Dynamic Stabilization of the Lumbar Spine: Indications and Techniques

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Chapter 168 Dynamic Stabilization of the Lumbar Spine

Indications and Techniques

William Welch, Boyle C. Cheng, Tariq E. Awad, M. Mason Macenski

Fusion is, has been, and will continue to be a widely accepted method of controlling pain and associated neurologic deficits due to lumbar spinal instability. It is widely recognized that unwanted consequences of otherwise successful lumbar fusion occur with some frequency. These “complications” include transfer of forces to adjacent segments, possibly resulting in expedited lumbar spondylosis, stenosis, and further instability and potentially necessitating further surgery. Less invasive surgical fusion techniques may reduce associated surgical trauma but ultimately do not alter the basic biomechanical changes associated with fusion.

Over the past 20 years, developments have been under way to produce spinal systems that control a painful “dysfunctional” spinal unit without causing fusion. Examples of this include facet replacements, interspinous process devices, and pedicle screw–based motion-controlling systems. These latter systems include motion-limiting systems (e.g., the Graf device), mechanical micromotion systems (e.g., Scient’x Isobar TTL), and the Dynesys system. This latter system is the most commonly utilized system in the world today and is reviewed in detail in this chapter. While the theory of these systems is to restore stability yet provide motion, their efficacy has not been completely proved.1 the U.S. Food and Drug Administration (FDA) has not approved any pedicle screw–based system as a stand-alone device for nonfusion applications. The systems available for use, including the Dynesys system and others, are cleared through the FDA 510(k) process for fusion applications as being substantially equivalent to existing pedicle screw–based fusion augmentation devices.

Biomechanical Rationale

The clinical advantage or utility of new technologies (i.e., motion preservation devices) may initially be unclear particularly in the absence of long-term, prospective, randomized, well-controlled clinical data. As the implants become commercially available, the suitability of specific patients for a particular implant may or may not be clear due to the generally recognized clinical inclusion criteria. For the subset of the population that the spine surgeon deems to consist of viable candidates for surgery, a decision tree should consider at least two factors: the severity of the disease and the surgical approach for optimal postoperative success. With the aid of a methodic, systematic approach in the identification of treatments and devices, the appropriate spinal implant technology suitable for the patient should become more evident. The information garnered from biomechanical studies may play a role in treatment of spinal pathologies but entirely depends on the patient’s symptoms, and triage requires a strategy that includes a multivariate analysis for the most effective clinical outcomes.

Both the spine surgeon and the implant design engineer benefit substantially from a clear understanding of the native function of the spine and the contribution of each of the spinal elements. The motion of the spine can be studied in the most basic form at a single level that begins with the functional spinal unit (FSU). The FSU comprises two vertebral bodies with their articulations, including the intervertebral disc, as well as the two posterior facet joints. The intervertebral disc forms an integral part of the FSU and has a high potential of becoming problematic, especially with age. Anatomically, the disc consists of fibrous layers arranged in alternating lamellar structures, in conjunction with a gelatinous inner core referred to as the nucleus pulposus. The other important articulations within the FSU are the facet joints, which are also susceptible to disease. Facets, in the normal condition, play a role in controlling the motion of the FSU. This three-joint complex within each FSU controls the kinematic response to load. The primary modes of loading taken into consideration are those associated with axial compression, flexion–extension bending, lateral bending, and axial torsion, as shown in Fig. 168-1.

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FIGURE 168-1 Demonstration of biomechanical testing parameters.

Over time, as degeneration occurs, the disc becomes fibrotic and less able to dissipate and distribute loads. Consequently, nonphysiologic loads are then distributed to the vertebral endplates and the annulus of the disc, which may lead to morphologic endplate changes and annular fissuring. With the onset of the degenerative cascade, both the intervertebral disc and then facets becomes compromised. Currently, it is unclear whether the onset of anterior column degeneration leads to posterior degeneration, and vice versa. Regardless, the degeneration within the FSU may lead to the inability to withstand even physiologic loads, and eventually, depending on the severity, instability may develop. Both clinically and biomechanically, instability can be defined by the inability of the FSU to control physiologic displacement. With instability, the neurologic structures are prone to impingement and injury. Instability of the intervertebral discs shifts greater-than-normal motion and load to the facet joints and ligamentum flavum. With time, these structures all undergo hypertrophy with narrowing of the central neural canal, as well as of the lateral recesses and neural foramina.

Fusion has been the gold standard in the surgical treatment of certain spinal pathologies, including degenerative disc disease. Fixation instrumentation including screws, rods, plates, and intervertebral cages have served as adjuncts to fusion and positively contributed to radiographic outcomes (i.e., radiologic fusion assessment). However, the techniques have not necessarily improved patient clinical outcomes—hence, the advent of posterior, dynamic stabilization constructs that provide immediate stability but not necessarily rigidity that would allow transmission and propagation of nonphysiologic loads to adjacent segments.

