The bones of the vertebral column are connected by a series of very tough fibroelastic cartilage plates called intervertebral discs. Little motion occurs at each individual articulation, combined, however, all vertebral articulations permit great mobility.

Figure 1-2
Thoracic Vertebrae, lateral view showing articulations and structures.

Most vertebrae have two superior and two inferior processes forming freely moveable synovial joints that articulate with vertebrae above and below. The intervertebral foramina are formed by intervertebral articulations. Each vertebra has at least six articulations (two slightly moveable and four freely movable). Each thoracic vertebra has two more freely moveable articulations with ribs via costal facets. (Figure 1-2)

Three special articulations of the vertebral column:

• Two exist at the superior end of the column, (C2), the atlantoaxial joint between the atlas and axis and (C1), the atlanto-occipital joint between the atlas and the occipital condyles of the skull.
• At the inferior end of the column the sacrum articulates with the ilia of the hip bones forming the sacroiliac joint.

Figure 1-3
Atlas and axial apposition. A) Posterior view. B) Midsagittal view

The atlanto-occipital (C1-Skull) articulation and the atlantoaxial (C1-C2) articulation involve extensive vertebral modification. The first cervical vertebra or atlas has become a simple ring of bone, essentially a vertebra without a body. The two superior articulating processes of the atlas are modified to form two biaxial joints with the occipital condyles of the skull. These biaxial articulations are restricted to the action of nodding the head affirmatively, the "Yes" joint.

The second cervical vertebra, the axis, has a strange tooth-like projection called the dens, projecting superiorly from its vertebral body. The dens extends into the ring of the atlas forming a uniaxial pivot joint between the two vertebrae. This permits the rotation of the head as in saying no. The dens represents the missing body of the atlas. (Figure 1-3).

Figure 1-4
Anterior view of the sacro-iliac joint.

The sacrum is a triangle with the apex pointing inferiorly. This shape allows the sacrum, under the weight of the upper body, to wedge itself between the two wings of the iliac bones to form a strong sacroiliac joint. (Figure 1-4)

With all these articulations, no wonder the back is susceptible to "cracking and popping".