Auditory Ossicles

The development of the middle ear:

• The ossicles are full-sized cartilage models by 15 weeks of gestation.
•  Endochondral ossification is complete by 25 weeks.
• The middle ear is adult-sized at birth.

Ear Ossicles 

“The tympanic cavity and mastoid antrum , auditory ossicles and structures of the internal ear are all almost fully developed at birth and subsequently alter little.” 

The tympanic cavity and mastoid antrum , auditory ossicles and structures of the internal ear are all almost fully developed at birth and subsequently alter little.

The most common site of dbease is promontory in the region of the anterior margin of otal window, and in advanced cases the stapes become ankylosed in position by a mass of new sponglt bone.

Maximum hearing loss occurs when there is an ossicular interruption with intact tympanic membrane (54dB).

Ossicular interruption with perforation results in 38dB hearing loss, hearing loss in otitis media with effusion averages about 26-30 dB.

Partial or complete fixation of the stapes results in conductive hearing loss that ranges from 5dB to 60dB.

By 15 weeks, the ossicles have attained adult size, and ossification begins, first in the incus and last in the stapes.

At the same time, the tensor tympani and stapedius muscles develop from the mesenchyme of the first and second branchial arches, respectively.

The adult configuration of the malleus and incus is achieved by 20 weeks, whereas the stapes continues to evolve into its adult dimensions into the 32nd week. 

The facial nerve passes in a canal situated in the posterior and medial walls of the middle ear. It is not associated with the anterior wall. 

• Otosclerosis is suggested by a positive family history, 
• a tendency toward onset at an earlier age, 
• the presence of conductive hearing loss, or
•  bilateral symmetric auditory impairment
• Treatment with a combination of sodium fluoride, calcium gluconate, and vitamin D may be effective. If not, surgical stapedectomy should be considered.

By 12 to 15 weeks, the ossicles are more clearly differentiated and approximate adult size. By the sixteenth week of gestation, the ossicles reach adult size. Shortly following this stage, ossification begins, first in the incus and last in the stapes.

The external auditory meatus and tympanic membrane also reach adult size before term.

Maxillary antrum reaches adult size by 15 yrs of age. 

A i.e. First arch

D i.e. Stapes

D i.e. Petrous temporal

D i.e. Maxillary antrum & orbit 

• The tympanic cavity, Tympanic (or mastoid) antrum, auditory (ear) osicles, and internal ear structures are all almost fully developed (i.e. of adult size) at birth and subsequently alter little. 
• Mastoid antrum is an air sinus in the petrous part of temporal bone. Its adult capacity is variable, but on average is 1 mL, with a general diameter of 10 nun. Unlike other air sinuses in the skull (e.g. maxillary antrum), it is present at birth, and indeed of almost adult sizeQ, although it is at a higher level relative to the external acoustic meatus than it is in adults.

• The lateral wall of mastoid antrum, which offers the usual surgical approach, is only 2 mm thick at birth but increases at an average of 1 mm a year, attaining final thickness of 12-15 mm.
• Though the mastoid process antrum is well developed at birth, the mastoid process is absent at birth and develop in 2,0 year. The mastoid air cells are merely minute antral diverticula it birth. As the mastoid develops in 2^nd year, the air cells gradually extend into it and by the 4^th year they are well formed, although their greatest growth occurs at puberty (Gray’s-626). But BDC – 30 & Gray’s -417 say mastoid processes do not develop until the 2^nd year & they are invaded by air cells (pneumatized i.e. mastoid air cells appear) during 6^th year.
• The thinness of lateral antral wall & absence or under development of mastoid process means that the stylomastoid foramen & emerging facial nerve are very superficially situated. In adults lateral wall of mastoid antrum corresponds to macewen’s suprameatal triangle.
• The paranasal sinuses are rudimentary or absent and only the maxillary sinuses are identifiable at birth
• Ethmoid, orbital and upper nasal cavities have almost completed growth by the 7^th year, (Grays- 418). Orbit reaches its full adult size by 15-16 years of age (David Taylor – 205)
• At birth bones of cranial vault are unilaminar & lack diploe but the tabular structure with intervening diploe is generally apparent by about 4^th year. Differentiation reach maximum by about 35 years.

A i.e. Ear ossicles

Auditory (ear) ossicles attain full adult size at (or before) birth. Although mastoid antrum (air sinus) is well developed (& of adult size) at birth, but mastoid process & air cells develop later.

D i.e. Synovial joint

The ossicles are situated in the middle ear and suspended by ligaments. They articulate with each other through synovial joints to form a chain across the length of the middle ear from the tympanic membrane (laterally) to the oval window (medially)

• Superior Malleolar fold, like the superior malleolar ligament, extends between the superior surface of malleus head and superior attic wall (epitympanum)
• Superior incudal fold, like superior incudal ligament, extends between the superior aspect of incus body & superior attic wall.
• The medial incudal fold is between the long process of incus & tendon of stapedius muscle (as far as pyramidal eminence). The lateral malleolar fold is between the neck of malleolus & scutum forming the superior border of Prussak’s space.
• Prussak’s space lies between Shrapnell’s membrane & lateral malleolar ligament.
• Anterior & posterior malleolar ligaments arise from the neck of malleolus from anterior & posterior aspects respectively. The anterior malleolar ligament extends from the long process of malleolus towards the anterior attic wall
• Discomalleolar ligament & anterior malleolar ligament damage during temporomandibular joint surgery may cause damage to the middle ear. This may be a risk factor in the dissemination of infection from one area to the other. Interossicular fold lies b/w malleolus handle and the long process of the incus.

