(Polenov Research Neurosurgical Institute, Saint Petersburg,
Faculty of Operative Surgery and Clinical Anatomy, Saint Petersburg State Medical University)
Parameters of an extended subfrontal approach (anterior transbasal approach according to Derome, 1988) were estimated depending on data of surgical anatomy. This approach, consisting in bifrontal trephination of the skull, formation of an additional naso-orbital flap, extradural exposure of the anterior cranial fossa (ACF), an excess to the sphenoid sinus after resection of posterior segments of the cribriform lamina, planum sphenoidale and saddle tubercle, permits to remove tumors in the area of the clivus and paranasal sinuses under direct visual control. However, it has some serious disadvantages: excessive traction of frontal lobes leads to intellectual and mnemic disorders in a postoperative period (1); extensive resection of the ACF complicates its plasty and results in nasal liquorrhea (2); a naso-orbital flap can be infected in a postoperative period.
Modeling of this approach on embalmed “head-neck” preparations of middle-aged males and females was carried out. The goal of the study was to determine indications and to give anatomic substantiation of approach modifications with the purpose of reducing the rate of main complications. The problems to be solved were as follows:
1. Revealing main areas of the ACF resection, depending on a skull base structure.
2. Determination of anatomic indications for mobilization of a naso-orbital flap.
3. More precise description of high-risk zones, which can be encountered in tumor removal with the help of this approach.
Results. It has been found out, that an upward shift of frontal lobes with a spatula should not be excessive. An angle of 40-35o between it and the ACF surface is quite sufficient. Optimum parameters of the approach are achieved, when a spatula and the clivus form an angle of 175o. There are 3 standard zones of the ACF resection, which ensure an approach to the sphenoid sinus and body of the sphenoid bone. A choice of a resection zone is first of all dependent on a ratio between a length of the planum sphenoidale and cribriform lamina. When the former is longer, its resection (zone 1) provides enough space for an exposure of two thirds of the clivus dura length. Sometimes the above structures are of equal length or the cribriform lamina is longer. In this case it is necessary to broaden a resection zone of the ACF in a forward direction so that it reached a half of the cribriform lamina (zone 2). A basilar angle value has almost no effect on a necessary zone of the ACF resection. The third zone goes from the saddle tubercle up to the cecal foramen. It is used only in removal of a tumor from the ethmoidal labyrinth or nasal cavity. Mobilization of a naso-orbital flap is most frequently necessary in the ACF resection of the second type. In this case an attack angle is limited by a posterior border of a naso-orbital flap, but not by an anterior margin of the ACF resection. A standard transbasal approach with the third zone of the ACF resection (up to the cecal foramen) and a mobilized naso-orbital flap does not allow to expose all dura of the clivus (from the dorsum up to the great foramen). There is always the so-called dead zone of a higher surgical risk. Here an anterior genu of the internal carotid artery protrudes into the sphenoid sinus cavity and dura of a medial surface of posterior segments of the cavernous sinus forms a fold near the nasopharynx vault. It is an area of tranformation of the ICA intrapetrous segment into its intracavernous segment. Landmarks in removal of tumors, located in these areas, can be as follows: a beginning of an intracanal segment of the optic nerve (1), the nasopharynx vault (2). A width of possible exposure of the clivus dura is limited by a projection of the lower cavernous sinus. If a tumor spreads into lateral segment of the sphenoid sinus or the pyramid apex, parameters of the approach under discussion are unsatisfactory. It is necessary to use lateral approaches for removal of tumors from these anatomic areas.