In 1868, Helmholtz ⁴ defined the principles of impedance matching by the middle ear. He proposed three levers to accomplish the required pressure transformation. The tympanic membrane, which is rigidly fixed at the periphery and mobile centrally, acts as a catenary lever, exerting its force on the malleus handle in response to pressure changes transmitted through the EAC.⁵ A twofold gain in sound pressure level is generated at the malleus. Although the pars flaccida seems to allow free movement of the malleus head in the epitympanum and may decrease middle ear/external ear atmospheric pressure differences, it has only a minor acoustic role in humans.⁶

The ossicular lever is produced when the incus and the malleus rotate as a unit around an axis extending from the anterior mallear ligament through the incudal ligament.⁷ The ossicular lever taken alone produces a small mechanical advantage for sound transmission. Further studies by Tonndorf and Khanna⁵ showed the catenary lever was tightly coupled to the ossicular lever because the tympanic membrane is extensively adherent to the malleus handle. Corrected calculations revealed a combined catenary and ossicular lever ratio of 1 : 2.3. According to studies done on fresh cadaver human temporal bones, the mean sound pressure gain produced by the human middle ear is 26.6 dB, and is centered around its resonant frequency (0.9 to 1 kHz). Beyond 1 kHz, the pressure gain measured at the stapes footplate decreases at a rate of −8.6 dB/octave.⁸

The hydraulic lever assists the transmission of the sound pressure, which is collected over the larger tympanic membrane and passed on to the much smaller stapedial footplate. For the hydraulic lever to be functional, the stapes should move in a “piston-like” fashion, which has been elegantly described by Guinan and Peake.⁹ The increased force is proportional to the ratio of the areas of the tympanic membrane to the footplate. The area of the tympanic membrane is approximately 85 mm², and that of the stapes footplate is 3.2 mm².¹⁰ After studying 43 human temporal bones, Saunders¹¹ concluded that the “areal ratio” was the most important component of the middle ear impedance matching system. Ratios varied from ear to ear, but averaged 20.8 : 1.

In a nondiseased ear, sound is transmitted to the oval window with increased energy and at an earlier phase than to the round window. Intensity differences and phase protection cause differential displacement of cochlear fluids, which sets a fluid wave in motion along the tectorial membrane, producing mechanical stimulation of sensory hair cells.

The pinnae gather sound from an arc of 135 degrees relative to the direction of the head and increase sound pressure by 6 dB. They reject sounds arriving from the rear, which assists in the determination of sound origin.² The EAC is a closed cylinder approximately 2.5 to 3.5 cm in length. Because ear canal resonance occurs at a frequency whose wavelength is four times the canal length, the corresponding resonant frequency is roughly 3500 Hz. The addition of the conchal component of the pinna increases the EAC length, resulting in a lower resonant frequency, which has been measured at 2700 Hz. The overall increase in sound pressure transduction caused by external ear structures and measured at the tympanic membrane is estimated at 15 to 22 dB.^10,¹²

The increased gain in sound pressure level from the impedance matching system of the middle ear and tympanic membrane measures roughly 34 dB. The EAC and auricle, primarily the conchal component of the pinna, provide an additional increase of up to 15 dB, depending on the azimuth of sound delivery and functional length of the canal.

Specific Defects in the Conductive Auditory Periphery and Resultant Hearing Loss

In his 1978 review, Austin ¹ identified five categories of anatomic defects in the middle ear sound-conducting system and described each within the context of the associated prototypic hearing loss (Table 143-1). In the first category, tympanic membrane perforation with undisturbed ossicular continuity produced a hearing loss that was linearly proportional to the size of the perforation (loss of areal ratio plus loss of catenary lever). The degree of hearing loss, flat across speech frequencies, was not altered by the location of the perforation on the drumhead.

Table 143-1 Specific Lesions of the Conductive Apparatus and Associated Hearing Loss

Classification	Component Disrupted	Expected Loss (dB)
Perforation of tympanic membrane	Loss of areal ratio, catenary lever	Proportional to size of perforation
Perforation of tympanic membrane with ossicular interruption	Hydraulic lever, areal ratio, catenary lever	38.3
Total loss of tympanic membrane and ossicular chain	Hydraulic lever, areal ratio, catenary lever, phase cancellation	50
Ossicular interruption with intact tympanic membrane	Hydraulic lever, areal ratio, catenary lever, phase cancellation, reflection of sound energy away from middle ear at tympanic membrane	55-60
Ossicular interruption with intact tympanic membrane and closure of oval window (distinct congenital malformation)	Hydraulic lever, areal ratio, catenary lever, phase cancellation, reflection of sound energy away from middle ear at tympanic membrane	55-60

From Austin DF. Sound conduction of the diseased ear. J Laryngol Otol. 1978;92:367; and Austin DF. Acoustic mechanisms in middle ear sound transfer. Otolaryngol Clin North Am. 1994;27:641.

