Phase I: The Bayesian Optimal Interval (BOIN) design is adopted to evaluate the volume escalation of liposomal bupivacaine (10, 15, 20, 25, 30, 35, 40 mL). There are 3 cases in each group, with an initial volume of 20 mL (referring to previous ACB studies d). The target incidence of adverse events (quadriceps femoris muscle strength decline ≥ 25%) is set at 10%, and the maximum volume is limited to 40 mL to avoid local anesthetic toxicity.

Phase II: Cohort expansion is carried out at volume levels lower than the MTV. The Bayesian Optimal Phase II (BOP2) design is adopted, with each group expanded to 12 cases to evaluate the balance point between block success rate and motor function preservation.

Patient Positioning: Supine with the surgical limb slightly flexed and externally rotated.

Operator Setup: The operator stands on the surgical side, with the ultrasound machine positioned contralaterally. The anterior thigh is sterilized and draped.

Ultrasound Technique: A high-frequency (5-12 MHz) linear ultrasound probe (Sonosite M-Turbo), protected by a 3M Tegaderm dressing, is placed perpendicular to the skin at the midpoint between the femoral trochanter and the superior patellar margin. The probe is maneuvered to visualize the adductor canal structures.

Needle Insertion: After local anesthesia at the insertion site, a 22G 80 mm needle is advanced using an in-plane technique until the tip reaches the triangular hypoechoic area lateral to the femoral artery. Following negative aspiration for blood, 1.33% liposomal bupivacaine is injected around the target site.

The maximum voluntary isometric contraction (MVIC) of the quadriceps femoris muscle is measured using a handheld dynamometer (HHD) and then re - measured 30 minutes after the block. A decrease in muscle strength of ≥ 25% is defined as a significant impact on motor function.

Sensory block at 30 minutes after surgery (pinprick test at several sites in the area innervated by the saphenous nerve) and a resting NRS score ≤ 3 points. The sensory block is evaluated using a pinprick test for sensation. This test is performed before the adductor canal block (baseline) and every 5 minutes after the nerve block until 30 minutes have elapsed or the surgery begins. The sensory test is carried out in the distribution area of the saphenous nerve (medial side of the lower leg), as well as on the anterior, medial, lateral, and posteromedial sides of the surgical knee joint, and above the patella. A sensory score of 2 indicates normal sensation, 1 indicates hypoesthesia, and 0 indicates no sensation. We define a successful adductor canal block as the occurrence of sensory block in the surgical knee joint within 30 minutes after the nerve block, that is, the loss of sensation to the pinprick (sensory score of 1 or lower).

The NRS (Numeric Rating Scale) scores range from 0 to 10 in total. A score of 0 represents no pain, while a score of 10 indicates severe pain. A score of 0 to 3 indicates mild pain, which is tolerable and does not affect sleep. A score of 4 to 6 indicates moderate pain, which is more obvious and will somewhat interfere with the patient's daily life and sleep, but is still bearable. A score of 7 to 10 indicates severe pain, which is intense and difficult to endure, seriously affecting the patient's daily life and sleep. The lower the score, the better the analgesic effect of the adductor canal block performed with liposomal bupivacaine.