This study is designed to explore the effects of peri-neural dexmedetomidine on the duration and motor sparing potentials of adductor canal block in adult patients undergoing arthroscopic medical meniscectomy in the setting of multimodal analgesia. The investigators ultimate goal is to increase the postoperative analgesic time and to preserve quadriceps muscle strength to promote safe and early ambulation.

Peripheral nerve blocks using long-acting local anesthetics are commonly utilized as the sole anesthetic technique or as an adjuvant to general anesthesia for post-operative pain management. However, the duration of sensory block after single dose of long acting local anesthetics is not consistently sufficient to avoid the use of postoperative opioids. Many adjuvants were added to local anesthetics to augment the potency and prolong the duration of peripheral nerve blocks. Alpha-2 adrenoceptor agonists such as clonidine have been shown to increase the duration of peripheral nerve block. Dexmedetomidine is a more potent and selective α-2-adrenoceptor compared to clonidine. Peri-neural dexmedetomidine was evaluated in animal studies where it prolonged the duration of sensory and motor blocks of local anesthetics without any evidence of neurotoxicity for up to 14 days after initial administration. The effectiveness of perineural dexmedetomidine in augmenting the duration of sensory block of upper limb extremity blocks is based on good quality clinical evidence.

Arthroscopic knee surgery is associated with severe postoperative pain which could be adequately managed by femoral nerve block. However femoral nerve block has been associated with significant motor block and decreased quadriceps muscle strength which may delay ambulation and increase the risk of falling. More recently, adductor canal block (ACB) emerged as a selective motor sparing effective variant of femoral nerve block. Compared with baseline values, the adductor canal block reduces quadriceps muscle strength by 8%, versus 49% for the femoral nerve block. A recent dose finding MRI study reported that 20 ml of local anesthetic is the closest volume to the ED95 for adductor canal block with minimal proximal spread and an estimated success probability of 95.1% (95% credibility interval: 91-98%).

Inclusion Criteria:

• Adult patients of aged (18-45), ASA physical status I or II undergoing arthroscopic medial meniscectomy.

Exclusion Criteria:

• Body mass index greater than 35 kg/m2.
• Pregnancy.
• Unstable coronary artery disease, congestive heart failure, or arrhythmias.
• Baseline heart rate (HR) less than 60 beats/min or baseline systolic blood pressure less than 100 mmHg.
• Pre-existing neurological deficits or neuropathy.
• Significant psychiatric or cognitive conditions interfering with consent or assessment.
• Significant renal or hepatic impairment;
• Severe bronchopulmonary disease, including chronic obstructive pulmonary disease and obstructive sleep apnea.
• Known contraindications to peripheral nerve block, including local skin infections, bleeding diathesis, and coagulopathy.
• Allergies to local anesthetics, dexmedetomidine, or any component of multimodal analgesia.