Biomedical Engineering Reference
In-Depth Information
Uniaxial tensile testing was performed on whole tendons, split tendons and iso-
lated individual fascicles from rabbit patellar tendons [
250
]. A comparison of
quasi-static stress-strain curves indicated that the intact tendons were stiffer than
split tendons, which were in turn stiffer than individually isolated tendon fascicles.
Stress relaxation testing revealed that whole tendons had a larger stress relaxation
magnitude and lower stress relaxation rate compared to individual fascicles. A
similar study reported that whole porcine cruciate ligaments were stiffer than split
ligaments and isolated fascicles [
107
]. The results of these studies seem coun-
terintuitive, in that macroscopic structures were stiffer than the constituents. A
parallel spring mechanism was proposed to explain this, but this awaits experi-
mental verification [
107
].
The opposite trend was observed in similar studies. In one such study, human
patellar tendons were sectioned into half, quarter and individual fascicles [
16
]. A
comparison of the elastic modulus, relaxation magnitude and rate revealed a strong
dependence on cross sectional area. As the sample cross sectional area decreased,
the linear modulus increased and the rate and magnitude of stress relaxation
decreased. This result is supported by another study in which macroscopic human
Achilles tendon samples were clamped and subjected to multiple quasistatic tensile
testing experiments [
126
]. For each test, a fascicular bundle was severed and
another stress-strain test was performed. The construct stiffness increased as the
cross sectional area decreased. Another finding from this study was that fascicles
within the tissue did not appear to bear load evenly, with some fascicles carrying
considerably more load than others. This may explain the macroscale observation
of inhomogeneity in strain distribution.
The preceding paragraphs highlight a discrepancy in the literature regarding the
variation of stiffness across scale levels. Some studies report increasing stiffness
with increasing scale level (e.g. [
107
,
162
,
250
]), while other report the opposite
(e.g. [
16
,
126
]) Although the cause of this discrepancy is unclear, differences may
arise from the use of animal versus human tissue, clamping methods and methods
used for sectioning and separating the tendons. In either case, it is clear that the
uniaxial tensile behavior in tendon depends on the physical scale.
It has also been shown that the shear behavior displays a scale dependence [
96
].
In this study two adjacent fascicles in human patellar tendon were isolated from
the whole tendon. The preparations were subjected to repeated tensile loading. On
the first cycle, both fascicles were intact. On the second cycle, a single fascicle was
cut on one end. On the last cycle, the second fascicle was cut on the opposing end
such that force could only be transmitted through an inter-fascicle shearing
mechanism. The results indicated that very little load was transferred through
shearing of adjacent fascicles, suggesting that fascicles are free sliding and largely
independent in the transmission of tensile forces across the tendon.
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