One hundred twenty patients with knee osteoarthritis scheduled for unilateral primary TKA were randomized to receive: (1) ropivacaine-only cocktail (control; n = 40), (2) ropivacaine + dexamethasone 10 mg (n = 40), or (3) ropivacaine + betamethasone 10.52 mg (n = 40). The primary outcome was postoperative VAS pain; secondary outcomes included knee ROM, straight leg raise (SLR), inflammatory markers (CRP, ESR, IL-6), and adverse events. Outcomes were assessed at 6, 12, 24, 48 hours and 2 weeks (VAS), and at 24, 48 hours and 2 weeks (ROM/SLR). Overall, the results revealed that betamethasone produced lower dynamic VAS at 12, 24, 48 hours and 2 weeks versus dexamethasone and control, and better ROM at 48 hours and 2 weeks versus dexamethasone, without excess complications. These findings suggest betamethasone is the more effective local glucocorticoid for multimodal periarticular analgesia after TKA.