Ossification of the pterygoalar and pterygospinous ligaments: a computed tomography analysis of infratemporal fossa anatomical variants relevant to percutaneous trigeminal rhizotomy

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OBJECTIVE

Ossification of pterygoalar and pterygospinous ligaments traversing the superior aspect of the infratemporal fossa results in formation of osseous bars that can obstruct percutaneous needle access to the trigeminal ganglion through the foramen ovale (FO), interfere with lateral mandibular nerve block, and impede transzygomatic surgical approaches. Presence of these ligaments has been studied on dry skulls, but description of their radiological anatomy is scarce, in particular on cross-sectional imaging. The aim of this study was to describe visualization of pterygoalar and pterygospinous bars on computed tomography (CT) and to review their prevalence and clinical significance.

METHODS

The authors retrospectively reviewed 200 helical sinonasal CT scans by analyzing 0.75- to 1.0-mm axial images, maximum intensity projection (MIP) reconstructions, and volume rendered (VR) images, including views along the anticipated axis of the needle in percutaneous Hartel and submandibular approaches to the FO.

RESULTS

Ossified pterygoalar and pterygospinous ligaments were readily identifiable on CT scans. An ossified pterygoalar ligament was demonstrated in 10 patients, including 1 individual with bilateral complete ossification (0.5%), 4 patients with unilateral complete ossification (2.0%), and 5 with incomplete unilateral ossification (2.5%). Nearly all patients with pterygoalar bars were male (90%, p < 0.01). An ossified pterygospinous ligament was seen in 35 patients, including 2 individuals with bilateral complete (1.0%), 8 with unilateral complete (4%), 8 with bilateral incomplete (4.0%), 12 with bilateral incomplete (6.0%) ossification, and 5 (2.5%) with mixed ossification (complete on one side and incomplete on the contralateral side). All pterygoalar bars interfered with a hypothetical needle access to the FO using the Hartel approach but not the submandibular approach. In contrast, 54% of complete and 24% of incomplete pterygospinous bars impeded the submandibular approach to the FO, without affecting the Hartel approach.

CONCLUSIONS

This study provides the first detailed description of cross-sectional radiological and applied surgical anatomy of pterygoalar and pterygospinous bars. Our data are clinically useful during skull base imaging to predict potential obstacles to percutaneous cannulation of the FO and assist in the choice of approach, as these two variants differentially impede the Hartel and submandibular access routes. Our results can also be useful in planning surgical approaches to the skull base through the infratemporal fossa.

ABBREVIATIONS CT = computed tomography; FO = foramen ovale; LPP = lateral pterygoid plate; MIP = maximum intensity projection; VR = volume rendered.

Article Information

Correspondence Tomasz Matys: University of Cambridge, United Kingdom. tm418@cam.ac.uk.

INCLUDE WHEN CITING Published online May 10, 2019; DOI: 10.3171/2019.2.JNS182709.

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

    Lateral oblique (A) and caudocranial (B) schematic views of the region of the foramen ovale (FO) and associated ligaments. The pterygoalar ligament (PA) attaches to the root of the lateral pterygoid plate (LPP) and to the undersurface of the sphenoid bone anterolateral to foramen spinosum (FS), coursing inferolateral to the exocranial opening of the FO. The pterygospinous ligament (PS) courses from the LPP to the sphenoid spine (SS) attaching medial to the FS. Note the different distances of the PA and PS in relation to FO and different courses in relation to the sagittal plane. CC = carotid canal; FL = foramen lacerum; M = mastoid process; SP = styloid process.

  • View in gallery

    A: Demonstration of pterygoalar bars on axial CT images. Coronal view through the plane of the FO (asterisks), and corresponding axial images at the level of the FO (0 mm) and 2 and 4 mm below this. When the imaging plane is moved inferiorly, no bone is seen anterolaterally to the FO just below the skull base (−2 mm, arrowheads). When the imaging plane is moved further caudally, a pterygoalar bar comes into view on the right side (−4 mm, long arrow; compare with usual appearance on the left [right side of image]). B: Demonstration of the pterygoalar bar on 5-mm coronal oblique MIP image obtained in the plane between the base of the lateral pterygoid plate and the medial aspect of the mandibular head (arrowheads). A pterygoalar bar (long arrow) and the resulting pterygoalar foramen are seen just below the skull base on the reconstructed image (lower panel). C: Examples of complete and incomplete pterygoalar bars (long arrows) on axial images (upper row) and corresponding oblique coronal MIPs (lower row). Double arrows indicate corresponding planes.

  • View in gallery

    A: Axial MIP (25-mm horizontal slab) placed at the level of the skull base allows reliable demonstration of even subtle calcification. There is a complete pterygospinous bar on the left (long arrow) and a partial pterygospinous bar on the right (short arrow). Note that the posterior attachment of the bar is just medial to the foramen spinosum (black arrowhead). B: A 5-mm MIP image obtained in the plane of the pterygospinous bar (traversing the base of the lateral pterygoid plate and styloid process, shown as a shaded area in upper panel; bar indicated by long arrow in lower panel) shows its relationship to the skull base and the lateral pterygoid plate (asterisk). C: Several examples of complete and partial pterygospinous bars (all left-sided) shown (arrows) on axial MIP images and corresponding oblique sagittal MIP images, demonstrating their variability in size and shape. Double arrows indicate corresponding planes.

  • View in gallery

    VR images showing views along hypothetical needle axes using the Hartel and submandibular approaches. In each row, the shaded area in the first image represents the magnified area shown in the remaining images, and relevant osseous structures are indicated by black arrowheads. Upper: Views along a hypothetical needle axis using the Hartel approach to the FO. In the absence of ossification of either the pterygoalar (PA) or pterygospinous (PS) ligament, access to the FO (white asterisk) is unobstructed. Note the thick complete PA bar causing almost complete obliteration of the FO lumen and narrowing of the lumen by the partial PA bar. In contrast, neither complete nor partial PS bars obstruct a hypothetical needle access using the Hartel approach, as they are projected medial to the foramen. Lower: Views along the hypothetical needle axis using the submandibular approach to the FO (white asterisk) in which the needle is introduced medial to the angle of the mandible. Apart from partial narrowing of the lumen by a complete PA bar shown here, none of the observed complete or incomplete PA bars interfered with needle access from this approach. However, both the complete and partial PS bars can interfere with needle access from this approach. In the examples shown, the PS bars would prevent cannulation of the FO.

  • View in gallery

    A and B: VR images showing coexistence of pterygoalar and pterygospinous osseous bars. Note the different distance and angulation of the pterygoalar (black arrowhead) and pterygospinous bar (open arrowhead). Access to the FO using either the Hartel or submandibular route would be impossible. C and D: Very wide and laterally angulated lateral pterygoid plate impeding lateral access to the infratemporal fossa (C, black asterisk), and completely precluding cannulation of the FO from submandibular approach (D, black arrowhead).

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