Spine and Spinal Cord Surgery

Spring 2001
Volume 12, Number 1

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Future of Spine Care

Brian H.Wieder, MD

The most common ailment affecting the population today remains neck and back problems. Yet our ability to address such pathology effectively has lagged behind our ability to treat other diseases. In the new millennium we are finally demonstrating outcomes and data that illustrate our greater understanding of spine pathology, yet there is still work to be done. The first decade of the new millennium has been denoted Decade of the Spine by the Joint section of the American Association of Neurological Surgeons and the North American Spine Society. The Decade of the Spine Initiative will guide and focus our progress in the new millennium. All disciplines that address back, neck, and spinal cord ailments will merge into a more rounded multi-disciplinary approach and allow collaboration on treatment fronts in the area of orthobiologics, neuroaugmentation, spinal fixation, prosthetics, surgical technique, and adjuvant therapies.

Introduction. This is an extraordinary time in the spinal sciences. In the past, care of those with spinal pathology has been marred by unclear indications, questionable results and inappropriately conservative or aggressive treatment paradigms. Similar pathologies were prescribed wildly divergent treatment alternatives. This lack of uniformly accepted treatment strategies and indications results from the various subspecialties approaching spinal pathology from vastly differing philosophies. Surgeons search for anatomically correctable lesions. Chiropractors aim for maintenance of balance. Psychiatrists focus on the somatization and behavioral response to stressors, Physiatrists focus on postural stabilization. The fact of the matter is that such approaches have their merits for specific patients, yet no subspecialty can hope to offer the answer to all patients. Pathology of the spine and spinal cord is simply too complex. With the acknowledgment of this dilemma in the last decade, we have seen early convergence of these subspecialties. The bringing together of minds from various backgrounds has catalyzed the progression of treatment technology.

These newer technologies represent a more well-rounded combination of holistic and scientific understanding of the spine and spinal cord. They represent an intersection of the cumulative understanding of biomechanical and neuromuscular basis of motion, behavioral dynamics, and cellular function. These technological advancements of the new millennium can be categorized into the categories of Orthobiologics, Neurophysiologic stimulation, absorbable fixation, prosthetics, surgical access minimization tools, and adjuvant therapies.

Orthobiologics. The orthobiologic advancements focus on stimulation, modification or control of the human bodies own reparative capabilities and may serve as temporary bio- scaffolding for later autologous replacement. In clinical and in vitro studies that are currently on-going in spine and spinal cord research centers throughout the world, numerous technologies are showing some promise and may very well play a role in treatment in the near future. The most promising of these technologies includes the isolation and application of recombinant Bone Morphogenic proteins, maximization of biosynthetic oteoconductive and inductive matrices, the isolation of nerve growth factors and neuromodulators for neural repair, synthetic ligaments, intervertebral disc regeneration, and epidural fibrois barriers.

The availability of recombinant and autologous Bone Morphogenic Proteins (BMPs) will improve fusion rates and minimize the necessity for other autologous bone harvest. As many as a dozen BMP’s are being investigated clinically after animal studies have shown significant promise. Two such proteins are currently being investigated clinically in spinal fusion, OP-1 and rhBMP2. Stryker Biotech is currently investigating BMP-OP-1 (osteogenic Protein) in FDA sanctioned clinical trials involving lumbar fusion1 without autograft augmentation in combination with a variety of biomaterials including collage sponges and interbody fusion cages. Stryker Biotech reports significant progress with possible availability within 1 or 2 years. There are however significant cost issues to overcome. Current treatment costs are estimated to exceed $3000 to $4000 per fused level. This may be partially offset by shorter hospital stays, shorter surgical time, and possibly lower incidence of metal implant utilization. Until this is commercially available and cost benefit effective, DePuy AcroMED and others have developed a technique for harvesting and concentrating autologus bone growth factors from phoresed platelet rich plasma. This is currently available clinically and available at Swedish Medical Center.

