All patients in the arm will receive a single intraneural injection of 12 ml Ropivacaine 1% BBraun, Germany, in the sciatic nerve under echographic guidance.

All patients in the arm will receive a single extraneural injection of 12 ml Ropivacaine 1% BBraun, Germany, around the sciatic nerve under echographic guidance.

BBraun Ropivacaine 1% 12ml injection divided in each sciatic components under echographic guidance with a PAJUNK SonoPlex 22G needle.

Pinprick test evaluation on sciatic nerve territory:

With a 22G needle the blinded operator will assess sensitiveness to puncture and then assign a score every 2 minutes for 30 minutes. A score of 0 will describe no sensation to puncture, on the other hand a score of 1 will be assigned if sensation to puncture is still present.

Evaluation of motor blockade:

Plantar and dorsal flexion of the foot will be evaluated for motor blockade of sciatic nerve. A score of 1 will be assigned if foot movement is conserved, 2 if is impaired and 3 when disappears.

Onset time is described as the time at which pinprick score turns to 0 and motor blockade score turns to 3.

Success rate is described as the percentage of subjects with an onset time within 30 minutes in a group among all subjects treated in that group