Stereotactic anatomy of the posterior cranial fossa. A study of the transcerebellar approach to the brainstem.

The stereotactic transcerebellar (suboccipital) approach to the brainstem is one of the most accessible routes to reach targets located in the pons or in the medulla oblongata. Its use requires a perfect knowledge of the stereotactic anatomy of the posterior cranial fossa, and a standard reference system related to structures of the brainstem itself which can easily be visualized by Nuclear Magnetic Resonance. The present work consists in a three-dimensional variability study of the human posterior cranial fossa with its main contents, based on the investigation of 30 normal specimens (hemifossae). A new stereotactic reference system is proposed for the pons and the medulla, comprising three orthogonal** planes: the midsagittal plane, the IVth ventricular floor plane and the pontomedullary junction plane. A standard "safety access tunnel" was defined for the transcerebellar approach to the pons and, with some limitations, to the medulla. A complementary investigation, based upon angiographic studies, was carried out to define a safe "entry zone" to the posterior cranial fossa of the occipital bone beneath the transverse venous sinus.