Other biologic agents are focused at preventing the sequelae of scar tissue. Scar tissue is currently an unavoidable by product of spine surgery. In 5% of patients this scar tissue can become symptomatic. Current approaches employ the use of pharmacological barriers that prevent epidural fibrosis. ADCON is currently the only commercially available form of this agent. However, use is not without complications and physicians using this need to understand what effects it may have. One experimental model utilizes low-dose radiation applied 24 hours prior to surgery to inhibit fibrosis in canines. The results have been promising and this strategy may be useful in secondary surgeries performed for those patients with complications associated with scar tissue, but it is not likely that exposure to any significant amount of radiation will be deemed acceptable in practical use.1

A very difficult problem to address is osteoporotic compression fractures. It is estimated that the prognosis of those with this ailment is on a par with breast cancer. Treatment strategies are limited by the extreme porosity of the spine and its inability to provide the support that normal physiologic loads require. Surgery is a poor option as it weakens the spine further. Bone strength may be insufficient to hold stabilizing metallic implants. Such patients are usually elderly and represent high surgical risks. To address this problem, percutaneous vertebral strengthening procedures are being investigated. One such approach currently available is injection with a plastic polymer cement, Methylmethacrylate. It is injected percutaneously through the pedicle into the vertebral body. Short-term results are quite good. Long-term results are disappointing as the underlying pathology is not addressed and further collapse generally occurs. Other modalities, such as vertebral kyphoplasty, focus on not only strengthening the fracture zone, but also correcting any kyphotic deformity. This is currently being investigated and preliminary results have been disappointing. Injection of other bio-compatible materials is being studied in humans in France. Osteoconductive granular coral has been injected into ewes and this showed new bone growth 2 months after injection. This may offer some promise, but needs to be investigated in osteopenic models.3 If this approach works, it has the benefit of promised improved long term results.

Also being investigated are a variety of nerve growth factors. This topic is of such interest that it will be dealt with in a separate section.

Neurophysiologic Stimulation. Neurophysiologic stimulation encompasses a variety of electromotive devices based on scientific studies that demonstrate that specific frequencies of electrical stimulation presented to biologic tissues can modulate their reparative capabilities, mask the perception of sensory input, and form the basis of robotic control of limb movement in spinal cord injured patients. Such neurophysiologic stimulation takes the form of bone growth stimulators, Spinal cord stimulators for the masking of pain, and computer controlled sequential motor group or locomotor group stimulation for the electrical control of artificial gait in paraplegics.

Bone growth stimulators have been shown to augment callous formation rate in long bones and early data suggests that it is useful in spine surgery as well. Bone growth stimulators apply a variety of differing currents, Direct Current, Inductive passive electromotive field, Capacitive coupling, and combine magnetic fields. There is on-going research now to determine the field with the most advantageous result. These studies will be changing clinical approaches to bone growth stimulators in the next 1 to 4 years.

Sequential locomotor stimulation allows artificial gait in paraplegic recipients. Complex locomotion software sends out an array of signals which mirror the motor plan established in the brain for ambulation through electrodes implanted in key motor groups associated with gait. Computer assisted gait is the result. More recently the discovery of the locomotion center in the spinal cord has raised questions as to whether a similar approach can be used to stimulate the spinal cord below the level of injury to induce a mechanical gait. The majority of this work has been accomplished at the Miami Project to Cure Paralysis.

Fixation. Spinal fixation is required for the treatment of a variety of ailments. It is analogous to the casting of a fractured arm, which promotes a stable environment for timely healing. Unfortunately, simple casting of the spine is not sufficient. The development of implantable metallic instrumentation over the last 20 years has improved our control of the spinal structures during healing, however the use of implants that do not possess the same biomechanical modulus of our inherent tissues has led to a variety of other problems. There is much interest in the development of spinal fixation instrumentation that is temporary like a cast would be. It will be in the form of bio-absorbable implants. Strategies to approach this problem are on the minds of physicists, engineers, and other investigators, and as of yet are highly secretive. No data could be obtained as to the approach or feasibility of this strategy other than it is currently being investigated at the early stages and would promise to be huge advance if found feasible. I estimate a 5 to 10 year time frame before this is known.

Prosthetics. Other centers are aggressively pursuing the field of prosthetics. Much like artificial hips and knees, prosthetic discs hope to offer the advantage of preserving spinal motion segments. Current techniques for treating painful discopathies include discectomy and fusion. They yield relatively good short term results, but the biomechanical changes that they produce accelerate adjacent degenerative processes. Disc replacement strategies have the added advantage of limiting the stress on adjacent motion segments and preventing the accelerated degenerative processes seen years after fusion procedures are carried out.4 These prosthetics may take the form of mechanical disc replacements, nucleus propulsis replacement pillows, or annular patches.

