Taking a Look at Cervical Disc Replacement: The Big PictureWhen a new treatment like disc replacement comes along, it takes a while before it's clear who should have this surgery. So patient selection is extremely important. At first, it's just a limited group of patients who qualify for the procedure. That's okay because surgeons want the best results for their patients.
In this article, a neurosurgeon familiar with cervical disc arthroplasty (neck disc replacement) summarizes the use of this device. Pros and cons for its use, patient selection, details of surgery, and postoperative complications are discussed.
Artificial disc replacement (ADR) is relatively new. In June 2004, the first ADR for the lumbar spine (low back) was approved by the FDA for use in the US. Replacing a damaged disc in the cervical spine (neck) is a bit trickier. The disc is part of a complex joint in the spine. Making a replacement disc that works and that will last is not an easy task.
The artificial disc is inserted in the space between two vertebrae. The goal is to replace the diseased or damaged disc while keeping your normal neck motion. Disc replacement can be done instead of fusing the neck and losing neck motion.
With cervical fusion, there is concern that the spine will develop similar problems over time above and below the affected spinal level. This is called adjacent segment disease. The hope is that disc replacement will help avoid such problems. A few studies comparing fusion to disc replacement have been published. The short-term results seem to show that adjacent segment disease occurs less often in patients with disc replacements compared with cervical fusion. Time will tell as long-term results are observed and reported.
Who's the best candidate for a cervical disc replacement? Well, for starters, anyone who is also a candidate for cervical fusion. These patients have obvious cervical disc disease with herniated disc, bone spurs, and/or neck/arm pain from myelopathy or radiculopathy. Myelopathy is any damage to the spinal cord as it travels down the spine. Pressure on the spinal cord can cause significant symptoms and disability. Radiculopathy is pressure or irritation of the spinal nerve roots as they exit the spinal cord and travel down the arms (or legs).
In Europe where disc replacements have been done much longer than in the U.S., there is a broader patient selection. For example, patients with a failed cervical fusion, patients with multilevel degenerative disease, and patients with segmental disease after fusion are also candidates for cervical disc replacement.
Surgeons now have more choice in selecting the right device for each patient. Newer, improved implants are available compared to when the first disc replacement came out. There are different basic designs in how the implant is held between the vertebrae and how movement is produced. Different materials are used such as titanium, cobalt, chrome, or combinations of these alloys.
In fact, there are many things to consider when designing a perfect disc replacement. The pore size and surface coating of the implant can influence how fast and how well the bone grows in and around the device. That's important in holding the implant in place and avoiding complications such as implant migration (moving) or subsidence (sinking). Implants that are held in place with teeth or spikes seem to take a while to settle into the endplates. The endplates are fibrous cartilage between the discs and the bone.
Certain patients may not be good candidates for this type of surgery. Brittle bones from osteoporosis or other significant diseases may keep a patient from having this type of surgery. Local problems in the spine such as infection or severe degeneration of the facet (spinal) joints are also possible contraindications (reasons NOT to have the operation).
If the spine has already started to fuse itself (autofusion) with bone spurs crossing the disc space, then disc replacement is not advised. And if the patient has an unnaturally straight cervical spine called kyphosis, cervical disc replacement may not be a good choice. The kyphosis can get much worse after surgery. The result can be worse neck pain than before surgery. In time, with the right design and placement, this problem may be resolved. But for right now, preoperative kyphosis is considered a contraindication for cervical disc replacement.
Not much is known about how to prevent subsidence or other complications with cervical disc replacements. The few studies that have been done offer surgeons some ideas. First, save the endplate and use them to help anchor the implant. A wider, thinner implant might be helpful when the disc space is small to begin with. Patients with a collapsed disc fall into this category. Anything that changes bone structure is considered a problem. This could be osteopenia (a pre-osteoporotic state), any other bone disease, and/or medications that reduce bone quality.
Migration hasn't been reported as a major problem with artificial cervical replacements. Spontaneous fusion seems to be more common. So much bone forms around the implant that motion is lost. Once again, surgeons are encouraged to avoid drilling into the endplate as a way to prevent this complication. Selecting the right size implant also seems to be helpful. Too small of an implant may result in the body filling in with bone and eventual fusion.
Despite the fact that it sounds like complications are common with this procedure, many patients do have a successful result. The outcomes are equal to (if not better than) cervical spine fusion. Surgeons in the United States are slowly expanding use of the implants for multisegmental (more than one level) spinal disease.
With long-term studies, it will eventually be possible to answer the question of whether adjacent segmental disease can be prevented with disc replacements. That would definitely give implants the advantage over cervical fusion.
It's still not entirely clear whether disc replacement is a better treatment approach than just doing a fusion. And with improved materials, fixation methods, and surgical techniques, the overall results with disc replacement may improve enough to put all these questions aside in the end.
Neil Duggal. Cervical Disc Arthoplasty: A Practical Overview. In Current Orthopaedic Practice. May/June 2009. Vol. 20. No. 3. Pp. 216-221.
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