Sixty-one patients with medial compartment osteoarthritis undergoing UKA were randomized to receive a modified local infiltration analgesia (LIA) (n=30) or a conventional LIA (n=31). The primary outcomes were postoperative pain (VAS at rest/motion) and PCIA consumption. Secondary outcomes included time to first rescue analgesia (PCIA), knee range of motion (ROM), daily ambulation distance, time to discharge, and complications. VAS was recorded at 3, 6, 12, 24, 36, 48, and 72 hours; ROM and ambulation were tracked over postoperative days (POD) 1–3. Overall, the modified LIA produced lower VAS on POD1–2 (notably at 24 h and 36 h), delayed first rescue, and reduced PCIA consumption, with better ROM/ambulation and shorter length of stay; complications were similar between groups. These findings suggest the magnesium+bicarbonate cocktail prolongs analgesia beyond the first 12 hours and facilitates early recovery after UKA.