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L. M. Thomas, V. L. Roberts and E. S. Gurdjian

a three-dimensional model of the skull. Pressure gradients have been postulated and demonstrated on the basis of fluid mechanics. 6 The shape of the gradients and the location of the nodal point have not been demonstrated experimentally. The experimental data presented here demonstrate the existence of a pressure gradient across the fluidfilled human skull as measured along three orthogonal axes. Method The experimental model consisted of a human skull from which the soft tissue and brain had been removed. In one model, the falx and tentorium had been

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B. H. Dawson, E. Dervin and O. B. Heywood

visual indication on brain maps of the electrode tip position for any chosen instrument coordinates 3. To show the precise direction, the degree of angulation and the pathway of the electrode on three-dimensional models of the brain 4. To correlate during the operation physiological data with anatomical data (spontaneous electrical activity records and clinical effects of brain stimulation). Other advantages of the monitor technique are: 5. There is no need for any calculation at any stage during the stereotaxic procedure. 6. No special arrangements

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Anthony J. Raimondi

specialization within the field of neurological surgery, within the field of radiology, in a field of its own? Should patients be referred directly to the neuroradiologist by either the internist or the neurologist and, if so, how will a neurosurgical resident learn to evaluate clinical problems, to select contrast studies, to determine when one study should be abandoned and another begun? 4) If the involvement of the neurosurgical resident in neuroradiologic procedures is less than total, how will he learn to see the brain as a three dimensional model consisting of parenchyma

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Russell A. Brown

components and phantom tumor spheres. Right: The edge outlines of both frame components and phantom tumors extracted from the scan shown left . See text. The outlines from a given slice are transferred from the coordinate system of that slice to the coordinate system of the frame and its visual simulation ( Fig. 2 ). This is done using the diagonal 5, 8 and vertical localizing rods attached to the frame ( Fig. 1 upper right ). Transferring the target sphere outlines from the 21 CT scans produced a three-dimensional model of the spheres superimposed on the frame

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Xiaoping Hu, Kim K. Tan, David N. Levin, Simranjit Galhotra, John F. Mullan, Javad Hekmatpanah and Jean-Paul Spire

which was made translucent. It is clear that the lesion was undermining the right sensory strip. Fig. 4. Case 2. Upper: “Plane-picker” display of cross-sectional images which were updated as the mouse-driven cursor was moved through the three-dimensional model of the brain. Lower: “Rehearsal” of the craniotomy which was subsequently performed on the patient. Case 3 This 58-year-old man underwent MR imaging ( Fig. 5 upper left ) which demonstrated a mass in the right cerebral hemisphere. The exact relationship between the lesion and

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Mario Ammirati, Jianya Ma, Melvin L. Cheatham, David Maxwell, Joseph Bloch and Donald P. Becker

wall and the nonampullated portion of the posterior semicircular canal, which is the only labyrinthine structure contained in this area. Drilling of the posterior pyramid remains insidious; topographical bone landmarks, while not absolute, may give the surgeon some useful guidelines. Three-dimensional models of individual petrous pyramids 8 or real-time intraoperative imaging systems represent future technologies that may be of value. References 1. Al-Mefty O , Fox JL , Smith RR : Petrosal approach for petroclival meningiomas

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Kim K. Tan, Robert Grzeszczuk, David N. Levin, Charles A. Pelizzari, George T. Y. Chen, Robert K. Erickson, Douglas Johnson and George J. Dohrmann

positioned so that its axis intersected the tumor and was normal to the scalp. The skin was then marked with an “x.” Upper Right: The position and orientation of the stylus were tracked in real time on this computer display. The upper panels show where the extrapolated stylus axis (while cursor) intersected cross-sectional image planes fixed during the procedure. The lower panels show the orientation of the stylus axis (black cursor) on three-dimensional models of the brain and skin, which the computer automatically displayed in the most en face view . The tumor is

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Masashi Tamaki, Warren McDonald, Verena R. Amberger, Emi Moore and Rolando F. Del Maestro

tumor cells. 2, 6, 9, 11, 14, 30 Monolayer and cell suspension systems do not reconstruct the cellular microenvironments present in the tumor mass or the three-dimensional nature of the extracellular matrix of the brain. Three-dimensional models, in which individual tumor cells, aggregates of these cells, or tumor tissue invade extracellular matrices 2, 30 or fetal brain, 6, 11 appear to solve some of these problems. Fundamental to the study of the invasive paradigm is the need to reconstruct the extracellular matrix independently to assess cell—matrix interaction

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Rolando F. Del Maestro

a Trattati di Architetturo (Treatise on Architecture) . 27 Francesco may have inherit- ed from another artisan, the architect Mariano di Jacopo, known as Taccola (A.D. 1381–ca. 1453), the idea of cutaway and transparency that allowed visualization of internal structures without the need for three-dimensional models. Taccola's cutaway architectural drawings not only allowed the external structure of an object to be seen, but resulted in the appreciation of internal hidden structures at the same time. 18 Francesco also exploited the concept of the exploded view

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R. Scott Graham, Eric K. Oberlander, John E. Stewart and David J. Griffiths

Object. The finite element (FE) method is a powerful tool for the analysis of stress patterns of anatomical structures. In this study a highly refined FE model of C-2 was created and validated. The model was then used to characterize stress patterns, predicted fracture patterns, and transitions between Type II and Type III odontoid fractures.

Methods. An anatomically accurate three-dimensional model of C-2 was created from computerized tomography data obtained from the Visible Human Project. The C-2 model was broken down into an FE mesh consisting of 32,815 elements and 40,969 nodes. For validation, the FE model was constrained and loaded to simulate that used in previous biomechanical studies. The validated model was then loaded in an iterative fashion, varying the orientation of the load within the validated range. A matrix of stress plots was created for comparative analysis.

Results of the validation testing closely correlated with those obtained in previous biomechanical testing. Pure extension loading produced a Type III stress pattern with maximum stress of 134 MPa. Loading at 45° produced a Type II stress distribution with a maximum stress of 123 MPa. These stresses are within 3% and 11%, respectively, of the reported yield stress of cortical bone (138 MPa). In the second portion of the study, systematic variation in the orientation of the load vector revealed that higher stresses were associated with increased lateral angulation and increasing upward inclination of the load vectors. A transition from a Type III to Type II pattern occurred with lateral orientations greater than 15° and with compressive loads of 45°.

Conclusions. The validated C-2 FE model described in this study both qualitatively and quantitatively was able to simulate the behavior of the C-2 vertebra in biomechanical testing. In this study the authors demonstrate the utility of the FE method when used in conjunction with traditional biomechanical testing.