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Research Papers

Roentgen Stereophotogrammetric Analysis Methods for Determining Ten Causes of Lengthening of a Soft-Tissue Anterior Cruciate Ligament Graft Construct

[+] Author and Article Information

Biomedical Engineering Program, University of California, One Shields Avenue, Davis, CA 95616

M. L. Hull1

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

S. M. Howell

Department of Mechanical Engineering, University of California, One Shields Avenue, Davis, CA 95616

1

Corresponding author.

J Biomech Eng 130(4), 041002 (May 16, 2008) (10 pages) doi:10.1115/1.2904897 History: Received January 16, 2007; Revised September 19, 2007; Published May 16, 2008

Abstract

There are many causes of lengthening of an anterior cruciate ligament soft-tissue graft construct (i.e., $graft+fixationdevices+bone$), which can lead to an increase in anterior laxity. These causes can be due to plastic deformation and∕or an increase in elastic deformation. The purposes of this in vitro study were (1) to develop the methods to quantify eight causes (four elastic and four plastic) associated with the tibial and femoral fixations using Roentgen stereophotogrammetric analysis (RSA) and to demonstrate the usefulness of these methods, (2) to assess how well an empirical relationship between an increase in length of the graft construct and an increase in anterior laxity predicts two causes (one elastic and one plastic) associated with the graft midsubstance, and (3) to determine the increase in anterior tare laxity (i.e., laxity under the application of a $30N$ anterior tare force) before the graft force reaches zero. Markers were injected into the tibia, femur, and graft in six cadaveric legs whose knees were reconstructed with single-loop tibialis grafts. To satisfy the first objective, legs were subjected to $1500cycles$ at $14Hz$ of $150N$ anterior force transmitted at the knee. Based on marker 3D coordinates, equations were developed for determining eight causes associated with the fixations. After 1500 load cycles, plastic deformation between the graft and WasherLoc tibial fixation was the greatest cause with an average of $0.8±0.5mm$ followed by plastic deformation between the graft and cross-pin-type femoral fixation with an average of $0.5±0.1mm$. The elastic deformations between the graft and tibial fixation and between the graft and femoral fixation decreased averages of $0.3±0.3mm$ and $0.2±0.1mm$, respectively. The remaining four causes associated with the fixations were close to 0. To satisfy the remaining two objectives, after cyclic loading, the graft was lengthened incrementally while the $30N$ anterior tare laxity, $150N$ anterior laxity, and graft tension were measured. The one plastic cause and one elastic cause associated with the graft midsubstance were predicted by the empirical relationships with random errors (i.e., precision) of $0.9mm$ and $0.5mm$, respectively. The minimum increase in $30N$ anterior tare laxity before the graft force reached zero was $5mm$. Hence, each of the eight causes of an increase in the $150N$ anterior laxity associated with the fixations can be determined with RSA as long as the overall increase in the $30N$ anterior tare laxity does not exceed $5mm$. However, predicting the two causes associated with the graft using empirical relationships is prone to large errors.

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Figures

Figure 8

Plot showing the increase in anterior tare laxity versus the intra-articular graft force. The minimum increase in anterior tare laxity before the graft force reaches 6N is 5.3mm.

Figure 7

Empirical relationship between the increase in length of the graft construct and increases in the 150N anterior laxity. The 95% prediction interval is plotted and is ±0.9mm. The random error (i.e., 1 standard deviation) in using this relationship to predict the increase in length of the graft construct is 0.5mm.

Figure 6

Empirical relationship between the increase in length of the graft construct and the increase in the 30N anterior tare laxity. The 95% prediction interval is plotted and is ±1.8mm. The random error (i.e., 1 standard deviation) in using this relationship to predict the increase in length of the graft construct is 0.9mm.

Figure 5

The eight causes of lengthening of an ACL soft-tissue graft construct associated with the fixations versus the number of load cycles. The four plastic causes are (1) between the femoral fixation and bone (PFXB), (2) between the graft and femoral fixation (PFGX), (3) between the graft and tibial fixation (PTGX), and (4) between the tibial fixation and bone (PTXB). The four elastic causes are lengthening due to a decrease in elastic stiffness (1) between the femoral fixation and bone (EFXB), (2) between the graft and femoral fixation (EFGX), (3) between the graft and tibial fixation (ETGX), and (4) between the tibial fixation and bone (ETXB). Negative values indicate shortening. The standard deviations are shown. Causes with the same letter were not statistically different (α=0.05).

Figure 4

Tibial (left) and femoral (right) fixation devices. The WasherLoc was used in our study as illustrated. A custom femoral fixation was used, which allowed the length of the graft construct to be adjusted and the graft tension to be measured (13). The cross bar of the custom femoral fixation, around which the graft is looped, was similar in design to the EZLoc, a femoral fixation commonly used clinically. Also, like the custom femoral fixation, the EZLoc is supported by the cortical bone.

Figure 3

A‐P view of the knee showing marker placement. T1–T6 are markers implanted in the tibia, F1–F6 are markers implanted in the femur, G1–G8 are markers implanted in the graft, and EL and WL are markers attached to the femoral and tibial fixations, respectively. F and T are vectors aligned with the axes of the femoral and tibial tunnels, respectively.

Figure 2

Photograph of a cadaveric leg specimen in the loading apparatus. A pneumatic actuator applied either a posterior or anterior force to the tibia while the thigh and ankle were fixed with the specimen in a supine position and the knee was flexed to 25degs. Two load cells measured the applied force and the reaction force at the ankle joint. The shear force transmitted at the knee was computed from the measured loads in conjunction with the weight of the specimen.

Figure 1

Diagram depicting ten causes of lengthening of a soft-tissue ACL graft construct, which include five plastic causes and five elastic causes. The five plastic causes are nonrecoverable deformation (1) between the femoral fixation and bone (PFXB), (2) between the graft and femoral fixation (PFGX), (3) between the graft and tibial fixation (PTGX), (4) between the tibial fixation and bone (PTXB), and (5) of the graft substance between the fixations (PG). The five elastic causes are lengthening due to a decrease in elastic stiffness (1) between the femoral fixation and bone (EFXB), (2) between the graft and femoral fixation (EFGX), (3) between the graft and tibial fixation (ETGX), (4) between the tibial fixation and bone (ETXB), and (5) of the graft substance between the fixations (EG). Of these ten causes, the eight causes associated with the fixations were measured using RSA.

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