With the exception of the cervical vertebrae, the hyoid bone is the only bone located in the anterior neck. Unlike other bony structures, the hyoid bone does not directly articulate with other bones. Instead, it is connected to neighbouring bones by muscular and ligamentous attachments. Muscles that insert on the upper surface of the bone are known as suprahyoid muscles, while those attached to the lower surface are infrahyoid muscles. This article will explore the embryology, anatomy and muscular attachments of this structure.
The hyoid bone is a U-shaped bone that is held in place by the strap muscles of the anterior triangle of the neck. The bone has a central body (forming the center of the “U”) with two smaller protruding structures on the superior surface (lesser horns) and two larger bony protrusions from the body (greater horns).
The body is quadrilateral, laterally stretched and irregular in shape. It forms the convexity of its classical U-shape, with its outer (anterior) border forming the outer convexity and the inner (posterior) border forming the concavity. There is a vertical median ridge located in the midline of the body that rarely projects to the lower surface. The body is situated at an oblique angle with the anterior end pointing superiorly and the posterior end pointing inferiorly. Immediately posterior to the body of the hyoid bone is a bursa, thyrohyoid membrane and areolar tissue that separates the hyoid bone from the epiglottis.
The greater cornua (horns) extend from the lateral extremity of the body in a posterolateral direction. They are wider proximally and become increasingly narrow until they terminate in tubercles. They are also flattened horizontally.
At the junction of the body and each greater cornu is a conical protruding bone known as the lesser cornua (horns). Fibrous tissue connects the lesser cornua to the body of the hyoid bone. There are also instances where each lesser cornu articulates with the ipsilateral greater cornu by way of a synovial joint that ossifies in later decades of life.
Embryology and Ossification
The second and third pharyngeal arches give rise to the horns of the hyoid bone. The greater horn arises from the third arch, while the lesser horn originates from the second arch. Fusion of both cornua results in the formation of the body of the hyoid. Chondrocytes begin the process of making the tissue cartilaginous (chondrification) by the fifth week of intrauterine life. The chondrification process usually ends between the third to fourth gestational months. The greater cornua are the first to begin ossifying (becoming bone) with two ossification centres – one in each cornu – closer to the end of intrauterine life. The body of the hyoid bone (also with two ossification centres) follows shortly after in the perinatal (shortly before or after birth) period. The lesser cornua are the last to begin ossifying during the peripubescent period. Like the other parts of the hyoid bone the lesser cornua have two ossification centres, giving a total of six ossification centres. The apical part of the greater cornua may not complete ossification until the patient is in their twenties.
The hyoid bone has superior and inferior muscular attachments that keep it in place in the anterior region of the neck. There are one membranous and two ligamentous attachments to the bone. The thyrohyoid membrane is an extrinsic membrane of the larynx that connects the superior border of the thyroid lamina and its superior horns to the superior aspect of the greater cornua and body of the hyoid bone. It accesses the superior surface of the hyoid bone by passing behind the bone and the bursa. The thyrohyoid membrane functions as the lateral walls of the piriform recess. The internal laryngeal nerve, along with the superior laryngeal arteries and veins, also pierce the membrane. Its primary responsibility is anchoring the laryngeal skeleton to the hyoid bone.
The hyoepiglottic ligaments are one of several extrinsic ligaments of the larynx. The bilateral ligaments are collections of fibrous tissue that are responsible for attaching the anterior and lateral aspects of the epiglottis to the body and greater cornu of the hyoid bone. Consequently, they give rise to the glossoepiglottic folds that surround the valleculae (depressions at the root of the tongue).
The stylohyoid ligament commences at the apex of the styloid process and inserts in the lesser cornu of the hyoid bone. It is crossed inferiorly by hyoglossus. The fibrous ligament, which is closely related to the lateral oropharyngeal wall, provides a point of attachment for the middle pharyngeal constrictors and some fibers of styloglossus. The ligament is a derivative of the second branchial arch and may be calcified.
