Development of a mathematical formula and online tool to calculate the potential maximum flap width to allow for primary anterolateral thigh donor-site closure in Caucasians

Background Primary anterolateral thigh (ALT) flap donor-site closure is crucial to achieve patient satisfaction, avoid burdensome secondary surgeries, and avert poor outcomes. Only vague maximum flap width recommendations have been suggested, which fall short of acknowledging individual patient habitus and thigh morphology. Therefore, we aimed at identifying a user-friendly preoperative calculation of maximum flap width for primary closure. Methods A total of 429 ALT free flaps performed between 2009 and 2020 were analyzed. A total of 350 donor-sites were closed primarily (82%) and 79 (18%) were split-thickness skin-grafted (STSG). Patient demographics including sex, age, and BMI, operative details, and flap characteristics were compared to assess their impact on the outcome variable. Receiver operating characteristic (ROC) curves were plotted for all significant predictors discriminating between closure and STSG. Areas under the curve (AUCs) were calculated for each parameter combination and optimal cutoffs were determined using Youden's Index. Results Sex, age, BMI, and flap width alone were poor discriminators. Dividing flap width by BMI and logarithmized BMI yielded AUCs of 0.91 and 0.94, respectively. Including patient sex yielded the best fitting regression model (chi(2) = 251.939, p < .0001) increasing the AUC to 0.96 (95% CI: 0.93-0.98, p < .0001). The optimal cutoff value discriminated between primary closure and STSG with 90% sensitivity and 89% specificity. An online calculator of patient-individual maximum ALT width was then programmed. Conclusions Sex and BMI are reliable predictors of successful primary ALT donor-site closure in Caucasians. We devised a novel formula for calculating patient-individual maximum ALT widths preoperatively, predicting failure of primary closure with 90% sensitivity in our cohort, available at: