Thursday, September 13, 2007

External Ear

Reading Dr Rob on the physical exam of the ears, I noticed that there are a couple of young men who could use a setback otoplasty. But before we get to that procedure and other ear reconstruction procedures, let's start with the anatomy.

The external ear consists of the expanded portion which is called the auricula or pinna, and the external acoustic meatus (ear cannel). The pinna projects from the side of the head and serves to collect the vibrations of the air by which sound is produced. The ear cannel leads inward from the bottom of the auricula and conducts the vibrations to the tympanic cavity.

The auricula is of an ovoid form, with its larger end directed upward. Its lateral surface is irregularly concave and directed slightly forward. The prominent rim of the auricula is called the helix. In the region where the helix turns downward, a small tubercle, Darwin's tubercle, is frequently seen. Another curved prominence, parallel with and in front of the helix, is called the antihelix; this divides above into two crura (leg-like part), between which is a triangular depression, the fossa triangularis (triangular fossa). The narrow-curved depression between the helix and the antihelix is called the scapha. The antihelix describes a curve around a deep cavity, the concha. In front of the concha, and projecting backward over the meatus, is a small pointed eminence, the tragus, (from the Latin tragos, goat). Opposite the tragus, and separated from it by the intertragic notch, is a small tubercle, the antitragus. Below this is the lobule, composed of tough areolar and adipose tissues, and wanting the firmness and elasticity of the rest of the auricula.
The backside (cranial surface) of the auricula presents elevations which correspond to the depressions on its front (anterior surface) and after which they are named, e. g., eminentia conchæ, eminentia triangularis, etc.


The cartilage of the auricula consists of a single piece. It provides the form to the external ear. It is absent from the lobule. It is also deficient between the tragus and beginning of the helix, the gap being filled up by dense fibrous tissue. The posterior (cranial) aspect of the cartilage exhibits a transverse furrow, the sulcus antihelicis transversus, which corresponds with the inferior crus of the antihelix and separates the eminentia conchæ from the eminentia triangularis. The skin is thin, closely adherent to the cartilage, and covered with fine hairs furnished with sebaceous glands, which are most numerous in the concha and scaphoid fossa.


Sensibility of the normal external ear is derived from several cranial and extracranial nerves. Cervical nerves (the great auricular nerve, C2 to C3) and the lesser occipital nerve (C2) innervate the posterior aspect of the auricle and lobule. In the majority of dissections, the lesser occipital nerves have been found to be dominant and innervate the superior ear and the mastoid region, whereas the inferior ear and a portion of the preauricular area are supplied by the great auricular nerve. The anterior surface and the tragus are supplied by the trigeminal nerve (auriculotemporal nerve V3). The auricular branch of the vagus nerve (Arnold’s nerve) provides sensibility to the external auditory meatus.
Two separate but intercommunicating arterial networks to the external ear are formed from the external carotid system. One network supplies the triangular fossa-scapha, and the other supplies the concha. The triangular fossascapha network is derived from one subbranch of the upper auricular branch of the superficial temporal artery and from branches of the posterior auricular artery, which come through the earlobe and triangular fossa and over the helical rim. The conchal network is derived from perforators (usually two to four vessels) of the posterior auricular artery. The superficial temporal artery also sends several small auricular branches to supply the anterior surface of the ear. The rich communications between the superficial temporal and postauricular arterial systems allow for either system to support the ear. Venous drainage flows through the posterior auricular veins into the external jugular, the superficial temporal, and the retromandibular veins.

The relationships, dimensions, and proportions of the external ear have been thoroughly reviewed by Tolleth. At birth the ear is the same size in both sexes, but by the end of the first year boys' ears are larger than girls'. The full width of a boy's ear develops by age 13 and the full length by age 15. In girls, width is complete at age 10 and length at age 13. Ear width is approximately 55 percent of length. The long axis of the ear is tilted posteriorly from the vertical axis of the face at an angle ranging from 2-30 degrees. The axis of the ear and the nasal bridge, although similar, are not identical. The angle between them approximates 15 degrees, with the ear more vertical. The helical rim protrudes 1 to 2 cm from the skull, with the projection increasing from superior to inferior. In a normal ear, the rim is positioned 10 to 12 mm from the mastoid at the superior helix, 16 to 18 mm from the mastoid at midear, and 20 to 22 mm from the mastoid in the lower third. Although these measurements are most commonly used as a reference in setback otoplasty to avoid the classic
telephone” deformity, they must also be carefully assessed and reproduced for an anatomically correct ear reconstruction in patients with microtia.

References

  • Grey's Anatomy, The External Ear (both photos from this link)
  • Core Curriculum Syllabus: Review of Anatomy - Temporal Bone and Ear--Baylor College of Medicine
  • Basic Ear Anatomy--a Power Point Presentation
  • Auricular Reconstruction for Microtia: Part I. Anatomy, Embryology, and Clinical Evaluation; Beahm, Elisabeth K. M.D.; Walton, Robert L. M.D.; Plastic & Reconstructive Surgery. 109(7):2473-2482, June 2002.
  • Artistic anatomy, dimensions, and proportions of the external ear.; Tolleth, H; Clin. Plast. Surg. 5: 337, 1978.

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