Quantitative Anatomical Study of Tailored Far-Lateral Approach for the VA-PICA Regions

Abstract

Objective The extent of the far-lateral approach (FLA) has not yet been quantified for the region of the vertebral and posterior inferior cerebellar arteries (VA-PICA). We quantitatively analyzed six main sequential steps of the FLA. Methods A modified small FLA (msFLA) and a classic large FLA (clFLA) were performed sequentially on both sides of five cadaveric heads. A frameless navigational system was used to quantify the angle of attack for the origin (T1) and lateral medullary segment (T2) of the PICA and the surgical area of exposure above and below the vagus nerve (cranial nerve [CN] X). Results The total area of exposure above CN X increased significantly (p < 0.05) from the msFLA to the clFLA. However, the surgical exposure area below CN X did not change (p > 0.05). C1 hemilaminectomy increased (p < 0.05) the vertical angle of attack, and drilling the posteromedial third of the occipital condyle increased (p < 0.05) the horizontal angle of attack to the origin of the PICA. Conclusions For the VA-PICA region, the msFLA offered a similar practical surgical working area and similar angles of attack when compared with the clFLA. The FLA should be tailored based on the location, size, and pathology of lesions and on the exposure required for effective surgical treatment.

Keywords: far-lateral approach; practical surgical working area; quantitative anatomical study.

Fig. 1

Illustration showing the modified small far-lateral approach (step 1, green), the classic large far-lateral approach (step 2, blue), and the extent of a C1 hemilaminectomy (steps 2 and 5) and posterior condyle drilling (steps 3 and 6, red). (Used with permission from Barrow Neurological Institute.)

Fig. 2

Cadaveric dissections of the vertebral and posterior inferior cerebellar arteries region demonstrating the six sequential steps of approach. (A) Step 1: modified small far-lateral approach (msFLA) with C1 arch and occipital condyle intact. (B) Step 2: msFLA with C1 laminectomy. (C) Step 3: msFLA with C1 laminectomy and removal of posterior third of occipital condyle. (D) Step 4: classic large far-lateral approach (clFLA) with C1 arch and occipital condyle intact. (E) Step 5: clFLA with C1 laminectomy. (F) clFLA with C1 laminectomy and posterior third of occipital condyle removal. AOJ, atlanto-occipital joint; HC, hypoglossal canal; JT, jugular tubercle; OC, occipital condyle; PCEV, posterior condylar emissary vein; PICA, posterior inferior cerebellar artery; PSA, posterior spinal artery; SS, sigmoid sinus; VA, vertebral artery; VII and VIII, seventh and eight cranial nerve complex; IX and X, glossopharyngeal and vagal nerves; XI, spinal accessory nerve; XII, hypoglossal nerve. (Used with permission from Barrow Neurological Institute.)

Fig. 3

Reference points used to measure areas of exposure. (A) Region below cranial nerve (CN) X The area of exposure was limited by three fixed reference points: (1)* the exit point of CN IX from brainstem, (2)* the point where the VA penetrates the dura and (3)* the jugular foramen, and by two variable points, (4) the lowest visible point of the fascicle of CN XI on the brainstem and (5) the most lateral point at the level of the tip of the condyle. (B) Region above CN X. The area of exposure was limited by four fixed reference points, (6)* exit point of CN X from brainstem, (7)* exit point of CN VII from brainstem, (8)* internal auditory meatus, and (9)* the jugular foramen, and two variable points, (10) the uppermost point visible in the direction of cerebellum and (11) the uppermost point visible in the direction of petroclival area. The lines connecting the points in (A) and (B) lie in the same planes. (Used with permission from Barrow Neurological Institute.)

Fig. 4

Angles of attack for target point 1 (T1), posterior inferior cerebellar artery origin on right-sided specimen. The horizontal angle of attack (solid black line) was parallel to the cranial base, whereas the vertical angle (dashed white line) was on a plane perpendicular to the cranial base. The medial and lateral points limited the horizontal angle; the caudal and cranial points limited the vertical angle. For the horizontal angle, there was no significant difference (p < 0.05) between steps 1 and 2 and 4 and 5; however, there was a significant increase (p < 0.001) between steps 2 and 3 and 5 and 6. The vertical angle increased between steps 1 and 2 (p = 0.010) and between groups (p < 0.05), but there was no significant difference (p > 0.05) noted between steps 2 and 3, 4 and 5, or 5 and 6. (Used with permission from Barrow Neurological Institute).

Fig. 5

Bar graph of the surgical exposure areas: above and below cranial nerve X and total area. Total area increased (p < 0.05) between steps 1 and 3 and 4 and 6 and also between the modified small far-lateral approach (msFLA) (steps 1–3) and classic large far-lateral approach (clFLA) (steps 4–6) groups (p < 0.05). Area below CN X increased (p < 0.05)* between steps 1 and 3, and 4 and 5, but no increase (p > 0.05) was observed between msFLA and clFLA. Area above CN X did not increase (p > 0.05) between steps 1 and 3 and 4 and 6, but a significant increase was noted from the msFLA to clFLA. (Used with permission from Barrow Neurological Institute.)

Fig. 6

Bar graphs demonstrating angles of attack in horizontal and vertical planes for target point 1, origin of posterior inferior cerebellar artery (PICA) (T1), and for target point 2 (T2), lateral medullary segment of PICA. (A) Horizontal angle of attack at T1 increased (p ≤ 0.001)* between steps 1 and 3, 2 and 3, 4 and 6, and 5 and6, but there was no significant difference (p > 0.05) between the modified small far-lateral approach (msFLA) (steps 1 to 3) and classic large far-lateral approach (clFLA) (steps 4 to 6) groups. (B) Vertical angle of attack at T1 significantly increased (p < 0.05)* from steps 1 and 2 and from msFLA to clFLA. (C) Horizontal angle of attack at T2 did not differ significantly between any of the steps (p > 0.05). (D) Vertical angle of attack at T2 did not differ significantly (p > 0.05) between the steps within the msFLA and clFLA groups. There were significant changes (p < 0.05)* between steps 1 and 4 and 2 and 5, but no significant increase (p = 0.142) between steps 3 and 6. (Used with permission from Barrow Neurological Institute.)