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Technical Brief

Effect of Potting Technique on the Measurement of Six Degree-of-Freedom Viscoelastic Properties of Human Lumbar Spine Segments

[+] Author and Article Information
Dhara B. Amin

Biomechanics and Implants Research Group,
School of Computer Science,
Engineering and Mathematics,
The Medical Device Research Institute,
GPO Box 2100,
Adelaide, South Australia 5001, Australia
e-mail: dhara.amin@flinders.edu.au

Isaac M. Lawless

Biomechanics and Implants Research Group,
School of Computer Science,
Engineering and Mathematics,
The Medical Device Research Institute,
GPO Box 2100,
Adelaide, South Australia 5001, Australia
e-mail: isaac.lawless@flinders.edu.au

Dana Sommerfeld

Institute of Biomechanics,
TUHH Hamburg University of Technology,
Denickestr. 15,
Hamburg 21073, Germany
e-mail: dana.sommerfeld@tuhh.de

Richard M. Stanley

Biomechanics and Implants Research Group,
School of Computer Science,
Engineering and Mathematics,
The Medical Device Research Institute,
GPO Box 2100,
Adelaide, South Australia 5001, Australia
e-mail: richard.stanley@flinders.edu.au

Boyin Ding

School of Mechanical Engineering,
The University of Adelaide,
Engineering South, L1,
Adelaide, SA 5005, Australia
e-mail: boyin.ding@adelaide.edu.au

John J. Costi

Biomechanics and Implants Research Group,
School of Computer Science,
Engineering and Mathematics,
The Medical Device Research Institute,
GPO Box 2100,
Adelaide, South Australia 5001, Australia
e-mail: john.costi@flinders.edu.au

1Corresponding author.

Manuscript received May 22, 2014; final manuscript received January 12, 2015; published online February 20, 2015. Assoc. Editor: James C. Iatridis.

J Biomech Eng 137(5), 054501 (May 01, 2015) (8 pages) Paper No: BIO-14-1220; doi: 10.1115/1.4029698 History: Received May 22, 2014; Revised January 12, 2015; Online February 20, 2015

Polymethyl methacrylate (PMMA) and Wood's Metal are fixation media for biomechanical testing; however, the effect of each potting medium on the measured six degree-of-freedom (DOF) mechanical properties of human lumbar intervertebral discs is unknown. The first aim of this study was to compare the measured 6DOF elastic and viscoelastic properties of the disc when embedded in PMMA compared to repotting in Wood's Metal. The second aim was to compare the surface temperature of the disc when potted with PMMA and Wood's Metal. Six human lumbar functional spinal units (FSUs) were first potted in PMMA, and subjected to overnight preload in a saline bath at 37 °C followed by five haversine loading cycles at 0.1 Hz in each of 6DOF loading directions (compression, left/right lateral bending, flexion, extension, left/right axial rotation, anterior/posterior, and lateral shear). Each specimen was then repotted in Wood's Metal and subjected to a 2-h re-equilibrating preload followed by repeating the same 6DOF tests. Outcome measures of stiffness and phase angle were calculated from the final loading cycle in each DOF and were expressed as normalized percentages relative to PMMA (100%). Disc surface temperatures (anterior, left/right lateral) were measured during potting. Paired t-tests (with alpha adjusted for multiple DOF) were conducted to compare the differences in each outcome parameter between PMMA and Wood's Metal. No significant differences in stiffness or phase angle were found between PMMA and Wood's Metal. On average, the largest trending differences were found in the shear DOFs for both stiffness (approximately 35% greater for Wood's Metal compared to PMMA) and phase angle (approximately 15% greater for Wood's Metal). A significant difference in disc temperature was found at the anterior surface after potting with Wood's Metal compared to PMMA, which did not exceed 26 °C. Wood's Metal is linear elastic, stiffer than PMMA and may reduce measurement artifact of potting medium, particularly in the shear directions. Furthermore, it is easier to remove than PMMA, reuseable, and cost effective.

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Figures

Grahic Jump Location
Fig. 1

Photograph showing the 6DOF hexapod robot testing system. A bone is shown installed in the hexapod for illustration purposes only. The specimen was inserted between the rigid base and mobile upper plate (arrow). A six axis load cell (indicated in photo behind actuator frame) is connected between the mobile upper plate and actuator frame. Six ball screw actuators produce the required displacement or rotation. Displacements and rotations of the specimen were directly measured by six linear optical encoders (arrow) that were positioned independently to the loading frame. Therefore, system compliance was eliminated from the measurement of specimen behavior.

Grahic Jump Location
Fig. 2

Recorded data averaged across all specimens and corresponding 95% confidence interval (CI), comparing the viscoelastic (load versus displacement) response for each potting procedure (PMMA, Wood's Metal). Solid lines indicate average response, dotted lines indicate upper, and lower 95% CI responses. Data taken from the final loading and unloading curves were each fitted to sixth order polynomials.

Grahic Jump Location
Fig. 3

Mean (95% CI) stiffness values for all 6DOF tests. The mean percentage (95% CI) normalized fractions are shown for each test and represent the Wood's Metal stiffness relative to the PMMA stiffness, and p values are also shown for each DOF.

Grahic Jump Location
Fig. 4

Mean (95% CI) phase angle values for all 6DOF tests. The mean percentage (95% CI) normalized fractions are shown for each test and represent the Wood's Metal phase angle relative to the PMMA phase angle, and p values are also shown for each DOF.

Grahic Jump Location
Fig. 5

Mean (95% CI) temperature values (°C) for each potting medium taken at three disc regions (anterior, left lateral, and right lateral). * denotes significant differences between groups.

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