The external ear comprises of two components. There is a cartilaginous portion, known as the pinna or auricle and a bony, tubular segment called the external acoustic meatus. The former portion of the auditory system is the first point at which sound wave modification begins. It collects sound waves in the series of “hills and valleys” and channels them into the external auditory canal.
Previous articles have detailed the structures found in the middle and inner ear regions. This one will complete the series by looking at the components of the outer ear along with references to embryology, neurovasculature and clinically relevant points.
The outer ear is situated superficially next to several bony landmarks. It is posterior to the zygomatic process of the temporal bone as well as the proximal part of the mandibular process and the auricular surface of the mandibular notch. Superior to it is the squamous part of the temporal bone, while the styloid process is located inferiorly. Finally, the mastoid process of the temporal bone lies posteroinferiorly with respect to the outer ear. There is very little subcutaneous tissue between the auricular scaffold and the overlying skin of the ear. The skin, however, continues into the external acoustic meatus and covers the lateral surface of the tympanic membrane.
Before the different parts of the external ear can be discussed, it is important to note that the left and right ears are optical isomers of themselves (similar to the left and right hands). This is relevant because the different parts of the external ear can be related to a clock face. However, the position of one structure, for example the tragus, will be in a 9 o’clock position on the left, but in a 3 o’clock position on the right. The left ear will be used as an example throughout, but keep in mind the mirroring necessary to discuss the right ear.
On that note, the tragus is the first of several cartilaginous flaps in the ear. As previously mentioned, it projects posteriorly from the 9 o’clock position to provide a lateral border to the distal end of the external acoustic meatus. Posteroinferior to the tragus in the 5 o’clock position is the antitragus. It is also a cartilaginous structure that is separated from the tragus by an intertragic notch. Superior to the tragus in the 10 o’clock position is the crus of the helix. It starts at the concha of the auricle (depression at the orifice of the external auditory meatus that corresponds with the protruding eminentia conchae on the posterior surface of the ear) and extends anterosuperiorly in the 10 o’clock position (superior to the tragus) and continues as the prominent ridge known as the helix. The helix is the outermost concaved section of the ear and it has a small protuberance at the 2 o’clock position called the auricular tubercle. The helix continues inferiorly and ends in the 4 o’clock position at the tail of the helix. Another raised cartilaginous structure called the antihelix mirrors the path of the helix. Apically, the antihelix has two crura surrounding a depression known as the triangular fossa. The antihelix is separated from the helix by the scaphoid fossa (corresponding to the eminentia scaphae on the posterior aspect of the external ear). Finally, in the 6 o’clock position is the soft, fibrofatty structure called the lobule.
There are two groups of muscles associated with the external ear. The intrinsic group of muscles consists of the tragicus and antitragicus, obliquus auriculae, transversus auriculae and the helicis major and minor. The tragicus muscle lies vertically on the lateral aspect of the tragus. It is a flat, short muscle that is perpendicular to (but not in contact with) the antitragicus muscle. Antitragicus originates from the anteroinferior part of the antitragus to the tail of the antihelix and helix. Helicis minor travels obliquely along the crus of the helix, while helicis major courses over the anterior boundary of the helix after travelling vertically over its spine. The obliquus auriculae and transversus auriculae muscles are found on the posterior (cranial) part of the external ear. The former extends from the upper part of the eminentia scaphae to the eminentia conchae, while the latter bridges the inferior aspect of the same two structures.
The extrinsic muscle group includes the anterior, posterior and superior auricular muscles. Auricularis posterior originates as three aponeurotic muscular fasciculi from the mastoid process to insert in the ponticulus (oblique ridge on the eminentia conchae) on the cranial aspect of the external ear. Auricularis anterior begins in the epicranial aponeurosis and inserts in the spine of the helix. It is the smallest of the three extrinsic muscles. Thirdly, auricularis superior is the largest of the extrinsic muscles. It also begins at the epicranial aponeurosis and inserts in the cranial part of the auricle.
