Quantitative Analysis of Surgical Working Space during Endoscopic Skull Base Surgery

Background The transsphenoidal endoscopic approach has widely been accepted as standard practice for accessing the majority of tumors of the skull base. Over the years, varying approaches have been described to improve visualization and maximize working space within the sinonasal corridor. Specifically, the approach may include a limited or wide posterior septectomy and the partial resection of one or both middle turbinates. No study to date has quantitatively assessed the improvement in the overall distance of working space, and distance between instruments/endoscope with these various maneuvers. Our study sought to calculate these measurements and determine the sequential quantitative improvement in working space.

Methods Following placement of fiducial markers, cone beam computed tomography (CT) scans of four cadaveric heads were obtained for registration of an optical tracking system with calibrated tracked endoscope and pointer. The nasal sill was defined as the primary reference point for all measurements. Each head was sequentially dissected: (1) sphenoidotomy and limited posterior septectomy, (2) unilateral middle turbinectomy, (3) bilateral middle turbinectomy, and (4) wide posterior septectomy. After each subsequent dissection, the maximal craniocaudal and mediolateral distance (mm) and angles (degrees) reached by an optical tracker was calculated at the level of the sphenoid face and sella, first through the left, and then right nare. These measurements were collected both with optical tracking via CT guidance, and under direct visualization with an endoscope. In two specimens, an additional measurement of the distance between the pointer and the tip of the endoscope was calculated. Statistical analysis was completed using SPSS 17.0.

Results With each subsequent dissection, a significant improvement in access, both craniocaudal (17 ±  3; 20 ±  3; 22 ±  10; and 22 ±  5 mm) and mediolateral distances (21 ±  3; 24 ±  3 mm; 26 ±  3; and 29 ±  5 mm), was observed at the level of the sphenoid face (p < 0.05). This same effect was observed at the level of the sella in the mediolateral dimension (23 ±  4 vs. 20 ±  4 mm; p < 0.05) with wide posterior septectomy, but not for uni- or bilateral middle turbinectomy. A small increase in the craniocaudal and mediolateral angles was observed at the level of the sphenoid face and sella with each subsequent dissection, but did not reach significance. A significant improvement in the mean distance between the optical tracking pointer and endoscope (sphenoid: 47 ±  10 mm; sella: 72 ±  8 mm) was found when a wide posterior septectomy was performed compared with limited posterior septectomy (sphenoid: 18 ±  2 mm; sella: 37 ±  5 mm) (p < 0.001). This effect was not observed for either uni- or bilateral middle turbinectomy. Overall, the side and degree of visualization (blind or direct visualization) had no impact on any of the measurements.

Conclusion Compared with limited posterior septectomy, resection of uni- or bilateral middle turbinates, and/or performing a wide posterior septectomy, maximizes access for working at the level of the skull base. In addition, a wide posterior septectomy will enhance the field of view by permitting a greater distance between working surgical instruments and the endoscope.