PART II THE VERTEBRAL COLUMN Compression load End plate Facet joints
Nucleus pulposus
Anulus fibrosus
End plate A A Compression load Compression load Disc
Anulus fibrosus
B
Figure 5.5 (A) Sagittal view of an intervertebral disc with end plate attachments to the vertebrae. Discal spacing assists with alignment of the facet joints and formation of the intervertebral foramen. (B) Frontal view of an intervertebral disc.
over which the vertebral body pivots and moves. The nucleus is surrounded by a series of concentric fi brocar- tilage rings called the anulus fi brosus (Fig. 5.5B). These type I collagen rings have a lower fl uid composition than the nucleus pulposus. The anulus fi brosus resists tensile stresses on the discs by stretching and bulging to absorb the forces. Each superior and inferior surface of the disc is connected to a vertebral end plate, which connects to the bony aspect of the vertebral body. Between each IVD and the vertebral body is an interbody or intervertebral joint. These fi brocartilaginous joints are symphysis-type joints that have movement in all three planes and non- linear anterior and posterior gliding. Although the move- ment between each intervertebral joint is slight, their combined movement allows the spine to move in long arcs of motion.
As loading forces are applied to the spine by muscle contractions and lifting external objects, compression on one area of the disc produces tension on the anulus fi bro- sis opposite to the compression force (Fig. 5.6A and B). The nucleus pulposus, because of its fl uid composition, is displaced by the compression away from the force and toward the anulus fi brosus. During spinal fl exion, the anterior portion of the disc and anterior spinal structures are compressed, and the posterior structures are stretched (Fig. 5.6C). With extension, the reverse occurs as the posterior structures are compressed and the anterior
Nucleus pulposus
B
Flexion
C
Extension
D
Figure 5.6 (A) Axial compression of a disc. (B) Lateral forces produce disc displacement in the opposite direction. (C) Spinal flexion moves disc material in a posterior direction. (D) Spinal extension bulges the disc in an anterior direction.
