Chapter 159 Biomechanics of Motion Preservation Techniques

For many years, cervical and lumbar arthrodesis has been the gold standard of treatment for a wide range of degenerative, traumatic, deformational, and oncologic spinal disorders. Although spinal arthrodesis has been used with great success, from a biomechanical perspective, it significantly alters the regional and global balance of physical forces and moments. As a consequence, several studies have demonstrated accelerated degenerative changes at levels directly adjacent to rigid spinal fusions.^1–3 This is commonly termed adjacent-segment degeneration (ASD). Specifically, it is thought that the stiffness imposed by a rigid fusion on an adjacent vertebral functional spinal unit (FSU) alters stress transfer between the vertebral segments and predisposes adjacent segments to accelerated degeneration. As a result of this, motion-sparing technologies have emerged in an attempt to preserve the native motion of the spinal unit, predominantly at the disc interspace. Theoretically, if a motion-sparing implant closely simulates the biomechanical properties of the intact FSU, the incidence of ASD would be expected to decrease.

The earliest and most traditional form of motion preservation surgery came in the form of laminectomy, laminoplasty, and laminoforaminotomy. These techniques were designed to treat underlying spinal pathology while preserving as much of the natural motion of the spine as possible. They involve a small amount of bone removal and preservation of the facets and many of the supporting ligaments, and do not violate the disc. In particular, the laminoforaminotomy has been shown to induce the least disruption in normal spinal mobility and stiffness compared with traditional laminectomy.⁴ There is, however, a limited range of pathologic processes for which these techniques are indicated (and effective).

To best understand the biomechanics of motion preservation techniques, one must appreciate the biomechanical forces in play in and about the intervertebral disc, with particular attention to load transfer, intradiscal pressures, and the neutral zone. Biomechanical principles of the degenerating disc, including the concepts of dysfunctional motion and ASD, must also be considered. Once these concepts are understood, one can then examine the biomechanical principles behind motion preservation technologies.

Biomechanics of the Functional Spinal Unit

The FSU is defined as the smallest motion segment of the spine that exhibits biomechanical characteristics representative of the physiologic motion of the whole spine. Most authorities consider it to be synonymous with the spinal motion segment. It consists of two vertebrae, the intervertebral disc, the facet joints, and the associated supportive ligaments. A firm grasp of the biomechanics of the FSU is necessary to design implants aimed at replicating physiologic motion.

Physical Principles

A vector is a force oriented in a fixed direction in three-dimensional space. All forces acting on the spine can be described in terms of their component vectors. A vector may act on a lever (moment arm), resulting in a bending moment. A bending moment applied to a point in space causes rotation (or a tendency to rotate) about an axis, called the instantaneous axis of rotation (IAR). Movement about the IAR can be described in terms of the cartesian coordinate system (Fig. 159-1). The IAR is the point or location in three-dimensional space about which each vertebral segment rotates at any given instant. The location of the IAR is not fixed, but rather is variable (mobile) depending on the intrinsic curvature of the spine and any other intrinsic or extrinsic forces that may act on the spine⁵ (Fig. 159-2).

FIGURE 159-1 There are 12 potential movements about the instantaneous axis of rotation: 2 translational movements about the x, y, and z axes, and 2 rotational movements around each axis.

(From Benzel EC: Biomechanics of spine stabilization, Rolling Meadows, IL, 2001, American Association of Neurological Surgeons, distributed by Thieme, New York, with permission.)

FIGURE 159-2 The bending moment (M) may be considered mathematically as the product of the distance (D) from the instantaneous axis of rotation (IAR) to the point of perpendicular application of force (F) and the magnitude of the force.

(From Benzel EC: Biomechanics of spine stabilization, Rolling Meadows, IL, 2001, American Association of Neurological Surgeons, distributed by Thieme, New York, with permission.)

