Biomechanical Properties and Behaviours of the Wrist Joint - Computational Biomechanics of the Wrist Joint - page 18

complex problem, in which the structures are divided into smaller pieces so that
complex mathematics formula can be occupied to calculate and simulate the
required problems [ 27 ]. In other words, it uses a complex system of points called
nodes which make a grid known as mesh. The mesh elements are connected to
their adjacent nodes to react upon conditions assigned, including the material
properties, the contact modelling as well as the boundary condition representing
the real condition of that particular problem. This section has information on some
aspects in FE analysis used to study the behaviours of the wrist, which covers
types of mesh element and von Mises stress criterion. Information on findings and
procedures used in previous studies on FE analyses were sufficiently provided in
the following sections.

2.4.2.1 Mesh Element Types

There are two main types of element that have been widely used: tetrahedral and
hexahedral. Tetrahedral with tetrahedron shape of elements can be either first order
(with 4 nodes) or second order (with 10 nodes), and is preferable to model cur-
vatures. Hexahedral, on the other hand, found to have better accuracy due to its
nature of having relatively more degrees of freedom [ 28 ]. This brick shaped of
element could also be categorized into first order (with 8 nodes) and second order
(with 16 nodes) [ 29 ]. Despite of having greater accuracy, this type of element
found to have less efficiency in performing contact analysis due to high number of
nodes resulted in more tendency to distort. Therefore, tetrahedral was the better
option for contact analysis.

2.4.2.2 Von Mises Yield Criterion

To analyse ductile material, it was often suggested to use von Mises stress which
was formulated from a theory named the von Mises stress yield criterion [ 27 ]. This
theory used to describe plasticity, and has been widely employed in material
science and engineering field. Usually, prediction on yielding of material under
any loading conditions was made based on this theory. The von Mises stress is
calculated by combining stresses in the dimensions (x-, y- and z-directions) at any
point on a structure, see Eq. ( 2.1 ). It was useful not only to determine the level of
plastic deformity, but also to investigate the fatigue strength. Based on the theory,
the material said to yield when the von Mises stress reaches the yield strength.

q

r 1 r 2 r vm ¼ 1

Þ 2 þ r 2 r 3 Þ 2 þ r 3 r 1 Þ 2

p 2 ð ð ð ð 2 : 1 Þ