The extrinsic muscles have limited impact on the movement of the external ear. The intrinsic muscles, however, are able to modify the shape of the auricle to a minor degree. Helicis major is known to move the auricle anteriorly and superiorly.
The external acoustic meatus is a bony-cartilaginous, tubular component of the external ear. It is about 4 cm in length when measured from the tragus, and 2.5 cm when measured from the concha of the auricle. It can be divided based either on its structural composition or its shape. Structurally, the medial two-thirds of the canal – about 16 mm in length – is osseous, while the lateral third – about 8 mm – is cartilaginous. Geometrically, it is an S-shaped tubular structure divided into the pars interna (anteromedially and inferiorly), pars media (posteromedial and superior) and pars externa (medial, then anterior then slightly superior). The external acoustic meatus is occluded medially by the tympanic membrane (see Middle Ear).
The major supply to the external ear is the posterior auricular branch of the external carotid artery. The superior auricular artery bridges the superficial temporal and posterior auricular arteries to provide a dependable collateral blood supply. The anterior auricular branch of the superficial temporal artery also brings blood to the anterior aspect of the external ear. There is also a branch of the occipital artery that perfuses the region. The posterior auricular artery also supplies the external acoustic meatus along with the auricular branches of the maxillary and superficial temporal arteries.
The veins of the external ear are analogous to the arteries of the region. There is also a well-established arterio-venous system that is believed to be involved in thermoregulation. Venous drainage of the external acoustic meatus goes to the pterygoid plexus, external jugular and maxillary veins.
Motor innervation to the extrinsic muscles is achieved by the temporal branch of the facial nerve (CN VII; to auriculares anterior and superior) and the posterior auricular branch of the facial nerve (auricularis posterior). For the intrinsic muscles, motor supply to the cranial group of muscles is also via the posterior auricular branch of the facial nerve, while those on the lateral surface are innervated by the temporal branch of the facial nerve. Sensory innervation, though not fully understood, is thought to originate from the great auricular nerve on the cranial and parts of the lateral surfaces, the lesser occipital nerve to the superior part of the cranial surface, and the auriculotemporal nerve to the tragus and crus of the helix. Sensory information from the external acoustic meatus is relayed by the auriculotemporal branch of the mandibular nerve (anterior and superior walls) and the auricular branch of the vagus nerve (CN X; posterior and inferior walls).
Traditionally, it was believed that the tragus and superior portion of the pinna drain their lymph to the pre-auricular group of nodes. The cranial portion of the pinna drains to the group of nodes at the tip of the mastoid bone. The remainder of the auricle drains its lymph to the upper deep cervical lymph nodes. However, recent studies now propose that there are four lymph draining vessels responsible for the lymphatic drainage of the external ear:
- There is an anterior branches that is spread over the anterior region of the external ear. They converge into a single lymph vessel that drains into the pre-auricular lymph nodes.
- The superior branches originate at the superior aspect of the helix and may course along the posterior part of the ear towards the infra-auricular lymph nodes.
- Vessels that originate at the scaphoid fossa (adjacent to the auricular tubercle) are known as the middle branches. They also travel along the back of the external ear to gain access to the infra-auricular nodes.
- Finally, the lobular (inferior) branches begin as a network of vessels in the auricular lobule. After converging, they travel toward the infra-auricular lymph nodes
One of the most dominant medical complications of the outer ear is foreign body occlusion. It is often seen in young children who can get small objects stuck in the external acoustic meatus. Other causes of obstruction can arise from a build-up of cerumen (wax) in the passage. There are several developmental abnormalities that can affect the external ear. A condition in which the external ear fails to develop is known as anotia and most likely a result of anomalies taking place between the 7th and 8th weeks of pregnancy. Conditions such as gestational diabetes, foetal alcohol syndrome, or use of teratogenic drugs such as isotretinoin (anti-acne drug) or the immunosuppressant thalidomide, have all been associated with microtia, also called diminutive ear. Polyotia occurs when the pre-auricular tissue persists anterior to the tragus instead of being absorbed by the pinna. It is also possible for the cranial part of the auricle to be tucked away under the temporal region in a condition known as cryptotia.