Evaluating changes in tendon crimp with fatigue loading as an ex vivo structural assessment of tendon damage - PubMed (original) (raw)

Evaluating changes in tendon crimp with fatigue loading as an ex vivo structural assessment of tendon damage

Benjamin R Freedman et al. J Orthop Res. 2015 Jun.

Abstract

The complex structure of tendons relates to their mechanical properties. Previous research has associated the waviness of collagen fibers (crimp) during quasi-static tensile loading to tensile mechanical properties, but less is known about the role of fatigue loading on crimp properties. In this study (IACUC approved), mouse patellar tendons were fatigue loaded while an integrated plane polariscope simultaneously assessed crimp properties. We demonstrate a novel structural mechanism whereby tendon crimp amplitude and frequency are altered with fatigue loading. In particular, fatigue loading increased the crimp amplitude across the tendon width and length, and these structural alterations were shown to be both region and load dependent. The change in crimp amplitude was strongly correlated to mechanical tissue laxity (defined as the ratio of displacement and gauge length relative to the first cycle of fatigue loading assessed at constant load throughout testing), at all loads and regions evaluated. Together, this study highlights the role of fatigue loading on tendon crimp properties as a function of load applied and region evaluated, and offers an additional structural mechanism for mechanical alterations that may lead to ultimate tendon failure.

Keywords: collagen; imaging; ligament; patellar tendon; polarized light.

© 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.

PubMed Disclaimer

Figures

Figure 1

Figure 1. Experimental setup of a plane polariscope

The setup (A) consists of a backlight, two linear polarizers on either side of the tendon(s), and a camera. (B) The polarizer (P) and analyzer (A) are crossed at 90° and are oriented at the angle θ at which maximal extinction in the dark crimp bands occurred at preload.

Figure 2

Figure 2. Mechanical testing and image capture protocol

(A) Tendons were preloaded (a), preconditioned (b), imaged at three loads (0.1N, 0.5N, and 2.0N) (c), and fatigue loaded (d). After 10, 100, and 1000 cycle intervals of fatigue loading, images were captured at these three loads to quantify tendon crimp properties in the toe, transition, and linear regions of a representative load-displacement curve (B). This process was repeated until tendons reached 1000 fatigue loading cycles. (C) Four ROIs were selected representing the midsubstance (orange), insertion (yellow), center (solid), and lateral (dashed) regions of the tendon. ROIs were low pass filtered to enhance the visibility of light and dark bands and intensities were averaged across the ROI width (red dashed line) before being highpass filtered (blue line). From these spectra, the crimp amplitude and frequency were computed.

Figure 3

Figure 3. Effect of fatigue loading on tendon mechanical properties

Cycle number was a significant (p<0.001) factor for peak strain, tangent stiffness, hysteresis, and laxity. Individual lines indicate each specimen tested. With fatigue loading, peak strain, tangent stiffness, and laxity increased, whereas the hysteresis decreased.

Figure 4

Figure 4

Δ Crimp amplitude (ΔAcrimp) increased with fatigue loading when assessed at (A) 0.1N (representative of the toe region of the force-displacement curve), (B) 0.5 N (representative of the transition region of the force-displacement curve), and (C) 2.0 N (representative of the linear region of the force-displacement curve). The ΔAcrimp demonstrated a load-dependent response, with lower values at higher loads. Bars indicate significant paired differences (p<0.0125) between the center and lateral ROIs and their corresponding insertion and midsubstances for a tendon after 10, 100, or 1000 cycles of fatigue loading. “u” indicates an intensity unit ranging between 1 and 256. *a,b,c,d indicates significant differences (p<0.0083) in the ROI when compared to 0, 10, 100, and 1000 cycles, respectively. “#” indicates trends (p<0.017).

Figure 5

Figure 5

Δ Crimp frequency (ΔFcrimp) decreased with fatigue loading when assessed at 0.1N. Bars indicate significant paired differences (p<0.0125) between the center and lateral ROIs and their corresponding insertion and midsubstances for a tendon after 10, 100, or 1000 cycles of fatigue loading. *a,b,c,d indicates significant differences (p<0.0083) in the ROI when compared to 0, 10, 100, and 1000 cycles, respectively. “#” indicates trends (p<0.017). Data for ΔFcrimp at 0.5 and 2.0N are not shown since the power of crimp frequencies decreases to near the power of noise (high frequencies) at high loads. This is an unavoidable trade off with our high resolution images that does not exist in the evaluation of the ΔAcrimp.

Figure 6

Figure 6

Tendon laxity (defined as the ratio of displacement from gauge length at a set threshold to the tissue displacement and displacement at a set threshold after the first cycle of fatigue loading) was strongly correlated to the change in crimp amplitude at 0.1N as assessed at 0, 10, 100, and 1000 cycles of fatigue life. This same relationship held at both higher loads (0.5N and 2.0N). “u” indicates an intensity unit ranging between 1 and 256.

Similar articles

Cited by

References

    1. Franchi M, Fini M, Quaranta M, et al. Crimp morphology in relaxed and stretched rat Achilles tendon. J Anat. 2007;210:1–7. - PMC - PubMed
    1. Miller KS, Connizzo BK, Feeney E, et al. Characterizing local collagen fiber realignment and crimp behavior throughout mechanical testing in a mature mouse supraspinatus tendon model. J Biomech. 2012;45:2061–2065. - PMC - PubMed
    1. Diamant J, Keller A, Baer E, et al. Collagen; ultrastructure and its relation to mechanical properties as a function of ageing. Proc R Soc Lond B Biol Sci. 1972;180:293–315. - PubMed
    1. Rigby BJ, Hirai N, Spikes JD, et al. The Mechanical Properties of Rat Tail Tendon. J Gen Physiol. 1959;43:265–283. - PMC - PubMed
    1. Soslowsky LJ, Thomopoulos S, Tun S, et al. Neer Award 1999. Overuse activity injures the supraspinatus tendon in an animal model: a histologic and biomechanical study. J Shoulder Elbow Surg. 2000;9:79–84. - PubMed

Publication types

MeSH terms

LinkOut - more resources