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Portrait of Stapes

Anatomy · Eye, ear & senses

Stapes

The 'stirrup' — the innermost auditory ossicle

Middle ear · ossicular chain

The innermost bone of the middle ear, shaped exactly like a stirrup: a small head that meets the incus, a short neck, two curved limbs (the anterior and posterior crura) and a flat, oval base, the footplate, that fits the oval window of the inner ear. The stapedius muscle, the smallest skeletal muscle, pulls on its neck. The gap between the limbs is the obturator foramen, a relic of an embryonic artery.

How to spot them

  • ▸Stirrup-shaped: a head, a short neck, two curved arms and a flat oval footplate
  • ▸The footplate is held in the oval window by an annular ligament
  • ▸The stapedius tendon emerges from a bony peg (the pyramidal eminence) to attach to its neck
  • ▸The hole between its arms, the obturator foramen, is spanned by a thin membrane

What to know

The gap between its two arms once held an embryonic artery that normally disappears before birth

The stapes — the obturator foramen surrounds the stapedial artery in the embryo

A tendon from a bony peg on the back wall of the cavity attaches to its neck; the muscle is supplied by the facial nerve

The stapedius muscle inserts on the neck of the stapes

After paralysis of the seventh cranial nerve, ordinary sounds may become painfully loud because the muscle that dampens it no longer works

Hyperacusis after facial nerve palsy, from loss of the stapedius reflex

A flat oval plate held in a window by a ring of ligament, with an area of only about three square millimetres

The footplate of the stapes, sealed in the oval window by the annular ligament

Replaced by a Teflon piston in John Shea's operation of 1956 in Memphis (1956)

Stapedectomy — removal of the stapes and replacement with a prosthesis

Develops from the cartilage of the second pharyngeal arch (Reichert's cartilage), with its footplate partly from the ear capsule

The stapes derives from Reichert's cartilage and the otic capsule

First described by Giovanni Filippo Ingrassia, a Sicilian anatomist, in Naples in 1546 (1546)

The stapes; he is usually credited with the first description

Key ideas

  • The footplate is much smaller than the drum. Concentrating the drum's vibration onto so small an area raises the pressure about seventeenfold, enough to overcome the resistance of the fluid in the inner ear.
  • Two small muscles protect the ear: tensor tympani stiffens the malleus and stapedius pulls the stapes out of the oval window, damping loud sound at the acoustic reflex.

Quick-fire facts

  • •About 3 mm tall, with a footplate about 3 mm long and 1.4 mm wide, and a mass of only a few milligrams
  • •Stapes is Latin for stirrup
  • •It is the smallest bone in the human body, and its muscle, the stapedius, is the smallest skeletal muscle
  • •The stapedius is supplied by the nerve to stapedius, a branch of the facial nerve (VII)
  • •Persistence of the embryonic stapedial artery into adult life is rare, found in well under one per cent of ears
  • •Otosclerosis, abnormal bone growth that fixes the footplate, is the classic cause of stapes fixation and is usually treated by stapedectomy or stapedotomy
  • •Samuel Rosen of New York pioneered mobilisation of the stapes in 1952; John Shea performed the first stapedectomy in 1956
  • •The acoustic (stapedial) reflex is triggered by loud sound, roughly 70–80 dB above the hearing threshold, and is a routine test of the facial and vestibulocochlear nerves

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