One such prosthetic, a pillow, the Aquarelle Hydrogel Disc Nucleus (Stryker Howmedica osteonics Rutherford, NJ) is being investigated in primates. It is a polyvinyl alcohol and water pillow that has been shown to restore intradiscal height and biomechanical function in pre-clinical trials.5 Another approach, the mechanical disc, is being investigated in clinical trials in the Netherlands. The modular type SB Charite III has been studied and results have been published on the first 50 patients. They report a 70% satisfactory result in patients treated with single level discopathies, but with a 13% rate of permanent side effects or complications. Their conclusion was that mechanical disc prosthesis is potentially a viable alternative to fusion, but that patient selection is critical to success.6, 7 In a novel approach, investigators in Japan, using an animal model, have reinserted herniated nucleus pulposis back into the disc space through an annular defect and patched the annulus. They were able to show a slower rate of degeneration when compared to discs where the herniated material was simply removed but failed to show any functional benefit.8

Other prosthetic devices hope to replace damaged or denervated muscles or ligaments. Ligaments serve to limit range of motion analogous to a tension band, and in this capacity, offer physiologic non-rigid spinal stabilization. By replacing ligaments removed in destabilizing traditional spinal surgery, the goal is to prevent degeneration caused by destabilization and solve the difficult problems caused by the difference in biomechanical properties between physiologic tissues and metal spinal implants.9 The Leeds-Keio artificial ligament was evaluated in invitro animal models. Their conclusion is this technique is effective in initially stabilizing destabilized segments and offers long term improved stabilization after cyclical loading when compared to controls. This is not currently being investigated in clinical trials.10

Surgical Access Minimization. All of the above technologies represent attempts to mirror more closely the complex forces, biomechanical stresses, and synergistic events that occur during spinal motion in a biomechanically analogous solution to what evolution has given us. Certainly, if implementation can be facilitated with minimization of surgical invasiveness this will also offer an advantage. This is the most rapidly advancing front on the treatment of spine pathology. Surgical access minimization includes application of any of the above treatment options through smaller less biodestructive openings than traditional surgical exposures. Endoscopic and percutaneous techniques augmented with the exquisite accuracy of neuro-navigational computer assisted image guidance and robotics will bring us closer to the Star Trek model of diagnosis and treatment than ever before. Most early strategies to decrease invasiveness also added an element of increased complexity and risk. Therefore its widespread implementation has been slow. Acceptance of such advanced technology is also fraught with additional complicating factors ranging from physician acceptance, high start up costs, and drawn out learning curves, but it is alluring once such obstacles are overcome.

Endoscopic spinal surgery has been utilized on a variety of pathology with successful outcomes. It promises to decrease pain, shorten hospital stays, and provide more cosmetically acceptable results. However, with the advent of any new technology, initially the risks are higher and will remain higher until the technology has advanced enough to overcome some of the inherent risks associated with limited visualization of critical structures. Because of this phenomenon, fewer surgeons are excited about endoscopic technology as there were 5 years ago. Although there are some procedures that suit themselves well for endoscopic spine surgery, most procedures are simply too complex for this approach. This has created a new approach, Portal surgery. With portal surgery slightly larger dilating cannulas are used to dissect an adequate portal or window bluntly through soft tissue and a high power microscope or endoscope is used to visualize important structures through the portal.11 This new strategy is far from percutaneous surgery, but does offer a more cosmetic result and speedier recovery. This approach will be adequate for the next few years until computer assisted robotic navigation is perfected. Units are already installed in Germany and are pending FDA approval in the US.

This advancement is the next generation of currently available frameless stereotactic image guidance technology. With the initial systems, CT and MRI images are fed into the system. Fiducials or markers are then placed on the patient. The coordinants of the fiducials are fed into the computer. Specialized software matched the patient’s coordinates with the CT or MRI. The computer is then able to track the instruments in space and present visualization on the monitor of where those instruments are related to the pathology at hand. This is a big step forward, but the time for setup, cost, and relatively low accuracy prevented wide acceptance.12

With second-generation systems, real time fluoroscopic images of the patient were used parallel to CT or MRI images to improve the location registration of the patient in space. This was a promising strategy and offered many benefits over the old techniques, however accuracy is still shy of perfection. The next generation is not yet available in the US, but will be in the coming year and promises to address all the limitations of current image guidance technology. In a recent trip to Munich, Germany, I had an opportunity to evaluate this technology and see it in action.

The latest generation uses an infrared wand that passes over the relevant bony structures and via triangulation feedback to a camera positioned near the ceiling gives an infinite number of fiducial readings from all elements under the wand and can track them through the expected minor motion of these structures during surgery. This produces a computer-generated image of the bony structures which is then matched with the fluoroscopic images point to point. This information is then morphed onto the CT and/or MRI images so all information is present in a computer generated 3-dimensional model of the spine that can be rotated about any axis.13 The surgical plan can then be completed on the computer in entirety and manually carried out with the computer tracking every movement of the instrument with relation to the computer generated 3-dimensional image of the patient. Future technology promises to replace the infrared wand with an ultrasonic probe that passes over the body and feeds the infinite pattern of fiducial points into the system without requiring any surgical exposure at all. This approach has succeeded in decreasing surgical set up time, operative time, and increased accuracy yet another level.

