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.

Inclusion Criteria: * Age 55 to 75 years * Scheduled for elective, primary, unilateral total knee arthroplasty * Surgery planned under spinal anesthesia * American Society of Anesthesiologists (ASA) physical status I, II or III * Able to walk without assistance or with a cane and to perform the Timed Up and Go test before surgery * Written informed consent Exclusion Criteria: * Revision or bilateral knee arthroplasty * Contraindication to spinal anesthesia or peripheral nerve block (bleeding disorder, anticoagulant therapy not discontinued according to guidelines, infection at the needle insertion site) * Known allergy to local anesthetics or to other study drugs * Body mass index above 40 kg/m² * Deformity, previous surgery or other pathology of the thigh that may hinder the block * Pre-existing neurological deficit or neuromuscular disease in the operative lower extremity * Regular opioid use for more than 3 months before surgery, or alcohol or substance dependence * Severe renal or hepatic failure * Dementia, psychiatric disease, cognitive impairment, or inability to understand pain scoring and the Timed Up and Go test despite preoperative instruction * Contraindication to tramadol, paracetamol or nonsteroidal anti-inflammatory drugs (e.g., uncontrolled epilepsy, use of monoamine oxidase inhibitors, active peptic ulcer or history of gastrointestinal bleeding, NSAID allergy) * Regular preoperative use of gabapentinoids (pregabalin or gabapentin)