24 Microsurgical Anatomy of the Middle Fossa and Petrous Apex
Abstract
The middle cranial fossa represents an anatomical challenge for neurosurgeon because of its neurovascular relationships such as the cavernous sinus.
Throughout history, there has been countless publications and studies with changes in the term or description of this region; nonetheless, the understanding of the neuroanatomy and the adjacent structures of the middle fossa and parasellar region has been critical for the evolution of neurosurgical approaches.
We describe some of the landmarks and key aspects of the middle fossa, its correlation with surgical approaches, and the importance of pathologies of this area.
Keywords: middle cranial fossa, parasellar region, petrous apex, microsurgical anatomy
24.1 Background
The microsurgical anatomy of the middle cranial fossa and its medial portion known as the parasellar region has been extensively described throughout the history.
The parasellar compartment was described initially by Claudius Galen who labeled it as a “parasellar carotid retia bathed in venous blood,” a description that persisted for more than 15 centuries.1 In 1684, Raymond de Vieussens, in the Neurographia Universalis publication, described the parasellar area as a “receptacle” for the cranial nerves that entered the “Turkish saddle.” In 1695, Humphrey Ridley published The Anatomy of the Brain Containing Its Mechanisms and Physiology, in which he described the circular sinus and its communications with the parasellar compartment and the internal carotid artery (ICA), a description that excluded any presence of other tissues.
It was with Jacobus Benignus Winslow’s description of the lateral sinuses as “a spongy or cavernous substance full of blood, much like that of the spleen or corpus cavernosum”, that the term cavernous sinus was coined.2
Throughout history, there has been countless publications and studies with changes in the term or description of this region; nonetheless, the understanding of the neuroanatomy and the adjacent structures of the middle fossa and parasellar region has been critical for the evolution of neurosurgical approaches.
24.2 Anatomy
24.2.1 Endocranial Surface and Its Limits
To better describe the middle fossa, we must recognize its limits. Rhoton divided the middle fossa into two portions: lateral and medial. The lateral portion contains the middle cranial fossa and the upper surface of the temporal bone; the medial portion corresponds to the sellae and parasellar region, where the pituitary gland and cavernous sinus reside3 (Fig. 24.1).
Fig. 24.1Rhoton’s division of the middle fossa as a lateral portion (green) and a medial portion (blue).
Anteriorly, the middle fossa is limited by the lesser sphenoid’s wing, forming medially the roof of the optic canal and the optic strut. These two structures connect the lesser wing with the body of the sphenoid bone and will form the superior and medial margins of the superior orbital fissure (SOF) (Fig. 24.2).
Fig. 24.2Anatomical description of the bone components of the middle fossa and its margins. ACP, anterior clinoidal process; FO, foramen ovale; FR, foramen rotundum; MMA, middle meningeal artery; PCP, posterior clinoidal process; SOF, superior orbital fissure.
The posterior limit of the middle fossa is formed by the superior border of the petrous portion of the temporal bone, in which the impression of the trigeminal nerve and the groove of the superior petrosal sinus can be identified (Fig. 24.3).
Fig. 24.3Anatomical description of the posterior limit of the middle fossa in which the petrous apex can be identified with the impression of the fifth nerve and the groove of the superior petrosal sinus. FO, foramen ovale; IAC, internal auditory canal; PCP, posterior clinoidal process; SPS, superior petrosal sinus.
The lateral limit is formed by the endocranial surface of the squamous portion of the temporal bone and its connection with the sphenoidal wing, which will form the margins of the superior orbital fissure.3 The medial margin of the middle fossa can be described categorically as the pituitary fossa sellae and parasellar regions.
The endocranial surface of the middle fossa is marked by the impression of the middle meningeal artery (MMA), the impression of the gyrus of the temporal bone, and the different foramens that compose the medial aspect of the middle fossa (Fig. 24.4).
