ANATOMY OF THE EXTERNAL EAR

By

Dr. Syed Salman Hussaini

INTRODUCTION

Three parts – external, middle and internal.Early warning system to threatening environmental sounds.Accurately and rapidly detecting head movement, to help fixing the gaze.Major part of balance system, important information about sudden changes in the environment, both external and self.Major part of communication system.

EXTERNAL EAR

AURICLE (PINNA) Projects at a variable angle from the

side of the head, functions in collecting the sound.

The lateral surface of the pinna is dominated by concavities, in particular the concha.

Curved rim – helix, Darwin's tubercule.

Anterior and parallel to helix – antihelix.

Superiorly divides in to two crura, between which is the triangular fossa.

The scaphoid fossa lies above the superior of the two crura.

In front of the antihelix – concha.

Divided into two portions by crus of the helix (descending limb of anterior superior portion of the helix), rests above the EAC.

Smaller superior portion – cymba conchae, is the direct lateral relation to the suprameatal triangle.

Larger inferior portion – cavum concae.

Tragus – small blunt triangular prominence pointing posteriorly.

Opposite the tragus, at the inferior limit of antihelix – antitragus.

Lobule lies below the antitragus – soft, composed of fibrous and adipose tissue.

Medial (cranial) surface – has elevations corresponding to the depressions on the lateral surface.

Possesses corresponding names, eg. Eminentia conchae

CARTILAGE

Body of auricle formed by elastic fibrocartilage. Forms a continous plate except between the tragus and the anterior crus of helix, where it is replaced by dense fibrous tissue band.

Site of endaural incision – will not damage cartilage or its perichondrium, allows wide exposure of deeper parts. Allows wide exposure of deeper parts after splitting the soft tissue ring surrounding the bony ear canal.

Cartilage extends around 8 mm to form lateral 1/3rd of EAC.

Cartilage is covered by perichondrium, from which it derives its blood supply, cartilage itself is avascular.

Stripping of the perichondrium from the cartilage – follwing injuries – haematoma – cartilage necrosis – boxer's ear.

Skin of the pinna is thin and closely attached to the perichondrium on the lateral surface. On the medial surface, there is subdermal adipose tissue, that allows for dissection during pinnaplasty. Skin of auricle is covered with fine hair, mostly at the concha and scaphoid fossa. Sebaceous glands open into the root canals of these hair. Thick coarse hair form over the tragus and intertrgic notch in middle and old aged males.

LIGAMENTS Cartilage of auricle is connected to the temporal bone by two extrinsic ligaments. Anterior ligament – runs from tragus and (cartilaginous spine on anterior rim) crus of helix to the root of zygomatic arch. Posterior ligament – runs from medial surface of the concha to the lateral surface of the mastoid prominence. Intrinsic ligaments connect various parts of the cartilaginous auricle, between helix and tragus and another from the antihelix to the posteroinferior portion of the helix.

MUSCLES

Extrinsic and intrinsic muscles are attached to the perichondrium of the cartilage. Temporal and posterior auricular branches of the facial nerve supplythe extrinsic muscles and, while being functionally unimportant, they do give rise to the post-auricular myogenic response following appropriate auditory stimulation.

Three extrinsic muscles: auricularis anterior, superior and posterior, the last being supplied by the posterior auricular branch of the facial nerve. All three radiate out from the auricle to insert into the epicranial aponeurosis.

The intrinsic muscles - six in number – are small, inconsistent and without useful function.

ARTERIAL SUPPLY External carotid artery. The posterior auricular artery (dominant artery) supplies the medial surface (except the lobule), the concha, the middle and lower portions of the helix and the lower part of the antihelix. The anterior auricular branches of the superficial temporal supply the upper portions of the helix, antihelix, triangular fossa, tragus and lobule. The superior auricular artery has a constant course and connects the superior temporal artery and the posterior auricular artery network. This branch can provide a reliable vascular pedicle for retroauricular flaps. A small auricular branch from the occipital artery may assist the posterior auricular in supplying the medial surface.

SENSORY INNERVATION

Both cranial branchial nerves and somatic cervical nerves supply the auricle. Their distribution is heterogeneous with extensive overlap.

The greater auricular nerve prevails on the lateral and medial surfaces.

The great auricular nerve originates from C2-3 branches of the cervical plexus; the sensory territory of this nerve is the lower two thirds of the anterior and posterior external ear.

