Accuracy is critical to all surgeries, particularly those involving the nervous system, where the slightest error can cause permanent damage. While neurosurgeons operating on the brain have dramatically improved surgical accuracy over the last several years, similar advances in surgeries involving the spinal cord have proven more vexing. One reason is that while surgeons can navigate inside the skull using an external reference-a head ring fixed to the cranium-it's far more difficult for them to find their way about when doing spinal operations. As a result, there is a significantly high misplacement rate associated with the fixing of screws in the spine-a common procedure to correct deformities and fractures.

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