The investigators believed the analgesic efficacy of adductor canal block on patients receiving total knee arthroplasty. However, the analgesic effects of different delivery regimens and duration of effects are variable. The investigators hypothesize that using continuous infusion and shorter interval bolus of local anesthetics to perform adductor canal block will reduce pain scale and opioid consumption in patients receiving total knee arthroplasty compared with longer interval bolus of local anesthetics.

Sensory innervations contributing pain after total knee arthroplasty (TKA) include branches of femoral, obturator and sciatic nerves. Branches of femoral nerve contribute the most pain sensation in TKA including nerves to the vastus medialis, intermedius, and lateralis, medial and intermediate femoral cutaneous, and saphenous nerves. Smaller contribution of pain sensation from branches of fibular and tibial nerves, and posterior branch of obturator nerve. Multiple techniques of nerve block could anesthetize some or all of the sensory innervations, but analgesia with motor sparing is important for early recovery and rehabilitation after TKA. For both pain reduction and motor function, adductor canal block (ACB) combined with local infiltration analgesia is considered more feasible than other peripheral nerve blocks.

ACB could anesthetize nerves beyond in adductor canal. Anatomical studies revealed the extended spreading of local anesthetics (LA) beyond adductor canal when performing ACB, and caudal spreading could reach popliteal fossa through adductor hiatus. Cephalad spreading of LA in ACB is limited and rarely extending to femoral triangle even when injecting from proximal adductor canal, but the cephalad spreading also depends on the volume of injectants and using tourniquets.

In clinical studies, both ACB injection site and volume of injectants were investigated. Clinical trials and systematic reviews revealed the similar efficacy of analgesia when ACB injection at proximal and distal adductor canal, although the volume and pattern of injection (bolus or continuous) were variable. Regarding to the volume of injectants, 20ml injectant of local anesthetics would be adequate without prominent motor impairment compared with smaller volume.

Previous systematic reviews and meta-analysis have confirmed better analgesia with continuous infusion of ACB than single shot, but few studies explored the difference of intermittent bolus and continuous infusion. One clinical trial compared continuous infusion and intermittent bolus of ACB in patients receiving TKA, two other trials investigated the difference in healthy volunteers and patients receiving knee arthroscopy. All these three studies concluded no difference of analgesic efficacy. However, no consistent volume and frequency of injection was studied. Whether longer interval of intermittent bolus was the same with continuous infusion in analgesic efficacy is still need to be further verified.

Inclusion Criteria:

• Adults receiving unilateral total knee arthroplasty under spinal anesthesia
• American society of anesthesiologists 1-3

Exclusion Criteria:

• Could not cooperate
• Allergy to medicines used in the study
• Chronic pain
• Long term opioid use
• Neuromuscular disease
• Surgical complication: massive bleeding, postoperative ICU, unanticipated procedure