Biomedical subjects

Steven K Boyd

Publications and source records attributed to Steven K Boyd.

8 recordsLinked to original sources

PubMed2007

Accuracy of high-resolution peripheral quantitative computed tomography for measurement of bone quality.

The introduction of three-dimensional high-resolution peripheral in vivo quantitative computed tomography (HR-pQCT) (XtremeCT, Scanco Medical, Switzerland; voxel size 82 microm) provides a new approach to monitor micro-architectural bone changes longitudinally. The accuracy of HR-pQCT for three important determinants of bone quality, including bone mineral density (BMD), architectural measurements and bone mechanics, was determined through a comparison with micro-computed tomography (microCT) and dual energy X-ray absorptiometry (DXA). Forty measurements from 10 cadaver radii with low bone mass were scanned using the three modalities, and image registration was used for 3D data to ensure identical regions were analyzed. The areal BMD of DXA correlated well with volumetric BMD by HR-pQCT despite differences in dimensionality (R(2) = 0.69), and the correlation improved when non-dimensional bone mineral content was assessed (R(2) = 0.80). Morphological parameters measured by HR-pQCT in a standard patient analysis, including bone volume ratio, trabecular number, derived trabecular thickness, derived trabecular separation, and cortical thickness correlated well with muCT measures (R(2) = 0.59-0.96). Additionally, some non-metric parameters such as connectivity density (R(2) = 0.90) performed well. The mechanical stiffness assessed by finite element analysis of HR-pQCT images was generally higher than for microCT data due to resolution differences, and correlated well at the continuum level (R(2) = 0.73). The validation here of HR-pQCT against gold-standards microCT and DXA provides insight into the accuracy of the system, and suggests that in addition to the standard patient protocol, additional indices of bone quality including connectivity density and mechanical stiffness may be appropriate to include as part of a standard patient analysis for clinical monitoring of bone quality.

Absorptiometry, Photon↗

PubMed2006

Regional trabecular morphology assessed by micro-CT is correlated with failure of aged thoracic vertebrae under a posteroanterior load and may determine the site of fracture.

INTRODUCTION: Spinal mobilization is commonly used in the treatment of patients with back pain, including individuals with osteoporosis. Previous data indicated that traditional predictors of skeletal failure-lateral or anteroposterior bone mineral density (BMD) by dual energy X-ray absorptiometry (DXA) or geometry of the spinous process or vertebral body-do not predict failure load during posteroanterior spinal mobilization. Morphological differences and inhomogeneities in BMD may have important effects on vertebral strength but integral BMD values by DXA cannot reflect these potentially important differences. We investigated the determinants of spinal fracture using muCT. MATERIALS AND METHODS: We measured failure load and failure site in 11 T5-8 cadaveric specimens (mean age 78 years) when a posteroanterior load was applied at the spinous process of T6 using a servohydraulic material testing machine. Radiography and CT scan were used to verify failure site. We observed no damage to the adjacent T7 vertebrae following the T6 posteroanterior failure test. The T7 vertebrae were sectioned to produce regional samples of the spinous process, the lamina and a vertebral body core. Each sample was scanned with muCT to measure bone microarchitectural parameters. We segmented and analysed four trabecular regions (spinous process base and middle, central lamina and central vertebral body). We used one-way repeated measures ANOVA to compare regions and computed Pearson correlations to assess the relation between PA failure load of T6 and the morphological parameters of T7. RESULTS: The BV/TV at the base or middle of the T7 spinous process (fracture sites), Tb.N and Tb.Th at the base were significantly correlated with posteroanterior failure load of T6 (BV/TV base: r=0.74, p=0.01; BV/TV middle: r=0.73, p=0.01; Tb.N base: r=0.64, p=0.03; Tb.Th base: r=0.65, p=0.03). The Tb.Th of the lamina was significantly greater than Tb.Th of the spinous process base (p=0.002). CONCLUSIONS: Whereas previous data indicated that BMD by DXA was not a good predictor of posteroanterior failure load, regional BV/TV of the spinous process base and middle regions, the sites of fracture, are correlated with posteroanterior failure load. Trabecular thickness differed significantly between the base of the spinous process and the lamina, and may have influenced the site of fracture.

