Effects of intervertebral disk behavior on the load distribution and fracture risk of the vertebral body

Osteoporosis is characterized by low bone mass and an increased fracture risk. Measurements of bone mass alone, however, will not provide adequate information about the fracture risk, because the trabecular architecture or spatial distribution of the bone density has an important effect on the strength. We have developed a method to estimate the tissue strength of trabecular bone directly from 3D reconstructed axial CT-scans in combination with a finite element model. The method provides the stress distribution throughout the structure which can be used as a measure for the strength and fracture risk of the bone. A matter of concern with this method are the external loading conditions placed on the vertebral body, which might be strongly affected by the behavior of the intervertebral disk. In this study we have tested the effects of various intervertebral disk models on the load distribution through the vertebral body. A 3D model of a vertebral body was developed based on serial axial CT-scans which were converted to a 3D finite element model. The model was augmented with intervertebral disks at the upper and lower endplates. The disks contained a nucleus and an annulus region. The properties of the nucleus were varied to study the effects of a healthy disk with a functional nucleus pulposus and a degenerated disk with virtually no load bearing of the nucleus pulposus. The methods introduced in this study can be used to estimate load transfer through the vertebral body directly from CT-scans and, thereby, assessing the fracture risk of the bone and thus the status of osteoporosi

The influence of microcomputed tomography threshold variations on the assessment of structural and mechanical trabecular bone properties.

In this study, we investigate how morphological parameters and mechanical properties derived from microcomputed tomography (microCT) are affected by small errors in threshold value when variable bone structures and different bone volume fractions are involved. For this purpose, biopsies of vertebrae of 6-, 23-, and 230-week-old female pigs were scanned using microCT. For each specimen, five threshold values were determined within the range of thresholds that an observer could select realistically, in steps of 0.5%. The scans were converted to microfinite-element (microFE) models, used to determine the elastic moduli. A variation of 0.5% in threshold resulted in a 5% difference in bone volume fraction and 9% difference in maximal stiffness for bone cubes with a volume fraction of 0.2, these differences were only 2% and 3%, respectively. For all bone cubes, the differences for trabecular thickness and bone surface density were <3%. The effects on morphological anisotropy and trabecular number were negligible for threshold variations of 0.5%. These findings suggest that threshold selection is important for the accurate determination of volume fraction and mechanical properties, especially for low bone volume fractions; the architectural directionality is less sensitive to changes in threshold

The influence of microcomputed tomography threshold variations on the assessment of structural and mechanical trabecular bone properties

In this study, we investigate how morphological parameters and mechanical properties derived from microcomputed tomography (microCT) are affected by small errors in threshold value when variable bone structures and different bone volume fractions are involved. For this purpose, biopsies of vertebrae of 6-, 23-, and 230-week-old female pigs were scanned using microCT. For each specimen, five threshold values were determined within the range of thresholds that an observer could select realistically, in steps of 0.5%. The scans were converted to microfinite-element (microFE) models, used to determine the elastic moduli. A variation of 0.5% in threshold resulted in a 5% difference in bone volume fraction and 9% difference in maximal stiffness for bone cubes with a volume fraction of 0.2, these differences were only 2% and 3%, respectively. For all bone cubes, the differences for trabecular thickness and bone surface density wer

Validated Spine Model to Assess Degenerated Spine Mechanics

Development of a validated spine model to assess the mechanics of the degenerated spin

The dependence of the elastic properties of osteoporotic cancellous bone on volume fraction and fabric.

Item does not contain fulltextOsteoporosis is a progressive systemic skeletal condition characterized by low bone mass and microarchitectural deterioration, with a consequent increase in susceptibility to fracture. Hence, osteoporosis would be best diagnosed by in vivo measurements of bone strength. As this is not clinically feasible, our goal is to estimate bone strength through the assessment of elastic properties, which are highly correlated to strength. Previously established relations between morphological parameters (volume fraction and fabric) and elastic constants could be applied to estimate cancellous bone stiffness in vivo. However, these relations were determined for normal cancellous bone. Cancellous bone from osteoporotic patients may require different relations. In this study we set out to answer two questions. First, can the elastic properties of osteoporotic cancellous bone be estimated from morphological parameters? Second, do the relations between morphological parameters and elastic constants, determined for normal bone, apply to osteoporotic bone as well? To answer these questions we used cancellous bone cubes from femoral heads of patients with (n=26) and without (n=32) hip fractures. The elastic properties of the cubes were determined using micro-finite element analysis, assuming equal tissue moduli for all specimens. The morphological parameters were determined using microcomputed tomography. Our results showed that, for equal tissue properties, the elastic properties of cancellous bone from fracture patients could indeed be estimated from morphological parameters. The morphology-based relations used to estimate the elastic properties of cancellous bone are not different for women with or without fractures

