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Research Papers

Time-Dependent Ultrasound Echo Changes Occur in Tendon During Viscoelastic Testing

[+] Author and Article Information
Sarah Duenwald-Kuehl

 Department of Biomedical Engineering, University of Wisconsin, Madison, WI 53706

Hirohito Kobayashi

 Department of Orthopedics, University of Wisconsin, Madison, WI 53706

Roderic Lakes

 Departments of Biomedical Engineering and Engineering Physics, University of Wisconsin, Madison, WI 53706

Ray Vanderby1

 Departments of Biomedical Engineering and Orthopedics, University of Wisconsin, Madison, WI 53706vanderby@ortho.wisc.edu

1

Corresponding author.

J Biomech Eng 134(11), 111006 (Oct 26, 2012) (8 pages) doi:10.1115/1.4007745 History: Received April 24, 2012; Revised August 01, 2012; Posted September 29, 2012; Published October 26, 2012; Online October 26, 2012

The viscoelastic behavior of tendons has been extensively studied in vitro. A noninvasive method by which to acquire mechanical data would be highly beneficial, as it could lead to the collection of viscoelastic data in vivo. Our lab has previously presented acoustoelasticity as an alternative ultrasound-based method of measuring tendon stress and strain by reporting a relationship between ultrasonic echo intensity (B mode ultrasound image brightness) and mechanical behavior of tendon under pseudoelastic in vitro conditions [Duenwald, S., Kobayashi, H., Frisch, K., Lakes, R., and Vanderby Jr, R., 2011, “Ultrasound Echo is Related to Stress and Strain in Tendon,” J. Biomech., 44 (3), pp. 424–429]. Viscoelastic properties of the tendons were not examined in that study, so the presence of time-dependent echo intensity changes has not been verified. In this study, porcine flexor tendons were subjected to relaxation and cyclic testing while ultrasonic echo response was recorded. We report that time- and strain history-dependent mechanical properties during viscoelastic testing are manifested in ultrasonic echo intensity changes. We also report that the patterns of the echo intensity changes do not directly mimic the patterns of viscoelastic load changes, but the intensity changed in a repeatable (and therefore predictable) fashion. Although mechanisms need further elucidation, viscoelastic behavior can be anticipated from echo intensity changes. This phenomenon could potentially lead to a more extensive characterization of in vivo tissue behavior.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Ultrasound bath setup used for mechanical and ultrasound testing; tendons were gripped in a stainless steel bone block (stationary) and a custom soft tissue grip (attached to actuator), the ultrasound transducer was fixed to a platform for repeatable positioning. The bath was filled with saline to maintain tissue hydration and facilitate ultrasound wave propagation.

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Figure 2

Ultrasound parameters collected during (a) stress relaxation and (b) recovery testing, and mechanical parameters collected during (c) stress relaxation and (d) recovery testing. Ultrasound parameters include maximum echo intensity change (comparable to max stress in (c), echo change during relaxation (comparable to stress decrease in (c), and echo change during recovery [comparable to stress increase in (d)]. Note that the initial jumps in echo intensity and stress correspond to the jump in load that accompanies the step displacement input.

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Figure 3

Cyclic testing parameters for (a) ultrasound testing and (b) mechanical testing, including maximum echo intensity change (comparable to peak stress) and echo change between cycles (comparable to stress decrease)

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Figure 4

Echo intensity and stress changes during stress relaxation and recovery from relaxation. (a) Echo intensity increases during stress relaxation at 4% strain; (b) echo intensity decreases during recovery (at 2% strain) from relaxation. (c) Maximum stress reached and stress decreases during relaxation at 4% strain; (d) stress increases during recovery (at 2% strain) from relaxation. Results demonstrate the stress or echo intensity changes of one representative specimen (open circles indicate data points) together with the average results of all 10 specimens, including average increase/decrease (dotted arrow) and average maximum echo intensity (filled circle with error bars). Error bars indicate one standard deviation. Note that the echo intensity response includes a sharp increase/decrease during step displacement as well as steady increase/decrease during relaxation/recovery. Also note that the echo intensity during recovery does not return fully to zero in the 5 s plotted, which would be anticipated based on the slower rate of recovery [23]. Echo intensity changes are negatively correlated with stress changes during (e) relaxation (R2  = 0.79435) and (f) recovery (R2  = 0.90622).

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Figure 5

(a) Echo intensity changes and (b) mechanical stress changes during cyclic testing to 4% strain. Results demonstrate the echo intensity and stress changes of one representative specimen (open symbols represent data points) together with the average results of all specimens in the group, including average maximum echo intensity change and maximum stress (filled symbols with error bars) and average increase/decrease (dotted arrow). Error bars indicate one standard deviation. (c) Echo intensity changes are negatively correlated with stress changes during cyclic testing (R2  = 0.82809).

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Figure 6

(a) Echo intensity changes and (b) mechanical stress changes during stress relaxation at various strains. Results demonstrate the echo intensity and stress changes of one representative specimen (open symbols represent data points) undergoing relaxation at 1%, 2%, 3%, 4%, 5%, and 6% strain together with the average results of all specimens in the group for maximum echo intensity change and maximum stress (filled symbols with error bars) at each of the strain levels. Error bars indicate one standard deviation. (c) A logarithmic relationship exists between maximum echo intensity change and strain level (R2  = 0.8983).

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