Application of finite element modeling to optimize flap design with tissue expansion

Abstract

Background: Tissue expansion is a widely used technique to create skin flaps for the correction of sizable defects in reconstructive plastic surgery. Major complications following the inset of expanded flaps include breakdown and uncontrolled scarring secondary to excessive tissue tension. Although it is recognized that mechanical forces may significantly impact the success of defect repair with tissue expansion, a mechanical analysis of tissue stresses has not previously been attempted. Such analyses have the potential to optimize flap design preoperatively.

Methods: The authors establish computer-aided design as a tool with which to explore stress profiles for two commonly used flap designs, the direct advancement flap and the double back-cut flap. The authors advanced both flaps parallel and perpendicular to the relaxed skin tension lines to quantify the impact of tissue anisotropy on stress distribution profiles.

Results: Stress profiles were highly sensitive to flap design and orientation of relaxed skin tension lines, with stress minimized when flaps were advanced perpendicular to relaxed skin tension lines. Maximum stresses in advancement flaps occurred at the distal end of the flap, followed by the base. The double back-cut design increased stress at the lateral edges of the flap.

Conclusions: The authors conclude that finite element modeling may be used to effectively predict areas of increased flap tension. Performed preoperatively, such modeling can allow for the optimization of flap design and a potential reduction in complications such as flap dehiscence and hypertrophic scarring.

Figure 1

Demonstration of complications of tissue expansion due to excessive tension. a) Tissue expanders inserted in thigh in preparation for of a giant pigmented nevus and resurfacing with advancement flap. b) Dehiscence of flap secondary to tension. c) Patient with scar hypertrophy and widening due to tension following expanded flap advancement for resurfacing of back following resection of a giant pigmented nevus; additional tissue expansion will be required to address residual pigmented nevus of lower back.

Figure 2

Two commonly used flap designs after tissue expansion. a) Direct-advancement flap. Two parallel cuts are made along the sides of the expanded skin. The extra tissue is discarded and the resulting flap is stretched to cover the defect. b) Double back-cut flap. The expanded skin is cut along the sides from the front to the middle and then perpendicular cuts towards the center are made. The flap is advanced at the front but there is rotation at the edges.

Figure 3

Direct-advancement flap oriented parallel to relaxed skin tension lines. Consecutive time frames show the evolution of the stress distribution in skin as the flap is pulled over the defect. Maximum stresses of 2.00MPa occur at the base and at the distal end of the flap. Arrow: Direction of flap advancement. Solid lines: Direction of relaxed skin tension lines.

Figure 4

Direct-advancement flap oriented perpendicular to relaxed skin tension lines. Consecutive time frames show the stress distribution in skin as the flap is pulled over the defect. Maximum stresses of 0.75MPa occur at the base and at the distal end of the flap. Arrow: Direction of flap advancement. Solid lines: Direction of relaxed skin tension lines.

Figure 5

Collagen fiber orientations for direct-advancement flap oriented a) parallel and b) perpendicular to relaxed skin tension lines. Dashed lines highlight the suture regions. Collagen fibers maintained their initial orientations and rotated only marginally upon flap advancement.

Figure 6

Double back-cut flap oriented parallel to relaxed skin tension lines.. Consecutive time frames of the stress distribution in skin as the direct-advancement flap is pulled over the defect. Maximum stresses of 1.50MPa occur at the base and at the distal end of the flap. Additional stress concentrations of 1.50MPa occur at the lateral sides, in regions where the tissue is rotated. Arrow: Direction of flap advancement. Solid lines: Direction of relaxed skin tension lines.

Figure 7

Double back-cut flap oriented perpendicular to relaxed skin tension lines. Consecutive time frames show the stress distribution in skin as the flap is pulled over the defect. Maximum stresses of 1.50MPa are locally concentrated at all four corners of the resurfaced region, while the base and the distal end experience stresses of only 0.75MPa. Arrow: Direction of flap advancement. Solid lines: Direction of relaxed skin tension lines.

Figure 8

Collagen fiber orientations for double back-cut flap oriented a) parallel and b) perpendicular to relaxed skin tension lines. Dashed lines highlight the suture regions. Different fiber orientations meet at the suture lines because of collagen fibers rotation.