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For other articles and previous issues click here. October 17, 2005
Flexible Fix A treatment similar to kyphoplasty may add flexibility to treating vertebral compression fractures. Nearly 800,000 people suffer vertebral compression fractures (VCF) in the United States each year. Primarily caused by osteoporosis, these fractures are most prevalent among older women who are both postmenopausal and have low bone mineral density. VCFs also occur in men and in people of any age who have been taking steroids or other drugs over a long term. Falls, car accidents, cancer, and bone infections also cause intensely painful spinal fractures. Traditional treatments—bed rest, bracing, physical therapy, and pain medication—resolve most cases. Severe cases require surgery to prevent the spine from pressing on the spinal cord or to support the fractured vertebra. Most physicians view surgery as a last resort because it is difficult and risky. Two minimally invasive interventional procedures—vertebroplasty and kyphoplasty—are gaining favor as an alternative to traditional surgery when conventional therapies fail. Vertebroplasty is an image-guided interventional procedure. Physicians stabilize the collapsed vertebra by injecting polymethylmethacrylate (PMMA) medical-grade bone cement into the spine. The cement fills the spongy bone cavity, and any cracks and crevices within, hardening rapidly. Kyphoplasty, developed not long after vertebroplasty, is similar
except that the physician inserts an inflatable balloon to create a cavity in
which the cement is injected. Kyphoplasty has the reported advantage of restoring
the height of the vertebrae. Alternative Approach The OptiMesh system was invented by the late Stephen Kuslich, MD, an orthopedic surgeon who specialized in treating disorders of the lumbar spine. Kuslich actually was looking for an improved interbody application that would require less damage to anatomical structures than traditional fusion surgery, says Karen Roche, vice president of operations and technology for Spineology. The OptiMesh system is similar to kyphoplasty, but instead of a balloon and bone cement, the surgeon prepares a cavity in the vertebra and uses a flexible mesh container with a biologic fill material (bone graft), which is delivered under pressure to fill the cavity and support the vertebra. Bone Graft Fill Pressurizing the bone graft into the mesh causes the mesh to deploy and conform to the prepared cavity. It also opens the mesh pores. The pores, nominally 1,500 microns in size, allow bone growth and graft incorporation to occur. Roche says the approach takes advantage of the physics of granular materials. The injected bone graft is finely ground and morselized. Pressure—and containment provided by the mesh—causes the granular material to change from a flowable liquid to a rigid solid. In its solid phase, the bone graft material is able to withstand significant compression loads. Like kyphoplasty, the procedure takes approximately 20 to 30 minutes. “It is ordinarily done under general anesthesia,” Roche says. “However, there are a number of physicians who do use conscious sedation.” It can be a same-day procedure or require an overnight stay, Roche says. “It depends on the nature of the fracture, but it’s not unusual in compression fracture treatments to have patients up and discharged by the next day.” Roche says the OptiMesh system has several advantages over vertebroplasty or kyphoplasty, the most significant being its use of a biologic material, which allows bone to regrow and the vertebrae to heal over time. “Acrylic cements do not support bone growth,” she says. However, some interventional radiologists are skeptical of the
system because the larger size of the instruments makes it a more invasive procedure
and because they don’t see much improvement on the success or safety of
vertebroplasty or kyphoplasty. The subsequent fractures occur because the cement hardens the vertebrae, according to Roche. “We believe, based on biomechanical testing, that this risk may be reduced by using bone graft instead of cement. With bone graft, the vertebrae are filled with a material that has more similar load sharing characteristics to that of normal bone. It’s less stiff, more ‘natural,’” she says. Also, Roche says, the cement becomes hot as it is setting and has the potential to harm any tissue with which it comes in contact. “Bone cement that flows up against a nerve or vessels creates cellular damage. The flow of cement can be difficult to predict or control.” Bone cement also has some monomer toxicity, which is why some physicians are cautious about doing multiple-level repairs, she says. However, that’s not an issue with bone graft. Height Restoration Restoring height is important because severe curvature of the spine can cause pulmonary and other complications, Roche says. “Loss of height, or increased angulation, alters the biomechanical loading pattern experienced by adjacent levels, potentially placing them at increased risk for fracture.” Roche says