Intrathecal Morphine Versus Morphine-dexmedetomidine Combination for Postoperative Pain Control After Total Knee Replacement

105 patients, aged \>50 years, with ASA Physical Status Class II and III, scheduled for TKR under spinal anesthesia will be included in this study. 105 Patients will be randomly allocated into three equal groups (35 patients each): * Group morphine (35 patients): patients will be received given 0.5% heavy bupivacaine (3.5 ml) plus 0.1 mg of morphine. * Group morphine-Dex (35 patients): patients will be received given 0.5% heavy bupivacaine (3.5 ml) plus 0.1 mg of morphine plus 5 mcg of dexmedetomidine. Group Dex (35 patients): patients will be received given 0.5% heavy bupivacaine (3.5 ml) plus 5 mcg of dexmedetomidine Measurements: 1. Demographic data as age, BMI, ASA status, duration of surgery 2. During surgery, systolic blood pressure (SBP), diastolic blood pressure (DBP), and heart rate (HR) will be recorded at the 1st, 5th, 15th, 30th, and 60th minutes. 3. Postoperative pain severity assessed by the visual analog score for pain at rest (ranging from 0 to 10, where 0 no pain and 10 maximum pain) will be evaluated postoperatively at 30 min and 2, 4, 6, 12, and 24 h postoperative. Accordingly, the patient is requested to verbally express his degree of pain using this scale. Patients with the visual analog score≥ 4 will be received 3 mg morphine IV and will be recorded. Patients who complained of pain (the visual analog score\< 3) and needed analgesics will be treated intravenous infusion every six to eight hours with Diclofenac Na(75mg). 4. The total dose of morphine used postoperatively will be observed and recorded for the 1st, 2nd, 6th, 12th, and 24th postoperative hours. 5. The period from the moment the intrathecal injection will be made postoperatively until the first analgesic became necessary will be recorded 6. Any recorded postoperative complication as sedation, nausea and vomiting, respiratory depression. 7. The patient's level of sedation will be assessed at the same time points using Ramsay Sedation Scale (score 3-4 mean adequate sedation). (17)