Stapes

Stapes

Also known as: stirrup, stapes footplate, otosclerosis

The smallest bone in the body ("stirrup", ≈3 mm, ≈3 mg); its footplate fits into the oval window and transmits ossicular vibration directly to the perilymph of the inner ear.

Show on body map Ear — Coronal Section

Overview

Height≈3 mm

Mass≈3 mg (smallest bone)

Footplate area≈3.2 mm²

The stapes has a head articulating with the lenticular process of the incus, a neck into which the stapedius tendon inserts, anterior and posterior crura (limbs) and an oval footplate (base) of ≈3.2 mm² that occupies the oval window (fenestra vestibuli). The footplate is held in the window by the elastic annular ligament, which allows it to rock and piston into the vestibule, setting the perilymph in motion. The space between the crura is closed by the obturator membrane; the stapedial artery runs through it in the embryo (persistence is a rare surgical hazard).

The ≈17:1 ratio between the effective area of the tympanic membrane and the footplate is the main source of middle-ear pressure gain. Contraction of stapedius tilts the footplate posteriorly and stiffens the annular ligament, reducing transmission (acoustic reflex).

Location: Medial-most ossicle: horizontal in the middle ear, footplate in the oval window on the medial wall, above and behind the promontory, just below the facial canal.

Parts & subdivisions

Head
Articulates with the lenticular process of the incus.
Neck
Insertion of the stapedius tendon.
Anterior and posterior crura
Arch enclosing the obturator foramen.
Footplate (base)
≈3 × 1.4 mm oval plate (≈3.2 mm²) sealed into the oval window by the annular ligament.

Anatomy

Innervation: Stapedius muscle — nerve to stapedius (facial nerve).
Articulations: Incudostapedial joint (ball-and-socket synovial); syndesmosis of footplate with oval window margin (annular ligament).
Ligaments: Annular (stapediovestibular) ligament.
Development: Second pharyngeal arch (Reichert cartilage); the footplate has an additional contribution from the otic capsule.

Function

Transmit vibration to the perilymph of the vestibule (oval window)
Concentrate force onto a small area (impedance matching)
Modulate transmission via stapedius contraction

Clinical significance

Otosclerosis

Disordered bone remodelling (otospongiosis) of the otic capsule, typically starting just anterior to the oval window (fissula ante fenestram), fixes the stapes footplate. Features: progressive conductive hearing loss in young adults (20–40 years), often bilateral, female predominance, worsens in pregnancy, autosomal dominant with incomplete penetrance; normal drum (occasionally a pink Schwartze sign); audiogram shows a Carhart notch (bone-conduction dip at ≈2 kHz). Patients may hear better in noisy places (paracusis of Willis). Treated with hearing aids or stapedotomy (prosthesis through a hole in the footplate).

Perilymph fistula

Trauma or barotrauma may rupture the annular ligament or round window, leaking perilymph — fluctuating hearing loss and vertigo provoked by pressure changes.

Test yourself

Which ossicle develops from the second pharyngeal arch?

Malleus
Stapes
Incus
Both malleus and incus

Answer: Stapes

The stapes (and stapedius, supplied by VII) derives from Reichert cartilage of the second arch; the malleus and incus (and tensor tympani, supplied by V3) derive from Meckel cartilage of the first arch.

What is the main mechanism by which the middle ear increases sound pressure delivered to the cochlea?

The ≈17:1 area ratio of tympanic membrane to stapes footplate
Resonance of the external acoustic meatus
Contraction of the stapedius muscle
Air in the mastoid air cells amplifying vibration

Answer: The ≈17:1 area ratio of tympanic membrane to stapes footplate

Concentrating force from the ≈55 mm² effective drum area onto the ≈3.2 mm² footplate gives ≈17×; the ossicular lever adds ≈1.3×, for a total gain of ≈22× (≈25–30 dB). Stapedius contraction reduces transmission.

A young woman has progressive bilateral conductive hearing loss that worsened during pregnancy; the tympanic membranes are normal. The most likely cause is:

Otitis media with effusion
Otosclerosis
Presbycusis
Ménière disease

Answer: Otosclerosis

Otosclerosis fixes the stapes footplate in the oval window by abnormal otic-capsule bone remodelling; it affects young adults, women more than men, worsens with pregnancy and shows a Carhart notch at 2 kHz. Treatment: hearing aid or stapedotomy.