Carotid Artery Mobilization to Increase the Working Zone Area in Optic Carotid Triangle and Carotid Oculomotor Triangle (A Technical Note of Proximal and Distal Dural Ring Release)

Background: The internal carotid artery (ICA) is attached to the anterior clinoid process and to the dural rings. Mobilization of the ICA in order to work on in the opticocarotid and carotid oculomotor triangle without dural ring release is a demanding procedure. Methods: In this paper we describe the technique of dural rings release, which is adopted by the senior author. Results: Intraoperative pictures of various sellar and parasellar lesions are shown with adequate view of the paraclinoid spaces. Conclusion: Dural ring release is crucial for widening the working paraclinoid triangles especially in cases of short supra-clinoidal portion of ICA.


Short Communication
Optic carotid and carotid oculomotor triangles are the main corridors for reaching sellar and parasellar lesions as well as basilar apex and paraclinoidal aneurysms. The ICA is fixed to the anterior clinoid process and to the dural rings [1]. Thus, its mobility is restricted, and surgery performed through these triangles is demanding. This will be evident especially in cases of short supraclinoidal portion of ICA [2]. Several anatomic morphometric studies showed the effect of anterior clinoidectomy on improving the working area in both triangles [3][4][5]. However, the surgical technique of paraclinoidal carotid artery mobilization via dural rings release is not clearly described in the literature. In fact, the additional space created by anterior clinoidectomy alone is not sufficient to mobilize the paraclinoidal portion of ICA. We here illustrate the technique of releasing both rings for mobilizing ICA.

Dural Rings Anatomic Review
The lateral wall of cavernous sinus consists of 2 layers, dura propria and a thin layer called the membranous layer. These two dural layers separate the cavernous extradural ICA from its subdural portion. At the lateral border of the anterior clinoid process (ACP) these dural layers split to cover its upper and lower surfaces. The layer, which covers the upper and lateral surface, forms the distal dural ring and continues medially to the diaphragma sellae to become falciform ligament. On the other hand, the layer which covers the lower and medial surfaces of the ACP is called proximal dural ring, this surrounds the ICA in a loose manner in contrast to the distal dural ring which is firmly adhered [6].

Surgical Technique Philosophy
The philosophy of this technique is based on the creation of a wide clinoidal space. This is the area occupied by ACP, set free by clinoidectomy itself. This space is bounded superiorly by distal dural ring and inferiorly by the proximal dural ring and constitutes the working channel for dural rings release.

Surgical Technique
Step  It is better demonstrated in modified orbito-zygomatic craniotomy).

Optic Nerve
After standard dural incision and distal Sylvian fissure dissections, the optic nerve is identified medial to the ICA and the oculomotor nerve is identified on the lateral side. Opening all basal cisterns will slack the brain further ( Figure 5).

Step 5: Intra Dural Cutting of the Falciform Ligament and Optic Nerve Sheath
Under high magnification by using blunt hook, the falciform ligament is elevated away from optic nerve and cut using arachnoid knife. It allows detaching the medial end of distal dural ligament.
Then this cut is extended laterally above the ICA leaving a cuff of dura over it. Exposing the carotid axilla between the C2 andC3 is a sign of complete distal dural ring release ( Figure 6).

Figure 6:
The steps of cutting the distal dural ring using the diamond knife and blunt hook in a case of paraclinoid artery aneurysm. Firstly, the optic sheath is elevated using the blunt hook then is cut by knife. Extension of the cut laterally towards the carotid. Note the remanents of dural ring on the aneurysm dome and the space created between optic nerve and the aneurysm.

Discussion
The most common technique adopted to increase the working area in carotid oculomotor triangle and optic carotid triangle is anterior clinoidectomy [3,[7][8][9][10]. Anterior clinoidectomy gives a twofold increase in exposure of the optic nerve length and the Oculomotor Carotid Triangle, as well as a three to four fold increase in the maximum width. This substantially improves the surgical exposure of the suprasellar and periclinoid regions [3], however, the ICA is still fixed at the dural ring even after cutting the falciform ligament. Because the mobility of the ICA is restricted, surgery through both spaces will be demanding especially for basilar tip and paraclinoidal aneurysms. These findings agree with Matsuyama's observations on basilar tip aneurysms [3]. The present technique facilitates risk-free mobilization of ICA, which can be dislocated as necessary without undue traction, as it is obligatory if both rings are not fully released. The present paper, in our opinion, is a further contribution to the proper clarification of the complex anatomy of this region and adds useful information for surgeons dealing with demanding pathological processes of the antero-lateral skull base.