A paramedian approach is used to enter the intrathecal space. Under fluoroscopic guidance, the catheter is threaded to the level of the sub-stantia gelatinosa at which pain transmission is modulated. Contrast is often injected to confirm appropriate catheter placement and to document free flow of fluid. After a tract of local anesthetic is applied, the stylet of a Tuohy needle is placed beside the first needle and threaded laterally. A second 17-gauge Tuohy needle is advanced over the stylet. At this point the first (intrathecal) Tuohy needle is withdrawn and the catheter is threaded down the second needle. After the patient has completed the trial, the original catheter cannot be internalized and must be removed and discarded. This approach has the advantage of not creating incisional pain that can confuse the trial; another benefit is that the device can easily be removed if the trial is unsuccessful. On the other hand, instrumenting the spine twice is necessary if the patient does well and then undergoes implantation.
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