The sphenoid bone is one of the eight bones that make up the cranium - the superior aspect of the skull that encloses and protects the brain. Its name is derived from the Greek ‘sphenoeides’, to mean wedge-shaped. In this article, we shall look at the anatomy of the sphenoid bone - its location, structure, and clinical significance.
The sphenoid bone is often said to be 'butterfly-shaped'. It consists of a body, paired greater and lesser wings, and two pterygoid processes. The body lies at the centre of the sphenoid bone, and is almost completely cubical in shape. It contains the sphenoidal sinuses, which are separated by a septum - meaning that the sphenoid body is essentially hollow. The body articulates with the ethmoid bone anteriorly, and it is here that the sinuses open up into the nasal cavity.
The superior surface of the sphenoid body contains some important bony landmarks - including the Sella turcica, a saddle-shaped depression. This has three parts - the Tuberculum sellae, Hypophyseal fossa, and the Dorsum sellae. The Sella turcica is surrounded by the anterior and posterior clinoid processes. The Tuberculum sellae forms the anterior wall of the Sella turcica, and the posterior aspect of the chiasmatic groove. The Hypophyseal fossa is the deepest part of the Sella turcica, where the pituitary gland is located. The Dorsum sellae forms the posterior wall of the Sella turcica, and the Chiasmatic groove is a sulcus formed by the optic chiasm (where the optic nerves partially cross).
The greater wing extends from the sphenoid body in a lateral, superior and posterior direction. It contributes to three parts of the facial skeleton - the floor of the middle cranial fossa, the lateral wall of the skull, and the posterolateral wall of the orbit. There are three foramina present in the greater wing – the foramen rotundum, foramen ovale and foramen spinosum. They conduct the maxillary nerve, mandibular nerve and middle meningeal vessels respectively.
The lesser wing arises from the anterior aspect of the sphenoid body in a superolateral direction. It separates the anterior cranial fossa from the middle cranial fossa. It also forms the lateral border of the optic canal, which is completed by the medial border formed by the body of the sphenoid. Between the lesser and greater wings of the sphenoid lies the superior orbital fissure, a 'slit-like' gap through which numerous structures pass to reach the bony orbit.
The pterygoid process descends inferiorly from the point of junction between the sphenoid body and the greater wing. It consists of two parts - the medial and lateral pterygoid plates. The medial pterygoid plate supports the posterior opening of the nasal cavity, while the lateral pterygoid plate is the site of origin of the medial and lateral pterygoid muscles.
The lateral and medial pterygoid muscles, which form some of the muscles of mastication, originate from the lateral pterygoid plate of the sphenoid bone.
The sphenoid is an unpaired bone. It sits anteriorly in the cranium, and contributes to the middle cranial fossa, the lateral wall of the skull, and the floor and sides of both orbits. It has articulations with twelve other bones - including four unpaired bones (the Occipital, Vomer, Ethmoid, and Frontal bones) and eight paired bones (the Temporal, Parietal, Zygomatic, and Palatine bones).
The sphenoid bone is an unpaired bone situated anteriorly in the cranium, with articulations to twelve other bones. It contributes to the facial skeleton, specifically the middle cranial fossa, the lateral wall of the skull, the floor, and both sides of the orbit. It is also an important site for surgical access to the pituitary gland.
The superior surface of the sphenoid body contains multiple important bony landmarks, including the Sella turcica - a saddle-shaped depression, that is divided into three parts: the Tuberculum sellae - which forms the anterior wall of the sella turcica, and the posterior aspect of the chiasmatic groove; the Hypophyseal fossa - the deepest part of the sella turcica, where the pituitary gland is located; and the Dorsum sellae - which forms the posterior wall of the sella turcica, and contains the Chiasmatic groove - a sulcus formed by the optic chiasm.
The sella turcica is surrounded by the anterior and posterior clinoid processes. The anterior clinoid processes arise from the sphenoidal lesser wings, and the posterior clinoid processes are the superolateral projections of the dorsum sellae, which serve as attachment points for the tentorium cerebelli, a membranous sheet that divides the brain.
The greater wing extends from the sphenoid body in a lateral, superior and posterior direction. It contributes to the facial skeleton by forming the floor of the middle cranial fossa, the lateral wall of the skull, and the postolateral wall of the orbit. Additionally, there are three foramina present in the greater wing: the foramen rotundum, foramen ovale and foramen spinosum. These allow the passage of the maxillary nerve, mandibular nerve and middle meningeal vessels respectively.
The lesser wing arises from the anterior aspect of the sphenoid body in a superolateral direction. It separates the anterior cranial fossa from the middle cranial fossa. It also forms the lateral border of the optic canal - through which the optic nerve and ophthalmic artery travel to reach the eye; the medial border of the optic canal is formed by the body of the sphenoid.
In between the lesser and greater wings of the sphenoid is a 'slit-like' gap known as the superior orbital fissure. Numerous structures pass through this gap to reach the bony orbit.
The pterygoid process descends inferiorly from the point of junction between the sphenoid body and the greater wing. It consists of two parts: the Medial pterygoid plate - which supports the posterior opening of the nasal cavity; and the Lateral pterygoid plate - the site of origin of the medial and lateral pterygoid muscles.
The lateral and medial pterygoid muscles, which are some of the muscles of mastication, originate from the lateral pterygoid plate of the sphenoid bone.
The sphenoid bone shares a close anatomical relationship with the pituitary gland, and its surgical access is known as endoscopic trans-sphenoidal surgery (ETSS). This type of surgery is the usual treatment of choice for pituitary adenomas, allowing for the management of pituitary pathology without the need for a more extensive craniotomy.
In transsphenoidal surgery, the surgeon passes instruments through the sphenoid bone and sinus to gain access to the pituitary gland. This method can safely remove tumors from the pituitary, restore hormone levels, relieve pressure on the optic nerves, and reduce the symptoms of pituitary adenomas.
The success rate for transsphenoidal surgery is very high, with most patients able to make a full recovery without any major complications. In some cases, the pituitary gland may be damaged and hormone replacement therapy may be necessary, but this is rare.
The sphenoid bone is an incredibly important structure for facial anatomy and pituitary surgery. It also plays a role in the support and protection of the eyes, optic nerve, and other vital structures of the brain. While it is a key site for endoscopic transsphenoidal surgery, its importance should not be overlooked.
