Development of a nomogram to predict surgical site infection after closed comminuted calcaneal fracture

With the approval of the Institutional Review Committee, we retrospectively collected patients with unilateral closed comminuted calcaneal fractures from the database of the Second Affiliated Hospital of Wenzhou Medical University from 2017 to 2020. A total of 214 patients with unilateral closed comminuted calcaneal fractures were included in this study and divided into two groups based on the incidence of surgical site infection: the SSI (Surgical site infection) group and the non-SSI group. Preoperative clinical characteristics, laboratory parameters, and operative records were collected for all patients. These baseline variables included age, body mass index (BMI), injury mechanism, injury-surgery interval, operative time, operative blood loss, gender, types of admission, current drinking, current smoking, injured feet, artificial bone graft, surgical approach, anesthesia method, preoperative mannitol, postoperative antibiotic, postoperative drainage tube, hypertension, diabetes, fatty liver, kidney stone, pneumonia, venous thrombus embolism (VTE), education level, prealbumin, total protein, albumin, globulin, albumin/globulin (A/G), aspartate aminotransferase (AST), alanine aminotransferase (ALT), ASL/ALT, alkaline phosphatase (ALP), gamma-glutamyltransferase (GGT), total bilirubin, direct bilirubin, indirect bilirubin, fasting blood glucose (FBG), blood urea nitrogen (BUN), serum creatinine (Scr), BUN/Scr, cystatin-C, creatine phosphate kinase (CPK), homocysteine, lactate dehydrogenase (LDH), cholinesterase, blood uric acid, serum sodium, serum kalium, serum calcium, serum chlorine, white blood cell (WBC), neutrophils, lymphocyte, monocyte, eosinophilic granulocyte, basophilic granulocyte, red blood cell (RBC), hemoglobin, hematocrit, mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpusular hemoglobin concerntration (MCHC), platelet count, plateletcrit, platelet distribution width (PDW) and mean platelet volume (MPV).

SSI diagnosis after surgery was based on CDC's National Healthcare Surveillance Network (NHSN) [9, 10]. Superficial SSI mainly involves the skin or subcutaneous tissue of the incision, and its diagnosis requires at least one of the following: (1) purulent exudate from the superficial incision, (2) positive microbial culture of the superficial exudate, and (3) redness, swelling, heat and pain of the incision. Deep SSI mainly involves deep soft tissue, and its diagnosis requires at least one of the following: (1) purulent exudate from the deep incision, (2) local abscess requiring debridement or plate removal, (3) positive microbial culture of deep exudate, (4) abscesses or other signs of infection detected by radiological or histopathological examination.

The inclusion criteria were: (1) Unilateral comminuted calcaneal fracture, (2) No other fractures, 3) age > 18 years, (4) The diagnostic criteria for SSI were in accordance with clinical guidelines. The exclusion criteria were: (1) open fracture, (2) with other fractures, (3) pathological fracture, (4) missing clinical characteristics or laboratory parameters.

The normality of the data distribution was tested using the Shapiro-Wilk test. Patient characteristics were described using median (interquartile range [IQR]) and mean ± standard deviation, frequency, and percentage when appropriate. A nonparametric test (Mann-Whitney U test or Kruskal-Wallis test) was applied for data with non-normal distribution or heterogeneity of variances. Categorical variables were expressed as percentages and analyzed using the Pearson Chi-squared test. Univariate logistic regression analysis was used to determine the independent risk factors for incision infection. Moreover, risk factors significantly associated with SSI in the univariate analysis (P < 0.1) were included in the multivariable logistic regression. Receiver operating characteristic (ROC) curves were applied to analyze predictive value indicators for closed comminuted calcaneal fracture patients with SSI. All statistics were calculated using SPSS software (version 26.0; SPSS Inc., Chicago, IL, USA). Besides, the final risk factors were incorporated into R4.1.2 software (R Foundation for Statistical Computing, Vienna, Austria) to establish a nomogram prediction model. The consistency index (C-index) was used to evaluate the model's prediction performance, and the correction curve was used to judge the prediction consistency [11]. The range of the C-index value was 0.5 to 1.0, and accuracy was positively correlated with the value. The calibration curve included an image. The calibration curve included an image comparison of predicting risk and SSI risk. The closer the predicted risk to the standard curve, the better the conformity of the model.

