Disc replacements were first designed for the low back (lumbar spine). Their success in restoring pain free motion led to the development of a similar device for the neck (cervical spine). Although not everyone qualifies for a cervical intervertebral disc replacement, they seem to work well for those patients who are good candidates.
In doing a literature search, we did find a recent review of cervical disc replacements (CDRs) by two surgeons at the William Beaumont Hospital in Michigan. Their intent was to bring surgeons up-to-date on the current status of these implants but much of the information they provided will be of interest to you (and other potential patients).
There are now five different models and designs to choose from. Some are made of a titanium-ceramic material. Others are a titanium-alloy outer part with a polyurethane (plastic) core. Shapes vary and include saddle-shaped, triangular, round, square, and square with rounded edges.
The goals of a good fitting design are first to maintain the space between two vertebra (in other words maintain disc height). Preserving motion at that segment is equally important. And providing shock absorption while keeping the proper spinal alignment is important, too. The implant should be durable (last a long time) with few (hopefully no) complications or problems.
A good fit can depend on how the device sits in-between the two vertebral bones. Different methods of “fixation” have been tried. Some implants are serrated while others have teeth or keels to help them grab hold of the bone and stay where placed. Screws and cement have been used to aid fixation.
But the method with the best results (fewest complications, minimal debris, lowest rate of adjacent segmental disease) has yet to be determined. Studies are ongoing comparing cervical disc replacement with the standard treatment (anterior cervical discectomy and fusion or ACDF). And now with five CDR devices to choose from, research is being done to compare the results among the currently available implants.
Overall, research results show that patients who are treated with either fusion (the ACDF procedure) or cervical disc replacement (CDR) all get better. They all have less pain and fewer neurologic symptoms (e.g., neck and arm pain, numbness, or tingling down the arm).
There is always a concern for adjacent segment disease or ASD. ASD is defined as degeneration of the disc at the level next to the fusion or disc implant. This seems to be improving with cervical disc replacements. These results may be explained by the fact that the implant preserves motion, so there is less pressure on the discs above and below the surgical level.
Other benefits of the cervical disc replacement (CDR) (over fusion) include fewer revision (second) surgeries, faster return to work, overall greater improved function, and maintenance of the improvements in pain and function over time. Longer-term studies (two years or more) tend to show fairly equal results between fusion and CDR as time goes by.
At first, cervical disc replacement was only done at one level. But now, with improved implant design and increased surgeon experience, multilevel procedures (up to three levels) are being done. Even with the increased risk of complications with multiple level implants, survivorship of the devices and patient satisfaction are high (95 per cent).
Reported complications with either procedure include difficulty swallowing, vocal cord paralysis, penetration of the esophagus or dura (lining around the spinal cord), infection, and hardware failure. In the case of disc replacement, there have been rare episodes of device migration (disc implant shifts or moves significantly), spinal cord compression, and bone spur formation around the implant. Some of these problems required removal of the disc implant.
Long-term concerns include adjacent segment degeneration and wear debris from tiny flecks of metal getting into the area from the implant. There have been some questions raised about the long-term safety of disc implants from studies that showed chronic inflammation around the implant and in the spinal cord. All metal implants have an increased risk of a hypersensitivity (serious allergic) reaction.
One final area the authors of this review article considered was the cost of cervical disc replacement (CDR) versus the fusion (anterior cervical discectomy and fusion or ACDF). At $2500 for a fusion compared with $4000 for the implant, fusion surgery is less expensive in the short-term. But if a second surgery after fusion is needed later because of increased wear and degeneration at the next segment, then in the long-run, disc replacement is less expensive.
Some insurance companies are refusing to pay for the implant surgery until the benefits of disc replacement (over fusion) are clearly proven. Long-term studies are needed to evaluate all factors related to these two very different treatment approaches to disc degeneration. In time, it may become clear which treatment will provide the most treatment benefit.