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RESEARCH PAPERS

An MRI-Based Method to Align the Compressive Loading Axis for Human Cadaveric Knees

[+] Author and Article Information
K. J. Martin

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

C. P. Neu

Department of Orthopedics, University of California at Davis Medical Center, 2315 Stockton Boulevard, Sacramento, CA 95817

M. L. Hull1

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

1

Corresponding author.

J Biomech Eng 129(6), 855-862 (Apr 30, 2007) (8 pages) doi:10.1115/1.2800765 History: Received June 01, 2006; Revised April 30, 2007

Abstract

There is a need to align the mechanical axis of the tibia with the axis of loading for studies involving tibiofemoral compression to interpret results and to ensure repeatability of loading within and among specimens. Therefore, the objectives of this study were (1) to develop a magnetic resonance imaging (MRI)-based alignment method for use with apparatuses applying tibiofemoral joint compression, (2) to demonstrate the usefulness of the method by aligning cadaveric knees in an apparatus that could apply tibiofemoral joint compression, and (3) to quantify the error associated with the alignment method. A four degree-of-freedom adjustable device was constructed to allow determination and alignment of the mechanical axis of the tibia of cadaveric knee joints with the axis of loading of an apparatus applying tibiofemoral joint compression. MRI was used to determine the locations of bony landmarks in three dimensions defining the mechanical axis of the tibia relative to an initial orientation of the four degree-of-freedom device. Adjustment values of the device were then computed and applied to the device to align the mechanical axis of the tibia with the axis of a compressive loading apparatus. To demonstrate the usefulness of the method, four cadaveric knees were aligned in the compressive loading apparatus. The vectors describing the mechanical axis of the tibia and the loading axis of the apparatus before and after adjustment of the four degree-of-freedom device were computed for each cadaveric knee. After adjustment of the four degree-of-freedom device, the mechanical axis of the tibia was collinear with the loading axis of the apparatus for each cadaveric knee. The errors in the adjustment values introduced by inaccuracies in the MR images were quantified using the Monte Carlo technique. The precisions in the translational and rotational adjustments were $1.20mm$ and $0.90deg$ respectively. The MR-based alignment method will allow consistent interpretation of results obtained during tibiofemoral compressive studies conducted using the apparatus described in this paper by providing a well-defined loading axis. The alignment method can also be adapted for use with other apparatuses applying tibiofemoral compression.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

Figure 1

Local coordinate systems for tibia and femur. See text for the definition of coordinate systems.

Figure 2

Diagram of the plastic version of the 4-DOF adjustable attachment device with components in non-neutral positions. The aluminum version of the device had the same dimensions, but did not have compartments encasing Delrin spheres surrounded by silicone gel. Instead the aluminum version had a pin fixed to the end of the hinge, which was used to fix the device and, thus, the specimen to a loading apparatus. The pin on the aluminum device was collinear with the vector on the analogous plastic device from the center of rotation to the single sphere at its end shown here. M-L and A-P translations of each device were achieved by sliding slotted plates relative to the first plate, which was attached to the distal end of the tibia. F-E and V-V rotations were achieved by revolving the hinge and pivot block respectively about the two axes of the universal joint.

Figure 3

Specimen aligned in an apparatus (top and front parts of the frame are removed) that can apply tibiofemoral compression such that the mechanical axis of the tibia coincides with the axis of loading. The specimen is flexed 10deg from its 0deg reference position.

Figure 4

Orientation of the loading axis of the 4-DOF device with the tibial mechanical axis of Specimen 2 before and after alignment of the device. After alignment, the mechanical axis was collinear with the loading axis.

Figure 5

Diagram illustrating the location of the spatial points and bases used to compute translation and rotation adjustments for the 4-DOF adjustable attachment device. Translation adjustments are also shown here while rotation adjustments are shown in Fig. 6.

Figure 6

Angles used to determine V-V and F-E adjustments of 4-DOF devices once A-P and M-L translations were computed

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