Which Middle Ear Ossicle Is Attached To, And Transmits Vibratory Motion To, The Oval Window?

Introduction

The center ear functions to connect the sound waves from the external environment and transfer them to the inner ear for auditory transduction. The auditory ossicles (malleus, incus, and stapes) play a key office in this function. The malleus connects to the tympanic membrane transferring auditory oscillations to the incus and then the stapes. The stapes connects to the oval window assuasive for mechanical energy to exist transferred to the fluid-filled inner ear. Middle ear anatomy consists of intricate relationships between nerves, blood supply, and muscles. The ossicles play a role in multiple clinical scenarios, including otosclerosis, choleastoma, and facial nerve palsy.

Structure and Function

The ear structures are classically divided into 3 parts: the external ear, middle ear, and inner ear. The middle ear consists of the tympanic membrane and the bony ossicles called the malleus, incus, and stapes. These iii ossicles connect the tympanic membrane to the inner ear allowing for the transmission of audio waves.

The malleus is the get-go ossicle and attaches to and moves with the tympanic membrane. Information technology is divided into four parts: the caput, neck, short procedure, and handle. The handle of the malleus attaches loosely to the tympanic membrane. The head of the malleus attaches to the incus and forms the incudo-malleolar joint.[1] The incus is the 2d ossicle and is divided into the body, brusk process, and long process. The incus connects with the stapes and forms the incudo-stapedial joint. The stapes is the third ossicle and consists of the head, legs, and footplate. It is known as the smallest bone within the human body. The stapes articulates with the oval window of the inner ear.[1]

The chief office of the centre ear is to transmit the sound waves from the external environment to the inner ear. The sound waves initially make contact with the external ear and tympanic membrane. The tympanic membrane vibrates, leading to the movement of the chain of ossicles. The zipper of the footplate of the stapes to the oval window allows the displacement of fluid within the inner ear.

On boilerplate, the malleus has a height of virtually 8.0 millimeters and about 2.vii millimeters in width; the incus has most a height of 6.8 millimeters and a width of about 5.iii millimeters; the stapes has a elevation of most iii.five millimeters and a width of about two.4 millimeters. The stapes has an bending of approximately 10.vii degrees to the surface of the tympanic membrane. Incudostapedial joint (ISJ) is organized every bit a synovial joint, with the presence of a meniscus and intra-articular fluids, a capsule; the joint has the typical characteristics of a viscoelastic articulation. The incudomallear joint is a saddle articulation, which starts to move with sounds starting at 2 kHz.

The ossicular chain is held in its position by the connections of the malleus handle with the tympanic membrane, the annular ligament of the stapes, and the ligaments of the malleus and incus.

At that place are two muscles of the ossicular chain, namely the tensor muscle of the tympanum or malleus and the stapedius or stapes musculus.

Embryology

The three ossicles are derived from the neural crest of the outset and second pharyngeal arches during the sixth week of development. The malleus and incus are predominantly derived from the starting time curvation from a construction chosen Meckel's cartilage. The stapes is formed from the second arch from a structure called Reichert'due south cartilage. Interestingly, outer portions of the stapes footplate are derived from the mesoderm. Throughout the fetal catamenia, the ossicles fully congeal through a process of endochondral ossification. This process of ossification occurs initially with the incus at xvi weeks, malleus at 16-17 weeks, and the stapes at eighteen weeks. Ossification continues for upwards to 26 weeks.[2][3]

Blood Supply and Lymphatics

The anterior tympanic artery supplies the malleus, incus, and stapes. This artery branches off of the mandibular portion of the maxillary artery. The anterior tympanic avenue is also idea to supply portions of the tympanic membrane.[4]

The venous system of the tympanic cavity leads, superiorly, into the center meningeal veins and the superior petrosal sinus; inferiorly, in the bulb of the internal jugular vein and in the pterygoid, pharyngeal and internal carotid plexuses.

The lymphatic vessels of the tympanic cavity become to the parotid, retropharyngeal and mastoid lymph nodes.

Fretfulness

The tensor tympani muscle attaches to the malleus to prevent excessive oscillations produced past loud sounds. A co-operative of the mandibular nerve innervates the muscle. The mandibular nerve arises from the third co-operative of the trigeminal nerve. The stapedius is innervated by a branch of the facial nerve.

Other of import fretfulness within the centre ear are the chorda tympani and facial nervus. While these nerves are not directly involved with the ossicles, they have important anatomical relationships with the ossicles. The chorda tympani travels closely to the ossicular chain and is often stretched and damaged during surgery.[1]

Muscles

Inside the centre ear, there are two muscles that are integrated with the bony ossicles. The tensor tympani attaches to the malleus at the handle and functions to dampen the vibrations from the tympanic membrane. When the tensor tympani contracts, the malleus is moved medially, leading to the tympanic membrane becoming tauter. This functions to protect the ear from loud and dissentious sounds.

