Figure 1: Osseous anatomy of the middle cranial base. Note the relationship of the foramen ovale to the surrounding foramina and its position when viewed from above (A) and below (B)(Images courtesy of AL Rhoton, Jr).

Figure 2: An oblique view of the pterygomaxillary fissure. Note the relationship of the foramen ovale (arrow) to the lateral pterygoid process and mandibular fossa. The vertical shape of the posterior lip of the foramen may complicate needle entry into the foramen if the needle does not enter the foramen at the appropriate inferior-to-superior trajectory (Image courtesy of AL Rhoton, Jr).

Figure 3: The dissection of the Meckel’s cave and cavernous sinus on the right side after removing the periosteal layer of the dura mater. Placement of the needle or cannula on the medial section of the foramen allows the balloon, electrode, or glycerol solution to make the most contact with the trigeminal roots and enter the trigeminal cistern. Images reproduced with permission from Peris-Celda M, Graziano F, Russo V, Mericle RA, Ulm AJ. Foramen ovale puncture, lesioning accuracy, and avoiding complications: Microsurgical anatomy study with clinical implications. J Neurosurg 2013;119: 1176-1193.

Figure 4: The safe zones and danger zones along the periforaminal space. The green zone defines the safe zones to “walk” the needle along the skull base to find the foramen. The orange zone defines danger zones where the carotid artery or jugular vein could be injured. It is best to aim the needle to enter the region more anteriorly and then adjust the needle tip more posteriorly until the foramen is cannulated.

Figure 5: An oblique radiograph coaxial to the needle aimed at the left foramen ovale (within the dotted outline). Note the approximate locations of the internal jugular vein (blue) and the internal carotid artery (red) in relation to the foramen ovale.

Figure 6: The operating room setup for a percutaneous rhizotomy is shown. The surgeon stands by the patient’s affected side (in this case, left side) and the surgical assistant stands at the head of the table to adjust the patient’s head during fluoroscopy. The anesthesia team is situated across from the surgeon. The stimulator apparatus is by the patient’s feet and cables are passed under the drapes.

A C-arm fluoroscope with an image intensifier is placed around the patient’s head and can be rotated freely to obtain lateral (lower right image) and oblique views parallel to the needle’s long axis (lower left image) as the needle is advanced toward the foramen. Note the position of the patient’s head during these two important imaging sequences; in the latter case, the head is extended. The X-ray viewer is placed across from the surgeon so both the surgeon and assistant can easily view the monitor.

Figure 7: Härtel’s anatomic landmarks are used for skin entry and needle trajectory. After injecting local anesthetic, the skin is entered 2.5 cm lateral to the corner of the mouth. The needle trajectory is toward a point in line with the medial ipsilateral pupil and 2.5–3 cm anterior to the external auditory canal. In other words, the needle trajectory is aimed at the intersection of two planes: one sagittal plane passes through the ipsilateral pupil (blue), and the other coronal plane (green) is 3 cm anterior to the external auditory canal.

Figure 8: The skull bone is transposed on the patient’s image to illustrate the bony anatomic landmarks used to reach the foramen.

Figure 9: The needle is advanced to reach the ideal position on the lateral X-ray. Note that the tip of the needle is aimed at the junction of the clivus and petrous bone. This is the most important landmark on the lateral fluoroscopic image guiding the surgeon to reach the foramen. The needle tip is advanced onto the junction and upon entering the foramen should rest between 5-15 mm below the sellar floor.

If the needle is not aimed directly at the petrous-clival junction, the needle needs to be repositioned. During repositioning, I withdraw the needle to the skin level and redirect its tip. If the needle is not adequately withdrawn, repositioning will lead to retraction of the mucosae by the needle and resultant deflection of the needle tip. This phenomenon will prevent reliable cannulation of the foramen.

Figure 10: Location of the needle and an intraoperative lateral X-ray (inset) illustrating the final location of the needle near the skull base along the petrous-clival junction (arrow).

Figure 11: The C-arm is then rotated to offer a tunnel view and the X-rays are beamed in a coaxial manner parallel to the axis of the needle. This view exposes the foramen at an oblique angle and allows exact localization of the needle tip in relation to the foramen. Most often, slight changes in the fluoroscopic beam are required to take a “face-on” image of the foramen. This methodology identifies the foramen between the ramus of the mandible laterally and the maxilla medially.

Figure 12: An oblique X-ray projection relatively parallel to the long axis of the needle demonstrates that the needle tip is just posterior to the foramen (white arrow, left image). Slight adjustment of the needle ensures that the needle is within the foramen (right image).

Figure 13: The needle is advanced slightly into the foramen. A lateral fluoroscopic image confirms the final location of the needle tip beyond the skull base (left image). In the case of a balloon compression rhizotomy, the balloon may be inflated within the Gasserian ganglion cistern as demonstrated (right image). The final location of the balloon demonstrates the slight extension of the balloon into the foramen rotundum (blue arrow).

Figure 14: Note the presence of a bony ridge along the medial aspect of the right foramen ovale at the tip of the black arrow (left image). This ridge will complicate cannulation of the foramen by the needle. Intraoperative CT-guided navigation assisted with placement of the needle in this patient (right image).