Name: a method to estimate cadaveric femur cortical strains during fracture testing using digital image correlation
Uploaded: 2017-09-14T12:30:00
Description: Watch this Scientific Journal Video about A Method to Estimate Cadaveric Femur Cortical Strains During Fracture Testing Using Digital Image Correlation at JoVE.com

The authors would like to thank the Materials and Structural Testing Core at Mayo Clinic for their technical support in performing the fracture testing. In addition we would like to thank Ramesh Raghupathy and Ian Gerstel for their assistance in developing the DIC scripts and specific details of the DIC protocol during their tenure at Mayo Clinic, and the Victor Barocas Research Group, University of Minnesota for the underlying open source software that performs the core of the digital image correlation strain calculations11. This study was financially supported by the Grainger Innovation Fund from the Grainger Foundation.

All experiments were conducted with Institutional Review Board approval. The samples were obtained from anatomical research labs in collaboration.
1. Preparing Specimens for Testing
<ol>
	<li>Thaw the femora at RT for 24 h.</li>
	<li>When femur is in queue for testing, remove any wrap that was applied prior to freezing and wipe the femur with a dry towel to remove any remnant moisture, fatty deposits, or soft tissues. Pot the greater trochanter into a prefabricated aluminum cup with bone cem...

<ol>
	<li>Peters, W., Ranson, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Digital+imaging+techniques+in+experimental+stress+analysis.">Digital imaging techniques in experimental stress analysis.</a> Opt Eng. 21 (3), 213427-213427 (1982).</li><li>Kwon, O., Hanna, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Enhanced+Digital+Image+Correlation+Technique+for+Feature+Tracking+During+Drying+of+Wood.">The Enhanced Digital Image Correlation Technique for Feature Tracking During Drying of Wood.</a> Strain. 46 (6), 566-580 (2010).</li><li>Sutton, M. A., Orteu, J. J., Schreier, H. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Image+Correlation+for+Shape,+Motion+and+Deformation+Measurements.">Image Correlation for Shape, Motion and Deformation Measurements.</a> Adv of Opt Methods in Exp Mech. 3, (2009).</li><li>Grassi, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=How+accurately+can+subject-specific+finite+element+models+predict+strains+and+strength+of+human+femora?+Investigation+using+full-field+measurements.">How accurately can subject-specific finite element models predict strains and strength of human femora? Investigation using full-field measurements.</a> J Biomech. 49 (5), 802-806 (2016).</li><li>Den Buijs, J. O., Dragomir-Daescu, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Validated+finite+element+models+of+the+proximal+femur+using+two-dimensional+projected+geometry+and+bone+density.">Validated finite element models of the proximal femur using two-dimensional projected geometry and bone density.</a> Comput Methods Programs Biomed. 104 (2), 168-174 (2011).</li><li>Keyak, J. H., Rossi, S. A., Jones, K. A., Skinner, H. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Prediction+of+femoral+fracture+load+using+automated+finite+element+modeling.">Prediction of femoral fracture load using automated finite element modeling.</a> J Biomech. 31 (2), 125-133 (1998).</li><li>Lotz, J. C., Cheal, E. J., Hayes, W. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fracture+Prediction+for+the+Proximal+Femur+Using+Finite-Element+Models+.+1Linear-Analysis.">Fracture Prediction for the Proximal Femur Using Finite-Element Models . 1Linear-Analysis.</a> J Biomech Eng-T Asme. 113 (4), 353-360 (1991).</li><li>Cody, D. D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Femoral+strength+is+better+predicted+by+finite+element+models+than+QCT+and+DXA.">Femoral strength is better predicted by finite element models than QCT and DXA.</a> J Biomech. 32 (10), 1013-1020 (1999).</li><li>Dragomir-Daescu, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Robust+QCT/FEA+models+of+proximal+femur+stiffness+and+fracture+load+during+a+sideways+fall+on+the+hip.">Robust QCT/FEA models of proximal femur stiffness and fracture load during a sideways fall on the hip.</a> Ann Biomed Eng. 39 (2), 742-755 (2011).</li><li>Bettamer, A., Hambli, R., Allaoui, S., Almhdie-Imjabber, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Using+visual+image+measurements+to+validate+a+novel+finite+element+model+of+crack+propagation+and+fracture+patterns+of+proximal+femur.">Using visual image measurements to validate a novel finite element model of crack propagation and fracture patterns of proximal femur.</a> Comput Methods Biomech Biomed Eng Imaging Vis. , 1-12 (2015).</li><li>Raghupathy, R., Barocas, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Robust Image Correlation Based Strain Calculator for Tissue Systems. , (2016).</li><li>Taddei, F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Subject-specific+finite+element+models+of+long+bones:+An+in+vitro+evaluation+of+the+overall+accuracy.">Subject-specific finite element models of long bones: An in vitro evaluation of the overall accuracy.</a> J Biomech. 39 (13), 2457-2467 (2006).</li><li>Grassi, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Accuracy+of+finite+element+predictions+in+sideways+load+configurations+for+the+proximal+human+femur.">Accuracy of finite element predictions in sideways load configurations for the proximal human femur.</a> J Biomech. 45 (2), 394-399 (2012).</li><li>Gerstel, I., Raghupathy, R., Dragomir-Daescu, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Digital+Image+Correlation+Identifies+Quantitative+Characteristics+in+Proximal+Femur+Fracture+Crack.">Digital Image Correlation Identifies Quantitative Characteristics in Proximal Femur Fracture Crack.</a> ORS Annual Mtg. , (2012).</li></ol>

