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TECHNICAL PAPERS: Soft Tissue

Long Durations of Immobilization in the Rat Result in Enhanced Mechanical Properties of the Healing Supraspinatus Tendon Insertion Site

[+] Author and Article Information
J. A. Gimbel, J. P. Van Kleunen, G. R. Williams

McKay Orthopaedic Research Laboratory, University of Pennsylvania, Philadelphia, PA 19104-6081

S. Thomopoulos

 Washington University, St. Louis, MO 63130

L. J. Soslowsky1

McKay Orthopaedic Research Laboratory, University of Pennsylvania, Philadelphia, PA 19104-6081soslowsk@mail.med.upenn.edu

1

Corresponding author.

J Biomech Eng 129(3), 400-404 (Oct 12, 2006) (5 pages) doi:10.1115/1.2721075 History: Received August 17, 2005; Revised October 12, 2006

Rotator cuff tears frequently occur and can lead to pain and decreased shoulder function. Repair of the torn tendon back to bone is often successful in relieving pain, but failure of the repair commonly occurs. Post-operative activity level is an important treatment component that has received minimal attention for the shoulder, but may have the potential to enhance tendon to bone healing. The objective of this study was to investigate the effect of short and long durations of various activity levels on the healing supraspinatus tendon to bone insertion site. Rotator cuff tears were surgically created in Sprague–Dawley rats by detaching the supraspinatus tendon from its insertion on the humerus and these tears were immediately repaired back to the insertion site. The post-operative activity level was controlled through shoulder immobilization (IM), cage activity (CA), or moderate exercise (EX) for durations of 4 or 16 weeks. The healing tissue was evaluated utilizing biomechanical testing and a quantitative polarized light microscopy method. We found that activity level had no effect on the elastic properties (stiffness, modulus) of the insertion site at four weeks post injury and repair, and a decreased activity level had a positive effect on these properties at 16 weeks (IM>CA=EX). Furthermore, a decreased activity level had the greatest positive effect on these properties over time (IM>CA=EX). The angular deviation of the collagen, a measure of disorganization, was decreased with a decrease in activity level at 4 weeks (IM<CA=EX), but was similar between groups at 16 weeks (IM=CA=EX). It appears from this study that decreasing the activity level by immobilizing the shoulder improves tendon to bone healing, which progresses by first increasing the organization of the collagen and then increasing the mechanical properties. Future studies in this area will investigate the effect of passive motion and remobilization on both tendon to bone healing and shoulder function.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 3

The angular deviation (AD) of the collagen near the healing insertion site at 4 and 16 weeks following repair for the immobilization (IM), cage activity (CA), and exercise (EX) groups. A larger AD represents a tissue that is more disorganized. Significance (p<0.017) between activity levels is designated with a (* ) and significance over time within an activity level is designated with a (#). A trend (0.017⩽p<0.05) is designated with a (†). The data are presented as mean ± standard deviation and “deg” represents degrees.

Grahic Jump Location
Figure 2

The change in cross-sectional area, modulus, stiffness, and angular deviation (AD) of the healing insertion site over time represented as a ratio from the 16 week to the 4 week time points. A ratio greater than one would designate an increase over time. A (* ) designates a significant (p<0.017) difference between activity levels and a (†) designates a trend (0.017⩽p<0.05). The data are presented as mean ± standard deviation.

Grahic Jump Location
Figure 1

The (a) stiffness; (b) modulus; and (c) cross-sectional area of the healing insertion site at 4 and 16 weeks following repair for the immobilization (IM), cage activity (CA), and exercise (EX) groups. The value for the uninjured control (CTL) is shown at the top of each plot to make it easier to visualize the differences between the experimental groups. Significance (p<0.017) between activity levels is designated with a (* ) and significance over time within an activity level is designated with a (#). A trend (0.017⩽p<0.05) is designated with a (†). The properties from all repair groups were significantly different from the uninjured control with the exception of the stiffness for the IM and EX groups at 16 weeks. The data are presented as mean ± standard deviation.

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