Concepts and Mechanisms of Spinal Biomechanics

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CHAPTER 265 Concepts and Mechanisms of Spinal Biomechanics

Vincent J. Miele, Tarun Bhalla, G. Alexander Jones, Edward C. Benzel

Over the past decade, increased understanding of the spinal column has rewarded the neurosurgeon with a plethora of new techniques and instrumentation. Along with these advances, however, comes the need to understand the principles of the various methods of spine stabilization available so that decision making is appropriate in choosing which of these tools to use. Essential to this process is a solid understanding of the concepts and mechanisms of spinal biomechanics. This chapter attempts to provide a foundation for this subject and is presented in three sections. The first reviews the relevant anatomy and basic biomechanical principles. The effects of the aging process and spinal pathophysiology on spinal biomechanics are the focus of the second section. Section three focuses on surgical planning and includes a discussion of newer technologies such as motion preservation devices.

Basic Biomechanical Principles

Biomechanically Relevant Anatomy

Spinal stability is maintained by a variety of anatomic structures that have evolved to provide resistance against deforming forces. These structures include both osseous and ligamentous components.

The vertebral body (VB) is the main axial load–bearing structure of the spine. Its cylindric shape, bounded peripherally by cortical bone and rostrocaudally by end plates, confers it with superior biomechanical properties. The width and depth of VBs increase as one descends in the spine to accommodate increased axial load (Figs. 265-1 and 265-2). The relative weakness of the L5 vertebra can be explained by the asymmetry in height between the ventral and dorsal cortical walls.

FIGURE 265-1 Vertebral body diameter versus spinal level. The width (blue line) and depth (red line) of the vertebral bodies are depicted separately.

(From Benzel EC. Biomechanically relevant anatomy and material properties of the spine and associated elements. In: Benzel EC, ed. Biomechanics of Spine Stabilization. Rolling Meadows, IL: AANS Publications; and New York: Thieme; 2001:1. Reprinted by permission.)

FIGURE 265-2 Vertebral compression strength versus spinal level.

The intervertebral disk serves as a shock absorber and is the primary stabilizing structure of the motion segment. It is composed of the nucleus pulposus (a hydrated core of proteoglycans suspended in a loose collagen network) located in the posterocentral area of the disk and the annulus fibrosus (a fibrocartilaginous ring designed to provide structural support). The loads that these structures must tolerate cyclically for decades is impressive. Compressive loads on the lumbar intervertebral disks are 1.0 to 2.5 times body weight during normal walking. During the lifting of 14- to 27-kg objects, axial compressive loads in the lumbar spine increase up to nearly 10 times body weight, with anteroposterior shear loads approaching double body weight.^1,² Concentric axial loads cause equally distributed forces within the disk, whereas eccentrically placed loads result in bulging of the annulus on the side of the applied force along with associated displacement of the nucleus to the opposite side (Fig. 265-3). Shearing and rotational forces are resisted by the annular fibers, which lie at a 30-degree angle with respect to each other. As the disk deteriorates, its isotropic load transfer properties are lost and load transfer becomes concentrated at the periphery (annular insertion) of the vertebral end plates.^3,⁴

FIGURE 265-3 The intervertebral disk. The fibers of the annulus fibrosus are oriented radially and in opposite directions throughout several layers. A, The nucleus pulposus (dashed outline) is contained (in nonpathologic situations) by the annulus fibers. B, Bearing of an axial load results in an even distribution of force. C, An eccentrically borne load results in bulging of the annulus fibrosus on the concave side of the resultant spinal curve and tension on annulus fibrosus on the convex side of the curve. The nucleus pulposus, however, tends to move in the opposite direction as the annulus fibrosus bulge when an eccentric load is borne (solid to dashed outline) (D).

In conjunction with the intervertebral disk, the facet joints provide additional load-bearing and stabilizing functions between segmental levels. Their orientation (Fig. 265-4) serves to facilitate or limit degrees of motion (Fig. 265-5) and therefore plays an important role in spinal stability. The cervical facets are coronally oriented and resist translation while facilitating flexion, extension, and rotation. Conversely, the lumbar facets are sagittally oriented (with the exception of L5-S1) and resist rotation while allowing significant flexion and extension. The thoracic facets are intermediately oriented and thus provide an “intermediate” restriction of translation and rotation. Both extension and ventral translation tend to load facets, whereas flexion and dorsal translation unload them. Degeneration of the intervertebral disk, loss of disk height, and alterations in sagittal alignment result in greater load transfer to the facet joints.

FIGURE 265-4 Facet joint orientation: relative coronal plane orientation in the cervical region (A), intermediate orientation in the thoracic region (B), and relative sagittal orientation in the lumbar region (C). The facet joint orientation changes substantially in the lumbar region; here, the facet joint angle (with respect to the midline) is depicted versus the spinal level (D).

FIGURE 265-5 Segmental motions allowed at the various spinal levels (combined flexion and extension, pink line; unilateral lateral bending, red line; and unilateral axial rotation, gray line).

