Civil Engineering Department Catholic University of Rio de Janeiro - Brazil THE USE OF FINITE THE USE OF FINITE ELEMENTS IN ELEMENTS IN SURGICAL SURGICAL PROCEDURES PROCEDURES Djenane C. Pamplona Laboratory of Membranes and Biomembranes Slide 1
Rio/Cambridge Rio/Cambridge Slide 2
Djenane Djenane Age: 23 Age: 30 Age: 44 Prof.Ivo Pitanguy Age: 54 Slide 3
Book Slide 4
Topics Topics � The Use of Skin Expanders and a Constitutive The Use of Skin Expanders and a Constitutive � Equation for the Skin. Equation for the Skin. � Breast Reconstruction with Adjustable Implants: Breast Reconstruction with Adjustable Implants: � a Numerical Analysis. a Numerical Analysis. � Numerical Model to Describe the Surgery of Numerical Model to Describe the Surgery of � Breast Reduction. Breast Reduction. Slide 5
THE USE OF SKIN EXPANDERS AND A CONSTITUTIVE EQUATION FOR THE SKIN Doctorate Student: Claudio Ribeiro Carvalho Slide 6
Skin Expanders Skin Expanders Skin expander is a silicone bag implanted under the skin which is gradually (weekly) filed with a saline solution. The objective is to expand the skin. Natural skin expansion occurs in pregnancy. Slide 7
Skin Expansion is used to: Skin Expansion is used to: � Obtain extra skin to reconstruct burned areas; � breast reconstruction; � eliminating tattoos and scars; � extra skin with the desired color and texture. Slide 8
The aim of this research is: The aim of this research is: � To understand the behavior of the skin under expansion. � To identify the the material parameters for the skin under expansion. � To help doctors to know the number and shape of the skin expanders they should use to obtain an extra amount of skin. Slide 9
Our study in the clinic Our study in the clinic To study the behavior of the skin (human scalp) under expansion, it was necessary to develop an apparatus to measure the pressure inside the skin expander while it was inflated. Expansion of the Filling the scalp expander Slide 10
Reconstruction Surgery Reconstruction Surgery Slide 11
The apparatus The apparatus The apparatus is coupled to a syringe and a needle in a way that the pressure inside the skin expander can be measured before, during and after inflation. Connecting the Measuring syringe. pressure. Slide 12
Experimental Results: Experimental Results: Curves Pressure x Volume Curves Pressure x Volume Skin Expansion - 6 stages Volume ( ml) Slide 13
The skin is considered : � A non linear viscoelastic material ; � incompressible ; � homogeneous ; � isotropic ; � large deformations. The model is simplified since the skin is not homogeneous nor isotropic and its properties varies from one site to the other. It changes with sex, age, race and exposure to the sun. Slide 14
Viscoelastic Behaviour Behaviour: : Viscoelastic Delfino’ ’s s Constitutive Equation Constitutive Equation Delfino a b = − − W exp( ( I 3 ) 1 1 b 2 W : Constitutive Relation. a and b : Parameters of the material. I 1 : First principal invariant of the Cauchy-Green deformation tensor. Integral formulation for relaxation Integral formulation for relaxation t ε d ∫ σ = − ( t ) 2 G ( t t ' ) dt ' dt ' 0 G ( t ) : Relaxation function Slide 15
Finite Element Model ABAQUS Circular skin expander Boundary Triangular and quadrilateral Condition membrane and fluid elements. Slide 16
Animation for a circular expander Animation for a circular expander Slide 17
Numerical Results 1 st st stage stage Numerical Results 1 Using the least-square fit procedure minimizing the error, the material coefficients at the first stage (volume 80 – 110 ml) are : a= 0.213 MPa and b=31.5. 3,50 3,00 Experimental Pressure (N/cm2) 2,50 C1 2,00 C2 C3 1,50 C4 1,00 C5 0,50 0,00 70,00 80,00 90,00 100,00 110,00 120,00 Volume (ml) The same procedure was used in the following stages, until volume from 538 ml is reached. Slide 18
Results for the Scalp Results for the Scalp Experimental results - Pacient 1 Experimental results Pacient 2 Fase Volume (ml) Constants Fase Volume (ml) Constants initial - final Vfinal/Vo a (Mpa) b initial - final Vfinal/Vo a (Mpa) b A 0 80 x x x A 0 214 x x x 1 80 110 0.28 0.213 31.5 1 214 254 0.85 0.636 42.6 2 110 140 0.35 0.222 33.4 2 254 298 0.99 0.491 48.2 B 140 380 x x x 3 298 338 1.13 1.007 49.8 3 380 425 1.06 0.986 51.3 4 338 376 1.25 1.500 65.6 4 425 465 1.16 1.298 75.2 5 465 500 1.25 1.463 134.5 6 500 538 1.35 1.787 120.5 Constant a scalp Constant b scalp 3.000 80.0 2.500 Constant a (MPa) Experimenta l experimental 2.000 60.0 Constant b 1.500 40.0 1.000 0.500 20.0 0.000 0.00 0.50 1.00 1.50 0.0 Current volume/initial volume (X100) 0.00 0.50 1.00 1,50 Current volume / initial volume (X100) Slide 19
