Integrated Presentation of CT/MR Images, Force and Position for Endoscopic Surgery Training

A writing instrument is described, which includes a writing assembly, a holder for an ink reservoir joined to the writing assembly, a removable barrel secured by rotation to the holder, and a removable cap which, in the closed position, fits over and protects the writing assembly. The cap and writing assembly are rotatably integrated in both rotational directions when the cap is in the closed position, so that removal of the barrel is effected simply by rotating the barrel relative to the cap without it being necessary to remove the cap and hold the writing assembly in the hand.