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Biomechanics of Motion Preservation Techniques


Summary of Key Points

  • Motion-preserving surgery has been developed in an attempt to reduce the likelihood of intervertebral disc degeneration at segments adjacent to a fusion.

  • The functional spinal unit (FSU) is defined as the smallest motion segment of the spine that exhibits biomechanical characteristics representative of the physiological motion of the whole spine. As the disc degenerates, a cascade of events ensues that eventually may result in symptomatic degenerative disc disease.

  • A dysfunctional motion segment is defined as an FSU that, for one reason or another, can no longer bear and resist the loads placed on it during daily activity without exhibiting some degree of abnormal motion, and often pain.

  • Fusion changes the biomechanics, in that the loads that were previously borne by the fused segment are now shared with the adjacent intact segments. This, many believe, is the driving force behind adjacent segment degeneration.

  • The ideal motion-sparing implant replicates the anatomy, motion, and mechanics of the intact, healthy FSU.

  • Developments in total disc arthroplasty devices have shown significant improvement, and these implants closely replicate the biomechanics of the normal intervertebral disc.

  • At long-term follow-up intervals, cervical disc arthroplasty has been shown to have a decreased rate of adjacent segment surgery compared with anterior cervical discectomy and fusion.

  • Compared with lumbar fusion, lumbar disc arthroplasty has been shown to have a decreased rate of adjacent level disease; however, further research is necessary to definitively compare the two procedures.

For many years, cervical and lumbar arthrodesis has been the gold standard of treatment for a wide range of degenerative, traumatic, deformational, and oncological 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. 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, 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 replicates the biomechanical properties of the intact FSU, the incidence of ASD would be expected to decrease. It is worth mentioning, however, that the exact cause of adjacent segment disease is unknown, and a large component may be related to a patient’s predisposition to degenerative disc disease.

Laminectomy, laminoplasty, and laminoforaminotomy are motion-preserving surgeries. These techniques allow for neural decompression while preserving segmental spinal motion. These techniques minimize bone removal, preserve the facet joints and most 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. There is, however, a limited number of pathological processes for which these techniques are 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.

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 physiological motion of the whole spine. It is synonymous with the spinal motion segment. The FSU consists of two vertebrae (including their facet joints), the intervertebral disc, and the associated supportive ligaments.

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 produces 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. 16.1 ). 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 usually 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 spine ( Fig. 16.2 ). In healthy individuals without axial pain the IAR of the cervical motion segment is located in the posterior half of the inferior vertebral body. In regards to the cranial–caudal location, at lower levels of the cervical spine the IAR is more cranial and closer to the intervertebral disc. , In the lumbar spine the axis of rotation is also within the posterior half of the inferior vertebral body, with the L5‒S1 axis being the most posterior. In regards to the cranial–caudal location of the axis, the location is at or near the superior endplate of the inferior vertebral body, except with L5‒S1, where the axis of rotation is located within the L5‒S1 intervertebral disc.

Fig. 16.1, There are 12 potential movements about the instantaneous axis of rotation: two translational movements about the x , y , and z axes, and two rotational movements around each axis.

Fig. 16.2, 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.

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. 16.3 ). 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.

Fig. 16.3, 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 (CD) is entered. At this point, increased stress results in permanent deformation of the solid up to the point of failure, when the solid yields (D).

Intervertebral Disc

The intervertebral discs contain a peripherally located annulus 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 annulus are arranged radially in orthogonal directions about the vertebral body interspace between the end plates. The annulus contains two types of laminated (mostly collagenous) fibers: radial fibers that attach to the cartilaginous end plates (called inner fibers), and a second set that attach 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. 16.4 ). 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.

Fig. 16.4, Annular 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 annular fibers are lax, torsional resistance is lost ( C ), and subluxation may ensue ( D ).

The pristine intervertebral disc can resist significant axial loads; 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 annulus 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 annular bulge) ( Fig. 16.5 ).

Fig. 16.5, 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 annulus. Asymmetrical loading ( C ) causes the annulus to bulge on the same side as the force application. However, the nucleus pulposus will migrate in the opposite direction ( D ).

Studies of the nucleus pulposus in vitro demonstrate its importance in intervertebral disc biomechanics. Mechanical denucleation of intervertebral discs in cadaveric lumbar spinal 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 annulus has a chance to be engaged in a more effective tensile load-bearing state.

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