Foot Model for Investigating Foot Biomechanics and Footwear Design - Computational Biomechanics of the Musculoskeletal System - page 10

FIgure 1.4 (See color insert.) Foot and foot orthosis model.

table 1.2
material Properties and element types defined in the Finite element model

Component element type young's modulus e

(mPa)

Poisson's ratio v Cross-sectional area

(mm 2 )

Bony structures 3D-Tetrahedra 7,300 0.3 -

Encapsulated soft
tissue

3D-Tetrahedra Hyperelastic - -

Cartilage 3D-Tetrahedra 1 0.4 -

Ligaments Tension-only Truss 260 - 18.4

Fascia Tension-only Truss 350 - 58.6

Ground Support 3D-Brick 17,000 upper layer

1,000,000 lower
layer

0.1 -

Sources: Bones (Nakamura, Crowninshield, and Cooper, Bulletin Prosthetics Research , 18, 27-34, 1981); cartilage
(Athanasiou et al., Clinical Orthopaedics and Related Research , 348, 269-81, 1998); ligaments (Siegler, Block,
and Schneck, Foot & Ankle , 8, 234-42, 1988); plantar fascia (Wright and Rennels, Journal of Bone and Joint
Surgery, American Volume , 46, 482-92, 1964).

The Young's modulus and Poisson's ratio for the bony structures were assigned as 7300 MPa
and 0.3, respectively (Nakamura, Crowninshield, and Cooper 1981). The Young's modulus and
Poisson's ratio for foot bony structures were obtained by averaging the elasticity values of corti-
cal and trabecular bones in terms of their volumetric contribution. The Young's modulus of the
cartilage (Athanasiou et al. 1998), ligaments (Siegler, Block, and Schneck 1988), and the plantar
fascia (Wright and Rennels 1964) were selected from the literature. The cartilage was assigned
a Poisson's ratio of 0.4 for its nearly incompressible nature. The ligaments and the plantar fascia
were assumed to be incompressible. The encapsulated soft tissue of the FE model was defined
as nonlinearly elastic. The stress-strain data on the plantar heel pad were adopted from in vivo
ultrasonic measurements (Lemmon et al. 1997) to represent the stiffness of the encapsulated soft
tissue.
ABAQUS offers a hyperelastic material model to simulate highly incompressible, elastic mate-
rials. The hyperelastic material model defined in ABAQUS is isotropic and nonlinear, which is