Chapter 164 Dorsal Dynamic Spine Stabilization
An estimated 1 million spine procedures are performed each year in the United States.1,2 The primary strategies used in these procedures include decompression of neural elements or stabilization using rigid fusion techniques. Spine stabilization surgeries are performed to correct or prevent deformity, diminish pathologic motion, compensate for iatrogenic destabilization, or eliminate pain generators. The introduction of rigid instrumentation to these procedures dramatically improved the rate and success of fusion.3,4 The use of rigid instrumentation provides an optimal environment for intervertebral or posterolateral fusion to occur by immobilizing two or more spinal segments. In addition, spinal instrumentation has allowed for more aggressive decompression and direct reduction or stabilization of spondylolisthesis.
The initial outcomes of fusion surgeries dramatically improved with rigid instrumentation, and long-term consequences have since been recognized.4 Patients undergoing arthrodesis are exposed to numerous long-term morbidities, including pseudarthrosis, fixed sagittal alignment that cannot adapt to postural change, and adjacent-level degeneration. The risk to adjacent levels is the most concerning of these because the degenerative changes are noted to be significantly accelerated compared with the natural history.5,6 Rigid titanium or stainless steel constructs have a supraphysiologic degree of stiffness that far exceeds normal bone or a noninstrumented fusion. This stiffness at the instrumented levels has a direct relationship to the stress load on the adjacent disc and facet joints.4,7 Over time, this additional stress can result in segment hypermobility, osteophyte formation, facet hypertrophy, disc herniation, and stenosis.4
Although the reported findings of radiographically evident adjacent-level disease are widely variable in the literature, the incidence of symptomatic adjacent-level disease is higher in patients who have undergone an instrumented fusion (12.2–18.5%) than in patients who have undergone a noninstrumented fusion (5.2–5.6%) in the lumbosacral spine.4,8,9 Other contributors to adjacent-level disease include muscular and ligamentous disruption, bone removal, and the underlying disease process; however, the supraphysiologic biomechanical stress created by rigid fixation seems to be the most influential factor.3–57 Many patients subsequently need further surgery that may require an extension of their fusion.10
Posterior dynamic stabilization of the spine is a rapidly evolving technique in thoracolumbar spine procedures. The concept of dorsal dynamic stabilization was introduced as a potential alternative for treating spine disorders that would avoid the long-term morbidities associated with instrumented fusion constructs. These constructs are likely far more rigid than is necessary to augment a fusion. One objective of dynamic dorsal instrumentation is to move the construct toward a more optimal degree of stiffness that reduces the risk of developing adjacent-level disease, while still promoting fusion. This construct would provide sufficient immobilization for fusion, while diminishing the degree of supraphysiologic stress on adjacent joints. The ideal device would have an elastic modulus close to that of bone to replicate physiologic behavior best. The altered load transfer of the treated spinal segment theoretically would improve on the long-term results of rigid fusion.
An alternative goal of dynamic stabilization is to restore the function of segmental mobility by replicating the behavior and biomechanics of a healthy spinal segment. Intervertebral motion is maintained or restored, while restricting the extremes of movement or dampening the kinetic energy of motion.2 Dynamic stabilization systems may help spine pain syndromes by restricting movement to a range in which only normal loading may occur, preventing the spinal segment from reaching any position where abnormal loading generates pain.11 In addition, a dynamic stabilizing device must withstand physiologic static and dynamic loads in any plane.10
The advantages associated with using dynamic constructs are primarily anticipated over the long term. Implantation of most dorsal dynamic stabilizing devices has similar immediate postoperative effects to a rigid construct. Both approaches have the capacity to restore lumbar lordosis, sacral tilt, and foraminal diameter in the immediate postoperative period, showing that dynamic devices have the potential for equally good short-term results regarding sagittal spine alignment and decompression of neural elements.12 The only short-term advantage lies in the elimination of morbidities associated with harvesting autograft when the dynamic device is replacing a potential fusion construct.13
Pseudarthrosis is a significant factor associated with poor outcomes after surgical fusions. The incidence rate for pseudarthrosis varies widely depending on the location, surgical technique, and number of levels fused. A meta-analysis on pseudarthrosis after lumbar arthrodesis reported a rate of 14%.14 Most reported incidence rates of pseudarthrosis after circumferential fusion are less than 10%.13 Anterior cervical discectomy and fusion has a low rate of pseudarthrosis (<10%), although this is increased if multiple levels are involved. Regardless of this variability, pseudarthrosis is a clinically significant adverse outcome after spine fusion. Dynamic devices used in place of fusion constructs eliminate the potential adverse outcome of pseudarthrosis.
