Far lateral transcondylar approach: dimensional anatomy

The far lateral extension of the classic suboccipital craniectomy has been found to reduce the depth of the field and improve the angle of surgical perspective to the ventrolateral clivus. The goal of the present study is to determine and compare the dimensions of the far lateral transcondylar approach with the suboccipital craniectomy.

Ten cadaveric specimens were used to study the anatomy at the foramen magnum (FM), occipital condyle (OC), and vertebral artery. The distances from the posterior midline of the FM to the medial and lateral borders of the OC were measured. The distance of the vertebral artery from the transverse foramen of C1 to its dural entry and the distance from the dural entry to the posteroinferior cerebellar artery were measured. The amount of OC removal that was necessary to expose the contralateral jugular tubercle was determined. A reference line was constructed from the posterior margin of the FM to the border of the OC. From this line, the angle of surgical approach provided by each exposure was measured.

The mean distance of the vertebral artery from the transverse foramen of C1 to its dural entry was 22+/-3 mm (standard deviation), and the distance from the dural entry to the posteroinferior cerebellar artery was 17+/-8 mm. The distance from the posterior midline of the FM to the medial border of the OC was 27+/-0.5 mm; the distance from the posterior midline of the FM to the lateral border of the OC was 40+/-0.4 mm; and the long axis of the OC was 30+/-0.4 mm. The amount of OC removal to expose the contralateral jugular tubercle without brainstem retraction was 17+/-1 mm. The angle of surgical approach versus the reference line decreased from 88+/-2 degrees with the suboccipital craniectomy alone to 47+/-2 degrees for the far lateral transcondylar exposure (P < 0.001). This angle decreased an average of 2.4 degrees per millimeter of OC removal.

Understanding the dimensions of the craniovertebral junction has clear implications for surgery in this area. If a lesion may be approached through a perpendicular exposure, the suboccipital craniectomy alone may be sufficient. Additional exposure of the ventrolateral clivus without brainstem retraction requires condylar removal. A more limited condylar removal than the 17 mm described in this report may be adequate if the entire 47-degree angle is not needed.