The sensory innervation to the anterior superior one third of the ear (tragus, crus helix, superior helix) is the auriculotemporal nerve (branch of V3).

The lesser occipital nerve derives its origin from the C2 branch of the cervical plexus and supplies sensory innervation from the posterior (cranial) surface of the superior one third of the external ear.

The auricular branch of vagus nerve (Arnold nerve) innervates the external auditory canal floor and concha.

Facial N. - Probably supplies small region in the root of concha

LYMPHATIC DRAINAGE

Lymphatic drainage from the posterior surface is to the lymph nodes at the mastoid tip. From the tragus and the upper part of the anterior surface to the preauricular nodes. From the rest of the auricle to the upper deep cervical nodes.

EXTERNAL AUDITORY CANAL The external auditory canal extends from the concha of the auricle to the tympanic membrane and is approximately 2.4 cm long.

The supporting framework of the canal wall is cartilage in the lateral one-third and bone in the medial two-thirds.

The diameter of the canal varies greatly between individuals and between different races.

In adults, the cartilaginous portion runs inwards, slightly upwards and backwards. The canal is straightened, therefore, by gently moving the auricle upwards and backwards to counteract the direction of the cartilaginous portion.

In the neonate, there is virtually no bony external meatus as the tympanic bone is not yet developed, and the tympanic membrane is more horizontally placed so that the auricle must be gently drawn downwards and backwards for the best view of the tympanic membrane.

In the adult, the lateral cartilaginous portion is about 8 mm long and is continuous with the auricular cartilage.

It has two deficiencies – the "fissures of Santorini" in this part of the cartilage and through them the parotid or superficial mastoid infections can appear in the canal, or vice versa.

The bony canal wall, about 1.6 mm long, and is narrower than the cartilaginous portion and becomes smaller closer to the tympanic membrane.

Anteroinferior part of the bony canal may present a deficiency (foramen of Huschke) in children up to the age of four or sometimes in adults, permitting infections to and from the parotid.

The medial end of the bony canal is marked by a groove, the tympanic sulcus, which is absent superiorly.

The tympanic bone forms the greater part of the canal, and also carries the tympanic sulcus. The squamous bone forms the roof.

There are two suture lines in the canal wall – the tympanosquamous anteriorly and the tympanomastoid posteriorly.

The suture lines may be more or less developed; they project into the canal with overlying closely adherent skin, which can makes raising an intact tympanomeatal flap a difficult.

The tympanomastoid suture is a complex suture line between the anterior wall of the mastoid process, a portion of the squamous bone and the tympanic bone.

There are two constrictions in the canal:

At the junction of the cartilaginous and bony portions and

The isthmus, 5 mm from the tympanic membrane where a prominence of the anterior canal wall reduces the diameter.

Deep to the isthmus, the anteroinferior portion of the canal dips forward forming a wedge-shaped anterior recess between the tympanic membrane and the canal. This recess can be a difficult spot for access and also acts as a cesspool for discharge and debris in cases of external and middle ear infections.

The external canal is lined with skin.

Instead of skin maturation taking place directly towards the surface, there is outward, oblique growth of the epidermis of the canal skin and pars flaccida so that the surface layers effectively migrate towards the external opening of the canal.

The normal rate of migration is about 0.1 mm/day, this range is hugely variable and in some conditions there is complete failure of migration with a consequent build-up of shed keratin in the ear canal.

The skin of the pars tensa has a different derivation from that of the deep canal and cell divisions occur randomly within the layer of basal cells.

The effect of this is to create outward mass migration of the skin of the pars tensa.

Ink dots applied to the surface have an outward pattern of movement.

If a hole is made in the tympanic membrane and a graft laid underneath the membrane (an underlay graft) then migration of the skin from the outer edge of the perforation is directed centrally to cover the graft.

This occurs because the boundary conditions have altered and provides the basis for the healing of grafts and for the re-epithelialization of mastoid cavities.

Even a small piece of pars tensa skin has this ability and so is a precious material and needs to be preserved during ear surgery if a bare area needs covering.

The property of canal skin to migrate can also lead to formation of cholesteatoma if the skin becomes displaced into the middle ear cleft.

The absence of hair serves to distinguish the bony and cartilaginous canals.

At the outer limits of the ear canal are some short hairs that project towards the opening of the canal.