Fig. 24.4Endocranial surface of the middle fossa with the impression of the temporal lobe gyri and the middle meningeal artery (*).
24.2.2 The Foramens of the Middle Fossa
As previously described, the parasellar region represents a challenging anatomical landmark for the neurosurgeon and it has been the focus of attention of several studies throughout history.2 A key component of this region is its different foramens and the different structures that travel through them. We describe these foramens from an anterior to posterior fashion as depicted on a surgical approach.
Meningo-orbital Band and the Superior Orbital Fissure
When approaching the middle fossa, the more anterior and lateral foramen is the superior orbital fissure (SOF). The SOF is delimited by the margins of the greater and lesser sphenoidal wings, forming a triangular aperture, and at its more lateral aspect the meningo-orbital band (MOB) can be identified (Fig. 24.5).
Fig. 24.5Cadaveric dissection of the superior orbital fissure by removal of the roof and lateral wall of the orbit. The meningo-orbital band can be identified as the fusion of periosteal dura of the sphenoidal wings.
The MOB represents the fusion of the periosteal dura from the sphenoidal wings and the periorbital periosteum. It contains the meningo-orbital artery and some dural veins. By dissecting the MOB, the SOF can be visualized exposing the anterior clinoid process.
The SOF can be identified by its triangular form, where the annular tendon (AT) attaches at its medial border. The AT divides the SOF into three compartments: (1) lateral—containing the trochlear, frontal, and lacrimal nerves and the superior ophthalmic vein; (2) central—containing oculomotor, nasociliary, and abducens nerves and ciliary ganglion; and (3) inferior—containing the inferior ophthalmic vein4 (Fig. 24.6).
Fig. 24.6Components of the superior orbital fissure divided by its three portions: lateral with the trochlear nerve, frontal nerve, and the superior ophthalmic vein; central containing the two components of the oculomotor nerve, the abducens, and nasociliary nerves and the ciliary ganglion; and the inferior containing the inferior ophthalmic vein.
Foramen Rotundum
Approximately 2 to 3 mm posterior and lateral to the SOF, the foramen rotundum (FR) can be identified. The FR is the exit gate of the maxillary branch of the trigeminal nerve (V2). It is located medially, below the SOF, and lateral to the sella turcica and the anterior clinoidal process. As its name implies its diameters anteroposteriorly and lateromedial are mostly equal, giving its name5 ( Fig. 24.7a, b).
Fig. 24.7 (a) Endocranial surface of the middle fossa demarcating the foramen rotundum (FR). (b) Cadaveric dissection of the middle fossa, demarcating the FR (red circle) as the exit point of the second branch of the fifth nerve (V2). III, third nerve; IV, forth nerve; ICA, internal carotid artery; ON, optic nerve; SOF, superior orbital fissure; V1, ophthalmic nerve; V2, maxillary nerve; V3, mandibular nerve.
Foramen Ovale
Situated posterior to the FR, the mandibular branch of the trigeminal nerve (V3) runs through the foramen ovale (FO) accompanied in some cases by the accessory middle meningeal artery, lesser petrosal nerve, and small emissary veins6 (Fig. 24.8).
Fig. 24.8Endocranial surface of the middle fossa demarcating the foramen ovale (red circle).
Foramen Spinosum
The foramen spinosum (FS) is an important landmark for middle fossa surgery, through which travels the middle meningeal artery, the middle meningeal veins, and a recurrent branch of the mandibular nerve (nervus spinosus).7 It receives its name due to the affinity to the “spinous process” or “spine of the sphenoid” described by Wood Jones in 1931.8
The middle meningeal artery (MMA) branch of the maxillary artery enters the middle fossa by the FS; at its entry point, it can give some small branches that irrigate the trigeminal ganglion, adjacent dura, and a branch to the greater superficial petrosal nerve (GSPN). In its trajectory, in a groove on the greater wing of the sphenoid, it is divided into two branches: one anterior and one posterior9 (Fig. 24.9).
