Strength of the Quadriceps Muscle is key to recover from Total Knee Arthroplasty. In this study we compare two techniques for pain control after total knee arthroplasty in terms of preserving the motor function (the strength of the quadriceps muscles of the thigh) which will facilitate the rehabilitation and accelerate the hospital discharge. We will use ultrasound guided technique to block either the femoral nerve in the control group or the saphenous nerve in the experimental group. The primary outcome is the functional capacity and the impact of each block on the ability to walk by measuring the distance patients are able to walk in two minutes in post operative day one in the two groups. The secondary outcome is the morphine consumption in the two groups.

Inclusion Criteria:

- Subject age over 18 years coming for total knee arthroplasty secondary to osteoarthritis

Exclusion Criteria:

• ASA 4-5
• Abnormal liver enzymes, hepatic failure, renal failure and cardiac failure if they become contraindication for the use of regional anesthesia
• Contraindications for spinal anesthesia
• Morbid obesity
• Organ transplant
• Neuropathic pain
• History of stroke or major neurological deficit
• Sensory and motor disorders in the operated limb
• Previous drug dependency
• Chronic use of opioids
• Allergy to local anesthetics
• Inability to comprehend pain assessment