A survey of micro-finite element analysis for clinical assessment of bone strength: The first decade
Section snippets
Rik Huiskes and micro-FE: A historical perspective (by Bert van Rietbergen)
The title of this paper is loosely based on that of a paper that Rik Huiskes wrote some 30 years ago, at the time that finite element analyses became a well-accepted tool in the field of biomechanics (Huiskes and Chao, 1983). Ten years later, this paper was followed by a second one: “From structure to process, from organ to cell: recent developments of FE-analysis in orthopaedic biomechanics” (Huiskes and Hollister, 1993). Even though micro- finite element analyses had only just been developed
The methodology and the issues
Due to the fact that only one type of HR-pQCT machine has been used so far, clinical application of micro-FE analysis is a relatively well standardized technique. In most of the clinical studies, a standard image processing workflow is used and the segmented image obtained in this way is used as the geometry of the micro-FE model. The micro-FE analysis itself, however, is less standardized. Different software packages and different procedures have been used, and different output parameters are
Image generation and processing
With the standard workflow, a 9 mm region is scanned at the distal radius or tibia at an isotropic resolution of 82 μm, thus generating a stack of 110 slices. The scan region is chosen at a specific distance from the joint surface as measured from a scout view and all other image settings are standardized as well. The image processing procedure is also standardized, although over time optimized versions have been developed. With the most common workflow, the grey-level image obtained from the
Micro-FE modeling
Typically, micro-FE models are generated directly from the segmented images using a voxel conversion approach. With this approach, voxels representing bone tissue are converted to equally sized brick elements whereas voxels representing the bone marrow are ignored. For typical HR-pQCT scans, the voxel conversion approach will generate models in the range of 1–4 million elements for the radius and in the range of 4–9 million elements for the tibia. This large number complicates the use of
How accurate are the results?
The accuracy of the micro-FE approach to analyze bone stiffness and strength has been investigated in a number of ex-vivo experimental studies, although all of these studies focused on the radius only. The set-up of all these studies is more-or-less the same: a large number of cadaver bones is collected and scanned with the HR-pQCT device using the same settings as used in clinical studies. Following, the bones are compressed to failure to measure their strength and, if possible, their
How reproducible are the results?
Whereas the studies on cadaver bones provide some information on reproducibility of strength predictions, the actual reproducibility in a clinical setting is dependent on more factors. Such factors include misalignment errors, movement artifacts, (cross-) calibration errors and errors related to differences in image analyses by different operators. In this section, several studies that investigated the reproducibility of micro-FE results in a clinical setting are summarized.
MacNeil and Boyd
Is it better than DXA?
One of the major findings in all validations studies is that micro-FE results better predict bone failure load measured in an experiment than any DXA or other bone density-based parameter. In the original paper by Pistoia et al. (2004) a coefficient of determination of 0.66 was reported when comparing the micro-FE estimated and measured failure loads, whereas those for BMC and aBMD were much less (0.48 and 0.31, respectively). In the study by Varga et al. (2010), these differences were less
Alternative techniques and emerging developments
In this survey, only results obtained with the most common micro-FE approach, that uses the voxel conversion technique, were discussed. Other FE approaches have been tested as well, some of them even in clinical studies. Among these are techniques that can significantly reduce the computer time needed to solve the FE problem by representing the actual architecture by a simpler one built of plate and beam elements (Liu et al., 2012). Also, recently, FE approaches incorporating cohesive elements
Conclusion and recommendations
Whereas this review made clear that much information about the accuracy and reproducibility of the micro-FE technique now is available, it also made clear that there are several important technical issues that need to be addressed. One of the major issues with accuracy will be the calibration of the models. Presently, the tissue Young’s modulus is used as a sort of calibration parameter. As a result of this, using different segmentation settings, using different boundary conditions, or using a
Disclosures
Bert van Rietbergen is a consultant for Scanco Medical AG.
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