The device was returned for evaluation.Initial observation shows the locking cap having an indentation at the center of the face that contacts the rod.The imaging provided shows the engagement between the rod and the locking cap to be around the region of the rod where it's conical end taper starts.The length of the rod seems to be sufficient, but the rod overhang beyond the screw heads was observed to be much longer at the rod attachment end compared to its conical tip end.It's possible that the superimposing images of the left and right side rods may have made it difficult to determine the rod overhang using the lateral x-ray.At the observed position of the rod, the start of the taper may lay around the center of the locking cap, resulting in non-uniform forces exerted by the rod on the locking cap, thereby contributing towards loosening the locking cap.The indentation observed at the center of the flat face of the locking cap is further indictive of this possibility.Although the images show the rod not fully seated into the screw head prior to the second surgery, it is not certain whether the image was taken post-operatively immediately after rod placement or later after reporting the locking cap migration.An exact cause of the reported issue could not be determined.
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