Accuracy of spinal navigation for magerl screws.

The influence of a protocol of preoperative computed tomography scanning and a special registration technique was assessed on the accuracy of navigation for implanting Magerl C1-C2-screws. The use of navigation systems for implanting Magerl screws could help to decrease the risk of complications and to reduce the required skin incision. Two parameters conceivably affecting the accuracy are the protocol of preoperative computed tomography scanning and the registration technique. Four cervical spine segments of human cadavers were scanned with two computed tomography protocols. Registration was done based on anatomic landmarks or using a specially designed percutaneous registration device. For the accuracy check, the pointer tip was placed exactly on the markers. The displayed distance on the monitor was referred as an estimate of accuracy. Varying the computed tomography protocol did not significantly affect the accuracy. The mean accuracy was improved from 3 mm after anatomic pair-point matching to 1.5 mm after matching using the percutaneous registration device. The accuracy obtainable seems to be sufficient for implanting Magerl screws by using frameless stereotactic navigation. Three-millimeter slice thickness and 2-mm table increment is a proper protocol for preoperative computed tomography scanning. Fiducial markers improve the accuracy significantly.