Genioglossus is an extrinsic muscle of the tongue that has its origins from a short tendon at the superior genial tubercle of the mandible (near the symphysis menti). There are three sets of fibers that make up genioglossus:
- Inferior fibers attach to the anterosuperior surface of the body of the hyoid bone, adjacent to the midline via a thin aponeurotic process
- Intermediate fibers extend posteriorly to the posterior aspect of the tongue
- Superior fibers traverse the entire ventral surface of the tongue from the base to its apex
The muscle is innervated by the hypoglossal nerve (CN XII) and produces forward protrusion of the tongue. Bilateral contraction generates a concave appearance of the tongue, with the midline depressed. Unilateral contraction produces contralateral deviation of the tongue.
Hyoglossus is a quadrilateral muscle originating from the anterior body of the hyoid bone and its greater cornu. The thin muscle passes between styloglossus and stylohyoid to enter the side of the tongue. Hyoglossus is able to depress the tongue through motor supply from CN XII.
Mylohyoid is a thin pair of diaphragm-like muscle that forms the floor of the mouth. Each mylohyoid muscle has its origin at the mylohyoid line. This line commences at the third molar tooth and continues towards the mental spines on the inner surface of the mandible. They interdigitate in the midline raphe anteriorly and posteriorly, they insert into the anterior part of the body of the hyoid bone. The mylohyoid branch of the inferior alveolar division of the mandibular arm of the trigeminal nerve (CN V3) innervates the muscle. The muscle elevates the floor of the mouth and raises the hyoid bone during swallowing.
Geniohyoid originates at the genial tubercle (inferior mental spine) of the mandible. The slender muscle continues inferiorly to insert on the anterior surface of the body of the hyoid bone. The bilaterally paired muscle lies between mylohyoid and genioglossus (base of the tongue). C1 fibers of the hypoglossal nerve (CN XII) innervate the muscle. It acts by protracting and elevating the hyoid during swallowing. If the hyoid bone is fixed, then the muscle acts by depressing the mandible.
Stylohyoid commences its journey at the base of the styloid process at the base of the cranium. It takes an anteroinferior route along the superior border of the digastric muscle. The fleshy belly of the muscle progresses to a tendinous structure that bifurcates around the tendon of the digastric muscle. Both slips of the bifurcated tendon insert at the junction of the greater cornu and body of the hyoid bone. A branch of the facial nerve provides motor innervation to the muscle that allows it to retract and elevate the hyoid bone during swallowing.
The digastric muscle has a posterior and an anterior belly. The posterior belly originates in the digastric groove on the medial aspect of the mastoid process. It travels anteroinferiorly toward its insertion in the intermediate tendon at the junction of the greater cornu and body of the hyoid bone. The anterior belly continues from the intermediate tendon, courses deep to mylohyoid and inserts in the digastric fossa of the posterior surface of the mandible. The muscle is able to depress and retract the chin and facilitate opening of the mouth through innervation from the facial nerve (posterior belly) and the nerve to mylohyoid (anterior belly).
The middle constrictor of the pharynx has several attachments. It is attached posteriorly to the median pharyngeal raphe. Anteriorly, the fibers are connected to the greater and lesser cornua and the distal stylohyoid ligament. The inferior fibers integrate with those of the inferior constrictor of the pharynx, while the superior fibers overlap with the fibers of the superior constrictor. The cranial part of the spinal-accessory nerve (CN XI), via the pharyngeal plexus, innervates the middle constrictor. This muscle is responsible for constricting the middle part of the pharynx during swallowing.
A flat strap muscle, known as omohyoid, arises from the inferolateral surface of the hyoid bone. It takes an inferolateral course deep to sternocleidomastoid and gradually becomes tendinous as it passes over the internal jugular vein (within the carotid sheath). The previously described segment of the muscle constitutes the superior belly of omohyoid. The inferior belly continues from the intermediate in a horizontal direction to insert in the transverse scapular ligament and the superior border of the scapula. The upper root of ansa cervicalis (C1) innervates the superior belly, while C1, C2 and C3 innervate the inferior belly.
Sternohyoid is another strap muscle coursing from the manubrium sterni and sternoclavicular joint to insert in the inferior surface of the hyoid bone. Innervation to the muscle is via ansa cervicalis.
The primary role of the hyoid bone is to provide stability to adjacent structures via the attached muscles. It also serves as a surgical landmark when approaching thyroglossal cysts and the tongue base. Furthermore, it plays an integral role in movement of the upper airway and depression of the hyoid has been implicated in obstructive sleep apnoea. It has therefore been suggested as a possible therapeutic point in the surgical management of the disease.