Cone of Light

• Seen in anteroinferior quadrant of the tympanic membrane is actually the reflection of the light projected into the ear canal to examine it.
• This part reflects it because it is the only part of tympanic membrane that is approximately at right angles to the meatus.
• This difference in different parts of the tympanic membrane is due to the handle of malleus which pulls the tympanic membrane and causes it to tent inside.
• Thus, the handle of malleus causes tenting and because of tenting the anteroinferior quardrant is at right angles to the meatus and thus reflects the light (leading to cone light).

The ossicles begins to form during 4th week of gestation from the mesenchymal tissue.

They originate as cartilaginous models that reach adult size by the 18th week of gestation.

Ossification of malleus begins at 15th week gestation, while stapes begins to ossify at 18th week of gestation. At birth. the ossicles are of nearly adult size.

i. Conduction of sound:

• It is done mainly by middle ear. Middle ear not just simply conducts the sound but converts sound of great amplitude and less force to that of less amplitude and greater force.
• This function of the middle ear is called as impedance matching mechanism or the transformer action.

ii. Transduction of mechanical energy to electrical impulse:

• Movements of the stapes footplate causes vibrations in scala vestibuli followed by scala tympani and is transmitted to the cochlear fluids which brings about movement of the basilar membrane.
• This sets up shearing force between the tectorial membrane and the hair cells.
• The distortion of hair cells gives rise to electrical nerve impulse.

A sound wave, depending on its frequency, reaches maximum amplitude on a particular place on the basilar membrane, and stimulates that segment (traveling wave theory of von Bekesy).

Higher frequencies are represented in the basal turn of cochlea and the progressively lower one toward the apex.

Ans. D None Ref. Dhingra sth/ed pg 61-62, 6th/eq P?5

Retracted Tympanic Bembrane

It is the result of-negative intratympanic pressure when Eustachian tube is blocked

Characteristics

It appears dull and lusterless 

Cone of light is absent or interrupted 

Handel of malleus appears foreshortened 

Lateral process of malleus becomes more prominent 

Anterior and posterior malleal folds become sickle shaped 

It is immobile or has limited mobility when tested with pneumatic otoscope or siegle’s speculum.

Features of Normal Tympanic Membrane

It is shiny and pearly grey in colour

Has concavity on its lateral surface

Cone of light seen inantero – inferior quadrant

It’s transparency varies

It is mobile when tested with pneumatic otoscope or siegle’s speculum.

Ans. is. B. Long process of incus

Ans. C Cochlea removed

Explanation

1. Radical mastoidectomy is a procedure to eradicate disease from middle ear and mastoid without any attempt to reconstruct hearing.
2. It is rarely done these days – Its only indications are:

– Malignancy of middle ear

– When cholesteatoma cannot be removed safely eg if it invades eustachian tube, round window or perilabrynthine cells

If previous attempts to eradicate cholesteatoma have failed

Following steps are done in radical mastoidectomy:

1. Posterior meatal wall is removed and the entire area of middle ear, attic, antrum and mastoid is converted into a single cavity, by removing the bridege and lowering the facial ridge.
2. All remnants of tympanic membrane, ossicles (except the stapes foot plate) and mucoperiosteal lining are removed (Not cohlea)
3. Eustachian tube is obliterated by a piece of muscle or cartilage
4. The diseased middle ear and mastoid are permanently exteriorised for inspection and cleaning.

Remember: Bridge is the most posterosuperior part of bony meatal wall lateral to aditus and anturm, which overlies the Notch of Rivinus while facial ridge lies lateral to fallopian canal. Bridge is removed and ridge is lowered in radical or modified radical operation.

Ans. C i.e. Marfan’s syndrome

Marfan syndrome is a disorder of connective tissue, manifested principally by changes in the skeleton, eyes, and cardiovascular system. It is transmitted by autosomal dominant inheritance.

The ossicles (auditory ossicles) are the three smallest bones in the body, the malleus, the incus, and the stapes.

They are contained within the middle ear space and serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea).

Ans. is ‘a’ i.e., 1^st Arch

lst (mandibular arch):- 

Muscular Contribution:-Muscles of mastication, Anterior belly of the digastric, Mylohyoid, Tensor tympani, Tensor veli palatini.

Skeletal  Contributions:-  Maxilla, mandible (only as a model for mandible ), Incus and malleus, Meckel’s cartilage, Ant. ligament of malleus, Sphenomandibular ligament.

Nerve:-  Trigeminal nerve (V2 and V3).

Artery:-  Maxillary artery, external carotid artery.

• Fossa incudis contains short process of Incus.
• Head of malleus is attached to epitympanum by ligament of head of malleus.
• Long process of incus is attached to head of stapes.
• Footplate of stapes lies over oval window.

The cartilage of second pharyngeal arch (hyoid arch) is known as Reicherts cartilage and contributes as :- (i) Stapes (except footplate), (ii) Styloid process, (iii) Stylohoid ligament, (iv) Lesser cornu and superior part of body of hyoid bone.

However, footplate of stapes develop from otic capsule.