PaThe 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 in the temporomandibular joint, pierces the cartilage or bone of the external meatus and supplies the anterior meatal wall skin.25 The auricular branches of the posterior auricular artery pierce the cartilage of the auricle and supply the posterior portions of the canal.
Venous drainage
Veins drain into the external jugular vein, the maxillary vein and the pterygoid plexus.25
Lymphatic drainage
The lymphatics of the tragus and anterior external portion of the auricle drain into the superficial parotid lymph nodes, the posterior external and medial aspect of the auricle into the retroauricular lymph nodes and of the lobule and inferiorly of the external auditory canal drains into the superficial cervical group of nodes.27
Tympanic membrane
The tympanic membrane, forming the lateral wall of the mesotympanum and a small part of the epitympanum, separates the tympanic cavity from the external
meatus. It is a thin, nearly oval disc slightly broader above than below, forming an angle of about 55 with the floor of the meatus. Its longest diameter from posterosuperior to anteroinferior is 9-10 mm. while perpendicular to this the shortest diameter is 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. This sulcus does not extend to the roof of the canal, which is formed by the squamous portion of the temporal bone. Form the superior limits, the annulus becomes fibrous bands which run centrally as anterior and posterior malleolar folds to the lateral process of malleus. The malleolar folds divide the tympanic membrane into pars flaccida and pars tensa. Pars flaccida is a triangular region of tympanic membrane above the malleolar folds, while pars tensa forms rest of tympanic membrane. The handle of malleus lies within the layers of the tympanic membrane.25,27
Histology: The pars tensa is taut and consists of three layers:
• lateral epidermal layer,
• medial mucosal layer, and
• intermediate fibrous layer – lamina propria.
The epidermal layer continues with the skin of external auditory canal.The mucosal layer is continuous with the mucosa of the middle ear.The intermediate layer is made of fibrous tissue arranged in inner circular and outer radial strata. These structures are interconnected by a parabolic arrangement of collagen fibers.
The pars flaccida, although lax, is actually thicker than the pars tensa. First described by Shrapnell, it also consists of epidermal, fibrous, and mucosal layers. The epidermis is composed of 5 to 10 layers of epithelial cells, the fibrous layer is less marked and consists of irregularly arranged collagen and elastic fibers and the mucosal layer is composed of simple endothelial cells, as in the pars tensa.
Blood supply:
Vessels supplying the tympanic membrane lie in connective tissue layer of the lamina propria. This layer has peripheral rim of arteries connected by radial anastomosis. with one
or two arteries that run down each side and around the tip of the malleus handle The arteries involved are 1) Deep auricular branch of maxillary artery on lateral surface 2) Medial surface
(a) Posteriorly by stylomastoid branch of the posterior auricular artery.
(b) Anteriorly by Tympanic branch of maxillary artery.
(c) Twigs from middle meningeal artery.
Nerve supply
Lateral surface: a) Anterior half by auriculotemporal branch of the fifth cranial nerve. B)Posterior half by auricular branch of vagus nerve. C) Medial surface is supplied by tympanic branch of the glossopharyngeal nerve.
Venous drainage
Veins drain into external jugular vein, transverse sinus, dural sinus and venous plexus around the eustachian tube.
DEVELOPMENT OF THE MIDDLE EAR
The middle ear cavity is endodermal. It originates at about four weeks from the first pharyngeal pouch, which grows laterally, and expands rapidly to pre-form two fundamental structures: the distal part forms the tubotympanic recess,which will become the primitive tympanic cavity; and the proximal part constricts to form the fibrocartilaginous Eustachian tube. The primitive tympanic cavity gradually expands like a growing bud to include the ossicles and their associated muscles and blood vessels. The accompanying dissolution of the mesenchyme facilitates this progression. Starting in the inferior half of the future tympanic cavity, this extension is hindered, higher up by a projection of the otic capsule: the superior periotic process, which will constitute the superior wall of the tympanic cavity, and lower down by the bony wall of the floor of the tympanic cavity, originating from separate bone or from a lamellar projection of the petrous pyramid. Possible progression occurs in the sagittal plane leading to, late in fetal life only,epitympanic recesses,antrum, and mastoid gas cells. Expansion of the tympanic cavity is virtually complete by about thirty-three weeks. The epitympanum follows approximately four weeks later.
Malleus and Incus
The anlage of the ossicles has been the subject of much discussion. The consensus now holds that the ossicles have multiple origins. It is believed that the manubrium of the malleus and the long process of the incus arive from the hyoid visceral bar, while the head of the malleus and body of the incus differentiate from the mandibular visceral bar.
