Mechanical Thresholds for Initiation and Persistence of Pain Following Nerve Root Injury: Mechanical and Chemical Contributions at Injury

[+] Author and Article Information
Beth A. Winkelstein

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

Joyce A. DeLeo

Departments of Anesthesiology & Pharmacology & Toxicology, Dartmouth-Hitchcock Medical Center, Lebanon, NH 03756

J Biomech Eng 126(2), 258-263 (May 04, 2004) (6 pages) doi:10.1115/1.1695571 History: Received June 14, 2002; Revised December 01, 2003; Online May 04, 2004
Copyright © 2004 by ASME
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The upper set of in vivo images on the left shows the initial unligated (reference) configuration (left) and the nerve root immediately following ligation (right). Superimposed on each image are the set of digitized boundary points along the nerve root (white dots) and the bony positional markers (black dots). The corresponding curve fits for the digitized nerve root boundaries are also shown in the set of images below them. The schematic on the right illustrates the geometric definitions for calculating radial strains along the nerve root. The radial strain magnitudes reported in this work were calculated in the region of maximal compression, indicated in this figure by the “ligation” region.
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Mean mechanical sensitivity for all injury groups (low, tight, chromic, sham, and chromic exposure) of animals. Foot lift response frequency to stimulation with 12 gm von Frey filament is depicted over the 14 days of the study. There was a significant (p<0.0005) increase in mean mechanical allodynia for the tight and chromic ligation animals compared to both sham groups and the low silk ligation animals. Of note is the finding that while the allodynia of the tight and chromic animals were not significantly different, the applied nerve root strain in the tight group was significantly higher (p=0.001) than that applied in the chromic ligation animals. The total number of responses resulting from 30 stimulations per animal was recorded and the group average and standard error are reported here. Behavioral responses for 2 gm von Frey filament testing were similar and are not shown here.
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The overall mechanical sensitivity shows no significant correlation with the applied ligation strain in the presence of inflammatory chromic gut material. Overall mechanical sensitivity is measured by total number of paw withdrawals when tested using a 12 gram von Frey filament. The correlation coefficient (r) of this relationship is 0.151.
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In contrast to the chromic ligations, overall mechanical sensitivity is significantly correlated with the applied ligation strain in the absence of inflammatory material. Overall mechanical sensitivity is measured by total number of paw withdrawals when tested using a 12 gram von Frey filament. The correlation coefficient (r) of this relationship is 0.776 and is significant at p<0.05.
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This plot demonstrates the logistic regression analysis of mechanical tissue strains required to produce persistent pain in this ligation model. Each ligation injury was given a pain score of 1 or 0 based on the existence (1) or absence (0) of persistent mechanical allodynia over 14 days. Scores were potted versus the applied ligation strain. This analysis predicted the 50th percentile strain of 20.8% applied strain and the 95th percentile threshold for pain persistence at a strain of 22.2%. These thresholds are indicated by dotted and straight lines on this plot. Individual animal responses are shown as squares. Thresholds for pain initiation were determined using similar approaches (not shown).




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