Incudostapedial joint is a small,
synovial ball-and-socket joint
The ball-and-socket joint (or spheroid joint) is a type of synovial joint in which the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone. The distal bone is capable of motion around an indefinite number of ...
between the ''
incus
The ''incus'' (plural incudes) or anvil is a bone in the middle ear. The anvil-shaped small bone is one of three ossicles in the middle ear. The ''incus'' receives vibrations from the ''malleus'', to which it is connected laterally, and transmit ...
'' (anvil) and the ''
stapes
The ''stapes'' or stirrup is a bone in the middle ear of humans and other animals which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the foo ...
'' (stirrup). The joint's function is to transfer vibrations between the two
ossicles
The ossicles (also called auditory ossicles) are three bones in either middle ear that are among the smallest bones in the human body. They serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea). The absence of the auditory ...
. The incudostapedial joint lies between the long leg of the incus (long crus, or ''crus longum incudis'') and the head of the ''stapes'' (''caput stapedis'').
The long leg moves with the rest of the ''incus'' and a small knob, the lenticular process, articulates with the head of the ''stapes''.
Biophysics
Although the joint is synovial, it nonetheless allows only for a very limited, linear range of motion. All of the ossicles move almost as a single unit.
As the transfer of kinetic energy from the incident sound waves to the
perilymph
Perilymph is an extracellular fluid located within the inner ear. It is found within the scala tympani and scala vestibuli of the cochlea. The ionic composition of perilymph is comparable to that of plasma and cerebrospinal fluid. The major ca ...
of the inner ear involves a loss of energy, the ossicular system functions to compensate for the loss by decreasing the
lever
A lever is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or ''fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effort, the lever is div ...
ratio between the surface of the eardrum and base of the ''stapes'', but also through the ratio between the ''manubrium mallei'' (the handle of the ''malleus'') and the long leg of the ''incus''. The ossicular system thus increases the
force
In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a p ...
of transmitted oscillations by some 25-times between the tymphanic membrane and the base of the ''stapes''.
See also
*
References
{{Authority control
Joints of the head and neck
Ear
Joints
Human head and neck