One Advantage of Cervical Disc Replacement Over Spinal FusionSurgeons are actively studying the advantages and disadvantages of neck fusion compared with disc replacement. In this study, they take a closer look at the amount of ossification of the surrounding ligaments in two groups of patients after surgery. Ossification refers to the formation of bone within soft tissue structures such as muscles and ligaments.
The exact cause of ossification isn't quite clear. It could be part of the healing response of the soft tissues after being cut and moved away from the bones. There is some thought that the pins used in the procedure might generate this type of response. Not everyone develops ossification. When they do, there aren't always any symptoms (such as pain) or obvious effects (further loss of motion). So just how much of a problem this problem is remains under investigation.
In this study comparing the amount of ossification between neck fusion and disc replacement, each patient had the selected procedure at one level in the cervical spine (neck). Everyone had tried conservative care first before resorting to surgery. The first group had an anterior cervical discectomy and fusion (ACDF). In this procedure, the surgeon comes in from the front of the neck (anterior) and removes the disc (discectomy). Then the two vertebrae (one above the disc and one below the disc) are held together (fused) with a metal plate and screws (also called pins). Only one level (between C3 and C7) of the cervical spine was operated on.
The second group had a disc replacement with a particular disc replacement product (the Bryan disc) made by the Medtronic Company in Memphis, Tennessee. As with the fusion procedure, only one cervical level was replaced in the disc replacement group. For both groups, X-rays were taken two years after the initial surgery. X-rays were repeated four years after the procedures. The location and amount of ossification were recorded. No one had any signs of ossification before the surgeries and no effort was made to prevent ossification after surgery.
As it turns out, patients in both groups developed some ossification. But the fusion group had significantly more ossification when compared with the implant (artificial disc) group. Here are a few statistical details to give you an idea of the differences between the two groups. At the end of two years, 64 per cent of the fusion group had ossification of the anterior longitudinal ligament at the levels adjacent (above and below) the fusion segment. By comparison, only 25 per cent of the artificial disc group developed ossification at the two-year check-up point.
At the end of the study (four years after the surgery), 84 per cent of the fusion group showed signs of ossification. This compared with 52 per cent in the disc replacement group. In both groups, there were patients who didn't develop any ossification at all. That finding (and those patients) is the focus of future research. If scientists can identify why some patients don't develop this problem at all, it might be possible to prevent it in everyone.
Scientists have thought that perhaps the closeness of the metal plate to the next disc space might be a cause of the ossification. There was one previous study where it looked like placement of the metal plate less than five millimeters away from the disc resulted in more cases of ossification. But the results of this new study did not support that finding. The authors of this study are still thinking it's more likely that preserving motion with a disc replacement is the key factor in preventing (and possibly delaying) ossification.
One other aspect that remains a mystery is the fact that if patients don't develop adjacent-level ossification 12 to 24 months after a disc replacement, then they aren't likely to develop it later. Other causative factors being considered include whether it's the plates that stimulate the ossification, the pins holding the plates in place, neither, or both. Alternately, it could be just a matter of changing the biomechanics of the cervical spine that is really the start of the ossification process.
For now, this study doesn't answer the question of why ossification develops. But it is an important study because it's the first time anyone has looked at the Bryan implant and compared it to anterior cervical discectomy and fusion (ACDF) in relation to ossification. There's plenty of room for follow-up studies to identify cause and effect in an effort to prevent ossification after fusion and/or after disc replacement. For now we can say there is much more ossification with cervical fusion compared with the Bryan disc replacement.
Ben J. Garrido, MD, et al. Adjacent-Level Cervical Ossification After Bryan Cervical Disc Arthroplasty Compared with Anterior Cervical Discectomy and Fusion. In The Journal of Bone and Joint Surgery. July 6, 2011. Vol. 93-A. No. 13. Pp. 1185-1189.
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