Our Patients Until Now Our Patients Until Now Patient 1 Patient 2 Patient 3 Patient 5 Patient 4 Slide 20
Discussion Discussion � The numerical model developed was successful in its purpose to understand the process of skin expansion. This is the first step towards this research. Other patients and other sites are been studied. � It was possible to understand the limiting stress during the skin expansion (pain). � A complete model should be developed to help the surgeon to decide the number and shape of the expanders in view of the desired final size of skin. � Other constitutive equations should be used. Slide 21
Conclusions Conclusions � Different regions: Scalp: more rigid larger values for constant a. Abdomen and leg : similar values for constant a. � A parammetric study involving the thickness of the skin: showed the necessity to have it correctly measured during the surgery. � Fatty regions: It was verified the importance of the amount of fatty tissue in the behaviour of the skin expansion. Slide 22
Future Work Future Work � More patients, with different regions of the body, discriminating race and age must be done.. � Experimental study to verify the importance of the elastic foundation (fatty base) in the expansion. � Reliable skin thickness to work with.. � Measure the relaxation process. � Propose number, shape and size of the skin expander for doctors. Slide 23
BREAST RECONSTRUCTION WITH BREAST RECONSTRUCTION WITH ADJUSTABLE IMPLANTS: ADJUSTABLE IMPLANTS: A NUMERICAL ANALYSIS A NUMERICAL ANALYSIS Doctorate Student: Cristina Cristina de A. de A. Alvim Alvim Doctorate Student: Slide 24
Breast Reconstruction with Expanders Breast Reconstruction with Expanders and Implants and Implants Relative Simplicity; Covering tissue with similar texture, color and sensation; Less scars; Avoidance of distant donor-site morbidity; Reduced surgery and post-surgery recovery time. Slide 25
Expanders and Implants Expanders and Implants Round Expanders and Implants: Anatomic Expanders and High and low profile; cheaper; Implants: Natural appearance, used in round breast without but more expensive. ptosis. Slide 26
Steps: Steps: 1 . After mastectomy, an expander is inserted beneath the skin, near the region to be reconstructed. 2. The expander is gradually filled, in periodical steps, until reaching the double of the desired volume (volume of the implant) . 3. The expander is removed and a permanent implant is then inserted. A third surgery may be recommended to reconstruct the nipple. Slide 27
Objectives: Objectives: Analyze the stresses and strains that occur in skin immediately after breast reconstruction, to suggest the refinement of surgical techniques and search for new expanders and implants. Verify the stresses distribution with the use of radiotherapy and the influence of the scar position. Improve the expansion process, optimizing the number, shape and size of the expander, according to the patient. Slide 28
Computational Modeling of Computational Modeling of Breast Reconstruction Breast Reconstruction Expander Anatomic Implant A B C 570 ml 285 ml R= 12 cm A = 11.3 cm B = 10.3 cm C = 4.3 cm Slide 29
Skin: mechanical behavior Skin: mechanical behavior Hyperelastic term: Skin µ 1 = 0,788 kPa Ogden’s Contitutive Equation: α 1 = 6.927 ( ) µ N 2 ∑ α α α = λ + λ + λ − i W 3 Scar i i i 1 2 2 α 2 µ 1 = 2.3464 kPa = i 1 α 1 = 6.927 Viscoelastic term (relaxation function) Prony: Skin and Scar ( ) N g 1 = 0.386 kPa ∑ − τ = − − t g ( t ) 1 g 1 e i τ 1 = 43.22 R i = i 1 Slide 30
Bilateral mastectomy Breast been expanded Slide 31
Finite Element Model ABAQUS/Explicit: Finite Element Model ABAQUS/Explicit: Solid Implant Skin 500 Solid Element (C3D8) with fluid 749 Membrane Element properties. (M3D3) (h = 6 mm; 8 mm in the scar) Slide 32
Expansion; Prosthesis + ; Relaxation With Scar � Maximum stresses (N/cm 2 ) Slide 33
Expansion; Prosthesis + ; Relaxation With Scar � Maximum stresses (N/cm 2 ) Slide 34
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