The ossicles are a chain of three tiny bones located in the middle ear: the malleus, incus, and stapes. They are housed within the air-filled middle ear cavity, behind the eardrum. Their purpose is to convert sound waves gathered from the air into mechanical vibrations. These vibrations are then transferred efficiently to the fluid-filled inner ear. The three ossicles are the smallest bones in the human body and perform a function necessary for hearing.
Anatomy and Location of the Ossicles
The three ossicles are named for their resemblance to common objects: the malleus (hammer), the incus (anvil), and the stapes (stirrup). The chain begins with the malleus, the largest of the three, which has a handle (manubrium) firmly attached to the inner surface of the tympanic membrane (eardrum). This attachment captures the initial vibratory motion from sound waves and transmits it into the middle ear space.
The central bone is the incus, which articulates with the head of the malleus at the incudomalleolar joint. The incus acts as a bridge, receiving vibrations from the malleus and passing them along its long process. The final bone in the sequence is the stapes.
The stapes articulates with the incus and terminates in a footplate (base). This footplate rests against the oval window, a membrane-covered opening leading into the inner ear. This precise arrangement within the air-filled middle ear cavity allows the bones to function as a delicate lever system.
The Mechanism of Sound Transmission
The function of the ossicles is to transmit and intensify the mechanical energy of sound from the eardrum to the inner ear’s fluid. When sound waves strike the tympanic membrane, the resulting vibration is relayed to the attached malleus. The malleus moves the incus, which then pushes and pulls the stapes footplate against the oval window.
This process is necessary because sound traveling from air (low-resistance) to the fluid in the inner ear (high-resistance) would normally result in a massive loss of energy. If sound bypassed the middle ear, about 99.9% of the sound energy would be reflected, leading to significant hearing loss. The coordinated action of the ossicles overcomes this problem through a mechanism called impedance matching.
Impedance matching is achieved through two physical principles. The most significant is the difference in surface area between the eardrum and the oval window. The eardrum is much larger than the stapes footplate, which concentrates the force of vibration onto a smaller area, dramatically increasing the pressure applied to the inner ear fluid. A secondary factor is the lever action created by the length difference between the malleus and incus, which further amplifies the force. Together, these mechanisms provide a significant pressure gain, ensuring that sound is efficiently transferred to the cochlea.
Conditions Affecting Ossicle Movement
The efficiency of sound transmission relies on the ossicles’ ability to move freely, and several conditions can impair this movement, resulting in conductive hearing loss. One common issue is otosclerosis, an abnormal bone growth that causes the stapes footplate to become fixed in the oval window. This fixation prevents the stapes from effectively vibrating the inner ear fluid, leading to a progressive loss of hearing. Otosclerosis is believed to have a genetic component, and hormonal changes may also play a role in its development.
Another problem is ossicular discontinuity, which involves the physical separation or fracturing of the ossicular chain. This condition can occur following severe head trauma, such as a temporal bone fracture, or due to rapid pressure changes. Discontinuity most frequently affects the delicate joints, particularly those between the incus and the stapes, or the incus and the malleus.
Chronic otitis media, a persistent middle ear infection, can also severely impact ossicle function. Long-term inflammation and the presence of erosive disease, such as cholesteatoma, can lead to the destruction of the ossicle bone tissue. Studies show that the incus is often the most susceptible to erosion. The resulting erosion or separation of the bones prevents the proper transfer of vibrations, causing a substantial loss of hearing.