Aged↗

PubMed2006

Evaluation of three-dimensional image registration methodologies for in vivo micro-computed tomography.

The advent of in vivo micro-computed tomography (micro-CT) provides a novel approach to measure the temporal adaptation of bone micro-architecture within an individual. Spatial alignment in the scanner between serial scans is challenging, but three-dimensional image registration can be used to superimpose the resulting image data, thus ensuring consistent regions of interest (ROI) for analysis. There have been several approaches to image registration developed, yet little is known about their application to high resolution micro-CT data. The purpose of this study was to explore combinations of three image registration similarity measures and three image interpolators, in addition to multi-resolution registration configurations, for assessment of computational efficiency and accuracy on both in vitro and in vivo micro-CT data. Accuracy measures were assessed by comparison with a gold-standard reference transform based on attached fiducial markers. It was concluded that a mutual information registration similarity measure with a linear image interpolator, applied at steps of increasing image resolution, provided the best compromise between accurate and efficient results. In vivo registration of tibial bone microstructure measured in an ovariectomized rat model provided consistent ROI thus demonstrating the usefulness of three-dimensional image registration for in vivo experimental and clinical micro-CT research. It is a technique that is poised to become commonly utilized for analysis of micro-CT data to diagnose and monitor efficacy of therapy in bone diseases.

Animals↗

PubMed2006

Establishment of an architecture-specific experimental validation approach for finite element modeling of bone by rapid prototyping and high resolution computed tomography.

A new experimental validation method for assessing the accuracy of large-scale finite element (FE) models of bone micro-structure at the apparent and tissue level was developed. Augmented scaled bone replicas were built using rapid prototype machines based on micro-computed tomography (micro-CT) data. The geometric accuracy of the model was evaluated by comparing experimental tests with the replicas to the FE solution based on the same micro-CT data. A new version of the large-scale FE solver was developed to incorporate orthotropic material properties, hence the experimentally determined properties of the rapid prototype material were input into the FE models. The modified FE solver predicted the experimental apparent level stiffness within less than 1%, and the difference between experimental strain gauge measurements and FE-calculated surface stresses was 7% and 49% on a flat and curved surface region, respectively. While absolute error estimates of surface stresses were limited due to strain gauge errors, the relatively larger difference on the curved surface is indicative of the limitations of a hexahedron FE model for representing such geometries. Although the validation approach is applied here for hexahedron based meshes, the method is flexible for varying bone architectures and will be important for validation of future large-scale FE modeling developments that utilize techniques such as mesh smoothing and tetrahedron elements.

Bone and Bones↗

PubMed2006

Monitoring individual morphological changes over time in ovariectomized rats by in vivo micro-computed tomography.

The ovariectomized (OVX) rat is a well established model for osteoporosis research. The recent development of in vivo micro-computed tomography (micro-CT) provides new possibilities to monitor individual bone changes over time. The purpose of this study was to establish the normal time course of bone loss in the OVX rat model, and to determine the ability to detect morphological changes in vivo compared to cross-sectional study designs where animals are sacrificed at each time point. Eight-month-old female Wistar rats were randomly assigned to one of two groups: OVX (N = 10) or sham-operated (N = 10). In vivo micro-CT scanning of the right proximal tibial metaphyses occurred at 1-month intervals for 6 months. Morphological analyses were performed at each time step for every animal, and a two-way ANOVA with repeated measures was used to analyze the data. A second statistical analysis was performed without repeated measures for analysis as a cross-sectional study design. The repeated measures analysis was more sensitive to early changes than the cross-sectional study analysis. Changes were detected by longitudinal analysis in the sham-operated and OVX animals over time (P < 0.001) with the exception of trabecular separation in the sham animals. The OVX animals had decreases of bone volume ratio of 33% after 1 month, and 72% after 3 months relative to baseline measurements. Significant changes in bone volume ratio, trabecular number and separation were detected early using a longitudinal analysis, thus in vivo assessment is well poised to enable the study of early treatment protocols on the effects of bone architecture. The in vivo analysis found significant changes in the sham animals which were not detected by the cross-sectional analysis, and the changes to the OVX animal morphology was detected sooner. A substantial variation of baseline morphometry within the homogenous group of rats and response to OVX was observed, thus emphasizing the advantage of performing in vivo analysis where each animal acts as its own control. These data provide new insight into individual bone changes following OVX, and can be used as baseline information upon which future in vivo studies can be designed.

