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Modeling Tissue Expansion with Isogeometric Analysis: Skin Growth is Correlated with Increased Latency After Expansion
Elbert E. Vaca, MD1, Adrian B. Tepole, PhD2, Taeksang Lee, MS2, Joanna K. Ledwon, PhD1, Hanah Bae, BS1, Jolanta M. Topczewska, PhD1, Arun K. Gosain, MD1.
1Division of Plastic & Reconstructive Surgery, Lurie Children's Hospital of Chicago, Chicago, IL, USA, 2Purdue University, School of Mechanical Engineering, Lafayette, IN, USA.

Purpose: Tissue expansion (TE) often has high complication rates, particularly in anatomically critical regions such as the face and the extremities with limited donor sites. Improved understanding of the biomechanics of TE to optimize true skin growth (i.e. non-reversible deformation) vs. elastic skin stretch could serve to minimize undue tissue necrosis and resultant infection. The aim of the present study is to differentiate the levels of skin growth vs. skin stretch at various time points after a single large volume expansion, and to correlate biomechanical growth to histologic and gene transcriptome changes during TE.
Methods: Two 5-6 week old minipigs were each tattooed with 4 grids, with a tissue expander implanted under 2 of these grids and the contralateral side serving as an internal control (Fig 1). Each of the 4 expanders were inflated once with 60cc of saline 1 hour, 24 hours, 3 days, and 7 days prior sacrifice. 3D photographs were obtained immediately before and after each expansion and the day of sacrifice, both in vivo and ex vivo. Isogeometric analysis of reconstructed 3D skin grids was preformed to calculate skin growth and stretch. Briefly, prestrain was calculated by amount of “snap-back” of control patches after being excised. In expanded patches, the “snap-back” after skin excision determines stretch, after controlling for prestrain. After controlling for natural growth, the remaining skin size increase following tissue expander removal determines growth. Epidermal thickness was evaluated via histology for skin grids expanded 1 & 24 hours prior to sacrifice. Histology for skin grids expanded 3 & 7 days prior to sacrifice is pending.
Results: Total deformation of the 4 expanded skin patches was similar. Total deformation, however, was attributable to increasing degrees of skin growth with increased latency period after expansion (1.12 for 1 hour, 1.11 for 24 hours, 1.06 for 3 days, 1.34 for 7 days; p<0.0001, Figure 2). Collagen fibers appeared more randomly oriented in apical expander skin compared to control skin where fibers appears to be oriented parallel to the epidermis. There was no significant difference in epidermal thickness 1 hour or 24 hours post expansion compared to controls (Figure 3).
Conclusions: While similar degrees of deformation were seen on all 4 stretched skin grids, skin growth was correlated with increased latency after a single expansion. Our investigation is ongoing regarding histological evaluation of collagen architecture, immunohistochemistry, and gene transcriptome analyses. Furthermore, we are actively investigating growth vs. stretch in animals undergoing serial expansions with the goal of optimizing the common clinical practice of TE.


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