Biomechanical Properties and Behaviours of the Wrist Joint - Computational Biomechanics of the Wrist Joint - page 15

Fig. 2.1 The principal load
behaviour of ligaments with
respect to their elongation
during constant elongation-
rate [ 13 - 15 ]
2.3 Biomechanical Consideration of the Ligamentous
Structure
Besides of the anatomical structure of the ligaments, several studies were also
performed to investigate its mechanical properties [ 13 - 15 ]. Viscoelastic stress
relaxation, nonlinear stress-strain behaviour, hysteresis and rate dependent stress-
strain behaviours were successfully found under constant or cyclic loadings [ 12 ].
Based on the typical stress-strain (force-elongation) curves during constant
elongation rate obtained from various carpal ligaments, three main subsequent
phases were found. It starts with a ''toe region'', followed by a quasi-linear region,
and ended up with a failure region (Fig. 2.1 ). Characterization of the curves has
been made based on ligament stiffness in both the quasi-linear region and ultimate
strengths.
There was also a report on the dependencies of these characterizing values on
elongation rate which also consisted of the load relaxation behaviour over the
course of time (Fig. 2.2 )[ 14 ]. It was shown that after preconditioning at low loads,
the specimens were elongated to 50 N at 50 mm/min. The specimens were held at
a specific elongation, and the relaxation behaviour of the ligaments over a period
of 100 s was obtained. After that, the specimens were elongated at 100 mm/min
until failure. An issue was arisen as the presented results were problematic due to
various measurement techniques applied. For instance, a study performed on the
scapholunate ligament to investigate the slope of the quasi-linear region (related to
ligament stiffness) used elongation rate of 50 mm/min [ 14 ], whereas an elongation
rate of 100 mm/min was applied purposely in another study to determine the slope
of various ligaments [ 16 ]. As the previous two studies were independent, another
biomechanical study made the elongation rate to be dependent on the ligament
length resulted in a rate ranging from 5 to 25 mm/s [ 17 ].