Support Structures

The spine is constructed of a series of vertebral bones that are stacked on one another to form the spinal column that runs from the pelvis to the head. A vertebral bone, or vertebra, is shown in Figure 7–2. The large round portion of the bone is the vertebral body and represents the major load-bearing structure of the spinal column. The outer portion of this bone is composed of a thin yet very strong layer of cortical bone. Cortical bone, also known as compact bone, forms a protective outer shell, has a high resistance to bending and torsion, and provides strength in situations where bending would be undesirable. The inner portion of the bone consists of a spongy matrix of cancellous bone. This type of bone is less dense and more elastic than cortical bone. Cancellous bone forms the interior scaffolding of the structure and helps the bone to maintain its shape despite compressive forces. This structure is composed of bundles of short and parallel strands of bone fused together.

FIGURE 7–2 Lumbar vertebra and its posterior elements.

(Adapted from Marras WS: The Working Back: A Systems View. Hoboken, John Wiley & Sons, 2008.)

Posterior of the vertebral body are bony structures that constitute the posterior elements and form a protective channel or tunnel for the spinal cord (see Fig. 7–1B). The biomechanical role of the posterior elements is to control the position of the vertebral bodies. These elements provide attachment points for muscles to control the position of the vertebra and supply lever arms to provide the system with mechanical advantage. In addition, these structures control motion and provide mechanical “stops” to prevent excessive movement of the vertebral body. A significant portion of the mechanical load is borne by the posterior elements, relieving the disc of excessive loading.

As shown in Figure 7–2, toward the top of the posterior surface of each vertebra are pedicles. The pedicles provide a robust support structure (a type of pillar) to transmit force between the posterior elements and the vertebral body. Projecting out from each pedicle are the lamina structures that come together at the midline of the body and form a neural arch. This arch is a strong structure that provides protection to the spinal cord in the form of a channel (vertebral foramen).

Emanating out from the junction of the two laminae at the midline of the body is a bony protrusion called the spinous process. Projecting laterally on each side of the structure at the junction of the pedicle and the laminae is another bony structure called the transverse process. These processes provide muscle attachment surfaces and mechanical advantage for control of the spinal column.

Two sets of articulating surfaces are also present in the posterior elements. Projecting out from each of the cephalic lateral corners of the lamina is a bony extension called the superior articular process. A portion of this surface is covered by articular cartilage. Emanating from the caudal lateral corner of the lamina on each side are the inferior articular processes. The superior articular process from the lower vertebra interacts with the inferior articular process of the vertebra above it to form a synovial joint known as the zygapophyseal joint. This joint is also referred to as the facet joint. The inclination of the facet joint changes from the cervical spine to the thoracic spine to the lumbar spine. This joint is defined as a plane surface in the cervical and thoracic joints, but becomes a curved surface in the lumbar spine. In the lumbar spine, the inferior facets are convex in shape, whereas the superior facets have a concave shape. In addition, the angle of these surfaces relative to the sagittal plane changes (increases) as one moves down the lumbar spine. The differences in orientation of these facet joints restrict movement in different planes of motion. They serve an important function in that they permit certain motions and limit other motions of the spine. They can be thought of as the guidance system of the spine.

Collectively, the posterior elements can provide a significant load path for the forces running through the spinal column. Approximately one third of a spinal load is carried through the posterior elements in the upright posture. The nature of the load transmission can be altered when spine degeneration occurs by altering the vector of force and magnitude of force transmitted through these posterior elements. This load path can be disengaged, however, when the spine is in a flexed posture, and the load can be entirely passed through the disc.

Disc

The vertebral bodies are connected by discs that serve several biomechanical purposes. First, the discs act as shock absorbers between the vertebrae, absorbing a portion of the mechanical forces transmitted through the spine. Second, they can transmit a portion of the mechanical load between vertebrae. Third, the discs are able to permit and govern motion between the vertebral bodies. Functionally, the discs are intended to provide a separation between consecutive vertebrae. This separation provides space between vertebrae so that the vertebral bodies can independently change their orientation and execute bending movements. With this arrangement, a pliable and deformable spinal structure is possible.

