Stereotactic surgery: a note on instrumentation.

Stereotactic surgical instruments have developed along two lines. The more traditional and accurate machines were devised by Spiegel and Wycis (1952), Spiegel, Wycis, Marks, and Lee (1947), Bailey and Stein (1951), Talairach, HeI-{aen, David, Monnier, and Ajuriaguerra (1949), Leksell (1949), Riechert, and Mundinger (1954), and others. Most of these machines are fixed to the skull for stability, and they employ a variety of geometric methods for correcting the distortion and head rotation occurring in cranial radiographs. A stereotactic surgeon, utilizing one of these traditional machines with its accompanying method for correcting radiological distortion, is able to hit a radiologically visualized intracranial target with a high degree of accuracy. (This leaves out entirely the question of geographic variation of the anatomical target structures within the intracranial cavity in their relation to points that can be visualized by air or opaque ventriculography.) Unfortunately, most of the traditional stereotactic machines are complicated and time consuming to use. On the other hand, Parera and Cooper (1960) devised needle guiding machines, used principally in the treatment of Parkinson's disease. These machines can be applied quickly and easily and some of them are fixed to the skull. Their accuracy, however, is not as great as the traditional stereotactic machines, and they are often used in conjunction with uncorrected measurements taken directly from cranial radiographs. A stereotactic machine, like any other surgical tool, requires constant refinement. It was felt that a stereotactic machine that combined the accuracy of the traditional, skull-fixed radiologically orientated machines with the simplicity and flexibility of the needle guides would be a welcome addition to the neurosurgical armamentarium. In 1954, our group described a new method for correcting distortion in cranial radiographs with special reference to the type I McPherson stereotactic machine (Mark, McPherson, and Sweet, 1954). This method employs a skull-fixed tubular aluminum stereotactic machine with attachable lead grids. When the distances between the x-ray tube, the lead grid lines, and the x-ray plate are fixed, one can use a geometric method to plot the position of any desired target point between the grids, i.e., intracranial point. With this method, it is possible to correct the magnification error inherent in all cranial radiographs with relatively short tube-film distances and, in addition, compensate for the degree of rotation or tilt of the head. By means of the geometric plots, it is thus possible to translate radiological measurements into settings for the stereotactic machine that will hit a given intracranial point with an error of less than one half millimetre. Although this method is easy to use, the type 1 McPherson stereotactic machine itself is cumbersome. The surgical procedure with this machine has to be done in two stages: one stags for the drilling of tiny openings in the outer table of the skull and the performance of a ventriculogram, and the second stage for the insertion of the electrodes into the target areas (Mark, Ervin, and Hackett, 1960). The halo, utilized as the base of the stereotactic machine, completely surrounds the head in a wide band and, therefore, limits the possible positions and angles at which an electrode can enter the cranial vaults to reach an intracranial target. We decided, then, to design a new stereotactic machine which would overcome the defects of the first model without sacrificing its potential accuracy.