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Initial Tension and Anterior Load-Displacement Behavior of High-Stiffness Anterior Cruciate Ligament Graft Constructs
Ari Karchin, MS1; M.L. Hull, PhD1; S.M. Howell, MD2
1 Biomedical Engineering Program, One Shields Avenue, University of California, Davis, CA 95616. E-mail address for M.L. Hull: mlhull@ucdavis.edu
2 Department of Mechanical Engineering, One Shields Avenue, University of California, Davis, CA 95616. E-mail address for M.L. Hull: mlhull@ucdavis.edu
View Disclosures and Other Information
In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from the Whitaker Foundation. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.
Investigation performed at the Biomedical Engineering Program and the Department of Mechanical Engineering, University of California, Davis, California

The Journal of Bone and Joint Surgery, Incorporated
J Bone Joint Surg Am, 2004 Aug 01;86(8):1675-1683
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Abstract

Background: Because the tension that exists in an anterior cruciate ligament graft when the knee is unloaded (the initial tension) affects the surgical outcome and because high initial tension has a number of adverse consequences, the primary purpose of this study was to determine quantitatively how much less initial tension was required for a high-stiffness construct than for a low-stiffness construct. A secondary purpose was to determine how the stiffness of the graft construct affects the anterior load-displacement behavior of the knee from 0° to 90° of flexion.

Methods: Anterior-posterior load-displacement was measured in each of ten intact cadaveric knee specimens, the anterior cruciate ligament was excised, and the anterior cruciate ligament was reconstructed with a double-loop bovine tendon graft. Graft constructs of different stiffness were created with use of six springs, ranging in stiffness from 25 to 275 N/mm to simulate the fixation stiffness. After adjusting the initial tension of the graft so that the anterior-posterior laxity of the reconstructed knee matched that of the intact knee, the 0-N posterior limit and the 225-N anterior limit were measured at 0°, 30°, 60°, and 90° of flexion.

Results: The highest stiffness fixation (275 N/mm) required an average of 73 N of initial tension, which was more than three times less than the average of 242 N of initial tension required by the lowest stiffness fixation (25 N/mm). The 225-N anterior limit was overconstrained an average of 1.0 mm with the highest stiffness fixation (275 N/mm), which was 3.6 mm less than the overconstraint with the lowest stiffness fixation (25 N/mm). Likewise, the posterior limit was overconstrained an average of 2.6 mm with the highest stiffness fixation (275 N/mm), which was 3.8 mm less than the overconstraint with the lowest stiffness fixation (25 N/mm).

Conclusions: The initial tension for a high-stiffness graft construct is more than three times less than that for a low-stiffness construct. The initial tension for a high-stiffness graft construct better restores both the 225-N anterior limit and the 0-N posterior limit to normal than the initial tension for a low-stiffness graft construct over the range of flexion from 0° to 90°.

Clinical Relevance: Because a high-stiffness graft construct requires substantially less initial tension than a low-stiffness graft construct, the tension pattern in a high-stiffness graft construct better matches the pattern in the intact anterior cruciate ligament. This tension pattern may avoid adverse consequences to both the knee joint function and the graft, which have been linked to high initial graft tension when the initial tension is maintained postoperatively. When the initial tension is not maintained postoperatively, a high-stiffness construct may be advantageous in avoiding a recurrence of knee instability.

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    These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.
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