This report is being resubmitted for an incident that occurred earlier.The locking cap and mating screw were returned for evaluation.The bottom surface of the locking cap was polished around the outside and a crack had partially propagated from the thread start across the minor diameter.Marks and polished surfaces from rod contact and rubbing were found on the screw on both sides of the rod slot and on one side of the saddle.The observed wear marks are consistent with rubbing between the rod and screw head after the locking cap had loosened.Witness marks typically observed during proper tightening could not be verified.The fracture across the thread start may have been caused by contact with the rod and fatigue loading after the locking cap had loosened, or by damage to the thread start during implantation.The observed pseudoarthrosis and two years of patient loading indicate that the implants may have been subjected to fatigue loading in excess of the anticipated life of the implant however, the exact cause of the reported issue could not be determined.
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