Experimental characterisation of breast tissues and its application to a numerical model of a healthy breast

Abstract

In this work, a great contribution to the knowledge of the biomechanics of the breast has been done. None of the previous works were valid to simulate the real movement of the breast and a little knowledge existed about many factors which play an important role in the behaviour of the breast. We have gone in depth in this field and provided the path that must be followed in the future computational and experimental studies about the breast. Many possibilities have been discarded shedding light on the way the development of the finite element model of the breast should be carried out. Moreover, the difficult interaction between the adipose tissue and the pectoral muscle has been pointed out as key in the process. Concerning the experimental work, a lot of relaxation test have been done in human adipose tissue, in the abdomen and in the breast. Different patients and areas have been tested. Two different viscoelastic models (with several hyperelastic strain energy functions) were adjusted to the experimental tests, providing very good results. For both of them, the independency of their constants with respect to the strain rate was proved, although the independency with respect to the strain level was not accomplished. That means that it is not possible to define a unique set of constants for the adipose tissue with these two models. Probably the tissue is suffering some damage at the different strain levels, making the constants dependent on it. This damage should be studied more carefully and a damage model should be used in case the degradation was confirmed. In spite of this fact, for a certain strain level, both models are valid, being the internal variables viscoelastic model, with an Ogden strain energy function, the formulation that adjusted the experimental results more accurately. Several comparisons have been made to analyse the differences between the mechanical behaviour of the adipose tissue of different areas and individuals. The mechanical behaviour of different individuals' abdominal fat was compared. Inter-individual differences were found in both, the elastic and the viscous constants, confirming that it was a correct decision to extract the specimens from the same individual in the subsequent statistical analyses, for example, to check the validity of the QLV and IVV models or to compare the adipose tissue from different anatomical regions. It is therefore advisable to extract the specimens from the same patient if avoiding the inter-individual effects is desired. In the comparison of the mechanical properties of the adipose tissue of several regions of the abdomen and the breast, differences were found between the superficial breast and three groups of the abdomen: superficial-medial, deep-medial and deep-lateral. However, there are no differences with the superficial-lateral group. No differences were detected between the deep breast and the rest of the groups either. These conclusions have a high relevance for the breast reconstruction surgery with autologous abdominal tissue. The breast adipose tissue can be considered as a unique tissue from the mechanical point of view. Moreover, in the breast reconstruction surgery, the deep breast fat can be replaced by any part of the abdomen, since no significant differences exist in the mechanical properties. However, the superficial breast fat should be replaced, if possible, by the superficial lateral region of the abdomen. Also important although with less clinical relevance are the differences found between regions of the abdominal adipose tissue. It seems that no differences exist in the mechanical properties between both regions of the superficial layer, between both parts of the deep layer and between both parts of the medial regions. However, differences between the deep and superficial layers were found, that is to say, the mechanical properties of the abdominal adipose tissue seem to change with the depth. Comparing the elastic part of the final model presented here for the breast adipose tissue with the Samani's constants (which are the only ones found in the literature for the breast fat), it can be seen that the behaviour modelled with the Samani's constants is much stiffer than that measured experimentally in this thesis. Also supported by the results obtained in the FE models, it seems that the constants provided by Samani do not correspond to the real behaviour of the breast adipose tissue. Regarding the computational work, several models, boundary conditions and strain energy functions for different materials have been tried. The deformed shape of the deformed breast model from supine to prone position has been improved much from the initial models, although some issues need to be solved yet to improve the models and finally obtain a deformation which can be considered valid. It has been found that none of the material models and boundary conditions proposed in the literature produce reasonable results. In particular, they yield an excessively stiff behaviour. The interaction between the muscle and the breast tissue has been detected to play a key role in the deformation of the breast. The properties of the muscle are not determinant, as the muscle is not activated during the change of position modelled here and, moreover, it is stiffer than the rest of the involved tissues. Of course, if an activity in which the dynamics matter, like running, is under study, the role of the muscle can change. The skin properties can be important in the global behaviour because it is the tissue which surrounds the organ. In general, a deeper knowledge about the mechanical properties of the materials is needed. Normally, these properties have been determined in the literature with one “simple" experimental test, whereas the deformation in the breast is not simple whatsoever and quite different from the experimental test which was carried out. This can lead to wrong results when using these mechanical properties in a computational model. In addition, more studies are needed about the internal structure of the tissues and their connections.