STUDY ON PREDICTING THE IDEAL VENTRICULAR FREEHAND PASS TRAJECTORY USING OSIRIX SOFTWARE AND THE ROLE OF OCCIPITAL SHAPE VARIATIONS.

Background Cannulation of lateral ventricles via a posterior approach is a common neurosurgical procedure. It is often believed that a single entry and fiducial point applies to all. No importance is given to skull shape variations, which can lead to wrong shunt positions and revisions. Objective A virtual-reality study was conducted to find the ideal entry point, ideal forehead fiducial point, and ideal angulation of the ventricular catheter and variations in these with changes in skull shapes. Methods Fifty human cadaveric skulls were used to measure anteroposterior (AP) diameter and width and to classify shape of skulls into 4 types. Hydrocephalus (100 cases) and normal magnetic resonance images (50 cases) were studied from a PACS (Picture Archiving and Communication System) database. An Osirix DICOM Viewer (3.9.4) was used to reconstruct the images and estimate the ideal, 90°, and midline shunt trajectory and correlate the same with AP/width ratios and skull shapes. Results Contrary to popular practice, the vertical distance from the inion for ideal trajectory placement was 4 cm in all shapes and ratio groups for hydrocephalus and nonhydrocephalus cases, respectively. As the AP/width ratio increases, the fiducial needs to be placed at a higher distance from the nasion and the distance of the entry point also increased from the inion. A rounder or more dolichocephalic skull dictates a 90° approach to be better, especially as the first pass. Conclusions No magical external entry point uniformly applicable for all cases exists. Hence, there is a need to classify skulls according to shapes/ratios and to use a tailored approach for a freehand pass to cannulate the ventricles.