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Technical Brief

Comparison of Strain Rosettes and Digital Image Correlation for Measuring Vertebral Body Strain

[+] Author and Article Information
Hannah Gustafson

Department of Mechanical Engineering,
University of British Columbia,
818 West 10th Avenue,
Vancouver, BC V5Z 1M9, Canada
e-mail: hgustafs@interchange.ubc.ca

Gunter Siegmund

MEA Forensic Engineers & Scientists,
11-11151 Horseshoe Way,
Richmond, BC V7A 4S5, Canada;
School of Kinesiology,
University of British Columbia,
210-6081 University Boulevard,
Vancouver, BC V6T 1Z1, Canada
e-mail: gunter.siegmund@meaforensic.com

Peter Cripton

Department of Mechanical Engineering,
University of British Columbia,
818 West 10th Avenue,
Vancouver, BC V5Z 1M9, Canada
e-mail: cripton@mech.ubc.ca

Manuscript received August 31, 2015; final manuscript received February 9, 2016; published online March 15, 2016. Assoc. Editor: Kristen Billiar.

J Biomech Eng 138(5), 054501 (Mar 15, 2016) (6 pages) Paper No: BIO-15-1430; doi: 10.1115/1.4032799 History: Received August 31, 2015; Revised February 09, 2016

Strain gages are commonly used to measure bone strain, but only provide strain at a single location. Digital image correlation (DIC) is an optical technique that provides the displacement, and therefore strain, over an entire region of interest on the bone surface. This study compares vertebral body strains measured using strain gages and DIC. The anterior surfaces of 15 cadaveric porcine vertebrae were prepared with a strain rosette and a speckled paint pattern for DIC. The vertebrae were loaded in compression with a materials testing machine, and two high-resolution cameras were used to image the anterior surface of the bones. The mean noise levels for the strain rosette and DIC were 1 με and 24 με, respectively. Bland–Altman analysis was used to compare strain from the DIC and rosette (excluding 44% of trials with some evidence of strain rosette failure or debonding); the mean difference ± 2 standard deviations (SDs) was −108 με ± 702 με for the minimum (compressive) principal strain and −53 με ± 332 με for the maximum (tensile) principal strain. Although the DIC has higher noise, it avoids the relatively high risk we observed of strain gage debonding. These results can be used to develop guidelines for selecting a method to measure strain on bone.

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References

Figures

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Fig. 3

Comparison of the RMS noise of the principal strains on the surface of the vertebrae measured by the strain rosette and DIC. For the DIC, the noise was measured both over the strain rosette and over the triangular region. Means (±1 SD) are shown and the raw data are plotted as points. The RMS noise was defined as the RMS error of the strain during 2 s of the trial where the load was held constant at 100 N. The DIC RMS noise levels are significantly different than the strain rosette.

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Fig. 2

Time history of the loading for each specimen. The pretest trial was used to ensure that the bone was seated to the PMMA. The prescribed loading was the same for all trials.

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Fig. 1

An example of one-camera view of the anterior surface of a prepared porcine vertebra. In this case, the strain rosette was applied on the right side. The bone was painted with a white layer with black speckle for DIC. Two sets of three dots were painted symmetrically about the centerline of the bone.

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Fig. 4

The difference at the loading peaks between the minimum principal strain measured by strain gage and DIC for trials (three peaks per trial, three trials). Peaks from trials that had no evidence of debonding are shown with circles while peaks from trials with evidence of debonding are shown as triangles.

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Fig. 5

Bland–Altman plots of the magnitudes of the (a) minimum (compressive) principal strain and (b) maximum (tensile) principal strain measured using a strain rosette and DIC at the peak load. The horizontal axis is the average values of the strain measured using the rosette and the average value of thestrains from DIC over the same area at the peak load. The vertical axis is the value measured using the rosette minus the strain from DIC. The solid lines are the average value of all thedifferences. The dashed lines are the average value of the differences ±2 SDs.

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Fig. 6

Example time histories of the minimum principal strains measured with DIC and strain rosette in one trial from each specimen in which debonding of the strain rosette was observed. For all trials where debonding was observed, the DIC measured higher absolute minimum principal strains than the strain rosette. Listed on each plot are the criteria met that demonstrate debonding.

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