We report on a technically simpler means of performing an extradural anterior clinoidectomy after a review of the anatomy of the ACP and its anatomical variations. The key element in the procedure involves cutting the MOB and its named vessels once the SOF is unroofed, which facilitates dural elevation as well as exposure of the ACP and the intracranial aperture of the optic canal. The ACP is safely disengaged from its three supporting structures: 1) the lesser wing of the sphenoid bone; 2) the roof of the optic canal; and 3) the optic strut. Finally, the ACP is centrally hollowed prior to its extirpation to avoid damage to the oculomotor nerve, which is very close to the lateral underside of the ACP. In this report, the original Dolenc procedure and its subsequent derivatives are compared and contrasted to our simpler and less laborious technique. We report on different clinical situations from our experience in 60 cases (40 aneurysm cases and 20 tumor cases) over a 4-year period.
Anatomy of the ACP
The ACP is a spine of bone that projects from the posteromedial border of the lesser wing of the sphenoid. This bone projection bridges the roof of the cavernous sinus and superiorly has the form of a triangular mass (Fig. 1A) that overhangs the proximal portion of the intradural carotid artery. Anteriorly, the base of the ACP has one lateral and two medial components: 1) laterally, it is the continuation of the lesser wing of the sphenoid bone overlying the SOF (Fig. 1B); and 2) medially, the base connects to the sphenoid bone via the anterior and posterior roots. The anterior root, located superiorly, is the continuation of the flat roof of the optic canal. The posterior root, located inferomedially, is the optic strut20 and connects the ACP to the basisphenoid bone. Parkinson20 describes the optic strut as a small pillar of bone with a rounded anterior aspect and a posterior sharp edge corresponding respectively to the leading and trailing edge of an aerofoil. The optic strut forms the lateral and ventral walls of the optic canal.13,18,20 It is also completely surrounded by dura mater from the middle and anterior fossae.
The ACP is generally composed circumferentially of dense cortical bone with an interior of cancellous bone that may contain small venous channels, which in turn communicate with the cavernous sinus, and the diploic veins of the orbital roof. On occasion, the ACP is pneumatized and contains air cells that communicate with the sphenoid sinus via the optic strut, which resembles a bone conduit.8,12,20 Although pneumatization is rare, its potential existence should be verified preoperatively on CT scans to take steps to avoid CSF leaks. The ACP is attached to a number of dural folds: 1) the falciform ligament extends from the planum sphenoidale over the roof of the optic canal and attaches to the base of the ACP; 2) the anteromedial aspect of the tentorium; 3) anterior and posterior petroclinoid ligament; and 4) interclinoid dural folds.
The course of the clinoidal segment of the ICA is intimately related to the inferior aspect of the ACP. The junction of the optic strut and the inferomedial aspect of the ACP are grooved by the clinoidal segment of the ICA. The posterior medial projection of the ACP, located posterior and superior to the optic strut, frequently contains a rounded impression, which accommodates the lateral surface of the clinoidal segment of the ICA. The oculomotor, trochlear, and ophthalmic divisions of the trigeminal and abducens nerves run together in a dural fold from the lateral wall of the cavernous sinus located just below the lateral side of the ACP to the medial wall of the SOF. Of these nerves, the oculomotor one is the closest to the body and base of the ACP. Nutik19 reports that the oculomotor nerve often grooves the inferolateral surface of the ACP. Note that the dural layer inferior to the ACP, which adjoins the lower dural ring of the clinoidal segment of the ICA to the dural covering of the oculomotor nerve, is referred to as the “carotico-oculomotor membrane.”10 This dural membrane can be thin and friable, although data from two different cadaveric studies indicate that the membrane is incomplete 16% of the time.12,21
The posterior tip of the ACP often projects medially behind the lateral portion of the clinoidal segment of the ICA and may be involved in an osseous bridge forming a bone foramen for the ICA. In a study involving a total of 70 ACPs, Kim and colleagues15 reported the occasional presence of an interosseous bridge from the ACP to either the middle (13%) or the posterior clinoid process (6%). The presence of an interosseous bridge from the ACP would prevent an extradural resection and necessitate a combined intra- and extradural procedure.
