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Research Papers

Estimation of In Vivo ACL Force Changes in Response to Increased Weightbearing

[+] Author and Article Information
Ali Hosseini

Department of Orthopaedic Surgery, Bioengineering Laboratory, Massachusetts General Hospital/Harvard Medical School, Boston, MA 02114; Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139hosseini@alum.mit.edu

Thomas J. Gill

Department of Orthopaedic Surgery, Bioengineering Laboratory, Massachusetts General Hospital/Harvard Medical School, Boston, MA 02114tgill@partners.org

Samuel K. Van de Velde

Department of Orthopaedic Surgery, Bioengineering Laboratory, Massachusetts General Hospital/Harvard Medical School, Boston, MA 02114svandevelde@partners.org

Guoan Li1

Department of Orthopaedic Surgery, Bioengineering Laboratory, Massachusetts General Hospital/Harvard Medical School, Boston, MA 02114gli1@partners.org

1

Corresponding author.

J Biomech Eng 133(5), 051004 (Apr 11, 2011) (9 pages) doi:10.1115/1.4003780 History: Received May 25, 2010; Revised March 02, 2011; Posted March 09, 2011; Published April 11, 2011; Online April 11, 2011

Accurate knowledge of in vivo anterior cruciate ligament (ACL) forces is instrumental for understanding normal ACL function and improving surgical ACL reconstruction techniques. The objective of this study was to estimate the change in ACL forces under in vivo loading conditions using a noninvasive technique. A combination of magnetic resonance and dual fluoroscopic imaging system was used to determine ACL in vivo elongation during controlled weightbearing at discrete flexion angles, and a robotic testing system was utilized to determine the ACL force-elongation data in vitro. The in vivo ACL elongation data were mapped to the in vitro ACL force-elongation curve to estimate the change in in vivo ACL forces in response to full body weightbearing using a weighted mean statistical method. The data demonstrated that by assuming that there was no tension in the ACL under zero weightbearing, the changes in in vivo ACL force caused by full body weightbearing were 131.4±16.8N at 15 deg, 106.7±11.2N at 30 deg, and 34.6±4.5N at 45 deg of flexion. However, when the assumed tension in the ACL under zero weightbearing was over 20 N, the change in the estimated ACL force in response to the full body weightbearing approached an asymptotic value. With an assumed ACL tension of 40 N under zero weightbearing, the full body weight caused an ACL force increase in 202.7±27.6N at 15 deg, 184.9±22.5N at 30 deg, and 98.6±11.7N at 45 deg of flexion. The in vivo ACL forces were dependent on the flexion angle with higher force changes at low flexion angles. Under full body weightbearing, the ACL may experience less than 250 N. These data may provide a valuable insight into the biomechanical behavior of the ACL under in vivo loading conditions.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 2

Virtual dual fluoroscopic system created based on the geometry of the actual experimental system (from Hosseini(30), with permission)

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Figure 3

(a) Robotic testing system with installed knee specimen (before removing soft tissues). (b) Stretching the ACL along its long axis using the robot arm.

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Figure 4

Schematic diagram showing the methodology used for determination of in vivo ACL tension. (a) Weightbearing elongation of the ACL from in vivo weightbearing. (b) Force elongation of the ACL from in vitro robotic test. (c) Estimation of in vivo ACL tension for each individual as a function of weightbearing. (d) Average in vivo ACL force-weightbearing data of all living knees using weighted mean statistical method. (These figures are only conceptual and do not present experimental results. BW: body weight.)

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Figure 5

(a) In vivo weightbearing-elongation behavior of the ACL at 15 deg, 30 deg, and 45 deg of flexion (results from previous study (30)). (b) In vitro force-elongation curves of the ACL at 15 deg, 30 deg, and 45 deg of flexion (standard deviation bars for 30 deg are not shown for the purpose of figure clarity).

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Figure 6

The increase in the in vivo ACL force when the subject placed full body weight on the force plate plotted with increasing assumed initial tension in the ACL

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Figure 7

(a) A knee specimen installed on the robotic testing system with the fluoroscopes to image the knee joint during applying load (validation study). (b) The in vitro ACL forces due to 130 N anterior tibial load measured by robot (hatched bars) and the corresponding estimation of the ACL forces with different ACL tensions under zero weightbearing at 15 deg, 30 deg, and 45 deg of knee flexion (solid bars).

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Figure 1

Schematic of the dual fluoroscopic imaging system

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