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Research Papers

Mechanical Compromise of Partially Lacerated Flexor Tendons

[+] Author and Article Information
Sarah Duenwald-Kuehl

Department of Biomedical Engineering,
University of Wisconsin-Madison,
Madison, WI 53705;
Department of Orthopedics and Rehabilitation,
University of Wisconsin-Madison,
Madison, WI 53705

Roderic Lakes

Materials Science Program,
University of Wisconsin-Madison,
Madison, WI 53705;
Department of Engineering Physics,
University of Wisconsin-Madison,
Madison, WI 53705

Ray Vanderby

Department of Biomedical Engineering,
University of Wisconsin-Madison,
Madison, WI 53705;
Department of Orthopedics and Rehabilitation,
University of Wisconsin-Madison,
Madison, WI 53705
Materials Science Program,
University of Wisconsin-Madison,
Madison, WI 53705
e-mail: vanderby@ortho.wisc.edu

Contributed by the Bioengineering Division of ASME for publication in the Journal of Biomechanical Engineering. Manuscript received June 5, 2012; final manuscript received November 2, 2012; accepted manuscript posted November 28, 2012; published online December 26, 2012. Editor: Victor H. Barocas.

J Biomech Eng 135(1), 011001 (Dec 26, 2012) (8 pages) Paper No: BIO-12-1218; doi: 10.1115/1.4023092 History: Received June 05, 2012; Revised November 02, 2012; Accepted November 28, 2012

Tendons function to transmit loads from muscle to move and stabilize joints and absorb impacts. Functionality of lacerated tendons is diminished, however clinical practice often considers surgical repair only after 50% or more of the tendon is lacerated, the “50% rule.” Few studies provide mechanical insight into the 50% rule. In this study cyclic and static stress relaxation tests were performed on porcine flexor tendons before and after a 0.5, 1.0, 2.0, or 2.75 mm deep transverse, midsubstance laceration. Elastic and viscoelastic properties, such as maximum stress, change in stress throughout each test, and stiffness, were measured and compared pre- and post-laceration. Nominal stress and stiffness parameters decreased, albeit disproportionately in magnitude, with increasing percent loss of cross-sectional area. Conversely, mean stress at the residual area (determined using remaining intact area at the laceration cross section) exhibited a marked increase in stress concentration beginning at 47.2% laceration using both specified load and constant strain analyses. The marked increase in stress concentration beginning near 50% laceration provides mechanical insight into the 50% rule. Additionally, a drastic decrease in viscoelastic stress parameters after only an 8.2% laceration suggests that time-dependent mechanisms protecting tissues during impact loadings are highly compromised regardless of laceration size.

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Figures

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Fig. 1

(a) Diagram of tendon gripped in soft tissue and bone grip. The soft tissue grip contains two rough surfaces to clamp the muscle side of the tendon and the potted bone fits snugly into the custom bone grip. (b) Diagram of the gripped tendon placed in a saline filled bath. The cut was produced 30 mm from the bone grip by the cutting tool shown. (c) Diagram showing the top view of the cutting tool and cross sections of three different tendons. The black section and percentages shown correspond to the cut area percentage.

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Fig. 2

Mechanical testing parameters for (a) cyclic testing (σpeak, σdecrease) and (b) relaxation testing (σmax, σdecay)

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Fig. 3

Representative force-displacement curves for pre- and post-laceration data for a tendon subjected to a 2.0 mm laceration. Data were obtained from the initial pull of the cyclic testing. The slopes of the linear regions of each curve (curve fits shown) were used to determine stiffness.

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Fig. 4

Representative stress-strain curves for pre- and post-laceration data for a tendon subjected to a 2.0 mm laceration. Data were obtained from the initial pull of the cyclic testing. Initiation of stress application occurs at a larger strain and nominal stress (force divided by original cross-sectional area) decreases post-laceration.

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Fig. 5

Representative pre- and post-laceration data for (a) cyclic and (b) relaxation testing of a tendon subjected to a 2.0 mm laceration. Nominal stress decreased after laceration for both tests.

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Fig. 6

Post-laceration nominal stress expressed as a percentage of pre-laceration stress plotted against the laceration area for (a) cyclic and (b) relaxation testing parameters. The expected change shown is based on the prediction that a proportional, linear correlation exists between damage and laceration area. Relaxation parameters exhibit a more linear relationship between post-laceration nominal stress percentage and laceration area.

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Fig. 7

Post-laceration parameter percentage of pre-laceration values for (a) nominal stress parameters, (b) MRA stress parameters, (c) stiffness, and (d) specified load parameters. (a) Nominal stress percentages decrease for both cyclic and relaxation parameters. (b) MRA stress parameters begin to increase at 2.0 mm laceration. (c) Stiffness decreases with increasing laceration size. (d) Strain and MRA stress increase post-laceration. Error bars represent standard error (n = 8) and differing Greek letters (α, β, γ) represent significant differences between laceration groups within each parameter determined by the Fisher's least significant difference post hoc analysis (p < 0.05).

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