Auditory Ossicles

Auditory Ossicles


AUDITORY OSSICLES

  • The ossicles (auditory ossicles) are the three smallest bones in the body, the malleus, the incus, and the stapes in the middle ear.
  • Synovial type of joint exists between the ossicles of the ear.
  • Function of Ossicles is to conduct sound energy from the tympanic membrane to the oval window and then to the inner ear fluid and Reduction of impedance to sound transmission.

DEVELOPMENT OF OSSICLES

  • Malleus and incus are derived from 1st Arch.
  • Stapes Suprastructure(except Footplate) is derived from Cartilage of Second pharyngeal Arch(Reichert’s Cartilage).
  • Footplate of Stapes develops from Otic Capsule.
  • Ear Ossicles attains adult size before birth.
  • 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 tympanic cavity and mastoid antrum , auditory ossicles and structures of the internal ear are all almost fully developed at birth and subsequently alter little. 

 MALLEUS

  • This bone is shaped like a hammer hence the name.
  • This is the largest of the three ossicles of the middle ear cavity.
  • It has a head, neck and three processes arising from below the neck.
  • The overall length of the malleus ranges between 7.5 – 9 mm.).
  • Superior malleolar ligament connects Head of malleus to roof of epitympanum
  •  During surgical procedures for attic cholesteatoma clipping of this head will improve the exposure in the attic region.
  • The head of the malleus on its posteriomedial surface has an elongated saddle shaped cartilage covered facet for articulation with the incus.
  • Below the neck the bone broadens and gives rise to the following: 
  • the anterior process from which a slender anterior ligament arises to insert into the petrotympanic fissure; 
  • the lateral process which receives the anterior and posterior malleolar folds from the annulus tympanicum, and 
  • the handle which runs downwards, medially and slightly backwards between the mucous and fibrous layers of the centre of tympanic membrane.
  • On the deep medial surface of the handle there is a small projection into which the tendon of the tensor tympani muscle inserts.
  • Cone of light is due to Handle of malleus.
  • Additionally the malleus is supported by the superior ligament which runs from the head to the tegmen tympani.

INCUS

  • This bone is shaped like an anvil.
  • It articulates with the malleus and has a body and two processes.
  • The body lies in the epitympanum and has a cartilage covered articular facet corresponding to that of the malleus.
  • The short process projects backwards from the body to lie in the fossa incudis. It is infact attached to the fossa incudis by a short ligament.
  • The long process of the incus descends into the mesotympanum behind and medial to the handle of the malleus. 
  • At its tip there is a small medially directed lenticular process which articulates with the stapes.
  • The long process of the incus has precarious blood suppy. 
  • This portion of the incus is most commonly involved in CSOM.

STAPES

  •  The stapes consists of a head, neck, two crura and a base (footplate).
  • The head of the stapes points laterally and has a small cartilage covered depression for articulation with the lenticular process of the incus.
  • The tendon of the stapedius muscle attaches to the posterior part of the neck and the upper part of the posterior crura.
  • The neck of the stapes gives rise to two crura which join the foot plate closing the oval window
  • The average dimensions of the foot plate is 3mm x 1.4mm. 

CLINICAL SIGNIFICANCE OF AUDITORY OSSICLES

  • Austin’s classification for ossicular chain defects depends on Malleus handle and stapes suprastructure.
  • Features of ossicles in moderately retracted tympanic membrane are Handle of malleus appearance foreshortened and Lateral process of malleus becomes more prominent.
  • Otosclerosis mostly affects Stapes.
  • Incus and head of malleus removed in modified radical mastoidectomy .
  • Ossicular disruption with intact tympanic membrane causes maximum hearing loss.
Exam Question
 
  • Ear Ossicles attains adult size before birth.
  • Synovial type of joint exists between the ossicles of the ear.
  • Malleus and incus are derived from 1st Arch.
  • Fossa incudis contains short process of Incus.
  • Austin’s classification for ossicular chain defects depends on Malleus handle and stapes suprastructure.
  • Function of Ossicles is to conduct sound energy from the tympanic membrane to the oval window and then to the inner ear fluid and Reduction of impedance to sound transmission.
  • Ossicle M/C involved in CSOM is Long process of incus.
  • Features of  ossicles in moderately retracted tympanic membrane are Handle of malleus appearance foreshortened and Lateral process of malleus becomes more prominent.
  • Ossicular disruption with intact tympanic membrane causes maximum hearing loss.
  • Body of incus lies in Epitympanum.
  • Cone of light is due to Handle of malleus.
  • Superior malleolar ligament connects Head of malleus to roof of epitympanum.
  • Otosclerosis mostly affects Stapes.
  • Incus and head of malleus removed in modified radical mastoidectomy .
  • Handle of Malleus is attached to the center of the tympanic membrane.
Don’t Forget to Solve all the previous Year Question asked on Auditory Ossicles
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