External and middle ear

Introduction

The primary function of the external and middle ear is to amplify sound. The anatomical structure of the outer ear, the auricle, reflects this primary function by gathering, amplifying and directing sound into the EAC. The EAC is a closed-ended cylinder in which resonances occur at a wavelength that is four times the length of the cylinder. For the EAC, this corresponds to a resonance frequency of 2.6–3 kHz, which contributes a 10 dB gain. The entire external ear produces a broad band 15–20 dB gain between 2 and 5 kHz, which includes the important speech frequencies.

Sound waves reach the tympanic membrane and are amplified by the ossicular chain, consisting of the malleus, incus and stapes. The middle ear transformer mechanism, formed by the middle and inner layers of the tympanic membrane and ossicles, serves as an impedance matching device. Acoustic energy in air is transferred to the fluid-filled cochlea. Impedance matching is effected by the relative size difference between the tympanic membrane and the stapes footplate. The lower two-thirds of the tympanic membrane, the pars tensa, is the most important area for vibration response to sound waves. The ratio of the vibrating portion of the tympanic membrane to that at the stapes footplate results in a 17 times amplification in sound energy. The lever action of the ossicular chain results in an additional 1.3-fold amplification of sound energy. Smaller amounts of energy transference are effected by the fluid wave created within the cochlea as transmissions are initiated in the oval window and travel to the round window membrane.

The ear surgeon’s primary role is in the extirpation of disease and re-establishment of the natural impedance matching characteristics of the outer and middle ear.

Surgical surface anatomy

The auricle is composed of elastic cartilage, whereas the cartilagin­ous portion of the EAC is composed of fibrocartilage. The auricle is attached to the EAC by an extension of cartilage from the meatus and by three ligaments. The anterior ligament is attached to the auricle from the root of the zygoma to the tragus and helical crus; the superior ligament extends from the EAC to the spine of the helix; a posterior ligament extends from the mastoid bone to the posterior portion of the concha. The auricle is attached to underlying periosteum by anterior, superior and posterior auricular muscles.

The auricle and lateral EAC are perfused by the superficial temporal and posterior auricular arteries. The medial EAC is perfused by the deep auricular artery, a branch of the first portion of the maxillary artery, which enters the EAC at the bony cartilaginous junction and extends branches along the superior bony EAC to supply the tympanic vascular ring: this surgically important portion of the superior posterior EAC is termed the ‘vascular strip’. The sensory innervation of the outer ear is variable. The EAC is innervated by branches of the trigeminal, facial, glossopharyngeal and vagus nerves. The auricular conchal bowl and posterior EAC are innervated by branches of the facial, glossopharyngeal and vagus nerves. The numerous sources of sensory innervation explain clinical findings such as the vesicular eruption with facial paralysis caused by herpetic infection of the geniculate ganglion (Ramsay Hunt syndrome) and hypaesthesia of the conchal bowl caused by facial nerve compression from a cerebellopontine angle mass (Hitselberger's sign)

The lateral third of the EAC is cartilaginous except in its superior border, where dense fibrous tissue attaches it to the squamous portion of the temporal bone. This superior defect, the incisura, is the surgical landmark for the endaural surgical approach to the EAC and middle ear. The medial two-thirds of the EAC are composed of a complete cylinder of bone lined only with periosteum and skin, with no intervening subcutaneous tissue. The anterior and inferior walls are composed of the tympanic portion of the temporal bone, and the superior and posterior walls are formed by the squamous and mastoid portions of the temporal bone. The salient surgical landmark, the tympanomastoid suture line, runs along the posterior inferior portion of the EAC and is evident when elevating a tympanomeatal flap. The surgical boundaries of the EAC are demarcated anteriorly by the mandibular fossa; inferiorly by the parotid gland; superiorly by the epitympanic recess (medially) and the cranial cavity (laterally); and posteriorly by the mastoid bone, forming the posterior wall.

External ear core surgical procedures