Chapter 19

Wullstein1 and Zollner² first introduced the term tympanoplasty in 1951 to describe surgical reconstruction of the middle ear hearing mechanism that had been impaired or destroyed by disease. Successful tympanoplasty requires a mobile tympanic membrane or graft and a secure sound-conducting mechanism between this mobile membrane and the inner ear fluids. Since the introduction of the concept of hearing restoration in surgery for chronic otitis media, numerous materials have been used to recreate the sound-conducting mechanism. Aeration of a mucosal-lined middle ear is essential for sound conduction. If this can be accomplished, then the most biocompatible implant material with appropriate design and weight must be used for the optimal hearing restoration.

This chapter reviews the mechanics of hearing and the history of ossicular chain reconstruction and implant design, and focuses on the authors’ preferred material, prostheses design, and evolution of technique.

ACOUSTIC MECHANICS

The middle ear functions to convey sound pressures from the air into the fluids of the inner ear via the ossicular chain. It is an impedance-matching system that ensures that energy is not lost. 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. The acoustic transformation theory states that this occurs via three lever systems: the tympanic membrane lever, the ossicular lever, and the hydraulic lever.³ As a result of these three lever systems, the acoustic transformer theory predicts a middle ear gain of approximately 27 to 34 dB.⁴

Implied in this transformer theory is the expectation that this gain is independent of frequency. Further investigations indicate that the acoustic transformer theory should be modified, proposing that middle ear sound transmission is actually frequency dependent.⁵ Three areas account for this: ossicular coupling, acoustic coupling, and stapescochlear input impedance.

TYMPANIC MEMBRANE LEVER

The anatomy of the tympanic membrane and bony tympanic annulus provide a mechanical advantage. The tympanic membrane is a tense surface formed by the fibers of the tympanic membrane, which are tightly stretched over the malleus. Sound energy is directed toward the center of the tympanic membrane so that the manubrium receives the greatest amount. This creates amplification of the energy and provides at least a twofold gain in sound pressure at the malleus.⁶

OSSICULAR LEVER

The anatomy of the ossicles lends itself to another mechanical advantage. The malleus and incus work as a unit, but the manubrium is 1.3 times longer than the long process of the incus. This provides a lever action ratio of 1.3 to 1 as sound energy is transferred.⁷

HYDRAULIC LEVER

The anatomy of the middle ear also lends itself to the third lever, the hydraulic lever. The area of the tympanic membrane is considerably larger than the area of 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. The average ratio has been calculated to be 20.8 to 1.2

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, however, is frequency dependent. The mean middle ear gain is approximately 20 dB at 250 to 500 Hz, reaches a maximum of about 25 dB around 1 kHz, and then decreases at about 6 dB per octave at frequencies above 1 kHz.⁸

Certain portions of the tympanic membrane move differently depending on the frequency of vibration presented. At low frequencies, the entire tympanic membrane moves in one phase. At higher frequencies, 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 to 2 kHz.⁴ 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.⁴

Loss of the tympanic membrane and ossicular chain can cause a hearing loss that exceeds 30 dB, because sound now has access to both the round and oval windows, which can decrease the movement of cochlear fluids.

ACOUSTIC COUPLING

Movement of the tympanic membrane produces a sound pressure in the middle ear that is transmitted to the oval and round windows. Acoustic coupling is due to the difference in sound pressures acting on these areas. 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.⁹ When this is performed, acoustic coupling plays a significant role in sound pressure conduction for cochlear stimulation.

IMPEDANCE AT THE OVAL WINDOW

At the oval window, sound impedance occurs by several structures, including the annular ligament, the viscosity of the cochlear fluids, and the round window membrane. The round window impedance contribution is negligible in the normal ear. When the round window niche is filled with fluid or fibrous tissue, the round window impedance increases, resulting in a conductive hearing loss.

MIDDLE EAR AERATION

The middle ear space must be well aerated to facilitate ossicular function and tympanic membrane motion. Middle ear air pressure is less than the external canal air pressure in normal conditions, providing an environment conducive to ossicular coupling. If middle ear aeration is poor and the space is reduced, the pressure of the middle ear increases relative to the external canal pressure as the impedance increases. The pressure difference leads to a reduction in ossicular and tympanic membrane motion. The minimal amount of air required to maintain ossicular coupling within 10 dB of normal has been estimated to be 0.5 mL.³

IMPLANT CHARACTERISTICS

Many materials have been used for ossicular substitution or reconstruction. The ideal prosthesis for ossicular reconstruction should be made of material that maintains its shape, rigidity, and acoustic properties, as well as being cost-effective. This material should also be biocompatible, safe, and easily inserted and modified (Table 19–1).

Ossicular reconstruction materials are categorized as autografts, homografts, and alloplastic prosthetics. Each of these materials has advantages and disadvantages for use in the middle ear (Table 19–2).

AUTOGRAFTS

Autograft material, such as cartilage and bone, was one of the first materials used for ossiculoplasty. Studies have shown that cartilage was unstable, loses rigidity, and resorption occurs.¹⁰ This loss of stiffness, due to ingrowth of blood vessels with subsequent chondritis, led to the conclusion that cartilage struts are unsatisfactory as long-term implants.^11,12

Schuknecht and Shi¹³ discussed the fate of bone middle ear implants. Incus and malleus grafts demonstrated no evidence of bone erosion and little resorption. Autologous incus grafts have been used for many years for middle ear reconstruction by modifying them to fit between the manubrium of the malleus and the stapes capitulum. These grafts may be used as a repositioned incus strut or as a total prosthesis. These bone implants maintain their contour, shape, size, and physical integrity for at least 11 years.^9,10 Today the preferred material for ossicular chain reconstruction when appropriate is the autograft biologic ossicle, if available, reshaped to fit the reconstruction needs of the surgery. In a series of 2200 cases, allograft and homograft pros-theses yielded better hearing results than even the most biocompatible allograft prostheses.¹⁴

Stay updated, free articles. Join our Telegram channel

Join