This technology also offers an additional level of safety by guiding the surgeon’s hand. When a specialized robotic arm is utilized with this system, it allows the surgeon full control of his instrumentation with certain movement constraints that prevent injury to vital structures, such as nerve roots, spinal cord, or blood vessels. This ability is extremely critical and when combined with ultrasonic registration may be the safety mechanism needed to allow percutaneous and endoscopic approaches to be done with the degree of safety that physicians and patients feel more comfortable with.

A similar approach is being investigated for removal of spinal cord tumors. The image information is fed into the computer and spinal cord ultrasound is used to identify the location of the tumor. Computer algorithms adjust for normal respiratory motion during surgery. A 3-dimensional image of the tumor is morphed onto real-time images of the spinal cord obtained through the microscope. Both the 3-D image and the microscopic view are visible through the eye pieces of the microscope.

Adjuvant Therapies. Another exciting advance is the application of spinal frameless stereotactic radiosurgery. Radiosurgical therapy for spinal cord tumors has been limited by the spine’s inability to accept a stereotactic frame. However, through ultrasound patient registration and image fusion morphing this is now available. Current techniques attempt to find an array of various sized circular beams of radiation that, when focused at a tumor, fills its volume at the focal point or isocenter. The radiation dose is additive at the isocenter limiting the dose to intervening normal tissues. The next generation called Dynamic Intensity Modulated Radiosurgery employs inverse planning algorithm where a computerized dose volume histogram is calculated and takes into account maximum and minimum doses to geographically eloquent areas. The computer generated and optimized radiosurgery plan is then utilized to generate a dynamically shaped beam of dose controlled radiation that matches the shape of the tumor when viewed by the radiation port at an infinite number of angles as the radiation port travels around the tumor isocenter. A high-resolution version of IMRT based on 3-D conformal radiosurgery technology developed at BrainLab makes sophisticated dose delivery for complex lesions, fast and efficient. This technology is currently under investigation at UCLA.

Another amazing technological feat is the development of the Independence 3000 IBOT™ Transporter. An engineering marvel, IBOT is a wheel chair for paraplegics that walks, climbs stairs, and stands for the patient. One of the psychological stresses facing paraplegic patients is the inability to get where their mobile counterparts can go and look at their peers at eye level. The advanced gyro-balanced system is designed to balance on 2 or 4 wheels. It instantly adjusts itself to balance and counter balance any movement of the seated user or changes in the center of gravity. In 2-wheel mode the seated user it is at eye level with average height individuals, in 4-wheel mode it effortlessly climbs stairs and traverses uneven terrain. More information can be obtained at http://jnj.com

Conclusion. The next 5 to 10 years promises to bring to the field of spine care a more well-rounded holistic approach to treatment. When more traditional treatments are required, they will focus on repairing and restoring function in a manner that more closely mimics our evolutionary design. Our greater understanding will lead to more uniformly accepted treatments, indications, and strategies. This will be reflected in our treatment outcomes. Until this time, it is vital that those currently treating such patients recognize the limits of our specific subspecialty approaches to spine pathology and maintain an open mind with respect to treatment approaches of allied subspecialties. We need to be acutely sensitive to the problem of inappropriately aggressive or conservative approaches that have led to a lack of credibility among the public with respect to spine care. Most importantly, it is imperative that we understand how our treatment strategies of today will affect our patients 5 or 10 years from now. Will an aggressive quick fix today make our patient ineligible for a more effective, longer lasting treatment that may be available a few years from now. Application of the innovations is only appropriate when there is a documented benefit over currently available methods with an acceptable risk profile and not just for the sake of change. The surgeon should undertake full assessment of why a new procedure is chosen over established procedure, what are the indications, and how much training is necessary before advocating the procedure to patients.14 Only by maintaining multi disciplinary open-mindedness and being one step ahead in our understanding of current technologies can we hope to reward our patients with maximized degree and duration of treatment outcome. The newer technologies must assume the burden of improving surgical efficiency and outcome with reduced risks, cost, and resource utilization.15

Address comments and questions to:

Brian Wieder, MD
701 E. Hampden Avenue, Suite 510
Englewood, CO 80113

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