In the second category, tympanic membrane perforation combined with ossicular disruption occurred in approximately 60% of Austin’s patients, and was the most common form of conductive hearing loss requiring surgical therapy. Incudostapedial joint erosion was the most frequent ossicular anomaly. Caution was taken when interpreting audiograms in these patients because underestimation of the magnitude of the conductive loss frequently occurred. Falsely depressed bone-conducted hearing levels (≤10 dB in speech frequencies) resulted from impedance variation occurring at the oval window caused by the ossicular discontinuity. Correction of the ossicular problem typically increased bone levels, reflecting the re-establishment of impedance matching at the oval window. The predicted 38.3-dB hearing loss, when adjusted for falsely depressed bone levels, was primarily a result of decreased function of the hydraulic lever added to dysfunction in the catenary/ossicular lever. Perforation size and the condition of the stapes footplate accounted for variations in this patient group.

In the third category, total loss of the tympanic membrane and ossicles created a condition wherein sound pressure contacted the oval and round windows simultaneously, resulting in partial phase cancellation of the sound wave in the cochlear fluids. Conductive hearing loss was flat across speech frequencies and averaged 50 dB when corrected for the previously described misleading bone levels. More complete phase cancellation caused increased hearing loss compared with patients with partial perforations.

The fourth category included patients with ossicular disruption behind an intact tympanic membrane. This defect resulted in a maximal conductive hearing loss of 55 to 60 dB. The intact eardrum reflected sound energy back into the EAC, causing an additional 17-dB conductive loss beyond what was expected from removal of the hydraulic and catenary/ossicular lever action. The decreased sound pressure also reached the round and oval windows nearly simultaneously, inducing phase cancellation in the labyrinthine fluids.

The fifth category described various congenital malformations with ossicular disruption and closure of the oval window; also included were cases of obliterative otosclerosis with closure of the oval and round windows behind an intact drum. The expected flat loss from such a defect is 60 dB.²

Diagnostic Evaluation

The use of advanced audiometric testing, laboratory studies, or diagnostic imaging is rarely indicated in patients with isolated conductive hearing loss. Complete assessment of a patient with hearing loss begins with a careful history. A thorough head and neck examination with cranial nerve testing and otoscopy is mandatory. Otomicroscopy is useful to remove foreign bodies and to examine the tympanic membrane and middle ear structures more precisely, and is a prudent step in preoperative evaluation. The use of 256-Hz, 512-Hz, and 1024-Hz tuning forks assists in the localization of the side of greatest auditory deficit and verifies audiometric findings.

The Weber test is effective in lateralizing hearing loss. The fork is gently struck by the examiner and placed on the forehead or on the vertex of the skull, and the patient states on which side he or she hears the tone more clearly. Placement of the tuning fork over the nasal bones or on the teeth may present a stronger stimulus for Weber testing in difficult-to-test patients.¹³ Lateralization of the tone represents either a conductive loss ipsilateral to the side of localization or a sensorineural loss greater in the contralateral ear. The 256-Hz tuning fork is the most sensitive of the available frequencies,¹⁴ but it can yield an unacceptable number of false-positive results. The most clinically useful single fork is the 512-Hz fork.¹⁵

Rinne testing using all three fork frequencies can accurately estimate the conductive component of a hearing deficit (Table 143-2).¹⁶ The test is performed by activating the tuning fork and by placing it firmly on the skin over the cribriform area of the mastoid bone. When the tone can no longer be heard, the fork is placed, tines oriented parallel to the head/frontal plane, in front of the meatus of the EAC. The patient indicates whether the tone is present or absent. A negative or abnormal Rinne result is found when bone conduction is perceived longer than air conduction. A positive, or normal, result occurs when the air conduction is greater than bone. Tuning fork testing should be carried out in any patient before surgery for a conductive anomaly to verify audiometric results and to establish a baseline for comparison for postoperative assessment.

Table 143-2 Degree of Hearing Loss Estimated by Rinne Testing

Rinne Test Result*	Estimated Conductive Loss (dB)
Negative, 256 Hz	Mild conductive loss of 20-30
Positive, 512 Hz and 1024 Hz
Negative, 256 Hz and 512 Hz	Moderate conductive loss of 30-45
Positive, 1024 Hz
Negative, 256 Hz, 512 Hz, 1024 Hz	Severe conductive loss of 45-60