Patient in this group will receive 25 ml bupivacaine 0.5% plus 0.5 ml normal saline peri-neurally and 0.5 ml subcutaneous normal saline.

Patients in this group will receive 25 ml of 0.5% bupivacaine plus 0.5 ml (50 microgram) Dexmedetomidine peri-neurally and 0.5 ml subcutaneous normal saline

Patients in this group will receive 25 ml bupivacaine plus 0.5 ml saline peri-neurally and 0.5 ml (50 microgram) subcutaneous Dexmedetomidine.

ultrasound quided femoral nerve block with injection of 15 ml bupivacaine 0.5% perineurally with 0.5 ml dexmedetomidine (50 microgram) either perineurally or subcutaneously.

After adequate assessment of femoral nerve block, patients will be transferred to the operating room to receive a standard general anesthesia

Over a period of 30 min after injection of the study medications, a blinded investigator will assess sensory block with pinprick test using a 3-point scale: 0 ═ complete loss of sensation, 1 ═ partial loss of sensation and 2 ═ normal sensation (10). Pinprick test will be done in comparison to the contralateral area at the sensory distribution of the femoral nerve

The onset time of sensory and motor block will be: "the time elapsed between the end of injection and the development of complete block (score of 0 at all dermatomes or no movement of the quadriceps muscle.

The duration of motor block will be defined as: "the time interval between the end of injection and the complete recovery of motor power of the quadriceps muscle)".