The Anatomy of the Stapes Bone

Smallest bone in the body plays role in hearing

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The stapes bone is one of three bones located the middle ear which together make up a sort of chain called the ossicles. The stapes is shaped like a stirrup and along with the other two ossicles, the malleus and incus is essential to the function of hearing.

Auditory ossicles maleus, incus, and stapes

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Anatomy

The stapes is the smallest bone in the human body. It is the innermost (most medial and nearest to the cochlea) of the three bones inside the middle ear which are called this ossicles. It is very much shaped like a stirrup. In fact, the name stapes actually means stirrup in Latin.

If you were to picture the stapes as a stirrup hanging from a saddle it is actually turned on its side, horizontally, with the top portion connecting with the incus and the part where your foot would rest, called the footplate, butting up against the oval window of the tympanic cavity.

The head of the stapes connects with the incus via the incudostapedial joint. On the head of the stapes are two limbs called the anterior and posterior limbs which attach to the oval shaped base of the incus. A ligament of sorts called the stapedial membrane aids this connection by spanning the width of the anterior and posterior limbs and base.

On the medial side of the stapes the stapedial annular ligament connects the base of the stapes to the oval window. The oval window is a thin membrane that covers the entrance to the cochlea.

The stapedius muscle is a muscle that runs from the pyramidal eminence of the mastoid to the neck of the stapes. When this muscle contracts, in response to loud noise, it pulls the annular ligament tight and prevents excessive movement of the stapes bone.

Function

The stapes is essential to our ability to hear. Sounds vibrate the tympanic membrane (the ear drum) and travel through all three bones of the middle ear, the malleus, incus and stapes. As the sound waves travel through the middle ear they are amplified.

The stapes, the last bone in the ossicular chain, hits the oval window which generates a wave in the fluid of the inner ear. This wave of fluid, set into motion by the stapes, is what allows the initiation of the process in the inner ear that converts the sound waves into electrical signals which can be interpreted by the brain.

Associated Conditions

Otitis Media with Effusion

Otitis media is another name for an ear infection. When there is also fluid in the middle ear it is called otitis media with effusion.

The presence of infection and fluid in the middle ear, especially over a prolonged period, can damage the delicate structures in the middle ear including the ossicular chain. This can lead to temporary or permanent hearing loss.

This occurs more often in children than adults but can happen to anyone. It is commonly associated with upper respiratory infections or allergies that can cause congestion and prevent the ears from draining properly.

Sometimes this condition requires surgery either to open up the auditory tube and allow the ear to drain. If damage to the ossicles does occur it can sometimes be surgically repaired.

Otosclerosis

Otosclerosis is a condition in which abnormal bone remodeling of the ossicles causes the stapes to become fixed in place. The stapes is then unable to strike the oval window and produce the fluid wave in the cochlea that results in sound.

The cause of otosclerosis is not well understood but it can be hereditary and most often affects middle-aged women. Symptoms of otosclerosis include: ringing in the ears (tinnitus), dizziness or balance issues, and most importantly hearing loss.

The hearing loss that occurs in this condition often begins in one ear and then moves to involve the other ear as well. Otosclerosis can be treated with hearing aids or a surgery called a stapedectomy.

Cholesteatoma

Cholesteatoma is a condition in which the skin of the eardrum grows abnormally. This can be congenital or caused by repeated ear infections usually with the presence of fluid in the ear and negative pressure inside the middle ear. Cholesteatoma can cause the ossicles including the stapes to deteriorate.

Symptoms of cholesteatoma may include ear drainage, pressure in the ears, hearing loss, and tinnitus. Without treatment cholesteatoma can lead to serious complications including deafness, meningitis, and brain abscess. The treatment depends on the extent of the growth and how much, if any, damage to the ear has been sustained.

Treatment

Treating chronic otitis media and otitis media with effusion in a timely and effective manner is imperative to preventing the deterioration and damage to the ossicular chain that can result in hearing loss. Several treatments may be used starting with antibiotics if infection is present to treating underlying allergic conditions that may be contributing and progressing to surgical intervention when necessary.

The surgical placement of synthetic ventilation tubes can open up the auditory tube allowing the ear to drain properly and restoring normal pressure in the middle ear. This is a relatively simple procedure that is frequently done in the United States with little risk of complications and a quick recovery time.

When damage does occur to the stapes bone as a result of cholesteatoma or other conditions a surgical procedure called a stapedectomy can sometimes restore hearing. This surgery involves removing some portions or all of the stapes bone and replacing it with a prosthetic.

While the success rate of this procedure is relatively high rare complications are possible and may lead to increased hearing problems. Your doctor should discuss these risks with you in detail before you make a decision to undergo a stapedectomy.

The best surgical outcomes occur when you select a surgeon who is experienced in stapedectomy procedures. Surgery is usually done as a same-day procedure. During the recovery time, you may experience some pain as well as temporary vertigo. Hearing may improve over time.

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