Mastoids and Organs of Hearing Fall 2007

Because of the density of the and relative location of the temporal bones, the mastoids are hard to visualize see with diagnostic radiography. They are more commonly examined with CT and MRI. The organs of hearing and equilibrium are the main structures within the petrous ridges of the temporal bone.

There are 3 divisions of the ear: External begins with the auricle or pinna on each side of the head. The tragus is on the external portion of the ear as well and acts as a shield to the opening of the ear. The EAM is the opening or canal of the ear which is approximately 2.5 cm long. Half of it is bony and half is cartilage. The mastoid process and tip are posterior and inferior to the EAM. The styloid process is inferior and slightly anterior to the EAM. The EAM narrows as it gets closer to the tympanic membrane or eardrum. The eardrum is at an oblique angle and forms a depression at the medial end of the meatus. 2) Middle ear is irregularly shaped air containing cavity. It is made up of 3 parts: tympanic membrane, auditory ossicles, and the tympanic cavity. Tympanic cavity is divided into two parts: tympanic cavity proper and the attic (epitympanic recess). The tympanic cavity communicates anteriorly with the nasopharynx by way of the eustachian tube (or auditory tube). The auditory ossicles are made up of 3 small bones: malleus (hammer), incus (anvil), and stapes (stirrup). The ossicles are located in the attic and partly in the tympanic cavity proper and in charge of vibrations. 3) Internal ear contains the essential apparatus for both hearing and equilibrium. It is located in the densest part of the petrous portion and is divided into two parts: osseous (bony labyrinth) and membranous labyrinth. The bony labyrinth is divided into 3 parts: cochlea, vestibule, and semicircular canals. Cochlea relates to sense of hearing. Semicircular canals relate to equilibrium.

Consists of two parts : auricle and EAM. Auricle is the fibrocartilaginous, sound collecting organ on each side of the head. EAM is the sound conducting canal. It is about 2.5 cm long. EAM ends at the tympanic membrane of the middle ear.

Passageway between the middle ear and nasopharynx Passageway between the middle ear and nasopharynx. It is about 3 cm long and serves to equalize pressure. This is where you get the sensation of popping after diving, flying or high elevations. The pressure being adjusted protects the eardrum from damage. The problem with there being a passageway between nasopharynx and middle ear is that it is an easy way for organisms to infect. Ear infections often accompany sore throats, especially in children.

The middle ear has 3 divisions: tympanic membrane or eardrum, tympanic cavity and auditory ossicles. The middle ear communicates between the mastoid antrum and the auditory eustachian tube. Tympanic membrane is a disk with a concave surface and serves as a partition between the external and middle ear. The function is the transmission of sound vibrations. The tympanic cavity communicates with the nasopharynx through the eustachian (auditory tube). The mastoid antrum is the large air cavity situated in the temporal bone above the mastoid air cells and directly behind the posterior wall of the middle ear. The auditory ossicles are made up of the malleus (hammer), incus (anvil) and stapes (stirrup). They bridge the middle ear cavity for the transmission of sound vibrations from the tympanic membrane to the inner ear.

The malleus is connected to the tympanic membrane and it articulates with the incus. The incus connects to the stapes and its base is fitted into the oval window of the inner ear.

The inner ear contains the essentials for hearing and equilibrium The inner ear contains the essentials for hearing and equilibrium. It lies in the most dense portion of the petrous ridges. It consists of a bony labyrinth and membranous labyrinth. Bony labyrinth consists of three parts: 1) Cochlea- which communicates with the middle ear through the membranous covering of the round window. 2) Vestibule communicates with the middle ear by way of the oval window. 3) Semicircular canals- from its cranial orifice the internal acoustic meatus passes inferiorly and laterally for a distance of about ½ “. It is through this canal that cochlear and vestibular nerves pass from their fibers in the respective parts of the membranous labyrinth into the brain. The cochlea is for hearing and the semicircular canals are for equilibrium.

Round window is how the cochlea of the inner ear communicates with the middle ear.

A second direct communication into the middle ear occurs posteriorly to the mastoid air cells. The aditus is the opening between the epitympanic recess and the mastoid portion of the temporal bone. The aditus connects directly to a large chamber within the mastoid portion called the antrum. The antrum connects to various mastoid air cells. This passageway allows infection into the middle ear, which may have originated in the throat to enter the mastoid area. Once the infection is in the mastoid area it is very easy for the infection to spread to brain tissue. The tegmen tympani is the thin plate of bone forming the roof of the antrum, aditus and attic area of the tympanic cavity.

