Biomechanics of the Spinal Motion Segment

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CHAPTER 7 Biomechanics of the Spinal Motion Segment

In biomechanics, information from the biologic sciences and engineering mechanics is integrated for the purpose of analyzing and quantifying the function of and forces occurring on tissue under various conditions. With an understanding of the natural behavior mechanics of the spinal motion segment, it can be possible to understand better the limitations of the system and the conditions under which tissue damage occurs and subsequent pain would be likely. Biomechanical assessments provide a quantitative means by which to accomplish this goal.

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.

With these goals in mind, this chapter (1) considers the physical characteristics of the spinal tissues that could influence function, (2) assesses the motion characteristics (kinematics) of the different portions of the spine, and (3) summarizes the ability of the spine to withstand forces that it is supporting (load tolerance). Collectively, this chapter shows, from a biomechanical perspective, how the spine functions and how it breaks down.

Assessing the Biomechanics of the Spinal Motion Segment

Ideally, it would be desirable to measure directly the forces imposed on the various tissues within the spine. With current technology, invasive measures would be required, however, to understand the loading imposed on the various spinal tissues. Such invasive measures would disrupt the tissues of interest and would most likely alter the very factors that one is attempting to measure. Direct biomechanical measurements of the spine in vivo are rare and currently difficult in live humans. Subsequently, much of the biomechanical information about the human spinal motion segment is based on in vitro studies. This information must be considered with caution because the properties of the spine derived from cadaveric studies are understood to be different in many respects from those of a live individual.

An alternative to direct measurement of spine tissue loading is the prediction of tissue loads based on biomechanical models. A biomechanical model is a conceptual representation and prediction of how the forces within the biomechanical system interact ultimately to impose force on a particular tissue of interest. Biomechanical analyses assume that the body behaves according to the laws of newtonian mechanics that must govern the distribution of forces within the musculoskeletal system. The object of interest in spinal biomechanics is a precise quantitative assessment of the movement behavior and mechanical loading occurring within the tissue of the musculoskeletal system. Biomechanical modeling permits one to estimate the direction and magnitude of forces acting on the spinal motion segment and allows one to estimate when natural motion tolerances have been exceeded and when damage or degeneration would be expected to occur. Biomechanical assessments help one understand potential pathways of low back disorders and can potentially help surgeons understand how contemplated surgical interventions might affect the health of the spine. Biomechanical modeling is outside the scope of this chapter, however.

Ultimately, biomechanical assessments are intended to determine “how much loading of the tissues within the spinal motion segment is too much loading?” This high degree of precision and quantification is the characteristic that distinguishes biomechanical analyses from other types of analyses.

Physical Characteristics of Spine Structures

The spine is composed of four types of vertebrae classified according to their regional location along the spinal column—cervical, thoracic, lumbar, and sacral. There are 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae. In addition, the sacrum consists of five immobile or “fused” vertebrae, and the coccyx (often referred to as the tailbone) is a fusion of four coccygeal vertebrae at the very base of the spine. Each vertebra is referenced according to a nomenclature system wherein the spine region (e.g., cervical, thoracic) is followed by a numbering system that refers to the vertical position of the vertebral body along the spine (beginning with the vertebra closest to the head) (e.g., first cervical vertebra, or C1). Disc levels are referenced relative to the vertebral levels surrounding the disc. The lowest lumbar vertebra (fifth lumbar vertebra, or L5) is adjacent to the first sacral vertebra (S1), and the disc between these vertebrae is referred to as L5-S1.

The shape of the vertebrae changes from level to level in the spine. The vertebral body shape and the orientation of the posterior elements change. In particular, the orientation of the bony structures that compose the posterior elements change in their shapes and contact angles. These subtle changes permit or restrict motions in different directions along the human spine.

Several physiologic curves are also characteristic of the upright spine (Fig. 7–1A). The curves within the cervical and lumbar regions of the spine are referred to as cervical lordosis and lumbar lordosis, whereas the thoracic and sacral curves are referred to as thoracic kyphosis and sacral kyphosis because these curves bow in the opposite direction of the lordotic curves. These curves work collectively to accommodate pelvic orientation under different conditions. When sitting, the pelvis rotates backward and the lumbar curve flattens. When the pelvis is rotated forward, the lumbar curve is accentuated. Collectively, the spinal curves balance each other and form a stable system that maintains the center of gravity in a balanced state.

FIGURE 7–1 A and B, Arrangement of the vertebral bones and spinal curves (A) and a functional spinal unit or spinal motion segment (B).

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

The “building blocks” of the spine are the spinal motion segments (Fig. 7–1B), also known as the functional spinal unit. This unit consists of two vertebrae and the disc in between them. This unit represents the central focus of biomechanical functioning and clinical assessment. This chapter explores the spinal motion segment from a biomechanical perspective with the intent of understanding the significance of features that may influence status.

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.