A neurosurgeon will only rely on a neuronavigation system for his orientation if the system is fairly accurate. The inaccuracy of neuronavigation is an accumulation of small inaccuracies involved with each part of the technique. Brainshift' denotes the changes in anatomy that occur after the CT or MR images have been made. Since the entire system is based on preoperative CT or MR images, displacement of tissue during the operation will not be accounted for. This effect is exaggerated when a large amount of fluid is drained, like cerebrospinal fluid from the ventricular system or tumor fluid from a cyst.
Spinal surgery presents even more problems.
First, the registration-procedure is more difficult to perform since fiducials on the spinal column can only be used after they have been cleared of surrounding tissue. Secondly, the vertebrae can move a little relative to each other, which necessitates the performance of a new registration-procedure for each vertebral level. Thirdly, this registration holds true for the vertebrae themselves, but hardly for the nervous tissue within the spinal canal. In other words, the relationship between the spinal cord and the spinal column is less rigid than the relationship between the brain and the skull. Therefore, the use of neuronavigation has only been reported in spinal operations concerning the vertebrae and not in operations concerning the spinal cord.