Ossiculoplasty

The earliest recorded attempt to re-establish a connection between the tympanic membrane and the oval window in the case of a missing ossicle was in 1901. Since then, numerous materials have been used to re-create the middle ear sound-conducting mechanism. Many materials have been used for ossicular substitution or reconstruction, including both biologic and alloplastic materials. Biologic materials include autograft or homograft ossicles, cortical bone, teeth, and cartilage.

The most commonly used autograft material has been the incus body, which is often reshaped to fit between the manubrium of the malleus and the stapes capitulum. Autograft materials are not always available, or—as in patients with cholesteatoma—an ossicle may have microscopic squamous epithelium infiltration that precludes such use. Autografts have several disadvantages, including lack of availability in chronically diseased ears, prolonged operative time to obtain and shape the material, resorption and/or loss of rigidity (especially with cartilage), and possible fixation to the walls of the middle ear. Additionally, osteitis may exist within the ossicles, and the risk of residual cholesteatoma may be increased in patients with cholesteatoma.

Irradiated homograft ossicles and cartilage were first introduced in the 1960s in an attempt to overcome some of the disadvantages of autograft implants. Homograft ossicles or cartilage may be presculpted by the manufacturer, or they may be sculpted during surgery. Since 1986, homograft materials rarely are used because of the risk of disease transmission (eg, AIDS, Creutzfeldt-Jakob disease).

Because of the disadvantages of autograft materials and the potential risk of infection from homograft implants, alloplastic materials are the most commonly used materials for ossicular reconstruction today. Alloplastic materials can be classified as biocompatible, bioinert, or bioactive. In the late 1950s and the 1960s, biocompatible material, such as polyethylene tubing, Teflon, and Proplast, were used. Ossicular reconstruction with these materials often resulted in migration, extrusion, penetration into the inner ear, or significant middle ear reactivity. For these reasons, use of these solid polymeric substances was eventually abandoned.

In the late 1970s, a high-density polyethylene sponge (HDPS) that had nonreactive properties was developed. HDPS has sufficient porosity to encourage tissue ingrowth. The original form was a machined-tooled prosthesis (Plasti-Pore); a more versatile manufactured thermal-fused HDPS (Polycel) arrived later. This latter form permitted coupling with other materials, such as stainless steel, thus lending itself to a wide variety of prosthetic designs. A high incidence of extrusion occurs when either Plasti-Pore or Polycel is placed in contact with the tympanic membrane. Extrusion is reduced considerably when cartilage is placed between a Plasti-Pore or Polycel prosthesis and the tympanic membrane.

Silastic, stainless steel, titanium, and gold are other examples of biocompatible materials used for ossicular reconstruction.

Bioinert implants are materials that do not release detectable trace substances. The prototype bioinert material is dense aluminum oxide ceramic (Al2O3). This material was popular in Germany and Japan in the 1970s. The implant can be fit to the undersurface of the tympanic membrane without cartilage coverage.

Bioactive implants react favorably with the body's tissues to promote soft tissue attachment. The attachment is a direct chemical bond to the surface of the material, not merely a mechanical attachment that occurs with bioinert and biocompatible materials. Bioactive implants were introduced in the 1970s with the hope that this new material would have a lower incidence of extrusion than the porous polyethylene implants. The first of the bioactive implants were bioactive glasses (Bioglass and Ceravital). Bioactive glasses enjoy limited use today because of the difficulty in trimming the glass prostheses and their instability in infected environments.

Hydroxylapatite is another bioactive material. From a compatibility standpoint, hydroxylapatite is the most promising implant material currently in use. The most common form of hydroxylapatite for middle ear reconstruction is the dense form. The nonporous and homogenous nature of dense hydroxylapatite resists penetration by granulation tissue. This aspect can clearly be seen using scanning electron microscopy. Hydroxylapatite can be placed directly under the tympanic membrane without increased risk of extrusion.

The goal of this article is to review some of the more common materials and techniques for ossicular chain reconstruction currently in use. An exhaustive review of all materials and techniques is not feasible in a single article.

Ossiculoplasty is defined as the reconstruction of the ossicular chain. For purposes of this discussion, reconstruction of the stapes (stapedectomy/stapedotomy) is not presented in this article.

The ideal prosthesis for ossicular reconstruction should be biocompatible, stable, safe, easily insertable, and capable of yielding optimal sound transmission. When the surgeon chooses a particular prosthesis, selection must be based on several factors, including compatibility and ease of configuring the prosthesis during surgery.

