Regional variation in vertebral bone morphology and its contribution to vertebral fracture strength

MEM Research Center, University of Bern, Stauffacherstrasse 78, CH 3014, Bern, Switzerland.
Bone (Impact Factor: 4.46). 01/2008; 41(6):946-57. DOI: 10.1016/j.bone.2007.08.019
Source: PubMed

ABSTRACT Vertebral fractures may result in pain, loss of height, spinal instability, kyphotic deformity and ultimately increased morbidity. Fracture risk can be estimated by vertebral bone mineral density (BMD). However, vertebral fractures may be better defined by more selective methods that account for micro-architecture. Our aim was to quantify regional variations in bone architecture parameters (BAPs) and to assess the degree with which regional variations in BAPs affect vertebral fracture strength. The influence of disc health and endplate thickness on fracture strength was also determined. The soft tissue and posterior elements of 20 human functional spine units (FSU) were removed (T9 to L5, mean 74.45+/-4.25 years). After micro-CT scanning of the entire FSU, the strength of the specimens was determined using a materials testing system. Specimens were loaded in compression to failure. BAPs were assessed for 10 regions of the vertebral cancellous bone. Disc health (glycosaminoglycan content of the nucleus pulposus) was determined using the degree of binding with Alcian Blue. Vertebrae were not morphologically homogeneous. Posterior regions of the vertebrae had greater bone volume, more connections, reduced trabecular separation and more plate-like isotropic structures than their corresponding anterior regions. Significant heterogeneity also exists between posterior superior and inferior regions (BV/TV: posterior superior 12.6+/-2.8%, inferior 14.6+/-3%; anterior superior 10.5+/-2.2%, inferior 10.7+/-2.4%). Of the two endplates that abutted a common disc, the cranial inferior endplate was thicker (0.44+/-0.15 mm) than the caudal superior endplate (0.37+/-0.13 mm). Our study found good correlations between BV/TV, connective density and yield strength. Fracture risk prediction, using BV/TV multiplied by the cross sectional area of the endplate, can be improved through regional analysis of the underlying cancellous bone of the endplate of interest (R(2) 0.78) rather than analysis of the entire vertebra (R(2) 0.65) or BMD (R(2) 0.47). Degenerated discs lack a defined nucleus. A negative linear relationship between disc health and vertebral strength (R(2) 0.70) was observed, likely due to a shift in loading from the weaker anterior vertebral region to the stronger posterior region and cortical shell. Our results show the importance of considering regional variations in cancellous BAPs and disc health when assessing fracture risk.

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    ABSTRACT: Osteoporosis is defined by the National Institutes of Health as a skeletal disorder characterized by compromised bone strength, increasing fracture risk. Samples of vertebral cancellous bone were extracted from human cadavers from the T12 region. The comparative pre-selection of bone into the three groups was performed using the calcaneal ultrasonometry and evaluated by image analysis: microtomography, X-ray diffraction, optical microscopy e scanning electron microscopy. The results showed, for the diseased bone, degradation, reduction in the histomorfometric parameters and lower X-ray intensity. It is concluded that this characteristics indicates a validation in the assessment of fracture risk.
    Procedia Engineering 01/2013; 59:6-15. DOI:10.1016/j.proeng.2013.05.087
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    ABSTRACT: Life expectancy continuously increases but our society faces age-related conditions. Among musculoskeletal diseases, osteoporosis associated with risk of vertebral fracture and degenerative intervertebral disc (IVD) are painful pathologies responsible for tremendous healthcare costs. Hence, reliable diagnostic tools are necessary to plan a treatment or follow up its efficacy. Yet, radiographic and MRI techniques, respectively clinical standards for evaluation of bone strength and IVD degeneration, are unspecific and not objective. Increasingly used in biomedical engineering, CT-based finite element (FE) models constitute the state-of-art for vertebral strength prediction. However, as non-invasive biomechanical evaluation and personalised FE models of the IVD are not available, rigid boundary conditions (BCs) are applied on the FE models to avoid uncertainties of disc degeneration that might bias the predictions. Moreover, considering the impact of low back pain, the biomechanical status of the IVD is needed as a criterion for early disc degeneration. Thus, the first FE study focuses on two rigid BCs applied on the vertebral bodies during compression test of cadaver vertebral bodies, vertebral sections and PMMA embedding. The second FE study highlights the large influence of the intervertebral disc’s compliance on the vertebral strength, damage distribution and its initiation. The third study introduces a new protocol for normalisation of the IVD stiffness in compression, torsion and bending using MRI-based data to account for its morphology. In the last study, a new criterion (Otsu threshold) for disc degeneration based on quantitative MRI data (axial T2 map) is proposed. The results show that vertebral strength and damage distribution computed with rigid BCs are identical. Yet, large discrepancies in strength and damage localisation were observed when the vertebral bodies were loaded via IVDs. The normalisation protocol attenuated the effect of geometry on the IVD stiffnesses without complete suppression. Finally, the Otsu threshold computed in the posterior part of annulus fibrosus was related to the disc biomechanics and meet objectivity and simplicity required for a clinical application. In conclusion, the stiffness normalisation protocol necessary for consistent IVD comparisons and the relation found between degeneration, mechanical response of the IVD and Otsu threshold lead the way for non-invasive evaluation biomechanical status of the IVD. As the FE prediction of vertebral strength is largely influenced by the IVD conditions, this data could also improve the future FE models of osteoporotic vertebra.
    03/2013, Degree: PhD., Supervisor: Philippe K. Zysset
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