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Technical Briefs

Preconditioning is Correlated With Altered Collagen Fiber Alignment in Ligament

[+] Author and Article Information
Kyle P. Quinn

Department of Bioengineering,  University of Pennsylvania, Philadelphia, PA 19104

Beth A. Winkelstein

Department of Bioengineering, University of Pennsylvania, Philadelphia, PA 19104, Department of Neurosurgery,  University of Pennsylvania, Philadelphia, PA 19104winkelst@seas.upenn.edu

J Biomech Eng 133(6), 064506 (Jul 06, 2011) (4 pages) doi:10.1115/1.4004205 History: Received February 28, 2011; Revised May 09, 2011; Posted May 11, 2011; Published July 06, 2011; Online July 06, 2011

Although the mechanical phenomena associated with preconditioning are well-established, the underlying mechanisms responsible for this behavior are still not fully understood. Using quantitative polarized light imaging, this study assessed whether preconditioning alters the collagen fiber alignment of ligament tissue, and determined whether changes in fiber organization are associated with the reduced force and stiffness observed during loading. Collagen fiber alignment maps of facet capsular ligaments (n = 8) were generated before and after 30 cycles of cyclic tensile loading, and alignment vectors were correlated between the maps to identify altered fiber organization. The change in peak force and tangent stiffness between the 1st and 30th cycle were determined from the force-displacement response, and the principal strain field of the capsular ligament after preconditioning was calculated from the fiber alignment images. The decreases in peak ligament force and tangent stiffness between the 1st and 30th cycles of preconditioning were significantly correlated (R ≥ 0.976, p < 0.0001) with the change in correlation of fiber alignment vectors between maps. Furthermore, the decrease in ligament force was correlated with a rotation of the average fiber direction toward the direction of loading (R = −0.730; p = 0.0396). Decreases in peak force during loading and changes in fiber alignment after loading were correlated (p ≤ 0.0157) with the average principal strain of the unloaded ligament after preconditioning. Through the use of a vector correlation algorithm, this study quantifies detectable changes to the internal microstructure of soft tissue produced by preconditioning and demonstrates that the reorganization of the capsular ligament’s collagen fiber network, in addition to the viscoelasticity of its components, contribute to how the mechanical properties of the tissue change during its preconditioning.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 1

The change in force and stiffness in a representative ligament specimen during preconditioning. The peak force decreased by 1.54 N and the tangent stiffness at 0.5 mm decreased by 2.47 N/mm between the 1st and 30th cycles. The force-displacement curves from only zero-to-peak displacement are shown for each cycle here.

Grahic Jump Location
Figure 2

Changes in principal strain and collagen fiber alignment after preconditioning in the specimen shown in Fig. 1. (a) First principal strain remained in certain regions of the tissue after unloading. (b) A slight change in the direction and magnitude of fiber alignment vectors toward the direction of loading was detectable in some regions of the capsular ligament (circled) after preconditioning. Those regions typically corresponded to areas of the ligament where larger principal strains were also measured, as shown in (a). (c) Decreases in the correlation of fiber alignment were detected in all specimens following preconditioning. The locations with the largest principal strain and change in correlation are indicated by circles in (a) and (c).

Grahic Jump Location
Figure 3

Altered fiber alignment correlates with a change in mechanical response during preconditioning. The reduced correlation of fiber alignment vectors between maps acquired before and after preconditioning was strongly correlated (p<  0.0001) with (a) the decrease in ligament force between the 1st and 30th cycles and (b) the decrease in tangent stiffness from the 1st to the 30th cycles of preconditioning.

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