Osselets

Osselets, or auditory ossicles, are three tiny bones in the middle ear that transmit and amplify sound from the eardrum to the inner ear. The malleus (hammer), incus (anvil), and stapes (stirrup) form a connected chain that links the tympanic membrane to the oval window of the inner ear. This arrangement converts air vibrations into fluid movements within the cochlea and enhances sound pressure through a lever and impedance-matching mechanism.

The ossicles reside in the air-filled middle ear, held in place by ligaments and moved by two small muscles, the tensor tympani and the stapedius, which can dampen vibrations to protect the inner ear from loud sounds. Their articulations allow a precise transfer of mechanical energy from the eardrum to the inner ear.

Development and evolution: In mammals, the malleus and incus are derived from the jaw bones of reptilian ancestors (articular and quadrate), while the stapes originates from elements of the hyoid/anterior gill arch. This evolution expanded the efficiency of airborne sound transmission. Other vertebrates exhibit different middle-ear configurations, but all rely on some mechanism to convey sound to the inner ear.

Clinical relevance: Disruption or calcification of the ossicles can cause conductive hearing loss. Otosclerosis, a common condition, involves abnormal bone growth around the stapes, reducing its mobility. Trauma, infection, or congenital abnormalities can also impair ossicular function. Treatments range from hearing aids to surgical restoration of the chain.

Diagnosis and treatment: Evaluation typically includes audiometry and tympanometry, with imaging in complex cases. Surgical options include stapedotomy or stapedectomy to bypass or replace a stiff stapes, and ossiculoplasty to reconstruct or replace parts of the ossicular chain.