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Research Papers

Defining the Flexion-Extension Axis of the Ulna: Implications for Intra-Operative Elbow Alignment

[+] Author and Article Information
James R. Brownhill, Louis M. Ferreira

Bioengineering Research Laboratory, The Hand and Upper Limb Center, St. Joseph’s Health Care London, 268 Grosvenor Street, London, ON, N6A 4L6, Canada; Department of Biomedical Engineering, The University of Western Ontario, London, ON, N6A 4L6, Canada

James E. Pichora

Bioengineering Research Laboratory, The Hand and Upper Limb Center, St. Joseph’s Health Care London, 268 Grosvenor Street, London, ON, N6A 4L6, Canada; Department of Medical Biophysics, The University of Western Ontario, London, ON, N6A 4L6, Canada

James A. Johnson

Bioengineering Research Laboratory, The Hand and Upper Limb Center, St. Joseph’s Health Care London, 268 Grosvenor Street, London, ON, N6A 4L6, Canada; Department of Biomedical Engineering, Department of Surgery, and Department of Medical Biophysics, The University of Western Ontario, London, ON, N6A 4L6, Canada

Graham J. King1

Bioengineering Research Laboratory, The Hand and Upper Limb Center, St. Joseph’s Health Care London, 268 Grosvenor Street, London, ON, N6A 4L6, Canada; Department of Surgery, and Department of Medical Biophysics, The University of Western Ontario, London, ON, N6A 4L6, Canadagking@uwo.ca

1

Corresponding author.

J Biomech Eng 131(2), 021005 (Dec 10, 2008) (5 pages) doi:10.1115/1.3005203 History: Received February 07, 2008; Revised September 25, 2008; Published December 10, 2008

Abstract

The increased utilization of total elbow replacements has resulted in a correspondingly increased number of failed implants requiring revision. The most common reason for revision is aseptic loosening of the ulnar component due to polyethylene induced osteolysis. Implant malalignment is thought to be an important cause of bearing wear and implant failure. The ulnar flexion axis can be used to accurately align the ulnar component of the elbow implant; however, the optimal method of determining this axis intra-operatively is unknown. This in vitro study determined the relationship amongst kinematically and anatomically defined ulnar flexion axes in an effort to improve the accuracy of ulnar component positioning. Five different techniques were used to determine the ulnar flexion axis in 12 cadaveric specimens, 3 kinematic and 2 anatomic. The techniques were compared with the screw displacement axis from simulated elbow flexion. An anatomic measurement technique using the guiding ridge of the greater sigmoid notch of the ulna and the radial head was found to most accurately replicate the position and orientation of the screw displacement axis of the elbow $(p<0.05)$. Because an anatomically derived flexion axis can be determined using both pre-operative imaging techniques, as well as with intra-operative guides, it is more practical than kinematically derived techniques requiring tracking systems for clinical application and should provide reliable and consistent results.

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Figures

Figure 3

n and c represent the normal vector and center of the circle fit, respectively. (1) The deepest point in the radial head is projected onto the plane of the greater sigmoid notch. (2) The distance of the projection to the center is calculated as X. (3) The deepest point is then moved proximally by distance X.

Figure 2

During flexion of the radius and ulna, shown here as a progression from light to dark shading, the path of the distal ulnar styloid is recorded. This path is then projected onto a best-fit plane, and approximated as a circle.

Figure 6

In the absence of an intact radial head, an internal correction of 3deg from the plane of the greater sigmoid is required to align to the optimal ulnar flexion axis defined using the proximal radius. Note that the angle shown is exaggerated for illustrative purposes only.

Figure 1

The ulnar coordinate system’s origin is at the center of the greater sigmoid notch. I¯PD, I¯AP, and I¯ML are perpendicular to one another, and point proximally, anteriorly, and medially, respectively, for right ulnae. For left specimens the I¯AP points posteriorly.

Figure 4

Mean (+1SD) positions and angles for each of the techniques examined in the ulnar coordinate system relative to the active SDA in the (a) proximal-distal, (b) anterior-posterior, (c) varus-valgus, and (d) Internal-external orientations. The techniques shown are active SDA (SDA), passive SDA (1), active simple circle (2), passive simple circle (3), greater sigmoid notch (4), and greater sigmoid notch and radial head (5). For techniques 4 and 5, the position of the axis is at the origin of the coordinate system by definition, and the medial-lateral axis is collinear with the normal vector from the circle fit, so the values were always 0 with no deviation.

Figure 5

The mean locations and orientations of the flexion axes generated from each of the six techniques are shown to scale relative to one another on a computed tomography image of a sample specimen. The dotted and dashed lines are used for clarity.

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