We introduced a protocol to consistently prepare femoral samples for high contrast imaging during fracture testing which were then used to estimate full field strain distributions with DIC. This protocol ensured appropriate contrast texture of black tracking speckles against a solid white background on the bone surface. Following this protocol, we successfully replicated the estimation of strains using DIC analysis for eighty-nine femora.
DIC is an optical method which involves placing a mesh ...

Digital Image Correlation (DIC) is an image post-processing method that is used in the current protocol to estimate the full field surface strain of cadaveric femoral test specimens from time-sequence images obtained during mechanical fracture tests. The technique was first developed and applied in experimental stress analysis in the 1980&#39;s and has experienced a rapid increase in use in recent years1,2,3. It has several key advantages over more traditional approaches of mounting strain gauges on a structure including increased spatial distribution of the strain field, finer gauge lengths through increased camera resolution, and avoiding issues with strain gauge glue adhesion or compliance. A major advantage of DIC for biological tissues, such as bone, is that it can be applied to irregular geometries comprising of highly heterogeneous material properties4,5. Its primary drawback over traditional strain acquisition methods is that it requires expensive high speed video cameras of sufficient resolution for the measurement of the region of interest to achieve sufficient spatial and temporal sampling to accurately estimate strain fields.
The primary application of the temporal strain fields obtained from bone fracture DIC analysis is to validate the strain estimations in QCT/FEA models of femoral strength5. Such validation is the focus of many orthopedic research groups which predominantly utilize remote measurements of force and displacement from load cells and displacement transducers6,7,8. In addition, post-fracture image analysis of the fracture pattern has been combined with these remote measurements as further means of model validation9. More recently, the DIC method was applied to validate an FEA model of fracture and crack propagation in the proximal femur10. By utilizing strain correlation between models and experiments, even more confidence in the validity of computational models of proximal femora will be obtained and further advance the QCT/FEA diagnostic method closer to clinical use.
This work explains a detailed protocol to incorporate the necessary steps for DIC analysis in fracture testing of proximal femora. The procedure included the bone preparation steps of spraying a white paint on the bone surface and then speckling black spots on the dried white surface of the bone, methods of obtaining images with sufficient spatial and temporal resolution using high speed video cameras, and the process and tools we used for computing strain fields from these images. We also explained several caveats that may affect the quality of the measurements.