Recent attention has been focused on preventing adjacent-level degeneration, as well as obtaining a positive functional outcome in the treatment of degenerative disc disease. To this end, numerous implants have been developed with the aim of effectively controlling physiologic loading and ensuring the appropriate kinematic motion at the index level. Posterior pedicle-based dynamic stabilization—as well as other motion preservation devices, such as total disc replacement, nucleus augmentation, total facet arthroplasties, and combinations thereof—are at various stages of evaluation and consideration, including as potential alternatives to fusion.

There has been a proliferation of pedicle screw–based designs over the past 20 years. These systems all have some degree of similarity in that they employ pedicle screws connected to a rod, which allows some form and degree of motion. While many of the early products, in theory, provided true motion preservation (i.e., maintenance of a significant proportion of the preoperative segmental motion), this has not been proved clinically. Most available systems appear to reduce preoperative motion. As such, the concept of a true pedicle-based motion preservation device seems faulty. Instead, these devices appear to limit segmental motion but provide varying degrees of “stability” to the spine (biomechanically equivalent to increased stiffness in one or more of the six generally assessed biomechanical ranges of motion). For the purposes of this chapter, devices designed for purposes other than fusion are considered “nonfusion stabilization” devices.

Another design concern that arose during testing of these types of devices was that of robustness. Essentially, these devices are cantilever in design. They transfer loads from the vertebral bodies through the pedicle screws and connecting units. Although this may “offload” the anterior and middle columns of the spine to a certain extent, the devices are theoretically required to bear this load for the life of the patient. Interestingly, traditional pedicle screw devices used to augment fusion are FDA approved as “temporary” fixation devices. They are tested and expected to perform for 2 years, after which they may be removed. Complicating the testing procedures for these devices, the number of cycles that the lumbar spine undergoes in the normal patient over the course is unknown. Estimates vary from 250,000 to 2 million cycles per year.

A robust nonfusion stabilization device may need to withstand millions of cycles without breakage to last for the life of the patient. The device would need to do this without producing harmful wear debris or the patient suffering implant failure. This is a difficult design paradigm to fulfill.

Another design parameter that a potential manufacturer needs to address is that of the amount of stiffness required in an individual patient. Many in vitro biomechanical studies have demonstrated that the degree of inherent stiffness in the spine varies among individual specimens. Generally, older cadaveric spine specimens are more rigid than younger spines. Clinically, this has traditionally been assessed by the Kocher Clamp Test, described by Albert Key in 1944 (personal communication). Essentially, the surgeon grasps the posterior spinous processes and pulls. This provides a tactile indication of the spinal stiffness, which Key used to compare to prior experiences. A more objective measure of assessment was described by Brown et al., who utilized an FDA-approved intraoperative spinal stiffness gauge that measured force displacement (Spinal Stiffness Gauge, Mekanika, Boca Raton, FL)2 (Fig. 168-2). Their findings of 655 motion segments demonstrated wide variability in degrees of stiffness. Generally, the L5–S1 segment was stiffer than L4-5, which was stiffer than L3–4. Male patients had stiffer spinal segments than did female patients. Older patients had stiffer spinal segments than did younger patients. Finally, stiffness was found to decrease by 20% following decompression. These basic findings raise the issue of patient-specific requirements and the potential need to customize implants based on intraoperative findings.

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FIGURE 168-2 In vitro biomechanical testing probe placed between posterior spinous processes to determine spinal stiffness.

Another design concern is the amount of stability that an individual patient may require to achieve the best clinical outcome. That is, all other variables being equal, it remains unknown as to the required stiffness in an individual situation that will lead to pain relief and enough stabilization so as to prevent recurrence of symptoms and clinical (and/or potential radiographic) instability.

Nonfusion pedicle screw–based stabilization systems have the potential clinical advantage of system revision. Should the nonfusion construct fail to provide the expected clinical improvement, these systems can be converted to fusion with the addition of bone and potentially interbody spacers to promote fusion. Another potential advantage to nonfusion systems is that they can be placed using less invasive techniques with concordantly less tissue disruption.

Most available systems provide the surgeon with the ability to a perform fusion, a nonfusion, or a combination hybrid construct. This latter approach allows the surgeon to “top off” a fusion level with a nonfusion construct, with the thought that the nonfusion device may help mitigate the stresses on the adjacent level and reduce adjacent-level facet and disc disease.

Lumbar posterior, dynamic stabilization, motion preservation devices have the potential to address neurologic deficits without arthrodesis. Although the design intent shares similar clinical goals with fusion devices (i.e., relief of patient pain symptoms), an important consideration in the development of any motion preservation device is the need for a preponderance of evidence supporting the safety and efficacy of the device design. Pedicle screw–based, posterior, dynamic motion preservation devices rely on the perpetual integrity of the construct, whereas fixation devices have the luxury of becoming obsolete once the fusion mass forms and subsequent off-loading of the spinal implant occurs.

