Prior to placing the screws, the surgeon used "landmark check" for confirming accurate registration.The working method of the surgeon included the following steps: using burr for the penetration point, saving the virtual trajectory, using gearshift and then placing the screw.3 screws were placed successfully in l4 and l5 on the right side, and in l4 on the left side.On the final screw (left, l5) the surgeon was struggling to place the screw and align it to the trajectory he had made.In addition it was noted that unlike the screws that were positioned on the right side, there was no retraction, however no movement of the clamp was noticeable.A feeler confirmed being in the bone on all sides of the trajectory.This was confirmed by both surgeons.Following this, the screw was placed while aligning it to the trajectory that had been saved.The surgeon verbally confirmed he had "great bite" in bone.The system demonstrated that the screw was in bone, however 2d confirmation shots that were taken using the c-arm, demonstrated that the last positioned screw (l5 left) was not in bone.Landmark check that was performed on the screw tip, demonstrated that the system was accurate, however, a landmark check must be performed on a known anatomy landmark and not a the tool.Checking registration accuracy while touching the screw tulip may be misleading, as the tulip may be rotated to a different angle.Neuromonitoring confirmed that everything was stable.Screw was redirected using freehand technique.Case proceeded and ended with decompression as intended.No patient harm or adverse event was reported.The screw tip and trajectory coordinates as well as post-op scan were not provided, therefore the investigation was done based on the system's logs.If the information of the screw trajectory as detected by the system was provided, it could be compared to the trajectory found on the 2d scan, thus confirming that the system presents reality in an accurate way.Several possible causes (such as mechanical problems, inaccurate registration, clamp movement, camera tracking problem) were analyzed during the investigation.However, based on the available information, these causes are not likely to be the reason for the inaccuracy.2 additional possible reasons could be associated with the surgical technique: the screw driver / screw interface was loose allowing for a deformation / bend at the screw driver / screw interface during screw positioning, permitting that screw to take a different trajectory than displayed on the nav; and/or the starting point is either the same or slightly shifted inferiorly - phenomenon known as entry point shift or skiving which is a known issue and may result in inaccurate screw placement due to patient anatomy or human user error.As the surgeon complaint about struggling to place the screw and align it to the trajectory he had made and verbally confirmed he had a "great bite" in bone, this seems to be the most likely reason for the misplacement.Based on the available information, the cause of the problem can't be concluded.Since it was impossible to reach unambiguous conclusion that the problem can't be attributed to the xvs system, it was decided to report the complaint.
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