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Background and Aims: Successful total knee replacement (TKA) is dependent on balancing peak load at the medial and lateral tibiofemoral joint interfaces. This can be achieved using a sterile sensor system intra-operatively. On the request of one surgeon at our institution, we explored the feasibility and safety of spinal anesthesia with limited motor blockade. Methods: 25 patients were enrolled in an IRB-approved non-randomized pilot study. For spinal anesthesia, a solution consisting of 1 mL of 5 mg/mL isobaric bupivacaine with 1.5 mL sterile saline solution containing 7.5 mcg of sufentanil was administered. During surgery, after components were cemented, patients were awakened and asked to move their leg in order to measure pressure balance. If an imbalance was noted, the surgeon would make adjustments intraoperatively. Results: During the intraoperativewake-up test, 15 patients successfully flexed and extended at the knee on command, while 10 were too weak for meaningful testing. As we reduced the local anesthetic volume to 0.8 mL in patients shorter than 160 cm, that issue was eliminated. One patient had neutral recollection of the test on follow up. No patient had pain or other side effect. Conclusions: A spinal anesthetic using sufentanil in combination with lowdose local anesthetic appears feasible and safe to provide surgical anesthesia for TKA. By performing a motor-sparing spinal anesthetic with an intraoperativewake up test, we can allow surgeons to test active pressure balance to improve the accuracy of the pressure balancing technique. A randomized study is in preparation to determine whether long-term surgical result is improved

Introduction Successful total knee replacement (TKA) is dependent, among other factors, on balancing the pressure in both femoro-tibial compartments using judicious soft-tissue release and insert thickness. Recently, some surgeons have started to test passive, soft tissue controlled pressure balance with the assistance of a sterile sensor system intra-operatively. One surgeon at our institution requested for the anesthesia team to provide surgical anesthesia for his patients that would allow him to monitor active pressure balance intra-operatively, i.e., with the patient actively flexing and extending the knee during surgery. A high-dose intrathecal sufentanil in combination with low dose local anesthetic can provide a sensation-only block with no added medical or pain risk to the patient. By performing a motor-sparing spinal anesthetic with an intraoperative wake up test, we can allow surgeons to test active pressure balance to improve the accuracy of the pressure balancing technique. Materials and methods (NA for case report) NA Results/Case report A 46-year-old male with a history of right knee osteoarthritis underwent a right TKA. A spinal anesthetic was performed at the L3-L4 interspace using a mixture of 7.5 mcg of sufentanil diluted to 1.5 mL using normal saline, and 1mL of isobaric bupivacaine (5 mcg/mL). Sedation was maintained with a propofol infusion. Once components were cemented, sedation was paused and a wake-up test was performed. The patient was verbally responsive. The surgeon then asked the patient to lift his right leg up in the air and to flex and extend at the knee. The patient complied without signs of weakness or pain. The patient was then sedated again with 1 mg midazolam, 25 mcg fentanyl, and the propofol infusion was resumed for the remainder of the case. Post-operatively in PACU, the patient was hemodynamically stable, reported a pain score of 0, had intact lower extremity strength and no recollection of the intraoperative test. Discussion The current standard of care for patients undergoing a TKA under spinal anesthesia typically includes 2 to 3 mL of a long acting local anesthetic such as 0.5% bupivacaine. This dose used for single shot spinals is adequate for surgical anesthesia; however, patients will also exhibit motor weakness. As a result, a test of active soft tissue pressure balance will not be possible with this dose. Especially in obstetrics where motor function is important, neuraxial opioids can be combined with a lower dose local anesthetic to provide to surgical anesthesia and analgesia. Using a spinal solution with a low dose of local anesthetic in combination with an opioid such as sufentanil, the motor function of the operated leg is not inhibited during surgery. Other studies have used 5 mcg of intrathecal sufentanil, but due to a reduced dose of local anesthetic, we elected to increase the dose of sufentanil to ensure the patient would not experience pain. A prospective study examining the reliability of this technique as well as, from the orthopedic standpoint, the influence of balancing the pressures with active motion on the outcome of the TKA is planned