System Designs with Biomechanical and Clinical Outcomes

Complex biomechanical and clinical needs have been addressed by designers with varying degrees of success. As previously stated, all pedicel screw–based nonfusion stabilization devices are similar in that they anchor an implant that allows for some degree of mobility to pedicle screws. Perhaps the most basic design type was the Graf ligament. This system limited movement in flexion, and had good early clinical results, but ultimately it has not received larger clinical use.3

More complex designs have included multiarticulated metal systems that allow movements in all planes. These systems may include ball-and-socket components and spring assemblies. These systems were designed to meet many of the aforementioned biomechanical goals, but the systems were quite complex in their mechanical design. They also have not been widely applied in clinical settings.

The systems that are intermediate or simpler in their design complexity include the N-Hance spine system (Synthes, West Chester, PA), Scient’x Isobar TTL system (Alphatech Spine, Carlsbad, CA), Dynesys Spinal System (Zimmer Spine, Warsaw, IN), CD Horizon Legacy polyethylethylketone (PEEK) rods attached to titanium screws (Medtronics, Minneapolis, MN), and the Transition system (Globus, Audubon, PA). The Dynesys and Transition systems are similar in their design and are discussed later.

N-Hance Spine System

The N-Hance spine system is a unique system in that it comprises a solid titanium rod with a sliding component. The sliding component, which attaches to a pedicle screw, has polyurethane bumpers immediately above and below it. This system damps flexion and extension (biomechanically represented by an increase and decrease of the interpedicular distance) against the polyurethane stops (Fig. 168-3). The length of the fixed component of the rod can accommodate span multiple segments so that the system can be utilized for both rigid and nonrigid fixation. This combination of rigid stabilization (acting as a traditional pedicle screw fusion augmentation construct) and nonrigid stabilization (potentially utilized as a nonfusion spinal augmentation construct) is commonly described as a “hybrid” construct.4

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FIGURE 168-3 A, N-Hance spinal system with a solid rod (inferior) and a nonfusion component (superior). B, Demonstration of the N-Hance spine nonfusion system motion component.

The N-Hance system has been studied biomechanically. The salient feature of the system is that it is less rigid than the Dynesys system and allows for changes, per its design, in interpedicular distances. The system has been tested to 10 million cycles in vitro. Although this system has been implanted in a number of patients throughout the world, larger outcome studies are not available.

Isobar TTL

The Isobar TTL system was initially released in Europe and was introduced in the United States about a decade ago. The system has been cleared by the FDA as an adjunct to fusion but has not been tested in a premarket approval study. This device, like other devices that permit movement, is suggested to be beneficial to fusion augmentation procedures by allowing movement and load sharing, to some degree, with the anterior column. The FDA approval for this device is as a fusion adjunct with anterior column support. The device allows small amounts of motion through a coupling joint incorporated into the interpedicular rod (Fig. 168-4).

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FIGURE 168-4 A, Lumbar spondylolisthesis and degenerative disc disease corrected with a hybrid system in B. B, Isobar TTL system with an interbody spacer used in a hybrid construct for circumferential fusion at L4-5 and nonfusion at L3-4.

A recent biomechanical study utilizing a high-capacity, 1112N (250 pounds), subminiature load cell with endplate components designed to mimic an interbody spacer (utilized in a posterior transforaminal approach) were cut with a sawtooth-patterned, textured surface. Each specimen was implanted with the high-capacity subminiature load cell–integrated transforaminal lumbar interbody fusion (TLIF) construct. The frictional characteristics of the interbody implant reflected the condition following surgery but prior to the onset of bony ingrowth. The interbody spacer directly measured transmitted axial load through the L3–4 FSU; thus, the percentage of anterior column load sharing could be calculated in the axial compressive mode of loading. The removable endplates allowed the appropriate-size implant to be used for each FSU. The endplates were available in 1-mm increments up to 12 mm in height.

The results of this study demonstrated no statistically significant differences between Isobar TTL and rigid rod fixation in flexion–extension bending when compared through the range of motion (ROM) metric. However, in axial compression, a statistically significant, larger, mean anterior column load-sharing capability was observed in the Isobar TTL constructs compared to the rigid rod treatment measured by the load cell–integrated TLIF interbody cages. The dynamic stabilization device showed increased anterior column load sharing (69.2N ± 17.8N) when compared to the rigid rod (40.6N ± 20.2N) fixation (p = 0.024 with the Bonferroni adjustment). In essence the Isobar TTL provided immediate stabilization similar to that of a rigid rod but with significantly higher axial compressive load-sharing capability or increased anterior column support.

Clinically, when used as a fusion augment device with anterior column support, this device’s capacity for load sharing changes during the course of the healing process as the fusion mass becomes structurally competent. The Isobar TTL dynamic device can provide immediate graft loading, particularly in the axial compression. Over time, as the fusion mass becomes a competent load-bearing member within the FSU, the Isobar will become obsolete.

