ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

2. INTRODUCTION
Analyzing mechanical stress and strain distribution in human soft tissue is gaining
importance as its potential grows in evaluating stress induced trauma with
computational models. Most commonly, continuum models are employed to investigate
the mechanical behaviour in impact biomechanics and rehabilitation engineering,
including pressure-sore related tissue support evaluation (e.g. Gefen et al., 2005;
Linder-Ganz et al., 2007; Majumder et al., 2008; Ceelen et al., 2008). Mechanical data
from tissue employed for simulation is mainly based on ex vivo animal experiments
since data for in vivo human tissue properties is lacking. Especially, in vivo transient
large deformation data from human tissue is sparse.
Several animal studies have been performed investigating the viscoelastic properties of
skeletal muscle (Best et al., 1994, in rabbit; Bosboom et al., 2001, in rat; Van Loocke et
al., 2008, in swine) and of porcine adipose tissue (Geerligs et al., 2008; Sims et al.,
2010; Comley and Fleck, 2010). Some of these studies, employing ex vivo testing,
showed marked differences in the response of fresh and aged tissue. Additionally,
assessing the significance of effects such as interstitial fluid flow and pressure,
hydration or blood circulation, as present in the living organism, remains difficult.
Studies investigating the elastic mechanical in vivo response of human gluteal tissue
have been performed by some investigators (Todd and Thacker, 1994; Brosh and Arcan,
2000; Moes and Horváth, 2002).
To complement methodological information regarding hyperelastic long-term tissue
property evaluation in vivo, provided in Then et al. (2007), an in vivo indentation
method is presented to evaluate the viscoelastic relaxation behaviour of human gluteal
adipose (including skin) and passive skeletal muscle tissue. The adipose-muscle tissue
compound is not, as in most indentation approaches, considered as a single aggregate,
but evaluates the individual contribution of each tissue. This is accomplished in
combination with magnetic resonance imaging (MRI) for non-destructive tissue
displacement field evaluation, applying the assumption of continuous indenter force
transmission through all tissue layers. In addition, the method employs a separable
viscoelastic formulation with exponential, i.e. Prony series in conjunction with a
hyperelastic model formulation. Performing ramp-and-hold experiments, the material
parameters of the quasi-linear viscoelasticity model were derived based on in vivo
relaxation test data from the human buttocks. Test scenarios were modelled and
simulated and the numerical output was fitted to the experimental data. Separate
parameter sets describing transient human gluteal adipose and transversally loaded
passive muscle tissue properties were thus obtained.
The derived material parameters were used in interaction simulation comprising a finite
element model of the gluteal region based upon anatomically adequate, three
dimensional surface data obtained from magnetic resonance imaging.
3. MATERIALS AND METHODS
Stepwise and cyclic tissue indentation with holding periods was performed in the gluteal
region within the MR environment at a constant loading and unloading speed, to
separate the elastic from the inelastic tissue material properties. Here, the indenter
loading was applied orthogonal to the skin surface and transversal to the gluteal muscle
fibres. Indentation force and displacement were recorded and the deformed tissue at the
single loading steps was MR scanned to gain internal tissue displacement field