Spineology is working to reduce the size of its instruments by 25% to 30% to make the procedure less invasive. Also, she adds, while the instruments are bigger, they require only one incision whereas kyphoplasty requires two entry points. The company offers one or two OptiMesh training courses at different locations around the country each month, according to Roche. “We’re still a small company and have limited distribution. But we’re in the process of building that network so we have better exposure,” Roche says. Multiple Choice The younger and more mobile the patient, the more Beall leans toward the OptiMesh system. “What I base it on is patient condition,” he says. “If somebody is vigorous and healthy and puts a lot of biomechanical strain on the spine, you want to provide them with something that will take a lot of strain and that’s the OptiMesh. If they don’t really put a lot of stress and strain on their spine, if they are more fragile or very advanced in age, then cement is probably OK.” The location of the vertebral body is also a consideration in determining which procedure Beall will use. The vertebral body to be repaired has to be a certain height to allow the entry of the OptiMesh instruments. Also, it has to be located in the mid to lower thoracic spine. “It can’t be the upper thoracic spine,” Beall says. Beall says he tends to favor the OptiMesh system because of the leakage issue. “Anytime you use bone cement you have the potential to leak,” he says. “Extravasation of bone cement is a real risk, especially with vertebroplasty. It can leak in and around the nerve roots, go where it shouldn’t go. The risk shouldn’t be underplayed. It’s definitely real.” The OptiMesh system offers a better containment method and provides less of a chance of extravasation, he says. Beall has also used the OptiMesh system to deliver PMMA. When
he uses bone graft, the system is more expensive than kyphoplasty because bone
graft costs twice as much as cement. He says it takes him roughly the same time
to do a kyphoplasty as it does to use the OptiMesh system with cement. The procedure
takes longer if he’s using bone graft rather than cement, however. The
time-consuming part is filling the tubes to be inserted into the cavity with
the allograft. Prefilled allograft delivery tubes currently in development should
reduce the time of the procedure, he says. Beall also likes that the OptiMesh
instruments are made of stainless steel and are reusable. Invasive Issue Murphy is concerned that the tools are larger than the pedicles, which are the short hollow tubes surrounded by thick cortical bone attaching each vertebral body together. The pedicles could easily be fractured by such large instruments, he says. “It’s 6 or 7 gauge now, but it has to get down to 9 to 10 gauge before I’ll use it,” he says. Joshua A. Hirsch, MD, is director of interventional neuroradiology and chief of minimally invasive spine surgery at Massachusetts General Hospital in Boston. He is a well-known proponent of vertebral augmentation. He is not surprised that some interventional radiologists would find the OptiMesh system cumbersome because it was designed by orthopedic surgeons who often work with larger instruments. Hirsch doesn’t discount the proven track record of vertebroplasty and kyphoplasty. If Spineology argues that the rationale for using its OptiMesh system is enhanced safety over vertebroplasty, it will be disappointed, Hirsch says. “Radiologists do vertebroplasty/kyphoplasty by the thousands today, and they do them safely,” Hirsch says. “Further, the OptiMesh system is far more involved than vertebroplasty/kyphoplasty, and risks often increase in proportion to the complexity of the procedure.” Still, Hirsch expects that the OptiMesh system will be just one of many alternatives for minimally invasive spine treatments that are developed. “To think that 10 years from now we only want to be putting in PMMA is silly,” he says. The idea of new applications and new materials for minimally invasive spine treatments by interventional radiologists are exciting, Hirsch says. “OptiMesh may or may not be part of that solution; however, it is clear to me that companies will continue to find new materials to deliver percutaneously to spine patients and new methods of enhancing the delivery.” Interbody Applications Beall is optimistic about the concept of using the system for interbody fusion. The OptiMesh product would fuse the spine in front and the pedicle screws and rods would fuse the spine and the vertebral body in the back. “I’m in the process of being trained to put pedicle screws and rods in minimally invasively,” he says. It would be exciting to combine the OptiMesh product with pedicle screws and rods, he says. “It’s a very exciting application because there are lots of spine surgeries and a great need for interbody fusion,” he adds. “This would allow interbody fusion to be done percutaneously.” — Beth W. Orenstein of Northampton, Pa., is a freelance
health writer and regular contributor to Radiology Today. |
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