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Research Papers

Healing Ligaments Have Shorter Lifetime and Greater Strain Rate During Fatigue Than Creep at Functional Stresses

[+] Author and Article Information
Gail M. Thornton

McCaig Institute for Bone and Joint Health,
University of Calgary,
Calgary, AB T2N 4Z6, Canada;
Department of Orthopaedics,
University of British Columbia,
Vancouver, BC V5Z 1M9, Canada
e-mail: gail.thornton@ucalgary.ca

Soraya J. Bailey

McCaig Institute for Bone and Joint Health,
University of Calgary,
Calgary, AB T2N 4Z6, Canada

1Corresponding author.

Contributed by the Bioengineering Division of ASME for publication in the Journal of Biomechanical Engineering. Manuscript received September 25, 2012; final manuscript received May 31, 2013; accepted manuscript posted June 5, 2013; published online July 10, 2013. Assoc. Editor: James C. Iatridis.

J Biomech Eng 135(9), 091004 (Jul 10, 2013) (6 pages) Paper No: BIO-12-1436; doi: 10.1115/1.4024754 History: Received September 25, 2012; Revised May 31, 2013; Accepted June 05, 2013

Healing ligaments have compromised strength, which makes them susceptible to damage during daily activities at normal functional stresses. Daily activities expose ligaments to cyclic (fatigue) and static (creep) loading. A gap injury was created in the midsubstance of both hindlimb medial collateral ligaments of 40 female 1-year-old New Zealand White rabbits. After a 14-week healing interval, medial collateral ligament gap scars were exposed to long-term fatigue and creep loading over a range of functional force/stress levels. Lifetime and strain behavior were compared during fatigue and creep. The contribution of time-dependent mechanisms to fatigue lifetime was modeled using creep data. Fatigue-loaded healing ligaments had shorter lifetime, greater steady-state strain rate and greater increase in strain at 0.8 h than creep-loaded healing ligaments. The actual fatigue lifetime was less than the predicted fatigue lifetime which was derived from time-dependent damage alone, indicating an important role for cycle-dependent damage mechanisms in healing ligaments during fatigue loading. Cyclic loading decreased lifetime and increased strain rate and strain prior to rupture compared to static loading in healing ligaments. These findings suggest that, after a ligament injury, more care should be taken when exercises result in cyclic loading rather than static loading of the healing ligament even at functional stresses.

FIGURES IN THIS ARTICLE
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Copyright © 2013 by ASME
Topics: Creep , Fatigue , Stress
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References

Figures

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Fig. 1

Schematic of stress–strain curves of normal (solid line) and healing (dashed line) ligaments. The functional range is 5%UTSNORMAL to 10%UTSNORMAL.

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Fig. 2

Lifetime for fatigue-loaded (♦) and creep-loaded (•) healing ligaments at different force/stress levels: (a) 30%UTS, (b) 45%UTS, (c) 60%UTS, and (d) 60%FF. Note that some data points may overlap. *Fatigue less than creep (p < 0.01).

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Fig. 3

Representative increase in strain-time profile for healing ligaments at 60%UTS for fatigue (black line) and creep (gray line). X indicates rupture.

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Fig. 4

Increase in strain at 0.8 h for fatigue-loaded (♦) and creep-loaded (•) healing ligaments at different force/stress levels: (a) 30%UTS, (b) 45%UTS, (c) 60%UTS, and (d) 60%FF. Healing ligaments that ruptured before 0.8 h were not included in the analysis of increase in strain at 0.8 h: 45%UTS fatigue n = 2; 60%UTS fatigue n = 1; 60%UTS creep n = 2; 60%FF fatigue n = 5; and 60%FF creep n = 4. *Fatigue greater than creep (p < 0.05). When outliers [21] identified with ∧ are excluded, fatigue was still greater than creep (p < 0.02).

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Fig. 5

Creep lifetime versus initial strain for healing ligaments. All creep data were included except one 80%UTS creep-loaded healing ligament that ruptured during initial loading. The solid line is the exponential fit to the creep data to determine the constants A and B in Eq. (4).

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Fig. 6

Fatigue lifetime versus initial strain for healing ligaments. The solid line is the exponential fit to the fatigue data.

(5)
Tlife,fatigue=1386e-112.8ɛi,r2=0.62The dotted line is the prediction using time-dependent damage (Fig. 5 and Eq. (4)) and solving Eq. (3).

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