The spinal ligaments provide passive stabilization of the vertebral column. Their bone-to-bone interface and elastic properties provide both tension band and translational support. The tension band contribution to spinal stability is related to both the ligament’s tensile strength (Fig. 265-6) and the moment arm through which it acts. As discussed later, the moment arm is the perpendicular distance from the instantaneous axis of rotation (IAR) to the applied force vector. The amount of resistance (counterbending moment) that a ligament provides is proportional to its distance from the IAR (Fig. 265-7).

FIGURE 265-6 Failure strength of spinal ligaments versus the spinal region. ALL, anterior longitudinal ligament; CL, capsular ligament; ISL, interspinous ligament; LF, ligamentum flavum; PLL, posterior longitudinal ligament.

FIGURE 265-7 A, Relative lever arm (moment arm) length of ligaments causing flexion (or resisting extension). B, The ligaments and their effective moment arms. Note that this length depends on the location of the instantaneous axis of rotation (IAR). An average” location is used in this illustration. ALL, anterior longitudinal ligament; black dot, IAR; CL, capsular ligament; ISL, interspinous ligament; LF, ligamentum flavum; PLL, posterior longitudinal ligament.

As opposed to the skeletal muscles, which generate long bone motion and span one or two articulations, the paraspinous musculature (and associated abdominal musculature) spans multiple segments (Fig. 265-8). The primary function of the paraspinous musculature is to stabilize the spinal column rather than produce motion. An exception is the action of the erector spinae muscles when arising from a forward flexed position. In general, any imbalance in muscular forces causes movement about an axis. Conversely, a balancing of muscle and other intrinsic forces about an axis results in no net movement. The ventral abdominal musculature is critical in counterbalancing the erector spinae muscles to provide stability.

FIGURE 265-8 Effects of muscles on stability. A, Muscles provide stability by virtue of the orientation of their attachments to the spine. B, In some situations, as with the rectus abdominis muscle, the muscle may influence spinal movement indirectly (i.e., without direct attachment to the spine). Similarly, this muscle (as well as others) may stabilize the spine by balancing opposing muscle function, thereby resulting in no movement. A significant degree of stability is thus provided. Lateral bending is achieved by the contraction of muscles attached to the lateral aspect of the spine; for example, the quadratus lumborum, attached to the right and left muscle actions, likewise results in no movement.

(From Benzel EC. Stability and instability of the spine. In: Benzel EC, ed. Biomechanics of Spine Stabilization. Rolling Meadows, IL: AANS Publications; and New York: Thieme; 2001:40. Reprinted by permission.)

The rib cage, acting as a barrel attached to the spine, adds significant stability to the upper and middle thoracic segments. Both the costovertebral and costosternal joints are essential to this contribution (Fig. 265-9).

FIGURE 265-9 Illustration of the stability provided to the spine by the rib cage. A, A spine without a rib cage can bend excessively. B, Addition of the rib cage moderately increases stability. C, Sternal attachments are required for achievement of the full stabilization potential of the rib cage. Removal of the effects of either the sternum or the ribs causes a significant diminution in stability.

Basic Biomechanical Principles

Biomechanical analysis assesses the effects of energy and forces on biologic systems by using physics and physical principles that have well-accepted definitions. A reference for many of these definitions can be found in Table 265-1.^5,⁶

TABLE 265-1 Basic Biomechanics Definitions

Kinematics	Study of the motion of objects without considering the factors that cause or affect the motion. The latter is the subject of dynamics.
Momentum	Product of mass and velocity
Moment	Circular force creating a rotational vector around an axis
Torque/bending moment	Product of the force applied to the lever arm multiplied by the perpendicular distance from the axis
Coupling	When more than one non-collinear force acts about the same axis and the resultant force moment is the sum of the individual forces
Stress	Force/load applied to an object divided by its cross-sectional area
Strain	Change in length of an object secondary to a deforming force
Stress/strain	Aids in defining an object’s intrinsic material properties
Stiffness	Relationship of stress or force and strain or deformation
Deformation	Change in shape or size secondary to stress and strain on an object from applied forces and moments. It is a structural property of a material that depends on the shape, size, and intrinsic material properties.
Elastic deformation	Occurs when strain on a material is totally recovered once the stress is removed
Plastic deformation	Occurs at the point where stress is no longer proportional to strain
Yield point	Point at which elastic deformation becomes plastic deformation
Ultimate tensile strength/breaking point	Point at which an object fails
Strength	Maximum stress that a material can sustain—coincides with the area under the stress-strain curve to the point of its ultimate tensile strength
Intrinsic material properties	Independent of an object’s shape and size—thus, its study requires that the effect of the object’s shape and size (geometry) be eliminated.
Ductile	Materials with intrinsic properties that allow permanent deformation before failure
Brittle	Materials with intrinsic properties that cause failure before permanent deformation
Hooke’s law	The degree of elastic deformation of a solid object is proportional to the deforming force, and the elastic modulus is a measure of the deformability of a solid object.
Isotropic objects	Intrinsic material properties independent of the direction of loading and with a randomly dispersed internal structure (metal, glass, plastic)
Anisotropic objects	Intrinsic material properties dependent on the direction of loading and with an orderly internal structural arrangement (bone, intervertebral disks, ligaments/tendons)