Biomechanical Properties and Behaviours of the Wrist Joint - Computational Biomechanics of the Wrist Joint - page 20

on the biomechanical effect of the transverse carpal ligaments (TCL) release for
surgically treated carpal tunnel syndrome (CTS) has been performed considering a
complete model of the wrist joint. The TCL was constructed as a bundle of
elements (not as springs which other studies used). It was found that due to the
TCL release, the carpal bones at the radius side (the trapezium, trapezoid and
scaphoid) displaced more towards radius during loading. This abnormal translation
was doubt to be one of the implications of the postoperative surgery of the TCL
release. Of these few studies on the diseased wrist joint, there was none made on
the wrist joint affected by RA.

2.4.2.4 Ligaments Modelling

Tension-only spring elements were mainly used in the previous studies to simulate
ligament. Despite the nature of the ligaments with nonlinear viscoelastic proper-
ties, due to simplification as well as limited resources, the issue on the linearity of
the ligaments varied among previous studies. Linear properties of cables were used
in a study by Ezquerro et al. [ 48 ] where different positions of Kirschner wires for
scaphoid fracture fixation were analysed. However, most of them were used
non-linear properties of springs to simulate the ligaments [ 25 , 44 , 46 , 47 ].
The load-deflection behaviour discussed in Sect. 2.3 was used, governed by the
superposition of a linear and exponential function:

e
0 : 03

f ðÞ¼ k l o e 0 : 03 ð Þ þ 0 : 03 k l o exp ð 2 : 2 Þ

l lo
lo

e ¼ ð 2 : 3 Þ

with k is the linear ligament stiffness, l is the momentary spring element length
defined by the distance between the two element connections, and l o being the
specified initial length of each ligament. Each spring element starts to exert
nonlinear tensile force after crossing the specified initial ligament length l o . The
initial length represents a variable model parameter.
Regarding number of ligaments, previous study has shown that not all liga-
ments could be modelled. Since springs element were used, it was not possible to
model the wrapping of ligaments. For instance, a study by Carrigan et al. [ 47 ]
modelled 29 ligaments out of 64 ligaments available. Similarly, in a more recent
study by Fischli et al. [ 25 ], a bit more ligaments of 32 were developed. In addition,
number of springs used to model each ligament was also varied. There was a study
where only one spring was used to simulate one ligament for their kinematics
study [ 25 ]. Another study performed to analyse the carpal load transmission used
two springs [ 47 ] and the others used multiple springs in parallel [ 44 , 46 ].