Placement of Needle in the Foramen
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Placement of the needle within the foramen can be challenging because of variations of the foraminal anatomy and the presence of intraforaminal ridges. The experience of the surgeon plays an important role in successful foraminal cannulation.
To complete a successful percutaneous rhizotomy, the foramen ovale must be accessed using Härtel’s technique. This technique is used regardless of the percutaneous modality (balloon compression, radiofrequency lesioning, or glycerol rhizotomy). Performance of a rhizotomy via each modality is discussed separately.
The following images show the relevant anatomy of the foramen ovale.
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Operating Room Setup
The patient is placed supine on the operating table with a neck roll to achieve a 15- to 20-degree neck extension. The patient’s head is placed in a hollow headrest (“donut gelrest”). More neck extension may disorient the surgeon regarding the landmarks for penetrating the foramen. General anesthesia is administered for balloon compression and glycerol rhizotomies because, unlike radiofrequency lesioning, an awake or cooperative patient is not required for these procedures. The endotracheal tube should be secured on the side contralateral to the affected side of the patient’s face to allow working room in the patient’s mouth for the surgeon’s hand.
For patients undergoing radiofrequency rhizotomy, only light sedation is used because the patient needs to be awake during portions of the procedure. Patient cooperation is critical during the stimulation phase to ensure correct needle tip placement and localization of the radiofrequency rhizotomy lesion. For this reason, uncooperative, overly anxious, or cognitively compromised patients are not suitable candidates for radiofrequency rhizotomy.
I place my finger in the oral cavity to ensure that the buccal mucosa is not perforated during needle advancement and to palpate the pterygoid process since the foramen lies lateral to this landmark. I palpate the mandible and advance the needle just medial to it. A bite block may be used to avoid bite injury to the surgeon’s hand. The needle trajectory may be altered using lateral and oblique fluoroscopic views to gain access to the foramen.
If I inadvertently penetrate the mucosa with the needle, I exchange the needle for a new one and use a different submucosal tract to reach the foramen. I do not abort the procedure.
After the needle tip has reached the skull base at the desired junction of the clivus and petrous bone, an oblique X-ray with the projection of the X-ray beams coaxial to the long axis of the needle will guide the final minor lateral-to-medial needle tip adjustments to penetrate the foramen. During this oblique imaging, the patient’s head is turned 45 degrees contralaterally and the patient’s neck is extended.
As the foramen ovale is engaged, a trigeminal depressor response and contraction of the masseter and pterygoid muscles are frequently elicited. In difficult-to-reach foramina, these responses can guide needle placement as I carefully “walk” the needle tip along the safe portion of the skull base to find the foramen. Once the needle enters the foramen, it should be advanced only 0.5 cm until another lateral X-ray confirms the needle position.
Return of cerebrospinal fluid (CSF) after removal of the stylet assures placement of the needle within the Meckel’s cave and Gasserian cistern. This assurance is not required for confirmation of the needle tip location, and the operator should not insist on acquiring CSF in every case except in cases of glycerol injection into the trigeminal cistern. Chances of CSF flow are maximized with the needle hub facing medially.
Penetration of the medial aspect of the foramen ovale is also most desirable because all divisions of the trigeminal nerve can be contacted with this penetration. Lateral needle placement may not allow adequate contact with V1 or V2 divisions. Positioning of the needle should be done with anticipation that the balloon or radiofrequency electrode will extend 5-10 mm beyond the tip of the cannula.
In my experience, there are a few patients (<5%) whose foramen is difficult to cannulate through the submucosal Härtel’s method using fluoroscopy. Any history of significant facial fractures or craniofacial abnormalities should alert the operator to evaluate the bony skull base around the foramen ovale using a preoperative computed tomography (CT) scan to ensure that bony landmarks, including the foraminal anatomy, are not altered.
I often find that the usual oblique angle of needle entry into the foramen will in fact direct the needle tip toward the posterior lip of the foramen. In this situation, the needle should be maneuvered and redirected to enter the foramen in a more inferior-to-superior trajectory. The oral mucosa may interfere with this redirecting and deflect the tip of the needle away from the target. Therefore, I create a different tract within the mucosa to ensure that the needle tip follows my intended trajectory.
Prior difficulty with cannulating the foramen should also prompt a preoperative CT scan of the skull base. Ossified pterygospinous or pterygoalar ligaments or other intraforaminal bony ridges across the foramen will complicate needle entry into the foramen. In such situations, I stereotactically place the needle using intraoperative neuronavigation to accurately navigate the angle of needle penetration.
Pearls and Pitfalls
- Entrance into the foramen ovale can be difficult because of wide variations in foraminal anatomy. I use lateral and oblique fluoroscopic imaging to guide needle placement.
- Any history of significant facial fractures or craniofacial abnormalities require an evaluation of the bony skull base around the foramen ovale using a preoperative CT scan to ensure that bony landmarks, including the foraminal anatomy, are not altered.
- The needle should not be forced to enter the skull base as this maneuver will lead to precarious needle locations and resultant cerebrovascular injury.
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