The TTL system has been utilized as a nonfusion device. No larger outcome studies have been published describing its efficacy.

CD Horizon Legacy PEEK

PEEK materials have received widespread use in the spine as interbody spacers. PEEK has appeal for bone applications because it is relatively inert and its modulus of elasticity falls between that of cortical and cancellous bone. The CD Horizon Legacy PEEK system incorporates rods, which are oval-shaped in cross section. These rods, when connected to polyaxial titanium pedicle screws, allow flexibility during spine movement. They provide stiffness similar to that of 5.5-mm titanium rods.

This system is most commonly utilized as a fusion adjunct. Anecdotal reports exist of rod fracture when these rods are used as fusion augments. There are no larger series reported with this device as a nonfusion instrumentation system.

Dynesys

The Dynesys system was developed in the early 1990s by Giles Dubois in France and has been surgically implanted in patients since 1994. The goals of the system were to provide spinal stabilization without the need for fusion, augment deficient ligamentous structures, maintain spinal alignment, unload the disc, preserve motion, and maintain robustness for the expected life span of the implant. This system became known as a “dynamic neutralization system,” a name that implies the preceding qualities.

The system is elegant in its simplicity. It consists of three components: pedicle screws, a spacer, and a tensioning cord (Fig. 168-5). The pedicle screws are made of titanium alloy and have a textured surface that promotes bone on-growth. Recently, hydroxyapatite-coated screws were made available to further bone adhesion and promote ingrowth, potentially reducing screw-loosening concerns. The Dynesys screws have fixed heads and are self-tapping.

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FIGURE 168-5 Dynesys system placed at the L3-4 and L4-5 spinal levels.

The tensioning cord is constructed of multistranded Sulene® polyethylene terephthalate. This cord has been used as a supplement to repair torn tendons in orthopedic applications. The cords are used to resist tension.

The spacers are tubular in design so that the tensioning cord easily fits within the spacers’ longitudinal structure. The spacers are made of polycarbonate urethane (PCU). These spacers come in standard lengths but are trimmed at the time of surgery to the length deemed appropriate. The spacers are designed to absorb forces in compression.

A recent design change allows the surgeon to combine rigid rod technology with nonrigid stabilization. Specifically, the surgeon can use this system to place rigid rod fixation at one or more levels and nonrigid stabilization at one or more different levels. This allows the surgeon to use rigid rod fusion augmentation at selected levels and the Dynesys system for fusion or nonfusion at other levels. The screws used for the rigid rod are standard pedicle screws, which may have fixed or polyaxial screw heads.

Furthermore, as implied earlier, the surgeon can utilize the Dynesys system as an adjunct to fusion per the FDA guidelines. In those instances, the surgeon can implant the Dynesys system with the PCU spacers and Sulene® nonrigid cords and lay bone graft along the transverse processes, with the thought that the movement allowed by the system may accelerate fusion. Fusion can be further augmented with interbody cages or spacers if more rigidity is desired.

The specific biomechanical characteristics of the Dynesys implant are important to recognize, as well as the metrics used to describe the device. Traditional methods of comparing fixation implants for fusion procedures have relied on biomechanical characterization through the flexibility test protocol.5–7 The methodology of flexibility testing has been well described in the literature, originating with Panjabi’s early description of load input utilizing pure moments.8 The standardization of the pure-moment test protocol by Goel et al. has contributed to repeatability, despite biologic variability inherent to cadaveric testing.9 Advances in finite element modeling and the ability to determine the exact design effect on a generalized model provide additional important information.

Dynesys has been shown to provide immediate stabilization through cadaveric biomechanical testing.10 In that study, the ROM in pure-moment testing was used to assess and compare the performance of Dynesys and rigid rod fixation in providing immediate stabilization to an unstable FSU, referred to as the index level. With Dynesys instrumentation at the index level, L3-4, a mean ROM measurement of 1.28 ± 0.42 degrees in flexion–extension bending was reported. With rigid rod fixation, the mean ROM was 2.07 ± 1.21 degrees for the same instrumented L3-4 FSU. Dynesys, when characterized by a single-level FSU, the FSU appears biomechanically no different from the FSU in rigid rod fixation at the index level under pure-moment biomechanical testing parameters. In effect, when the treatment was limited to a single-level FSU, differences in the effects to the index level using the ROM metric alone were difficult to detect between rigid rod fixation treatments and Dynesys. This does not mean that differences do not exist but highlights the need for additional metrics to assess posterior, dynamic stabilization devices.

In addition, the effects at adjacent levels were examined in that study. The adjacent superior direction would have been L2-3 for both treatment groups. With Dynesys instrumentation, the mean ROM at the noninstrumented L2-3 was 4.91 ± 2.98 degrees. The rigid rod fixation constructs resulted in a mean ROM of 7.15 ± 2.77 degrees for the L2-3 level. For index instrumentation at L3-4 only, the biomechanical effects at L2-3 were statistically insignificant as measured by ROM. Based on ROM, both the index and the adjacent level exhibited similar characteristics, regardless of the treatment.

