Hulme PA, Boyd SK, Ferguson SJ. 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: 3.97). 01/2008; 41(6):946-57. DOI: 10.1016/j.bone.2007.08.019
Source: PubMed


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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    • "Hence, the challenge of incorporating an anisotropic model into continuum-based subject-specific FE models from clinical CT persists. Second, the anisotropy ratio itself can vary with density [11] and has also been shown to vary within a single vertebral bone based on location as well as regional density [5] [6]. Hence, applying a single anisotropic model for the whole vertebra may be insufficient. "
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    ABSTRACT: Most studies investigating human lumbar vertebral trabecular bone (HVTB) mechanical property- density relationships have presented results for the superior-inferior (SI), or "on-axis" direction. Equivalent, directly measured data from mechanical testing in the transverse (TR) direction are sparse and quantitative computed tomography (QCT) density-dependent variations in the anisotropy ratio of HVTB have not been adequately studied. The current study aimed to investigate the dependence of HVTB mechanical anisotropy ratio on QCT density by quantifying the empirical relationships between QCT-based apparent density of HVTB and its apparent compressive mechanical properties - elastic modulus (Eapp), yield strength (?y), and yield strain (?y) - in the SI and TR directions for future clinical QCT-based continuum finite element modeling of HVTB. A total of 51 cylindrical cores (33 axial and 18 transverse) were extracted from four L1 human lumbar cadaveric vertebrae. Intact vertebrae were scanned in a clinical resolution computed tomography (CT) scanner prior to specimen extraction to obtain QCT density, ?CT. Additionally, physically measured apparent density, computed as ash weight over wet, bulk volume, ?app, showed significant correlation with ?CT [?CT=1.0568x?app, r = 0.86]. Specimens were compression tested at room temperature using the Zetos Bone Loading and Bioreactor system. Apparent elastic modulus modulus (Eapp) and yield strength (?y) were linearly related to the ?CT in the axial direction [ESI = 1493.8x(?CT), r = 0.77, p < 0.01; ?YSI = 6.9x(?CT) - 0.13, r =0.76, p < 0.01], while a power law relation provided the best fit in the transverse direction [ETR = 3349.1x(?CT)1.94, r = 0.89, p < 0.01; ?YTR = 18.81x(?CT)1.83, r = 0.83, p < 0.01]. No significant correlation was found between ?y and ?CT in either direction. Eapp and ?y in the axial direction were larger compared to the transverse direction by a factor of 3.2 and 2.3 respectively, on average. Furthermore, the degree of anisotropy decreased with increasing density. Comparatively, ?y exhibited only a mild, but statistically significant anisotropy: transverse strains were larger than those in the axial direction by 30%, on average. Ability to map apparent mechanical properties in the transverse and axial directions from CT-based densitometric measures allows incorporation of transverse properties in FE models based on clinical CT data, partially offsetting the inability of continuum models to accurately represent trabecular architectural variations.
    Journal of Biomechanical Engineering 05/2014; 136(9). DOI:10.1115/1.4027663 · 1.78 Impact Factor
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    • "Consistently, trabecular separation (Tb.Sp) increased with aging. The age-related decrease in trabecular thickness (Tb.Th) was not statistically significant [14] [15]. The reduction of BV/TB with aging is associated primarily with reduced Tb.N and increased Tb.Sp [14] [17]. "
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    ABSTRACT: The elderly population has substantially increased worldwide. Aging is a complex process, and the effects of aging are myriad and insidious, leading to progressive deterioration of various organs, including the skeleton. Age-related bone loss and resultant osteoporosis in the elderly population increase the risk for fractures and morbidity. Osteoporosis is one of the most common conditions associated with aging, and age is an independent risk factor for osteoporotic fractures. With the development of noninvasive imaging techniques such as computed tomography (CT), micro-CT, and high resolution peripheral quantitative CT (HR-pQCT), imaging of the bone architecture provides important information about age-related changes in bone microstructure and estimates of bone strength. In the past two decades, studies of human specimens using imaging techniques have revealed decreased bone strength in older adults compared with younger adults. The present paper addresses recently studied age-related changes in trabecular and cortical bone microstructure based primarily on HR-pQCT and micro-CT. We specifically focus on the three-dimensional microstructure of the vertebrae, femoral neck, and distal radius, which are common osteoporotic fracture sites.
    International Journal of Endocrinology 03/2013; 2013(4):213234. DOI:10.1155/2013/213234 · 1.95 Impact Factor
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    • "An appreciation of bone architecture is essential for the assessment of fracture risk and interpretation of clinical data [12, 21]. It has been documented that human vertebral bodies are heterogeneous, with two distinct morphological zones (subendplate and central zones), each with very specific trabecular bone density and microstructure [12]. "
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    ABSTRACT: A validated ovine model of osteoporosis achieves severe bone loss in a relatively short period. This study investigated if osteoporotic features persist in this model after cessation of corticosteroid administration. Methods. Osteoporosis was induced in nine ewes by chronic corticosteroid injection, ovariectomy, and low calcium diet. Six ewes were used as controls. Bone mineral density (BMD) of the lumbar spine (LS) and body weight were assessed at regular intervals. After five months, corticosteroid treatment was withdrawn systematically over one month. Three months later, all animals were euthanised, and the LS was collected for histomorphometric analysis. Results. BMD in the LS of osteoporotic sheep was 25% lower than control sheep. Body weight of osteoporotic sheep was reduced in the first month of the corticosteroid withdrawal period but returned to baseline level thereafter. Trabecular bone volume of LS in osteoporotic sheep was 27% lower than controls and showed a heterogeneous structure. Conclusions. Osteoporotic characteristics remain in the vertebra after ceasing corticosteroid administration providing an opportunity to evaluate potential systemic or local treatments in vivo under realistic physiological conditions. The microstructural arrangement of vertebral trabecular bone in sheep is similar to humans demonstrating further relevance of this model for preclinical investigations.
    Journal of Osteoporosis 10/2012; 2012(5):182509. DOI:10.1155/2012/182509
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