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Computational Optimization of Flap Design Following Skin Expansion
Adrian Buganza-Tepole, MS1, Ellen Kuhl, PhD1, Arun K. Gosain, MD2.
1Stanford University, Stanford, CA, USA, 2Lurie Children's Hospital, Chicago, IL, USA.

Optimal flap design after skin expansion is crucial in reconstructive surgery. However, even after careful planning, skin necrosis and flap loss may occur. Although mechanical factors are known to play critical roles in flap necrosis, flap advancement has never been analyzed from a mechanistic perspective. The present study explores mechanical stress profiles associated with different designs for flap advancement to address the fundamental question: Can computational simulations inform flap design so as to decrease the risk of flap necrosis?
To create a model for expanded skin geometry we virtually implanted and inflated a rectangular expander using a computational skin expansion tool developed by our group. We modeled skin as anisotropic, stiffer along the relaxed skin tension lines, which presumably correlates with collagen fiber orientation. With this model, we simulated stress profiles for four different techniques of skin flap design in the expanded tissue model consisting of a direct-advancement flap and a double back-cut flap advanced either parallel or perpendicular to the relaxed skin tension lines (Figure 1).
Our stress profiles revealed three major clinical implications (Figure 2): i) Elevated stresses at the base of the flap were inherent to all flap designs, while stress concentrations at other zones were flap-specific: The direct-advancement flap induced high stress at its distal end. The double back-cut flap induced additional regions of stresses at the lateral sides. Stress concentrations at the distal end correlated with the zone of frequent flap necrosis. ii) Stress profiles were highly sensitive to the advancement direction with respect to the relaxed skin tension lines. Maximum stresses were twice as large for flap advancement parallel to the relaxed skin tension lines as compared to perpendicular. iii) Stress profiles were sensitive to the type of flap design.
We present the first study to apply computational models using virtual simulation for the prediction of stress profiles on a flap-specific basis. These findings indicate that: i) stress concentration can be reduced by planning tissue expander placement to allow flap advancement perpendicular to the relaxed skin tension lines; ii) regional stress concentration are intrinsic to specific flap design; iii) regions of high-stress concentration correlated with clinical zones of flap necrosis. The present simulation model can help to design expanded skin flaps tailored to the reconstruction of specific defects to minimize regions at risk for necrosis. Our computational model can serve as a valuable tool to minimize tissue stresses, reduce skin necrosis, accelerate healing, minimize scarring, and optimize design and planning of expanded skin flaps in clinical practice. As such, our simulations are a key first step towards optimizing individualized flap design based on patient-specific reconstructive needs.

Figure 1. Two common flap designs.

Figure 2. Stress analysis for four different models.

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