Research Papers

The Effect of Flash Freezing on Variability in Spinal Cord Compression Behavior

[+] Author and Article Information
Carolyn J. Sparrey

Department of Mechanical Engineering, University of California, Berkeley, Berkeley, 94720 CAcsparrey@me.berkeley.edu

Tony M. Keaveny

Department of Mechanical Engineering, University of California, Berkeley, Berkeley, 94720 CA; Department of Neurological Surgery, University of California, San Francisco, San Francisco, 94143 CA; Department of Bioengineering, University of California, Berkeley, Berkeley, 94720 CAtmk@me.berkeley.edu

J Biomech Eng 131(11), 111010 (Oct 26, 2009) (5 pages) doi:10.1115/1.4000079 History: Received December 02, 2008; Revised May 11, 2009; Posted September 01, 2009; Published October 26, 2009

The compression behavior of spinal cord tissue is important for understanding spinal cord injury mechanics but has not yet been established. Characterizing compression behavior assumes precise specimen geometry; however, preparing test specimens of spinal cord tissue is complicated by the extreme compliance of the tissue. The objectives of this study were to determine the effect of flash freezing on both specimen preparation and mechanical response and to quantify the effect of small deviations in specimen geometry on mechanical behavior. Specimens of porcine spinal cord white matter were harvested immediately following sacrifice. The tissue was divided into two groups: partially frozen specimens were flash frozen (60 s at 80°C) prior to cutting, while fresh specimens were kept at room temperature. Specimens were tested in unconfined compression at strain rates of 0.05s1 and 5.0s1 to 40% strain. Parametric finite element analyses were used to investigate the effect of specimen face angle, cross section, and interface friction on the mechanical response. Flash freezing did not affect the mean mechanical behavior of the tissue but did reduce the variability in the response across specimens (p<0.05). Freezing also reduced variability in the specimen geometry. Variations in specimen face angle (0–10 deg) resulted in a 34% coefficient of variation and a 60% underestimation of peak stress. The effect of geometry on variation and error was greater than that of interface friction. Taken together, these findings demonstrate the advantages of flash freezing in biomechanical studies of spine cord tissue.

Copyright © 2009 by American Society of Mechanical Engineers
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Grahic Jump Location
Figure 2

The predicted mechanical responses of the finite element models resulting from changes in friction and geometry, only friction, or only geometry for the higher stiffness (5.0/s) material. When friction and geometry effects are combined, the mean response closely matches the actual material. Half error bars represent the standard deviation of each population. Friction+geometry indicates results that varied both the coefficients of friction (μ=0–0.3) in the finite element model with the face angle (α=0–10 deg) of the specimens. Friction only is the mechanical outcome predicted for an ideal specimen geometry with only the coefficient of friction (μ=0–0.3) altered in the models. Geometry only is the mechanical outcome predicted for variations in face angle (α=0–10 deg) while contact is frictionless.

Grahic Jump Location
Figure 1

Flash freezing showed no effect on the mean response of the tissue when tested at strain rates of 0.05 s−1 (a) and 5.0 s−1 (b). p-values (‡) indicate the results of Fligner–Killeen statistical tests for differences in group variability. A p-value (‡)<0.05 indicates that the variation in the frozen group is significantly less than the variation in the fresh group. A significant effect of strain rate on the mechanical response was observed in both the fresh (c) and flash-frozen (d) groups. p-values ( ∗)<0.05 indicate a significant difference in the mean response. Error bars represent one standard deviation.



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