Depending on age, time of day, occupation, and disease state, the discs make up approximately 15–20 % of the length of the spinal column. Aside from absorbing biomechanical forces, each disc permits movement of the spinal column. Undoubtedly, flexibility decreases with age, while spinal movements at all stages of life can be severely limited by disease. Since vertebrae themselves are relatively inelastic, movement in the spine is mediated notably by the tissues of the intervertebral disc. Although the mobility of contiguous vertebrae (motion segments) can be viewed as limited, the integrated motion of the 33 intervertebral discs together with movement at the zygapophyseal joints permits all of the critical movements of the spine without compromising nerve or muscle function.

The famous English anatomist Henry Gray (1827–1861) classified articulations between vertebrae as “amphiarthroses in which the contiguous bony surfaces are either connected by broad flattened discs of fibrocartilage, of a more or less complex structure.” By definition, these joints permit very little motion. Shapiro et al. (2012) compared the structure-function relationships of both the intervertebral disc and synovial joints. On first consideration, the intervertebral disc could be seen as being very different from the generic synovial joint. However, on reflection, the separate tissues of the intervertebral disc are very similar to that of the diarthrodial joint: both types of joints are lined by cartilage, they are limited by an external ligament, and the joint space contains molecules that promote lubrication (lubricin and hyaluronan) and elevate the osmotic pressure (aggrecan). Indeed, even the presence of a band of nucleus pulposus tissue across the joint is not out of line with what is known of complex ­diarthrodial joints that contain cartilage and fibrocartilage discs and menisci. Related to the function of the nucleus pulposus and the inner annulus, it is not yet clear whether a distinct synovial-like membrane exists in the intervertebral disc. Whether inner annulus is derived from the notochordal sheath has not been determined. Nevertheless, like the cells of the synovium, the resident disc cells do have the ability to mount a robust defense against bacterial attack (Nerlich et al. 2002; Jones et al. 2008).

In terms of movement, the current classification of the disc as an amphiarthrosis would indicate very limited mobility. However, biomechanical studies of the motion segment with or without contributions from the zygapophyseal joints indicate that there is wide range of motion between vertebrae. Moreover, the actual movement of the cervical, thoracic, and lumbar vertebrae includes flexion-extension, axial rotation, and lateral bending, as well as translatory motions. These three-dimensional movements are more in line with those of a diarthrodial joint rather than an amphiarthrosis where movements are slow and motion is limited. Probably the major difference between appendicular diarthrodial joints and the axial intervertebral joints lies in their development. Although the joints originate from different mesenchymal elements, the nucleus pulposus is derived from a unique embryonic tissue, the notochord; deletion studies indicate here too there are considerable similarities in the expression of genes that govern organ development and maturation. Recent investigations indicate that joint formation and even function are dependent on the expression of a number of genes including those of the Hox family, BMPs, and GDF5 (Brunet et al. 1998; Archer et al. 2003; Pacifici et al. 2005). Indeed, deletion of Ext1 influences not just the development of limb joints but also the formation of the intervertebral disc (Mundy et al. 2011). This topic is considered further in Chap.​ 3.

Based on the overt structural and functional similarities between the intervertebral disc and the synovial joint and recognizing that while some differences exist between these articulations, it would seem logical to place the disc in the same grouping as the diarthrodial joint. Further, since the intervertebral motion segment displays movement in three dimensions and the spine itself provides further rotatory movements, Shapiro et al. (2012) were of the opinion that it should be classified not as an amphiarthrosis, “a slightly moveable joint,” but as a complex polyaxial joint.

While the intervertebral discs and the zygapophyseal joints provide sites for vertebral motion, the overall shape of the spine as well as curvatures in specific regions of the spine is dependent on intrinsic genetic factors as well as biomechanical forces mediated through the pull of muscles, ligaments, and gravity. Encoded curvatures are seen in the cervical, lumbar, and sacral regions of the spine. On adoption of a vertical stance, and with maturation, these curvatures become more distinct (Fig. 1.1c). However, about 2–3 % of the population exhibit deficits in axial curvature, which vary from simple bending with little functional implications to excessive bending which impacts not just locomotor activities, but other critical functions associated with the spinal nerves. The “hunchback” spine, kyphosis, is due to excessive posterior bending of the thoracic motion segments (Fig. 1.2a); when the cervical and lumbar anterior spinal curvatures become excessive, this condition is termed lordosis (Fig. 1.2b). While these latter conditions are deviation in the anterior-posterior (cephalic-caudal) axis of the spine, abnormal bending is also seen in the lateral (side-to-side) dimensions. Scoliosis can affect any part of the spine; the most common regions are in the thoracic and the lower lumbar spine. These exaggerated musculoskeletal warps in spinal architecture are evident almost entirely in human populations even in royalty (Richard III); their occurrence in rodents is infrequent. Thus, from an experimental viewpoint, rodents and lagomorphs make useful models to investigate the molecular control of spinal curvature.

