Design and Validation of a Machine for Reproducible Precision Insertion of Femoral Hip Prostheses for Preclinical Testing

[+] Author and Article Information
S. A. Maher, P. J. Prendergast, A. J. Reid, D. V. Waide

Department of Mechanical and Manufacturing Engineering, Trinity College, Dublin 2, Ireland

A. Toni

Laboratorio di Tecnologia Medica, Istituti Ortopedici Rizzoli, Via di Barbiano 1/10, 40136 Bologna, Italy

J Biomech Eng 122(2), 203-207 (Nov 30, 1999) (5 pages) doi:10.1115/1.429645 History: Received March 17, 1999; Revised November 30, 1999
Copyright © 2000 by ASME
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Three stages during an animation of an insertion of a femoral prosthesis into a composite femur. Throughout insertion, contact between the prosthesis and the inner cortical wall is avoided. The animated insertion can be seen on http://www.mme.tcd.ie/Groups/Bioengineering/insertion.html.
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Two points (points A and B) are linked to the prosthesis profile. They plot two trajectories as the prosthesis follows the specified insertion path. The two trajectories are exported to a drawing package, where they are offset (illustrated as a thickened line) to compensate for the diameter of the rollers that will follow the profiled plate. The trajectories are joined together to produce a closed profile. The profile is programmed to a CNC milling machine, and the profile is machined from mild steel plate, producing a cam.
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An assembly drawing of the femoral insertion machine. Two rollers (A and B) are seated on the cam. They are connected via cheek plates to produce a carriage. The machine is of mild steel box section construct, with a wide base for good stability. The fixtures are detachable and adjustable so that the insertion of different prosthesis geometries is possible.
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The prosthesis is located relative to a fixed reference, called an alignment femur, prior to insertion. The schematic shown illustrates the methodology behind locating two fixed points relative to the alignment femur in the coronal and sagittal planes. The positions of two fixed points on the prosthesis are determined from the animation software. Micrometer heads push against the surface of the prosthesis, so that the lower fixed point on the prosthesis is pushed into its predetermined position, and locked in place. The upper point is then pushed into position, so that the prosthesis can rotate about its lower fixed point. The micrometer heads act through slots that have been milled through the faces of the femur.
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Diagram indicating the procedure followed to quantify the reproducibility and the accuracy of insertions, in the coronal plane. Two lines are drawn on the prosthesis. One at a height of 10 mm from the tip of the prosthesis and one at a height of 80 mm from the tip of the prosthesis (following recommendations for prosthesis alignment in ISO 7206-3 16). At 20 mm increments on lines drawn perpendicular to the reference line, the distance between the prosthesis surface and the outer cortical bone is measured on the radiographs and in the animation. A similar procedure is followed in the sagittal plane.




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