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Technical Briefs

The Effect of Prior Compression Tests on the Plantar Soft Tissue Compressive and Shear Properties

[+] Author and Article Information
Paul T. Vawter

VA RR&D Center of Excellence for Limb Loss
Prevention and Prosthetic Engineering,
Seattle, WA 98108;
Department of Mechanical Engineering,
University of Washington,
Seattle, WA 98195

William R. Ledoux

VA RR&D Center of Excellence for Limb Loss
Prevention and Prosthetic Engineering,
Seattle, WA 98108;
Department of Mechanical Engineering,
University of Washington,
Seattle, WA 98195;
Department of Orthopaedics and Sports Medicine,
University of Washington,
Seattle, WA 98195
e-mail: wrledoux@u.washington.edu

1Corresponding author.

Contributed by the Bioengineering Division of ASME for publication in the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received August 3, 2012; final manuscript received April 19, 2013; accepted manuscript posted May 16, 2013; published online July 10, 2013. Assoc. Editor: Barclay Morrison.

J Biomech Eng 135(9), 094501 (Jul 10, 2013) (5 pages) Paper No: BIO-12-1342; doi: 10.1115/1.4024572 History: Received August 03, 2012; Revised April 19, 2013; Accepted May 16, 2013

Changes in the shear plantar soft tissue properties with diabetes are believed to play a role in plantar ulceration, yet little is known about these properties. Our group recently conducted shear tests on specimens previously tested in compression to fully characterize the tissue under both these loading modes. However, previously tested specimens may not necessarily provide representative mechanical properties as prior testing may have altered the tissue to an unknown extent. Thus, the purpose of this study was to test the effect of prior compression testing on both the plantar soft tissue shear and compressive properties using paired specimens. First, one specimen from each pair was subject to compression using our standard protocol with modifications to compare compressive properties before and after the protocol while the other specimen from each pair was left untested. Then, both specimens (i.e., one previously compression tested and one previously untested) were subject to shear testing. The results indicate that prior compression testing may affect the tissue compressive properties by reducing peak stress and modulus; however, additional testing is needed since these results were likely confounded by stress softening effects. In contrast, neither the elastic nor the viscoelastic plantar soft tissue shear properties were affected by prior testing in compression, indicating that previously compression tested specimens should be viable for use in future shear tests. However, these results are limited given the small sample size of the study and the fact that only nondiabetic specimens were examined.

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Figures

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Fig. 1

Experimental setup with (a) paired specimen locations (only one from each pair was tested in compression) at the lateral midfoot (la) and calcaneus (ca), and (b) sample specimen before skin removal as well as (c) specimen in environmental chamber between sand-paper covered platens (prior to sealing chamber to maintain in vivo conditions of near 100% humidity and 35 °C)

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Fig. 2

Illustration of compression protocol showing compressive strain as a function of time starting with (a) 1 Hz triangle waves to 40% strain to obtain previously untested properties followed by (b) the standard compression protocol (specimen-specific load control sine waves to a target strain, machine tuning, preconditioning sine waves, a ramp, and hold, and triangle waves at 1, 2, 3, 5, and 10 Hz) and finally after recovery (c) a repeat of the initial 1 Hz triangle wave test to 40% strain to obtain previously compression tested properties

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Fig. 3

Shear test preparation showing (a)–(b) use of custom guillotine device to ensure uniform thickness before placing (c) specimen in environmental chamber and adhering to bottom platen using sandpaper and cyanoacrylate and (d) then sealing chamber and pumping moist warm air into the testing chamber to maintain in vivo conditions of near 100% humidity and ∼35 °C. Note: the stain on the specimens in (b) and (c) was used to indicate specimen orientation in subsequent work by our group but not in this study (see limitations section of Discussion).

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Fig. 4

Illustration of shear protocol showing shear strain as a function of time (after applying static compression) including (a)–(c) triangle waves at each frequency of 1, 2, and 3 Hz, and (d) a ramp and hold to the target shear strain of 81%

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Fig. 5

Sample stress–strain hysteresis curve for 1 Hz triangle wave compression testing for the same specimen from one foot (A1) showing large differences between treatments where U = previously untested and C = compression tested

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Fig. 6

Sample stress–strain hysteresis curves in shear showing variability between specimens from two different feet (B1 versus B3). U = previously untested, C = compression tested. Note that the curves for both test groups lay on top of each other for the remaining two pairs of specimens (not shown).

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Fig. 7

Normalized stress relaxation curves for all four paired specimens on both (a) a log time scale and (b) regular time scale. U = previously untested, C = compression tested. Note: All curves shown are for decimated data for plotting purposes only (all viscoelastic parameters were computed from the original curves with 300,000 data points); B2_C was a potential outlier compared to the other specimens.

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