Other studies have discussed the difficulty of attributing additional disease to the adjacent level from prior rigid fixation treatments.11,12 Clearly, it can be shown that both Dynesys and a rigid rod construct provide immediate stability in flexion–extension at the instrumented levels. Dynesys provided increased stiffness in lateral bending but no increased stiffness on axial rotation. Biomechanically, immediate stabilization is important; however, long-term, posterior, dynamic stabilization devices are designed to continually withstand the repeated loading often characterized by bench-top fatigue cycling techniques.

The Dynesys system has been implanted worldwide in more than 40,000 patients. Most constructs performed outside of the United States have been applied for nonfusion, spinal stabilization of the lumbar spine. The indications for use have been highly varied, as have the clinical results. Reported uses have included treatment of spinal stenosis, degenerative lumbar instability, lumbar spondylosis, internal disc derangement, recurrent disc rupture, massive disc rupture, and adjacent-level degeneration prophylaxis. An early article by Dubois et al. reported its use in 57 patients with degenerative disc disease.13 These patients underwent decompression and nonfusion spinal stabilization. Mean follow-up was 13 months. Of the patients, 63% were pain free and 30% had mild pain. Also, 85% had a good or excellent results based on their MacNab scores.

Another study with long-term follow-up was published by Stoll et al. in 2002.11 The authors performed an examination of 83 patients who underwent decompression and Dynesys nonfusion stabilization for lumbar spinal stenosis, degenerative spondylolisthesis, and degenerative disc disease. The mean follow-up on these patients was 38 months. The authors demonstrated a statistically significant improvement in the Oswestry Disability Index (ODI) score and improvements in the visual analogue score (VAS) for lower back and leg pain. Putzier et al. compared 35 patients who underwent nucleotomy and Dynesys placement for disc rupture to 84 patients who underwent nucleotomy alone.14 Again, improvements were noted in VAS lower back and leg scores and in ODI outcomes at 34 months. Bordes-Monmeneu et al. showed similar results in 94 patients treated for lumbar spinal stenosis, degenerative disc disease, and disc herniation.15

A smaller study was performed on 26 patients treated for lumbar spinal stenosis and degenerative spondylolisthesis.16 These patients underwent stabilization with placement of the Dynesys system in all cases and decompression in some cases. The patients were followed for 2 years. This study showed significant improvements in VASs and walking distances but revealed that patients who underwent decompression and Dynesys fared better than patients who underwent Dynesys placement alone. Grob et al. had less positive results with application of the Dynesys system.17 In this study of 31 patients, 13 patients had Dynesys application alone and 18 underwent decompression with Dynesys supplementation. Results at 2-year follow-up showed that two thirds of the patients improved in VAS back and leg pain scores but only half of the patients said that the operation had helped and had improved their quality of life. Again, it was felt that Dynesys supplementation alone was inadequate and that decompression was a necessary to improve surgical outcomes.

As part of the FDA application process, the Dynesys system underwent a single-blinded, multicenter, prospective randomized, investigative device trial. The study group included patients 20 to 80 years of age with degenerative spondylolisthesis (anterolisthesis or retrolisthesis) up to grade I and a stenosing spinal lesion (neuroforaminal, lateral recess, or central stenosis). The patients had to experience greater leg symptoms than lower back symptoms. All patients entered into the study were felt, by the enrolling surgeon, to be candidates for single- or contiguous two-level lumbar fusion between L1 and S1. The minimum preoperative leg pain score was 40 mm on a 100-mm VAS,18 and an ODI version 219 score of 30 was required. Degenerative back pain more severe than the leg pain was the primary exclusion criterion. Patients who had pain not attributed to instability or stenosis, who had radiculopathic signs at more than two levels, or who had previously undergone lumbar fusion procedures, facetectomy, or trauma at the index level were also excluded.

Patients were blinded to their treatment until after surgery and were randomly assigned to receive either Dynesys or posterolateral fusion (PLF) with iliac crest autograft bone harvest utilizing a standard, rigid pedicle screw system. A 2:1 Dynesys-to-PLF randomization schema was utilized. This was a noninferiority trial, and a fixed noninferiority margin of 10% was required by the FDA. The final sample size was based on a 15% noninferiority margin.

Standard radiographic studies were obtained pre- and postoperatively. These studies included neutral, flexion, and extension studies. The radiographs were evaluated by an FDA-approved commercial vendor with high-precision, computer-generated outcomes.

We reviewed the preliminary (1 year) results on 101 patients in 2007.20 That review concentrated on the clinical results from six investigational device exemption (IDE) sites of the larger FDA IDE trial. The study included 53 women and 48 men with a mean age of 56 years. There were 43 prior surgeries in the group. Of the patients, 13% had secondary legal issues and 21% were smokers. Preoperative symptoms were present for an average of 5.3 years with mean ODI score of 56%. The mean VASs were 80 for leg pain and 54 for lower back pain.

