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Mechanical Loading Causes Detectable Changes in Morphometric Measures of Trabecular Structure in Human Cancellous Bone

[+] Author and Article Information
Yener N. Yeni

e-mail: yeni@bjc.hfh.edu

Daniel Oravec

Bone and Joint Center,
Henry Ford Health System,
Detroit, MI 48202

1Corresponding author.

2Present address: Section of Biomechanics, Bone and Joint Center, Henry Ford Hospital, 2799 West Grand Boulevard, Detroit, MI 48202.

Contributed by the Bioengineering Division of ASME for publication in the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received November 13, 2012; final manuscript received March 21, 2013; accepted manuscript posted April 4, 2013; published online April 24, 2013. Assoc. Editor: Tammy Haut Donahue.

J Biomech Eng 135(5), 054505 (Apr 24, 2013) (5 pages) Paper No: BIO-12-1554; doi: 10.1115/1.4024136 History: Received November 13, 2012; Revised March 21, 2013; Accepted April 04, 2013

The relationships between mechanical loads and bone microstructure are of interest to those who seek to predict bone mechanical properties from microstructure or to predict how organization of bone microstructure is driven by mechanical loads. While strains and displacements in the material are inherently responsible for mechanically caused changes in the appearance of the microstructure, it is the morphometric measures of microstructural organization that are often available for assessment of bone quality. Therefore, an understanding of how strain history is reflected in morphometric measures of bone microstructure has practical implications in that it may provide clinically measurable indices of mechanical history in bone and improve interpretation of bone mechanical properties from microstructural information. The objective of the current study was to examine changes in morphometric measures of cancellous bone microstructure in response to varying levels of continuum level strains. The experimental approach included stereologic analysis of microcomputed tomography (μCT) images of human cancellous bone samples obtained at sequentially increasing levels of strain in a custom-made loading apparatus mounted in a μCT scanner. We found that the degree of anisotropy (DA) decreased from baseline to failure and from failure to postfailure. DA partially recovered from postfailure levels upon unloading; however, the final DA was less than at failure and less than at baseline. We also found that average trabecular thickness (Tb.Th.Av) increased with displacements at postfailure and did not recover when unloaded. Average trabecular number decreased when the specimens were unloaded. In addition, the heterogeneity of Tb.Th as measured by intra-specimen standard deviation (Tb.Th.SD) increased and that of trabecular number (Tb.N.SD) decreased with displacements at postfailure. Furthermore, the intraspecimen coefficient of variation of trabecular number decreased at postfailure displacements but did not recover upon unloading. Finally, the coefficient of variation of trabecular separation at unload was less than that at baseline. These measures can be developed into image-based indices to estimate strain history, damage, and residual mechanical properties where direct analysis of stresses and strains, such as through finite element modeling, may not be feasible. It remains to be determined how wide a time interval can be used to estimate strain history before remodeling becomes an overriding effect on the trabecular architecture.

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References

Figures

Grahic Jump Location
Fig. 5

(a) Changes in the standard deviation of trabecular number (Tb.N.SD) with different levels of strain representing baseline, elastic, postyield, postfailure, and unloaded states. Changes in the asterisks indicate significant changes in Tb.N.SD as strain levels change (from left to right). The levels of strain connected with a bar (–) are not significantly different. This graph indicates that there is no change in Tb.N.SD from baseline at the elastic and failure levels (the bar) but there is a change at the post-failure (PF) level (* versus **) after which no further changes are observed (the second bar). The error bars represent within-subject standard deviations which take into account the repeated nature of the measurements [18]. (b) Replot with error bars representing standard deviations without taking into account the repeated nature of the measurements (no test of significance is performed).

Grahic Jump Location
Fig. 4

(a) Changes in the standard deviation of trabecular thickness (Tb.Th.SD) with different levels of strain representing baseline, elastic, postyield, postfailure, and unloaded states. Changes in the asterisks indicate significant changes in Tb.Th.SD as strain levels change (from left to right). The levels of strain connected with a bar (–) are not significantly different. This graph indicates that there is no change in Tb.Th.SD from baseline at the elastic and failure levels (the bar), but there is a change at the postfailure (PF) level (* versus **) after which no further changes are observed (the second bar). The error bars represent within-subject standard deviations which take into account the repeated nature of the measurements [18]. (b) Replot with error bars representing standard deviations without taking into account the repeated nature of the measurements (no test of significance is performed).

Grahic Jump Location
Fig. 3

(a) Changes in average trabecular thickness (Tb.Th.Av) with different levels of strain representing baseline, elastic, postyield, postfailure, and unloaded states. Changes in the asterisks indicate significant changes in Tb.Th.Av as strain levels change (from left to right). The levels of strain connected with a bar (–) are not significantly different. This graph indicates that there is no change in Tb.Th.Av from baseline at the elastic and failure levels (the bar), but there is a change at the postfailure (PF) level (* versus **) after which no further changes are observed (the second bar).The error bars represent within-subject standard deviations, which take into account the repeated nature of the measurements [18]. (b) Replot with error bars representing standard deviations without taking into account the repeated nature of the measurements (no test of significance is performed).

Grahic Jump Location
Fig. 2

(a) Changes in the degree of anisotropy with different levels of strain representing baseline, elastic, postyield, postfailure, and unloaded states. Changes in the asterisks indicate significant changes in DA as strain levels change (from left to right). The levels of strain connected with a bar (–) are not significantly different. This graph indicates that there is no change in DA from baseline at the elastic level (the bar); however, there is a change at the failure (F) level (* versus **), further changes at the post-failure (PF) level (** versus ***) and further changes at the unload (U) level (*** versus ****) but that changes in DA are not fully recovered upon unloading (**** versus *). The error bars represent within-subject standard deviations which take into account the repeated nature of the measurements [18]. (b) Replot with error bars representing standard deviations without taking into account the repeated nature of the measurements (no test of significance is performed).

Grahic Jump Location
Fig. 1

Central slice from images of a typical specimen before (left, zero strain) and post-failure (right, unloaded). Encircled areas show the nature of deformation that led to the reported microstructural changes.

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