Lengthening of a Single-Loop Tibialis Tendon Graft Construct After Cyclic Loading: A Study Using Roentgen Stereophotogrammetric Analysis

[+] Author and Article Information
Conrad Kay Smith

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

M. L. Hull1

Department of Mechanical Engineering, and Biomedical Engineering Program,  University of California, Davis, CA 95616

S. M. Howell

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


Corresponding author; e-mail: mlhull@ucdavis.edu

J Biomech Eng 128(3), 437-442 (Nov 16, 2005) (6 pages) doi:10.1115/1.2187038 History: Received May 17, 2005; Revised November 16, 2005

Although single-loop tibialis tendon allografts have increased in popularity owing to their many advantages over patellar tendon and double-loop hamstring tendon autografts, some percentage of the patient population do not have clinically stable knees following anterior cruciate ligament reconstruction with single-loop tibialis tendon allografts. Therefore, it would be advantageous to determine the causes of increased anterior laxity which ultimately must be traced to lengthening of the graft construct. One objective of this study was to demonstrate the feasibility of using Roentgen stereophotogrammetric analysis (RSA) to determine the causes of lengthening of a single-loop graft construct subjected to cyclic loading. A second objective was to determine which cause(s) contributes most to an increase in length of this graft construct. Radio-opaque markers were inserted into ten grafts to measure the lengthening at the sites of the tibial and femoral fixations and between the sites of fixation. Each graft was passed through a tibial tunnel in a calf tibia, looped around a rigid cross-pin, and fixed to the tibia with a Washerloc fixation device. The grafts were cyclically loaded for 225,000 cycles from 20to170N. Prior to and at intervals during the cyclic loading, simultaneous radiographs were taken. RSA was used to determine the three-dimensional coordinates of the markers from which the lengthening at the sites of fixation and between the sites of fixation was computed at each interval. The sites of the femoral and tibial fixations were the largest contributors to the increase in length of the graft construct, with maximum average values of 0.68 and 0.55 mm, respectively, after 225,000 cycles. The graft substance between the sites of fixation contributed least to lengthening of the graft, with a maximum average value of 0.31 mm. Ninety percent of the maximum average values occurred before 100,000 cycles of loading for the largest contributors. RSA proved to be a useful method for measuring lengthening due to all three causes. Lengthening of the graft construct at the sites of both fixations is sufficiently large that the combined contributions may manifest as a clinically important increase in anterior laxity.

Copyright © 2006 by American Society of Mechanical Engineers
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Grahic Jump Location
Figure 1

Anterior view of the tibia showing the placement of tantalum markers in the bone (T1–T6), tendon markers in the graft (G1, G2, G3, G4), and a marker cemented to the support containing the rigid cross pin (R). The vector T is drawn to indicate the axis of the tibial tunnel. The axis of the tunnel coincides with the direction of loading.

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Figure 2

Contributions of lengthening at the sites of the fixations and between the sites of fixation to the total increase in length of single-loop tendon graft constructs after cyclic loading (n=10). The error bars indicate one standard deviation from the mean.

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Figure 3

Average lengthening at the sites of the fixation and between the sites of fixation in single-looped tendon graft constructs after cyclic loading. The horizontal axis is a linear scale. Lengthening at the sites of fixation reached approximately 86% of the final value by 50,000cycles while lengthening between the sites of fixation reach 80% of the final value after 100,000cycles.



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