The disc consists of two distinct portions, each of which is associated with a distinct mechanical function. The outer portion of the disc, called the anulus fibrosus, consists of alternating layers of fibers that are oriented at a 60- to 65-degree angle relative to the vertical. The anulus fibrosus consists of about 10 to 20 concentric, circumferential sheets of collagen called lamellae that are nestled together around the periphery of the disc (Fig. 7–3). The lamellae are stiff and can withstand significant compression loading. Given the collagenous nature of these lamellae, they are pliable and can also permit bending of the spinal column. If the structure were to buckle, however, it would lose its stiffness and would be unable to support compression. The second portion of the disc (nucleus pulposus) is designed to overcome this potential problem.

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.)

Within the anulus fibrosus is a gelatinous core referred to as the nucleus pulposus (see Fig. 7–3). When compressed, this core expands radially and places the anulus fibrosus in tension, providing stiffness. The integrity of the system changes throughout the day. The disc absorbs water while one is recumbent, which makes the system stiffer than when one is upright. Conversely, when one is upright, water is squeezed out of the disc, and the structure becomes more lax.

Finally, the endplate is located at the intersection of the disc and the vertebral body. The endplates are composed of cartilage and cover the superior and inferior portions of the disc. These structures bind the disc fibers to the vertebral bones and play a significant role in disc nutritional transport.

Spinal Ligaments

The spinal ligaments play a significant role from a biomechanical standpoint. Ligaments are most effective in supporting loads in the direction in which their fibers run. They support loads under tension and can buckle under compression. These structures can store energy and act much like a rubber band in that they can provide resistance to loads by developing tension.

The ligaments serve three roles. First, they permit motion and help orient the vertebrae without muscle recruitment. Second, ligaments protect the spinal cord by restricting spinal motion segment movement to within specific ranges. Third, they absorb energy and protect the spinal cord during rapid motions.

The spinal ligaments are shown in Figure 7–4. The arrangement of these structures provides support for the spine in different dimensions of loading. Because support is offered in the different directions of motion, these structures provide stability when the spinal system is intact.

FIGURE 7–4 Ligaments of the spine.

(From White AA III, Panjabi MM: Clinical Biomechanics of the Spine, 2nd ed. Philadelphia, JB Lippincott, 1990.)

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 7–5 Central or global coordinate system for the body.

All movements of the spine are described relative to the origin of the central coordinate system. Flexion and extension are typically described in the sagittal plane, lateral bending occurs in the coronal plane, and twisting occurs along the horizontal or transverse plane. In reality, most activities are combinations of movements in these planes.

Within the spinal motion segment or functional spinal unit, a local coordinate system can also be defined. The convention that defines this local coordinate system is shown in Figure 7–6. Movement of the spinal motion segments is defined relative to the subjacent vertebrae. Movements of the motion segment can be either translations (indicating straight line movements in any direction) or rotations (indicating movement around a point as when bending).

FIGURE 7–6 A and B, Spinal motion segment planes and directions of motion (A) and biomechanical coordinate system and direction of forces and moments (B). Motions and forces are described relative to this coordinate system.

(From Bogduk N: Clinical Anatomy of the Lumbar Spine and Sacrum, 4th ed. Edinburgh, Churchill Livingstone, 2005.)

Figure 7–6 indicates that forces and moments (torques) can develop along each dimension of the reference frame. Forces along the Z dimension are either compression or tension depending on whether they compress the spinal motions segment or pull on the tissues. These are typically the forces one is concerned about when lifting an object in the sagittal plane. Two types of shear forces are also of concern when evaluating the biomechanics of the spine. Anteroposterior shear force describes the forward or backward force in the Y-axis that can result from pushing or pulling activities. The lateral shear forces refer to the sideways forces acting along the X-axis and represent the forces that develop in the spinal motion segment when one pushes an object to the side of the body.