Increase in bone volume fraction precedes architectural adaptation in growing bone

In mature trabecular bone, both density and trabecular orientation are adapted to external mechanical loads. Few quantitative data are available on the development of architecture and mechanical adaptation in juvenile trabecular bone. We studied the hypothesis that a time lag occurs between the adaptation of trabecular density and the adaptation of trabecular architecture during development. To investigate this hypothesis we used ten female pigs at 6, 23, 56, 104, and 230 weeks of age. Three-dimensional morphological and mechanical parameters of trabecular bone samples from the vertebra and proximal tibia were studied using microcomputed tomography and micro-finite element analysis. Both bone volume fraction and stiffness increased rapidly in the initial growth phase (from 6 weeks on), whereas the morphological anisotropy started increasing only after 23 weeks of age. In addition, the anisotropy reached its highest value much later in the development than did bone volume fraction. Hence, the alignment of trabeculae was still progressing at the time of peak bone mass. Therefore, our hypothesis was supported by the time lag between the increase in trabecular density and the adaptation of the trabecular architecture. The rapid increase of bone volume fraction in the initial growth phase can be explained by the enormous weight increase of the pigs. The trabeculae aligned at later stages when the increase in weight, and thus the loading, was slowed considerably compared with the early growth stage. Hence, the trabecular architecture was more efficient in later years. We conclude that density is adapted to external load from the early phase of growth, whereas the trabecular architecture is adapted later in the development

Increase in bone volume fraction precedes architectural adaptation in growing bone

In mature trabecular bone, both density and trabecular orientation are adapted to external mechanical loads. Few quantitative data are available on the development of architecture and mechanical adaptation in juvenile trabecular bone. We studied the hypothesis that a time lag occurs between the adaptation of trabecular density and the adaptation of trabecular architecture during development. To investigate this hypothesis we used ten female pigs at 6, 23, 56, 104, and 230 weeks of age. Three-dimensional morphological and mechanical parameters of trabecular bone samples from the vertebra and proximal tibia were studied using microcomputed tomography and micro-finite element analysis. Both bone volume fraction and stiffness increased rapidly in the initial growth phase (from 6 weeks on), whereas the morphological anisotropy started increasing only after 23 weeks of age. In addition, the anisotropy reached its highest value much later in the development than did bone volume fraction. Hence, the alignment of trabeculae was still progressing at the time of peak bone mass. Therefore, our hypothesis was supported by the time lag between the increase in trabecular density and the adaptation of the trabecular architecture. The rapid increase of bone volume fraction in the initial growth phase can be explained by the enormous weight increase of the pigs. The trabeculae aligned at later stages when the increase in weight, and thus the loading, was slowed considerably compared with the early growth stage. Hence, the trabecular architecture was more efficient in later years. We conclude that density is adapted to external load from the early phase of growth, whereas the trabecular architecture is adapted later in the development

The fracture risk of adjacent vertebrae is increased by the changed loading direction after a wedge fracture

Item does not contain fulltextSTUDY DESIGN: In vitro biomechanical study. OBJECTIVE: To measure the effect that off-axis vertebral loading has on the stiffness and failure load of vertebrae. SUMMARY OF BACKGROUND DATA: Adjacent level vertebral fractures not only are common in patients who received a vertebroplasty treatment but also occur in patients with conservatively treated wedge fractures. The wedge-like deformity, which is present in both groups, changes the spinal alignment. The load of vertebrae adjacent to the fractured vertebra will change from perpendicular to the endplate to a more shearing, off-axis, load. This change may induce a higher fracture risk for vertebrae adjacent to wedge-like deformed vertebrae. METHODS: Twenty vertebrae, harvested from one osteopenic cadaver spine and three osteoporotic cadaver spines, were loaded until failure. The vertebrae were loaded either perpendicular to the upper endplate, representing vertebrae in a spine without wedge fractures (0 degrees group, n = 10), or at an angle of 20 degrees , representing vertebrae adjacent to a wedge fracture (20 degrees groups, n = 10). Vertebral failure load and stiffness were the most important outcome measures. RESULTS: The failure load was significantly higher (P 5 0.028) when tested at 0 degrees (2854 N, SD 5 622 N), compared with vertebrae tested at 20 degrees (2162 N, SD 5 670 N). Vertebrae were also significantly stiffer (P, 0.001) when tested at 0 degrees (4017 N/mm, SD 5 970 N/mm) than those tested at 20 degrees (2478 N/mm, SD 5 453 N/mm). CONCLUSION: The failure load of osteopenic/osteoporotic vertebrae was 24% lower under off-axis loads (20 degrees ) than under axial loads (0 degrees ). This study may lead to a better understanding of the etiology of adjacent vertebral fractures after wedge-like deformities and demonstrates the importance of height reconstruction of wedge fractures in order to normalize the loading conditions on adjacent vertebrae