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TECHNICAL BRIEFS

# Empirical Relationship Between Lengthening an Anterior Cruciate Ligament Graft and Increases in Knee Anterior Laxity: A Human Cadaveric Study

[+] Author and Article Information
Dustin Grover, Dustin Thompson

Biomedical Engineering Program, University of California, Davis, CA 95616

M. L. Hull1

Biomedical Engineering Program, University of California, Davis, CA 95616 and Department of Mechanical Engineering, University of California, Davis, CA 95616mlhull@ucdavis.edu

S. M. Howell

Department of Mechanical Engineering, University of California, Davis, CA 95616

1

Send all correspondence to Professor M. L. Hull.

J Biomech Eng 128(6), 969-972 (Apr 20, 2006) (4 pages) doi:10.1115/1.2378931 History: Received October 14, 2005; Revised April 20, 2006

## Abstract

Lengthening of an anterior cruciate ligament (ACL) graft construct can occur as a result of lengthening at the sites of tibial and/or femoral fixation and manifests as an increase in anterior laxity. Although lengthening at the site of fixation has been measured for a variety of fixation devices, it is difficult to place these results in a clinical context because the mathematical relationship between lengthening of an ACL graft construct and anterior laxity is unknown. The purpose of our study was to determine empirically this relationship. Ten cadaveric knees were reconstructed with a double-looped tendon graft. With the knee in 25° of flexion, the position of the proximal end of the graft inside the femoral tunnel was adjusted by moving the femoral fixation device until the anterior laxity at an applied anterior force of $134N$ matched that of the intact knee. In random order, the graft construct was lengthened 1, 2, 3, 4, and $5mm$ by moving the femoral fixation device distally along the femoral tunnel and anterior laxity was measured. The increase in the length of the graft construct was related to the increase in anterior laxity by a simple linear regression model. Lengthening the graft construct from 1 to $5mm$ caused an equal increase in anterior laxity (slope=$1.0mm∕mm$, $r2=0.800$, $p<0.0001$). Because an anterior laxity increase of $3mm$ or greater in a reconstructed knee is considered unstable clinically and because many fixation devices in widespread use clinically allow $3mm$ or greater of lengthening in in vitro tests, our empirical relationship indicates that lengthening at the site of fixation probably is an important cause of knee instability following ACL reconstructive surgery. Our empirical relation also indicates that an important criterion in the design of future fixation devices is that lengthening at the sites of fixation in in vitro tests should be limited to less than $3mm$.

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## Figures

Figure 1

Diagram of the reconstructed knee placed in the testing apparatus in 25° of flexion. The rigidly-fixed femur was aligned so that the flexion-extension axis is perpendicular to the sagittal (i.e., vertical) plane. The origin of the tibial coordinate system is the point where the longitudinal axis of the tibia (defined by the axis of the intramedullary rod) intersects a line that is perpendicular to the longitudinal axis of the tibia at the level of the joint line and that lies in the sagittal plane. The line lying in the sagittal plane and perpendicular to the longitudinal axis of the tibia at the level of the joint line is the A-P axis. The custom-made arthrometer was installed on the tibia with the load handle centered over the knee. Anterior and posterior forces were applied by pulling and pushing on the load handle. A load cell in series with the handle recorded the load. Anterior-posterior translation was measured with a linear potentiometer (ETI Systems, Carlsbad, CA) connected to a rigid arm, which was connected to the femoral fixture. The linear potentiometer was positioned above the tibia so that its axis was collinear with the A-P axis. The linear potentiometer was connected through spherical bearings at both ends so that it did not constrain the motion of the knee

Figure 2

Graph of the simple linear regression analysis showing that lengthening the graft 1 to 5mm causes an equal millimeter-for-millimeter increase in anterior laxity

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