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Technical Briefs

A Simple Method for In Vivo Measurement of Implant Rod Three-Dimensional Geometry During Scoliosis Surgery

[+] Author and Article Information
Remel A. Salmingo

Division of Human Mechanical Systems and Design,Graduate School of Engineering,  Hokkaido University, North 13 West 8 Kita-Ku, Sapporo, Japan 060-8628

Shigeru Tadano1

Division of Human Mechanical Systems and Design, Faculty of Engineering,  Hokkaido University, North 13 West 8 Kita-Ku, Sapporo, Japan 060-8628tadano@eng.hokudai.ac.jp

Kazuhiro Fujisaki

Division of Human Mechanical Systems and Design, Faculty of Engineering,  Hokkaido University, North 13 West 8 Kita-Ku, Sapporo, Japan 060-8628

Yuichiro Abe

Department of Orthopaedic Surgery,Eniwa Hospital, Kogane-Cho 2-1-1, Eniwa, Japan 061-1449

Manabu Ito

Department of Orthopaedic Surgery, Graduate School of Medicine,  Hokkaido University, North 15 West 7 Kita-Ku, Sapporo, Japan 060-8638

1

Corresponding author.

J Biomech Eng 134(5), 054502 (May 25, 2012) (5 pages) doi:10.1115/1.4006687 History: Received January 05, 2012; Revised April 11, 2012; Posted May 01, 2012; Published May 25, 2012; Online May 25, 2012

Scoliosis is defined as a spinal pathology characterized as a three-dimensional deformity of the spine combined with vertebral rotation. Treatment for severe scoliosis is achieved when the scoliotic spine is surgically corrected and fixed using implanted rods and screws. Several studies performed biomechanical modeling and corrective forces measurements of scoliosis correction. These studies were able to predict the clinical outcome and measured the corrective forces acting on screws, however, they were not able to measure the intraoperative three-dimensional geometry of the spinal rod. In effect, the results of biomechanical modeling might not be so realistic and the corrective forces during the surgical correction procedure were intra-operatively difficult to measure. Projective geometry has been shown to be successful in the reconstruction of a three-dimensional structure using a series of images obtained from different views. In this study, we propose a new method to measure the three-dimensional geometry of an implant rod using two cameras. The reconstruction method requires only a few parameters, the included angle θ between the two cameras, the actual length of the rod in mm, and the location of points for curve fitting. The implant rod utilized in spine surgery was used to evaluate the accuracy of the current method. The three-dimensional geometry of the rod was measured from the image obtained by a scanner and compared to the proposed method using two cameras. The mean error in the reconstruction measurements ranged from 0.32 to 0.45 mm. The method presented here demonstrated the possibility of intra-operatively measuring the three-dimensional geometry of spinal rod. The proposed method could be used in surgical procedures to better understand the biomechanics of scoliosis correction through real-time measurement of three-dimensional implant rod geometry in vivo.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Three-dimensional reconstruction from different views. (a) Projection of points in the image plane of each camera at different views. (b) Geometry relation of the points when viewed from the top

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Figure 2

(a) Curved spinal rod in space projected at different views. (b) Selected points (arbitrary) from the inferior to the superior endpoint in the left and right images of the rod for quintic polynomial curve fitting.

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Figure 3

Scanned image of implant rod with scale for validation of the two-camera method. The cross marks are selected arbitrary points for curve fitting.

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Figure 4

Left and right images of three different positions obtained by two cameras at different views (upper pictures are taken from the center of both left and right views)

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Figure 5

Typical reconstructed three-dimensional geometry of implant rod using two cameras. It can be seen that the distance of points in the y-axis is almost zero. This is in agreement with the actual spinal rod geometry.

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