What is an ossicle?

Ossicles: The Tiny Bones of the Middle Ear

Ossicles are the three small bones in the middle ear (malleus, incus, and stapes) that transmit sound vibrations from the tympanic membrane to the inner ear, forming a crucial link in the hearing process.

Anatomy and Structure

The ossicular chain consists of three distinct bones:

1. Malleus (hammer) - Attaches to the tympanic membrane (eardrum)

   • Has a head, neck, manubrium (handle), and processes (anterior and lateral)
   • The manubrium connects directly to the eardrum

2. Incus (anvil) - The middle ossicle

   • Connects the malleus to the stapes
   • Has a body, short process, and long process with lenticular process at its end
   • Considered the most stable ossicle morphologically 1

3. Stapes (stirrup) - The smallest bone in the human body

   • Has a head, neck, two crura (legs), and footplate
   • The footplate connects to the oval window of the cochlea
   • Shows the most morphological variation among the three ossicles 1

Function

The ossicles serve several critical functions:

• Sound transmission: They conduct and amplify sound waves from the tympanic membrane to the inner ear 2
• Impedance matching: They convert large-amplitude, low-force vibrations of the eardrum into smaller-amplitude, higher-force vibrations at the oval window
• Protection: The ossicular chain includes joints that can decouple during extreme pressure changes to protect the inner ear 3

Clinical Significance

Conductive Hearing Loss

Ossicular abnormalities are a common cause of conductive hearing loss (CHL), which must be distinguished from sensorineural hearing loss (SNHL) 2:

• Ossicular erosion or fusion can impair sound transmission
• Temporal bone CT is the first-line imaging modality for evaluating ossicular abnormalities 2
• CT provides excellent delineation of the ossicular chain and can identify:
  • Ossicular erosion or fusion
  • Changes of otosclerosis
  • Superior semicircular canal dehiscence 2

Otitis Media and Ossicles

In chronic or recurrent otitis media, ossicular damage can occur:

• Middle ear effusion can impair ossicular movement
• Persistent effusion may require surgical intervention with tympanostomy tubes 2
• Tympanostomy tubes bypass the eustachian tube dysfunction and allow ventilation of the middle ear, preserving ossicular function 2

Surgical Considerations

The high variability in ossicular morphology has important implications for middle ear surgery:

• Statistical shape modeling shows significant variation in the manubrium of the malleus, long process of the incus, and crura/footplate of the stapes 4
• When fixed at the malleus, variations along the chain result in different stapes positions 4
• This variability must be considered during ossiculoplasty and prosthesis selection

Evolutionary Significance

The presence of three ossicles is a defining feature of mammals:

• Non-mammalian vertebrates have only one middle ear ossicle (stapes/columella) 5
• The malleus and incus evolved from jaw joint bones (quadrate and articular) of reptilian ancestors 5
• This evolutionary adaptation allowed for improved hearing sensitivity, particularly at higher frequencies

Imaging and Evaluation

For evaluation of ossicular abnormalities:

• High-resolution CT of the temporal bone is the gold standard 2
• MRI is insufficient for evaluating ossicular details due to their bony nature 2
• CT should be performed with bone windows to visualize the small ossicular structures 2

Common Pathologies

1. Ossicular discontinuity - Disruption of the ossicular chain, often due to trauma
2. Ossicular fixation - Restricted movement, commonly from otosclerosis
3. Ossicular erosion - Often secondary to chronic otitis media or cholesteatoma
4. Congenital ossicular anomalies - May occur with other ear malformations

Understanding ossicular anatomy and function is essential for diagnosing and treating conductive hearing loss and other middle ear disorders.