Ear ossicles

Malleus, incus and stapes

What are auditory ossicles and how many are they?

The middle ear contains three small ossicles: the hammer (or malleus), the anvil (or incus), and the stirrup (or stapes). Named after their resemblance to three building/craft tools, these bones are the smallest in the human body, and their function is to receive sound waves, amplify them, and transmit the sounds from the ear drum into the fluid-filler the inner ear.

Where are the auditory ossicles located?

The three auditory ossicles function as a bridge between the eardrum and the oval window. The malleus is the most lateral middle ear ossicle, it is located in the epithympanic recess directly in contact with the eardrum. The stapes, located in a medial ear position, is connected to the cochlea and communicates with the malleus thanks to the incus, the intermediate ossicle located between the malleus and the stapes.

What are ear ossicles' names?

Malleus: functions and anatomy

The malleus is the largest ossicle in the ear and its main function is to relay the vibrations of the incoming soundwaves produced by the eardrum to the incus. The malleus is made up of 5 parts: the head, the neck, the handle, the anterior process and the lateral process.

  • - The head of the malleus is smooth, oval-shaped and attached to the incus. The head connects to the neck of the malleus which then continues down the handle of the malleus or manubrium, which connects to the tympanic membrane. The articular facet joint has a larger upper section and a smaller lower section that forms an angle of about 90°. On the opposite side, the lower margin of the facet joint gives rise to the so-called "tooth" or "spur" of the malleus. 
  • - The neck connects the head to the handle which extends upwards to the pars flaccida of the tympanic membrane, a small triangular-shaped area located at the top of eardrum.
  • - The handle, also known as the manubrium, is connected to the tympanic membrane from its lateral side. While Its upper end features a slight projection, its medial side is curved slightly and faces downward and backward. As you reach its unattached end, the handle is flattened transversely and decreases in size.
  • - The anterior process of the malleus is a fine thorn-like bone that originates in the underside of the neck and is connected to the Glaser petrotympanic fissure.
  • - The lateral process is a cone-shaped bony projection that originates at the base of the handle. It is laterally located and it is attached to the top of the tympanic membrane and to the ends of Rivino's sulcus.

Incus: functions and anatomy

The incus bone is located between the malleus and the stapes, it measures about 0.7 cm and its function is to receive the vibrations produced by sound waves. It was first described by Alessandro Achillini as a bone that resembles a molar tooth with a slightly flattened, cube-shaped body and two roots (also known as apophysis branches or processes). The short (or superior) root runs towards the posterior wall of the tympanic cavity; the long (or inferior) root runs downwards and ends with the lenticular process, consequently connecting the incus to the stapes.

Stapes: the smallest bone of the human body

The stapes bone is the smallest of the ossicles. It has the fundamental function of enabling communication between the middle ear and the inner ear, transmitting the vibrations carried by the chain of ossicles to the inner ear. It is located in the center of the ear, connecting with the incus on one side and the oval window on the other. It was discovered by Professor Gianfilippo Ingrassia of the University of Naples in 1546.

The stapes consists of a head, two arches and a base. The head is linked to the incus lenticular process, the two arches (anterior and posterior) join the head with the base, which is a thin elliptical bone that occupies the oval window.

What is the origin of ear ossicles?

As far as the phylogenetically formation of the ear bones is concerned, the incus and the malleus in mammals were formed respectively from the quadrate and the articular, the two bones of the oral arch that allow maxilla-mandibula articulation in fish. The formation of the stapes, on the other hand, derives from the mandibular bone, i.e., the dorsal portion of the hyoid arch that in fish serves as the suspension of the oral arch.

Ear bones common diseases

Mastoiditis: infection in the mastoid process

Mastoiditis is the inflammation of the mastoid process, the posterior lateral portion of the temporal bone, otherwise known as the area with a palpable bony protuberance behind the ear lobe. Often middle ear infections can extend to the mastoid through the mastoid antrum. The mastoid antrum is the microorganism responsible for this inflammation as well as as for otitis. The symptoms of mastoiditis usually appear days or weeks after the onset of acute otitis.

Although the most symptoms are intense, throbbing ear pain with decreased hearing, fever and headache, the infection can worsen to the point of perforating the tympanic membrane. Generally, intensive therapy with intravenous antibiotics is the most resorted to treatment for mastoiditis.

Otosclerosis: abnormal bone growth inside the ear

Otosclerosis is an ear disease that leads to progressive hearing loss. The disorder originates in the otic capsule, the structure that protects the ear labyrinth, where bone tissue begins to degenerate. Over time, a buildup of newly formed bone is created, eventually affecting the oval window, the opening between the middle ear and the vestibule of the inner ear.

Otosclerosis is most often an inherited condition that reduces the mobility of the stapes, causing hearing loss. It usually occurs in adults, more frequently in women. The progression of otosclerosis can be slowed by certain medical therapies, including the administration of sodium fluoride. A hearing aid or surgery can help in restoring hearing ability and rehabilitating movement between the bracket and the oval window.

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