Clinical analysis of the types of abnormal spinal bending indicates that the most common form of this condition is idiopathic, i.e., of unknown origin. Nevertheless, the etiology of this condition is likely to be multifactorial as both environmental and genetic factors have been implicated. A second form of scoliosis is neuromuscular which is secondary to other conditions, such as cerebral palsy or a myopathy. In the elderly, abnormalities in axial bending are often due to degenerative disc disease and spondylolisthesis. Possibly, the most intriguing form of scoliosis is ­congenital in origin, a rare condition that is evident early in childhood (usually within the first 6–8 weeks). Radiographically, the spine exhibits fused vertebrae, single or multiple hemivertebrae, a vertebral bar, block vertebrae, and wedge-shaped or butterfly vertebrae. If left untreated, almost all of these congenital anomalies result in deformities and loss of normal function. Since the anomalies occur early in development, this form of scoliosis has been linked to patterning, particularly during the period of somitogenesis (Chal and Pourquie’ 2009).

As will be discussed in considerable detail in Chap.​ 3, somitogenesis occurs at a very early stage in development and is the process whereby the mesoderm of the developing embryo undergoes a carefully timed segmentation process; somites are generated that specify skeletal muscles, dermis, vertebrae, ribs, and annulus fibrosus. Very recent work by Pourquie’ (2011) has shown that the trigger for the rhythmic production of somites involves three major signaling pathways: Notch (Jiang et al. 2000), Wnt/β-catenin (Dequeant et al. 2006), and fibroblast growth factor (Benazeraf et al. 2010) which are integrated into a molecular circuit. The oscillatory activities of this circuit generate a highly coordinated developmental event that serves as a traveling wave of gene expression along the anterior-posterior axis of the developing embryo. Pourquie’ (2011) refers to this synchronized change in gene expression in the pre-mesodermal cells as the “segmentation clock.” Clearly any activity that interferes with coordinated gene expression and the development of the waves of gene oscillations will impact somitogenesis which in turn will influence the subsequent formation of the vertebrae and the intrinsic curvature of the axial skeleton. Although this system was developed from studies in mice, it is most likely that these new understandings will impact our understanding and ultimately the treatment of congenital scoliosis.

The sizes of the discs in the human skeleton have been assessed by a number of investigators, especially in relationship with age, underlying conditions, and responses to surgery. Disc thickness can be assessed by radiography and other forms of imaging analysis. Frobin et al. (1997) made a determined effort to measure the disc and vertebrae height using archived radiographic measurements of the spine. This approach was complicated by a number of factors that included artifacts due to image distortion, axial rotation and lateral tilt, and even magnification. To account for these problems, algorithms were developed that generated dimensionless parameters. The study showed that lumbar vertebrae and discs were larger in males and females and in males there appeared to be no or little impact of age. More recently, magnetic resonance imaging (MRI) was used to provide direct information on the discs of seven healthy males aged 22–30 (Belavý et al. 2011).

Some general comments about disc dimensions are as follows. Disc height (cephalic-rostral dimensions) varies according to the spinal region. In the cervical spine, the disc height is about 3 mm, whereas in the lumbar spine, it is 9–17 mm; in the thoracic spine, the thickness is about 5 mm. In the cervical spine, the discs are thicker in the anterior region than posterior, thus helping to provide the curvature that is characteristic of the neck. In the thoracic spine, the discs are of constant thickness, whereas in the lumbar spine, they are again thickest anteriorly. Radiographs have been used to assess disc parameters in animals most commonly used in spine research (O’Connell et al. 2007).

The major functional role of the disc is mechanical: it allows movements between the axial and appendicular skeleton and the head; it accommodates applied loads; and to some extent the disc protects the spinal cord and nerve roots. The discs themselves are complex tissues comprising an outer circumferential ring of fibrocartilage, the annulus fibrosus which encloses a central proteoglycan-rich core, the nucleus pulposus. The nucleus is sandwiched caudally and cephalically by the cartilage endplates of the contiguous vertebrae. Since details of the biochemical, developmental, and biomechanical aspects of each of the disc tissues are provided in considerable detail in other chapters of this book, the sections below merely highlight the major characteristics of the endplate cartilage, nucleus pulposus, and the annulus fibrosus.