Hammer: Structure, Function & Diseases

The malleus is one of a total of three ossicles in the middle ear. It transmits the vibrations of the eardrum under amplification to the incus. The incus transmits the vibrations to the stapes, which transmits the mechanical vibrations via the oval window to the liquid medium perilymph and the cochlea. The malleus, along with the other two ossicles, is one of the smallest yet hardest bones in humans.

What is the malleus?

The tiny malleus in the middle ear is one of three ossicles that are hinged together and mechanically amplify the vibrations of the eardrum. The stapes transmits the vibrations at the oval window to the inner ear and the cochlea, where the conversion of the mechanical sound waves into electrical nerve impulses takes place. The malleus, together with the other two ossicles, is one of the smallest but also one of the hardest bones in the human body. Within this group of three, the malleus is the largest ossicle. The malleus is firmly fused to the eardrum with its “handle” so that it can directly take over the vibrations of the eardrum. The malleus transmits the vibrations to the incus via a special joint. The technical name Malleus for the hammer also stands in identical spelling for a bacterial disease that exclusively affects equids. The disease is also known as glanders.

Anatomy and structure

Anatomically, the ossicle malleus can be divided into the handle (manubrium), neck (collum), and head (caput). At the upper end of the malleus are two small processes, the anterior and lateral malleus, to which ligaments are attached to hold the malleus in position. With the handle in place, the malleus is firmly grown into the connective tissue layer in the middle of the tympanic membrane. From the outside, from the other side of the tympanic membrane, the ingrowth point of the hammer shows through as stria mallearis and is visible by otoscopy. The large head of the hammer is connected to the incus by a saddle joint (articulatio incudomallearis). The joint is tightly encapsulated and provided with so-called locking teeth, so that only small movements up to about 5 degrees are possible. It evolved in mammals from the original temporomandibular joint (primary temporomandibular joint), so the present temporomandibular joint of mammals is a relatively new development and is also called the secondary temporomandibular joint. Tiny muscles provide a permanent state of tension to the middle ear reaction chain, consisting of the tympanic membrane, ossicles, and oval window. The tympanic tensor (Musculus tensor tympani) pulls the hammer handle inward when tension is applied, thereby tightening the eardrum. The malleus – like the other ossicles – is covered by a mucous membrane.

Function and tasks

The main function and task of the malleus is to receive sound vibrations from the eardrum and transmit them to the incus, which in turn transmits them to the stapes, amplifying them. The hammer and anvil are mounted in such a way that their axes of rotation are each located at the center of gravity. This, in conjunction with their low weight, allows them to vibrate with the lowest possible mass acceleration and the least possible energy loss. Even the highest still audible tones in the range above 15,000 Hz to below 20,000 Hz at the limit of ultrasound can be picked up and transmitted by the hammer without any problems. The hammer is also capable of transmitting low frequencies in the infrasound range below 40 Hz without any frequency shifts or conversions. It is important for the recording and transmission of the vibrations of the eardrum that the movable joints between the ossicles and the ossicles themselves react very hard and elastically, because otherwise there would be considerable transmission losses. In vibration transmission, however, it is not only the frequency response of the tones and sounds that is important, but also the sound pressure acting on the eardrum. Within the hearing range, the sound pressure moves between the lower hearing threshold or hearing limit and the pain threshold. The range that is most perceptible to the human ear and at the same time shows a high tolerance until the pain threshold is reached is approximately 100 to 6,000 Hz.However, the task of the hammer in interaction with the other two ossicles is not only to transmit the sound waves as realistically as possible, but also to protect the sensory cells in the inner ear from overload. This means that sound transmission can be attenuated by reflexive tension of the tiny inner ear muscles, thus protecting the sensory cells.

Diseases

The most common ailments associated with vibration pickup by the malleus and transmission of the vibrations are caused by inflammatory processes in the middle ear. If left untreated, the inflammatory processes can lead to sclerotic changes in the ossicles, which are associated with a decrease in function and cause a corresponding hearing loss due to sound conduction problems. Inflammatory processes in the middle ear often lead to tympanic effusion, an accumulation of serous, mucous, bloody or purulent fluid in the tympanic cavity. Tympanic effusion is also usually accompanied by hearing loss because the sound conduction chain eardrum, ossicles is impaired in its function. If the symptoms are in the non-chronic stage, they may resolve on their own if the cause, of the tympanic effusion, has been successfully treated. Interestingly, hypersensitivity of hearing occurs when the trigenic nerve, the 5th cranial nerve is functionally impaired because a lateral branch of the nerve innervates the tensor tympani muscle (eardrum tensor). The muscle can then no longer respond to (too) loud sounds, so that the protective function of noise attenuation fails by reducing the efficiency of sound transmission.