(Polenov Research Neurosurgical Institute, Saint Petersburg)
Tumors of the skull base account for a rather numerous group of all neoplasms of the skull and brain. Efficacy of their removal has a direct effect on duration of a five-year survival of patients. A subtemporal preauricular approach via subtemporal and middle cranial fossae is the widest access, which allows to remove a tumor from the middle cranial fossa, infratemporal fossa, lateral segments of the sphenoid sinus, apex of the temporal bone pyramid and to examine anterolateral areas of the brain stem, lying ahead from acoustic-facial and caudal groups of nerves. It was proposed by L.N. Sekhar, L. Scheramm, N.F. Jones in 1987. Such an extended approach was achieved due to use of an original method of exposure and transposition of the ICA intrapetrosal segment. It permitted to perform subsequent resection of the whole apex of the temporal bone pyramid. The main stages of this access were described as independent approaches, i.e. a subtemporal orbito-zygomatic transsphenoidal approach (S.D. Pitelli, G.G. Almada, 1986) and an anterior transpyramidal approach (K. Sugita et al., 1979; Kawase et al., 1985; M. Samii, 1986).
When this approach is performed on the skull base of various forms and in compliance with one and the same landmarks, it can result in its different size: either too extended or too narrow. Unjustified resection of normal bones of the skull base makes an approach more traumatic and demands additional plasty of the skull base in order to prevent liquorrhea. Some experimental studies and the experience, accumulated by surgeons, served the basis for determining a minimum convenient angle of operation activity, equal to 15o. It allowed to perform exact manipulations, using microsurgical technique.
We performed 8 right-side and 7 left-side preauricular subtemporal approaches via subtemporal and middle cranial fossae. They were made on 15 cadavers in the Department of Pathoanatomy of Saint Petersburg Hospital N 17.
In performing a subtemporal orbito-zygomatic transsphenoidal stage, we divided all preparations into groups in accordance with two criteria. The first criterion was necessity of additional resection of the mandible coronoid process or its absence. The second criterion was a segment of the sphenoid sinus, exposed after resection of the external plate of the pterygoid process (”bone window” I). The results demonstrated, that visualization of middle segments of the sphenoid sinus was more probable, when the middle cranial fossa (45mm) and an upper margin of the temporal bone pyramid (60 mm) were longer. The short middle cranial fossa and upper margin of the temporal bone pyramid, a backward shift of the oval foramen from a line of the saddle dorsum and a basilar angle of more than 125o demanded not only resection within the limits of the lateral triangle, but also resection in the anterolateral triangle of the cavernous sinus, which allowed to approach middle segments of the sphenoid sinus.
In studying an anterior transpyramidal stage all preparations were divided into 2 groups, depending on exposure of the lower cavernous sinus or a failure to do it. It turned out, that probability of its exposure in performing a subtemporal anterior transpyramidal stage was higher in a bigger length of the middle cranial fossa (more than 45 mm) and a greater height of the pyramid apex (more than 8 mm), combined with a small angle of convergence of the pyramids (100o). In case of increase of the skull clivus length and width and an angle of convergence of the pyramids up to 110o, dimensions of exposed dura of an anterolateral surface of the posterior cranial fossa and clivus became smaller. As a result it was impossible to reach the lower cavernous sinus. This form of the skull base was characterized by a following peculiarity: better downward visualization of dura in the direction of the great foramen demanded exposure and transposition of a cavernous segment of the internal carotid artery. Dura areas, exposed after an anterior transpetrosal stage, and transposition of a cavernous segment of ICA were compared. All preparations were divided into 2 groups. The first group included those preparations, where an area of dura, exposed after transpetrosal resection within the limits of Kawase’s triangle was much bigger than an area of dura, exposed after transposition of a cavernous segment of ICA. The second group comprised preparations, where zones of exposed dura of an anterolateral surface of the posterior cranial fossa and clivus were approximately equal.
Analyzing changes of parameters of the skull base structure in groups with a lateral approach via the subtemporal and middle cranial fossae, we found out, that the most important thing was a form of the posterior cranial fossa. There were two conditional forms, which looked like “a glass” and “a saucer”. When the posterior cranial fossa had a form of “a glass”, an exposed area and, as a result, efficacy of an anterior transpetrosal stage were much greater in comparison with transposition of ICA. On the contrary, if the posterior cranial fossa looked like “a saucer”, addition of ICA transposition to anterior petrosal resection was extremely useful, as it increased an area of exposed dura by 40-50%. The given information can be important in choosing a version of a lateral subtemporal approach for removal of extensive neoplasms of lateral segments of the sphenoid sinus and tumors of the apex of the temporal bone pyramid with extracranial spread into the subtemporal fossa.