Conductive hearing loss from ossicular chain abnormalities may result from either discontinuity or fixation of the ossicular chain. In order of frequency, discontinuity most commonly occurs because of an eroded incudostapedial joint (occurring in approximately 80% of patients with ossicular discontinuity), an absent incus, or an absent incus and stapes superstructure. Ossicular fixation, exclusive of otosclerosis, most commonly occurs from malleus head ankylosis or from ossicular tympanosclerosis.

The problems associated with ossicular chain reconstruction in chronic otitis media are quite different from those in patients with a dry, infection-free middle ear. Some of the problems associated with chronic otitis media include tympanic membrane perforation, eustachian tube dysfunction, or cochlear deficits. These problems must also be considered to achieve optimal hearing.

Treatment of patients with cholesteatoma poses a unique set of problems. In order of importance, the goals of cholesteatoma removal are developing a safe ear, producing a clean dry ear, and improving or maintaining hearing. These goals sometimes are mutually exclusive. Specifically, a safe, dry ear may require removal of the posterior external auditory canal. Canal removal reduces middle ear volume, which may affect hearing.

In more than 80% of patients, the cause of ossicular damage (ie, discontinuity, fixation) is cholesteatoma or chronic suppurative otitis media. Trauma or congenital malformations account for most of the remaining causes of ossicular damage.

The normal human middle ear couples sound from the low impedance sound energy in the ear canal through the tympanic membrane and ossicles to the relatively high impedance of fluid within the cochlea. Recent investigations of human middle ear mechanics indicate that traditional teaching of middle ear mechanisms should be modified. To provide a more comprehensive description, both traditional and recent discussions of the physiology of middle ear sound transmission are briefly discussed in this section.

Traditional teaching states that the acoustic transformer system of the middle ear is divided into 3 systems: the catenary lever (due to the tympanic membrane), the ossicular lever (due to ossicular action), and the hydraulic lever (due to the difference in area between the tympanic membrane and the stapes footplate).

Catenary lever

The attachment of the tympanic membrane at the annulus amplifies the energy at the malleus because of the elastic properties of the stretched drumhead fibers. Because the annular bone surrounding the tympanic membrane is immobile, sound energy is directed away from the edges of the drum and toward the center of the drum. The malleus receives the redirected sound energy from the edge of the drum because of the central location of the manubrium. The catenary lever provides at least a 2-fold gain in sound pressure at the malleus.

Ossicular lever

The ossicular lever is based on the concept that the malleus and incus act as a unit. The malleus and incus rotate around an axis running between the anterior mallear ligament and the incudal ligament. The ossicular lever is the length of the manubrium of the malleus divided by the length of the long process of the incus (approximately 1.3:1). Since the malleus and tympanic membrane act as coupled system, some authors believe that the ossicular lever value of 1.3:1 should be reduced to 1.15:1. The reduction can be supported because of the different areas of curvature of the drum and how this affects the lever ratio. Together, the ossicular and catenary levers provide a sound pressure advantage of 2.3:1, which is more than twice that of the ossicular lever acting alone.

Hydraulic lever

The hydraulic lever acts because of the size difference between the tympanic membrane and the stapes footplate. Sound pressure collected over the area of the tympanic membrane and transmitted to the area of the smaller footplate results in an increase in force proportional to the ratio of the areas (also known as the areal ratio). The average ratio has been calculated to be 20.8:1.

According to traditional teaching, the acoustic transformer theory predicts a middle ear gain of approximately 27-34 decibels (dB). This figure is derived as a product of the action of the catenary, ossicular, and hydraulic levers. Implied in the transformer analogy is the expectation that this gain is independent of frequency.

Recent investigations of the human middle ear indicate that the acoustic transformer theory should be modified. Merchant et al (1997) summarized the latest reports of human middle ear sound transmission. ^[1] They proposed that middle ear sound transmission is the result of ossicular coupling, acoustic coupling, and stapes-cochlear input impedance. Middle ear aeration also is considered essential for proper middle ear sound conduction.

Ossicular coupling

Ossicular coupling refers to the sound pressure gain that occurs through the actions of the tympanic membrane and the ossicular chain. The pressure gain provided by the normal middle ear with ossicular coupling is frequency dependent. The mean middle ear gain is approximately 20 dB at 250-500 hertz (Hz), it reaches a maximum of about 25 dB around 1 kilohertz (kHz), and it then decreases at about 6 dB per octave at frequencies above 1 kHz.