When several forces act on a solid with a resultant net force of zero, the solid is deformed. A stress-strain curve describes this relationship (Fig. 159-3). Stress is defined as the force applied to an object (load). Strain is defined as the response of the object to the force (deformation). The neutral zone or zone of nonengagement describes the reaction of a solid to stress before deformation of that solid is realized. When the neutral zone is exceeded and stress continues, the solid enters into the elastic zone. The elastic zone describes applied stress (usually of lower magnitude) that results in strain that is completely recoverable after removal of stress. As the applied force increases, the elastic limit is reached and the plastic zone is entered. At this point, increased stress results in permanent deformation of the solid up to the point of failure, when the solid yields.

FIGURE 159-3 The neutral zone (AB) or zone of nonengagement describes the reaction of a solid to stress before deformation of that solid is realized. When the neutral zone is exceeded and stress continues, the solid enters into the elastic zone (BC). As the applied force increases, the elastic limit (C) is reached and the plastic zone is entered (CD). At this point, increased stress results in permanent deformation of the solid up to the point of failure, when the solid yields (D).

(From Benzel EC: Biomechanics of spine stabilization, Rolling Meadows, IL, 2001, American Association of Neurological Surgeons, distributed by Thieme, New York, with permission.)

Intervertebral Disc

The intervertebral discs contain a peripherally located anulus fibrosus and a centrally located nucleus pulposus. The cartilaginous end plates form the rostral and caudal limits of the intervertebral discs. The fibers of the anulus are arranged radially in opposite directions about the vertebral body interspace between the end plates, whereas the nucleus pulposus is contained within the anulus. The anulus contains two types of laminated (mostly collagenous) fibers: radial fibers that attach to the cartilaginous end plates (called inner fibers) and to the cortical bone on the edge of the vertebral bodies (called Sharpey fibers). These fibers are oriented roughly 30 degrees with respect to the end plate and opposite to each other, which gives the disc the ability to resist rotational forces (Fig. 159-4). The nucleus pulposus is a gel-like remnant of the embryonal notochord. It is composed of reticular bands surrounded by a thick mucoid ground substance.

FIGURE 159-4 Anular fibers are oriented 30 degrees with respect to the end plate. This orientation resists torsion (A), but does not effectively resist compression (B) or distraction. If the anular fibers are lax, torsional resistance is lost (C) and subluxation may ensue (D).

(From Benzel EC: Biomechanics of spine stabilization, Rolling Meadows, IL, 2001, American Association of Neurological Surgeons, distributed by Thieme, New York, with permission.)

The intervertebral disc can resist significant axial forces; however, this ability decreases with age. A pure (i.e., central) axial load causes symmetrical deformation of the disc because the intradiscal pressures are distributed symmetrically. In the presence of flexion, extension, or lateral bending (i.e., eccentric) forces, the intradiscal pressures are distributed asymmetrically and the normal disc will deform in a relatively predictable manner. The anulus bulges on the side of the deformation and stretches on the side opposite to the load, whereas the nucleus pulposus tends to move away from the area of applied force (in the opposite direction from the anular bulge)⁵ (Fig. 159-5).

FIGURE 159-5 The normal intervertebral disc (A). Pure axial loads (B) cause an even distribution of force and symmetrical displacement of the nucleus pulposus and bulging of the anulus. Asymmetrical loading (C) causes the anulus to bulge on the same side as the force application. However, the nucleus pulposus will migrate in the opposite direction (D).

(From Benzel EC: Biomechanics of spine stabilization, Rolling Meadows, IL, 2001, American Association of Neurological Surgeons, distributed by Thieme, New York, with permission.)

Studies of the nucleus pulposus in vitro demonstrate its importance in intervertebral disc biomechanics. Mechanical denucleation of intervertebral discs in cadaver lumbar spine segments results in increased range of motion and neutral zone compared with intact discs. Removal of the nucleus also results in decreased disc height and segment stiffness. Interestingly, the nucleus is most effective at lower loads, before the anulus has a chance to be engaged in a more effective tensile load-bearing state.⁶

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