Auditory Ossicles Osteology Notes

Auditory Ossicles Terminology

There are three ossicles malleus, incus and stapes.

These names are Latin in origin, the meanings of which are as follows:

  • Malleus: Hammer
  • Incus: An anvil
  • Stapes: A stirrup

Note: Remember that MIS is situated between the tympanic membrane and the oval window where M-Malleus, 1-Incus and S-Stapes.

Auditory Ossicles Features And Attachments 

Features And Attachments Of Malleus

It consists of a head, neck and handle.

  • Malleus Head
    1. It is the large upper end of the bone.
    2. It is located within the epitympanic recess.
    3. Its posterior surface articulates with the body of the incus.
  • Malleus Neck
    1. It is the constricted part below the head.
    2. Its medial surface is crossed by chorda tympani nerve.
  • Malleus Handle
    1. It is the lower elongated part of the malleus.
    2. It is embedded in the tympanic membrane and moves with it.
    3. Its upper end (root) shows the following features:
      • A slight projection on the medial aspect provides attachment to the tendon of the tensor tympani.
      • Anterior process projects forward. Anterior ligament of the malleus is attached to it. This ligament extends into the petrotympanic fissure.
      • The lateral process projects laterally from where extend anterior and posterior malleolar folds to the ends of the tympanic sulcus.

Auditory Ossicles Ossicles Of The Right Ear Lateral Aspect

Features And Attachments Of Incus

It has a large body and two processes (long and short).

  • Incus Body
    1. It is cubical in shape.
    2. Its anterior surface is concave and articulates with the head of the malleus.
  • Incus Processes
    1. Long Process
      • It projects downwards parallel to the handle of the malleus.
      • Its lower end (lenticular process) bears an articular surface on the medial aspect for articulation with the head of stapes.
    2. Short Process
      • It is directed backwards.
      • It is attached by a ligament to the fossa incudis just below the aditus.

Features And Attachments Of Stapes

It has a head, a neck, two limbs (anterior and posterior) and a foot plate (base).

  • Stapes Head
    1. It is rounded
    2. It articulates with the long process of incus.
  • Stapes Neck
    1. It is a constricted part adjacent to the head.
    2. The Tendon of the stapedius is attached to its posterior surface.
  • Stapes Limbs (Crura)
    1. Anterior and posterior limbs diverge from the neck.
    2. These two limbs are attached to the footplate.
  • Stapes Foot Plate (Base)
    1. It is oval in shape.
    2. It fits into the fenestra vestibuli.

Auditory Ossicles Ossification

  1. Malleus and incus develop from the dorsal end of Meckel’s cartilage.
  2. Stapes develop from the dorsal end of the hyoid arch cartilage.
  3. Malleus ossifies by two centres:
    • One endochondral centre near the neck.
    • One centre for the anterior process appears in dense connective tissue.
      1. Appearance
        • 4th month of intrauterine life.
      2. Fusion
        • 6th month of intrauterine life.
  4. Incus ossifies by the single endochondral centre in the upper part of the long process. This centre appears in the 4th month of intrauterine life.
  5. Stapes ossify by a single endochondral centre which appears in the base at 4th month of intrauterine life.
  6. At birth, the auditory ossicles are of almost adult size.

Auditory Ossicles Functions

The malleus functions as a lever as it is attached to the tympanic membrane.

  1. The base of stapes is considerably smaller than the tympanic membrane.
  2. Due to this fact, the vibratory force of the stapes is about 10 times that of the tympanic membrane.
  3. Thus the auditory ossicles increase the force but decrease the amplitude of vibrations transmitted from the tympanic membrane.

Auditory Ossicles Applied Anatomy

  1. Treacher-Collins syndrome is a condition in which there are abnormalities of ossicles and craniofacial skeleton.
  2. This may be one of the causes of congenital conductive deafness.
  3. Damage to ossicles in cases of head injury with fracture of temporal bone leads to very severe and permanent conductive deafness.
  4. Late conductive deafness due to aseptic necrosis of the long process of incus can occur some years after head injury.
  5. Ankylosis of stapes is a common occurrence in cases of otosclerosis.

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