Water Movement in Tendon in Response to a Repeated Static Tensile Load Using One-Dimensional Magnetic Resonance Imaging

[+] Author and Article Information
K. G. Helmer

Department of Biomedical Engineering, Worcester Polytechnic Institute, Worcester, MA 01609helmer@nmr.mgh.harvard.edu

G. Nair

Department of Psychiatry, University of Massachusetts Medical School, Worcester, MA 01655

M. Cannella, P. Grigg

Department of Physiology, University of Massachusetts Medical School, Worcester, MA 01655

J Biomech Eng 128(5), 733-741 (Mar 03, 2006) (9 pages) doi:10.1115/1.2244573 History: Received May 02, 2005; Revised March 03, 2006

Rabbit Achilles tendons (N=8) were subjected to tensile loading while internal water movements were followed using NMR. The distribution of the internal water in tendons was measured using a one-dimensional proton-density map that was collected along a radial line oriented transverse to the tendon’s long axis. The proton density map was created from fits to T2 relaxation data. The experimental design included two cycles of loading (7.5 N tensile load) and relaxation. The first load application was for 42.67 min: unloaded for 21.33 min, reloaded for 21.33 min, and then unloaded for 21.33 min. Water was redistributed in a time-dependent fashion upon loading: proton density decreased in the core region and increased in the rim region. In addition there was evidence that tensile loading caused water to become NMR visible. In separate, parallel experiments, we studied the mechanical behavior of tendons using identical conditions of uniaxial loading (N=7). The time constants of water movements were very different from the time constants of mechanical relaxation, indicating that water redistribution is not the sole determining factor of mechanical behavior.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

Schematic representations of the NMR imaging experimental configuration (a) and that for the creep measurements (b). Experimental details are given in the text. In (a), T is the tendon, S is the suture, R/C is the detection coil while IC is the inductively coupled coil used to excite the sample, and SM is the SmartMotor. In (b), T is the tendon, items labeled as C are the clamps, L is the load cell, and Aurora is the linear actuator.

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Figure 2

A representative axial image of a rabbit Achilles tendon (a) and the resulting linescan data (b). The gray scale has been inverted on the image to clearly show the extent of the tendon. The position of the slice profile is represented schematically by the box. The extent of typical rim and core regions are denoted by dashed lines. Note that the slice is positioned such that the rim and core signals can be differentiated.

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Figure 3

M0i time course plots for all eight tendons for the rim (a) and the core (b) regions. The loading protocol alternates loading and unloading, with the first loading period being twice the duration (42.67min) of the succeeding periods. Time periods of different loading are separated by vertical lines. Data collected before the load was applied are assigned negative times spaced by the time resolution of each point (40.0sec). Postload data times are based on the time at the beginning of the data acquisition for that point, i.e., the time of the first data point after load application is assigned to time of zero. (c) Time course data of M0i for each of the same eight tendons used to generate (a) and (b). The MRI data have been normalized to 100% at the second data point after the load is applied. (d) Time course data for the measured displacement for seven separate tendons used in the mechanical creep experiments. The creep data have been normalized to zero displacement at the third time point after the load is applied point to account for the compliance of the system. The time resolution for the mechanical data is 2seconds / point.

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Figure 4

Time courses of the mean M0ir(a), M0ic(b), and M0itot(c) data and displacement values arising from the creep measurements (d) for rabbit Achilles tendons. The mechanical experiments used the same loading protocol as the imaging experiments. Normalization points are identical to those of Fig. 3. Error bars for all plots are standard deviations.

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Figure 5

Examples of fits to individual data sets. (a) Exponential fit to a representative M0ir time course from the first loading period. (b) Biexponential fit to the same data set used in (a). (c) Exponential fit to the representative displacement data set from the first loading period of the creep tests. (d) Biexponential fit to the same data set used in (c).



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