The ossicles are located in the middle ear and serve to transmit mechanical vibrations.
What are the ossicles?
The ossicles, known as auditory ossicles in Latin, consist of tiny bones located in the middle ear that are responsible for transmitting mechanical vibrations to the inner ear. In the human ear, this term ossicles refers to ossicles such as the stapes, the malleus, and the incus. The ossicle, or tympanic cavity, is an air-filled space behind the eardrum where the ossicles are located.
Anatomy and structure
The ossicles are among the smallest bones in a person and connect the outer ear to the inner ear. They weigh only a few grams. The hammer weighs about 23 milligrams, the anvil 27 milligrams, and the stapes only 2.5 milligrams. The ossicles are very articulated and interconnected. They are attached by the ligamentous apparatus, which is located in the cavity of the middle ear. The mucous membrane of the middle ear covers the ossicles. The name of the anvil, stirrup, and malleus is derived from the shape of the ossicles, which are located between the eardrum and the inner ear in the following order: Mallet, Anvil, Stirrup. Partially embedded in the eardrum is the malleus. This transmits the vibrations from the eardrum to the other ossicles. The scientific name for the malleus is malleus, for the incus is incus and the stapes is stapes. The malleus is fused to the tympanic membrane by a malleus peduncle and is connected to the incus by the malleus head. The anvil in turn forms an anvil-stapes joint with the so-called stapes head.
Function and tasks
The ossicles are quite different from the other bones of the human body. Namely, they contain not only lamellar bones, but also cartilage, plexus bones, and strand bones. The strand bone is understood to be an embryonically formed bone substance in which the collagen fibrils interlace to form strands as in hair. Ossification of the malleus begins in the fetus as early as the fourth month and is almost complete by the seventh month. The incus, which arises from the first gill arch, also forms toward the end of the fifth month. Ossification of the stapes occurs toward the end of the fourth month and ossifies by about the end of the eighth month. At birth, the ossicles are then in the state of fully grown, completely ossified bones. The purpose of the small ossicles is to couple the corresponding vibrations emitted by the eardrum to the inner ear as well as possible and to protect the inner ear from loud sound pressure. They therefore act as impedance converters at the oval window with the tympanic membrane and atrial membrane, because a low sound pressure in front of the tympanic membrane is converted into high pressure at the oval window of the inner ear. That is, the conversion of acoustic vibrations in the ear canal are converted into mechanical vibrations by the ossicles into fluid vibrations. Since the eardrum is not fixed to the oval window, this has a positive effect on the ear. Otherwise, the transmission of sound would be about 30 decibels lower and soft sounds would hardly be perceived. The ossicles also have an important protective function. Two small muscles change the degree of deflection of the ossicles. For example, one muscle attaches to the malleus and tenses the eardrum; it also protects against overly violent movements of the ossicles and the eardrum, such as when sneezing. The second muscle, which attaches to the stapes, protects the very sensitive hair cells in the inner ear from excessive sound pressure.
Diseases
In otosclerosis, pathological hardening of the membrane occurs and leads to increasing hearing loss as a result of severe impairment of the transmission of vibrations from the ossicular chain to the inner ear. Triggers can be diseases such as measles or mumps, but also inflammatory diseases or autoimmune processes in the body. This process can be stopped by a microsurgical intervention in which the almost immobile stapes is replaced by a so-called stapes prosthesis, an artificial stapes. As many as 20 percent of Germans suffer from hearing loss. The proportion of men older than 65 who are hard of hearing reaches more than 50 percent.Hearing loss is often caused by a disease of the inner or middle ear. Disorders of the ossicles or the eardrum also promote hearing loss. Hearing improvement surgery can improve the quality of life as well as the hearing ability of those affected. Smallest instruments, which have a diameter of barely one millimeter, support interventions in which tumors as well as inflammations in the middle ear are removed, but also already destroyed parts of an ear are rebuilt, for example a reconstruction of the eardrum. Thanks to today’s medicine, it is even possible to replace a lost eardrum and insert it with grafts from muscle and cartilage tissue. It is also possible to reconstruct ossicles that have already been destroyed. Mastoiditis, a complication of a not completely healed middle ear infection, is rather rare today due to the administration of antibiotics. However, it can have a negative impact on the sound-conducting and sound-amplifying function of the middle ear and promote hearing loss. The inflammation may lead to unpleasant impairments such as dizziness or meningitis. It is important to consult an otolaryngologist for any problems or the vague suspicion, the feeling of worsening hearing and to clarify this. Progressive otosclerosis, if left untreated, can result in deafness in the worst case. Impairment due to tinnitus is also possible.
Typical and common ear diseases
- Ear drum injuries
- Ear flow (otorrhea)
- Otitis media
- Ear canal inflammation
- Mastoiditis
- Ear furuncle
