Study Variables

Patients who underwent lumbar arthrodesis through a TLIF approach alone were included (Current Procedural Terminology [CPT] code 22612). There is no CPT code to distinguish between patients undergoing open versus minimally invasive surgery (MIS) technique. In 2012, 2 CPT codes, 22612 (lumbar arthrodesis with lateral transverse technique) and 22630 (arthrodesis, posterior interbody technique), were merged into a single CPT code (22633). Cases with CPT codes 22630 and 22633 were excluded to focus the analysis on patients exclusively undergoing TLIF surgery. Only elective single level surgical cases were included. Exclusion criteria were emergency, urgent unplanned cases, and 2-level or multilevel fusion (secondary CPT codes: +22632, +22634, or +22614).

Variables were chosen with 2 general time categories: (1) preoperative period including patient characteristics and (2) operative and postoperative period. Body mass index (BMI) in kg/m² was calculated. Variables were identified through literature search and clinical knowledge of known predictors associated with readmission following elective lumbar spine surgery. The causes for admission were identified through 2 variables, using (1) the variable for the reason for readmission or (2) the International Classification of Diseases Version 9 (ICD-9). Grafting material was also explored (CPT codes: 20930, 20931, and 20936 to 20938).

Two variables in the database captured first time 30-day readmission: “readmission” and “readmission1.” These variables were combined to capture all 30-day readmission.