<table border="0" cellpadding="0" cellspacing="0" width="1549">
	<tbody>
		<tr height="25">
			<td height="25" style="height:25px;width:285px;">Krylon plastic primer white</td>
			<td style="width:360px;">Krylon, Peoria, AZ, USA</td>
			<td style="width:425px;">N/A</td>
			<td style="width:479px;">Used as a base coat for a smooth white finish on bone surface</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:285px;">Water-based acrylic white and black paint&#160;</td>
			<td style="width:360px;">Plaid Enterprises (Ceramcoat), Norcross, GA, USA</td>
			<td style="width:425px;">N/A</td>
			<td style="width:479px;">Paint source for white and black colors</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:285px;">Mixing bowl</td>
			<td style="width:360px;">Not specific (generic)</td>
			<td style="width:425px;">N/A</td>
			<td style="width:479px;">Used to mix and prepare paint</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:285px;">Foam brush</td>
			<td style="width:360px;">Linzer Products, Wyandanch, NY, USA</td>
			<td style="width:425px;">N/A</td>
			<td style="width:479px;">Used to apply paint on bone surface</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:285px;">Toothbrush</td>
			<td style="width:360px;">Colgate-Palmolive, New York, NY, USA</td>
			<td style="width:425px;">Firm bristle</td>
			<td style="width:479px;">Used to apply appropriate size and distribution of speckling pattern</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:285px;">Hygenic Orthodontic Resin (PMMA)</td>
			<td style="width:360px;">Patterson Dental, St Paul, MN, USA</td>
			<td style="width:425px;">H02252</td>
			<td>Controlled substance and can be purchased with proper approval</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:285px;">Kenmore Freezer</td>
			<td style="width:360px;">Sears Holdings, Hoffman Estates, IL, USA</td>
			<td style="width:425px;">N/A</td>
			<td>Used to maintain a -20oC storage enviroment for bone specimens</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:285px;">Physiologic Saline (0.9% Sodium Chloride)</td>
			<td style="width:360px;">Baxter Healthcare, Deerfield, IL, USA</td>
			<td style="width:425px;">NDC 0338-0048-04</td>
			<td>Used for keeping specimens hydrated</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:285px;">Scalpels and scrapers</td>
			<td style="width:360px;">Aspen Surgical (Bard-Parker), Caledonia, MI, USA&#160;</td>
			<td style="width:425px;">N/A</td>
			<td>Used to remove soft tissue from bone specimens</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:285px;">Fume Hood</td>
			<td style="width:360px;">Hamilton Laboratory Solutions, Manitowoc, WI, USA</td>
			<td style="width:425px;">70532</td>
			<td>Used for ventilation when preparing PMMA for potting of specimens</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:285px;">Lighting units</td>
			<td style="width:360px;">ARRI, Munich, Germany</td>
			<td style="width:425px;">N/A</td>
			<td>Needed for illumination of target for image capture</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:285px;">High-speed video camera</td>
			<td style="width:360px;">Photron Inc., San Diego, CA, USA</td>
			<td style="width:425px;">Photron Fastcam APX-RS&#160;</td>
			<td>Used to capture the high speed video recordings of the fracture events</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:285px;">Photron FASTCAM Imager and Viewer</td>
			<td style="width:360px;">Photron Inc., San Diego, CA, USA</td>
			<td style="width:425px;">Ver.3392(x64)</td>
			<td>Used to record and view the high speed video recordings</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:285px;">Camera lens</td>
			<td style="width:360px;">Zeiss, Oberkochen, Germany</td>
			<td style="width:425px;">Zeiss Planar L4/50 ZF Lens</td>
			<td>Needed for appropriate image resolution</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:285px;">ABAQUS CAE</td>
			<td style="width:360px;">Dassault System&#232;s, Waltham, MA, USA</td>
			<td style="width:425px;">Versions 6.13-4</td>
			<td>Used for defining region of interest and creating finite element mesh</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:285px;">MATLAB</td>
			<td style="width:360px;">Mathworks, Natick, MA, USA</td>
			<td style="width:425px;">Version 2015b</td>
			<td>Used for image processing and DIC analysis</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:285px;">TecPlot</td>
			<td style="width:360px;">TecPlot Inc., Bellevue, WA</td>
			<td style="width:425px;"></td>
			<td>Used for post processing of strain fields</td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;width:285px;">Strain Calculator Software</td>
			<td style="width:360px;">Victor Barocas Research Group, University of Minnesota, Minneapolis, MN, USA</td>
			<td style="width:425px;">http://license.umn.edu/technologies/20130022_robust-image-correlation-based-strain-calculator-for-tissue-systems</td>
			<td>Used to calculate strain field</td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:285px;">mov_frames.m</td>
			<td style="width:360px;">Matlab script, Mayo Clinic, Rochester, MN,USA</td>
			<td style="width:425px;">N/A</td>
			<td>Used to downsample uncompressed images from high speed video files</td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:285px;">convert_imagesize.m</td>
			<td style="width:360px;">Matlab script, Mayo Clinic, Rochester, MN,USA</td>
			<td style="width:425px;">N/A</td>
			<td>Used to register image pixel coordinates with mesh coordinates</td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;">rrImageTrackGui.m</td>
			<td style="width:360px;">Matlab script, Mayo Clinic, Rochester, MN,USA</td>
			<td style="width:425px;">N/A</td>
			<td>Used to perform the image cross-correlation to obtain deformations and run Strain Calculator</td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:285px;">analyzeFailurePrecursor.m</td>
			<td style="width:360px;">Matlab script, Mayo Clinic, Rochester, MN,USA</td>
			<td style="width:425px;">N/A</td>
			<td>Used to track the peak strain components temporally</td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:285px;">makeMovies.m</td>
			<td style="width:360px;">Matlab script, Mayo Clinic, Rochester, MN,USA</td>
			<td style="width:425px;">N/A</td>
			<td>Used to create portable *.avi movies of the deformation components, strain components, principal strains, von Mises strain, and strain energy</td>
		</tr>
	</tbody>
</table>

