Lower limb amputees often suffer skin and tissue problems from using their prosthesis which is a challenging biomechanical problem. The finite element method (FEM) has previously been applied to analyse internal mechanical conditions of the leg at prosthesis use. However, the representation of soft tissue was simplified to few layers and tissue types. The effects of such a simplification of human tissue is still unclear and the results from simplified models may be misleading. Thus, comparisons of the effects of using five versus six tissue types were performed on a transtibial cross section model exposed to three different socket designs. Skin, fat, vessels and bones were defined separately while muscle and fascia tissues were separate or merged. Nonlinear behaviour and friction between socket and skin were considered in the simulations. Contact forces as well as internal stresses and strains of each tissue type differed in both magnitude and maxima site for each material set within and between the different prosthetic socket conditions. Relative changes of stresses and strains by several hundred percent were found when fascia and muscle material properties were merged compared to when they were modelled separately. Thus, the level of tissue detail needs to be considered when creating limb models and interpreting results of related FEM simulations. Keywords: mechanical condition, finite element, soft tissue, material property, contact, prosthesis
Deep Dive into Comparison of mechanical conditions in a lower leg model with 5 or 6 tissue types while exposed to prosthetic sockets applying finite element analysis.
Lower limb amputees often suffer skin and tissue problems from using their prosthesis which is a challenging biomechanical problem. The finite element method (FEM) has previously been applied to analyse internal mechanical conditions of the leg at prosthesis use. However, the representation of soft tissue was simplified to few layers and tissue types. The effects of such a simplification of human tissue is still unclear and the results from simplified models may be misleading. Thus, comparisons of the effects of using five versus six tissue types were performed on a transtibial cross section model exposed to three different socket designs. Skin, fat, vessels and bones were defined separately while muscle and fascia tissues were separate or merged. Nonlinear behaviour and friction between socket and skin were considered in the simulations. Contact forces as well as internal stresses and strains of each tissue type differed in both magnitude and maxima site for each material set within a
Lower limb amputees who use a prosthesis often suffer blisters, oedema, skin irritation, dermatitis and pressure ulcers [1,2]. The load on the residual limb is unavoidable and biomechanical understanding of the interaction between the human tissue and the prosthetic socket is important in order to design a comfortable and practical socket with proper load distribution. [1]. Fergason &Smith defined three concepts of socket design used in a transtibial prosthesis, based on previous developments: total contact (TC), total surface-bearing (TSB) and hydrostatic (HS) sockets [3]. Liners, socks and padding may be used between the skin and the hard plastic socket depending on the concepts used, and individual adjustment. Despite these efforts of distributing loads on the residual limb, 15-82 % of lower limb amputees experience skin problems [4][5][6]. A person using a prosthetic socket is also at a risk of developing deep tissue injury (DTI) [7,8]. Excessive levels of tissue strains and stresses have been found at bony prominences [8], and support the theory that DTI starts in the deep tissues underneath the intact skin [9,10]. Thus, it is important to consider the internal mechanical conditions in the soft tissues while designing a prosthetic socket.
Finite element analysis (FEA) has been used in many studies to simulate the behaviour of soft tissues under external loading [7,[11][12][13]. Given realistic boundary conditions, geometries and material models, FEA can be used to predict the stress-strain distribution and enable parametric studies to optimize the design of sockets. In previous works, finite element (FE) models were employed to investigate contact stresses at the skin-socket interface in an attempt to optimize the design of a prosthetic socket [1,14]. The internal mechanical conditions in the limb were ignored. In more recent works, internal mechanical conditions in the lower limb under external loading were studied [7,8,[15][16][17][18]. The different layers of soft tissues were merged (lumped) together, most commonly into three tissue types. When merging tissue layers for representation with the same properties typically the tissue types are assigned with same properties from one of the types, e.g. merging/lumping geometries of muscles, connective tissue, vessels and nerves assigning all with muscle properties. The development of models with more detailed tissue geometries in the human lower limb have recently been published, e.g., cross section of tibial compartments with explicit skin, fat, fascia and muscles [19], and individual hamstrings, quadriceps and gluteal muscles [20]. Moerman et al. investigated the influence of individual gluteal geometries on tissue loads with FEA, using one set of material properties for the soft tissue segments skin, adipose fat and muscles [21]. They showed that damage risk volume (DRV) -volume of tissues exposed to shear strains above 50% -were geometry dependent and recommended future studies to further investigate the relative importance of varying material compositions and/or geometries. Material behaviour in simulated human soft tissue has been represented by different material models and related material parameters, e.g., linear elastic models [22,23], nonlinear by polynomial models [24,25] and hyperelastic by elastic strain energy functions such as James-Green-Simpson [26,27], Neo-Hookean [8,[28][29][30][31][32] and Ogden models [16,20,33]. The diverse use of models reflects the complexity of human soft tissue behaviour and the need for further development. Comparison of material models on soft tissue level is rare but is important in evaluating FE simulations due to soft tissue complexity and model limitations [13]. There is also a lack of comparison between different material representations for the same loading situation.
The aim of this study is to examine the mechanical consequences of combining fascia and muscle materials for respective regions compared to representing them separately, in a generic transverse cross section model of a lower limb exposed to three different prosthetic socket designs. Specifically, we aim to investigate mechanical conditions by internal stresses and strains, and by contact pressure between skin and socket models, simulating a section at the middle height of the calf in a non-weightbearing lower limb contained in different prosthetic sockets. A detailed FE model of a generic transtibial transversal section will be developed. Skin, fat, fascia, muscles, blood vessels and bones will be represented separately and material properties obtained from the literature. The contact interaction between the skin and socket will include friction. The resulting stresses, strains and contact pressures using two sets of material properties will be compared under the three socket design conditions..
The cross section of the lower limb considered in this study (Figure 1) is based on an anatomy illustration of the horizontal cross section jus
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