Personalized Computational Models of Tissue-Rearrangement in the Scalp Predict the Mechanical Stress Signature of Rotation Flaps

Abstract

Objective: To elucidate the mechanics of scalp rotation flaps through 3D imaging and computational modeling. Excessive tension near a wound or sutured region can delay wound healing or trigger complications. Measuring tension in the operating room is challenging, instead, noninvasive methods to improve surgical planning are needed.

Design: Multi-view stereo allows creation of 3D patient-specific geometries based on a set of photographs. The patient-specific 3D geometry is imported into a finite element (FE) platform to perform a virtual procedure. The simulation is compared with the clinical outcome. Additional simulations quantify the effect of individual flap parameters on the resulting tension distribution.

Participants: Rotation flaps for reconstruction of scalp defects following melanoma resection in 2 cases are presented. Rotation flaps were designed without preoperative FE preparation.

Main outcome measure: Tension distribution over the operated region.

Results: The tension from FE shows peaks at the base and distal ends of the scalp rotation flap. The predicted geometry from the simulation aligns with postoperative photographs. Simulations exploring the flap design parameters show variation in the tension. Lower tensions were achieved when rotation was oriented with respect to skin tension lines (horizontal tissue fibers) and smaller rotation angles.

Conclusions: Tension distribution following rotation of scalp flaps can be predicted through personalized FE simulations. Flaps can be designed to reduce tension using FE, which may greatly improve the reliability of scalp reconstruction in craniofacial surgery, critical in complex cases when scalp reconstruction is essential for coverage of hardware, implants, and/or bone graft.

Keywords: craniofacial reconstruction; finite element analysis; multi-view stereo; skin anisotropy; tissue mechanics.

Figure 1

Preoperative, patient-specific 3D geometry reconstruction of two clinical cases of melanoma resection: 73-year-old adult male (patient 1) and 62-year-old adult male (patient 2) were photographed from different angles and MVS was used to extract the preoperative 3D scalp geometry.

Figure 2

Preoperative patient-specific models were processed to virtually execute the treatment plan. Patient 1 underwent resection of two skin lesions, one posterior and one anterior, of 2.5 cm and 4.8 cm size respectively. A posterior 3.8 cm defect was removed in the scalp of patient 2. Sutures are imposed in a finite element framework to closely recreate the clinical setting.

Figure 3

General rotation flap design parameterized by three variables: rotation angle ϕ, orientation with respect to skin tension lines (RSTL) θ, and relative back-cut length c/r. The edge at the back-cut is referred to as proximal, while the edge that is rotated is referred to as distal.

Figure 4

Postoperative geometries for the two clinical cases presented in this study. The left column shows photographs taken immediately after the tissue rearrangement procedure. The results of the finite element (FE) simulation predict final geometries consistent with the real procedure as well as associated mechanical stress contours. For patient 1, a stress concentration occurs at the distal end, with maximum principal stress averaging 30 kPa. For patient 2, a band of high stress connects the proximal and distal regions.

Figure 5

General flap simulations show that flap orientation with respect to the relaxed skin tension lines (RSTL; anisotropy direction) is the most important parameter. Depending on flap orientation there is either no stress at the distal end, or, in the worst-case scenario, a stress band can extend from the proximal to the distal end.