Far lateral transcondylar approach: Dimensional anatomy

OBJECTIVE: The far lateral extension of the classic suboccipital craniectom y 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 s tudy is to determine and compare the dimensions of the far lateral transcon dylar approach with the suboccipital craniectomy. METHODS: Ten cadaveric specimens were used to study the anatomy at the fora men magnum (FM), occipital condyle (OC), and vertebral artery. The distance s 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 transv erse 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 r emoval that was necessary to expose the contralateral jugular tubercle was determined. A reference line was constructed from the posterior margin of t he FM to the border of the OC. From this line, the angle of surgical approa ch provided by each exposure was measured. RESULTS: The mean distance of the vertebral artery from the transverse fora men 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 media l border of the OC was 27 +/- 0.5 mm; the distance from the posterior midli ne of the FM to the lateral border of the OC was 40 +/- 0.4 mm; and the lon g 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 8 8 +/- 2 degrees with the suboccipital craniectomy alone to 47 +/- 2 degrees for the far lateral transcondylar exposure (P < 0.001). This angle decreas ed an average of 2.4 degrees per millimeter of OC removal. CONCLUSION: Understanding the dimensions of the craniovertebral junction ha s clear implications for surgery in this area. If a lesion may be approache d through a perpendicular exposure, the suboccipital craniectomy alone may be sufficient. Additional exposure of the ventrolateral clivus without brai nstem retraction requires condylar removal. A more limited condylar removal than the 17 mm described in this report may be adequate if the entire 47-d egree angle is not needed.