Fig. 1.1
Spine stability is guaranteed by the strict connection between three systems: column, muscles and central nervous system. Their activities are regulated on the basis of the reciprocal feedbacks
Column
Muscles and tendons
Central nervous system and spinal nerves
The column includes bones, discs, ligaments and joint capsules; these structures fulfil an intrinsic structural role [5] and contain mechanoreceptors which act as transducers, sending a continuous flow of proprioceptive information on loads, motions and posture through the spinal nerves to the central nervous system that, in turn, replies via an appropriate and coordinated feedback muscular action [6, 7].
Degeneration or any traumatic lesion to the bony and soft components of the spine tends to increase the demand on muscles and nervous systems in order to preserve or restrict the segmental instability [5].
1.2 Column
The intrinsic structural and stabilization role of the spine depends on:
Vertebral architecture
Disc-intervertebral joints
Facet joints
Ligaments
Curves
1.2.1 Vertebral Architecture
The passive load-bearing ability of the vertebral body depends on the size, shape, integrity of the trabecular system and bone density. The vertebral body mainly consists of spongy bone with a three-dimensional honeycomb structure that yields the best strength/weight ratio [8]. The progressive increase in body size downward in the spine is the only physiological answer to increasing weight loads [9].
The cancellous bone of any vertebral body has four main trabecular systems (Fig. 1.2):
Fig. 1.2
The four trabecular systems: vertical, horizontal and curved force lines are constant in all the vertebral components of the spine. The vertical system (red lines) supports the central portion of the soma lending resistance to cranio-caudal compression. The horizontal (yellow lines) and the two curved systems (blue lines) strongly attach the posterior arch to the soma
A vertical system extending between the endplates which accepts and transmits vertical loads
A horizontal system travelling in the posterior arch and joining the transverse processes
Two curved oblique systems, superior and inferior, starting from the endplates and crossing in the peduncles to end in the spinous and joint processes
Their function is to withstand the horizontal shear stresses ensuring the neural arch to the body.
Compared to spongy bone, the cortical bone presents much lower elasticity but is more resistant. The resistance of spongy bone also strongly depends on mineral density; indeed bone loss in osteoporosis results in a disproportionate exponential reduction of resistance: a bone loss of 25 % leads to a reduction of resistance of about 50 % [10].
1.2.2 Disc-Intervertebral Joints
Thanks to its peculiar structure, the disc has both the tension-resisting properties of a ligament and the compression-resisting properties typical of joint cartilage. The disc behaves as a ligament allowing for and controlling the complex three-dimensional movements of the spine: vertical compression and distraction, flexion–extension, lateral bending and axial rotation. The outermost fibres of the annulus are the first controller of abnormal micro-movements [11].
With the nucleus behaving like a pressured cylinder, the disc is also the main shock absorber of mechanical stresses transmitted during motions to the skull and brain. The biomechanical behaviour of the normal young nucleus is homogeneous and isotropic, equal in all its parts and all directions: independently from the spatial position of the spine, the load is transmitted on the endplates avoiding any focal concentration [12]. By contrast, in the degenerated disc the nucleus loses its normal fluidlike properties and loads asymmetrically assuming a solid-like behaviour.
The tensile circumferential properties of the annulus are also inhomogeneous, the anterior annulus being stiffer than the posterior annulus (Fig. 1.3) and the outer annulus stiffer than the inner ring [13]. When the normal disc is loaded, tensile circumferential loads are generated in the annulus because of the pressurization of the nucleus and the resistance of its fibres to stretching and bulging under axial compression [14].
Fig. 1.3
Framework of a normal disc: the central nucleus (azure) presents a homogeneous hydration acting as shock absorber; the circumferential annulus has fibres resistant to stretching, stiffer in the anterior portion (dark blue) than in the posterior (clear blue)
The water content and thickness of the disc continuously change during normal daily activities under the opposite influences of hydrostatic and osmotic pressures [15]; under load, the high hydrostatic pressure leads to a gradual release of water out of the disc whose thickness diminishes until it is counterbalanced by the osmotic pressure exerted by proteoglycans whose concentration increases progressively [15]; in the recumbent position the re-prevailing osmotic pressure again recalls water back into the disc.
1.2.3 Facet Joints
Facet joints fulfil two basic functions:
Control of direction and amplitude of movements
Sharing of loads
According to the three-column model of Louis, the weight of the head and trunk is transmitted first on two columns placed on the same frontale plane, the atlanto-occipital lateral joints, then, from C2 to L5, on three columns arranged like a triangle with an anterior vertex [8]. The anterior column is composed of the superimposing bodies and discs, the two posterior columns of the vertical succession of the facet joints (Fig. 1.4).
Fig. 1.4
Representation of the three-column model of Louis (from C2 to L5): the anterior column (dark blue circle) is composed of the superimposing bodies and discs, the two posterior columns (clear blue circle) of the vertical succession of the facet joints; these structures form a triangle with an anterior vertex (yellow triangle)
In physiological conditions a balanced action exists between the three columns so that the posterior facets accept from 0 % up to 33 % of the load depending on the posture [16]. Like the vertebral bodies the increasing size of the facet joints downward compensates the increasing functional demand. The spatial symmetry of the facets is an essential requirement for correct functioning: every significant asymmetry predisposes to instability and premature degeneration of the facets and discs.