214 patients were enrolled in this study, including 44 in the SSI group and 170 in the non-SSI group. There were statistically significant differences between the two groups in operative blood loss, operative time, injury mechanism, injury-surgery interval, types of admission, surgical approach, prealbumin, A/G, AST, cystatin-C, CPK, cholinesterase, and eosinophilic granulocyte (P_s < 0.05). Moreover, there were no statistically significant differences between the two groups in BMI, education level, gender, current drinking, current smoking, injured feet, artificial bone graft, anesthesia method, preoperative mannitol, postoperative antibiotic, postoperative drainage tube, hypertension, diabetes, fatty liver, pneumonia, kidney stones, VTE, total protein, albumin, globulin, ALT, ASL/ALT, ALP, GGT, total bilirubin, direct bilirubin, indirect bilirubin, FBG, BUN, Scr, BUN/Scr, homocysteine, LDH, blood uric acid, serum sodium, serum kalium, serum calcium, serum chlorine, WBC, neutrophils, lymphocyte, monocyte basophilic granulocyte, RBC, hemoglobin, hematocrit, MCV, MCH, MCHC, platelet count, plateletcrit, PDW and MPV. (Details are shown in Table 1).

The univariate logistics regressions analysis was applied to the baseline variables, laboratory tests, and comorbidities. Age, BMI, injury-surgery interval, injury mechanism, operative time, operative blood loss, gender, types of admission, current drinking, current smoking, injured feet, artificial bone graft, surgical approach, anesthesia method, preoperative mannitol, postoperative antibiotic, postoperative drainage tube, hypertension, diabetes, fatty liver, kidney stone, pneumonia, VTE, education level, prealbumin, total protein, albumin, globulin, A/G, AST, ALT, ASL/ALT, ALP, GGT, total bilirubin, direct bilirubin, indirect bilirubin, FBG, BUN, Scr, BUN/Scr, cystatin-C, CPK, homocysteine, LDH, cholinesterase, blood uric acid, serum sodium, serum kalium, serum calcium, serum chlorine, WBC, neutrophils, lymphocyte, monocyte, eosinophilic granulocyte, basophilic granulocyte, RBC, hemoglobin, hematocrit, MCV, MCH, MCHC, platelet count, plateletcrit, PDW and MPV were analyzed during the univariate analysis. Parameters significantly associated with SSI, including injury-surgery interval, high-fall injury, operative time, operative blood loss, types of admission, surgical approach, prealbumin, albumin, A/G, AST, total bilirubin, direct bilirubin, cystatin-C, CPK, LDH, cholinesterase, blood uric acid, and eosinophilic granulocyte (P < 0.1) (Table 2) were included in multiple logistic regression analysis. The results showed that the high-fall injury, operative time, prealbumin, AST, and cystatin-C were independent predictors of SSI in calcaneal fracture patients, with OR values of 5.565 (95%CI 2.220–13.951), 1.044 (95%CI 1.023–1.064), 0.988 (95%CI 0.980–0.995), 1.035 (95%CI 1.004–1.067) and 0.010 (95%CI 0.001–0.185) (P_s < 0.05) (Table 3).

Receiver operating characteristic curve (ROC) analysis was performed to assess the predictive value of the composite indicators of SSI in calcaneal fracture patients.

Taking the occurrence of SSI as the status variable and high-fall injury + operative time + prealbumin + AST + cystatin-C as test variables of the composite indicator, ROC curve analysis showed yielded an Area Under Curve (AUC) value of 0.860 (95%CI 0.794 -0.926, P < 0.001) for composite indicators for the prediction of SSI. In addition, the AUC of high-fall injury, operation time, prealbumin, AST and cystatin-C was 0.680 (95%CI 0.593–0.766), 0.756 (95%CI 0.672–939), 0.331 (95%CI 0.243–0.419), 0.605 (95%CI 0.512–0.698) and 0.319 (95%CI 0.226–0.413) (P_s < 0.05), respectively (Fig. 1).

A nomogram was established to predict the risk of SSI (Fig. 2).

The total scores of every single item, while the probability of SSI in calcaneal fracture patients is obtained by the total score. For example, for a patient with a unilateral closed comminuted calcaneal fracture, preoperative prealbumin of 200 g/L, AST of 30 and cystatin-C of 0.8, fracture caused by a fall from a height of 2 m, and an operative time of 100 min, the probability of SSI was approximately 70% (Fig. 3). The C-index of the model was 0.860 after 1000 bootstrap self-sampling replicates, which indicated that the consistency between the predicted value and the actual observation value is by the standard and has a standard resolution. Furthermore, the coefficient of determination (R²) of the calibration curve (Fig. 4) was 0.443, suggesting that the curve was an excellent fit.