Modeling Of Bone Material Properties From Ct Scans

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Download or read book Bone Volume Fraction Measured Via Micro-CT Imaging Is A Predictor For Hydraulic Permeability And Elastic Modulus Of The Trabecular Bone Of Human Lumbar Vertebral Bodies written by and published by . This book was released on 2017 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: INTRODUCTION: Thanks to the technological advances in computer science and technology, and medical imaging, 3D computational modeling has the potential for becoming a fundamental investigative and diagnostic tool in spine orthopaedics. A major problem related to the modelling approach is the accurate assessment geometric and mechanical properties of the bones. To adopt computational modeling in clinics, one should be able to determine geometry and mechanical properties of the vertebrae with minimally invasive tools. CT imaging is a mildly invasive routine diagnostic method that can provide accurate geometric information about vertebrae. In addition, radiographic density of the CT images have been related to the elastic properties of bones, such as the Youngu2019s modulus [1-3]. The cancellous bone is a porous solid structure saturated with interstitial fluid and, therefore, its mechanical behavior has been described as poroelastic [4]. The morphology of the solid skeleton (size and density of the pores) relates to how quickly the fluid moves into the bone, and the physical parameter that quantifies the ease of this fluid motion is the hydraulic permeability. To our best knowledge, very few studies have characterized hydraulic permeability in vertebrae [5, 6]. In particular, Baroud and co-workers have tried to establish a relationship between hydraulic permeability and bone mineral density measurements with unsatisfactory results [6]. The objective of this study was to use a mildly invasive approach, such as micro-CT imaging, to measure the local vertebral bone volume fraction, in order to establish a quantitative empirical relationship among this bone morphological characteristic and vertebral mechanical properties, such as hydraulic permeability and elastic modulus.METHODS: Vertebrae L1, L2 and L3 of bone mineral density 0.895, 1.043 and 1.106 g/cm2 were obtained from 54 y.o. white male donor. Vertebral bodies were excised, and their caudal and rostral cortical portions were removed. Subsequently, each vertebral body was cut along the plane orthogonal to the axial direction to yield 3 slices of ~5mm thickness. Nine vertebral body slices (3 vertebrae x 3 slices => n=9) were used for indentation tests and, subsequently, for micro-CT imaging, see Figure 1. For the indentation test, the slices were embedded in a physiological saline solution and compressed in displacement control at the left posterolateral region by a cylindrical indenter (5mm diameter) connected to a servoelectric testing system (Instron E3000, Norwood, MA) equipped with a 5kN load cell. More specifically, a compressive ramp of 0.25mm/s was applied until reaching a displacement equal to 5% of the initial height of the sample. At the end of the ramp, the final displacement was held for 800 seconds. The time dependent reaction of the sample was measured at a sampling rate of 10Hz. The relaxation over time of the reaction force of the sample was curve-fitted with the solution of a finite element model (FEBio 2.6.2, University of Utah, UT) simulating the indentation test on a vertebral slice. More specifically, the computational model schematized the vertebral slice as a biphasic material with an isotropic elastic solid phase and fluid flow governed by Darcyu2019s law with constant hydraulic permeability. The geometrical characteristics (shape and thickness) of the computational domain were similar to those of the sample tested. Each computational domain comprised about 27,000 tetrahedral elements used to discretize the vertebral slice and the indenter. The results of the curve-fitting provided hydraulic permeability (K) and elastic modulus (E) of the sample. After indentation testing, the slices were imaged via micro-CT (SkyScan1176, Bruker BioSpin Corp., Manning Park, MA) at 50kV with 18uf06dm resolution. An open source image processing software (ImageJ, NIH) was used to measure the bone volume fraction (BV/TV), defined as the ratio of the volume occupied by the bone to the total volume, in the left posterolateral region of the samples. A regression analysis was performed to determine whether BV/TV is a predictor for K and E. RESULTS: Values of hydraulic permeability and elastic modulus of the samples ranged from 4.3u00b710-3 to 3.3u00b710-1mm4u00b7N-1u00b7s-1 and 3.18 to 18.95MPa, respectively. The measured bone volume fraction ranged from 0.11 to 0.36. The regression analysis showed strong relationships between BV/TV and K (R2 = 87.1%), with the permeability exponentially decreasing as the bone volume increased, see Figure 2. Similarly, a strong relationship between BV/TV and E (R2 = 71.9%) was also found, with the elastic modulus linearly increasing with the bone volume, see Figure 3.DISCUSSION: This study aimed at verifying whether the bone volume fraction, when measured via micro-CT image analysis, can be a good predictor of fundamental vertebral mechanical properties such as hydraulic permeability and elastic modulus. The vertebral mechanical properties were measured via indentation tests: the observed magnitude of the hydraulic permeability was in good agreement with those reported for other trabecular bones [7], while the values of the elastic modulus were smaller than those reported for vertebrae [8]. Regression analyses indicated strong relationships among BV/TV and the two mechanical properties K and E. In particular, as previously observed [6], the relationship between the permeability and the bone volume was found to be non-linear. SIGNIFICANCE/CLINICAL RELEVANCE: This study identifies the bone volume fraction as a parameter measurable with the mildly invasive technique of micro-CT imaging analysis that can provide crucial information on the mechanical behavior of the human lumbar vertebrae. If similar results can be obtained with a routine diagnostic method as CT scanning, this study paves the way to the possibility of accurately modelling human vertebrae in vivo. REFERENCES: [1] Zannoni et al., Med Eng Phys, 1999. [2] Hodgkinson and Currey, J Mater Sci, 1992. [3] Rice et al., J Biomech, 1988. [4] Isaksson et al., J Biomech, 2006. [5] Ochia and Ching, J Biomech Eng, 2002. [6] Baroud et al., J Biomech, 2004. [7] Ochoa and Hillberry, Proc Ann Meet ORS, 1992. [8] Hernandez, Handbook of Biomaterial Properties, 2016. ACKNOWLEDGEMENTS: This study was supported by Miami CORE (Center for Orthopaedic Research and Education) at University of Miami.