The lack of suitable soft tissues is a common challenge facing the reconstructive surgeon. Tissue expansion, first described in 1957 by Neumann, is an ingenious technique that can cause body to grow additional skin tissue of similar texture and color to the defect to be repaired and reduce donor site morbidity at the same time. Generally, a silicone balloon expander is inserted under the skin and then gradually filled with saline or carbon dioxide over time, inducing tissue regeneration. Tissue expansion represents one of the major advances in reconstructive surgery and has been widely involved in all kinds of plastic and reconstructive endeavor. In the process of expansion, mechanical stretch also triggers tissue neovascularization and changes in hemodynamic characteristics. These alterations in expanded flap make it challenging to assess flap perfusion and predict postoperative blood flow disorders.

Traditionally, surgeons rely on the clinical experience for determining tissue perfusion, including flap color, capillary refill and pinprick bleeding. However, clinical judgment is highly subjective, difficult to acquire and may be misleading in nonconventional flaps like expanded flaps. Various technologies have been evaluated for perfusion monitoring, including fluorescein angiography, tissue oxygen saturation measurement and thermography. Nevertheless, these are rarely used as routine due to practical limitations, insufficient sensitivity and/or specificity. Recently, Indocyanine Green Angiography (ICGA) has been used for intraoperative evaluation of tissue perfusion. ICGA can give a real-time assessment of flap vascularity and perfusion by intravenously injecting a contrast agent indocyanine green that emits fluorescence when excited by a laser of specific wavelength. Several studies have validated its role in intraoperative evaluation of conventional free and pedicled flap perfusion to aid in surgical decision making and predict postoperative tissue viability. There is obvious difference in hemodynamic characteristics between conventional and expanded flaps so that the conclusion generated from conventional flap study can't be simply extended to expanded flap. In other words, the benefits of ICGA on predicting viability of expanded flaps remains to be validated.

The purpose of this study is to evaluate the effect of intraoperative ICGA on the prediction of flap necrosis in patients underwent tissue expansion. ICGA will be conducted intraoperatively, meanwhile, the possible area of necrosis will be marked according to clinical experience and the fluorescence value of each observation point will be recorded. After 1 week's follow up, the flap viability at each observation point will be assessed by clinical examination. Then, the corresponding fluorescence value will be determined by superimposing digital photography over ICGA imaging results. By analyzing the observation point representing different fate of flap tissue with Logistic regression analysis, ROC curve and area under curve (AUC) can be synthesized by SPSS. A cut-off point can be further identified to achieve both higher positive and negative predictive value, improving the utility and accuracy of ICGA in predicting the postoperative skin viability of expanded flaps.