middle ear ossicles
There are three tiny articulating bones in the middle ear known as ossicles (from lateral to medial):
Their role is to mechanically amplify the vibrations of the tympanic membrane and transmit them to the cochlea where they can be interpreted as sound. They are located in the middle ear cavity and articulate with each other via two tiny synovial joints. The stapes also articulates with the oval window via the stapediovestibular joint, which is a syndesmosis ; this joint transmits the ossicular vibrations to the endolymph in the vestibule.
Interestingly, they are the only bones in the body that do not grow after birth and are also the smallest bones in the body (variant tiny sesamoids aside).
On axial images (in the plane of the lateral semicircular canal), the normal landmarks of the ossicles can be remembered with the following mnemonic, from superior to inferior :
- ice cream cone (level of horizontal tympanic segment of facial nerve): incudomallear joint involving head of malleus (ice cream) and body of incus (top of cone), and short process of incus (tip of cone)
- two dots (level of oval window): neck of malleus (lateral dot) and long process of incus (medial dot)
- two dashes: handle/manubrium of malleus (lateral line), incudostapedial joint involving lenticular process of incus and head of stapes (medial line)