The malleus, incus, and stapes are a trio of minute bones in the middle ear, collectively known as the ossicles, and they represent the smallest bones found anywhere in the human body. These structures are the physical bridge that connects the airborne sound waves hitting the eardrum to the fluid-filled inner ear where sound is processed. The common names—hammer, anvil, and stirrup—are derived from the Latin terms for the bones, which accurately describe their distinct shapes. Their function is to mechanically transform and amplify sound, ensuring that faint vibrations are translated into signals the brain can interpret.
The Ossicles: Names and Position in the Ear
The three ossicles are suspended within the air-filled space of the middle ear cavity, positioned between the tympanic membrane (eardrum) and the entrance to the inner ear.
The first bone is the hammer, or Malleus, which is firmly attached to the inner surface of the eardrum. This connection allows the Malleus to move first when sound waves cause the eardrum to vibrate.
The middle bone is the anvil, or Incus, which articulates with the Malleus and the third bone. It acts as the central link in this mechanical transmission chain, distributing the force received from the Malleus.
The final and smallest bone is the stirrup, or Stapes, which is shaped like a horse’s stirrup. The Stapes has a flat base, known as the footplate, which rests directly against the membrane-covered opening called the oval window. This articulated chain spans the middle ear and is held in place by small ligaments and muscles.
How the Hammer, Anvil, and Stirrup Transmit Sound
The primary function of the ossicles is to overcome the significant difference in acoustic resistance, or impedance, between the air of the middle ear and the fluid that fills the inner ear. If sound waves were to hit the fluid directly, over 99% of the sound energy would be reflected away, resulting in a nearly complete loss of hearing. The ossicles counteract this loss by acting as a sophisticated mechanical transformer.
The sound transmission process begins when airborne vibrations strike the tympanic membrane, causing it to move. This movement is immediately transferred to the attached Malleus, initiating a mechanical vibration through the chain to the Incus and then the Stapes. The ossicles achieve sound amplification through two distinct mechanisms, effectively concentrating the sound energy.
Hydraulic Action (Area Ratio)
This mechanism results from the difference in size between the eardrum and the Stapes footplate. The surface area of the eardrum is approximately 20 times larger than the small area of the oval window membrane. This size difference focuses the force of the large eardrum vibration onto the tiny Stapes footplate, increasing the pressure by a factor of about 17 to 1.
Lever Action
The lever action is created by the relative lengths of the Malleus and Incus. These two bones pivot together, acting as a lever with the Malleus arm being slightly longer than the Incus arm. This length difference provides a mechanical advantage, increasing the force transmitted to the Stapes by about 1.3 times. When combined, the area ratio and the lever action result in a total pressure increase of approximately 22 times, which is enough to efficiently transfer sound energy into the dense fluid of the inner ear.
Conditions That Affect Ossicular Function
Disruptions to the ossicular chain lead to a form of hearing loss called conductive hearing loss, where sound transmission is impaired before reaching the inner ear sensors.
Otosclerosis
Otosclerosis is characterized by abnormal bone remodeling that causes the Stapes footplate to become fixed and rigid within the oval window. This fixation prevents the Stapes from moving freely, drastically reducing the efficiency of sound transfer, particularly for lower-frequency sounds.
Ossicular Discontinuity
This pathology refers to a break or dislocation in the chain, most commonly occurring at the joint between the Incus and the Stapes. Trauma to the head or severe chronic middle ear infections can cause this separation, which completely interrupts the mechanical path of sound from the eardrum to the inner ear. When the chain is broken, the amplified force cannot be delivered to the oval window, leading to a substantial drop in hearing across all frequencies.
Otitis Media (Middle Ear Infections)
Middle ear infections (otitis media) can impair function acutely or chronically. In acute cases, the accumulation of fluid in the middle ear cavity dampens the movement of the ossicles, preventing them from vibrating effectively. Chronic infections can lead to the erosion of the bones themselves. The Incus is the most susceptible ossicle to damage due to its relatively poor blood supply. Erosion eventually results in a discontinuity, permanently disabling the sound amplification mechanism.