The changes in gain above 1 kHz are caused by portions of the tympanic membrane moving differently than other portions, depending on the frequency of vibration. At low frequencies, the entire tympanic membrane moves in one phase. Above 1 kHz, the tympanic membrane divides into smaller vibrating portions that vibrate at different phases. Another factor for the change in gain above 1 kHz is slippage of the ossicular chain, especially at frequencies above 1-2 kHz. Slippage is due to the translational movement in the rotational axis of the ossicles or flexion in the ossicular joints. In addition, some energy is lost because of the forces needed to overcome the stiffness and mass of the tympanic membrane and ossicular chain.

Acoustic coupling

Acoustic coupling is the difference in sound pressures acting directly on the oval and round windows. Movement of the tympanic membrane produces a sound pressure in the middle ear that is transmitted to the oval and round windows. The pressure at each window is different because of the small distance between windows and the different orientation of each window relative to the tympanic membrane. In normal ears, the difference in pressures between the oval and round windows (acoustic coupling) is negligible.

In some diseased and reconstructed ears, the difference becomes significant and can greatly affect hearing. Specifically, when the ossicular chain is interrupted or absent, shielding of the round window results in redirection of all sound energy into the oval window, such as in Wullstein type IV tympanoplasty. When this is performed, acoustic coupling plays a significant role in sound pressure conduction for cochlear stimulation.

Stapes-cochlear input impedance

Stapes footplate motion is normally impeded by several anatomic structures, including the annular ligament, the cochlear fluids, the cochlear partition, and the round window membrane. Together, these structures result in stapes-cochlear input impedance. The round window impedance contribution is negligible in the normal ear. When the round window niche is filled with fluid or fibrous tissue, round window impedance increases, resulting in an increase in stapes-cochlear input impedance. Increases in this impedance cause conductive hearing loss.

Middle ear aeration

Ossicular coupling is impaired when the middle ear space (the air space of both the middle ear and the mastoid cavity) is reduced. The difference in sound pressures between the external auditory canal and the middle ear facilitates tympanic membrane motion. In the normal ear, the middle ear air pressure is less than the pressure in the external canal. When the middle ear space is reduced (eg, by chronic ear disease or canal wall down surgery), the impedance and pressure of the middle ear increase relative to the external canal because the impedance of the middle ear space varies inversely with its volume. The pressure difference between the external canal and the middle ear leads to a subsequent reduction in tympanic membrane and ossicular motion. The minimal amount of air required to maintain ossicular coupling within 10 dB of normal has been estimated to be 0.5 mL.

The clinical presentation of patients who would benefit from ossiculoplasty is quite variable. Conductive hearing loss may be the result of ossicular erosion or fixation from chronic ear disease, blunt or penetrating trauma, or congenital or neoplastic causes. It may also be associated with inner ear causes. These inner ear causes include superior semicircular canal dehiscence and an enlarged vestibular aqueduct.

The goal of ossicular chain reconstruction is better hearing, most typically for conversational speech. Ossiculoplasty is used to improve or to maintain the conductive portion of hearing loss. The aim of ossiculoplasty is not to close the air-bone gap per se but to improve the patient's overall hearing (ie, improve the air conduction score). A patient's perceived hearing improvement is best when the hearing level of the poorer-hearing ear is raised to a level close to that of the better-hearing ear. Small improvements in hearing are more likely to be appreciated by patients with bilateral hearing loss. Ossiculoplasty in children requires special considerations. ^[2]

A thorough knowledge of the anatomy of the tympanic membrane and the middle ear space is necessary prior to performing ossiculoplasty.

The tympanic cavity (middle ear) extends from the tympanic membrane to the oval window and contains the bony conduction elements of the malleus, incus, and stapes. The tympanic membrane is an oval, thin, semi-transparent membrane that separates the external and middle ear (tympanic cavity). The tympanic membrane is divided into 2 parts: the pars flaccida and the pars tensa. The manubrium of the malleus is firmly attached to the medial tympanic membrane; where the manubrium draws the tympanic membrane medially, a concavity is formed. The apex of this concavity is called the umbo. The area of the tympanic membrane superior to the umbo is termed the pars flaccida; the remainder of the tympanic membrane is the pars tensa.

For more information about the relevant anatomy, see Ear Anatomy.

Relatively few contraindications to ossiculoplasty exist. Acute infection of the ear is the only true contraindication. Acute infection would most likely result in poor healing, prosthesis extrusion, or both. Relative contraindications include persistent middle ear mucosal disease, tympanic membrane perforation, and repeated unsuccessful use of the same or similar prostheses.