Ear Bones: Things You Need To Know

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The ear is divided into three parts: the outer, middle, and inner ear. The outer ear contains the auricle, ear canal, and tympanic membrane. The inner ear is made up of the oval window, cochlea, subcircular ducts, and an auditory tube. The middle ear consists of the eardrum, tympanic cavity, and three tiny ear bones.

What Are Your Ear Bones?

The ear bones of the middle ear include the malleus, incus, and stapes. The Latin meanings for them are hammer, anvil, and stirrup, respectively. They are also known as the auditory ossicles. Ossicle means “tiny bone.” The purpose of the middle ear bones is to concentrate the energy and vibrations from the eardrum in order to create a greater force per unit area at the oval window. This is the process by which we hear. The ossicles of the middle ear amplify the vibrations of sound by around 30 decibels to modify the sound as it travels from the outer to inner ear.  

Incus

The incus is located between the malleus and the stapes. The incus consists of a short process, a long process, and a body. The body of the incus is connected to the head of the malleus. The most inferior part of the incus that attaches to the head of the stapes is known as the lentiform process or nodule. On axial CT, it appears as a ball of ice cream on a cone; this is the visual representation of the body and short process.

Malleus

The malleus is the most lateral ossicle of the middle ear. It is connected to the incus via the incudomalleolar joint and is attached to the tympanic membrane. It is divided into the head, neck and three processes: manubrium, anterior, and lateral. It has an oval-shaped head and a narrow neck just below it. The processes are connected via the neck.

Stapes

The stapes is the most medial bone of the middle ear. It connects to the incus via the incudostapedial joint and is attached to the oval window through the annular ligament. Additionally, it is attached through the stapedius muscles to the back of the neck. The footplate of the stapes connects to the oval window and allows for the travel of sound from the middle to the inner ear.

Together, these ear bones work as a lever system. At one end, they are used to apply a large force over a small distance by applying a smaller force over a longer distance at the opposite end. These shifting abilities are necessary to help generate the forces that allow us to hear. When sound enters the ear, the eardrum vibrates. The vibration of the eardrum causes the small bones of the ear to stimulate the cochlea. Then, the hair-like cells in the cochlea sense these vibrations, and the vibrations are transmitted to the brain to detect sound.

Health Issues And Your Ear Bones

There are many health conditions that can affect the bones of the middle ear. The ossicles may break, fuse together, or grow abnormally. When the bones of the middle ear fuse or fixate, they are no longer able to move and vibrate in response to sound waves, and this leads to hearing loss. Hearing loss is almost always a symptom when there is damage to the middle ear ossicles. Typically, hearing loss occurs gradually, except in cases of acute trauma. Here are a couple common issues affecting the ear bones. Of the three ossicles, the stapes is the one that is most often abnormal or deformed.

Otosclerosis

This condition affects the bones of the middle ear. Bone tissue grows back abnormally on and around the stapes. The stapes becomes restricted in movement. This leads to the three ossicles becoming unable to transmit sound waves efficiently to the inner ear, and hearing loss occurs. Over time, the stapes becomes so fixed that it cannot move at all, causing severe conductive hearing loss.

Other symptoms include dizziness or vertigo and sensations of ringing in the ears known as tinnitus. This condition may make it hard to hear normal conversations, especially when background noises are present. The first signs of otosclerosis may be someone being unable to hear a whisper or other sounds of low pitch. A whisper is heard at about 30 decibels, while normal conversation is heard at 60 decibels.

Those affected typically start noticing hearing loss in their late 20s or early 30s, but this symptom can begin as early as 15 or even younger. Gender, race, pregnancy, and a family history are all risk factors, although no exact cause has been identified. Stress fractures to the tissue in the ear, autoimmune diseases, and a previous measles infection are also thought to be risk factors.

White women are the most likely to develop otosclerosis. Hearing aids and surgery are two typical treatment options. One highly successful surgery may be a stapedectomy where the stapes is replaced with an artificial bone from metal or plastic that allows sound to travel through the ear. It can be diagnosed with hearing tests and a CT scan.

Ossicular Chain Dislocation 

Also known as ossicular chain discontinuity, this condition is the separation of the middle ear bones that results in conductive hearing loss. It is most often caused by chronic middle ear infections and sometimes cholesteatoma. Rarely, it can be caused by a lightening strike. A fracture to the bones or trauma, such as an injury from a cotton swab, can also cause this dislocation. The incudostapedial joint between the incus and the stapes is the most commonly affected joint. The incudomalleolar joint connecting the malleus and incus is the second most common joint affected.

Congenital Malformations Of The Ossicles

The middle ear bones are quite complex, therefore they may have developed abnormally in some cases. They may be fixed to the inner ear or surrounding skull. Additionally, they may not be able to properly conduct sound vibrations. Hearing loss is the result of the middle ear ossicles being unable to properly carry sound.

5 Facts About Your Ear Bones

1. The Ear Bones Are Some Of The Smallest In The Human Body

In fact, the absolute smallest bone in the human body is the stirrup. Together, the three ear bones are smaller than the size of a dime. As tiny as they are, these bones play a huge role in carrying vibrations from sound waves to the inner ear. When the auditory ossicles vibrate, the stapes vibrates quickly in and out causing the basilar membrane to vibrate down and up, all while the secondary tympanic membrane is vibrating in and out. This process can occur as often as 20,000 times per second.

2. The Ossicles Also Help Protect The Ear From Loud Noises

Particularly, the stapedial reflex protects the ears from sudden loud noises. It retracts or shifts backward in order to dampen the sound and protect our ears. It decreases the pressure put on the oval window. This ability is more pronounced in young people. In young children, it may even be possible for the stapes to completely break away from the inner ear. As we age, these structures become more rigid. Hence, older people are less tolerant of loud noises than their younger counterparts.

Unfortunately, this reflex cannot be held for long periods of time. It also cannot protect us from the sound of a gunshot as it surpasses the threshold at 140 decibels; pain will often be felt when this threshold is crossed. As noise-induced hearing loss is on the rise, it is important to protect the delicate middle ear ossicles. A rock concert is heard at about 110 decibels and can cause damage to the ears after only two minutes. A lawn mower, at about 90 decibels, can be dangerous after two hours. It is recommended that volume is kept under 90 decibels to protect the delicate middle ear bones.

3. If You Did Not Have Auditory Ossicles, You Would Have Moderate To Severe Hearing Loss

Without the middle ear ossicles, less than 1% of sound vibration would make it to the inner ear.

4. Humans, But Not All Animals, Have Middle Ear Bones

As humans, we can hear up to 20 kHz, but we typically hear sounds between 4 and 8 kHz for most of our lives. Not all animals have the middle ear bones that we do. As such, animals without these middle ear bones cannot hear sounds more sensitive than 1 kHz very well.

5. The Ear Bones Are The Only Bones That Do Not Continue Growing After Birth

In humans, the bones of the middle ear are among the first to develop in the fetus. Unlike every other bone, they do not continue growing after birth. 

Conclusion

The functional purpose of the ear bones is to transmit sound from the outer ear to the inner ear. Hearing loss occurs when these bones do not properly conduct sound. The bones may be fused together or separated. Otosclerosis is one such example in which progressive hearing loss occurs. Ossicular chain discontinuity may result from chronic ear infections or trauma. Furthermore, some children have congenital malformations of the ossicles at birth.