Relationship between CT intensity, micro-architecture and mechanical properties of porcine vertebral cancellous bone

doi:10.1016/j.clinbiomech.2005.11.001

Clinical Biomechanics

Volume 21, Issue 3, March 2006, Pages 235-244

https://doi.org/10.1016/j.clinbiomech.2005.11.001 Get rights and content

Abstract

Background

In vivo assessment of bone density is insufficient for the evaluation of osteoporosis in patients. A more complete diagnostic tool for the determination of bone quality is needed. Micro-computed tomography imaging allows a non-destructive method for evaluating cancellous bone micro-architecture. However, lengthened exposure to ionizing radiation prevents patients to be imaged by such a system. The aim for this study was to elucidate the relationships between image intensity (of Hounsfield units), cancellous bone micro-architecture and mechanical properties.

Methods

Using pig vertebral cancellous bone, the bone specimens were imaged using clinical and micro-computed tomography scanners and subsequently subjected to uniaxial compression testing.

Results

Results indicate that micro-architecture can be predicted using clinical image intensity. Micro-architectural parameters relevant to osteoporosis study, such as percent bone volume, trabecular bone pattern factor, structure model index, trabecular thickness and trabecular separation have shown significant correlation with R² values of 0.83, 0.80, 0.70, 0.72, and 0.54, respectively, when correlated to Hounsfield units. In addition, the correlation of mechanical properties (E, σ_yield, and σ_ult) in the superior–inferior direction (the primary loading direction), to micro-architecture parameters has also been good (R² > 0.5) for all except tissue volume, tissue surface and degree of anisotropy.

Interpretation

This proves that the predictive power of bone strength and stiffness was improved with the combination of bone density and micro-architecture information. This work supports the prediction of micro-architecture using current clinical computed tomography imaging technology.

Introduction

Extensive research has shown that compressive mechanical properties of cancellous bone are closely related to the physical bone density (Keller, 1994, Rice et al., 1988). Density measurements from radiological images, bone mineral density (BMD) information from dual-X-ray absorptiometry (DXA) scanning (Dougherty, 1996) and Hounsfield number (HU) from computed tomography imaging (Rho et al., 1995), also yield such close relationships. From these relationship studies, although strong correlations exist, there is still a 30–50% of unaccounted variance in mechanical properties from bone density measurement (Keaveny and Yeh, 2002, Van Lenthe et al., 2001). This phenomenon is clearly demonstrated clinically in osteoporosis study. Osteoporotic cancellous bone is characterized by low bone mass as well as a deterioration of the micro-architecture. Whether a patient is diagnosed osteoporotic depends only on his/her BMD and how this BMD value compares to a population average (Kanis, 2002). Clinical results have shown that BMD of patients with osteoporotic bone fractures and patients without such fractures can have a substantial overlap (Majumdar et al., 1997), causing variance similar to that in density–mechanical property relationship studies. The micro-architecture of cancellous bone has been largely attributed to this variance; density approximates the amount of bone tissue within a cancellous bone specimen but it does not quantify the micro-architecture that is inherent. Together with bone density readings, a quantitative measurement of micro-architectural parameters may improve our ability to better estimate bone strength (Genant et al., 1999).

Micro-computed tomography (micro-CT) scanners, with similar working principle as conventional clinical CT scanners, have been used to study microstructure of materials in three dimensions. Ulrich et al. (1999) used such a scanner to study the relationship between micro-architectural parameters (Parfitt, 1988) and mechanical properties of cancellous bone. Image datasets of human cancellous bone specimens at micron resolution (14 μm voxels) were acquired and from these datasets, they determined micro-architectural parameters as well as converted them into micro-finite element (FE) models. These micro-FE models were used to calculate orthotropic mechanical stiffness; employment of micro-FE models were to ensure that stiffness constants obtained was purely due to the micro-architecture, and not to trabeculae tissue properties and inherent experimental artifacts. They obtained good correlations between micro-architectural parameters and mechanical stiffness constants (R² > 0.67) for cancellous bone specimens from the calcaneous, and moderate correlations (R² > 0.52) for specimens from the femoral head, iliac crest and lumbar spine. Unfortunately, they were unable to obtain mechanical properties such as yield stress–strain and ultimate stress–strain. Micro-FE analyses simply do not allow such analyses and in the study of osteoporosis, such data is important.

The primary objective of this study is to study the possible relationship between information gathered from CT images, namely HU, to micro-architectural parameters. As a secondary objective, we would also like to study the relationship between micro-architectural parameters and mechanical properties (stiffness, yield strength/strain and compressive strength/strain) in the superior–inferior direction, as this is the primary loading axis for vertebrae in humans as well as animal.

Section snippets

Specimen preparation

Cancellous bone cubes, measuring 5 × 5 × 5 mm, were excised from fresh vertebral bodies extracted from the thoracic to the lumbar section of vertebral columns excised from two mature pigs (weighing 40 and 55 kg) (Fig. 1). Excising of bone cubes from the vertebral bodies was done using a slow speed diamond saw (Minitom, Struers A/S, Denmark) under constant irrigation. A total of 38 bone cubes were extracted for the experiment; cubes were stored and frozen individually to minimize thermal cycling. The 5

Micro-architectural parameters of pig cancellous bone

The mean, standard deviation and range for each of the micro-architectural parameters from the 38 cancellous bone cubes were tabulated (Table 1). We compared the results of selected micro-architectural parameters (BV/TV, BS/BV, Tb.Th, Tb.N, Tb.Sp and DA) obtained from this study against results reported for human femoral and lumbar spine cancellous bones (Müller and Rüegsegger, 1997) and sheep femoral cancellous bones (Mittra et al., 2005) (Table 2). These are standard parameters employed for

Discussion

Our primary objective of studying the relationship between HU, from CT images, and micro-architectural parameters is important for the realization of a diagnostic tool for in vivo assessment of cancellous bone quality. The results from this study have positively shown that in addition to cancellous bone density, bone micro-architectural parameters can also be predicted from clinical-CT imaging. We chose to use CT, as it is a three-dimensional measurement (Dougherty, 1996).

In a similar study,

Conclusions

This study was conducted to examine the possible relationship between HU, obtained from clinical-CT imaging, and micro-architectural parameters. These parameters better explain the quality of bone and complement the present standard quantifications of bone density for osteoporosis study. The results from this study show significant correlation between HU and the micro-architectural parameters as well as ultimate stress of pig cancellous bone. Therefore, HU is an attractive tool for the in vivo

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Relationship between CT intensity, micro-architecture and mechanical properties of porcine vertebral cancellous bone

Abstract

Background

Methods

Results

Interpretation

Introduction

Section snippets

Specimen preparation

Micro-architectural parameters of pig cancellous bone

Discussion

Conclusions

Med. Eng. Phys.

Bone

J. Biomech.

J. Biomech.

Lancet

J. Biomech.

J. Biomech.

J. Biomech.

J. Biomech.

J. Biomech.

J. Biomech.

Bone

Med. Eng. Phys.