The Anatomy of the Ossicles

Three Tiny Bones in the Middle Ear Essential for Hearing

The smallest 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.

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

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

Problems in the ossicles—such as congenital defects or ossicular chain discontinuity, in which these bones aren’t connected properly—can lead to partial or total deafness as well as other hearing problems. Given the crucial role these tiny, intricate bones play in hearing, it’s important to have a sense of their anatomy and function.

The middle ear - stock illustration

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Anatomy

Named for their respective shapes, the three ossicles are connected to each other via complementary joints, which allow these to vibrate and translate motion of the eardrum to the inner ear. The outermost of these is the malleus, followed by the incus, which is then connected to the stapes.

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, and a handle (called the manubrium), as well as 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 in turn extends downward, curling forward slightly and narrowing as it joins the lower portion of the eardrum at a tiny projection called the spatulate process of the malleus.

The tendon of the tensor timpani muscle (a small muscle that helps dampen sounds that are too loud) inserts into a small protrusion on the upper end of this feature. Just below the neck, the anterior process 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 and 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 middle of the ossicles, the larger, upper portion or body of the incus attaches to the malleus at the incudomalleolar joint. The body, 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 former of these is cone-shaped and moves towards the back of the body; 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 smallest of the ossicles—At about 3.2 millimeters (mm) in height, and 2.8 mm in width, the stirrup is the smallest of the ossicles, and the smallest bone in the body. It has four major features: the head (or capitulum), the base (footplate), as well as anterior and posterior limbs.

The head, 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.

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

Anatomical Variations

As can happen with other parts of the body, a small but not insignificant number of people—about one in 10,000—are born with malformations or variations in ossicle anatomy. There are many different types of abnormalities, and they can happen on one side (unilaterally) or on both (bilaterally).

The stapes is known to have the most variations, while the size and shape of the incus are the most stable. Many of these have very serious, debilitating effects on hearing function. Here are some of the more commonly seen variants:

  • Hypoplasia, which is the under-development of one or more ossicles, is associated with two hereditary conditions: Branchio-oto-renal syndrome and Crouzon’s syndrome.
  • Stapes ankylosis, or fixation of the stapes, is when this bone is locked in place and unable to move.
  • Absence of stapes has also been reported in the literature and had a profound effect on hearing.
  • Absence of the long limb of the incus which connects the body to the lenticular process, has also been reported as leading to significant hearing loss.
  • Stapes aplasia is a rare condition in which the stapes doesn’t develop properly, and the base is not connected to the rest of the bone. In these cases, patients progressively lose their hearing.  
  • Stapes hyperplasia is the overgrowth of the stapes, which is another rare abnormality that can cause deafness.

Function

To understand what the ossicles do is to understand how the human body starts to sense and processes sound. Their specialized structure ensures that soundwaves are efficiently 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 ear canal (clinically known as the external acoustic meatus), and are channeled to the eardrum, causing it to vibrate. From the other side, the ossicles pick up this motion.
  • Transmission and amplification: As the malleus is attached to the eardrum, it and the subsequent ossicles also begin to vibrate, essentially moving the signal to the inner ear. Crucially, because the base of the stapes attached to the oval window is so much smaller than the eardrum, 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 causes tiny waves to occur, which are picked up by the receptor cells, essentially converting mechanical information into electrical signals. These are then transmitted to the brain through a structure called the vestibulocochlear nerve.

Associated Conditions

Given how essential they are, it’s no wonder health conditions that impact the ossicles can seriously affect hearing.

The most common problem to arise is ossicular chain discontinuity, in which the bones are either fused together, limiting mobility, or set too far apart. As a result, vibrations of the eardrum are not translated to the oval window, leading to severe hearing loss, 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): This is when middle ear infections become chronic and don’t resolve, leading to long-term damage to ossicles over the long-term. Among the symptoms of COM is erosion of the malleus, which can spread infection to the meninges, the waxy covering surrounding the brain.
  • Otosclerosis: The abnormal growth of bone in the middle ear, called otosclerosis, can severely impact the mobility of the ossicles, leading to hearing loss. Most often, this leads to the base of the stapes being affixed to the oval window, preventing the transmission of sound vibrations.
  • Cholesteatoma: Non-cancerous growths in the middle ear, cholesteatomas are another cause of hearing loss. If untreated—and depending on their location—they grow and impact the ossicles, interrupting their function.
  • Congenital malformation: Birth defects can include insufficient formation or even absence of one or more of the ossicles, leading to ossicular chain discontinuity.
  • Trauma: Trauma due to head injury, such as skull fracture, also may lead to ossicular chain discontinuity, either directly or due to subsequent inflammation in the middle ear.
  • Hyperacusis: This condition occurs when the tensor timpani muscle, which dampens loud sounds, or the stapedius muscle that stabilizes the stapes become paralyzed. Resulting from infection or trauma, it leads to a hypersensitivity to louder noises.  

Rehabilitation

There’s no doubt that the hearing loss, deafness, or other auditory problems that arise due to problems can be very debilitating. However, there are a number of treatments that correct or 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 will help make up for their limited function.
  • Ossicular chain reconstruction surgery: This surgery, in which the ossicles are repaired and reattached, can effectively resolve ossicular chain discontinuity. Existing bone structures may be fixed or reinforced, or a prosthetic of the affected ossicle can be implanted.    
  • Stapesplasty: Otosclerosis is treated with a special type of ossicular chain reconstruction that removes and replaces the head and the limbs of the stapes. 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, surgery will be performed to remove it.
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