Long-standing remodelling and destabilization of the facet joints along with degenerative changes in posterior ligaments lead to degenerative spondylolisthesis with sagittal orientation of the facet joints acting as a predisposing factor [17]. An estimated 15–40 % of chronic low back pain cases are thought to be caused by lumbar facet joints due to joint capsule mechanical stresses and deformation with activation of nociceptors [18].
1.2.4 Ligaments
Ligaments are the passive stabilizers of the spine. The interspinous and supraspinous ligaments being located far away from the rotational axis and working with a long lever arm oppose spinal flexion more than the flava ligaments having a shorter lever arm [19]; on the other hand being very close to the spinal rotational axis and intrinsically less resistant, the posterior longitudinal ligament presents a double mechanical disadvantage.
1.2.5 Physiological Curves
Sagittal curves are acquired and represent the evolutionary response to the needs of the upright standing position [20]. Dorsal kyphosis is the only sagittal spinal curve present at birth. Cervical and lumbar lordoses develop with head rising and standing and walking.
Both in normal individuals and in pathologic conditions, sagittal spine curves are regulated by pelvic geometry expressed by different parameters, namely, pelvic incidence, sacral slope and pelvic tilt [20, 21]. Pelvic incidence is a fixed morphologic parameter which after birth remains unchanged in each subject: any sagittal balance change is obtained because of the adaption of other positional parameters [21].
Sagittal spinal curves also increase the resistance to vertical loads up to 17 times by directing deformations into pre-ordered directions which can be quickly controlled by the fast intervention of muscle contraction.
1.3 Muscles and Tendons
Muscles and tendons provide active stabilization of the spine under the control of the nervous system; their action stabilizes the spine during standing, lifting and bending activities. Without the muscles, the spine would be highly unstable, even under very light loads [5, 18].
The muscles may be divided into superficial (rectus abdominis, sternocleidomastoideus) and deep (psoas) flexors and superficial (long) and deep (short) extensors.
The function of the superficial, multisegmental muscles differs from that of deep unisegmental muscles. Being small and located very close to vertebral rotation axes, the short muscles (inter-transverse, interspinous, multifidus) globally act primarily as force transducers sending feedback responses to the central nervous system on the movement, load and position of the spine [22]. The long superficial muscles are the main muscles responsible for generating movements: the lumbar erector spinae and the oblique abdominal muscles produce most of the power forces required in lifting tasks and rotation movement, respectively, having only limited insertions on the lumbar motion segments, while the multifidus muscle acts as a dynamic stabilizer of these movements [22]; the oblique and transverse abdominis muscles are mainly flexors and rotators of the lumbar spine but stabilize the spine at the same time, creating a rigid cylinder around the spine by increasing intra-abdominal pressure and tensing the lumbodorsal fascia [23]. The complexity of the posterior musculature excludes any possibility of voluntary control upon single units.
The large number of muscles, the complex antagonistic activities and the variability of spine insertion control the determination of muscle force and its contribution to spinal loading.
All these components, together with joints and tendons of each segment, send inputs through the spinal nerves to the central nervous system that regulate and coordinate the muscle activity [5].
1.4 Spinal Nerves and Vertebral Pain
Spinal nerves are mixed nerves, which carry motor, sensory and autonomic signals between the spinal cord and the body. Each spinal nerve is formed from the combination of nerve fibres from its posterior and anterior roots. The posterior root is the afferent sensory root and carries sensory information to the spinal cord and then to the brain. The anterior root is the efferent motor root and carries motor information from the brain passing through the spinal cord. The spinal nerve emerges from the spinal column through the intervertebral foramen between adjacent vertebrae [24].
Outside the vertebral column, the nerve divides into branches: anterior and posterior.
The anterior ramus contains nerves that serve the anterior portions of the trunk and the upper and lower limbs, carrying visceral motor, somatic motor and sensory information to and from the ventrolateral body surface, structures in the body wall and the limbs.
The posterior ramus contains nerves that serve the posterior portions of the trunk carrying visceral motor, somatic motor and somatic sensory information to and from the skin and muscles of the back.
The vertebrogenic pain presents three origins:
Nociceptive, induced by direct stimulation of nervous branches (receptors) present in the structure involved in the pathology (cortical bones, periosteum, subchondral region)
Neuropathic, if there is a direct compression of the spinal nerve or nerve root
By breakthrough (oncology patient)
In the spine, nociceptors are localized in subchondral area, on the periosteum, near ligaments and the facet joints and around the disc; these are activated by direct physical damage or by chemical substances that are released by damaged tissues [25].
The nerves of Luschka (called also recurrent nerves because part of their fibres re-enter the intervertebral foramen) are small meningeal branches (Fig. 1.5) of the spinal nerves that branch near the bifurcation of the anterior and posterior rami; they are divided into two branches, superior and inferior, receiving fibres by the facet joints, the annulus fibrosus of the intervertebral disc, the ligaments and the periosteum of the spinal canal; furthermore Luschka nerves joined a nervous branch coming from the sympathetic chain. They present an anastomotic distribution so that a single nerve provides a sensitive innervation of multiple levels. They are crucial elements in the origin of the spinal pain: any type of mechanical or chemical stimuli that determines a modification of the structures innervated by Luschka nerve is responsible for the local spinal somatic pain [26].