With 80% follow-up at the 1-year postoperative period, there was a statistically significant reduction in VAS leg and lower back pain scores (26 and 29, respectively). There was also a statistically significant improvement in the mean ODI score to 26% (Fig. 168-6). Ten patients required further surgery for radiculopathy or increased lower back symptoms.

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FIGURE 168-6 Oswestry Disability Index score in patients receiving Dynesys as a nonfusion construct (rigid fixation with autograph fusion).

The complete FDA IDE study comprised 367 patients who were enrolled at one of 28 U.S. sites between March 2003 and June 2005. Ultimately, 253 patients received Dynesys, and 114 patients underwent pedicle screw–augmented PLF.

Clinically, both PLF and Dynesys constructs were safe and robust at the 2-year evaluation point. Dynesys-treated patients had a 4% rate of reoperation at the index level (4.4% in the PLF group) and a 1% revision/removal rate (3% in the PLF group). The rate of either removal and supplementation fixation or supplemental fixation alone was higher in the Dynesys group (4%) than the PLF group (2.7%). Screw breakage occurred in 2 of 253 Dynesys-treated patients (none occurred in the PLF group), and the screw-loosening rate was 2% in the Dynesys-treated group (4.4% in the PLF group).

Overall clinical success rates were not statistically significantly different between the Dynesys-treated and the PLF-treated groups. VAS leg pain scores improved greatly in both groups, as did the ODI score and neurologic status, but none of these differences reached statistical significance.

Radiographically, 86% of the PLF patients were determined to have obtained solid fusion at 2 years. Two Dynesys patients were determined to have solid fusion at 2 years. Motion at the Dynesys-stabilized spinal segments was reduced by approximately two thirds compared to the preoperative motion, but global rotation (L1-S1) was reduced by only approximately 2 degrees (5 degrees in the PLF group), suggesting that the nonfusion patients increased their mobility at other levels over time. Overall sagittal angle (measured from L1 to S1) was well maintained in both groups. Importantly, sagittal translation was also well controlled in both groups. The mean loss of disc height, as measured at the anterior or posterior aspects of the disc space, was less than 2 mm in either group.

Surgical Techniques and Complication Management

The surgical technique of insertion for all of these devices is straightforward. All devices are translational to any surgeon familiar with lumbar pedicle screw–augmented fusion. Patient selection is important to successful outcome. The patients who have had the best outcomes in the larger trials are those with minimal degenerative spondylolisthesis and associated stenotic symptoms. Patients with pure lower back pain fair less well.

The patients are prepared per the usual surgical standards. Preoperative antibiotics are administered. The patients are placed in a prone position, and a standard prep is performed. Intraoperative monitoring is used at the surgeon’s discretion. We generally employ intraoperative fluoroscopy for level localization.

The surgeon has the option of performing either a standard midline incision or lateral incisions and dissection through the lumbar musculature (Wiltse approach). The midline incision offers the advantage of easier laminar resection and decompression. The paraspinous approach allows easier lateral to medial screw placement and can be utilized for minimally invasive approaches.

The dissection is performed to the lamina and facet–transverse junction. Should fusion be desired, dissection is performed over the entire transverse process and the transverse process is decorticated. Laminar decompression is performed at the surgeon’s discretion but is generally recommended. The facet joints are preserved to the extent possible for biomechanical purposes.

Safe, accurate, and nonrevised placement of the pedicle screws is paramount for success of the nonfusion systems, because the surgeon is not relying on arthrodesis for stability. All efforts are made to maximize the bone–screw interface. Most surgeons use a pedicle probe to create the bony channel in the pedicle. This may be done under fluoroscopy for increased accuracy. The pedicle holes are created to maximize medial trajectory (Fig. 168-7). Tapping of the holes is to be avoided whenever possible. The largest and longest screw possible is placed into the pedicle to maximize the bone–screw interface. Furthermore, the depth of screw insertion is carefully observed so that the surgeon does not need to back the screw out (and subsequently reduce the screw–bone strength).

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FIGURE 168-7 Demonstration of the medial screw trajectory.

Once all screws are secured in the pedicles, the various systems are assembled per the manufacturer’s suggestions and the patient’s needs. Many systems require determination of the distance between pedicle screw heads, and this is easily determined with supplied measuring devices. Should distraction across the neuroforamina be desired, a slightly longer spacer or rod can be fashioned or selected. The system is secured to the pedicle screws, and the fusion bone, if desired, is placed. The wound is closed in the usual fashion. Lumbar bracing may be prescribed for 6 weeks to remind the patient to limit lumbar flexion and extension.