The anterior process of the malleus, however, emerges from intramembranous ossification distinct from the visceral bars. In this context, it is useful to draw a distinction between the mandibular and hyoid visceral bars as opposed to Meckel’s and Reichert’s cartilages . Lying within the branchial arches is a condensation of mesenchymal tissue. With maturation it differentiates into cartilage and eventually becomes bone in some, although not all, regions. Visceral bar is the term used to describe the entire masses of condensed mesenchymal tissue, whereas the terms Meckel’s and Reichert’s cartilages refer only to the cartilage formed from the ventromedial portions of these mandibular and hyoid visceral bars, respectively. At approximately four weeks of gestation , areas of condensation of the mesenchyme appear at the dorsolateral ends of the mandibular and hyoid bars. An interbranchial bridge is formed which connects the upper end of the mandibular visceral bar to the central region of the hyoid visceral bar; it is this bridge that gives rise to the blastemae of the malleus and incus.
Stapes:
The stapes, like the malleus and incus, has a dual origin first described by Gradenigo in 1887. Stapes development involves a complex morphogenesis starting as a blastema at 41weeks. A stapedial “ring,” which arises from mesenchyme of the hyoid visceral bar, gives rise to the capitulum, crura, and tympanic (lateral) surface of the footplate. The lamina stapedialis,which gives rise to the annular ligament and the labyrinthine (medial) surface of the footplate, develops from the otic capsule and retains some of its cartilaginous structure throughout life. A blastemal mass is all that is recognizable of the future stapes at the fourth week of gestation. This blastema is composed of the condensed mesenchymal cells of the dorsolateral end of the hyoid visceral bar, adjacent to the facial nerve, and the nascent stapedial artery. By the sixteenth week, the ossicles attain adult size, and ossification begins at discrete ossification centers.
SURGICAL ANATOMY OF MIDDLE EAR CLEFT
The middle ear cleft consists of the Eustachian tube, tympanic cavity and the mastoid air cell system. The tympanic cavity is a biconcave, air-filled space within the temporal bone. It is bounded by the osseous labyrinth medially and the tympanic membrane laterally. It contains the ossicles and their tendons that attach them to the middle ear muscles. The tympanic segment of the facial nerve, run along its medial walls to pass through the cavity.28
THE TYMPANIC CAVITY
The tympanic cavity is shaped like a biconcave disc which measures around 15 mm in the antero-posterior and vertical dimensions. In its transverse dimension, it expands superiorly to 6mm and inferiorly to 4mm from a constriction of 2mm. It is divided into three compartments: the epitympanum, the mesotympanum and the hypotympanum. The epitympanum or the attic is the portion of middle ear space above the level of malleolar folds. It contains the head of the malleus and the body/short process of the incus and communicates with the mastoid antrum via a narrow passage, the aditus and antrum. The mesotympanum lies immediately deep to the pars tensa of tympanic membrane and contains the manubrium of the malleus, long and lenticular process of the incus, the stapes, tensor tympani, stapedial tendon and the chorda tympani nerve. The hypotympanum is the portion of the middle ear below the floor of the external auditory canal (EAC) and contains the opening of the Eustachian tube anteriorly.29 The tympanic cavity can be linked to a six walled chamber consisting of the roof, floor, medial and lateral walls, anterior and posterior walls.
THE SUPERIOR WALL (TEGMEN TYMPANI) OR ROOF
The tegmen tympani is the bony roof of the tympanic cavity, and separates it from the dura of the middle cranial fossa. It is formed in part by the petrous and part by the squamous bone; and the petrosquamous suture line, unossified in the young, does not close until adult life. Veins from the tympanic cavity running to the superior petrosal sinus pass through this Suture line.
THE FLOOR OF THE TYMPANIC CAVITY
It separates the hypotympanum from the dome of jugular bulb. The tympanic branch of the glossopharyngeal nerve from its origin at the skull base enters the middle ear through a small opening at the junction of the floor and the medial wall of tympanic cavity.
LATERAL WALL OF TYMPANIC CAVITY
The lateral wall of the tympanic cavity is formed by the bony outer attic wall (scutum) superiorly, the tympanic membrane centrally and the bony lateral wall of hypotympanum
inferiorly.
