Research Papers

Quantification of Strain Induced Damage in Medial Collateral Ligaments

[+] Author and Article Information
Zheying Guo

Mechanics of Soft Biological
Systems Laboratory,
Department of Biomedical Engineering
and Mechanics,
Virginia Tech,
330 Kelly Hall,
Blacksburg, VA 24061
e-mail: guozhy@vt.edu

Joseph W. Freeman

Musculoskeletal Tissue Regeneration Laboratory,
Department of Biomedical Engineering,
Rutgers University,
599 Taylor Road,
Piscataway, NJ 08854
e-mail: jfreemn@rci.rutgers.edu

Jennifer G. Barrett

Marion duPont Scott Equine Medical Center,
Department of Large Animal Clinical Sciences,
Virginia-Maryland Regional
College of Veterinary Medicine,
Virginia Tech,
P.O. Box 1938,
Leesburg, VA 20176
e-mail: jgbarret@vt.edu

Raffaella De Vita

Mechanics of Soft Biological
Systems Laboratory,
Department of Biomedical Engineering and Mechanics,
Virginia Tech,
330 Kelly Hall,
Blacksburg, VA 24061
e-mail: devita@vt.edu

Manuscript received August 23, 2014; final manuscript received April 4, 2015; published online June 3, 2015. Assoc. Editor: David Corr.

J Biomech Eng 137(7), 071011 (Jul 01, 2015) (6 pages) Paper No: BIO-14-1415; doi: 10.1115/1.4030532 History: Received August 23, 2014; Revised April 04, 2015; Online June 03, 2015

In the past years, there have been several experimental studies that aimed at quantifying the material properties of articular ligaments such as tangent modulus, tensile strength, and ultimate strain. Little has been done to describe their response to mechanical stimuli that lead to damage. The purpose of this experimental study was to characterize strain-induced damage in medial collateral ligaments (MCLs). Displacement-controlled tensile tests were performed on 30 MCLs harvested from Sprague Dawley rats. Each ligament was monotonically pulled to several increasing levels of displacement until complete failure occurred. The stress–strain data collected from the mechanical tests were analyzed to determine the onset of damage and its evolution. Unrecoverable changes such as increase in ligament's elongation at preload and decrease in the tangent modulus of the linear region of the stress–strain curves indicated the occurrence of damage. Interestingly, these changes were found to appear at two significantly different threshold strains (P<0.05). The mean threshold strain that determined the increase in ligament's elongation at preload was found to be 2.84% (standard deviation (SD) = 1.29%) and the mean threshold strain that caused the decrease in the tangent modulus of the linear region was computed to be 5.51% (SD = 2.10%), respectively. The findings of this study suggest that the damage mechanisms associated with the increase in ligament's elongation at preload and decrease in the tangent modulus of the linear region in the stress–strain curves in MCLs are likely different.

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Grahic Jump Location
Fig. 3

Schematic of experimentally measured mechanical quantities. (a) Stress–strain curves starting from a 0 N load. These are examples of curves obtained by loading one FTMC to four consecutive displacements: d1 = 0.45 mm, d2 = 0.65 mm, d3 = 0.85 mm, d4 = 1.05 mm. Note that σ0.1N denotes the stress that corresponds to the 0.1 N load (preload). (b) Initial nonlinear load–strain data that are associated with the stress–strain curves shown in (a) (left) and measured mechanical quantities (right). Note that the strain is plotted versus the load. Δɛ0.1N(k) = ɛ0.1N(k)-ɛ0.1N(0) for k = 1,2,3,4 are shown. Recall that ɛ0.1N(k) is the strain at 0.1 N load obtained by loading the FTMC to the displacement dk and ɛ0.1N(0) is the strain at 0.1 N load obtained by loading the FTMC to the first displacement d1 = 0.45 mm. (c) Stress–strain curves shown in (a) but computed from a 0.1 N preload (left) with measured mechanical quantities (right). For k = 1,2,3,4,E(k) defines the tangent modulus of the linear region of the stress–strain curve obtained by loading the FMTC to the displacement dk and ɛmax(k) is the strain that corresponds to the maximum load achieved at dk.

Grahic Jump Location
Fig. 2

Schematic of the experimental protocol

Grahic Jump Location
Fig. 4

Typical tensile stress–strain data computed by loading one FMTC to consecutive and increasing displacements dk for k=1,2,3,4,5,6 starting from 0 N load. The values of these displacements are reported in the legend.

Grahic Jump Location
Fig. 5

Load and strain data of the initial nonlinear region for the stress–strain data presented in Fig. 4. The change in elongation at the 0.1 N load (preload) is measured by Δɛ0.1N(k) for k=1,2,3,4,5,6. This quantity is defined as the difference between the strain at 0.1 N load measured when loading the specimen up to the displacement dk, ɛ0.1N(k), and the strain at 0.1 N load measured when loading the same specimen up to the displacement d1=0.45 mm, ɛ0.1N(1). For example, Δɛ0.1N(5) is computed as shown. This increase in elongation was assumed to be induced by the displacement d4 (or the strain ɛmax(4)).

Grahic Jump Location
Fig. 6

Stress–strain data for one FTMC (same FTMC used to generate the data in Fig. 4) computed using the 0.1 N load (preload) state as the undeformed configuration for strain measurements

Grahic Jump Location
Fig. 7

Change in elongation at preload (0.1 N) (circular symbols) and tangent modulus of the linear region, E(k) (squared symbols), of the stress–strain curve measured when loading one FMTC to the displacement dk plotted versus the maximum strain ɛmax(k-1) obtained when loading the same specimen to the displacement dk-1 for k=1,2,3,4,5,6. (Note that ɛmax(0)=0 and Δɛ0.1N(1)=0). The data used to compute these quantities are shown in Figs. 4–and 6. Recall that ɛmax(k-1) was assumed to determine Δɛ0.1N(k) and E(k). One can note that the elongation at preload starts to increase at the threshold strain ɛmax(3)=4.62% (filled circular symbol) and the tangent modulus of the linear region of the stress–strain curve starts to decrease at the threshold strain ɛmax(4)=6.22% (filled squared symbol).

Grahic Jump Location
Fig. 8

Box plot of the threshold strains indicating the increase in elongation at preload and decrease in tangent modulus of the linear region of the stress–strain curves for n = 30 MCLs

Grahic Jump Location
Fig. 1

(a) FMTC attached to a frame made of a polyethylene terephthalate sheet at a 70-deg flexion. (b) MCL with black ink sprayed on its surface. Two ink markers were selected for strain measurement. (c) FMTC mounted on the tensile testing device.



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