The complications with the nonfusion systems are, in general, similar to those seen with pedicle screw systems used for fusion. Dural tears are the most common occurrence and are managed primarily. Symptomatic loss of the bone–screw interface is generally identified in the early (less than 3 months) postoperative time frame. Should the surgeon choose to correct this, larger screws can be used or fusion can be revised with larger screws and bony arthrodesis. Early infection can usually be managed with debridement, and device removal usually is not required. Extension of the system can performed in a manner similar to that of pedicle screw–based fusion systems.

Conclusions

Surgeons have established biomechanical and clinical paradigms for the “perfect” posterior pedicle screw–based device. These paradigms include maintaining control of segmental motion, neutralizing excessive forces, augmenting or supplementing for lost physiologic structures, maintaining or improving spinal alignment, preserving motion, and preventing adjacent-level disease. Design requirements are such that the system needs to be translational in its surgical approach, minimally invasive, easily revisable, and extremely robust.

Compromises need to be made to maximize clinical outcomes and minimize complexity of the systems. It appears that motion-preserving technologies allow noticeably quicker improvement of pain and return of physiologic motion. However, biomechanically, it has been difficult to quantify how these systems differ. Bench-top testing, cadaveric studies, and even animal studies present different aspects of the utility of these devices.

Posterior, dynamic stabilization devices have been utilized for motion preservation, as adjuncts to fusion, and in combinations of the two (e.g., hybrid constructs for multilevel lumbar pathologies). The metrics that are needed to assess the constructs’ ability to serve as adjuncts to fusion can be described best with load-sharing capability. Beyond ROM, anterior column loading is important for the index level, as well as adjacent levels. These differences augment the biomechanical characterization of posterior, dynamic constructs.

Clinically, most systems have been relatively translational in their application compared to fusion augmentation with pedicle screw–based systems. While overall clinical outcomes between systems are unknown, long-term follow-up is available on some systems, and the outcomes, at least in the case of the Dynesys system, are similar to fusion augmentation with pedicle screw–based systems. Robustness of these systems must continue to be carefully scrutinized and may be a deciding factor in their ultimate utilization.

Key References

Bordes-Monmeneu M., Bordes-Garcia V., Rodrigo-Baeza F., et al. System of dynamic neutralization in the lumbar spine: experience on 94 cases. Neurocirugia (Astur). 2005;16:499-506.

Brown M.D., Holmes D.C., Heiner A.D., et al. Intraoperative measurement of lumbar spine motion segment stiffness. Spine. 2002;27:954-958.

Chapman C.R., Casey K.L., Dubner R., et al. Pain measurement: an overview. Pain. 1985;22:1-31.

Cheng B.C., Gordon J., Cheng J., et al. Immediate biomechanical effects of lumbar posterior dynamic stabilization above a circumferential fusion. Spine. 2007;32:2551-2557.

Dubois B., de Germay B., Schaerer N.S., et al. Dynamic neutralization: a new concept for restabilization of the spine. In: Szpalski M., Gunzburg R., Pope M.H., et al. Lumbar Segmental Instability. Philadelphia: Lippincott, Williams & Wilkins; 1999:233-240.

Fairbank J.C., Pynsent P.B. The Oswestry Disability Index. Spine (Phila Pa 1976). 2000;25:2940-2952.

Gardner A., Pande K.C. Graf ligamentoplasty: a 7-year follow-up. Eur Spine J. 2002;11:157-163.

Goel V.K., Panjabi M.M., Patwardhan A.G., et al. Test protocols for evaluation of spinal implants. J Bone Joint Surg Am. 2006;88(Suppl 2):103-109.

Grob D., Benini A., Junge A., et al. Clinical experience with the Dynesys semirigid fixation system for the lumbar spine: surgical and patient-oriented outcome in 50 cases after an average of 2 years. Spine (Phila Pa 1976). 2005;30:324-331.

Maserati M.B., Tormenti M.J., Panczykowski D.M., et al. The use of a hybrid dynamic stabilization and fusion system in the lumbar spine: preliminary experience. Neurosurg Focus. 2010;28:E2.

Mihara H., Onari K., Cheng B.C., et al. The biomechanical effects of spondylolysis and its treatment. Spine. 2003;28:235-238.

Nockels R.P. Dynamic stabilization in the surgical management of painful spinal disorders. Spine. 2005;30:S68-S72.

Oda I., Cunningham B.W., Abumi K., et al. The stability of reconstruction methods after thoracolumbar total spondylectomy. An in vitro investigation. Spine. 1999;24:1634-1638.

Panjabi M.M. Biomechanical evaluation of spinal fixation devices: I. A conceptual framework. Spine. 1988;13:1129-1134.

Patwardhan A.G., Carandang G., Ghanayem A.J., et al. Compressive preload improves the stability of anterior lumbar interbody fusion cage constructs. J Bone Joint Surg Am. 2003;85-A:1749-1756.

Putzier M., Schneider S.V., Funk J.F., et al. The surgical treatment of the lumbar disc prolapse: nucleotomy with additional transpedicular dynamic stabilization versus nucleotomy alone. Spine (Phila Pa 1976). 2005;30:E109-E114.

