Fig. 1

Flow diagram of included and excluded patients. NSQIP = National Surgical Quality Improvement; TKA = Total Knee Arthroplasty. Of these, 2,034 patients had surgery with general anesthesia and 3,540 had surgery with spinal anesthesia. A cohort of 1,962 patients who underwent TKA under general anesthesia were propensity matched with 1,962 patients who underwent TKA under spinal anesthesia. Patients in the outpatient group who had general anesthesia were younger (64.4 vs. 66.3, P < 0.001), and more likely to have a greater body mass index (32.6 vs 31.1) (P < 0.001) (Table 1). Covariates were well balanced between the propensity matched cohorts, absolute standard mean difference < 0.1 for all covariates (Table 2)