Anatomy of the Incus

The central of three small bones in the middle ear

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The incus, also known as the “anvil,” is the middle of three small bones in the middle ear. The incus transmits vibrations from the malleus to the stapes. The vibrations then move to the inner ear. Conditions that affect the incus often affect the other ossicle bones.

Anatomy


The incus sits between the other two bones, known as ossicles, of the middle ear. The malleus (“hammer”) is the outermost bone and the stapes (“stirrup”) is the innermost. The ossicles are part of the auditory system, and together, they comprise an area no larger than an orange seed.

Incus is Latin for “anvil,” which is why it is sometimes referred to as such. It gets its name from the shape of the bone.

The ossicles are held in place in the middle ear by ligaments. The incus consists of four parts: the body, short limb, long limb, and lenticular process. Joints connect the ossicular bones. The body of the incus is connected to the malleus and the lenticular process connects to the stapes.

Anatomic Variations

Defects of the ossicles can include hypoplasia (under-development) and displacement. A birth defect called congenital aural atresia happens when the external auditory canal fails to fully develop. This condition may be associated with other congenital anomalies and is challenging to correct.

Function


Hearing is the primary role of the ossicle bones. The ossicles transmit sound waves from the outer ear to the inner ear by taking vibrations from the eardrum through the ossicles to the cochlea.

The ossicles also work to protect the ear from loud sounds. When the muscles of the middle ear contract in response to loud noise, the eardrum’s ability to vibrate is reduced. This reduces the movement of the incus and the other ossicles, limiting the damage that might otherwise occur from the impact of the noise.

Associated Conditions

Due to its role in transmitting sound, conditions of the incus often affect hearing. In fact, conditions of the incus are rarely isolated and most often involve the entire ossicle chain.

Otosclerosis

Otosclerosis is a condition that results in hearing loss from abnormal bone growth in one or more of the ossicles. The condition is marked by bones that become stuck together, which limits their movement and impairing hearing. Symptoms include hearing loss, tinnitus, and dizziness.

Diagnosis usually involves audiography and tympanometry, which help to determine hearing sensitivity. A computed tomography (CT scan) may also be done in order to view the ossicle bones and confirm the diagnosis.

Dislocation

Dislocation of any of the ossicle bones can occur as the result of trauma. Ossicular chain dislocation, as it’s known, can happen from a loud blast, a blow to the head, injury from air or water pressure, or injury from sticking something in the ear canal. Symptoms of dislocation might include hearing loss, tinnitus, vertigo, and facial paralysis.

A CT scan is usually used to diagnose dislocation of the ossicle bones. To determine the extent of hearing loss, tympanometry and audiography may also be used.

Cholesteatoma

When abnormal skin grows in the middle ear, it can damage the ossicles, especially if it gets very large. Cholesteatoma is noncancerous. Symptoms include hearing loss, ear pain and pressure, vertigo, drainage, and facial paralysis.

Cholesteatoma is diagnosed by using an otoscope to examine the ear. You may also have a CT scan to confirm the diagnosis.

Rehabilitation

Treatment for conditions of ossicles is provided by an otolaryngologist, a doctor that specializes in conditions of the ear, nose, and throat.

Surgery can be used to correct congenital aural atresia. It is one of the more challenging treatments for conditions affecting the ossicles. The surgery attempts to fully restore hearing and usually happens when a child is 6 or 7 years old. 

Non-surgical treatment for atresia includes bone conduction hearing aids, which transmit sound vibrations through bones in the head. These can be magnetic or surgically implanted.

Studies have shown these types of hearing aids to offer good hearing outcomes and recent advances have improved the technology. These devices should be placed as early as possible in order to be most effective.

Otosclerosis treatment options can be supportive (treating symptoms) or curative. Hearing aids and vitamin and mineral supplements are examples of supportive treatments. Curative treatments involve surgical restoration. In a stapedectomy, the damaged bone (usually the stapes) is removed and replaced with a synthetic implant. 

Ossicular chain dislocation most often requires a kind of surgery called ossiculoplasty. During the surgery, the ossicular chain is reconstructed in order to improve hearing. If the dislocation affects a person’s only hearing ear, surgery is usually contraindicated.

Surgical removal of abnormal skin growth is usually necessary for cholesteatoma. Antibiotics and ear drops are often prescribed prior to surgery in order to control infection and reduce swelling. 

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