The passive subsystem plays a structural role, ensuring spinal stability, namely, within the elastic zone. It also has a transducer function carried out from a number of mechanoreceptors located within the joint capsules, discs and ligaments which send a continuous flow of proprioceptive inputs about the load status, spatial position and movement from each MS to CNS. On the basis of this information flow, CNS replays through an appropriate and coordinate muscle activity [4].

The vertebral body is the key element of the load-bearing system. It mainly consists of spongy trabecular bone which plays a crucial role in determining the strength and resilience of the vertebral body. Compressive loads are first accepted by vertical trabecular struts joining directly the vertebral endplates. Vertical columns, if isolated, would tend to bow and lose their load-bearing capacity if these were not restrained and maintained right by tension of horizontal trabeculae (Figs. 1.2 and 1.3). It is this conversion of vertical loads in transverse tensions that confers weight-bearing resistance to vertebral body.

A solid bony block would be heavy and, like all crystalline structures, unable to bear dynamic loads which could fracture it, whereas, conversely, a box of cortical shell would easily collapse under compressive forces because of scarce resilience of cortical bone. Cancellous bone guarantees the best load/resistance ratio.

The resistance of vertebral spongiosa either depends on trabecular architecture and bone density.

The vertebral spongiosa of any vertebra contains four main trabecular systems having a quite constant orientation: a vertical system between endplates which transmits axial loads, a curved system running in neural arc and two curved systems joining the endplates with the articular and spinous processes which anchor the neural arc to vertebral body resisting shearing forces (Fig. 1.4).

The bone loss in osteoporosis results in an exponential reduction of resistance. During early stages of osteoporosis, the elective reabsorption of transverse connections leads to a progressive relative elongation of vertical columns, whose resistance decreases by the square of length. Later, the thinning of columns themselves also contributes to a quadratic loss of strength with a summation of both effects.

Out of any pathological change, vertebral endplates can fail by fracturing following repeated stresses.

When a vertebral body is compressed, a part of the energy applied to deform it will be lost and, when the load is removed, less energy is available for it to restore the initial shape, which is so not immediately regained. This mechanical phenomenon, defined as hysteresis, expresses the amount of energy lost during the deformation of any solid structure. By virtue of hysteresis, a structure regains its original shape at a rate and extent different to that at which it was deformed. If forces are applied with many repetitions and sufficient frequency, the vertebral body, like any material, is unable to recover and accumulate small weaknesses, losing gradually stiffness, and, finally, fails under a stress much less than that could damage it if applied a single time [5].

Failure fractures of the vertebral endplates may occur after as few as 30–80 applications of forces corresponding to 50–80 % of ultimate compressive strength (normally between from 3,000 to 10,000 N). These forces are within the ranges occurring during normal daily activities [6].

Endplate fractures are now considered a possible cause of internal disc disruption and discogenic pain [7].

The annulus acts as the first ligament for restraining the complex 3-D movements a vertebra can perform.

The oblique alternating arrangement of collagen fibres between adjacent lamellae (with a degree of 65–70° to endplate plane) and the degree of obliquity of fibres within each lamella both optimize the capacity to restrain movements in all directions. A steeper orientation of lamellar fibres would increase the resistance to distraction, but it would compromise that to sliding and twisting; a flatter inclination would enhance control of twisting, but it would lessen that of distraction and bending.

Adjacent to vertebral endplates, the disc acts as shock absorber of the forces transmitted to the head and brain during walking and jumping.

Both nucleus and inner annulus are involved in weight-bearing. The nucleus pulposus mainly works in the NZ bearing low axial loads, while the stiffer annulus fibrosus accepts a larger proportion of the highest loads. The densely packed lamellae confer compression stiffness to annulus, but, over time, an isolated annulus under sustained loads tends to buckle and collapse.

The nucleus provides an internal bracing mechanism. Like a ball of fluid, the nucleus can be deformed but not compressed. Under axial compressive load it expands radially stretching the annulus and pushing away the opposite endplates. The vertical pressure transmits part of load from one vertebra to the next, lessening the tension created in the annulus fibrosus. The radial pressure stretches and braces the annulus preserving the vertical orientation of lamellae, avoiding their buckling and increasing their capacity to transmit loads [8] (Fig. 1.5). The cooperative action of the nucleus and the annulus enhances the disc capacity of bearing higher and prolonged loads.

This mechanism is lost in case of degenerative depressurization of nucleus pulposus that leads to external and internal buckling of annular lamellae. Buckling accelerates and favours the further annulus disruption (Fig. 1.6).

In addition, under compression, a part of energy is used and stored to stretch the annulus and then released soon after the discharge of the spine. This momentary diversion of energy attenuates the speed of transmission of the force from a vertebra to another, preserving the adjacent endplates.