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Stapes

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What are Stapes?

MVSAT 2024

The ossicular chain in our body is made up of three bones; malleus, incus stapes which are ingrained deep inside the temporal bone within the tympanic cavity. All the three are joined together by two synovial joints called the incudostapedial and incudomalleolar joints. Malleus, incus stapes together form the sound-conduction apparatus in our system which transmits vibrations.


For sound to be recognized by the brain, sound waves need to enter the auditory canal and then go through the tympanic membrane (or eardrum), and finally enter the middle ear compartment. Once the sound waves reach the middle ear compartment, they vibrate the ossicular chain i.e. the three bones malleus, incus, and stapes. Out of these three bones, the stapes bone is the tiniest one which is also the smallest bone in the body.


Here we will find out what exactly stapes bone is, what is the stapes location, stapes function, and stapes superstructure.


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Stapes Bone in Human Body

With a dimension of 3 mm X 2 mm, stapes (plural stapedes) is the smallest known bone in the human body. Since the shape of stapes bone is like a stirrup it is also referred to as stirrup ear bone.


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  • Stapes is located in the middle ear along with the other two oscillatory bones; malleus and incus. Stapes is the most medial of all the three bones and closest to the cochlea.


  • Sound pressure waves are conveyed from the eardrum (outer ear) to the inner ear by the ossicles. These waves then reach the brain as nerve impulses to be identified as sound.


  • The components of the stapes bone are:

    • Base or stape footplate - This connects to the oval window through the annular ligament. The base conducts vibrations to the cochlea. 

    • Anterior crus

    • Neck - This is attached to the head via anterior and posterior crura.

    • Head - The head is concave in shape and sits on top of the neck. It is also called the capitulum. The head forms a joint with the incus through the incudostapedial joint.

    • Stapes superstructure - The head, neck, and the crura together form the stapes superstructure which is also sometimes referred to as suprastructure.


  • The malleus can be compared to a hammer as it strikes the anvil-shaped incus. The vibrations created by this movement travel to the stapes bone.


  • Stapes are like tuning forks with their horseshoe shape having two branches; inferior and posterior that convey the sound waves to the stapes footplate or base.


  • The stapes has numerous attachments:

    • Synovial or Incudostapedial Joint - This is the connection between stapes and incus bones.

    • Stapediovestibular Joint - It is a fully functional joint between the oval window and stapes. The main role of this joint is to transmit sound vibrations from the middle ear to the fluid in the vestibule (and from there to the cochlea).

    • Stapedius - This is a small muscle in the middle ear that is attached to the posterior part of the stapes. When this muscle contracts, it dampens the sound vibrations which are passing to the cochlea through the oval window. The muscle is supplied by a small branch in the facial nerve. This joint prevents excessive movement of the stapes bone.


Stapes Function

If for some reason (like severe head trauma) our stapes get damaged, we can lose some or all of our hearing abilities. Such is the importance of stapes functions in the human hearing system.

  • Stapes is the last bone in the ossicular chain so when it hits the oval window, a wave is generated in our inner ear’s fluid.

  • This fluid wave initiates the process in the inner ear which is responsible for the conversion of sound waves to electrical signals.

  • Our brain then interprets these electrical signals.


What is Otosclerosis? 

Sometimes an abnormal growth of extra bone can occur around the stapes bone. Due to this growth, the stapes bone is frozen in its place and unable to vibrate, which is its usual function. This condition is known as Otosclerosis. This results in hearing loss. Otosclerosis is a slow growth and generally happens in both ears. It is mostly a genetic trait that runs in the family. This condition can be corrected through a surgery called stapedectomy.


Conclusion

Stape bone is a vital bone in the human hearing system. They are the smallest bone known in the human body and are responsible for the translation of movement from the outer eardrums to the fluid in the inner ear which transforms sound waves into nerve impulses. These nerve impulses get interpreted by the brain. 

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FAQs on Stapes

1. What is cochlea and its role?

Cochleas is a snail-like structure within the inner ear. It is full of fluid which contains the organ Corti. Corti is the organ for hearing and has tiny hair cells lining the cochlea. These tiny cells convert sound vibration into electrical impulses and the sensory nerves carry these impulses to the brain.

  • When the fluid moves, it sets 25,000 nerve endings into motion.

  • The nerve endings change the sound vibrations to electrical impulses.

  • The eighth cranial nerve or the auditory nerve carries these impulses to the brain to be interpreted.

2. How is stapedectomy performed?

The process of stapedectomy is performed as illustrated below:

  • The surgeon first removes parts of or the entire original stapes bone.

  • The stapes bone is then replaced by an artificial device.

  • This artificial device enables sound waves to be sent to the inner ear.

  • The surgeon uses an operating microscope to perform this procedure through the ear canal.

  • S/he lifts the eardrum to expose bones in the middle ear.

  • After removing the stapes bone and placing the artificial device, the surgeon would put back the eardrum in place with packing material that would hold the eardrum in place. This packing material is later absorbed by the body.

  • The whole procedure of stapedectomy takes around 90 minutes. Most patients are able to return home on the same day of the surgery.

  • Local anesthesia is used for relaxing the patient. Patients are not put to sleep completely. The surgeon also uses numbing medicine at the site of surgery.

3. What is Otitis Media?

Otitis media is an infection of the middle ear which results in fluid build-up in the middle ear. If the fluid and infection remain in the middle ear for a long time, it could damage the structure of the ear, which includes the ossicular chain. Such infections might result in temporary or permanent hearing loss. Otitis media is mainly associated with allergies or infections in the upper respiratory tracts and happens mostly in children (but can affect adults too). It causes congestion and hampers proper drainage of the ears. The infection could be treated by a minor surgery where the auditory tubes are opened up to allow ear drainage.


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