In this region are clusters of ceruminous and sebaceous glands. The ceruminous glands are modified apocrine sweat glands that open into the root canal of the hair follicles and produce a watery, white secretion that slowly darkens, turning semi-solid and sticky as it dries.

These glands are apocrine sweat glands they respond to many stimuli such as adrenergic drugs, fever and emotion which, along with direct mechanical stimulation, can all produce an increase or altered secretion.

The sebaceous glands produce an oily material (sebum) from the breakdown of their fat-containing cells which is usually excreted into the root canals of the hair follicles.

The mixture of desquamated cells, cerumen and sebum forms wax.

Human earwax is a Mendelian trait consisting of wet and dry forms.

Dry wax, lacking cerumen, is yellowish or grey and brittle, while wet wax is brownish and sticky.

The wet phenotype is dominant over the dry type, and is frequently seen in populations of European and African origins.

East Asians show the dry phenotype and there are intermediate frequencies among the Native American and Inuit of Asian ancestry.

A singlenucleotide polymorphism in the ABCC11 gene is responsible for the determination of earwax type, with the AA genotype corresponding to dry wax and GA and GG to wet wax.

The areas of skin that take part in cerumen production have all the components of an active local immune system and probably protect the canal by an antibody-mediated local immune response.

Wax is not usually found in the deep ear canal and a lump of 'wax' overlying the upper portion of the tympanic membrane (pars flaccida or attic region) is rarely true wax, but is nearly always associated with an underlying cholesteatoma as it is, in fact, dried-up, oxidized keratin.

The sense of the old adage 'beware the attic wax' is still just as true today as it was in the past.

Arterial Supply: Derived from branches of the external carotid artery.

The auricular branches of the superficial temporal artery supply the roof and anterior portion of the canal.

The deep auricular branch of the first part of the maxillary artery arises in the parotid gland behind the temporomandibular joint, pierces the cartilage or bone of the external meatus and supplies the anterior meatal wall skin and the epithelium of the outer surface of the tympanic membrane.

The auricular branches of the posterior auricular artery pierce the cartilage of the auricle and supply the posterior portions of the canal.

The veins drain into the external jugular vein, the maxillary veins and the pterygoid plexus.

The lymphatic drainage follows that of the auricle.

Effective local anesthesia can be obtained by 1 to 2% lidocaine infiltration of the postauricular region accompanied by infiltration of the cartilaginous canal in a four-quadrant (ie, at the 2, 4, 8, and 10 o’clock positions) fashion.

Infiltration of the bony canal must be done gently to avoid bleb formation; if done properly, the anchoring of the skin of the bony EAC “outlines” the tympanomastoid and tympanosquamous sutures.

Relationships of the external auditory canal

TYMPANIC MEMBRANE The tympanic membrane lies at the medial end of the external

auditory meatus and forms the majority of the lateral wall of the tympanic cavity.

It is slightly oval in shape, being broader above than below, forming an angle of about 55° with the floor of the meatus.

Its longest diameter from posterosuperior to anteroinferior: 9-10 mm

Perpendicular to this the shortest diameter: 8-9 mm.

Most of the circumference is thickened to form a fibrocartilaginous ring, the tympanic annulus, which sits in a groove in the tympanic bone, the tympanic sulcus.

The sulcus does not extend into the notch of Rivinus at the roof of the canal, which is formed by part of the squama of the temporal bone.

Tympanic membrane (left).

1, Malleus; 2, anterior mallear fold; 3, posterior mallear fold; 4, pars flaccida (Shrapnel's membrane); 5, projection of the long process of the incus; 6, pars tensa; 7, annular ligament.

From the superior limits of the sulcus, the annulus becomes a fibrous band which runs centrally as anterior and posterior malleolar folds to the lateral process of the malleus

The handle of malleus is clearly visible within the tympanic membrane.

This leaves a small, triangular region of tympanic membrane above the malleolar folds within the notch of Rivinus, called the pars flaccida, which does not have a tympanic annulus at its margins.

The pars tensa forms the rest of the tympanic membrane and is concave towards the ear canal.

But each segment is slightly convex between the lateral attachment of the annulus and the centre of the membrane where the tip of the malleus handle is attached at the umbo.

Both the pars tensa and pars flaccida comprise three layers.

There is an outer epithelial layer, the epidermis,which is continuous with the skin of the external meatus; a middle, mainly fibrous layer, the lamina propria; and an inner mucosal layer continuous with the lining of the tympanic cavity.

