Stapes

The ''stapes'' or stirrup is a bone in the middle ear of humans and other tetrapods which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the footplate (or base) to transmit sound energy through the oval window into the inner ear. The ''stapes'' is the smallest and lightest bone in the human body, and is so-called because of its resemblance to a stirrup ().

Structure

The ''stapes'' is the third bone of the three ossicles in the middle ear and the smallest in the human body. It measures roughly , greater along the head-base span. It rests on the oval window, to which it is connected by an annular ligament and articulates with the ''incus'', or anvil through the incudostapedial joint. They are connected by anterior and posterior limbs ().

Development

The ''stapes'' develops from the second pharyngeal arch during the sixth to eighth week of embryological life. The central cavity of the ''stapes'', the ''obturator foramen'', is due to the presence embryologically of the stapedial artery, which usually regresses in humans during normal development.

Animals

The ''stapes'' is one of three ossicles in mammals. In non-mammalian tetrapods, the bone homologous to the ''stapes'' is usually called the columella; however, in reptiles, either term may be used. In fish, the homologous bone is called the hyomandibular, and is part of the gill arch supporting either the spiracle or the jaw, depending on the species. The equivalent term in amphibians is the '.

Variation

The ''stapes'' appears to be relatively constant in size in different ethnic groups. In 0.01–0.02% of people, the stapedial artery does not regress, and persists in the central foramen. In this case, a pulsatile sound may be heard in the affected ear, or there may be no symptoms at all. Rarely, the ''stapes'' may be completely absent.

Function

Situated between the incus and the inner ear, the ''stapes'' transmits sound vibrations from the incus to the oval window, a membrane-covered opening to the inner ear. The ''stapes'' is also stabilized by the stapedius muscle, which is innervated by the facial nerve.

Clinical relevance

Otosclerosis is a congenital or spontaneous-onset disease characterized by abnormal bone remodeling in the inner ear. Often this causes the ''stapes'' to adhere to the oval window, which impedes its ability to conduct sound, and is a cause of conductive hearing loss. Clinical otosclerosis is found in about 1% of people, although it is more common in forms that do not cause noticeable hearing loss. Otosclerosis is more likely in young age groups, and females. Two common treatments are stapedectomy, the surgical removal of the ''stapes'' and replacement with an artificial prosthesis, and stapedotomy, the creation of a small hole in the base of the ''stapes'' followed by the insertion of an artificial prosthesis into that hole. Surgery may be complicated by a persistent stapedial artery, fibrosis-related damage to the base of the bone, or obliterative otosclerosis, resulting in obliteration of the base.

History

The ''stapes'' is commonly described as having been discovered by the professor Giovanni Filippo Ingrassia in 1546 at the University of Naples, although this remains the nature of some controversy, as Ingrassia's description was published posthumously in his 1603 anatomical commentary '. Spanish anatomist Pedro Jimeno is first to have been credited with a published description, in (1549). The bone is so-named because of its resemblance to a stirrup (), an example of a late Latin word, probably created in mediaeval times from "to stand" (), as stirrups did not exist in the early Latin-speaking world.

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Auditory system
Skeletal system
Ossicles
Otorhinolaryngology
Otology
Human head and neck