Femoral nerve block (FNB) is a first-line analgesic technique for multimodal analgesia after total knee arthroplasty (TKA). Recent studies and meta-analysis indicate that dexmedetomidine combined with local anesthetics for FNB can prolong the analgesic duration, improve the analgesic efficacy, inhibit local inflammatory response, and reduce narcotic consumption. The investigators hypothesize that dexmedetomidine combined with ropivacaine for FNB can also improve functional recovery in aged patients after TKA.

Many patients following total knee arthroplasty (TKA) complain moderate to severe postoperative pain. Multimodal analgesia, a combination of different techniques and analgesic agents, plays an increasingly important role to relieve pain after TKA. Femoral nerve block (FNB) is a first-line analgesic technique for multimodal analgesia after TKA. But local anesthetics alone often exert limited potency of analgesia and are insufficient to avoid supplemental opioid usage. Dexmedetomidine, a selective alpha 2-adrenergic receptor agonist, is widely used in clinical settings due to its properties of sedation, anxiolysis, analgesia, and sleep promotion. Recent studies and meta-analysis indicate that dexmedetomidine combined with local anesthetics for FNB can prolong the analgesic duration, improve the analgesic efficacy, inhibit local inflammatory response, and reduce narcotic consumption. The investigators hypothesize that dexmedetomidine combined with ropivacaine for FNB can also improve the functional recovery in aged patients after TKA.

Inclusion Criteria:

• Age ≥65 years but <90 years;
• Scheduled to undergo unilateral total knee arthroplasty;
• Planned to use femoral nerve block and patient-controlled intravenous analgesia (PCIA) for multimodal analgesia.

Exclusion Criteria:

• Scheduled for bilateral total knee arthroplasty or revision surgery;
• Contraindications to femoral nerve block;
• Preoperative history of schizophrenia, myasthenia gravis, inability to communicate because of coma, severe dementia, or language barriers;
• Preoperative history of hemorrhagic disease or coagulopathy;
• Preoperative obstructive sleep apnea (diagnosed sleep apnea syndrome or a STOP-Bang score ≥3 combined with a serum bicarbonate ≥28 mmol/L);
• Sick sinus syndrome, severe sinus bradycardia (< 50 beats per minute), or second-degree or above atrioventricular block without pacemaker;
• Severe hepatic dysfunction (Child-Pugh class C), severe renal dysfunction (requirement of renal replacement therapy before surgery), or American Society of Anesthesiologists physical status >III;
• Preexistent delirium (diagnosed by Three-Dimensional Confusion Assessment Method);
• Under treatment with dexmedetomidine or clonidine.