Zone of Injury Determined by Free Air on Computed Tomography Scans Predicts Open OTA 42A-C Tibia Fracture Complications

OBJECTIVE: To define a computed tomography (CT)-derived zone-of-injury metric, incorporating normalized soft-tissue air extent and body mass index (BMI) and second to determine whether this metric was associated with adverse outcomes after an open OTA 42 A-C tibia fracture. DESIGN: Retrospective cohort study. SETTING: Level I trauma center. PATIENT SELECTION CRITERIA: A retrospective review of patients in a tibia fracture registry (2012-2024) meeting inclusion criteria (age ≥18 years old, open OTA 42A-C fractures, preoperative full length tibia CT imaging, ≥6-month follow-up) was performed. OUTCOME MEASURES AND COMPARISONS: The CT-based zone-of-injury (ZOI) was measured as the longitudinal extent of soft-tissue air (mm) normalized to tibial length (mm) (ZOI soft /Tibial Length). The primary outcome was composite complications including fracture-related infection, amputation, or nonunion. A logistic regression model using ZOI soft /Tibial Length and BMI generated predicted probabilities for composite complications. Model discrimination was assessed through area under receiver operating characteristic (AUROC) analysis and compared with Gustilo-Anderson classification using the DeLong test. An optimal probability threshold was derived statistically (Youden Index) for dichotomizing patients into high- and low-risk cohorts. RESULTS: Fifty-five patients (58 fractures) met inclusion criteria (mean age 40.0 ± 15.1 years; 81.0% men; mean follow-up 16.9 ± 9.5 months). Soft-tissue ZOI and BMI were the significant predictors of composite complications ( P = 0.006, 0.061). The CT-based ZOI model (log(p/1-p) = 0.601 + (3.343 × soft-tissue ZOI/Tibial Length) + (-0.106 × BMI) demonstrated superior discrimination (AUROC = 0.752) than GA classification (AUROC = 0.581, P = 0.042). Patients above the derived threshold (0.403) had significantly worse outcomes: composite complication rate 64.0% versus 18.2% ( P < 0.001) and nonunion (52.0% vs. 9.1%, P < 0.001). Amputation (20.0 vs. 3.0%, P = 0.075) and fracture-related infection (32.0% vs. 15.2%, P = 0.203) were not significant. CONCLUSIONS: A novel CT-based ZOI metric integrating soft-tissue injury extent as measured by soft-tissue air and BMI independently predicted overall complications risk. This newly described CT-based ZOI soft metric provided superior prognostic accuracy than Gustilo-Anderson classification and may enhance early risk stratification in open tibia fractures. LEVEL OF EVIDENCE: Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.