What is the sphenoid bone?
The sphenoid bone (os sphenoidale) is a central bone of the skull that is roughly shaped like a flying wasp with outstretched wings and drooping legs: It consists of the sphenoid body (corpus), two large sphenoid wings (alae majores), two small sphenoid wings (alae minores) and the downward-pointing wing-like projections (processus pterygoidei).
The body of the sphenoid bone (corpus)
The sphenoid body (corpus) has a roughly cube-like shape. Inside are two cavities separated by a septum, which are known as sphenoid sinuses.
The posterior surface of the sphenoid body forms a connection (initially cartilaginous, later bony) with the occipital bone.
The upper surface of the sphenoid body forms the so-called Turkish saddle (sella turcica) in the posterior area, in which the pituitary gland (hypophysis) is located. In the upper front area there is a connection to the ethmoid bone via a bony spine. In addition, a small, flat piece of bone (jugum sphenoidale) connects the two small wings of the sphenoid bone in front of the sulcus chiasmatis, in which the optic chiasm is located. The paired optic nerve (nervus opticus) runs through an opening in the bone, together with its artery.
The lower surface of the sphenoid body has a beak-like bony ridge pointing vertically downwards, the rostrum sphenoidale, which is surrounded by the wings of the plow bone and merges into the nasal septum.
The large sphenoid wings (Alae majores)
The large sphenoid wings are strong bony processes on the sides of the sphenoid body that are curved outwards and upwards. They have four surfaces, four edges and one angle.
The surfaces of the large sphenoid wings are called:
- Facies cerebralis (points upwards towards the brain)
- Facies temporalis (on the outer surface of the skull and separated from the underlying facies infratemporalis of the upper jaw by a bony ridge)
- Facies orbitalis (borders the eye socket with a flat, smooth bone surface)
- Facies maxillaris (directly under the facies orbitalis; represents the border to the maxilla)
The facies maxillaris contains the foramen rotundum – a round opening through which the second branch of the trigeminal nerve (a facial nerve) passes.
The edges of the large sphenoid wings are called:
- Margo frontalis (borders the frontal bone)
- Margo zygomaticus (borders the zygomatic bone)
- Margo parietalis (borders the parietal bone)
- Margo squamosus (adjacent to the temporal bone)
The small sphenoid wings (Alae minores)
The small sphenoid wings are thin, triangular bone plates that sit at the top front of the sphenoid body. They form the optic canal, through which the optic nerve passes from the cranial cavity into the eye socket. Their lower surface delimits the eye socket and their upper surface delimits the cranial cavity. Towards the middle and back they form short bony projections.
The wing process of the sphenoid bone
The pterygoid process is the name given by doctors to the wing-like projections that extend almost vertically downwards from the base of the large sphenoid wings on the body of the sphenoid bone. They consist of two bone plates, the lamina medialis (central plate) and the lamina lateralis (lateral plate).
Between these two is a fossa, the pterygoid fossa (wing palate fossa). The posterior part of this fossa is formed by the wing process of the sphenoid bone, the plate of the palatine bone stands vertically, and the anterior part is formed by the maxilla.
The base of the wing processes is pierced by a vascular-nerve canal, this pit is a central distribution point for vessels and nerves.
The medial lamella has a hook-shaped projection at the lower end. The tendon of the muscle that stretches the palate runs here.
The sphenoid sinus
What is the function of the sphenoid bone?
Like the other bones of the skull, the sphenoid bone serves to protect the brain and as an attachment point for various muscles (such as the muscles of mastication). It forms the rear section of the eye socket and – together with other bones – the base of the skull.
The function of the sphenoid sinus and the other paranasal sinuses is not yet fully understood. Presumably, the air-filled cavities reduce the weight of the skull and serve as a resonance chamber for the voice.
Where is the sphenoid bone located?
The sphenoid bone is the central bone of the skull and lies wedge-shaped between all the other bones of the skull, in front of the occipital bone (os occipitale) in the middle of the base of the skull. Until adolescence, the sphenoid bone is only connected to the occipital bone by cartilage; only in adults is there a bony connection.
What problems can the sphenoid bone cause?
Inflammation of the sphenoid sinuses is relatively rare. As they are connected to the upper nasal concha, viruses and bacteria that cause sinusitis can also lead to an infection here. There is a pressure-like pain in the back of the head and in the crown of the head because secretions accumulate in the cavity and exert pressure. This is accompanied by a cold and fever.
Extensive inflammation occasionally leads to an abscess or empyema (accumulation of pus) in the sphenoid sinus.
A sphenoid wing meningioma is a benign tumor of the temporal brain that spreads into the small wing of the sphenoid bone. It can also spread into the orbit or the palpebral fossa, leading to visual disturbances and nerve paralysis.
A fracture of the occipital bone can also involve the sphenoid bone.
