The architecture of a vertebral body is comprised of highly porous trabecular bone, but also of a fairly dense and solid shell. The shell is very thin throughout, on average only 0.4 mm . It is virtually indistinguishable from the tra-becular core but rather is a denser arrangement of trabec-ular elements forming solid and compact bone (histologi-cally different from cortical bone). Finite element analysis estimates the contribution of the shell to the overall load carrying capacity to be less than 15% [23, 35].
Regional variation in bone architecture also exist within any given vertebral body. The regions adjacent to the endplate feature more dense, rodlike trabecular structures. The regions far from the endplate, on the other hand, are less dense, with platelike shaped trabeculae. Mechanical properties tested in normal vertebrae for distinct regions of trabecular bone samples attribute higher strength, stiffness and bone mineral density (BMD) to central trabecu-lar regions [18, 19, 20]. Variability in mechanical properties can be interpreted as adaptive to the environment, in this case to higher vertical stresses transmitted by the central region adjacent to the nucleus pulposus, as opposed to the peripheral region adjacent to the annulus fibrosus. Keller et al.  have demonstrated for degenerated intervertebral discs a change in adjacent trabecular mechanical properties, suggesting a more uniform load distribution across the endplate in degenerated spines.
The apparent bone density varies widely (0.05 g/cm3 to 0.30 g/cm3) between individuals, between levels, but also as a function of age. Starting in the fourth decade of life, elderly men can easily lose up to 30% and elderly women up to 50% of bone density . Routine estimates of the apparent bone density are obtained using dual energy X-ray absorptiometry (DEXA). Although BMD or bone mineral content (BMC) are not volumetric parameters for bone, they still have proven to be useful predictors for ultimate vertebral strength, since the ultimate vertebral strength is dependent on both the vertebral geometry and the trabec-ular failure strength. To compare failure strength for vertebral samples from different spinal regions or from different individuals it is best to express the failure strength as a material property, normalized for the endplate's cross-sectional area , or expressed as compressive failure stress. The stress at failure for a lumbar vertrebral body is found to range from 1.0 to 5.0 MPa. This measure, however, does not differentiate between trabecular and compact elements of the vertebral body.
Keller  established from in vitro testing of isolated trabecular bone samples a relationship between apparent bone density and compressive failure strength. The exponential function [compressive strength=(97.8xapparent bone density)230] identifies trabecular bone with low apparent density (<0.10 g/cm3) to feature an ultimate compressive strength of less than 0.2 MPa, which puts this bone at risk to fracture already at axial loads seen in routine and low level daily activity. Resch et al.  have shown using quantitative computer tomography that men with 0.11 g/cm3 apparent bone density have a 25% vertebral fracture risk, whereas individuals with 0.05 g/cm3 bone density have a 99% vertebral fracture risk.
Osteoporosis is a disease that weakens the structural strength of bone to an extent that normal daily activity can exceed the vertebra's ability for carrying this load, resulting in vertebral fractures. The incidence of fragility fractures doubled within the last decade. It is predominant in women, with an osteoporotic fracture prevalence at age 50 years and above of over 40% ("Bone and joint decade," WHO 2003: http://www.boneandjointdecade.org/background/default. html). Clinically osteoporosis is characterized using DEXA measurements (BMD or BMC) of the lumbar spine that are 2.5 SD or more below the average value for a 30-year-old gender-matched individual . In women the risk for vertebral fractures rises 2.2-fold for every 1 SD loss in BMD or BMC .
Decreased structural strength is not only the result of reduced apparent bone density, but also of profound changes in the architecture and the bone remodeling and/ or repair rate, resulting in faster damage accumulation for continuous cyclic loading. The increase in bone fragility
Fig. 1 Normal (top) and osteoporotic (bottom) vertebral bodies. Decreased structural strength is not only the result of reduced apparent bone density but also changes in the architecture of the tra-becular bone. The increase in bone fragility is due to replacement of platelike close trabecular structures with more open, rodlike structures. The more porous cancellous bone appearance is the result of reduced horizontal cross-linking struts
Fig. 1 Normal (top) and osteoporotic (bottom) vertebral bodies. Decreased structural strength is not only the result of reduced apparent bone density but also changes in the architecture of the tra-becular bone. The increase in bone fragility is due to replacement of platelike close trabecular structures with more open, rodlike structures. The more porous cancellous bone appearance is the result of reduced horizontal cross-linking struts is due to replacement of platelike close trabecular structures with more open, rodlike structures. The more porous cancellous bone appearance is the result of reduced horizontal cross-linking struts, further reducing the buckling strength of vertically oriented trabeculae (Fig. 1).
The typical osteoprotic vertebral fracture leads to a height reduction of the anterior vertebral body, often leaving the posterior vertebral wall intact. This wedge-shaped deformity usually leads to a local increase in kyphosis, and with multiple adjacent vertebral fractures to a progressive kyphotic deformity with postural disfigurement. Multiple vertebral fractures are very common, since the fracture risk of neighboring levels have shown to have a fivefold increased fracture risk compared to normal vertebrae .
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