MECHANICAL BEHAVIOUR OF TENDON IN VITRO*
A PRELIMINARY REPORT
MICHAEL ABRAHAMS
Biomechanics Laboratory, University of California School of Medicine,
San Francisco, California 94122, U.S.A.
Abstract—The mechanical behaviour of horse and human tendon, as cbaracterised by the stress-strain curve, has been examined withrespect to load-strain cycling and strain rate. It was found that the tendon stress-strain curve forsuccessive cycles was reproducible provided
that strain on the specimen did not exceed 2.0—4.0~. If this strain level was exceeded,
a permanent deformation occurred. This phenomenon was verifiedby histological studies on
strained tendon which showed that some ofthe collagen fibres did not return to their original
orientation. Variation in the rate of strain was found to affect both the magnitude and the
shape of the stress-strain curve. Additionally, it was found that the stress relaxation phe-
nomenon for tendon was essentially the same as that found for other connective tissues.
………….
2. GENERAL METHODS OF PROCEDURE
All tests on human tendon were carried out at
37~ within 36 hr of autopsy; horse tendon was
tested at 38~ within 48 hr of autopsy. All
specimens were refrigerated in Ringer’s solution
from time of detachment until time of testing.
Each specimen was immersed in Ringer’s
solution at the temperature used for testing
for 15 min before the start of each test, unless
otherwise indicated.
All experimental testing was carried out on a
floor model Instron Testor, Type T.C., which is
shown in Fig. 1.
The machine was modified
slightly so that the tendon specimens could be
tested in Ringer’s solution at body temperature.
The immersion tank (shown in detail in Fig. 2) is
mounted on the stool below the large crosshead.
Experimental load and specimen strain can be
recorded continuously on a time-base chart by
the two-pen recorder which is contained in the
control console shown on the left side of Fig. 1.
In Fig. 2, a specimen of horse extensor tendon
is held in the specially designed grips Each grip
is made from stainless steel and consists of a
block which encloses two self-tightening springloaded wedge-shaped jaws.
The grips were
successful up to the 4-0-5.0% strain level;
thereafter, damage to the tendon fibres within
the jaw faces occurred, and in some cases the
specimen slipped from the grips.
……..
Effects of cycling and determination of "elastic limit"
RIGBY et al. (1959), PARTINGTONand WOOD
(1963), and RIGBY (1964) all found that if
rat-tail tendon was strained beyond a 2.0—4.0~
level, is suffered a permanent deformation.
Partington and Wood also observed that load
extension curves for successive cycles beyond
the 2.0~ level were displaced along the extension axis. To see if this overstrain phenomenon
9 occurred in horse and in human tendon, and to
ascertain the effects of repeated cycling, the
following experimental procedure was adopted.
Test specimen.
The results of repeated strain cycling to
different strain levels on one sample of horse
extensor tendon are shown in Fig. 4. This
specimen was cycled 10 times to the 2.0~ strain
level and then allowed to rest for 5 min while the
strain cam was reset to produce a 3.0~ strain.
It was then cycled 10 times to the 3"0~o strain
level, allowed to rest and the cam reset to give a
4.0~ strain. No rest period was allowed
between cycles of the same strain magnitude.
Curves nos. 1 and 10 are, therefore, the first
and last cycles to the 2"0~o strain level, plotted
as nominal stress against percentage strain. The
curves are, for practical purposes, identical,
because complete recovery was obtained after
each of the 10 cycles. The crosshead speed
selected, 2 in./min, produced an average specimen
strain rate of 45~o/min q-5~o/min. The total
time for 1 cycle to the 2.0~ strain level was
about 5.3 sec. The stress obtained at the
maximum level was approximately 1600 Ib/in~
(110 kg/cm~).
…..
The above results show that if tendon is
strained beyond the 2.0-3.0~ level, permanent
deformation will result. The tests also show that
if tendon is not strained beyond the 2~0-2.5~
strain level, then test results from one sample are
reproducible and, therefore, one sample can be
used for a series of tests as long as this level is
not exceeded.
The point at which "residual" or permanent
strain occurs is henceforth referred to as the
"elastic limit".
….
Rod Lakes