Anatomy & Organs

Stirrups – structure, function & diseases

Stirrup

In ENT medicine and audiometry , one of the three interconnected auditory ossicles in the middle ear is referred to as a stirrup . With a weight of only about 2.5 mg, the little bone, which is reminiscent of a stirrup used in equestrian sport, is the smallest bone in the human body and at the same time the hardest. Its main task is to transmit the vibrations of the eardrum to the inner ear through the oval window .

What is the stirrup?

The inner ear contains the three ossicles, which consist of the hammer (malleus), anvil (incus) and stirrup (stapes). 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 stirrup, which is reminiscent of a stirrup in equestrian sport, is the last link of the three auditory ossicles.

He sits with his foot on the oval window and transfers his 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 auditory 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 the sound from the gaseous medium of air to the liquid medium of perilymph in the inner ear. At the same time, the stirrup is able to protect the inner ear from damage caused by loud noises through the stapedius reflex, which is triggered by a sudden increase in noise level (bang).

Anatomy & Structure

With a weight of about 2.5 mg and an average length of 3.3 mm, the stapes is the smallest but also the hardest bone in the human skeleton . With its 3.2 square millimeter base, it rests against the oval window that forms the flexible connection to the inner ear. At the same time, the oval window 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 vestibular organs and cochlea of ​​the inner ear are surrounded by perilymph, while inside they contain endolymph, which is electrolytically distinct from perilymph.

With its upper end, the stapes head, the stapes is articulated to the anvil. The stirrup is connected to a tiny muscle called the stapedius muscle. In the case of suddenly occurring high sound pressures, e.g. B. with a loud bang, the so-called stapedius reflex is triggered. The reflex tightens the smallest striated muscle in the human body and jams the stirrup. This significantly reduces the sound transmission, resulting in a kind of overload protection for the inner ear.

Function & Tasks

The main task of the stapes is, in conjunction with the other two auditory ossicles, incus and malleus, to transmit the vibrations of the eardrum to the perilymph in the inner ear in a suitable form. In another function, the stapes protects the sensory cells in the cochlea from sudden overload caused by noises with extremely high sound pressure levels.

The problem of the phase transition arises when the vibrations of the eardrum are transmitted to the perilymph. Airborne sound, which is characterized by low sound pressures at high deflections due to the compressibility of the gaseous sound carrier, must be transformed into a form of high sound pressures with low deflections (impedance transformation) in order to do justice to sound transmission in incompressible liquids. This happens through the type of articulated connections between the three auditory ossicles using the law of the lever. The force exerted by the eardrum on the hammer, which absorbs the vibration of the eardrum, is amplified by a factor of 90 via mechanical leverage, with a corresponding reduction in deflection.

In this way, the stirrup converts the sound that arrives at the eardrum as airborne noise with almost no loss and transmits it to the inner ear through the stirrup at the oval window. To help protect the sensory cells in the cochlea from overload, the efficiency of sound transmission through the stapedius reflex can be reduced. The tiny stapedius muscle, which is connected to the stirrup, shortens when the reflex is triggered by a very loud noise (bang) and thus tilts the stirrup, which reduces the efficiency of sound transmission significantly.

Diseases

One of the most important and common diseases and disorders related to the function of the stapes is sclerotization of the membrane that holds the stapes foot to the oval window at the junction with the inner ear. This is an ossification, also known as otosclerosis , which leads to gradually increasing conductive hearing loss because the transmission of sound vibrations to the inner ear is disrupted.

In an advanced stage of the disease, an artificial stirrup (stapes prosthesis) can restore hearing to a certain extent by means of microsurgery. The reasons for the occurrence of otosclerosis are not (yet) sufficiently known and researched. Colds, middle ear infections , eustachian tube dysfunction, and similar conditions can lead to tympanic effusion , fluid buildup in the middle ear . The liquid can vary in consistency and is prone to inflammation .

The functional chain of sound transmission through the auditory ossicles is usually disturbed, so that conductive hearing loss also occurs in the affected ear , which is reversible if the problem that caused it, the tympanic effusion, can be cured. In rare cases, a tympanic effusion can also be caused by a tumor in the nasopharynx, which requires appropriate medication.

Typical & common ear diseases

Lisa Newlon
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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.