What is the Sphenoid Bone
Sphenoid is one of the eight cranial bones that make up the cranium. It is a large, complex, unpaired bone, deriving its name from Greek ‘sphenoeides’, meaning wedge-shaped. The bone forms a major portion of the middle part of the skull base, and floor of the middle cranial fossa.
It is also called the “keystone” of the cranial floor since it is in contact with all of the other cranial bones.
Where is the Sphenoid Bone Located
It sits anteriorly in the cranium, just in front of the occipital bone.
|How many are there in the human body||1|
|Articulates with||Frontal, parietal, ethmoid, zygomatic, temporal, occipital, palatine, and vomer bones|
It forms the base and lateral sides of the skull in combination with the orbital floor.
Anatomy: Parts and Structure of the Sphenoid Bone
The sphenoid bone consists of a central body, two lateral paired wings on either side – lesser wings and greater wings, and two pterygoid processes. This unique anatomy gives the bone a prominent bat-, butterfly-, or wasp-like appearance.
As stated, the body is positioned centrally, and is almost completely cubical in shape. The body contains the sphenoidal sinuses, which are separated by a septum, opening into the nasal cavity. Due to the presence of these sinuses, the sphenoid body becomes a hollow structure.
The superior surface of the sphenoid body contains the following bony landmarks:
1. Chiasmatic groove: It is a narrow transverse groove or sulcus formed by the optic chiasm that lies near the superior surface of the body. The groove ends on either side in the optic foramen, which transmits the optic nerve and blood vessels into the orbital cavity.
2. Sella turcica: It is a saddle-shaped depression, having three parts:
i. Tuberculum sellae – This part forms the anterior wall of the sella turcica, and the posterior aspect of the chiasmatic groove.
ii. Hypophyseal fossa – It is a small depression of the sella turcica, housing the pituitary gland.
iii. Dorsum sellae – It is a depression that slopes back at the base of the skull, forming the posterior wall of the sella turcica.
The sella turcica is surrounded by the anterior and posterior clinoid processes that arise from the lesser wings, and dorsum sellae, respectively. The tentorium cerebella muscle attaches here.
The inferior surface presents a triangular spine in the middle line, the sphenoidal spine or rostrum, which is continuous with the sphenoidal crest on the anterior surface. It also has a deep fissure between the alae of the vomer. On either side of the rostrum is a projecting lamina, the vaginal process, directed medialward from the base of the medial pterygoid plate.
Two paired, triangular lesser wings arise from the front of the body of the sphenoid bone in a superolateral direction. It separates the anterior cranial fossa from the middle cranial fossa. These wings form the lateral border of the optic canal, through which the optic nerve and ophthalmic artery pass and reach the eye. The inferior surfaces of the wings form the lateral margin of the orbit while the superior surface forms part of the cranial cavity.
The two greater wings emerge behind the lesser wings, extending in a lateral, superior and posterior direction. They form the three parts of the facial skeleton: the floor of the middle cranial fossa, lateral wall of the skull, and posterolateral wall of the orbit.
Greater wings feature three foramina, which are as follows:
- Foramen rotundum: The maxillary nerve passes through this foramen.
- Foramen ovale: It allows the passage of the mandibular nerve, accessory meningeal artery, lesser petrosal nerve and emissary vein.
- Foramen spinosum: This foramen conducts the middle meningeal vessels, and a branch of the mandibular nerve.
Between the body, lesser and greater wings is a slit-like gap known as the superior orbital fissure where numerous nerves and vessels pass through to reach the bony orbit. These vessels include the superior ophthalmic vein, ophthalmic nerve and its branches, abducent nerve, oculomotor nerve, and trochlear nerve.
The two pterygoid processes descend inferiorly from the point of junction between the sphenoid body and the greater wings. Each process consists of a medial pterygoid plate and a lateral pterygoid plate. The lateral plate is wider and shorter than the medial and serves as the origin of the medial and lateral pterygoid muscles. At the inferior tip of the medial pterygoid plate is the small hook-shaped projection, the pterygoid hamulus.
The processes also contain two canals known as the pterygoid canal and palatovaginal canal.
i. Pterygoid canal: It contains the major petrosal nerve,and deep petrosal nerve.
ii. Palatovaginal or pharyngeal canal: It contains the pharyngeal nerve.
- The temporalis muscle, which serves as one of the muscles of mastication, is attached to the temporal surface of the greater wing of the sphenoid.
- The upper fibers of the lateral pterygoid muscle attach to the infratemporal surface of the greater wing of the sphenoid as well as the infratemporal crest. This muscle also attaches to the lateral aspect of the lateral plate of the pterygoid process.
- The medial pterygoid muscle is attached to the medial aspect of the lateral plate as well as the pterygoid fossa.
- The medial division of the pterygoid process, called the medial plate, serves as a site of attachment to the superior pharyngeal constrictor muscle.
It has articulations with twelve other bones:
With Paired Bones: Temporal, parietal, zygomatic and palatine bones.
With Unpaired Bones: Occipital, vomer, ethmoid and frontal bones.