Anvil (Ear Bone)
Incus
location_on Middle ear cavity within the temporal bone, between the malleus and stapes
The incus is the middle ossicle of the auditory chain, articulating with the malleus laterally and the stapes medially. It resembles an anvil or a premolar tooth with two roots. The long process of the incus descends vertically and terminates in the lenticular process, which forms a delicate connection with the stapes head. This lenticular process is the most vulnerable point in the ossicular chain.
Key Anatomical Features
- Body articulates with the head of the malleus
- Short process (crus) projects posteriorly and is anchored by a ligament to the fossa incudis
- Long process (crus) descends vertically parallel to the manubrium of the malleus
- Lenticular process is a small knob at the tip of the long process articulating with the stapes
- Resembles a premolar tooth with two diverging roots
Muscle Attachments
| Muscle | Attachment | Action |
|---|---|---|
| No direct muscular attachments | The incus is stabilized by ligaments and its articulations | Transmits vibrations from malleus to stapes |
| Posterior incudal ligament | Short process to fossa incudis in posterior wall | Primary ligamentous support for the incus |
| Superior incudal ligament | Body to roof of epitympanum | Suspends the incus superiorly |
| Incudomalleolar joint capsule | Between malleus head and incus body | Maintains the malleus-incus articulation |
Joints and Articulations
| Joint | Type | Connects to |
|---|---|---|
| Incudomalleolar joint | Synovial saddle | Malleus (head) |
| Incudostapedial joint | Synovial ball-and-socket | Stapes (head) |
Common Pathologies
Incus necrosis
The long process of the incus is the most vulnerable part of the ossicular chain to necrosis from chronic middle ear disease, because its blood supply is relatively tenuous.
Incudostapedial joint erosion
Chronic otitis media or cholesteatoma can erode the lenticular process, disrupting the connection between the incus and stapes and causing significant conductive hearing loss.
Traumatic ossicular dislocation
The incudostapedial joint is the weakest link in the ossicular chain and is the most common site of dislocation from head trauma or barotrauma.
Clinical Relevance
The long process of the incus has the most tenuous blood supply of all the ossicles, making it the most common ossicle to undergo necrosis in chronic ear disease. In ossiculoplasty, the incus is frequently harvested, reshaped, and used as an autograft to reconstruct the ossicular chain (sculptured incus interposition). The incudostapedial joint is the most commonly disrupted point in traumatic ossicular dislocation.
Development and Ossification
The incus develops from the cartilage of the first pharyngeal arch, like the malleus. Ossification begins during the 4th fetal month from a single center. The incus is fully ossified and adult-sized at birth and does not grow postnatally.
Did You Know?
- Incus means anvil in Latin, though it more closely resembles a tooth with two roots
- The incudostapedial joint is the smallest synovial joint in the body
- The incus is frequently recycled during ear surgery: removed, reshaped, and reinserted as an autograft prosthesis
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