The Foramen Lacerum is a significant feature located at the base of the skull. It is an irregular, triangular opening situated in the floor of the middle cranial fossa. The name, derived from Latin, translates to “lacerated piercing,” reflecting its jagged edges. This bony defect is formed at the junction of three major cranial bones, serving as a landmark in skull base anatomy.
Anatomical Context and Boundaries
This opening is positioned within the middle cranial fossa, a depression in the base of the skull that houses several important neurovascular structures. The Foramen Lacerum is not a simple hole through a single bone, but rather a gap formed where three bones meet. It is situated directly anteromedial to the carotid canal, which is the bony tunnel that transmits the Internal Carotid Artery.
The boundaries of the foramen are defined by the convergence of the sphenoid bone, the temporal bone, and the occipital bone. The anterior border is formed by the body of the sphenoid bone. The posterolateral boundary is the apex of the petrous part of the temporal bone.
The posteromedial edge is defined by the basilar part of the occipital bone, often referred to as the clivus. This arrangement places the foramen at the intersection of the sphenopetrosal and petroclival sutures. The foramen typically measures approximately 7 to 9 millimeters in length and 5 to 7 millimeters in width, though its irregular shape makes precise measurement difficult.
Primary Structures Traversing the Area
The question of what passes through the Foramen Lacerum has a nuanced answer due to its unique anatomical structure. The Internal Carotid Artery (ICA) is often associated with the foramen but does not pass through it. Instead, the ICA runs along its superior aspect as it emerges from the carotid canal.
This segment, known as the lacerum segment, lies in a shallow groove formed by the bony edges, covered by a fibrous membrane above the foramen. The artery is merely passing over the opening, not through the tissue that fills it. The sympathetic nerve plexus accompanying the ICA also traverses this superior pathway as the artery makes its sharp turn toward the cavernous sinus.
The most significant structures directly related to the foramen form the nerve of the pterygoid canal, also known as the Vidian nerve. The greater petrosal nerve and the deep petrosal nerve meet and merge within the vicinity of the foramen. The greater petrosal nerve carries parasympathetic fibers, while the deep petrosal nerve carries sympathetic fibers.
After their union, the resulting nerve of the pterygoid canal then passes forward through its own separate bony channel, the pterygoid canal, which opens near the anterior boundary of the Foramen Lacerum. The artery of the pterygoid canal also travels along this path.
The Cartilage Filling and Functional Significance
The distinct anatomical feature of the Foramen Lacerum in an adult is that it is largely filled with a plug of fibrocartilage. This cartilaginous filling means that the bony opening is not an open passageway connecting the cranial cavity to the base of the skull, unlike many other cranial foramina. This unique occlusion is the primary reason why major neurovascular structures do not simply pass through the space.
In an anatomical sense, this structure is more accurately described as a synchondrosis, or a cartilaginous joint, rather than a true foramen. The cartilage plug develops after birth and acts as a barrier, effectively isolating the cranial cavity.
Small, minor structures do pierce the fibrocartilage plug to connect the intracranial and extracranial spaces. These include small meningeal branches of the ascending pharyngeal artery, which supply the dura mater. Emissary veins also traverse the cartilage, connecting the cavernous sinus with the pterygoid venous plexus.
These small emissary veins can be clinically significant because they provide a potential route for the spread of infection from the extracranial tissues into the cranial cavity. The Foramen Lacerum is also a notable landmark for skull base surgeons and can serve as a portal for the extension of certain tumors into the cranial cavity.