In otolaryngology and audiometry, a stapes is one of a total of three interconnected ossicles in the middle ear. Reminiscent in its shape of a stirrup from equestrian sports, the ossicle is the smallest bone in the human body, weighing only about 2.5 mg, and at the same time the one with the greatest hardness. Its main function is to transmit the vibrations of the eardrum to the inner ear via the oval window.
What is the stapes?
The inner ear contains the three ossicles, which consist of the malleus (hammer), incus (anvil) and stapes (stirrup). As a functional unit, they are responsible for transmitting the vibrations of the eardrum to the oval window, which is the connection to the inner ear. The stapes, whose shape is reminiscent of a stirrup in equestrian sports, is the last link of the three auditory ossicles. It attaches its foot to the oval window and transmits its vibrations to the oval window and thus to the perilymph of the inner ear. Due to the anatomy and the articulated connection of the three ossicles to each other, the sound vibrations arriving at the eardrum via air conduction are amplified by a factor of 20 to 30 in order to compensate for the reflection losses that occur during the transition of sound from the gaseous medium of air to the liquid medium of perilymph in the inner ear. At the same time, the stapes is able to protect the inner ear from damage caused by excessively loud sounds through the stapedius reflex, which is triggered by an abrupt increase in the noise level (bang).
Anatomy and structure
The stapes, weighing about 2.5 mg and with an average length of 3.3 mm, is the smallest but at the same time the hardest bone in the human skeleton. It rests with its 3.2 square millimeter foot on the oval window, which forms the flexible connection to the inner ear. The oval window also forms the transition from the gaseous medium air on the side facing the middle ear to the liquid medium perilymph on the side facing the inner ear. The organs of equilibrium and the cochlea of the inner ear are surrounded by the perilymph, while inside they contain endolymph, which is electrolytically different from the perilymph. With its upper end, the stapes head, the stapes is hinged to the anvil. The stapes is connected to a tiny muscle, the musculus stapedius. In the event of sudden high sound pressures, e.g. a loud bang, the so-called stapedius reflex is triggered. The reflex causes the smallest striated muscle in the human body to tense and tilt the stapes. This sensitively reduces sound transmission, resulting in a kind of overload protection for the inner ear.
Function and tasks
The main function of the stapes, in conjunction with the other two ossicles, the incus and malleus, is to transmit the vibrations of the tympanic membrane to the perilymph in the inner ear in a form suitable for this purpose. In another function, the stapes takes over the protection of the sensory cells in the cochlea from sudden overload by sounds with extremely high sound pressures. When the vibrations of the tympanic membrane are transmitted to the perilymph, the problem of phase transition arises. Airborne sound, which is characterized by low sound pressures at high excursions because of the compressibility of the gaseous sound carrier, must be transformed into a form of high sound pressures at low excursions (impedance transformation) to match sound transmission in incompressible fluid. This is accomplished by the nature of the articulated connections between the three ossicles, utilizing the law of leverage. The force exerted by the tympanic membrane on the hammer, which picks up the vibration of the tympanic membrane, is amplified by a factor of 90 via mechanical leverage, with a corresponding reduction in displacement. This ensures that the sound arriving at the eardrum as airborne sound is converted almost without loss and transmitted to the inner ear through the stapes at the oval window. To help protect the sensory cells in the cochlea from overload, the efficiency of sound transmission can be reduced by the stapedius reflex.The tiny stapedius muscle, which is connected to the stapes, shortens when the reflex is triggered by a very loud noise (bang), causing the stapes to tilt and sensitively reducing the efficiency of sound transmission.
Diseases
One of the most important and common diseases and disorders related to the function of the stapes is sclerotization of the membrane holding the stapes foot to the oval window at the junction with the inner ear. This is an ossification, also called otosclerosis, which leads to a gradually increasing conductive hearing loss because the transmission of sound vibrations to the inner ear is disturbed. In advanced stages of the disease, an artificial stapes (stapes prosthesis) can restore hearing to a certain degree by means of microsurgery. The reasons for the occurrence of otosclerosis are not (yet) sufficiently known and researched. Colds, middle ear infections, dysfunction of the Eustachian tube and similar conditions can lead to tympanic effusion, fluid accumulation in the middle ear. The fluid can vary in consistency and is prone to inflammation. The functional chain of sound transmission through the ossicles is usually disturbed in this case, resulting in conductive hearing loss in the affected ear as well, which is reversible if the causative problem, the tympanic effusion, can be cured. In rare cases, a tympanic effusion may also be due to a tumor in the nasopharynx, which requires appropriate medication.
Typical and common ear disorders
- Ear drum injuries
- Ear flow (otorrhea)
- Otitis media
- Ear canal inflammation
- Mastoiditis
- Ear furuncle
