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Heterotopic Ossification Higher Than Expected with Artificial Disc Replacement

Posted on: 08/12/2010
It's possible now to have a disc replacement in the cervical spine (neck). But the "kinks" aren't fully worked out yet. One of those kinks is the problem of heterotopic ossification (HO) (extra bone formation).

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.

But how often does this really happen? Is it enough to raise a red flag to suggest something should be done to prevent it?

According to the results of this study, it happens more often than expected. In a group of 170 patients treated in Korea, 40 per cent developed heterotopic ossification. And it was serious enough to reduce the life of the implant to an average of only 27 months (less than two and a half years).

The natural question to ask is: What causes this to happen? Can it be prevented? Are there certain patient factors that could be identified ahead of time signaling which patients might be at risk for the development of heterotopic ossification?

By collecting, examining, and analyzing data on this group of patients, the authors were able to provide some (but not all) answers to these questions. First of all, it's important to recognize that they only studied three of the many different types of disc replacement currently available for the cervical spine (neck). Differences were determined using the Bryan disc, the Mobi-C, and the ProDisc-C.

These three were chosen because they are different in how they are made (material) and how they work (motion system). The Bryan disc is made of titanium and polyurethane. Polyurethane is a resilient, flexible, and durable manufactured material that can be stretched, smashed, or scratched, and remain fairly indestructible.

The Mobi-C implant is composed of titanium and polyethylene (plastic component). And the ProDisc is cobalt chrome and polyethylene. On a continuum of motion provided by each artificial disc, at one end, there's the Bryan disc, which provides the most movement with the least constraint. At the other end, is the ProDisc device with the least motion. The Mobi-C falls somewhere in between.

X-rays were used to look for heterotopic ossification (HO) during the follow-up period after the joint implants were put into place. The length of time between surgery and the development of HO was also recorded.

Though this sounds like an easy task, there were some problems. For example, X-rays show bone formation in the front of the vertebra (anterior) much better than in the back (posterior). Sometimes it was difficult to tell the difference between bone spurs large enough and long enough to form a bridge and heterotopic ossification.

But the overall results showed there was more bone formation than expected and that the type of implant did make a difference. Occurrence rate was highest in the ProDisc-C group and lowest in the Bryan disc group. What made the difference? And what patient factors might account for no heterotopic formation in 60 per cent of the patients?

This study didn't provide all of the answers to those questions. There's plenty of room for future studies to explore both patient characteristics and implant factors that could account for the problem of bone formation.

For now, it looks like type of implant does make a significant difference in when heterotopic ossification develops. Implants that fit well up against the endplate seem to avoid the problem of bony overgrowth. The endplate is a circle of cartilage between the disc and the vertebral body.

As far as possible patient factors, the authors propose genetic traits might be at work. Specific patient characteristics (age, gender, lifestyle, body size, level of implant, diagnosis, type of surgery, and so on) were not compared.

Seong Yi, MD, PhD, et al. diffeence in Occurrence of Heterotopic Ossification According to Prosthesis Type in the Cervical Artificial Disc Replacement. In Spine

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