The other musculus, the stapedius, is located in the posterior portion of the tympanic membrane. It attaches to the stapes and functions to move the stapes posteriorly. Past serving this action, the stapes is restricted in its range of movement. Additionally, the stapedius muscle contracting tilts the base of operations of the stapes reducing the oscillatory range.[1]

Physiologic Variants

The anatomical human relationship between the malleus, incus, and stapes must exist intact for normal sound conduction. In the literature, at that place have been many variants of the ossicles.[v][6][vii] Out of the iii ossicles, the stapes is noted to be the most variable with respect to congenital abnormalities.[eight] In this written report, the incus was found to be the virtually stable with the least corporeality of anatomical variations establish.[8] The stapes was found to have variations in the shapes ranging from circular to triangular to oval. Additionally, cases with the absenteeism of the stapes ossicle and aplasia/hyperplasia were seen.

Ossicle variants are as well seen in certain built syndromes, including Teacher Collins syndrome. Acquired variants are as well seen with chronic conditions, including chronic otitis media.

Surgical Considerations

Heart ear surgery requires all-encompassing knowledge of the anatomical relationships between the tympanic membrane, ossicles, and inner ear. Surgical procedures are further complicated by the small-scale anatomical infinite and anatomical abnormalities. In improver, careful attention must be placed on the nerves and blood vessels that surround the ossicles. The chorda tympani nerve is closely associated with the malleus and is commonly injured during surgery. The facial nerve is institute in the posterior wall of the middle ear and runs inside the temporal bone. Surgical procedures, including ossiculoplasty and stapes surgery, require taking these considerations into account.[i]

Clinical Significance

Otosclerosis

Otosclerosis is defined every bit abnormal bony growth starting in the bony labyrinth and oval window moving toward the auditory ossicles leading to conductive hearing loss. The hardening of the structures results in loss of flexibility and ultimately prevents the stapes footplate from transmitting oscillations to the oval window. The pathologic process involves new bone formation and proliferation of the vasculature.

Histologically, increased osteoblasts and osteoclasts are seen with the presence of abnormal sclerotic bone.[9] Clinically, patients will present with hearing loss and, less usually, tinnitus and vertigo. A bulk of patients with otosclerosis will present with bilateral hearing loss. Evaluation of patients involves the Rinne and Weber tests, audiometry, and CT imaging. Handling can involve medical or surgical management. Medically, patients can be treated with bisphosphonates to preclude excessive bony growth. Surgically, patients can be treated with stapedotomy for patients who are refractory to medical management.[10]

Cholesteatoma

Choleastomas are benign growths of keratinized squamous epithelium that are commonly located within the eye ear. The aberrant growths can be either acquired or congenital. Choleastomas that abound chop-chop tin can damage the ossicles and lead to significant hearing loss. The location of the choleastoma plays a large role in which the ossicle may be damaged.[11] Treatment for these patients ordinarily involves surgery to remove the growth.

Facial Nerve Palsy

The stapedius muscle that attaches to the stapes is responsible for dampening sounds. In weather condition where the facial nervus is compromised, the stapedius muscle volition be nonfunctional. This leads to an increased range of movement of the stapes to sound, causing hyperacusis.[12]

Other Issues

From a functional point of view, the ossicular chain represents the near efficient system for transmitting sounds from the outer ear to the inner ear. The membrane of the eardrum (TM) vibrates and transmits its motion to the malleus, and this to the anvil and the incus to the stapes. When the TM moves medially (towards the hollow of the eardrum), the stapes also motion medially and deepen into the oval window; when the TM moves laterally (towards the external acoustic meatus), the stapes also moves in the same management. The displacements of the stapes are transmitted from its base of operations to the perilymph of the antechamber. However, the audio waves tin can also propagate to the inner ear by vibration of the air contained in the eardrum cavity due to the total vibration of the bone formations surrounding the cochlea.

Review Questions

Figure

The Auditory Ossicles, Left malleus, A; From behind, B; From inside. Contributed by Gray's Anatomy Plates

Effigy

The Auditory Ossicle, Left incus, A; From within, B; From the front. Contributed by Gray's Beefcake Plates

Figure

The Auditory Ossicles, A; Left stapes, B; Base of stapes, medial surface. Contributed by Grayness'south Anatomy Plates

Figure

The Auditory Ossicles, Chain of ossicles and their ligaments, seen from the front in a vertical; transverse department of the tympanum, Malleus, Incus, Crenel of Tympani, Stapes, Vestibules. Contributed by Gray's Beefcake Plates

References

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