Over the last several weeks, we’ve tackled the concept of motion preservation in the spine. At the start of the new millennium, the concept of motion preservation technology took the spine industry by storm. While clairvoyants preached the end of the world, pundits in the spine industry proclaimed “the end of fusion.” Neither prediction came true.
Artificial discs, interspinous process spacers, and dynamic constructs continue to evolve, but to what end? What will become the standard of care?
Based on our conversations with leaders in the field, pedicle screw based dynamic stabilization is re-emerging as the most logical solution for most surgeons and patients who seek to preserve motion and treat spinal instability. There is, in other words, a middle road between disc arthroplasty and spine fusion—a middle road that ultimately may be more consistently successful for patients and their surgeons.
Last week we discussed the troubling issue of screw loosening in pedicle-based motion preservation, and we heard the experts explain how some devices can overcome that problem. This week, in our third and final installment, we discuss patient selection and device differentiation.
The Surgeon and Indications
Different types of surgeons evaluate devices in a variety of ways. Their considerations can include: anatomical approach, ease of implantation, and published outcomes/complication rates. Academically based surgeons may evaluate devices differently than active, higher volume surgeons, who may primarily focus on approach and ease of implantation. These different types of surgeons may not have a single standard of evaluation for devices, but we still found that, based on our discussions with leaders in both the theoretical and design aspects of these technologies, the surgeon is the final decision maker regarding which cases require which device. In the end, each surgeon should decide which device he or she is most comfortable working with, but in order to objectively decide which device is best for each case, it helps to start with the on-label indications for each device.
Pedicle-based systems are designed to address spinal instability facilitated by surgical procedures (such as decompression) or by degenerative spine disorders. According to Dr. Avinash Patwardhan, Director of the Musculoskeletal Biomechanics Laboratory at Edward Hines Jr. VA Hospital and Professor of Orthopaedic Surgery at Loyola University Medical Center “the primary indication is back pain that is somehow related to spinal instability. Some people refer to this as activity related back pain. A secondary indication would be decompressive surgery that creates iatrogenic instabilities. Another indication would be as a ‘topping off’ device adjacent to a fusion.”
According to the FDA 510(k) summaries of the Dynesys, DSS, and Transition, the following are indications for patients:
- Patients who are receiving fusions with autogenous graft only
- Patients who are having the device fixed or attached to the lumbar or sacral spine
- Patients who are having the device removed after the development of a solid fusion mass
Table 1 lists the indications for several of these pedicle-based motion preservation devices. Note that almost all of these devices treat chronic instabilities or deformities of the spine adjunct to fusion. The Transition is on-label for use without interbody fusion, but it must be used with an autogenous bone graft. The Stabilimax NZ also stands out from the crowd in that it treats spinal stenosis adjunct to decompressive surgery.
Table 1: Pedicle-Based Motion Preservation Devices and Their Indications
|
Company |
Product |
Regulatory Status |
Indications* |
|
Applied Spine |
Stabilimax NZ |
IDE clinical trials ongoing |
Adjunct to decompressive surgery in patients with spinal stenosis |
|
Zimmer Spine |
Dynesys |
510(k) approved |
Adjunct to fusion in the treatment of chronic instabilities or deformities of the spine |
|
Globus Medical |
Transition |
510(k) approved |
Adjunct to fusion in the treatment of chronic instabilities or deformities of the spine. May be used without interbody fusion, but must be used with autogenous bone graft. |
|
Scient’x |
Isobar semi-rigid rod |
510(k) approved |
Adjunct to fusion in the treatment of chronic instabilities or deformities of the spine |
|
Paradigm Spine |
DSS Modular Stabilization System |
510(k) approved |
Adjunct to fusion in the treatment of chronic instabilities or deformities of the spine |
*Please see FDA documentation for full description of on-label indications
The indications are as clear as black and white, but a patient’s specific needs can often fall in the gray area. Patients present at varying points in the degenerative cascade and in the overall continuum of care. There are many variables that determine the appropriate use of the technology, and the ability of the surgeon to determine appropriate usage is of critical importance. In our view, it will be necessary to continually develop methods for accurately diagnosing spinal disorders in order to complement the technology that treats the disorders.
Differentiation Between Devices
The devices in Table 1 are all pedicle-based dynamic stabilization devices, and some of them share the same indications, but how do they match up against each other? Are there specific advantages and disadvantages to certain devices? While long-term outcome data isn’t yet available, researchers have analyzed key variants in these devices. Dr Vijay Goel, Co-Director at the Engineering Center for Orthopaedic Research Excellence at the University of Toledo, for example, has performed analysis highlighting some key characteristics of Stabilimax and Dynesys.
According to Dr. Goel, “Biomechanical studies have shown that the Dynesys is comparable to rigid instrumentation (i.e., fusion) particularly in flexion. So, the PET cord doesn’t seem to allow any motion. Early clinical results also indicate that the device behaves like a fusion device. Lack of interpedicular travel (due to lack of material compliance) seems to be the root cause.
