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 could be the most logical solution for most surgeons and patients who are seeking to preserve motion while still treating 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.
But one issue continues to emerge as a concern among surgeons with regards to pedicle based motion preserving technologies—pedicle screw loosening.
Pedicle Screws and Motion Preservation
Dr. Manohar Panjabi, Professor Emeritus,
Department of Orthopedics and Rehabilitation,
Yale University School of MedicinePedicle screws represent a potential concern when used to anchor a motion preserving device. Certainly, the performance of pedicle screws is well established in rigid fixation. But for motion preserving technologies, long-term data is lacking.
According to Dr. Manohar Panjabi, Professor Emeritus, Department of Orthopedics and Rehabilitation, Yale University School of Medicine, when surgeons use fixation for fusion, the pedicle screws essentially remain in use for only about six months, and thereafter the screws are not fully loaded. However, when surgeons use pedicle screws with motion preserving devices, those screws are fully loaded throughout the life of the device. How can researchers and device manufacturers ensure that dynamic screws will withstand carrying a full load for a device’s entire life-span?
Dr. Vijay Goel,
Co-Director at the Engineering Center for
Orthopaedic Research Excellence
at the University of ToledoDr. Vijay Goel, Co-Director at the Engineering Center for Orthopaedic Research Excellence at the University of Toledo notes that pedicle screw testing is more rigorous in dynamic stabilization devices than in rigid fixation devices. Researchers often test pedicle screws in dynamic stabilization devices for 5 million cycles, but they only test screws in rigid fixation devices for as low as 2.5 million cycles. Dynamic screws are sometimes slightly stronger at the head than rigid screws, which helps the dynamic screws hold greater loads throughout the life of the device.
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, also notes the importance of pedicle screw design in motion preserving technology:
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 biggest problem in dynamic stabilization devices is maintaining good fixation to bone. A very rigid device could loosen or break, as seen in the Dynesys device, because of the high rigidity of the device.”
“This was taken into account in the design of Stabilimax NZ. By providing a ball and socket joint at each pedicle screw-device junction, only the compression-tension force in the device is transmitted to the screw without causing any bending moments, which theoretically lowers the incidence of screw breakage. You also don’t want a dynamic stabilization device to bear the full load. You want it to share the load with the rest of the spine. If you put 100% of the load on the device, the chances of device failure increase.”
Dr Goel couldn’t agree more.
“Unlike posterior instrumentation in fusion systems, posterior dynamic stabilization (PDS) devices are expected to function throughout the life of the patient. This makes both the screw design and the implant design equally critical. A true dynamic stabilization implant (one that has an optimum stiffness profile, permits interpedicular travel, and thus maintains a near normal center of rotation) will impose lesser loads at the bone-screw interface as compared to a fusion system. You can further reduce the loads at this interface by using devices, such as in Stabilimax NZ, which avoid or reduce bending moments.”
“While conical screws are favorable mechanicallythey are also the least forgiving surgically. If, for example, the screw height needs to be adjusted during surgery, backing out a conical screw has a detrimental effect on its purchase. Hence, some combination of a cylindrical and conical geometry should be used for a PDS screw in order to maintain the most strength in the region where the screw holds the heaviest load.”
“Also, while a stiffer screw is less prone to fracturing, it could also be more prone to loosening, which is definitely a concern for PDS systems. This factor should be kept in mind while designing a PDS screw. HA (hydroxyapatite) coating theoretically seems beneficial for the loosening issue, but we need to see clinical data to validate that theory. Other surface treatments, like the dual shot peening of the Stabilimax second generation pedicle screw, may also help decrease screw loosening. These coatings can improve the surface roughness and fatigue life of the screw without necessarily making the screw stiffer.”
