Anatomy, Head and Neck, Ear Ossicles


Introduction

The middle 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 role 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 allowing for mechanical energy to be 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 three 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 three ossicles connect the tympanic membrane to the inner ear allowing for the transmission of sound waves.

The malleus is the first ossicle and attaches to and moves with the tympanic membrane. It is divided into four parts: the head, neck, short process, 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 second ossicle and is divided into the body, short 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 main function of the middle 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 attachment of the footplate of the stapes to the oval window allows the displacement of fluid within the inner ear.

On average, the malleus has a height of about 8.0 millimeters and about 2.7 millimeters in width; the incus has about a height of 6.8 millimeters and a width of about 5.3 millimeters; the stapes has a height of about 3.5 millimeters and a width of about 2.4 millimeters. The stapes has an angle of approximately 10.7 degrees to the surface of the tympanic membrane. Incudostapedial joint (ISJ) is organized as a synovial joint, with the presence of a meniscus and intra-articular fluids, a capsule; the joint has the typical characteristics of a viscoelastic joint. The incudomallear joint is a saddle joint, 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.

There are two muscles of the ossicular chain, namely the tensor muscle of the tympanum or malleus and the stapedius or stapes muscle.

Embryology

The three ossicles are derived from the neural crest of the first and second pharyngeal arches during the sixth week of development. The malleus and incus are predominantly derived from the first arch from a structure called Meckel's cartilage. The stapes is formed from the second arch from a structure called Reichert's cartilage. Interestingly, outer portions of the stapes footplate are derived from the mesoderm. Throughout the fetal period, the ossicles fully ossify through a process of endochondral ossification. This process of ossification occurs initially with the incus at 16 weeks, malleus at 16-17 weeks, and the stapes at 18 weeks. Ossification continues for up 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 artery is also thought to supply portions of the tympanic membrane.[4]

The venous system of the tympanic cavity leads, superiorly, into the middle 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 go to the parotid, retropharyngeal and mastoid lymph nodes.

Nerves

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

Other important nerves within the middle ear are the chorda tympani and facial nerve. 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

Within the middle 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 damaging sounds.

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

Physiologic Variants

The anatomical relationship between the malleus, incus, and stapes must be intact for normal sound conduction. In the literature, there have been many variants of the ossicles.[5][6][7] Out of the three ossicles, the stapes is noted to be the most variable with respect to congenital abnormalities.[8] In this study, the incus was found to be the most stable with the least amount of anatomical variations found.[8] The stapes was found to have variations in the shapes ranging from circular to triangular to oval. Additionally, cases with the absence of the stapes ossicle and aplasia/hyperplasia were seen.

Ossicle variants are also seen in certain congenital syndromes, including Teacher Collins syndrome. Acquired variants are also seen with chronic conditions, including chronic otitis media.

Surgical Considerations

Middle ear surgery requires extensive knowledge of the anatomical relationships between the tympanic membrane, ossicles, and inner ear. Surgical procedures are further complicated by the small anatomical space and anatomical abnormalities. In addition, 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 found in the posterior wall of the middle ear and runs within the temporal bone. Surgical procedures, including ossiculoplasty and stapes surgery, require taking these considerations into account.[1]

Clinical Significance

Otosclerosis

Otosclerosis is defined as 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 commonly, tinnitus and vertigo. A majority of patients with otosclerosis will present with bilateral hearing loss. Evaluation of patients involves the Rinne and Weber tests, audiometry, and CT imaging. Treatment can involve medical or surgical management. Medically, patients can be treated with bisphosphonates to prevent 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 middle ear. The abnormal growths can be either acquired or congenital. Choleastomas that grow rapidly 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 commonly involves surgery to remove the growth.

Facial Nerve Palsy

The stapedius muscle that attaches to the stapes is responsible for dampening sounds. In conditions where the facial nerve is compromised, the stapedius muscle will be nonfunctional. This leads to an increased range of motion of the stapes to sound, causing hyperacusis.[12]

Other Issues

From a functional point of view, the ossicular chain represents the most efficient system for transmitting sounds from the outer ear to the inner ear. The membrane of the eardrum (TM) vibrates and transmits its movement 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 move medially and deepen into the oval window; when the TM moves laterally (towards the external acoustic meatus), the stapes also moves in the same direction. The displacements of the stapes are transmitted from its base to the perilymph of the vestibule. However, the sound waves 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.



(Click Image to Enlarge)
The Auditory Ossicles, Left malleus, A; From behind, B; From within
The Auditory Ossicles, Left malleus, A; From behind, B; From within
Contributed by Gray's Anatomy Plates

(Click Image to Enlarge)
The Auditory Ossicle, Left incus, A; From within, B; From the front
The Auditory Ossicle, Left incus, A; From within, B; From the front
Contributed by Gray's Anatomy Plates

(Click Image to Enlarge)
The Auditory Ossicles, A; Left stapes, B; Base of stapes, medial surface
The Auditory Ossicles, A; Left stapes, B; Base of stapes, medial surface
Contributed by Gray's Anatomy Plates

(Click Image to Enlarge)
The Auditory Ossicles, Chain of ossicles and their ligaments, seen from the front in a vertical; transverse section of the tympanum, Malleus, Incus, Cavity of Tympani, Stapes, Vestibules
The Auditory Ossicles, Chain of ossicles and their ligaments, seen from the front in a vertical; transverse section of the tympanum, Malleus, Incus, Cavity of Tympani, Stapes, Vestibules
Contributed by Gray's Anatomy Plates
Article Details

Article Author

Tom George

Article Editor:

Bruno Bordoni

Updated:

5/4/2021 2:40:05 PM

References

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