The optic canal is located at the orbital apex. It is bounded by the body of the sphenoid bone medially, the lesser wing of the sphenoid bone superiorly, and the optic strut laterally and inferiorly (Fig. 1B). The optic canal assumes a vertically oval shape at its orbital end, where it measures approximately 5 to 6 mm in diameter. In its central portion it is round in cross-section, and on the cranial end it is oval in the horizontal plane. The canal attains its adult size by the age of 3 years.9 In approximately 4% of normal individuals the ophthalmic artery will notch the canal floor, forming a keyhole deformity.14 The optic canal is 8 to 12 mm in length.
The ACP has a mean length of 7.7 mm (range 3–8 mm)15 from the roof of the optic canal to its tip (Fig. 1A). Its mean width is 5.6 mm (range 4–7.8 mm) and mean height is 5.3 mm (range 4–6.8 mm) at the widest portion.12 In summary, the ACP has three supporting structures: 1) the lesser wing of the sphenoid bone; 2) the roof of the optic canal; and 3) the optic strut. Extradural removal of the ACP involves disengaging the ACP from these three structures.
Extradural Removal of the ACP
The extradural removal of the ACP is performed using microscopic magnification and illumination after the sphenoid ridge is flattened following a modified orbitozygomatic craniotomy.1 First, the dura is elevated and directed medially toward the orbital roof overlying the optic canal to delineate the intracranial roof of the optic canal (Fig. 1A and C). The SOF is then identified and unroofed (Fig. 1B and D), which involves removing two bone structures with either rongeurs or a diamond burr: 1) complete removal of the lesser wing of the sphenoid bone overlying the superolateral margin of the SOF—the first of three supporting structures for the ACP; and 2) partial removal of the greater wing of the sphenoid bone, which forms the inferolateral wall of the SOF. This step provides mobility for the contents of the SOF and exposes the dural fold at the apex of the SOF (Fig. 1D) where the MOB is located with its named artery and vein. There is no need to unroof the foramen rotundum unless required by the pathology at hand. The MOB together with its vessels can be coagulated and divided with impunity (Fig. 1E) for a distance of 3 to 5 mm, and a cleavage plane by sharp dissection is established between the dura propia of the temporal tip and the inner cavernous membrane. The temporal tip dura is then peeled away from the anterior aspect of the cavernous sinus and the orbital apex. This maneuver is key because it facilitates further dural elevation in a posteromedial direction along the lateral aspect of the ACP. It also facilitates dural elevation medially to identify the intracranial aperture of the optic canal. This cleavage plane can also be extended, if required, along the second and third divisions of the trigeminal nerve on the middle fossa floor and medially to the tentorial edge.
The optic canal—the second of three supporting structures for the ACP—is unroofed from a lateral to medial direction (Fig. 1A, hatched area with embedded arrow) by using a 2-mm diamond burr (Fig. 1F) and constant-cooling irrigation to prevent thermal damage of surrounding neural structures from the elevation in bone temperature. Care is taken not to open either the ethmoid or sphenoid sinus at the medial edge of the optic canal. If the sinuses are entered, we do not violate the mucous membrane and close the bone opening with wax. The ACP is now connected only at the floor of the optic canal by the optic strut (Fig. 1B)—the third of three supporting structures for the ACP—which is drilled off as needed. A final step involves centrally hollowing out the dense cortical bone in the center of the ACP with the aid of a diamond burr and constant-cooling irrigation. During this procedure, the surgeon must be ever cognizant about the relative positions of the optic nerve, the carotid artery, and the oculomotor nerve with reference to the ACP. A circumferential dissection plane between the surrounding dural folds and the centrally shelled ACP is established (Fig. 1G). Caution is exercised at the site of the carotico-oculomotor membrane, which is friable and occasionally incomplete,12,21 prior to extirpating the ACP by using fine rongeurs. As a direct extension of this procedure, the optic sheath and the distal dural ring are excised following dural opening, which allows early decompression and mobilization of the optic nerve with minimal brain retraction. This technique was successfully used during a 4-year period for the removal of the ACP in aneurysm pathology (40 cases) and selective tumorous lesions (20 cases).
The first and second authors (A.N. and V.B.) contributed equally to this work. We thank the medical photographers at Oregon Health & Science University, specifically Richard York, for photographic assistance and Shirley McCartney, Ph.D., for assistance in figure preparation and editing.
Dolenc VV: A combined transorbital-transclinoid and transsylvian approach to carotid-ophthalmic aneurysms without retraction of the brain. Acta Neurochir Suppl 72:89–971999Dolenc VV: A combined transorbital-transclinoid and transsylvian approach to carotid-ophthalmic aneurysms without retraction of the brain. Acta Neurochir Suppl 72: