Course of the V3 segment of the vertebral artery relative to the suboccipital triangle as an anatomical marker for a safe far lateral approach: A retrospective clinical study

Abstract

Background: The third segment of the vertebral artery (V3) is vulnerable during far lateral and retrosigmoid approaches. Although the suboccipital triangle (SOT) is a useful anatomical landmark, the relationship between V3 and the muscles forming the triangle is not well-described. We aimed to demonstrate the relationship between the V3, surrounding muscles, and SOT in clinical cases.

Methods: Operative videos of patients with the vertebral artery (VA) and posterior inferior cerebellar artery (PICA) aneurysms treated with occipital artery-PICA bypass through the far lateral approach were examined. Videos from January 2015 to October 2021 were retrospectively reviewed to determine anatomy of the V3 and the SOT.

Results: Fourteen patients were included in this study. The ipsilateral V3 was identified without injury in all patients using the bipolar cutting technique. The lateral 68.2% of the horizontal V3 segment, including the V3 bulge, was covered by the inferomedial part of the superior oblique muscle (SO). The medial 23.9% was covered by the inferolateral part of the rectus capitis posterior major muscle. The inferomedial part of the horizontal V3 segment is located within the SOT.

Conclusion: Most of the V3, including the V3 bulge, were located beneath the SO and the inferomedial part of V3 located within the SOT. Elevation of the SO should be performed carefully using the bipolar cutting technique to avoid injury to the V3. To the best of our knowledge, this is the first description of the V3 relative to the SOT in the clinical setting.

Keywords: Far lateral approach; Suboccipital triangle; Superior oblique muscle; V3 segment; Vertebral artery.

Figure 1:

(a) Semi-prone park bench position. (b) Reverse “L” shape incision relative to the mastoid tip (arrow), mastoid groove (arrowhead), and superior nuchal line (SNL). (c and d) After the sternocleidomastoid, splenius capitis, longissimus capitis, and semispinalis capitis muscles and the posterior belly of digastric muscle were elevated in layers, the transverse process of C1 (TPC1) and the spinous process of C2 (SPC2) were palpated and the suboccipital triangle (asterisk) formed by the superior oblique (SO in blue color), inferior oblique (IO in green color), and rectus capitis posterior major (RCPM in yellow color) muscles was identified.

Figure 2:

Intraoperative photograph demonstrating left suboccipital muscle dissection. (a) After inferolateral elevation of the superior oblique muscle (SO) from the rectus capitis posterior major muscle (RCPM), the inferolateral border (dashed line) of the RCPM was identified as the landmark of the third segment of the vertebral artery (V3) and the starting point for the bipolar cutting technique to separate the SO from the V3. (b) Magnified photograph of Panel A demonstrated the inferolateral border (dashed line) of the RCPM and the partially elevated SO.

Figure 3:

(a and b) After the SO (in blue color) was inferolaterally elevated, the V3 (red color) was identified beside the RCPM (yellow color) and IO (green color). The location of the center of the suboccipital triangle is marked with an asterisk. (c and d) After the RCPM (yellow color) was inferomedially elevated from the inferior nuchal line, the foramen magnum (FM) was exposed. The condylar fossa (CF), SNL, and mastoid groove (MG) were also identified. (e and f) After retrosigmoid craniotomy, removal of the posterolateral part of the FM, and drilling of the CF, the posterior part of jugular tubercle (JT), and the medial part of the occipital condyle, the transcondylar exposure was completed.

Figure 4:

Method of quantitative measurement of the V3 length on the left side. The superior oblique (SO) and rectus capitis posterior major (RCPM) muscles were identified. The whole length of the V3 was marked as a dashed line and measured as “c.” The length of the V3 covered by SO (blue color) and RCPM (yellow color) were measured as “a” and “b,” respectively. The center of the suboccipital triangle above the V3 is indicated with an asterisk.

Figure 5:

(a) Three-dimensional reconstructed computed tomography angiography of the left side of the craniocervical junction demonstrating the skin incision (blue line), mastoid tip (arrow), and mastoid groove (arrowhead). (b) An intraoperative photograph, similar to Panel A, demonstrating the mastoid tip (arrow), mastoid groove (arrowhead), the third segment of the vertebral artery (V3 and red color), the suboccipital triangle (green triangle), and the related suboccipital muscles. The lateral 68.2% and medial 23.9% of the V3 are covered by the superior oblique muscle (SO) and rectus capitis posterior major muscle (RCPM), respectively. The inferomedial part of the V3 is located within the suboccipital triangle (green triangle). (C1: Posterior arch of C1, FM: Foramen magnum, IO: Inferior oblique muscle, LGC: Longissimus capitis muscle, RCPM: Rectus capitis posterior major muscle, SO: Superior oblique muscle, SPC: Splenius capitis muscle, SSC: Semispinalis capitis muscle, TPC1: Transverse process of C1).

Figure 6:

Illustrative Case 1 (Patient 1) (a and b) Intraoperative finding of the left suboccipital muscle dissection and exposure for the far lateral transcondylar approach. (b) The superior oblique (SO) and rectus capitis posterior major muscle (RCPM) were elevated. The third segment of the vertebral artery (blue, red, and yellow colors) was exposed. The blue color was covered by the SO, which included the lateral 70% of the third segment of the vertebral artery. The yellow color was covered by the RCPM, which included the medial 24% of the third segment of the vertebral artery. The red color was located within the center of the suboccipital triangle (asterisk).

Figure 7:

Illustrative Case 2 (Patient 13). (a) Right semi-prone park bench position. (b) Skin incision (blue line) relative to the mastoid tip (arrow) and mastoid groove (arrowhead). (c-h) The steps to expose the third segment of the left vertebral artery are demonstrated, similar to Figure 1-3. The superior oblique (SO, blue color), inferior oblique (IO, green color), and rectus capitis posterior major (RCPM, yellow color) muscles; the third segment of the vertebral artery (V3, red color); and the center of the suboccipital triangle (asterisk) are shown. (I and j) The left far lateral transcondylar fossa approach.

Figure 8:

Illustrative Case 2 (Patient 13). (a and b) Intraoperative findings during the left suboccipital muscle dissection and exposure for the far lateral transcondylar approach. (b) The superior oblique (SO) and rectus capitis posterior major (RCPM) muscles were elevated. The third segment of the vertebral artery (blue, red, and yellow colors) was exposed. The blue color was covered by the SO, which included the lateral 68% of the third segment of the vertebral artery. The yellow color was covered by RCPM, which included the medial 24% of the third segment of the vertebral artery. The red color was located within the center of the suboccipital triangle (asterisk).

Figure 9:

Illustrative Case 3 (Patient 14). (a) Left semi-prone park bench position. (b) Skin incision (blue line) relative to the mastoid tip (arrow), mastoid groove (arrowhead), and superior nuchal line (SNL). (c-h). The steps to expose the third segment of the right vertebral artery are demonstrated, similar to Figures 1-3. The superior oblique (SO, blue color), inferior oblique (IO, green color), and rectus capitis posterior major (RCPM, yellow color) muscles; the third segment of the vertebral artery (V3, red color); and the center of the suboccipital triangle (asterisk) are shown. (i and j) Right far lateral transcondylar approach.

Figure 10:

Illustrative Case 3 (Patient 14). (a and b) Intraoperative findings during right suboccipital muscle dissection and exposure for the far lateral transcondylar approach. (b) The superior oblique (SO) and rectus capitis posterior major (RCPM) muscles were elevated. The third segment of the vertebral artery (blue, red, and yellow colors) was exposed. The blue color was covered by the SO, which included the lateral 68% of the third segment of the vertebral artery. The yellow color was covered by the RCPM, which included the medial 23% of the third segment of the vertebral artery. The red color was located within the center of the suboccipital triangle (asterisk).