The Anatomy of the Ossicles

The smallest bones in the body, the auditory ossicles, are three bones in each middle ear that work together to transmit soundwaves to the inner ear—thereby playing an essential role in hearing.

The malleus, incus, and stapes—commonly referred to by their shapes as the hammer, anvil, and stirrup, respectively—form an interconnected chain from the tympanic membrane (eardrum) to the oval window, which separates the middle ear from the inner ear.

When sound travels through the ear canal, the eardrum vibrates. The ossicles carry that vibration to the cochlea, a fluid-filled inner-ear structure that converts the message to electrical signals that are processed by the brain.

Problems affecting the ossicles can include congenital defects or ossicular chain discontinuity, in which the bones aren’t connected properly. These issues can lead to partial or total deafness, as well as other hearing problems.

Anatomy

The outermost ossicle is the malleus, followed by the incus, which is then connected to the stapes.

The three ossicles are connected to each other via joints, which allow the bones to move in response to the motion of the eardrum.

Structure and Location

Here’s a quick breakdown of each ossicle.

Malleus (Hammer)

The largest of the ossicles, the malleus has:

• A head
• A neck
• A handle (called the manubrium)
• Two other major extensions: the anterior and lateral processes

On the inner (anterior) side, the oval-shaped head connects to the incus via a small, saddle-shaped facet joint with a larger upper portion and a smaller lower portion.

The neck connects the head to the handle, which extends downward, curling forward slightly and narrowing as it joins the lower portion of the eardrum at the spatulate process of the malleus, which is a tiny projection.

The tensor timpani muscle is a small muscle that helps dampen sounds that are too loud. The tendon of the tensor timpani muscle inserts into a small protrusion. Just below the neck, the anterior process of the malleus is a tiny protrusion that terminates in a small fissure in the temporal bone (a part of the skull).

The lateral process is a larger, cone-shaped projection that extends upward from the root of the handle, attaching to the upper portion of the eardrum. It’s held in place by three ligaments: the anterior, superior, and lateral ligaments of the malleus.

Incus (Anvil)

The body of the incus is the larger, upper portion of this bone. It attaches to the malleus at the incudomalleolar joint.

The body of the incus, held in place by the superior ligament of the incus (running from the top of the ear canal to the bone), has two projections: the short and long limbs.

• The short limb is cone-shaped and moves towards the back of the body of the incus, providing stability. It’s attached to the rear wall of the eardrum via the posterior ligament.
• The long limb projects downward and then bends 90 degrees (towards the inside of the head) to form the lenticular process. This is connected to the stapes at the incudostapedial joint.

Stapes (Stirrup)

The stapes is the smallest ossicle bone, and the smallest bone in the body—measuring about 3.2 millimeters (mm) in height, and 2.8 mm in width.

It has four major features:

• The head (or capitulum)
• The base (footplate)
• Anterior and posterior limbs

The head of the stapes, connected to the incus at the incudostapedial joint, is round, and significantly smaller than the base. The anterior and posterior limbs curve away from the head symmetrically and attach to the oval-shaped base of the stapes, giving this bone the stirrup shape.

The tympanostapedial joint connects this bone to the oval window, which is the membrane that separates the middle ear from the cochlea. Notably, the stapes bone is also connected to the rear side of the malleus via the incudomalleolar joint.

Anatomical Variations

Approximately one in 10,000 people are born with malformations or variations in ossicle structure. There are many different types of abnormalities, and they can affect one side (unilaterally) or both (bilaterally).

The stapes bone has the most variations of the three, while the incus has the least. These structural variations can have very serious, debilitating effects on hearing function.

Here are some of the variants:

• Hypoplasia: Under-development of one or more ossicles is associated with two hereditary conditions—Branchio-oto-renal syndrome and Crouzon’s syndrome.
• Stapes ankylosis: Fixation of the stapes occurs when this bone is locked in place and unable to move.
• Absence of stapes: When the stapes is not present, hearing is profoundly impaired.
• Absence of the long limb: When this portion of the incus, which normally connects the body to the lenticular process, is not present, it can lead to significant hearing loss.
• Stapes aplasia: This is a rare condition in which the stapes doesn’t develop properly, and the base is not connected to the rest of the bone. This can cause progressive hearing loss.
• Stapes hyperplasia: This overgrowth of the stapes is a rare abnormality that can cause deafness.

