Comparison of Anesthetic Effects of Spinal Anesthesia at L5-S1 and L3-4 under Real-time Ultrasound Guidance in Knee Arthroplasty

Background: Spinal anesthesia at L5-S1 is rarely selected in clinical anesthesia, and many characteristics of L5-S1 spinal anesthesia are not well-documented in the literature. Objective： To investigate the efficacy and safety of real-time ultrasound-guided L5-S1 and L3-4 space spinal anesthesia in knee arthroplasty. Methods: A total of 46 patients who underwent elective knee arthroplasty from December 2023 to December 2024 were randomly divided into L5-S1 (group A) and L3-4 (group B) with 23 cases each. All patients were treated with paraveterbral approach spinal anesthesia under real-time ultrasound guidance, a sagittal puncture was performed in the lateral decubitus position of the affected extremity. Cerebrospinal fluid was observed in the lumbar puncture syringe to determine the success of the puncture. After successful puncture, 15mg of 0.75% bupivacaine (0.75% bupivacaine 2ml + cerebrospinal fluid 1ml) was injected. A third party recorded the highest sensory block level, the time required to reach the highest sensory block level, the incisional sensory block recovery time, the mean arterial pressure and heart rate and their changes at each time point during the operation, the operation time, the satisfaction rate of anesthesia, the use of vasopressors, and the incidence of post-operative complications (headache, parasthesias, hypotension, thrombotic events, nausea and vomiting, etc.). Results: In the L5-S1 group, the highest blocked spinal segment was lower and the total number of spinal segments blocked were fewer, the time to reach highest blocked spinal segment was longer, the recovery time of incisional sensory block was later (P < 0.001). The mean arterial pressure of patients in the L5-S1 group decreased at various time points such as 10 minutes after spinal anesthesia, at the beginning of the surgery, and before PACU discharge. However, the decrease in blood pressure was smaller (P < 0.05), with greater hemodynamic stability. Eight patients in the L3-4 group required vasopressors to maintain hemodynamic stability. The satisfaction rate of anesthesia in both groups was 100%. Conclusion: Under real-time ultrasound guidance, both L5-S1 and L3-4 intervertebral space spinal anesthesia can be safely used in knee joint replacement surgeries. Under the same dose of bupivacaine, the total number of spinal segments blocked in the L5-S1 were fewer, the recovery time of incision sensory block was longer, and the intraoperative hemodynamics were more stable. Trial registration: This trial was registered on July 21, 2025 at the Chinese Clinical Trial center (No. ChiCTR2500106275). Retrospectively registered.