The annular ligament (annulus fibrosus of the stapes) attaches the footplate of the stapes to the oval window (fenestra vestibuli). Otosclerosis preferentially involves which anatomical site of the temporal bone, and how does this impair conductive hearing?
- A Posterior wall of the external auditory canal; new bone compresses the tympanic membrane drum
- B Round window niche; occlusion prevents release of pressure wave from scala tympani
- C Petrous apex; demineralisation weakens the otic capsule causing sensorineural hearing loss only
- D Anterior margin of the oval window (fissula ante fenestram); spongy bone replaces normal endochondral bone, progressively immobilising the stapes footplate ✓
Explanation
Otosclerosis (otospongiosis) is a disease of the otic capsule in which normal lamellar bone is replaced by vascular spongy (woven) bone. The most common site is the fissula ante fenestram — a small fissure just anterior to the oval window, through which blood vessels pass. New spongiotic bone grows from this site to encase and fix the stapes footplate in the oval window, preventing its normal piston-like movement and causing progressive conductive hearing loss. If cochlear endosteum is involved (cochlear otosclerosis), sensorineural hearing loss also occurs. Stapedectomy/stapedotomy with prosthesis restores ossicular chain mobility.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.