Schmoelz W., Huber J., Nydegger T., et al. Influence of a dynamic stabilisation system on load bearing of a bridged disc: an in vitro study of intradiscal pressure. Eur Spine J. 2006;15:1276-1285.

Schnake K.J., Schaeren S., Jeanneret B. Dynamic stabilization in addition to decompression for lumbar spinal stenosis with degenerative spondylolisthesis. Spine. 2006;31:442-449.

Stoll T.M., Dubois G., Schwarzenbach O. The dynamic neutralization system for the spine: a multi-center study of a novel non-fusion system. Eur Spine J. 2002;11(Suppl 2):S170-S178.

Welch W.C., Cheng B.C., Awad T.E., et al. Clinical outcomes of the Dynesys dynamic neutralization system: 1-year preliminary results. Neurosurg Focus. 2007;22:E8.

Numbered references appear on Expert Consult.

References

1. Nockels R.P. Dynamic stabilization in the surgical management of painful spinal disorders. Spine. 2005;30:S68-S72.

2. Brown M.D., Holmes D.C., Heiner A.D., et al. Intraoperative measurement of lumbar spine motion segment stiffness. Spine. 2002;27:954-958.

3. Gardner A., Pande K.C. Graf ligamentoplasty: a 7-year follow-up. Eur Spine J. 2002;11:157-163.

4. Maserati M.B., Tormenti M.J., Panczykowski D.M., et al. The use of a hybrid dynamic stabilization and fusion system in the lumbar spine: preliminary experience. Neurosurg Focus. 2010;28:E2.

5. Mihara H., Onari K., Cheng B.C., et al. The biomechanical effects of spondylolysis and its treatment. Spine. 2003;28:235-238.

6. Oda I., Cunningham B.W., Abumi K., et al. The stability of reconstruction methods after thoracolumbar total spondylectomy: an in vitro investigation. Spine. 1999;24:1634-1638.

7. Patwardhan A.G., Carandang G., Ghanayem A.J., et al. Compressive preload improves the stability of anterior lumbar interbody fusion cage constructs. J Bone Joint Surg Am. 2003;85:1749-1756.

8. Panjabi M.M. Biomechanical evaluation of spinal fixation devices. I. A conceptual framework. Spine. 1988;13:1129-1134.

9. Goel V.K., Panjabi M.M., Patwardhan A.G., et al. Test protocols for evaluation of spinal implants. J Bone Joint Surg Am. 2006;88(suppl 2):103-109.

10. Cheng B.C., Gordon J., Cheng J., et al. Immediate biomechanical effects of lumbar posterior dynamic stabilization above a circumferential fusion. Spine. 2007;32:2551-2557.

11. Stoll T.M., Dubois G., Schwarzenbach O. The dynamic neutralization system for the spine: a multi-center study of a novel non-fusion system. Eur Spine J. 2002;11(suppl 2):S170-S178.

12. Schmoelz W., Huber J., Nydegger T., et al. Influence of a dynamic stabilisation system on load bearing of a bridged disc: an in vitro study of intradiscal pressure. Eur Spine J. 2006;15:1276-1285.

13. Dubois B., de Germay B., Schaerer N.S., et al. Dynamic neutralization: a new concept for restabilization of the spine. In: Szpalski M., Gunzburg R., Pope M.H., et al. Lumbar Segmental Instability. Philadelphia: Lippincott, Williams & Wilkins, 1999. pp. ∗∗∗

14. Putzier M., Schneider S.V., Funk J.F., et al. The surgical treatment of the lumbar disc prolapse: nucleotomy with additional transpedicular dynamic stabilization versus nucleotomy alone. Spine (Phila Pa 1976). 2005;30:E109-E114.

15. Bordes-Monmeneu M., Bordes-Garcia V., Rodrigo-Baeza F., et al. System of dynamic neutralization in the lumbar spine: experience on 94 cases. Neurocirugia (Astur). 2005;16:499-506.

16. Schnake K.J., Schaeren S., Jeanneret B. Dynamic stabilization in addition to decompression for lumbar spinal stenosis with degenerative spondylolisthesis. Spine. 2006;31:442-449.

17. Grob D., Benini A., Junge A., et al. Clinical experience with the Dynesys semirigid fixation system for the lumbar spine: surgical and patient-oriented outcome in 50 cases after an average of 2 years. Spine (Phila Pa 1976). 2005;30:324-331.

18. Chapman C.R., Casey K.L., Dubner R., et al. Pain measurement: an overview. Pain. 1985;22:1-31.

19. Fairbank J.C., Pynsent P.B. The Oswestry Disability Index. Spine (Phila Pa 1976). 2000;25:2940-2952.

20. Welch W.C., Cheng B.C., Awad T.E., et al. Clinical outcomes of the Dynesys dynamic neutralization system: 1-year preliminary results. Neurosurg Focus. 2007;22:E8.

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