Basic Science of Ligament Healing:: C. Anterior Cruciate Ligament Graft Biomechanics and Knee Kinematics

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Injury of the anterior cruciate ligament (ACL) can lead to knee instability associated with damage to other knee structures and the increased risk of osteoarthritis. Thus, surgeons have turned to the use of autografts as ACL replacements to restore knee function and expedite the patient's return to sports, as well as avoid the long-term complications associated with chronic knee instability. However, retrospective and prospective clinical studies conducted at short- and long-term follow-up have revealed that 15% to 25% of patients have less than satisfactory results. Whereas there can be numerous factors that contribute to this outcome, biomechanical studies have helped to gain a new perspective of the complex function of the ACL that these replacement grafts failed to restore. In this article, the readers are provided with a brief review of the experimental methodology used to evaluate the biomechanical properties and function of the ACL and its replacements. Specifically, the use of tensile tests to measure the structural properties of the graft tissue and graft-fixations constructs is described. Tensile testing has allowed for comparison of different graft materials, fixation methods, as well as provided insight into changes in mechanical behavior of the graft construct throughout graft remodeling and incorporation. A unique testing system utilizing a six degrees of freedom robotic manipulator and universal force-moment sensor (UFS) has been developed such that the knee measurements can be made in a non-contact fashion while allowing a series of experiments to be performed on the same knee. Using this robotic/UFS testing system, it has been possible to quantitatively assess the crucial roles of the ACL and their respective bundles in maintaining normal knee stability, especially under rotatory loading. The effects of several variables for reconstruction (graft selection, femoral tunnel positioning, anatomic “double bundle” reconstruction, and initial graft tension) on knee kinematics are then reviewed. To close, directions for future research that could potentially further improve the outcome of reconstructions are provided.

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