1. Biomechanics of the spine. Part I: Spinal stability
Roberto Izzo, Gianluigi Guarnieri, Giuseppe Guglielmi, Mario Muto 
European Journal of Radiology 
Volume 82, Issue 1, Pages (January 2013)
DOI: /j.ejrad
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2. Fig. 1
Lateral conventional radiograph of the cervical spine in flexion. During flexion–extension the vertebra moves around a transverse rotation axis placed in the subjacent vertebral body. Both the endplates and the facet joints perform two circumference arcs around the same rotation centre whose location changes according to level, being placed two vertebral bodies below in the superior cervical spine and in the subjacent vertebral body in the inferior cervical, in the dorsal and lumbar spine.
European Journal of Radiology  , DOI: ( /j.ejrad )
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3. Fig. 2
Load/displacement curve. The load/displacement curve of the spine is not linear. The range of motion of the spinal joints includes an initial neutral zone (NZ) with relatively large displacements at low load and an elastic zone (EZ) that requires more load per unit of displacement because of the tension of capsules and ligaments.
European Journal of Radiology  , DOI: ( /j.ejrad )
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4. Fig. 3
Three subsystems control the stability of the spine: the spinal column, the muscles, and the central nervous system. They are strictly related so any acute or chronic damage to one subsystem requires more compensatory work by the others.
European Journal of Radiology  , DOI: ( /j.ejrad )
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5. Fig. 4
The subsystems controlling spinal stability are functionally related. A continuous stream of proprioceptive information starting from the spinal mechanoreceptors muscle and tendons inform the CNS on the position, load and movement of each FSU. The CNS, in turn, answers through an appropriate and coordinated muscular activity.
European Journal of Radiology  , DOI: ( /j.ejrad )
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6. Fig. 5
(a and b) The vertical compressive loads are first accepted by vertical trabecular columns which transmit forces between the endplates. However the vertical struts alone would tend to bow (a). Their bowing is restrained by the presence of horizontal lamellae which join the vertical struts and by tension favour the radial dispersion of forces conferring resilience to the vertebral body (b).
European Journal of Radiology  , DOI: ( /j.ejrad )
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7. Fig. 6
(a–c) The load-bearing capacity of vertebral bodies heavy depends on the vertical trabecular struts joining the endplates (a). The resistance of a column decreases by the square of increasing length and by the square of decreasing cross section. During osteoporosis both processes occur with progressive elongation of the columns provoked by the resorption of the horizontal lamellae (b) and the thinning of the columns themselves (c). It results a disproportionate exponential reduction of bone resistance and load bearing capacity.
European Journal of Radiology  , DOI: ( /j.ejrad )
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8. Fig. 7
Owing to kyphosis the vertebrae of the dorsal spine are located distant from the body sagittal balance vertical axis which joins the external auditory canals and the centre of femoral heads passing through the C7–D1 and L5–S1 interspaces. Eccentric ventral and lateral axial loads and bending moments create which concentrate the stresses on the anterior parts of the bodies favouring their collapse and wedging. The larger the kyphosis, the greater the distance between vertebral bodies and the body balance axis and the greater the ventral concentration of stresses.
European Journal of Radiology  , DOI: ( /j.ejrad )
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9. Fig. 8
In case of damage to ligaments, discs, joint capsules and to the mechanoreceptors they contain, abnormal transducer signals are generated and sent to the CNS causing an altered motor response which, in turn, increases the mechanical stress of bony and joint spinal components and elicits an abnormal feedback response by FSUs and muscles themselves creating a vicious cycle ultimately leading to the development of inflammation, muscle fatigue, and activation of nociceptors with acute and chronic pain. CNS: central nervous system.
European Journal of Radiology  , DOI: ( /j.ejrad )
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10. Fig. 9
Sagittal midline FSE T2-w scan showing a posterior D12–L1 disc herniation with light compression of spinal cord. One of main drawbacks of the spinal fixations by rigid constructs is the transition syndrome caused by abnormal load stresses converging on motion segments adjacent to fixed segments. Two years after a fixation L1–S1 this middle-aged patient developed dorsal chronic pain. The image is degraded by susceptibility artefacts due to the construct.
European Journal of Radiology  , DOI: ( /j.ejrad )
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11. Fig. 10
(a and b) As a consequence of disc degeneration and collapse, higher loads are supported by vertically slipping neural arcs and facet joints (a). The implant of interspinous spacers (IS) shifts the instantaneous axis of rotation (IAR) backwards reducing the pressure between facets and in the posterior annulus, potential sources of acute and chronic pain (b). In case of listhesis the IS can reduce the anterior slippage of the vertebra. Vertical red arrows indicate compression; green arrows tension or distraction. Big red horizontal arrow: degenerative anthelistesis; green horizontal arrow: vertebral realignment.
European Journal of Radiology  , DOI: ( /j.ejrad )
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