Function

The specialized structure of the ossicles allows soundwaves to be converted to electrical signals for your brain to process. How does this work? Here’s a quick breakdown:

• Vibration of the eardrum: Soundwaves enter the external acoustic meatus (ear canal), and are channeled to the eardrum, causing it to vibrate. The ossicles pick up this motion.
• Transmission: The malleus is attached to the eardrum, and to the other ossicles in a chain-like sequence. When the malleus picks up the movement, it vibrates, leading to the vibration of the other ossicles too. This moves the signal to the inner ear.
• Amplification: The base of the stapes attached to the oval window is much smaller than the eardrum, and the force of the vibrations is increased. In this way, these bones act as a kind of amplifier of sound.
• Beyond the oval window: After the vibrations pass through the ossicles, they cause the oval window to move. Beyond it is the cochlea, a fluid-filled structure that’s shaped like a snail and has thousands of receptor cells. The motion of the stapes produces tiny waves, which are picked up by the receptor cells, essentially converting mechanical information into electrical signals. These are then transmitted to the brain through the vestibulocochlear nerve.

Associated Conditions

Health conditions that involve the ossicles can seriously affect hearing.

The most common problem affecting the ossicles is ossicular chain discontinuity, in which the bones are either fused together with limited mobility or set too far apart. This inhibits vibrations of the eardrum from being translated to the oval window. This can lead to impaired hearing, tinnitus (a persistent ringing in the ears), or total deafness.

A number of conditions can lead to ossicular chain discontinuity or other issues of the ossicles, including:

• Chronic otitis media (COM): Middle ear infections can frequently recur or might not resolve. This leads to damage of the ossicles over the long-term. COM can cause erosion of the malleus, which can spread an infection to the meninges, the protective covering surrounding the brain.
• Otosclerosis: This abnormal growth of tissue in the middle ear can severely impact the mobility of the ossicles, leading to hearing loss. Most often, this causes the base of the stapes to become affixed to the oval window, preventing the transmission of sound vibrations.
• Cholesteatoma: Cholesteatomas are noncancerous growths in the middle ear. If untreated—and depending on their location—they may enlarge and impact the ossicles, affecting their function and leading to hearing loss.
• Congenital malformation: Birth defects can include abnormalities such as malformation or absence of one or more of the ossicles, potentially with ossicular chain discontinuity.
• Trauma: Head trauma can lead to an injury, such as a skull fracture. Injuries may cause ossicular chain discontinuity, either directly or as a result of subsequent inflammation in the middle ear.
• Hyperacusis: Resulting from infection or trauma, hypersensitivity to noises can occur when certain muscles in the inner ear become damaged. The tensor timpani muscle normally dampens loud sounds, and its function can be impaired when it’s damaged. And the stapedius muscle, which normally stabilizes the stapes, can become weakened due to damage as well.

Rehabilitation

Hearing loss, deafness, or other auditory problems can be debilitating. A number of treatments can correct or help manage these issues, including:

• Hearing aids: The use of hearing aids is the most common intervention for hearing loss. While these won’t resolve the underlying condition affecting the ossicles, they can help make up for their limited function.
• Ossicular chain reconstruction surgery: During this surgery, the ossicles are repaired and reattached to resolve ossicular chain discontinuity. Existing bone structures may be fixed or reinforced, or a prosthetic of the affected ossicle can be implanted.
• Stapedectomy: Otosclerosis is treated with a special type of ossicular chain reconstruction in which the head and the limbs of the stapes are removed and replaced. Its base is then outfitted with a prosthetic that’s connected to the incus.
• Cholesteatoma surgery: If the presence of cholesteatoma in the middle ear is impacting hearing function, it can be surgically removed.