Long-Term Results of Cervical Spine Disc Replacement
The first report of long-term results with disc replacement for the cervical spine (neck) is now available. Patients in France treated with the Bryan Cervical Disc Arthroplasty were followed for at least eight years after surgery. This report gives us an idea of how well those 21 patients did past the first two to four years (which is the time period most other studies have reported on).
Artificial disc replacements were developed to avoid neck fusions and thus preserve neck motion. The idea came about when studies showed that cervical spine fusion often resulted in added wear and tear and degeneration of the adjacent (next level above or below) segment.
But there are concerns with the disc implants. They can migrate or move backwards in the spine. A position too far back can increase the risk of spinal cord or spinal nerve root compression. There is the possibility of subsidence (disc sinks down into the vertebral bone). Loosening of the implant and adjacent segment degeneration round out the list of possible problems or complications that can occur.
The disc system used (Bryan cervical disc arthroplasty) mimics the normal protective disc that acts like a cushion between two vertebral bodies. It is made of titanium on the outside with a plastic inside. The titanium endplate shells are covered in a double layer of porous coating to allow for better grip between the vertebrae.
Everyone in the study had good spinal alignment without any sign of instability. They had all tried conservative care without good enough results to bypass surgery. Patients ranged in age from 26 to 65 years old.
Results were measured using a variety of outcomes such as work status, activity level, symptoms (neck and/or arm pain), and neck motion. X-rays were used to assess the position of the device and to look for heterotopic ossification (HO).
With heterotopic ossification, there is additional bone formed outside the skeletal system. The extra bone usually forms in the surrounding soft tissues, especially the nearby muscles. It's a problem because the artificial disc replacement (ADR) is designed to maintain joint motion. The heterotopic ossification reduces soft tissue mobility and causes increased stiffness.
The eight-year results were excellent. Three-fourths of the patients (18 of the 21) were free of neck and/or arm pain. They could perform all daily activities and chores without any problems. There were no neurological symptoms (e.g., no numbness or tingling down the arms, no change in deep tendon reflexes or sensation).
Heterotopic ossification (HO) was a problem in almost half of the patients. More HO seems to develop as time goes by. Not only do more patients show evidence of HO in time, those who develop it early start to progress to more severe cases. Having a two-level disc replacement is also a risk factor for HO.
Increased pain and decreased neck motion were linked with HO. The authors suggest several reasons why there was so much heterotopic ossification in these patients. First, the disruption of soft tissues and damage to the muscles is greater when more than one level is involved.
Second, patients in this study were not given antiinflammatory medications, which are known to reduce the risk of HO. And third, removing the diseased disc and damaged bone involves a special milling process, which may contribute to the body's response making more bone.
As far as the other common complications developing (loosening, migration, subsidence, implant failure), there were none! There were three cases of spontaneous fusion (bone filling in around the implant) but no symptoms or functional problems as a result. No one had any further follow-up surgery.
About 19 per cent of the group did develop adjacent segment disease (ASD). That's a striking finding since it has been believed that fusion is what causes increased degeneration at the vertebral segments on either side of the fusion. There is some speculation that ASD develops because there were already some degenerative changes present before disc replacement.
In conclusion, artificial disc replacement for the cervical spine (neck) is safe and effective. But it may not be without its problems. Patients who chose to have this procedure should be aware of the potential problems, risks, and benefits. More suggestions and guidelines can be made as other long-term studies are published.
Gerald M. Y. Quan, MBBS, FRACS, PhD, et al. Eight-Year Clinical and Radiological Follow-Up of the Bryan Cervical Disc Arthroplasty. In Spine. April 15, 2011. Vol. 36. No. 8. Pp. 639-646.