50 patients, between the ages of 30 and 85 years, who had just undergone unilateral TKA (post-op day 1 or 2) were randomly allocated to 1 of 2 groups to determine the effect an adductor canal block (ACB) had on postoperative pain. Patients in group 1 were administered an active ACB immediately after inclusion into the study, while those in group 2 were administered a placebo immediately after inclusion and an active ACB 45 minutes after. Primary follow up was conducted at 45 and 90 minutes. Following the first injection period (t45), patients who received an active ACB experienced significant improvements in VAS pain score during active flexion of the knee and rest when compared to those who had not yet received their active ACB block (group 2). After both groups received their ACB injections, results regarding VAS pain were comparable. Individual analysis revealed that 25% of patients experienced no block effect during active flexion. Block success rate for both groups was 98%.