The lamina propria of the pars tensa has radially oriented fibres in the outer layers and circular, parabolic and transverse fibres in the deeper layer.

This arrangement probably accounts for the complex pattern of tympanic membrane displacement during sound stimulation.

In the pars flaccida, the lamina propria is less marked and the orientation of the collagen fibres seems random.

The arterial supply of the tympanic membrane arises from branches supplying both the external auditory meatus and the middle ear.

These two sources interconnect through extensive anastomoses within the connective tissue layer of the lamina propria.

The epidermal vessels originate from the deep auricular branch of the maxillary artery coming from the external auditory meatus,

The mucosal vessels arise from the

anterior tympanic branches of the maxillary artery,

stylomastoid branch of the posterior auricular artery,

middle meningeal artery.

Nerve Supply

Branches of the auriculotemporal nerve (Vc), the auricular branch of the vagus and the tympanic branch of the glossopharyngeal nerve supply the tympanic membrane.

These also run in the lamina propria and, while variations and overlap are considerable, both the vascular supply and innervation are relatively sparse in the middle part of the posterior half of the tympanic membrane.

DEVELOPMENT OF THE EXTERNAL EAR The development of the pinna commences at 4 weeks as tissue

condensations of the mandibular and hyoid arches appear at the distal portion of the first branchial groove.

Within 2 weeks, six ridges, known as the hillocks of His, arise from the tissue condensations.

One views suggests that the entire pinna except the tragus and anterior external auditory canal (of mandibular arch origin) arises from the hyoid (second branchial) arch.

Other view suggests a balanced participation of both the first and second branchial arches in the development of the pinna.

The hillocks fuse into an anterior fold of mandibular arch origin and a posterior fold of hyoid arch origin, oriented about the first branchial groove. The folds unite at the upper end of this groove.

Adult configuration is achieved by the fifth month, independent of developmental progress in the middle and inner ears. The darwinian tubercle, makes its appearance at roughly 6 months.

The six hillocks of His at approximately 6 weeks. At approximately 7 weeks, the six hillocks are fusing to form two folds, which will later fuse superiorly.

The adult auricle with the derivatives of the six hillock numbered

The dorsal part of the first branchial groove, which gives rise to the external auditory canal, progressively deepens during the second month.

The ectoderm of the groove briefly abuts on the endoderm of the tubotympanic recess (first pharyngeal pouch), but during the sixth week, a mesodermal ingrowth breaks this contact.

Beginning at 8 weeks, the inferior portion of the first branchial groove deepens again, forming the primary external auditory canal, which corresponds to the fibrocartilaginous canal of the adult.

In the next week of development, a cord of epithelial cells at the depths of the primary external auditory canal grows medially into the mesenchyme to terminate in a solid (meatal) plate.

The mesenchyme adjacent to the meatal plate gives rise to the lamina propria (fibrous layer) of the tympanic membrane and at 9 weeks is surrounded by the four membranous bone ossification centers of the tympanic ring.

Tympanic ring also functions to inhibit inward epithelial migration. Failure of this function may lead to cholesteatoma formation (ie, congenital cholesteatoma) at the junction of the first and second branchial arches.

The primary external auditory canal (EAC) is formed at 9 weeks with deepening of the first branchial groove. The meatal plate develops as epithelial cells grow medially toward the tympanic cavity.

By the tenth week, the tympanic ring elements fuse except superiorly, where a defect remains, the notch of Rivinus.

These elements then expand, accompanied by growth of the solid epithelial cord of cells.

It is not until after the fifth month that the cord splits open, initially at its medial terminus, forming the bony external auditory canal by the seventh month.

The cells remaining at the periphery form the epithelial lining of the bony external auditory canal, whereas those remaining medially form the superficial layer of the tympanic membrane.

The medial layer of the tympanic membrane derives from the epithelial lining of the first pharyngeal pouch.

These developmental changes in the external auditory canal occur at a time when the outer, middle, and inner ears are already well developed.

THE MIDDLE EAR CLEFT

The middle ear cleft consists of the tympanic cavity, the Eustachian tube and the mastoid air cell system.

The tympanic cavity is an irregular, air-filled space within the temporal bone between the tympanic membrane laterally and the osseous labyrinth medially.

It contains the auditory ossicles and their tendons that attach them to the middle ear muscles.

Other structures, including the tympanic segment of the facial nerve, run along its walls to pass through the cavity.