Differing Definitions of Outpatient Surgery May Influence Study Outcomes Related to ACL Reconstruction

<b>Background:</b> Anterior cruciate ligament reconstruction (ACLR), one of the most frequently performed orthopedic procedures, has experienced rising demand and escalating costs, driving efforts to reduce expenses through shorter hospital stays and an increased shift toward outpatient settings. This study aims to evaluate how differing definitions of "outpatient" surgery influence the interpretation of outcomes following ACLR. <b>Methods:</b> ACS-NSQIP was queried for patients undergoing primary ACL reconstruction between 2014 and 2023. Patients ≥ 18 years with CPT code 29888 were included. Patients with missing hospital length of stay (LOS) data or a LOS > 2 days (≥99th percentile) were excluded. Two definitions of "outpatient" surgery were evaluated: hospital-defined outpatient (HDO) and same-day discharge (SDD, LOS = 0). Propensity score matching of baseline demographics and comorbidities was used to compare HDO and SDD cohorts to their respective inpatient counterparts. Primary outcomes analyzed included 30-day readmission, reoperation, and postoperative complications. Univariate and multivariate analyses were performed to compare risks of complications for HDO and SDD cohorts compared to their inpatient counterparts. <b>Results:</b> A total of 37,546 patients were included in this study, with 35,334 HDO (94.1%) and 34,801 (92.7%) SDD cases. 1021 (2.9%) of the 35,334 HDO patients had an inpatient hospital stay of at least 1 night. In propensity-matched cohorts, hospital-defined inpatient ACLR was associated with significantly greater risk of 30-day reoperation (odds ratio [OR] 3.167, 95% CI 1.267-7.915, <i>p</i> = 0.009) and superficial surgical site infection (SSI) (OR 5.0, 95% CI 1.712-14.604 <i>p</i> = 0.001), while HDO ACLR was associated with increased risk of deep vein thrombosis (DVT) (OR 0.333, 95% CI 0.121-0.916, <i>p</i> = 0.025). Compared to the propensity-matched SDD cohort, inpatient ACLR was significantly associated with greater rates of 30-day readmission (OR 1.988, 95% CI 1.088-3.630, <i>p</i> < 0.001), reoperation (OR 3.222, 95% CI 1.528-6.794, <i>p</i> = 0.001), and superficial SSI (OR 3.286, 95% CI 1.412-7.644, <i>p</i> = 0.003). <b>Conclusions:</b> This study found differences in readmission and deep vein thrombosis between HDO and SDD cohorts when compared to inpatient ACLR. A standardized definition of outpatient surgery should be created to clearly distinguish same-day discharge from other outpatient categories, considering discharge timing and patient monitoring practices.