Although one postulated advantage of dynamic technology is the preservation of motion, the functional implications of this aspect are less relevant in the lumbosacral spine. However, retained mobility may contribute to the reduction of symptomatic adjacent-level disease—the most significant potential advantage of dynamic dorsal stabilization.
Because the prevalence of adjacent-level disease is so variable in the literature, it is difficult to make generalizations regarding its occurrence after fusion surgeries. The level, location, and length of the fusion all appear to have an impact on the likelihood of developing future degeneration at adjacent spinal levels. The reported incidence of the prevalence of adjacent-level disease during long-term follow-up after rigid spine fusion ranges from 32% to 36% in the lumbar spine.13 Regardless of the exact etiology, a significant percentage of patients are affected by this problem after a successful spine fusion.
Several retrospective studies evaluating various posterior dynamic devices have shown lower rates of reoperation for and prevalence of adjacent-level disease. The theoretical reduction of this problem has yet to be definitively proven in the literature, however, and long-term follow-up of randomized controlled trials is necessary to determine absolutely the advantage of dynamic technology and the appropriate patient population for its application.10,15,16
The indications for dorsal dynamic stabilization vary depending on the device being considered and are likely to expand as this new technology evolves. The longest-standing indication has been for augmentation of interbody fusion. The typical adjunct to interbody fusion is rigid fixation with pedicle screws and rods. The supraphysiologic rigidity of these constructs creates stress shielding of the interbody graft and likely contributes to a certain portion of resulting pseudarthroses.17 A dorsal dynamic stabilization device allows for an increase in anterior load sharing that augments fusion, while limiting any extremes of motion that could result in graft displacement.2
Dynamic devices may also be used in an iatrogenically destabilized spine to provide controlled motion. Decompressive surgeries that involve laminectomy and disruption of the facet joint are used to treat lumbar stenosis and lateral recess stenosis. If a significant amount of encroachment of the superior facet is involved, the decompression required may lead to iatrogenic destabilization. Typical management options for such a situation include primary fusion of the spine at the affected levels or observation for the development of sagittal plane imbalance or deformity.2 Dorsal dynamic stabilization provides another option after a potentially destabilizing procedure and could allow for controlled mobility at the treated spinal levels, while avoiding the need for arthrodesis.18
At the present time, the most widely applied use of this technology is for prevention of fusion-related sequelae. Adjacent-level disease and pseudarthrosis are the most commonly considered problems; however, rigid arthrodesis may also lead to loss of lumbar lordosis if excessive distraction is used. The loss of lumbar lordosis can cause a fixed sagittal imbalance or flat-back deformity that may create symptoms of fatigue, gait disturbance, or construct failure. When applied for this indication, dynamic stabilization devices may avoid the need for reintervention from development of any of these adverse sequelae.
Patients with osteoporosis or osteopenia may benefit from dorsal dynamic stabilization. These patients are especially prone to construct failure when rigid stabilization devices are used because the bone-metal interface creates significant bony destruction. Using less rigid fixation may be ideal in these patients and reduce the incidence of construct failure.2
A controversial and poorly understood indication for the use of dorsal dynamic stabilization involves its use to protect and restore degenerated facet joints and intervertebral discs. Rather than attempting to remove or destroy the pain generators in the disc or facet joints via fusion, the placement of a dorsal dynamic device may shield the disc and facet joints from destructive motion, allowing for a reduction in inflammatory processes and permitting self-repair mechanisms to operate.2 This application may become a more valid indication as understanding of the pathophysiology of back pain improves.
Finally, complete circumferential reconstruction of the motion segment may be possible with combined technologies. The presence of facet disease is a contraindication for total disc arthroplasty because this procedure alone is thought to accelerate facet degeneration. A dorsal dynamic stabilization device may be used in conjunction with a total disc replacement, allowing for reconstruction of all mobile joints in a spinal segment.2
The concept of dynamism involves controlled and limited deformation of the spine. An ideal dynamic implant prevents undesirable deformation, while allowing desirable deformation and controlling its extent.
Dynamic implants that are not intended for fusion need to bear substantial and repetitive loads for many decades. A thorough understanding of spinal biomechanics must be involved in the engineering of these implants. The vertebral motion segments are designed to provide stability, mobility, and load transmission to the spinal column. The primary joint responsible for load bearing and stability is the intervertebral disc. The secondary load-bearing and stabilizing structure is the facet joint. The anatomy of the facet joints varies depending on the region of the spine being considered.