“The Stabilimax design permits interpedicular travel and maintains a near normal center of rotation, which suggests that the stiffness profile is also optimal. The ball-socket design at the pedicle screw head also reduces bending moments at the bone screw interface. Dynesys lacks these characteristics, as indicated both by biomechanical testing and early clinical results. What’s interesting is that given the stress relaxation characteristics of the materials used in the Dynesys system, combined with evidence of “ball-socket like” wear patterns on the PCU spacer from long term retrievals, the Dynesys will probably behave more like the Stabilimax, but only after 8-10 years of implantation, which unfortunately might be too late to avoid adjacent level disease progression.
“The Dynesys also has an issue with screw loosening with their 1st generation screws. We will wait to see if HA coating in their 2nd generation screw addresses the issue. The Stabilimax, on the other hand, had issues with the surface finish of their 1st generation pedicle screws, resulting in some cases of screw fracture. Given the notch sensitivity of titanium, grit-blasting as a surface finish is detrimental to the fatigue life of the screw. The 2nd generation Stabilimax pedicle screws are dual shot peened, which improves the surface roughness and fatigue life of the screw, without necessarily making the screw stiffer. This is probably the right solution, as a stronger, stiffer screw could easily result in screw loosening, which is of primary concern with PDS systems.”
Dr. Paul McAfee, Chief of Spinal Surgery at Towson Orthopaedic Associates, suggested that the Transition device marketed by Globus Medical offers significant improvements over past tension band based devices such as the Dynesys. The device utilizes a PET cord, but it also contains two bumpers made out of polycarbonate polyurethane. There are titanium spools attached to the conventional pedicle screws. The spools have a collar on them which encircles the sleeves so that the device can maintain stability up to 10 million cycles. The device also works with a posterior disc implant.
Patient Expectations
Some orthopedic companies are now embracing a direct marketing model, advertising to consumers via the Internet and television. This has the potential to attract patients throughout the continuum of care, including those who are undergoing conservative, non-operative treatment as well as those with failed back syndrome or ongoing symptoms. What expectations do these patients have regarding these technologies?
Dr. Manohar Panjabi, Professor Emeritus, Department of Orthopedics at Yale University School of Medicine suggests that patients generally have high expectations regarding new technologies and treatments. Patients tend to be attracted to technologies that will decrease the pain while avoiding an irreversible procedure such as fusion. Patients who still have pain after surgical intervention may also be attracted to these new alternatives. Dr. Patwardhan added that an additional benefit to these devices is that the surgeon can use a minimally invasive procedure, reducing the incidence of muscle related trauma associated with fusion.
Moving Forward: A Hybrid Construct?
Treatment with pedicle-based dynamic stabilization technologies allows patients to avoid the potential effects of irreversible procedures such as fusion while still maintaining natural motion in the spine. In the future, dynamic stabilization technologies may work with other devices to not only avoid fusion, but to preserve anatomy. Nucleus replacement technologies seek to preserve the annulus fibrosis of the disc by only replacing the nucleus. Dr. Panjobi, who advocates preserving as much of the anatomy of the spine as possible during treatment, believes that there is the potential to use nucleus replacement technologies with posterior stabilization devices. However, he does not necessarily agree with combining total disc replacement (TDR) technologies with posterior dynamic stabilization because TDR can contribute to instability with the removal of the native disc and affect the anterior longitudinal ligament.
Dr. Goel believes that ultimately, a 360° motion preservation system could be the solution. While several combinations of systems are possible, he suggests that a posterior disc and a posterior stabilization system may be preferential, as an anterior implant would require an additional surgery.
Motion Preservation as Part of Fusion
One of the appeals of motion preservation technologies is that they represent an alternative to spine fusion. However, there is no reason why fusion and motion preservation cannot also co-exist. The focus should not be on which one will eliminate the need for the other. For example, severe spondylolisthesis may require fusion while the adjacent level is treated with a motion preserving device or a hybrid motion preservation construct. We submit that that there will likely be a place for both in the future, and that the technologies may be increasingly utilized together.
Final Words: The Preferred Spine Treatment
Motion preservation technologies are now a permanent part of the evolving spine technology landscape. Pedicle-based dynamic stabilization systems implanted from the posterior approach may prove to be an appealing treatment option for surgeons and patients in search of alternatives to fusion.
Researchers and manufacturers are designing dynamic constructs based on a growing body of quantitative analysis involving anatomical responses to injury in relation to the biomechanics of the spine. Pedicle-based dynamic stabilization technologies are evolving to match a greater understanding of the range of motion in the spine, more specificity regarding indications, and an advanced understanding of device materials. The industry is also exploring ways to use dynamic stabilization with other devices such as nucleus replacement in order to provide a complete solution to spine disorders.
Instead of simplifying spine treatment to a single best treatment option, the future of the spine industry will grow increasingly complex as researchers and surgeons combine devices and methods to find the best solution to each patient’s problem.
