Do Disc Replacements Get the Green Light?
Five years ago (in 2004), the Food and Drug Administration (FDA) approved the CHARITÉ artificial disc. This approval was made based on a two-year trial conducted by orthopedic surgeons at 14 different clinics around the United States. In this study, five-year results for the same patients are reported. This is the largest and longest randomized, multicenter study of artificial discs published in the United States so far.

All patients receiving the artificial disc had severe back pain. They had all tried (and failed to improve with) six months or more of conservative (nonoperative) care. The diagnosis for everyone was degenerative disc disease. Only patients with single-level disease between L4 and S1 were included.

The patients were randomly divided into two groups. One group was treated with a spinal fusion at the affected level. The fusion was done with an anterior approach (from the front of the spine). BAK cages and iliac crest autograft were used to fuse the spine. This means the damaged disc was removed and a titanium cage put in its place. The procedure can be done with a very small incision to limit tissue trauma. The cage has holes in it and the bone grows in and around it, holding it in place. Iliac crest autograft refers to the fact that bone was taken from the patient's own hip to help get the fusion process started.

The second group had the damaged disc removed and replaced with an artificial disc device. The CHARITÉ artificial disc was designed and tested first in Germany before it was brought to the United States. The CHARITÉ disc has three-pieces. There is a sliding core sandwiched between two metal endplates. The sliding core is plastic and the endplates are metal made from cobalt chromium. The endplates support the core. They have small teeth that hold them to the vertebrae above and below the disc space. The sliding core fits in between.

The main difference between spinal fusion and an artificial disc implant is motion. Fusion stops vertebral motion at the level of the fusion. The disc implant allows continued normal motion in all directions -- even including a small amount of segmental translation. Segmental translation refers to the sliding motion of one vertebral body over another. With a fusion, there is no segmental translation at the fused level. Just the right amount of translational movement is needed for normal spinal motion. Too much translation and the segment would be unstable.

Neither treatment (fusion or disc replacement) is perfect. There are concerns that over time, the artificial disc won't hold up and the patient will need another operation. But fusions limit motion and are known to cause adjacent-level disc degeneration and spinal joint degeneration. There are so many unknowns -- that's why a long-term study like this (comparing fusion to disc replacement) can be very helpful.

Both the authors and the editors reviewing this study raised serious concerns about the way the study was done and the significance of the findings. Let's look at the results and then discuss why the study design and methods might be under question.

Various measures were used to study the outcomes. Pain, range-of-motion, function, disability, work status, and reoperation rates were all compared between the two groups. For the most part, they found similar results in both groups. The same number of patients improved at the same time and in the same amount in both groups.

Digitized, computerized X-rays were taken to measure differences between flexion, extension, and translation of the vertebral segment operated on as well as the adjacent segments (above and below). The radiologist looked for bone spurs, osteoporosis (loss of bone mass), and heterotopic ossification (formation of bone in the nearby soft tissues).

The X-rays showed some loss of disc height over time with both the BAK fusion and the CHARITÉ implant. The BAK group also had a small amount of measurable translation (0.1 mm compared with 0.4 mm to 0.8 mm in the CHARITÉ group). Ossification in the disc area that could affect motion affected about one out of every five patients with the disc replacement.

The groups were very similar in terms of race, age, weight, activity level, and work status before surgery. The only significant difference was the larger number of patients in the disc replacement group who had prior (minor) back surgery. The CHARITÉ group was also more likely to be working full-time before surgery compared with the fusion group. After surgery, long-term disability was much more likely in the BAK group (20 per cent compared with eight per cent in the disc group).

And the most important finding was that the results for both groups after five years were the same as was observed after the first two years. The consistency of results from two to five years supports the idea that mid-term to long-term results are good for the disc replacements.

Now, what about the concerns raised concerning the study design? The authors reported a loss of two-thirds of the patients from the two-year study. In other words, only one-third of the patients in the original study participated in the full five-year review. And only eight of the original 14 clinical sites were involved.

The authors report that there were many legitimate reasons for these changes. But the editors question the fact that not all patients were contacted at five years. Those who were contacted had similar results between the BAK and the CHARITÉ. Whereas the authors say the disc replacement is just as good as a fusion, the editors point out that the outcomes were disappointing. In theory, the artificial disc replacement should produce better results than a fusion. In reality, only slightly more than half the group had improved function and that was very minimal.

The editors suggest that the results of both treatment methods weren't really acceptable. And the fact that the patients chosen for either surgery were highly selected raises some questions, too. If patients were so carefully selected but didn't really get the hoped for results, what does that suggest?

Although the authors say the disc replacement is safe and effective for this patient population, the editors advise caution. Future studies are needed to look at some of these problems and answer the difficult questions raised before giving artificial disc replacements the green light as safe and effective for long-term use.
Richard D. Guyer, MD, et al. Prospective, Randomized, Multicenter Food and Drug Administration Investigational Device Exemption Study of Lumbar Total Disc Replacement with the CHARITÉ Artificial Disc Versus Lumbar Fusion: Five-Year Follow-Up. In The Spine Journal. May 2009. Vol. 9. No. 5. Pp. 374-386.