Biomedical Engineering Reference
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conditions, and simulating the indentation test scenarios at 10-, 20-, 30- and 40 mm
ramp displacement. The simulated indenter force-time data was then compared to
the test data, Fig. 5.32 . Simulated displacement-time data is not depicted since the
indenter was displacement-driven and data values thus coincide. The observed
maximum displacement of the fat-muscle interface in the simulation during the hold
phase was found to be B0.5 mm. This is in agreement to cine-MR findings where
interface tissue displacement was not detectable at employed MR-image resolution.
Discussion and Critical Review: Employing the H OLZAPFEL -G ASSER -O GDEN -
model to describe tissue properties, the in vivo long-term shear moduli for human
gluteal skin/fat and muscle tissue can be calculated from the optimized long-term
parameters, using the relation G ¼ 2c 1 for skin/fat: G 1; S = F ¼ 1854 Pa and for
transversally loaded passive muscle tissue: G 1; M ¼ 881 Pa : This value is in good
agreement with ex vivo porcine data provided by (Palevski et al. 2006) and (Van
Loocke et al. 2008) who reported values for skeletal muscle long-term properties
in the transverse direction of 700 ± 300 Pa and 523 Pa, respectively. In addition,
(Gefen and Dilmoney 2007) deduced that long-term transverse shear moduli of
human skeletal muscles should be in the order of 250-1,200 Pa.
Derived long-term skin/fat shear modulus is in the range of ex vivo experimental
human adipose and porcine subcutaneous fat data supplied by (Samani et al. 2007)
and (Geerligs et al. 2008) who derived values of 1,073 and 5,600 Pa, respectively.
Here, the ex vivo human data agrees somewhat better with our value representing in
vivo human skin/fat properties. Similarly, in vivo long-term bulk moduli can be
derived from (3.332) 1 for skin/fat: K 1; S = F ¼ 719 kPa and for cross-fibre muscle
tissue: K 1; M ¼ 77 kPa : From (3.330) 1 instantaneous shear moduli can be derived
from our data at the employed ramp speed for skin/fat: G 0 ; S = F ¼ 12 : 7 kPaand for
cross-fibre muscle tissue: G 0 ; M ¼ 911 Pa. Both values are in agreement with col-
lected data supplied by (Gefen and Dilmoney 2007), summarizing instantaneous
shear moduli of human adipose breast tissue and human skeletal muscle in the order
of 500Pa-25kPa and 500Pa-2kPa, respectively.
The presented approach involves creep and relaxation testing at the gluteal
region from one subject and therefore may not generally represent human gluteal
soft tissue. Substantial individual variability in the mechanical properties of soft
tissue has been shown (cf. Sect. 5.2.2.4 ). The approach, therefore, presents a
methodological concept. Despite these limitations, however, the specified
parameter values provide reasonable agreement with literature data, and based on
our reduced amount of data, passive human cross-fibre skeletal muscle data
compares well to porcine skeletal muscle values.
The applied indentation test procedure provides force data resulting from a 3D
stress state constituted from intrinsic in vivo tissue material properties and the
prevalent interstitial fluid flow and pressure situation. This, test parameters, bio-
logical variability within one species as well as differences in human and animal
tissue material and ex vivo and in vivo differences may contribute to the differ-
ences in previously compared values. We presume that our assumptions regarding
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