Compression of the disc causes pressure within the nucleus pulposus in all directions, and this pressure places the anulus fibrosus under tension. As shown in Figure 7–7, the nucleus pressure can lead to deformation near the center of the endplate with this form of loading.

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 illustrates how shear, torsion, and tension influence the fibers of the anulus. Shear forces tense the fibers in the direction of movement and relax the fibers in the opposite direction. Similarly, torsion or twisting tenses the fibers that are lengthened by the movement and relaxes the remaining fibers. This differential of force among the fibers is believed to result in tissue damage. Finally, lengthening of the spine places the fibers under tension. This action increases the force on all the fibers regardless of their orientation.

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.)

Bending moments refer to forces acting around an axis in Figure 7–6. The curved arrows in this figure show the direction in which moments act around a spinal segment. A bending moment can be defined around the X-axis resulting in a movement in the sagittal plane (forward bending moment), or it can be defined around the Y-axis indicating a sideways or lateral bend. In either of these situations, the moment or toque around the central axis defines the loading of the segment. Twisting of the spine can result when forces are applied around the Z-axis of the spine. This situation results in what is typically referred to as torsional moment.

The forces and moment can be defined around each vertebra along the spine resulting in a very large number of forces and moments and numerous degrees of freedom. For practical purposes, the forces and moment are typically defined in most situations around one particular vertebra or disc (e.g., L5-S1) depending on the purpose of the study.

Movements between vertebral bodies can also be coupled. Coupling refers to the motion relationship of one vertebra around an axis relative to another vertebra around a different axis. In other words, coupling refers to the motion in different planes that occurs simultaneously. The spine can bend forward and twist at the same time: This is a coupled motion.

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

The forces represented in Figure 7–6 define the direction of load application and the magnitude of the force. The nature and temporal characteristics of the loading situation also define the probability that the load application will result in tissue damage. It is believed that tissue damage can result from several different “types” of trauma to the tissue. Each type of trauma is believed to be associated with very different tolerance levels. First, acute trauma is the most familiar type of loading. Acute trauma refers to a single application of force that exceeds the tolerance level of the tissue. This would be the case if a large load were imposed on the spinal motion segment and a rupture of the disc occurred. In this case, the magnitude of the force applied in a particular direction would far exceed the tissue strength of the disc resulting in a rupture.

Another well-recognized mechanism of tissue disruption involves repeated cumulative loading of the tissues. With cumulative trauma, moderate repetitive loads are applied to the tissues, and this repeated loading is believed to weaken the structure so that the tolerance of the tissue is reduced. Although moderate loading can cause the tissues to strengthen and adapt to load, repetitive loading without proper rest (adaptation) time can cause degeneration of the tissues. Repetitive application of force to a structure is believed to cause microtrauma, which weakens the structure and leads to failure at lower levels than would expected with an acute trauma to the tissue.

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

Many tissues in the spinal motion segment can be influenced by structure loading. These tissues include bones, discs, ligaments, tendons, and nerves. Tissue loading can result in a disruption of the tissue integrity. Bones can be cracked or broken, disc endplates can sustain microfractures, the disc can bulge or rupture, muscle can experience fiber tears, and blood flow to the tissues can be disrupted. All of these events are believed to be capable of initiating a sequence of events leading to back pain. The tolerance of many of these structures within the spine is reviewed in detail.

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

Much attention in spine biomechanics and clinical care has been focused on the intervertebral disc because disc disruption has been associated with pain. Over the past several decades, clinicians have also begun to understand how spine loading can initiate the degeneration process within the disc. To appreciate this process, the system behavior of the disc, vertebral body, and endplate must be considered in response to cumulative trauma. The disc receives no direct blood supply for nourishment. It relies heavily on nutrient flow and diffusion from surrounding vascularized tissue for disc viability. The nourishment is transported from the vertebral body through the endplate to the disc. The endplate is very thin (about 1 mm thick) and facilitates nutrient transport to the disc.

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