Spinal Motion Preservation: Update and Outlook—(Part 1 of 3)

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? Today we take a sobering look at motion preservation and at posterior pedicle based dynamic stabilization technologies. Based on our conversations with leaders in the field, pedicle screw based dynamic stabilization could be the most logical solution for most surgeons and patients who are seeking to preserve motion while still treating spinal instability and back pain. 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.

Introducing Motion to the Spine From the Posterior Approach

Beginning in 2004, with the FDA clearance of the Charité artificial disc, the era of motion preserving technologies implanted from the anterior approach began. Proponents of artificial discs point out that the anterior approach provides superior access to the disc space, which is critical to the success of the surgery. Beyond this, some spine surgeons are familiar with the anterior approach based on experience with cervical spine fusion, where the anterior approach is commonly employed. Indeed, all of the approved lumbar artificial discs and the majority in development are designed to be implanted from the anterior. However, Dr Vijay Goel, Co-Director at the Engineering Center for Orthopaedic Research Excellence at the University of Toledo, related that neurosurgeons tend to be uncomfortable with the anterior approach. He also cited that the literature has shown evidence of facet syndrome in patients who have received a total disc replacement. What about the prospect of introducing motion preservation technologies from the posterior approach?

Based on our analysis, there are few artificial discs designed to be implanted from the posterior. During our conversation with 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, we learned that posterior discs have good rationale from a clinical point of view versus an anterior device, but there are still design issues that need to be settled.

This leaves interspinous process spacers as the primary motion preserving technology which is exclusive to the posterior. However, interspinous process spacers such as the X-Stop are primarily indicated to treat spinal stenosis in elderly patients. In addition to being a fairly rigid device (made of PEEK or titanium), interspinous process devices do not address the full range of motion in the spine. According to Dr. Manohar Panjabi,  Professor Emeritus Department of Orthopedics and Rehabilitation Yale University School of Medicine, the device allows for flexion, but very little extension, essentially taking away some of the natural motion of the spine.

While total disc replacement and interspinous process spacers will remain an option for the appropriate indications as determined by the surgeon, there remains a potentially better technology when introducing motion to the spine from the posterior approach. Based on our research, pedicle based dynamic stabilization systems may be poised for resurging interest among surgeons as the best technology to introduce motion into the spine from the posterior approach (which is used in 70% of all lumbar spine fusion surgeries), while maintaining spinal stability. 

Pedicle Screw Based Dynamic Stabilization Technology as a Solution

Essentially, the goal of spine fusion is to eliminate motion in the spinal segment that is causing the pain by inducing bone growth between two vertebrae. However, based on our conversation with Dr. Panjabi, the theory that increased range of motion leads directly to back pain has largely been discarded, as it has been shown that you can decrease back pain without eliminating motion, and, more importantly, elimination of gross motion by spine fusion does not always take away back pain. However, fusion does have consequences. The elimination of motion by fusion at one level, may cause hyper-mobility at the adjacent segment and accelerate the degenerative cascade. This is called adjacent segment disease and is one of the key consequences of fusion that motion preservation technologies seek to prevent. Other goals of the technology, besides helping reduce or eliminate back pain, include: controlled motion in the destabilized spine, limiting stress shielding, increasing anterior load sharing, protecting the facet joints, and protecting the musculature.

Players in the Spine Industry Are Taking Notice

Companies in the spine industry are increasingly gravitating toward dynamic stabilization. Perhaps the most well known companies that have dynamic stabilization technologies in their portfolios include Zimmer Spine, Scient’x, and Applied Spine. While these technologies essentially seek to meet the goals mentioned previously, approaches and designs vary by technology. The Dynesys Dynamic Stabilization System, by Zimmer Spine, is held in place by titanium pedicle screws and is considered a tension band device due to the polyethylene cord used in the device. Scient’x markets the Isobar semi-rigid rod system, which utilizes a mobile joint within the rod (dampener) to allow motion. Applied Spine has developed the Stabilimax NZ, which is held in place by pedicle fixation. Motion is driven by dual springs which are utilized in the system though connecting rods. Other devices in development include the DSS by Paradigm Spine and the NFix II now market by Synthes Spine.

How Much Motion Is Enough?

Given the variability of spine conditions and various stages of degenerative spine disorders, the key question becomes: how much motion is enough?

According to Dr. Goel, it depends. Because “we do not know what magnitude of motion we need to preserve and what should be the shape, NZ (neutral zone) reduction is important and/or the overall range of motion (ROM); axial and lateral bending stabilization may also become essential.” So, said Dr. Goel, surgeons should consider five elements in their surgery planning:

1. What is the grade and characteristic of the patient’s degenerative instability? Low grades may be treated with an insterspinous process spacer, while high grades like spondylolisthesis may need fusion.
2. Motion variations as a function of the individual patient are important, and should be assessed on an individual basis by the surgeon in order to determine if the motion preservation device is appropriate.
3. What type of decompression is needed (laminectomy, facetectomies, etc.)?
4. What is the relationship between disc degeneration, stenosis, and instability?
5. What combination of implants with different stabilizing capabilities may be the best solution?

Device Design and Biomechanical Considerations

Pedicle based dynamic stabilization expands upon traditional, rigid fixation systems by introducing motion. This new paradigm forces bioengineers and surgeons to think differently in evaluating design characteristics, materials, and device testing.

Design Considerations

Dr. Goel suggested several key design considerations:

1. The device should be able to control the motion in the spine.
2. The device should bring the motion back to “normal” (restore the motion) and reduce instability. This also means restoring the center of rotation.
3. It should produce load sharing between the disc and the device so that the disc is unloaded, allowing it to begin to regain its height and be regenerated.
4. Specifications may depend on the application (requirements for the device stiffness will vary). If the goal is to restore stability alone vs. a procedure that creates additional instability such as a discectomy/laminecomty/facetectomy
5. The device should seek to minimize stress at the bone-screw interface.

In addition, Dr. Patwardhan adds that pedicle-to-pedicle excursion (interpedicular travel) should be monitored as well. Finally, the quality of motion is just as important if not more so than the quantity of motion.

Part 2—Next week, the persistent issue of using pedicle screws as part of motion preservation.

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