Comprehensive anatomy of the foramen ovale critical to percutaneous stereotactic radiofrequency rhizotomy: cadaveric study of dry skulls

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OBJECTIVE

Percutaneous stereotactic radiofrequency rhizotomy (PSR) is often used to treat trigeminal neuralgia, a serious condition that results in lancinating, episodic facial pain. Thorough understanding of the microsurgical anatomy of the foramen ovale (FO) and its surrounding structures is required for efficient, effective, and safe use of this technique. This morphometric study compares anatomical and surgical orientations to identify the variations of the FO and assess cannulation difficulty.

METHODS

Bilateral foramina from 174 adult human dry skulls (348 foramina) were analyzed using anatomical and surgical orientations in photographs from standardized projections. Measurements were obtained for shape, size, adjacent structures, and morphometric variability effect on cannulation. The risk of potential injury to surrounding structures was also assessed.

RESULTS

The authors identified 6 distinctive shapes of the FO and 5 anomalous variants from the anatomical view, and 6 shapes from the surgical view. In measurements of surface area of this foramen obtained using the surgical view, loss (average 18.5% ± 5.7%) was significant compared with the anatomical view. Morphometrically, foramen size varied significantly and obstruction from a calcified pterygoalar ligament occurred in 7.8% of specimens. Importantly, 8% of foramina were difficult to cannulate, thus posing a 12% risk of inadvertent cannulation of the foramen lacerum.

CONCLUSIONS

Significant variability in the FO’s shape and size probably affected its safe and effective cannulation. Preoperative imaging by 3D head CT may be helpful in predicting ease of cannulation and in guiding treatment decisions, such as a percutaneous approach over microvascular decompression or radiosurgery.

ABBREVIATIONS AP = anteroposterior; FO = foramen ovale; ICA = internal carotid artery; LPP = lateral pterygoid plate; PSR = percutaneous stereotactic radiofrequency rhizotomy; TN = trigeminal neuralgia.

Article Information

Correspondence Jeffrey T. Keller: c/o Glia Media, Cincinnati, OH. mary.kemper@gliamedia.com.

INCLUDE WHEN CITING Published online April 19, 2019; DOI: 10.3171/2019.1.JNS18899.

Disclosures The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    FO examined from the exocranial anatomical (A) and surgical (B) views. Published with permission from Mayfield Clinic, from Tew et al.: Percutaneous stereotactic rhizotomy in the treatment of intractable facial pain, in Quinones-Hinojosa A (ed): Schmidek & Sweet Operative Neurosurgical Techniques, 6th edition. Philadelphia: Elsevier Saunders, 2012, pp 1409–1418.

  • View in gallery

    Illustrations of Härtel’s 3 skin landmarks made on the patient’s face to help the surgeon guide the needle into the FO. A: Beneath the medial aspect of the pupil on the lower eyelid (1); 3 cm anterior to external auditory meatus (2); and 2.5 cm lateral to the oral commissure (3). Surgeon inserts his gloved index finger into the patient’s mouth along the side of the molars to rest against the lateral pterygoid. This finger position helps guide the cannula toward the foramen ovale and prevents penetration of the oral mucosa. B: Cannula is inserted 2.5 cm lateral to the oral commissure (3) and is aimed at the intersection of midpupillary line (1) and a point 3 cm anterior to the external auditory meatus (2). Published with permission from Mayfield Clinic, from Tew et al.: Percutaneous stereotactic rhizotomy in the treatment of intractable facial pain, in Quinones-Hinojosa A (ed): Schmidek & Sweet Operative Neurosurgical Techniques, 6th edition. Philadelphia: Elsevier Saunders, 2012, pp 1409–1418.

  • View in gallery

    Six classic FO shapes viewed from an anatomical perspective. Arranged (left to right) according to frequency: oval, crescent, cordate, almond, elongated, and round. An oval shape, which classically defines the FO, occurred in 38.2% of the foramina (less than expected). A crescent-shaped foramen, defined by a straight anterolateral border, occurred in 31.2% of foramina. A cordate (heart-shaped) foramen, with a small convexity or bar on its anterior border, formed in 12% of foramina. An almond shape, defined by an acute asymmetrical narrowing anteromedially that can limit cannulation of the medial border, occurred in 9.4% of foramina. An elongated foramen, defined by a foraminal length exceeding its width, was identified in 7.2% of foramina. Finally, a perfectly round foramen formed in 2.9% of foramina.

