1. Results and Discussion
Characterization of Creep and Stress Relaxation in Porcine Achilles
Tendon and Ligaments
Brandon Nelson1, Bryn Brazile2, Lysmarie Figueroa Rios3, Sallie Lin2, Dr. Jun Liao2
1Department of Chemical & Petroleum Engineering, University of Pittsburgh,3700 O’hara Street, Pittsburgh, PA 15261, USA
2Tissue Bioengineering Laboratory, Department of Agriculture & Biological Engineering, Mississippi State University, MS, USA
3Department of Biology, University of Puerto Rico, Ave Santiago De Los Caballeros Ponce 00734, PR
Discussion and Conclusions
References
Acknowledgements
Stress Relaxation and Creep Tests
• Both tendon and ligaments experienced similar levels of Stress
Relaxation at both loads (Table 2).
• Both tendon and ligaments experience similar Creep at each
load; however, with increasing load the level of creep increases
(Table 2).
• Future studies are required to evaluate the viscoelastic properties
of these tissues beyond their normal physiological range.
Figure 3: (A) Representative Stress Relaxation test at
300g. (B) Representative Creep test at 300g. (c)
Representative Stress Relaxation test at 1000g. (d)
Representative Creep test at 1000g.
A B
C D
Figure 1: Anatomy of the ankle and foot in humans. The
Achilles runs down the back of the lower leg and connects
the calf muscle to the heel bone, while the posterior and
anterior tibiofibular ligaments form a joint between the tibia
and fibula just above the ankle.
Materials & Methods
Figure 2. Test Resources uniaxial
testing system with a PBS bath
was used for the stress relaxation
and creep testing. Loads were
examined at 300g and 1000g.
Sixteen samples total were
analyzed, as four samples of
tendon and four samples of
ligaments were tested at each load
level.
1. R. Sopakayang, R. De Vita. A mathematical model for creep, relaxation and strain
stiffening in parallel-fibered collagenous tissues. Medical Engineering & Physics 33 (2011),
p.1056-1063.
2. James E. Carpenter, Kurt D. Hankenson. Animal models of tendon and ligament injuries
for tissue engineering applications. Biomaterials 25 (2004), p. 1715-1722.
The authors would like to acknowledge Dr. James N. Warnock of the Bagley College
of Engineering at Mississippi State University. This research was conducted as part
of the 2015 Physical Properties of Materials Research Experience for
Undergraduates (REU) and funded by the National Science Foundation (Award#:
DMR-1359437).
Objective
• The goal of this study to observe and characterize the
viscoelastic properties of Achilles tendon and tibiofibular
ligament samples obtained from domestic pigs to serve as a
model human tissue behavior.
• We aim to determine whether the tendons and ligaments
demonstrate similar viscoelastic behavior and to observe
whether the stress relaxation and creep in these tissues varies
among different loads.
Introduction
• Creep is defined by a long continuous increase in strain
over time when subjected to a constant stress.
• Stress relaxation is defined as a slow continuous
decrease in stress over time, when subjected to a constant
strain (1)
• Connective tissue can be exposed to high load forces
during motion and are susceptible to injury, especially
amongst athletes (2)
• It is estimated that there are more than 250,000 significant
Achilles tendon sports injuries in the U.S. per year.
• Whereas, reports estimate that 25,000 Americans will suffer
from an ankle sprain each day, and ankle sprains account
for half of all sports injuries that require time off.
• These structures, ligaments and tendons, are comprised of
water, a degree of elastin fibers, collagen fibers, and a
surrounding proteoglycan-rich matrix (1).
• Due to this composition, tendons and ligaments have been
found to exhibit long-term viscoelastic properties, which
include creep and stress relaxation.
Specimen Collection
• Porcine shanks were obtained from a local abattoir.
• Porcine Achilles Tendons and Tibiofibular ligaments were
dissected from each shank.
• Specimens were subsequently dissected in the fiber preferred
direction and left in rectangular shaped samples for mechanical
testing.
Uniaxial Mechanical Testing
• Samples were divided into two groups: 300g and 1000g targeted
load levels for mechanical testing.
• Uniaxial mechanical testing was carried out in the Test Resources
Uniaxial Tensile Testing Machine model 100R, version 1.03.01
machine (Fig 3), with a 22lbf load cell, in a 1X PBS bath.
Protocols for Creep and Stress Relaxation Tests
Creep Stress Relaxation
Pre-condition
•Load adjusted 1g/sec until load is
2g sampling at 30 samples/sec
•Position adjusted sinusoidally with
amplitude 1.0mm and period 1.0
sec sampling at 30 samples/sec
for 10 cycles.
Run Creep
•Load adjusted 10.0g/sec until load
is @ target load sampling at 30
samples/sec
•Load held constant until Time is
15 min sampling 30 samples/sec
Pre-condition
•Load adjusted 1g/sec until load is
2g sampling at 30 samples/sec
•Position adjusted sinusoidally
with amplitude 1.0mm and period
1.0 sec sampling at 30
samples/sec for 10 cycles.
Run Stress Relaxation
• Load adjusted 10.0g/sec until
load is @ target load sampling at
30 samples/sec
• Position held constant until Time
is 15 min sampling 30
samples/sec
Table 1. Average sample dimensions with standard deviations
Sample Dimensions Tendon Ligament
300g 1000g 300g 1000g
Thickness (mm) 3.329 ±
0.770
2.899 ±
0.517
2.877 ±
0.827
3.540 ±
0.483
Width (mm) 4.414 ±
0.359
4.014 ±
0.372
3.977±
0.692
4.518 ±
0.483
Grip-2-Grip (mm) 10 10 10 10
Area (mm
2
) 14.798 ±
4.363
11.531 ±
1.510
11.852 ±
4.731
15.959 ±
2.067
Ligament
Stress Relaxation AVG
(mm) ± STDEV
Creep AVG (mm) ±
STDEV
300g 53.4035 ± 8.2209 12.3508 ± 2.4753
1000g 57.4956 ± 2.5644 13.928 ± 3.479
Tendon
Stress Relaxation AVG
(mm) ± STDEV
Creep AVG (mm) ±
STDEV
300g 59.9359 ± 5.9919 24.9227 ± 8.1941
1000g 56.319 ± 4.3394 30.1129 ± 7.7877
Table 2. Tests Results (Average ± standard deviations)