a method to estimate cadaveric femur cortical strains during fracture testing using digital image correlation

This protocol describes the method using digital image correlation to estimate cortical strain from high speed video images of the cadaveric femoral surface obtained from mechanical testing. This optical method requires a texture of many contrasting fiduciary marks on a solid white background for accurate tracking of surface deformation as loading is applied to the specimen. Immediately prior to testing, the surface of interest in the camera view is painted with a water-based white primer and allowed to dry for several minutes. Then, a black paint is speckled carefully over the white background with special consideration for the even size and shape of the droplets. Illumination is carefully designed and set such that there is optimal contrast of these marks while minimizing reflections through the use of filters. Images were obtained through high speed video capture at up to 12,000 frames/s. The key images prior to and including the fracture event are extracted and deformations are estimated between successive frames in carefully sized interrogation windows over a specified region of interest. These deformations are then used to compute surface strain temporally during the fracture test. The strain data is very useful for identifying fracture initiation within the femur, and for eventual validation of proximal femur fracture strength models derived from Quantitative Computed Tomography-based Finite Element Analysis (QCT/FEA).

Before the speckling process, the femur is cleaned from excess fat and soft tissues, and the greater trochanter is potted in an aluminum cup. During solidification of polymethylmethacrylate (PMMA), the bone is wrapped in a saline soaked cloth to avoid tissue dryness. Once PMMA is solidified, the bone is cleaned again right before spraying (Figure 1). Then, the bone surface is sprayed or brushed with a water-based plastic white color. Once dri...

Watch this Scientific Journal Video about A Method to Estimate Cadaveric Femur Cortical Strains During Fracture Testing Using Digital Image Correlation at JoVE.com

A Method to Estimate Cadaveric Femur Cortical Strains During Fracture Testing Using Digital Image Correlation

In this protocol, the femur surface strains are estimated during fracture testing using the digital image correlation technique. The novelty of the method involves application of a high-contrast stochastic speckle pattern on the femur surface, carefully specified illumination, high speed video capture, and digital image correlation analysis for strain calculations.

This protocol describes the method using digital image correlation to estimate cortical strain from high speed video images of the cadaveric femoral ...

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