There are three holes in the bone of the medial surface of the lateral wall of the tympanic
cavity. The petrotympanic fissure is a slit (2 mm long), which opens anteriorly just above the tympanic membrane. It receives the anterior malleolar ligament and transmits the anterior tympanic branch of the maxillary artery to the middle ear cavity. The chorda tympani, which carries taste sensation from the anterior two-thirds of the tongue of the same side and secretomotor fibres to the submandibular gland, enters the medial surface of the fissure through a separate anterior canaliculus (canal of Huguier) which is sometimes merged with the fissure. Then it turns posteriorly and runs between the fibrous and the mucosal layer of the tympanic membrane. It crosses the handle of malleus and reaches the posterior bony canal wall just medial to the tympanic sulcus, enters the posterior canaliculus and then runs obliquely downwards and medially through the posterior wall of the middle ear cavity until it reaches the facial nerve.
THE MEDIAL WALL OF THE TYMPANIC CAVITY
The medial wall separates the tympanic cavity from the inner ear. Most of the central part of
the medial wall is occupied by a rounded elevation called promontory. It covers the basal coil of the cochlea and has grooves on its surface for nerves which form the tympanic plexus.
Behind and above the promontory is the fenestra vestibuli (oval window),a nearly kidney-shaped opening that connects the tympanic cavity with the vestibule, but which in life is closed by the base of the stapes and its surrounding annular ligament.
The size of the fenestra vestibuli naturally varies with the size of the base of the stapes, but on average it is 3.25 mm long and 1.75 mm wide. Above the fenestra vestibuli is the facial nerve. Anterior to the oval window is a curved hook like projection called processus cochleariformis, which houses the tendon of tensor tympani.
The fenestra cochleae (round window), is closed by the secondary tympanic membrane (round window membrane), lies below and a little behind the fenestra vestibuli from which it is separated by a posterior extension of the promontory, called the subiculum. A spicule of bone extending from the promontory above the subiculum upto the pyramid on the posterior wall of the cavity is called the ponticulus. The round window lies completely under cover of the overhanging edge of the promontory in a deep niche which is most commonly triangular in shape. The shape of the secondary tympanic membrane varies from round to oval and kidney shaped, with average longest diameter of 2.30 mm and shortest diameter of 1.87mm.15
The ampulla of the posterior semicircular canal is the closest vestibular structure to the membrane and its nerve (the singular nerve) runs almost parallel to and 1 mm away from the medial attachment of the deep portion of the posterior part of the membrane. The membrane is therefore a surgical landmark for the singular nerve. The facial nerve canal runs above the promontory and fenestra vestibuli in an antero-posterior direction. It has a smooth rounded lateral surface that is occasionally deficient, and is marked anteriorly by the processus cochleariformis.29 The dome of the lateral semicircular canal extends a little lateral to the facial canal and is a major feature of the posterior portion of epitympanum.28
THE ANTERIOR WALL OF THE TYMPANIC CAVITY:
The lower portion of the anterior wall is larger than the upper and consists of a thin plate of bone covering the carotid artery as it enters the skull and before it turns anteriorly. This plate is perforated by the superior and inferior caroticotympanic nerves carrying sympathetic fibers to the tympanic plexus, and by one or more tympanic branches of the internal carotid artery. The upper part of the anterior wall has two parallel tunnels placed one above the other. The lower opening is flared and leads into the bony portion of the Eustachian tube. Just above this is the canal for the tensor tympani muscle.28,29
THE POSTERIOR WALL OF TYMPANIC CAVITY:
The posterior wall is wider above than below and has in its upper part the opening [aditus] into the mastoid antrum. This is a large irregular hole that leads back from the posterior epitympanum. Below the aditus, the pyramid is present that contains the stapedius muscle and the tendon which runs forward to insert into the neck of stapes. Below the aditus is a small depression, the fossa incudis, which houses the short process of the incus and the suspensary ligament connecting the two. The facial recess is bounded laterally by the tympanic annulus and medially by the facial nerve, chorda tympani nerve runs through the wall between the two, with a varying degree of obliquity.28
Deep to both the pyramid and the facial nerve is a posterior extension of the mesotympanum — the sinus tympani which is bounded by the ponticulus above and subiculum below .28 This extension of air cells into the posteriorwall can be extensive and Anson and Donaldson (1981) reported that when measured from the tip of the pyramid, the sinus can extend as far as 9 mm into the mastoid bone.28
THE CONTENTS OF TYMPANIC CAVITY:
The tympanic cavity contains a chain of three small movable bones – the malleus, incus and stapes – two muscles, the chorda tympani nerve and the tympanic plexus of nerves. The ossicles are the malleus, incus and stapes; the malleus is the most lateral and is attached to
the tympanic membrane, whereas the stapes is attached to the oval window.
THE OSSICLESste your essay in here…