Stirrup (Ear Bone)
Stapes
location_on Middle ear cavity within the temporal bone, the innermost ossicle against the oval window
The stapes is the smallest and lightest bone in the human body, weighing only about 3 milligrams and measuring roughly 3mm in height. Shaped like a stirrup, it consists of a head, neck, two crura (legs), and a footplate that sits in the oval window of the inner ear. The stapes transmits amplified sound vibrations to the fluid-filled cochlea, converting air-conducted sound waves into fluid waves essential for hearing.
Key Anatomical Features
- Smallest bone in the human body at approximately 3mm tall and 3mg in weight
- Footplate sits in the oval window of the vestibule, sealed by the annular ligament
- Two crura (anterior and posterior) connect the neck to the footplate, resembling a stirrup
- Head articulates with the lenticular process of the incus
- Stapedius tendon attaches to the posterior surface of the neck
Muscle Attachments
| Muscle | Attachment | Action |
|---|---|---|
| Stapedius | Posterior neck of stapes via a tiny tendon | Tilts the stapes to dampen excessive vibration and protect the inner ear from loud sounds (acoustic reflex) |
| Annular ligament | Margin of footplate to oval window | Holds the footplate in the oval window while allowing piston-like movement |
| Incudostapedial joint capsule | Head of stapes to lenticular process of incus | Maintains the articulation between incus and stapes |
Joints and Articulations
| Joint | Type | Connects to |
|---|---|---|
| Incudostapedial joint | Synovial ball-and-socket | Incus (lenticular process) |
| Stapediovestibular joint (oval window) | Syndesmosis (annular ligament) | Oval window of vestibule |
Common Pathologies
Otosclerosis
Abnormal bone growth that fixes the stapes footplate in the oval window, causing progressive conductive hearing loss. Affects about 0.5-1% of the population, most commonly young white women.
Stapes fracture
Rare fracture of the stapes crura from barotrauma or direct trauma, potentially causing a perilymphatic fistula with sensorineural hearing loss and vertigo.
Congenital stapes fixation
Developmental anomaly where the stapes footplate is fixed or the crura are malformed, causing conductive hearing loss from birth.
Clinical Relevance
Otosclerosis is the most common cause of progressive conductive hearing loss in young adults. Stapedectomy (replacement of the stapes with a prosthesis) or stapedotomy (creating a small hole in the fixed footplate and inserting a piston prosthesis) restores hearing in over 90% of cases. The stapedius muscle reflex (acoustic reflex) is tested during tympanometry and is absent in otosclerosis and facial nerve palsy. Carhart notch (apparent sensorineural loss at 2000 Hz on audiometry) is characteristic of otosclerosis.
Development and Ossification
The stapes develops from the cartilage of the second pharyngeal arch (Reichert's cartilage), unlike the malleus and incus which develop from the first arch. Ossification begins during the 4th fetal month from a single center. The stapes is fully formed at birth and is the smallest bone throughout life.
Did You Know?
- The stapes is the smallest bone in the human body, weighing about the same as a grain of sand (3 milligrams)
- Unlike the malleus and incus (first arch), the stapes develops from the second pharyngeal arch, which also gives rise to the hyoid bone
- Stapedectomy for otosclerosis was one of the first successful microsurgical procedures, pioneered in the 1950s
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