Sphenoid Bone

The sphenoid bone has a complex shape that is often compared with a butterfly having two wings on each side, a central body and a couple of extensions inferiorly that might be considered the legs. It is the central bone of the cranium and thus articulates with all of the cranial bones and in addition articulates with the zygomatic, vomer and palatine bones of the facial skeleton. It also has many foramina to allow the passage of nerves and blood vessels. It is one of seven bones found in the orbit and makes up much of the posterior wall of that structure. In addition, the sphenoid contains one of the paranasal sinuses.

Parts – As mentioned above the sphenoid bone has a very complex shape. The centrally located body of the sphenoid contains the sphenoid paranasal sinus. It also has an indentation on the cranial surface that encases the hypophysis (also known as the pituitary gland). This indentation, known collectively as the sella turcica consists of three sections, a raised part at the anterior end, the tuberculum sellae; a depression in the middle where the gland sits, the hypophysial fossa; and a raised part posteriorly known as the dorsum sellae.

Starting at the centrally located body the lesser wings project outwards to from the posterior part of the anterior cranial fossa. The greater wings are found in the middle cranial fossa.

Figure 12.

Figure 12.

Figure 13.

Figure 13.

Figure 14.

Figure 14.

Openings – There are a number of openings in the sphenoid bone. Three of them are associated with passage of the three roots of the trigeminal nerve. The first division, the ophthalmic nerve, exits through the superior orbital fissure along with cranial nerves II, IV and VI. The second division, the maxillary nerve, passes through the foramen rotundum. The third division of the trigeminal, the mandibular nerve, passes through the foramen ovale. Another important opening is the optic canal which allows for passage of the optic nerve which carries visual information from the retina to the brain.

Figure 15.

Figure 15.

There are also a number of openings that lie at the junction of the sphenoid bone and other bones. The largest of these is the inferior orbital fissure found where the sphenoid, the maxilla, the palatine bone and the zygomatic bone meet. This does not open into the cranial cavity as the other openings do but rather connects the orbit to the pterygopalatine fossa. Structures that pass through there include the maxillary nerve, artery and vein which change name at the border to the infraorbital nerve, artery and vein.

The foramen lacerum is found at the junction point of the sphenoid, temporal and occipital bones. This is an odd foramen as it is closed superiorly by the carotid artery which passes directly over it and is mostly sealed with connective tissue. Only minor blood vessels and a small nerve pass through it despite its size. The carotid continues over the sphenoid bone and forms the carotid groove just anterior to the foramen lacerum.

Muscle attachments – The lateral pterygoid plate plays an important role in the stomatognathic system being an attachment point for both the lateral pterygoid muscle and confusingly enough the medial pterygoid muscle also. There are four muscles of mastication so these two muscles comprise half of the muscles of mastication.