CHAPTER 7 Biomechanics of the Spinal Motion Segment
From a biomechanical standpoint, the spine seems to accomplish three major functions.1 First, the spine provides a structure by which loads can be transmitted through the body. Second, the spine permits motion in multidimensional space. Third, the spine provides a structure to protect the spinal cord. To appreciate the ability of the spine to accomplish these functions, we need to understand the natural movements of the spine and the ability of the spine to withstand forces or loads that are transmitted through the structure.
Physical Characteristics of Spine Structures
Support Structures

FIGURE 7–2 Lumbar vertebra and its posterior elements.
(Adapted from Marras WS: The Working Back: A Systems View. Hoboken, John Wiley & Sons, 2008.)
Disc

FIGURE 7–3 A, Disc, vertebral endplate, and vertebral body. B, Construction of intervertebral disc.
(Adapted from Marras WS: The Working Back: A Systems View. Hoboken, John Wiley & Sons, 2008; Bogduk N: Clinical Anatomy of the Lumbar Spine and Sacrum, 4th ed. Edinburgh, Churchill Livingstone, 2005.)
Coordinate System and Force and Movement Definitions
A biomechanical assessment of the spine is concerned with the assessment of movements and forces developing within the spine as it is exposed to activities of daily living and other work or environmental conditions. Movements or motions are compared with the natural limits of movement, and forces imposed on a tissue (also called tissue loading) are compared with the tissue tolerances (magnitude of load at which damage occurs). To describe movement and force transmission through tissue accurately, it is necessary to describe precisely direction of movement and direction and magnitude of the force application on the tissue. Direction is defined relative to a coordinate system or reference frame. The central (global) coordinate system of the body is shown in Figure 7–5. The origin or center of this coordinate system is located at the base of the spine. Figure 7–5 describes the coordinate system (used in this chapter) as a traditional three-dimensional cartesian coordinate system with three mutually perpendicular axes oriented with a vertical Z-axis. Some references have adopted the ISB coordinate convention, where the Y-axis is defined as the vertical axis.

FIGURE 7–7 Compression of disc leading to increased pressure in disc nucleus and deformation of endplate.
(From White AA III, Panjabi MM: Clinical Biomechanics of the Spine, 2nd ed. Philadelphia, JB Lippincott, 1990.)

FIGURE 7–8 The effects of shear (A), torsion (B), and tension (C) on the fibers of the anulus fibrosus.
(From Adams MA, Bogduk N, Burton AK, et al: The Biomechanics of Back Pain, 2nd ed. Edinburgh, Churchill Livingstone, 2006.)
The amount of displacement between the neutral position of the vertebra and the point at which resistance to physiologic motion is experienced is referred to as a neutral zone.2 Neutral zones can be defined for translational and rotational movements. The neutral zone can be described for each of 6 degrees of freedom.
Tissue Load Characteristics
More recently, a third type of biomechanical trauma (instability) has received much attention in the literature.3–8 Stability is the ability of a system to respond to a perturbation and reestablish a state of equilibrium.2 Instability of the spine refers to the abnormal displacement of spine under physiologic loading. The abnormal displacement can occur in translation or rotation, but most likely would be some combination of these two types of motions. These abnormal motions are often small in magnitude, but the displacement may be enough to stimulate pain in sensitive tissue. Stability is significant because it is often the initiator of tissue damage when the system is out of alignment or when the musculoskeletal system overcompensates for a perturbation.2 When the supporting musculature cannot offer adequate stability to a joint (owing to improper muscle recruitment, fatigue, structure laxity, or weakness), the structure may move abnormally and result in sudden and unexpected force applications on a tissue. This type of trauma is similar to the acute trauma pathway, but is initiated by a miscalculation of the muscle recruitment pattern.
Mechanical Degeneration—Tissues at Risk
Clinicians are beginning to understand that low back disorders can occur before tissue damage. Biochemical studies have shown that these types of tissue insults can result in an upregulation of proinflammatory cytokines. This upregulation may result in tissue inflammation at much lower levels of load than would occur under normal conditions. This inflammation makes nociceptive tissues more sensitive to pain and may initiate back pain.9
When endplate loading exceeds its tolerance limit, microfractures can occur in the structure. Microfracture of the endplate itself usually does not initiate pain because few pain receptors reside within the disc and endplate. Repeated microfracture of this vertebral endplate can lead to the formation of scar tissue and calcification that can interfere with nutrient flow to the disc fibers. Because scar tissue is thicker and denser than endplate tissue, the scar tissue interferes with nutrient delivery to the disc. This reduced nutrient flow can lead to atrophy and weakening of the disc fibers and disc degeneration. Because the disc has relatively few nociceptors except at the outer layers, this degenerative process is usually not noticed by the individual until the disc is weakened to the point where bulging or rupture occurs, and surrounding tissues that are rich in nociceptors are stimulated. Figure 7–9 describes this sequence of events that are believed to lead to disc degeneration.9

FIGURE 7–9 Sequence of events associated with cumulative or repeated trauma leading to disc degeneration.