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P339: COMPARATIVE ANALYSIS OF ENDOSCOPIC ENDONASAL FAR-MEDIAL, FAR-LATERAL TRANSCONDYLAR AND COMBINED APPROACHES TO THE FORAMEN MAGNUM REGION: VOLUMETRIC ANALYSIS AND QUANTITATIVE INSIGHTS ON SURGICAL EXPOSURE AND MANEUVERABILITY
Sonia Ajmera, MD¹; Rachel Blue, MD¹; Yan Zhou²; Kamila Bond, MD, PhD¹; Reinier Alvarez, MD²; Patrick Hosokawa, MS²; A. Samy Youssef, MD²; ¹University of Pennsylvania Health System; ²University of Colorado

Background: Surgical access to the anterior craniovertebral junction (CVJ) and foramen magnum region remains technically challenging due to dense neurovascular anatomy and limited working corridors. Although the far-lateral transcondylar and endoscopic far-medial approaches are both employed clinically, direct quantitative comparisons of maneuverability and volumetric exposure are still wanting.

Objective: To quantitatively assess and compare surgical maneuverability, anatomic limits and volume of exposure of the endoscopic far-medial, far-lateral, and combined approaches. 

Methods: Five latex-injected cadaveric heads underwent dissection via an endoscopic endonasal far-medial approach (EEFM), contralateral far-lateral transcondylar approach (FL), and a combined approach. Surgical freedom, horizontal and vertical angles of attack, and volumetric exposure were measured using neuronavigation and 3D CT-based segmentation. Site-specific measurements were obtained at the hypoglossal canal, jugular foramen, and vertebrobasilar junction. Statistical comparisons were made using Wilcoxon rank-sum tests.

Results: The FL demonstrated significantly greater surgical freedom (363 mm² vs. 6.8 mm², P = .01), horizontal angle of attack (99° vs. 8.1°, P = .01), and vertical angle of attack (78° vs. 15.4°, P = .01) compared to the EEFM. The EEFM yielded the smallest mean total volume (5.53 ± 2.01 cm³), primarily concentrated in the anteromedial compartment (4.70 ± 1.67 cm³). The FL offered a significantly larger mean total exposure (15.12 ± 1.93 cm³, p < 0.001), dominated by posterolateral access (12.60 ± 2.16 cm³, p < 0.001). The combined approach provided the greatest overall exposure volume (18.76 ± 3.58 cm³), with substantial access to both the posterolateral (12.90 ± 2.32 cm³) and anteromedial (5.86 ± 1.95 cm³) compartments. 

Conclusion: The far-lateral approach offers superior surgical freedom and posterolateral exposure, while the far-medial endoscopic approach provides a direct but limited midline corridor predominantly anteromedial foramen magnum lesions. The combined approach maximizes volumetric access and should be considered for extensive foramen magnum lesions particularly engulfing or anterior to neurovascular structures. 

Figure 1

Far Lateral Cadaveric Anatomical Dissection. CF: Condylar Fossa, OC: Occipital Condyle, MB:

Muscular Branch, VA: Verterbral Artery, PMA: Posterior Meningeal Artery, CV: Condylar Vein, JT: 

Jugular Tubercle, HC: Hypoglossal Canal, JF:Jugular Foramen, LN: Lower Cranial Nerves, XI: Cranial 

Nerve 11, XII: Cranial Nerve 12

Figure 2

Far Medial Cadaveric Anatomical Dissection. P: Pituitary, CA: Carotid Artery, V: 

Vidian Nerve, C: Clivus, JT: Jugular Tubercle, HC: Hypoglossal Canal, OC: Occipital

Condyle, VII-VIII: Cranial Nerves 7-8, LN: Lower Cranial Nerves, VA: Vertebral 

Artery, XII: Cranial Nerve 12

 

Figure 3

Surgical freedom and angles of attack in EEFM and FL approaches. The vertical and horizontal angles of attack define the limits of maneuverability and instrument reach at a fixed target point. The projected working envelope represents the total degrees of freedom, serving as a surrogate for surgical freedom. These parameters were quantitatively measured using neuronavigation for both far-medial endoscopic and far-lateral approaches to evaluate access to key anatomical targets 

Figure 4

Brain Lab 3D volumetric exposure between the Far Lateral exposure (A), Endoscopic Endonasal Far Medial exposure (B) and Combined exposure (C). 

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