To date, 3D visualization model becomes more popular in the study of surgical anatomy for the merits of time-saving, reproducible, and manipulation without intra-operative pressure[19-21]. Based on the 3D visualization model containing bony and neurovascular tissues, the volumetric analysis can provide detailed information in the surgical anatomy in the skull base. We thus performed this anatomical study for the far-lateral approach using the 3D model.
Value of simulative surgical corridor
The pyramidal corridor simulating surgery can be understood as an illuminationrangelight source of operating microscope. The vertex of pyramid is the point directed by the light source. The bottom of pyramid represents the movement range of operating microscope. The surgical space in the pyramidal corridor therefore provides an intra-operative exposure range for anatomical structures. We designed two pyramidal corridors to simulated far-lateral approach exposing clivus, of which the surgical space in corridor 2 was approximate twofold than that in corridor 1.
The 3D simulative surgical corridor offers a critical condition for volumetric analysis. The result of morphometric comparative analysisonly showed that partial CN X was contained in the corridor 2 instead of corridor 1. Whereas, the detail information of increased anatomical exposure in the corridor 2 was demonstrated objectively by the volumetric analysis.
Volumetric analysis of bony tissues
Regarding bony removal, the postoperative stabilization of the craniovertebral junctionis still a concern of life quality for the patient. Stability rebuilding of craniovertebral junction was required for both corridor 1 and 2 following the intra-operative damage of occipitoatlantal joint. As the result of volumetric comparison showed, the more damage of occipitoatlantal joint occurred in corridor 2 which would face more difficulties in the stability reconstruction.
In addition, It was time-consuming procedure to perform bony drilling. Obviously, the corridor 2 faced more difficulties in such manipulation for more volume of bony tissues involved. The more disturbance caused by the extra-cranial VA during bony drilling also increased the difficulty of corridor 2.
Volumetric analysis of neurovascular tissues
The surgical spaces of the two corridors exposed the cistern lateral and anterior to the cerebellum and medulla oblongata without disturbing the brain tissues, which may represent the condition of epidural manipulation to reach clivus in the far-lateral approach. When passing through the lateral edge of foramen magnum, the posterior portion of glomus jugulare and the inferior end of sigmoid sinus was involved, which has potential risk of troublesome bleeding[24, 25]. The increasing extent of venous structures in the surgical space of the corridor 2 was the most.
Compared with the approximate twofold increase of surgical space, the unmatched less increase of CN XI and XII was shown in the corridor 2, which implies that the difficulty of CN XI and XII protection was nearly similar in both corridor 1 and 2. Meanwhile，the CN X was only involved in the corridor 2. Careful manipulation should be performed to protect the cranial nerves.Due to the characteristics of position offset of the BA, only 16 sides of surgical surface involved partial BA of which the volume was slightly more in the corridor 2. The mobilization of BA can be used to improve the access.
This anatomy study without real operative practice limit the clinical value. The regular geometric corridor simulating surgery is unable to complete in the current real operative conditions. We thus only demonstrate an objective comparison by volumetric analysis via regular corridors.This article highlights the relevant valuesof quantification forthe surgical space and anatomical exposure in reference to the technical development of 3D visualization and neuronavigation.