are also included. Few reports in the literature fulfill such strict criteria. The success rate of ossiculoplasty, if defined as an air-bone gap (ABG) closure to within 20 dB, was roughly 80% in cases where the stapes superstructure was present and about 60% when it was absent. There was no alloplastic material free from extrusion, even if the head plate was covered by a sheet of cartilage. Because the long-term success of ossiculoplasty is often more dependent on the middle ear pathology than the prosthesis biomaterials, it should also be noted that comparisons of one prosthesis material versus another require careful stratification of subjects according to middle ear risk factors to be meaningful.
