Mechanical characterisation of ex-vivo periprosthetic tissue and Finite Element Analysis (FEA) of a 3D breast model = Mechanical characterisation of ex-vivo periprosthetic tissue and Finite Element Analysis (FEA) of a 3D breast model

Following the breast implant surgery, whether in the case of breast augmentation or reconstruction mastoplasty, a normal inflammatory reaction is generated. The immune system's response induces the formation of a fibrotic capsule separating the breast tissue and the newly implanted prosthesis. The periprosthetic capsule can be considered physiological if it is thin. However, if the inflammatory reaction continues, it will cause an increase in the thickness of the physiological capsule and a modification of it, leading to a pathological state of capsular contracture. Patients with breast implants undergo diagnostic imaging tests, the most widely used for them is mammography. This test employs low-energy X-rays to examine the human breast. The mammography aims to the early detection of breast cancer, generally by detecting masses or characteristic microcalcifications, in some cases it can be used for control tests such as magnetic resonance imaging and ultrasound are also integrated. This thesis work was born from a clinical need expressed by Dr. Federica Bergamin, breast reconstruction specialist surgeon at Ivrea Hospital (ASL TO4), who evidenced her concern about the side effects of mammography in patients with implanted prostheses, especially in the case of postmastectomy breast reconstruction. This study aimed to experimentally evaluate the mechanical properties of periprosthetic capsule samples subjected to uniaxial tensile tests. The periprosthetic tissue samples used in this study were supplied by Ivrea Hospital. The mechanical properties obtained from the tensile test, in particular the elastic modulus, resulting from data processing, of the first set of samples is 24 MPa, which is consistent with the value of 28 MPa that could be found in the literature. Subsequently, the experimental data were used to mimic the physiological behaviour of capsule’s tissue during finite element simulations. Typically, biological tissues are considered nearly incompressible, with a Poisson’s ratio of 0,5, for this reason in FE simulations a Poisson’s ratio of 0,49 was set. Rhinoceros 3D CAD software was used to design two models of prostheses, round and anatomical, created from measurements of real prostheses from a catalogue of GC Aesthetics (a company that manufactures breast implants) as well as their respective periprosthetic capsules. The prosthesis was divided into two separate models, shell and filling, the shell has a thickness of 0.8, while what represents the filling is a solid model of the total size of the prosthesis minus the thickness of the shell. The two in-silico models of the breast implant virtually tested are differentiated from each other by two materials. In particular, two types of PDMS were used: for the shell, a material with a Young's modulus of 750 kPa and Poisson's ratio v = 0.47, and for the filling, a material with a Young's modulus of 250 kPa and Poisson's ratio v = 0.49. Finally, the numerical analysis was carried out using ANSYS Mechanical software, the boundary conditions to solve the problem were set in order to simulate the breast in standing position and during a mammographic exam, using a simplification with only capsule and prosthesis. The future developments of this research could foresee testing more periprosthetic capsule samples to obtain statistically robust and more accurate data and the influence of the histological composition of capsule samples on the mechanical properties found during the tests