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 Liao21Department 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 TestsCreep 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 deviationsSample Dimensions Tendon Ligament

300g 1000g 300g 1000gThickness (mm) 3.329 ±

0.7702.899 ± 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 10Area (mm2) 14.798 ±

4.36311.531 ±

1.51011.852 ±

4.73115.959 ±

2.067

Ligament Stress Relaxation AVG (mm) ± STDEV

Creep AVG (mm) ± STDEV

300g 53.4035 ± 8.2209 12.3508 ± 2.47531000g 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 ± 8.19411000g 56.319 ± 4.3394 30.1129 ± 7.7877

Table 2. Tests Results (Average ± standard deviations)