Ear

Temporal Bones Situated on each side of cranial base between greater wings of sphenoid and occipital bone Form large part of middle cranial fossa and a small part of posterior cranial fossa

Temporal Bones Consist of Squamous portion Tympanic portion Styloid process Zygomatic process Petromastoid portion which contain the organs for hearing and equilibrium

Temporal Bones Tympanic portion = located below squama and in front of petromastoid portion Forms anterior wall, inferior wall and part of posterior walls of the EAM Styloid process = slender, pointed bone projecting inferiorly, anteriorly, and slightly medially from inferior surface of tympanic portion

Temporal Bones Petromastoid portion combines petrous and mastoid portions Forms the inferior, posterior part of the temporal bone Articulates with parietal bone at its superior border and with occipital bone at its posterior border Usually contains air cells, which vary greatly in size, number, and pneumatization

Temporal Bones Mastoid process = conical process projecting from mastoid portion Petrous portion projects medially and anteriorly between greater wing of sphenoid and occipital bone Also called petrous pyramid Conical or pyramidal in shape Thickest and densest portion of cranium Contains the organs of hearing and balance

Temporal Bones Auditory ossicles = bones of middle ear Malleus Incus Stapes Temporal bone articulates with the parietal, occipital, sphenoid, zygoma, and mandible

Temporal Bones Lateral view Squamous portion Mastoid portion EAM Tympanic portion Styloid process Temporomandibular fossa Articular tubercle Zygomatic process Lateral view

Temporal Bones Anterior view Squamous portion Mastoid tip D.Masoid process E. Styloid process Petrous portion Anterior view

Essential Projections: Temporal Bone Axiolateral oblique (modified Law) Axiolateral oblique (Stenvers) Axiolateral oblique (Arcelin)

Axiolateral Oblique (Modified Law) Prone or seated upright Tape auricle forward Place head in true lateral position with affected side closer to IR IOML parallel with IR IPL perpendicular to IR MSP angled 15 degrees toward IR

Axiolateral Oblique (Modified Law) CR Angled 15 degrees caudad to midpoint of IR Exits mastoid tip closer to IR about 1 (2.5 cm) posterior to EAM Enters 2 (5 cm) posterior to and superior to upper EAM

Axiolateral Oblique (Stenvers) Prone or seated upright Rest head on forehead, nose, and cheek with side of interest on IR IOML parallel with transverse axis of IR MSP at 45-degree angle from IR

Axiolateral Oblique (Stenvers) CR Angled 12 degrees cephalad Enters about 3 to 4 (7.6 to 10 cm) posterior and ½ (1.3 cm) inferior to upside EAM Exits 1 (2.5 cm) anterior to downside EAM IR and CR centered

Axiolateral Oblique Temporal Bone (Arcelin) Supine MSP centered to midline Rotate face away from side of interest to place MSP at 45-degree angle with plane of IR IOML perpendicular to IR

Axiolateral Oblique Temporal Bone (Arcelin) CR Angled 10 degrees caudad Enters temporal area at 1 (2.5 cm) anterior to EAM and ¾ (1.9) cm above it Center IR and CR

Axiolateral Oblique Temporal Bone (Modified Law) Mastoid process closer to IR Air cells centered to IR Opposite mastoid process inferior and anterior mastoid of interest Auricle of ear not superimposing mastoid Superimposition of IAM and EAM TMJ visible anterior to mastoid process Close beam restriction

Axiolateral Oblique Temporal Bone (Stenvers) Petromastoid portion in profile Lateral border of skull to lateral border of orbit Petrous ridge extended to a point about two thirds up lateral border of orbit Mastoid process in profile below cranium Posterior margin of mandibular ramus superimposing lateral border of C-spine Mandibular condyle projecting over atlas near petrosa Close beam restriction

Axiolateral Oblique Temporal Bone (Arcelin) Petromastoid portion in profile Lateral border of skull to lateral border of orbit Petrous ridge lying horizontal about two thirds up lateral border of orbit Mastoid process in profile below cranium Posterior margin of mandibular ramus superimposing lateral border of C-spine Mandibular condyle projecting over atlas near petrosa Close beam restriction