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Research Papers

Capturing Three-Dimensional In Vivo Lumbar Intervertebral Joint Kinematics Using Dynamic Stereo-X-Ray Imaging

[+] Author and Article Information
Ameet K. Aiyangar

EMPA (Swiss Federal Laboratories
for Materials Science and Research),
Mechanical Systems Engineering (Lab 304),
Ueberlandstrasse 129,
Duebendorf 8400, Switzerland
Department of Orthopaedic Surgery,
University of Pittsburgh,
3820 South Water Street,
Pittsburgh, PA 15203
e-mail: ameetaiyangar@gmail.com

Liying Zheng

Department of Orthopaedic Surgery,
Musculoskeletal Modeling Laboratory,
University of Pittsburgh,
3820 South Water Street,
Pittsburgh, PA 15203
e-mail: zlyreed@gmail.com

Scott Tashman

Department of Orthopaedic Surgery,
Department of Bioengineering,
Orthopaedic Biodynamics Laboratory,
University of Pittsburgh,
3820 South Water Street,
Pittsburgh, PA 15203
e-mail: tashman@pitt.edu

William J. Anderst

Department of Orthopaedic Surgery,
Orthopaedic Biodynamics Laboratory,
University of Pittsburgh,
3820 South Water Street,
Pittsburgh, PA 15203
e-mail: anderst@pitt.edu

Xudong Zhang

Department of Orthopaedic Surgery,
Department of Bioengineering,
Department of Mechanical Engineering and Materials Science,
Musculoskeletal Modeling Laboratory,
University of Pittsburgh,
3820 South Water Street,
Pittsburgh, PA 15203
e-mail: xuz9@pitt.edu

There were other tasks performed by the subject, which were not reported here. The total effective radiation dose was estimated to be well under 3.6 mSv.

1Corresponding author.

Contributed by the Bioengineering Division of ASME for publication in the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received May 3, 2013; final manuscript received October 14, 2013; accepted manuscript posted October 22, 2013; published online November 26, 2013. Assoc. Editor: Brian D. Stemper.

J Biomech Eng 136(1), 011004 (Nov 26, 2013) (9 pages) Paper No: BIO-13-1215; doi: 10.1115/1.4025793 History: Received May 03, 2013; Revised October 14, 2013; Accepted October 22, 2013

Availability of accurate three-dimensional (3D) kinematics of lumbar vertebrae is necessary to understand normal and pathological biomechanics of the lumbar spine. Due to the technical challenges of imaging the lumbar spine motion in vivo, it has been difficult to obtain comprehensive, 3D lumbar kinematics during dynamic functional tasks. The present study demonstrates a recently developed technique to acquire true 3D lumbar vertebral kinematics, in vivo, during a functional load-lifting task. The technique uses a high-speed dynamic stereo-radiography (DSX) system coupled with a volumetric model-based bone tracking procedure. Eight asymptomatic male participants performed weight-lifting tasks, while dynamic X-ray images of their lumbar spines were acquired at 30 fps. A custom-designed radiation attenuator reduced the radiation white-out effect and enhanced the image quality. High resolution CT scans of participants' lumbar spines were obtained to create 3D bone models, which were used to track the X-ray images via a volumetric bone tracking procedure. Continuous 3D intervertebral kinematics from the second lumbar vertebra (L2) to the sacrum (S1) were derived. Results revealed motions occurring simultaneously in all the segments. Differences in contributions to overall lumbar motion from individual segments, particularly L2–L3, L3–L4, and L4–L5, were not statistically significant. However, a reduced contribution from the L5–S1 segment was observed. Segmental extension was nominally linear in the middle range (20%–80%) of motion during the lifting task, but exhibited nonlinear behavior at the beginning and end of the motion. L5–S1 extension exhibited the greatest nonlinearity and variability across participants. Substantial AP translations occurred in all segments (5.0 ± 0.3 mm) and exhibited more scatter and deviation from a nominally linear path compared to segmental extension. Maximum out-of-plane rotations (<1.91 deg) and translations (<0.94 mm) were small compared to the dominant motion in the sagittal plane. The demonstrated success in capturing continuous 3D in vivo lumbar intervertebral kinematics during functional tasks affords the possibility to create a baseline data set for evaluating the lumbar spinal function. The technique can be used to address the gaps in knowledge of lumbar kinematics, to improve the accuracy of the kinematic input into biomechanical models, and to support development of new disk replacement designs more closely replicating the natural lumbar biomechanics.

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Figures

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Fig. 1

(a) Dynamic stereo radiography (DSX) system configured for the functional lifting task. (b) Lateral view shows positioning of the pelvic rest and radiation attenuator. (c) and (d) Illustration of the radiation attenuator.

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Fig. 2

Graphical representation of the volumetric model-based bone tracking process

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Fig. 3

Vertebral anatomical coordinate system with origin located at the center of the vertebral body. Origin is defined as the mean of the eight landmark (red) points. Axis1 is defined as the vector connecting the anterior and posterior points on the superior endplate. Temporary axis is defined as the vector connecting the two lateral points. Vertical axis (Axis2) is defined as the cross product (Axis1 × temporary axis). Axis3 is then defined as the cross product (Axis1 × Axis2).

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Fig. 4

Continuous extension (ad) and AP translation (eh) data for individual participants during the lifting task. Intervertebral segmental motion is plotted against L2–S1 ROM. Each participant's L2–S1 ROM is normalized to the upright posture recorded during the static test (100%), with the initial position in the lifting task set as 0%.

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Fig. 5

Segmental distribution of L2–S1 rotation (a) and translation (b). Bars delineate the percentage contribution from each lumbar segment to the overall L2–S1 motion at every 10th percent of L2–S1 rotation during the functional lifting task. 100% motion is defined as the L2–S1 pose in the static upright posture. 0% is the initial position at the beginning of the lifting task. Error bars are 95% confidence intervals.

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Fig. 6

Slope of the segmental rotation with respect to the overall lumbar (L2–S1) extension, shown for each segment. L2–S1 extension is normalized to the upright static posture (100%), with the initial position set as 0%. Slopes are calculated based on linear fit to the data between 20% and 80% of the overall lumbar motion. Error bars are 95% confidence intervals. No statistical differences were detected in the slope means between the segments by ANOVA (p = 0.4).

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