The cervical spine facet joints are oriented in a plane that is midway between a coronal and an axial plane. They resist extension and anterior translation.19,20 The facet joints are loaded by anterior shear forces during extension in the cervical spine. Facet joints in the lumbar spine are oriented in a plane that is midway between the sagittal and coronal planes with a slight anterior incline. Lumbar facet joints resist extension and bear large compressive loads while in extension. During flexion or axial compression, lumbar facets are unloaded.20 Anterior shear forces load the lumbar facet joints, whereas posterior shear forces unload them. The prevalence of substantial and repetitive physiologic loads on the lumbar facets is clinically relevant given their source as major pain generators.19 The posterior supraspinous and intraspinous ligaments resist posterior translation and bear tensile loads.
The biomechanics of dynamic posterior stabilization constructs vary among devices. Typically, the pain-generating tissue is left in situ, while the device restricts certain types of motion and alters load transfer.19 Engineering of posterior dynamic stabilization devices must be done with the kinematics of a functional spinal unit in mind. The posterior and anterior elements of a motion segment move in harmony with each other, and a significant disruption of one could result in excessive loads being placed on the other. Dynamic implants should be constructed with the capacity to maintain range of motion similar to a healthy spinal segment. Excessive motion could lead to degeneration that may include facet arthrosis or hypertrophy, ligamentum flavum hypertrophy, or disc degeneration. Inadequate motion prevents the goals of the dynamic device from being met.19
Some applications of dynamism apply the phenomenon of Wolff’s law to promote bone healing. Wolff’s law states that bone remodels and becomes stronger under increased loads to resist those loads. The reverse is also true, and decreasing the load on a bone results in its weakening. When applied to spine fusions, this theory states that transmitting forces to an intervertebral graft and avoiding stress shielding increase the rate and success of arthrodesis.18 Rigid posterior instrumentation may unload an intervertebral graft, resulting in fusion failure. However, a dynamic device loads the graft when used as a posterior tension band supplement, increasing the likelihood of fusion.
Avoiding adjacent-level disease is one of the driving goals behind the development of dynamic devices. The biomechanics behind this adverse effect have been studied in cadaver and animal models.19,21,22 Fusion of a spinal segment increases the stress on the anulus and end plates of adjacent levels and increases intradiscal pressures. Restricted motion at the fused segment also leads to higher mobility at adjacent segments. In flexion and extension, the loss of mobility across a rigidly fused segment is compensated predominantly in the first rostral adjacent segment.4 In a dynamic stabilization, the compensation is distributed across the first and second rostral segments and in the caudal adjacent segment.23 Additionally, the fixed sagittal alignment prevents accommodation of regional alignment changes that occur with different postural positions. If a segment is fused with suboptimal sagittal alignment, degeneration of adjacent levels may be accelerated further.19
A wide variety of dorsal dynamic stabilization devices are in various stages of development and clinical investigation and use. Any implant that is not rigid and allows some motion provides some degree of dynamism. An absorbable implant could be considered dynamic because it permits delayed spine movement after its structural integrity is lost.24 Absorbable implants in the posterior spine, in particular, the lumbosacral spine, are rare, and the focus of this classification is on deformable implants. Deformable implants may permit either angular deformation or axial deformation. The generalized objectives considered when designing these devices include avoidance of fatigue failure, maintenance of normal spine resting posture without excessive kyphosis or lordosis, prevention of abnormal load distributions, and easy salvage in case of construct failure.
Some earlier dorsal dynamic implants that were used in the cervical spine include cerclage wiring with submaximal tension and lateral mass plates with nonfixed moment arm cantilevers. These types of constructs allowed for kyphotic deformation and encouraged fusion by permitting load sharing through the bone graft.
Available dorsal dynamic neutral fixation devices allow for limited movement to occur between the screw and the plate. The most common mechanism is a screw head that can pivot in a concave bed within the plate, allowing a “rocking” motion. Although this type of permissive motion may minimize the chance of failure at the screw-plate interface, the cyclic loading at the screw-bone interface causes degradation, and screws have the potential to pull out under flexion or axial loads. The weak point with this type of fixation is the screw-bone interface. In contrast, in rigid techniques, the weakest point is the screw-plate or screw-rod interface.25 Considering this difference, a dynamic neutral plating technique in a patient with osteoporotic bone would lead to a high likelihood of construct failure given the poor screw pull-out resistance and the dynamic nature of the construct.25 A dorsal dynamic neutral construct should always be coupled with sufficient axial load resistance to be stable in flexion. If either the intrinsic axial load-resisting abilities are insufficient or interbody support is not present, excessive flexion can lead to construct failure and potential spinal canal compromise.