The OPTOTRAK gives closer distances to the true 10 mm spacing of the holes in the calibration bar than the FARO arm, the mean error being 0.09 mm. It also delivers smaller spreads for each hole. The measured accuracy of both the OPTOTRAK and FARO arm devices closely matches the manufacturer's specifications. Generally the FARO arm determinations of hole spacing are within 0.5 mm, with a mean value of 0.24 mm, and have spreads of 0.5 mm. However, some distances are determined more accurately, with 0.1 mm not being uncommon.
Although the accuracy of the surgical arm has been accessed for only a small fraction of the entire spatial range of the device, we do not expect large differences in its behavior in other regions.
Even though this experiment shows that the OPTOTRAK is a more accurate localizing device than the FARO surgical arm, we believe that the latter is adequate for surgical applications because its accuracy is significantly better than the current accuracy level observed in based on MR images. However, in any attempt to increase the accuracy of the issue of the localizing device accuracy must be addressed more thoroughly.