According to Dr. Goel, manufacturers and designers employ a wide range of materials in dynamic stabilization devices, but it is important to differentiate the device from the screw. Screws are primarily made of titanium. The devices may be comprised of polymers, titanium, cobalt chromium, carbon fiber, and PEEK (polyaryletheretherketone) which has the advantage of being radiolucent. Another emerging biomaterial that manufacturers could use in dynamic stabilization devices is nitinol. Nitinol is a shape memory alloy made of nickel and titanium originally developed by the U.S. Navy. However, Dr. Goel notes that nitinol is a difficult material for manufacturers to use during fabrication, and there could be issues with fatigue resistance. Still, Dr. Goel expressed his confidence that these issues would eventually be overcome, allowing for nitinol to play a greater role in orthopedic devices in the future.
The Neutral Zone and Pedicle Screw Based Dynamic Stabilization Technology
Dr. Panjabi’s research has focused on the elements of motion in the spine and spinal instability as it relates to back pain. In his two-part article entitled “The Stabilizing System of the Spine” published in the Journal of Spinal Disorders, he postulates that the spinal stabilizing system consists of three subsystems: the passive subsystem (spinal column), the control subsystem (neural) and the active subsystem (spinal muscles). If any one of these subsystems dysfunctions, then the others may compensate, maintaining normal functionality of the spine. However, injury to one or more of these subsystems can lead to low back pain.
Based on extensive experimental studies conducted by him and his colleagues, Dr. Panjabi has identified what he terms the “neutral zone” in the spine. The neutral zone (NZ) is the initial part of the range of motion within which spinal motion is produced
Figure 1
Stabilimax NZ (Applied Spine Technologies)with minimal internal resistance. It is the micro-motion that the spinal segment inherently exhibits around its neutral posture. Other experimental studies of spinal injuries have shown that the NZ is a more sensitive indicator of spinal instability than the full range of motion. Dr. Panjabi postulated that an increase in NZ could lead to back pain. If one could preferentially stabilize the NZ, then one could reduce the back pain and simultaneously allow spinal motion.
The Stabilimax NZ (Applied Spine Technologies), displayed in Figure 1, is based on Dr. Panjabi’s theories regarding the interaction of subspinal systems and the experimental findings concerning the neutral zone. With two uniquely arranged springs, this device can preferentially decrease the neutral zone.
Do PEEK and Tension Band Technologies Allow Enough Motion?
Dr. Patwardhan expresses doubts as to whether pedicle based dynamic stabilization devices incorporating PEEK or tension bands truly allow enough motion. He describes devices based on PEEK as “bogus, ” and states, “If you just take a PEEK rod it will not stretch enough to allow the motion that is sought after in a true dynamic stabilization device, and it will not allow enough excursion. In the case of a tension band device such as Dynesys, there is an elastic bumper at the end of the band which compresses to allow some excursion, but the Dynesys has very little motion because it doesn’t allow adequate pedicle to pedicle travel.”
Dr. Patwardhan suggests that the real key is to incorporate a spring into pedicle based dynamic stabilization devices. The spring can be made out of titanium or cobalt chrome, depending on the fatigue characteristics.
Is There a Future for Tension Band Technologies?
Dr. Paul McAfee, Chief of Spinal Surgery at
Towson Orthopaedic AssociatesBased on our conversations with leaders in the design and theory behind pedicle based dynamic devices, implants such as the Dynesys that utilize tension bands may not provide the desired degree of motion in technologies of this nature. According to Dr. Paul McAfee, Chief of Spinal Surgery at Towson Orthopaedic Associates, the basic problem with the Dynesis is that it is too stiff to provide the appropriate amount of motion. The other issue is that the PET (polyethylene terephthalate) cord on the Dynesys could stretch out over time and contribute to instability.
However, the industry is not giving up on tension band devices. The Transition posterior dynamic stabilization system marketed by Globus Medical incorporates tension band technology and overcomes several of the drawbacks seen in the Dynesys device. The Transition system has an extra bumper as compared to the Dynesys device which allows for additional motion. Additionally, Dr. McAfee notes that the device allows for a “soft stop” which decreases the chances of screw loosening.
Next Week: We close our three-part series with a discussion of the use of dynamic stabilization as a hybrid device and the resurgence of dynamic stabilization as the future of motion preservation in the spine.