  • View in gallery

    Aberrant variants of the FO shape in anatomical view in order of frequency (left to right). We identified a primitive foramen (F.) lacerum medius (persistent embryologic foramen in which FO is completely confluent with the foramen lacerum and foramen spinosum) in 3% of FOs; a pithecoid foramen (FO is incomplete or present as a notch) in 2.4%; and a confluent foramen spinosum (FO communicates with the foramen spinosum by a small channel or is entirely confluent with it) in 1.6% of specimens. Presence of the foramen of Vesalius (an aberrant foramen located anteromedial to the FO) and duplicate FO (its venous portion is separated from the remainder of the foramen by a bony spicule) was exceedingly rare, occurring in 0.3%.

  • View in gallery

    Six distinct FO shapes in the surgical view arranged by frequency (left to right). Slanted, formed primarily by the lateral pterygoid obstructing the medial border of the FO, in 34.2% of specimens. Oval, with almost no overlap from surrounding bone, in 27.3%. Eclipse, formed by a prominent bony arch resulting from either an incomplete or complete ossification of pterygospinous ligament spanning from the spine of Civinini on the lateral pterygoid to the angular spine of the sphenoid, in 20.9%. These bony structures significantly limited the width of the FO at both ends. Other forms included tear-shaped in 9.1%, slit-shaped in 6.4%, and a rare obstructed type in 2.1%—in which the foramen itself was obstructed by an ossified pterygoalar ligament that spanned across the face of the FO from the lateral pterygoid plate to the greater wing of the sphenoid at the anterolateral margin of the foramen spinosum.

  • View in gallery

    In the surgical view, bony spicules were associated with the anterior and posterior borders of the FO in 27% and 15.5% of the foramina studied, respectively; 7.5% of specimens had both types. Although these spicules appeared to significantly reduce foraminal surface area, they never obstructed cannula passage. The pterygoalar ligament runs from the region of the spine of Civinini to the greater wing of the sphenoid bone lateral to the foramen spinosum (see Agazzi et al.). Complete ossification of the pterygoalar ligament resulting in a pterygoalar bar (A, arrow) formed in 7.8% of skulls. In such cases, the cannula passed through the pterygoalar foramen above the bar before entering the FO. Pterygoalar bar visually obscured FO in 3% of foramina and restricted access to FO in 1.5% of foramina. The pterygospinous (Civinini) ligament runs from the spine of Civinini of the lateral pterygoid plate to the sphenoidal spine medial to the foramen spinosum. Although complete ossification of the pterygospinous ligament formed a pterygospinous bar (B, arrow) in 8% of specimens, this did not prevent access to the FO. In the surgical view, the pterygoalar ligament tended to course across the face of the FO, whereas the pterygospinous ligament coursed posterior to the FO.

  • View in gallery

    Skull base foraminae may be punctured with a cannula when targeting the FO. Inferior orbital fissure (IOF) can be hit by a cannula aimed too anterior and superior to the FO. Jugular foramen (JF) or the petro-occipital fissure (POF) can be hit by a cannula aimed too posterior and inferior. The ICA can be hit at the foramen lacerum (FL) or carotid canal (CC). Cannulation of a dehiscence in the floor of the foramen lacerum was surprisingly easy in many skulls but was only possible in a few skulls with a petro-occipital fissure, or with dehiscence between the clivus and occipital bone. Although the cannula easily passed directly into the foramen lacerum in 12% of the FOs, it could not easily pass into the cranial vault in most cases because of a bony roof over the foramen lacerum. Rarely (in 0.86% of cases), a cannula projected posterior to the FO could easily pass through a dehiscence of the petro-occipital fissure. Published with permission from Mayfield Clinic, from Tew et al.: Percutaneous stereotactic rhizotomy in the treatment of intractable facial pain, in Quinones-Hinojosa A (ed): Schmidek & Sweet Operative Neurosurgical Techniques, 6th edition. Philadelphia: Elsevier Saunders, 2012, pp 1409–1418.

  • View in gallery

    Correlation of anatomical shapes (A) of the FO to surgical perspective (B). Oval, crescent, and cordate anatomical shapes most often appeared as a slanted-shaped FO in the surgical view. Almond and elongated shapes in the anatomical view most commonly appeared oval in the surgical view. Printed with permission from Mayfield Clinic.

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