Age Factors↗

PubMed2006

Smooth surface meshing for automated finite element model generation from 3D image data.

Finite element (FE) modelling based on data from three-dimensional high-resolution computed tomography (CT) imaging systems provides a non-invasive method to assess structural mechanics. Automated mesh generation from these voxel based image data can be achieved by direct conversion to hexahedron elements, however these model representations have jagged edges. This paper proposes an automated method to generate smoothed FE meshes from voxel-based image data. Mesh fairing processes are utilized that allow constraints that control the smoothing process, and are computationally efficient. Surfaces of the mesh on the exterior, as well as interfaces between two tissues, can be smoothed by varying fairing parameters and constraint criteria. The method was tested on a variety of real and simulated three-dimensional data sets, resulting in both hexahedron and tetrahedron meshes. It was shown that the fairing process is linearly related to the number of smoothing iterations, and that peak stresses are reduced in FE simulations of the smoothed models. Although developed for micro-CT data sets, this fast and reliable mesh smoothing method could be applied to any three-dimensional image data where node and element connectivity have been defined.

Animals↗

PubMed2004

A finite element beam-model for efficient simulation of large-scale porous structures.

This paper presents a new method for the generation of a beam finite element (FE) model from a three-dimensional (3D) data set acquired by micro-computed tomography (micro-CT). This method differs from classical modeling of trabecular bone because it models a specific sample only and differs from conventional solid hexahedron element-based FE approaches in its computational efficiency. The stress-strain curve, characterizing global mechanical properties of a porous structure, could be well predicted (R(2)=0.92). Furthermore, validation of the method was achieved by comparing local displacements of element nodes with the displacements directly measured by time-lapsed imaging methods of failure, and these measures were in good agreement. The presented model is a first step in modeling specific samples for efficient strength analysis by FE modeling. We believe that with upcoming high-resolution in-vivo imaging methods, this approach could lead to a novel and accurate tool in the risk assessment for osteoporotic fractures.

Alloys↗

PubMed2002

Mechanical and architectural bone adaptation in early stage experimental osteoarthritis.

The purpose of this study was to quantify mechanical and architectural changes to knee joint periarticular subchondral cancellous bone in early stage experimental osteoarthritis (OA). Unilateral anterior cruciate ligament transection (ACLX) was performed on 10 dogs that were assigned randomly to two groups: 3 weeks or 12 weeks post-ACLX. Cylindrical bone cores excised from the medial condyle of the distal femur after death were scanned using high-resolution microcomputed tomography (muCT) and subsequently failed under unconstrained uniaxial compression. The apparent-level elastic modulus was less in the ACLX femur compared with the contralateral control, and the decrease was significant (-45%; p < 0.05) by 12-weeks post-ACLX. A finite element (FE) analysis based on muCT data simulated the uniaxial compression tests on a specimen-by-specimen basis to determine tissue modulus. No change in tissue modulus was detected, and a single tissue modulus of 5100 MPa (95% CI, +/- 600 MPa) explained the apparent-level modulus changes observed in the disease-related bone adaptation. The three-dimensional (3D) connectivity was evaluated from the original muCT data to quantify architectural alterations in contrast to tissue alterations. Significantly increased connectivity (through plate perforations) occurred as early as 3 weeks post-ACLX and was as high as 127% by 12 weeks post-ACLX in the distal femur. These measured changes indicated that architectural adaptation predominated over tissue modulus changes affecting apparent-level elastic modulus in the early stage of experimental OA and suggests that to maintain normal cancellous bone after a traumatic injury, early intervention should focus on preventing the substantial architectural alterations.

Adaptation, Physiological↗