GENIVS Block Versus Adductor Canal Block After Total Knee Arthroplasty

The goal of this clinical trial is to learn whether the ultrasound-guided GENIVS block is not worse than (non-inferior to) the adductor canal block in reducing opioid use after total knee arthroplasty performed under spinal anesthesia. The main question it aims to answer is: \- Is total opioid consumption (intravenous tramadol, mg) in the first 24 hours after surgery with the GENIVS block not clinically worse than with the adductor canal block? Researchers will also compare pain scores, time to the first analgesic request, the need for rescue pain medication, mobilization (Timed Up and Go test and stair climbing), quality of recovery (QoR-15) and opioid-related side effects between the two groups. Participants will: * have elective, primary, one-sided knee replacement surgery under spinal anesthesia; * be randomly assigned to receive either a GENIVS block or an adductor canal block, with the same local anesthetic dose, at the end of surgery; * use an intravenous tramadol patient-controlled analgesia pump and receive standard pain treatment for 72 hours; * rate their pain and complete mobility tests and questionnaires at scheduled times during the first 72 hours after surgery.