Postoperative delirium is a geriatric syndrome occurring after anesthesia and surgery which manifests as acute alterations in mental status, involving changes in cognition, attention, and levels of consciousness that tend to fluctuate The use of dexmedetomidine with the intention to prevent postoperative delirium remains controversial. Where it has shown to be effective in older adult patients undergoing cardiovascular and non-cardiovascular surgeries

On the day of surgery and after arrival to the theatre, all patients will have supplemental oxygen by nasal cannula at 5 L min-1 and will be connected to the standard monitoring non-invasive blood pressure, electrocardiography, and pulse oximetry. crystalloid 10mg/kg will be administered then all patients will receive spinal anaesthesia in the sitting position, under complete aseptic technique. The midline or Para median spinal puncture will be performed. volume of 3 ml of hyperbaric bupivacaine (Marcaine® Spinal Heavy 0.5%, AstraZeneca) will be injected using a 25 G spinal needle . After establishment of spinal anesthesia , A loading dose of dexmetomedine (1 µg/kg) will be administered for 10 min for all the patients then Three types of syringes will be prepared on 50 ml syringes Group A: saline 50 ml, Group B: a 50 ml mixture of saline 49 ml and dexmedetomidine 100 μg, and Group C: a 50 ml mixture of saline 48 ml and dexmedetomidine 200 μg. the patients then they will be randomized into 3 groups

Inclusion Criteria:

• ASA physical status I- II
• Age from 65to 75 years old,
• Elective total knee arthroplasty under spinal anaesthesia.

Exclusion Criteria:

• Bleeding disorders
• Allergy to any of the drugs used in the study
• Renal insufficiency
• Liver failure-Neurological abnormalities -patients with hearing and visual impairment