Objective:To explore the predictive factors for prepatellar subfascial gas in patients with closed patellar fracture and their impacts on the early infection following internal fixation.Methods:A retrospective analysis was performed in the 148 patients with closed patellar fracture who had been treated at Department of Orthopaedic Surgery, Zhangjiagang Hospital Affiliated to Soochow University from January 2018 through December 2021. All patients underwent preoperative three-dimensional CT examination of the knee joint and was treated by open reduction and internal fixation of patellar fractures. According to the presence or absence of gas in the prepatellar fascia, the patients were divided into 2 groups. In the gas group of 18 patients, there were 12 males and 6 females with an age of (58.3±14.5) years; in the gas-free group of 130 patients, there were 57 males and 73 females with an age of (60.5±14.6) years. The risk factors for prepatellar subfascial gas were screened out by comparing the gender, age, body mass index, injury mechanism, AO/OTA classification, diabetes, primary hypertension, neutrophil percentage, lymphocyte percentage, white blood cell count, neutrophil count, lymphocyte count, C-reactive protein, erythrocyte sedimentation rate, procalcitonin, and albumin before operation between the 2 groups. A receiver operating characteristic (ROC) curve for risk factors were made to identify the best screening points. The impacts of prepatellar subfascial gas were analyzed on early infection after internal fixation.Results:The preoperative neutrophil percentage was the risk factor for prepatellar subfascial gas ( P<0.05). The area under the ROC curve of preoperative neutrophil percentage for prediction of prepatellar subfascial gas was 0.700 (95% CI: 0.554 to 0.847), the optimal critical value was 78.45%, and the sensitivity and specificity were 0.556 and 0.831, respectively ( P=0.006). In the gas group, the incidence of early postoperative infection was insignificantly higher ( P=0.058) , but the time for postoperative antibiotic use was significantly longer and the dressing changes were significantly more frequent than those in the gas-free group ( P<0.05). Conclusions:In patients with closed patellar fracture, preoperative neutrophil percentage >78.45% can be used as an effective non-imaging indicator for prepatellar subfascial gas. A patient with prepatellar subfascial gas could be more prone to early postoperative infection.