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Technical Forum

Deconstructing the Anterior Cruciate Ligament: What We Know and Do Not Know About Function, Material Properties, and Injury Mechanics

[+] Author and Article Information
Scott G. McLean

Human Performance Innovation Laboratory,
School of Kinesiology,
University of Michigan,
Ann Arbor, MI 48109
e-mail: mcleans@umich.edu

Kaitlyn F. Mallett

Department of Mechanical Engineering,
University of Michigan,
Ann Arbor, MI 48109
e-mail: kmallett@umich.edu

Ellen M. Arruda

Department of Mechanical Engineering,
Department of Biomedical Engineering,
Program in Macromolecular Science
and Engineering,
University of Michigan,
Ann Arbor, MI 48109
e-mail: arruda@umich.edu

Manuscript received August 8, 2014; final manuscript received November 25, 2014; published online January 26, 2015. Editor: Beth A. Winkelstein.

J Biomech Eng 137(2), 020906 (Feb 01, 2015) (19 pages) Paper No: BIO-14-1378; doi: 10.1115/1.4029278 History: Received August 08, 2014; Revised November 25, 2014; Online January 26, 2015

Anterior cruciate ligament (ACL) injury is a common and potentially catastrophic knee joint injury, afflicting a large number of males and particularly females annually. Apart from the obvious acute injury events, it also presents with significant long-term morbidities, in which osteoarthritis (OA) is a frequent and debilitative outcome. With these facts in mind, a vast amount of research has been undertaken over the past five decades geared toward characterizing the structural and mechanical behaviors of the native ACL tissue under various external load applications. While these efforts have afforded important insights, both in terms of understanding treating and rehabilitating ACL injuries; injury rates, their well-established sex-based disparity, and long-term sequelae have endured. In reviewing the expanse of literature conducted to date in this area, this paper identifies important knowledge gaps that contribute directly to this long-standing clinical dilemma. In particular, the following limitations remain. First, minimal data exist that accurately describe native ACL mechanics under the extreme loading rates synonymous with actual injury. Second, current ACL mechanical data are typically derived from isolated and oversimplified strain estimates that fail to adequately capture the true 3D mechanical response of this anatomically complex structure. Third, graft tissues commonly chosen to reconstruct the ruptured ACL are mechanically suboptimal, being overdesigned for stiffness compared to the native tissue. The net result is an increased risk of rerupture and a modified and potentially hazardous habitual joint contact profile. These major limitations appear to warrant explicit research attention moving forward in order to successfully maintain/restore optimal knee joint function and long-term life quality in a large number of otherwise healthy individuals.

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References

Figures

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Fig. 1

External forces and moments acting on the human leg

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Fig. 3

AMB separated from the PLB via a transection of the tibia at their natural separation [67]

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Fig. 4

Load–unload responses of ovine AMB and PLB. This is representative data from a study of bundles from six ovine knees.

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Fig. 8

Axial (vertical) false color strain contours on the surface of a sheep AMB (a) and PLB (b) [67]

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Fig. 7

The initial (red) and stress-softened via preconditioning (blue) load–unload response of a hypothetical viscoelastic tissue. The equilibrium response of this tissue is illustrated by the black dashed line.

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Fig. 6

The strain rate dependent uniaxial loading response of bovine ACL. Adapted from Ref. [154].

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Fig. 5

Viscoelastic response of a human AMB. Nonlinear stress relaxation experiments from various initial strain levels (a). The initial and equilibrium (elastic) responses from the initial and final stress versus strain pairs in the stress relaxation experiments (b). Relaxation modulus function at 0.18 strain plotted on a logarithmic scale to demonstrate its three distinct relaxation regions (c).

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Fig. 2

For uniaxial ACL loading, the fully extended knee in the anatomical position (a) undergoes a posterior and lateral translation of the tibia relative to the femur (b) followed by a 90 deg internal rotation of the tibia (c). Adapted from Ref. [67].

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