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The development of increasingly complex spinal surgical techniques and instrumentation has meant that 2D lateral intraoperative fluoroscopy is now considered to be insufficient for safe and effective insertion of implants. The application of the principles of intracranial neuronavigation to the spine is not straightforward, for several reasons. Registration of the spine cannot reliably depend on skin markers or fidu-cials, in view of the high mobility of the spinal column and the overlying skin. Indeed, registration needs to be performed intraoperatively on the exposed spinal anatomy of the segment requiring surgery, using points that are easily and accurately identifiable on the exposed spine and on the pre-operatively acquired images. These may include the superior and inferior portions of the spinous processes and the medial and lateral limits of the facet joint in the cervical spine, and the posterolateral aspect of the transverse process tips on each side in the thoracic spine. Problems may arise when the posterior elements of the relevant segment are disrupted by trauma or previous surgery. A minimum of three points is required for the vertebra in question. Because each level in the spine represents a separate and distinct anatomical structure, each vertebra should ideally be registered and tracked individually during surgery. The spine is mobile within the body and therefore the reference arc or LED array must be attached to the spine itself. In practice, this is clamped to a spinous process of the same, or an adjacent, vertebra for posterior approaches, and to a Caspar retractor for anterior approaches [25]. This is essential because the spatial relationships of adjacent spinal segments during pre-operative image acquisition (in the supine position) may be different from those during surgery, often in the prone position, particularly in situations of spinal instability such as highgrade spondylolisthesis and spinal fractures. Frequent perioperative confirmation of registration accuracy is advised; this is readily done by placing the activated probe on an easily identifiable bony point within the operative field and by ensuring that the cursor on the computer points to the corresponding point on the pre-operative image. A significant decrease in accuracy, usually recorded at around 2 mm, should prompt re-registration. The absence of clear bony landmarks and the flat 2D nature of the anterior vertebral bodies imply that accurate registration of the spine for anterior or antero-lateral approaches remains difficult.

Virtual fluoroscopy technology (FluoroNav, Medtronic Sofamor Danek Inc., Memphis) does not require a pre-operative image. The C-arm is equipped with an LED attachment and a fiducial display that acts as a calibration target. The bony anatomy is first exposed; a reference arc is then attached to the spine. A lateral or antero-posterior image is taken. The fiducial display is used by the computer to register the anatomy instantaneously. Data from the optical camera allow the computer to identify the spatial relationships between the C-arm and the reference arc attached to the spine. This registered image then forms the basis on which navigation, using LED-marked instruments that appear on the image in real time, can proceed.

Probably the most important spinal application of image-guided surgery is the insertion of pedicle screws [26,27]. Pedicle screws confer high rigidity to a spinal construct, allowing the insertion of a shorter and more reliable construct, with maximal preservation of movement at the adjacent segments. In the lumbosacral spine, using perioperative radiography only, the rate of penetration of the pedicular cortex has been shown to be between 21% and 31%. Poor pedicle screw insertion is associated not only with neural injury, but also with fixation failure, particularly if the pedicle is fractured. Image guidance allows evaluation of the pedicular anatomy, the selection of the appropriate screw entry point, the identification of the optimal trajectory in the axial and sagittal planes, and also the ideal depth of insertion, allowing the longest bone purchase in the best-quality bone.

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