Comparative Anatomic Study Between Intradural Suboccipital Retrosigmoid and Extradural Transzygomatic Middle Fossa Approaches to Expose the Trigeminal Nerve

Background: Meckel cave tumors are relatively rare, especially trigeminal nerve (TN) schwannomas. These tumors frequently project through the trigeminal pore, occupying the middle and posterior fossae. The most used routes to this region are the suboccipital retrosigmoid intradural approach (SORSA) and the transzygomatic middle fossa approach (TZMFA). Both approaches allow further exposure by adding intraoperative techniques, such as removing the suprameatal tubercle (retrosigmoid intradural suprameatal approach [RISA]) and the petrous apex (TZMFA-PA), respectively. This study aims to understand how TN exposure differs between both surgical approaches and how it increases by adding specific surgical maneuvers to these techniques.

Methods: Five formalin-fixed adult cadaver heads were submitted to high-resolution computed tomography and their images were loaded into the neuronavigation device. Anatomic key points were defined along the outline of the TN, and their three-dimensional spatial locations were collected following each surgical approach. This process allowed the calculation of the TN exposed area obtained through each technique.

Results: The mean areas of exposure of the TN were 125.9 mm² with SORSA and 208.9 mm² with RISA, which represents an additional mean gain of 61.92% (P = 0.047). Using TZMFA, a mean exposure of 419.24 mm² was obtained. When TZMFA-PA was used, the mean exposed area was 486.03 mm², representing a mean gain in the exposure area of 16.81% (P = 0.072).

Conclusions: Our study suggests that TZMFA allows better exposure of TN ganglionic and postganglionic segments, and the removal of the PA adds the preganglionic segment visualization, although with less TN exposed area compared with RISA. With SORSA, the additional suprameatal tubercle removal shows the trigeminal pore and the medial margin of the central portion of the TN ganglionic segment, making it possible to expose the mouth of the Meckel cave and part of its contents.